Synthesis, reactivity, and catalytic behavior of iron/zinc-containing species involved in oxidation of hydrocarbons under Gif-type conditions

Article abstract of DOI:10.1021/ja970562r

The present study explores the nature and reactivity of iron- and zinc- containing species generated in hydrocarbon-oxidizing Gif(IV)-type solutions (Fe catalyst/Zn/O₂ in pyridine/acetic acid (10:1 v/v). The ultimate goal of this investigation is to unravel the role of metal sites in mediating dioxygen-dependent C-H activation, which in the case of Gif chemistry demonstrates an enhanced selectivity for the ketonization of secondary carbons. Reaction of [Fe₃O(O₂CCH₃)₆(py)₃]·py (1) with zinc powder in CH₃CN/CH₃COOH or CH₂Cl₂/CH₃COOH affords the trinuclear compound [Zn₂Fe(II)(O₂CCH₃)₆(py)₂] (2). Single-crystal X-ray analysis confirms that one monedentate and two bidentate acetate groups bridge adjacent pairs of metals with the iron atom occupying a centrosymmetric position. The analogous reduction of 1 in py/CH₃COOH (10:1.5:1,2:1 v/v) yields [Fe(II)(O₂CCH₃)₂(py)₄] (3), [Fe(II)₂(O₂CCH₃)₄(py)₃](n) (4), and [Zn(O₂CCH₃)₂(py)₂] (5) depending on the isolation procedure employed. Compound 3 possesses a distorted octahedral geometry, featuring a C₂ axis bisecting the equatorial, pyridine-occupied plane, whereas the two acetate groups reside along the perpendicular axis. Compound 4 is a one-dimensional solid constructed by asymmetric diferrous units. Two bidentate and one monodentate acetate groups bridge the two iron sites, with the monodentate bridge also acting as a chelator to one ferrous center. The two iron centers exhibit weak antiferromagnetic coupling. Compounds 3 and 4 are also accessible from the reduction of 1 with iron powder or treatment with H₂/Pd. Solutions of 3 and 4 in pyridine or py/CH₃COOH react with pure dioxygen or air to eventually regenerate 1 in a concentration-dependent manner. Oxidation of 2 in py/CH₃COOH with pure dioxygen or air yields [Fe(2.22(2)(Zn(0.78(2))O(O₂CCH₃)₆(PY)₃]·py (1') and [Zn₂(O₂CCH₃)₄(py)₂] (6). Compound 1' is isostructural to 1, exhibiting rhombohedral symmetry at 223 K. The filtrate of the reduction of 1 with zinc in neat pyridine, when exposed to dioxygen, affords dichroic red-green crystals of monoclinic [Fe₂ZnO(O₂CCH₃)₆(py)₃]·py (1'). Species 1' yields products identical with those provided by 1 under reducing conditions. Compounds 2-6 are related by pyridine-dependent equilibria, as demonstrated by mutual interconversions and electronic absorption data in pyridine and py/CH₃COOH solutions. In non-pyridine solutions, Zn-containing species 5 and 6 rearrange to the crystallographically characterized species [Zn- (O₂CCH₃)₂(py)](n) (7) and [Zn₃(O₂CCH₃)₆(py)₂] (8). Compound 7 is a one-dimensional solid featuring a chain of Zn sites linked by a bidentate acetate group while additionally coordinated by a chelating acetate. Compound 8 is isostructural to 2. Further perturbations of the described structures are apparent in ionic iron-containing species, such as the pseudo-seven- coordinate iron in [Ph₃P=N=PPh₃][Fe(II)(O₂CCH₃)₃(py)] (9), which is obtained from the reaction of 3 with [PPN] [O₂CCH₃], and the water- coordinated iron in [Fe(II)(H₂O)₄(trans-py)₂][O₂CCH₃]₂ (10), which reveals an extensive two-dimensional network of hydrogen-bonding interactions. The pyridine-free species [Fe(II)₃(O₂- CCH₃)₆(OS(CD₃)₂)₂](n) (11) is isolable upon extensive incubation of 3 in (CD₃)₂SO. Compound 11 exhibits a remarkable one-dimensional structure, featuring four different types of acetate groups. Catalytic oxidations of adamantane, isopentane, benzene, toluene, cis-stilbene, and pyridine mediated by the system 1 (or 2-4)/Zn/O₂ in py/AcOH (10:1) afford product profiles which are not fully compatible with the reported outcome of analogous oxidations by hydroxyl radicals or biologically relevant high-valent iron- oxo species alone. The intermolecular deuterium kinetic isotope effect for the oxidation of adamantane to adamantanone is small (k(H)/k(D) = 2.01(12)) by comparison to values obtained for oxidation of hydrocarbons by biological oxygenases. Employment of hydrogen peroxide, t-BuOOH, or peracetic acid as potential oxo donors does not provide viable shunt pathways in the catalytic oxygenation of adamantane. The nature of active oxidant in Git(IV)-type oxidation is discussed in light of these structural and functional findings.

Full text of DOI:10.1021/ja970562r

7030
J. Am. Chem. Soc. 1997, 119, 7030-7047
Synthesis, Reactivity, and Catalytic Behavior of
Iron/Zinc-Containing Species Involved in Oxidation of
Hydrocarbons under Gif-Type Conditions
Bharat Singh,† Jeffrey R. Long,‡ Fabrizia Fabrizi de Biani,§ Dante Gatteschi,§ and
Pericles Stavropoulos*,†
Contribution from the Departments of Chemistry, Boston UniVersity, Boston,
Massachusetts 02215, HarVard UniVersity, Cambridge, Massachusetts 02138, and
UniVersita` degli Studi di Firenze, I-50144 Florence, Italy
ReceiVed February 21, 1997X
Abstract: The present study explores the nature and reactivity of iron- and zinc-containing species generated in
hydrocarbon-oxidizing GifIV-type solutions (Fe catalyst/Zn/O2 in pyridine/acetic acid (10:1 v/v). The ultimate goal
of this investigation is to unravel the role of metal sites in mediating dioxygen-dependent C-H activation, which in
the case of Gif chemistry demonstrates an enhanced selectivity for the ketonization of secondary carbons. Reaction
of [Fe3O(O2CCH3)6(py)3]‚py (1) with zinc powder in CH3CN/CH3COOH or CH2Cl2/CH3COOH affords the trinuclear
compound [Zn2FeII(O2CCH3)6(py)2] (2). Single-crystal X-ray analysis confirms that one monodentate and two bidentate
acetate groups bridge adjacent pairs of metals with the iron atom occupying a centrosymmetric position. The analogous
reduction of 1 in py/CH3COOH (10:1, 5:1, 2:1 v/v) yields [FeII(O2CCH3)2(py)4] (3), [FeII (O2CCH3)4(py)3]n (4), and
2
[Zn(O2CCH3)2(py)2] (5) depending on the isolation procedure employed. Compound 3 possesses a distorted octahedral
geometry, featuring a C2 axis bisecting the equatorial, pyridine-occupied plane, whereas the two acetate groups
reside along the perpendicular axis. Compound 4 is a one-dimensional solid constructed by asymmetric diferrous
units. Two bidentate and one monodentate acetate groups bridge the two iron sites, with the monodentate bridge
also acting as a chelator to one ferrous center. The two iron centers exhibit weak antiferromagnetic coupling.
Compounds 3 and 4 are also accessible from the reduction of 1 with iron powder or treatment with H2/Pd. Solutions
of 3 and 4 in pyridine or py/CH3COOH react with pure dioxygen or air to eventually regenerate 1 in a concentration-
dependent manner. Oxidation of 2 in py/CH3COOH with pure dioxygen or air yields [Fe2.22(2)Zn0.78(2)O(O2CCH3)6-
(py)3]‚py (1′) and [Zn2(O2CCH3)4(py)2] (6). Compound 1′ is isostructural to 1, exhibiting rhombohedral symmetry
at 223 K. The filtrate of the reduction of 1 with zinc in neat pyridine, when exposed to dioxygen, affords dichroic
red-green crystals of monoclinic [Fe2ZnO(O2CCH3)6(py)3]‚py (1′′). Species 1′′ yields products identical with those
provided by 1 under reducing conditions. Compounds 2-6 are related by pyridine-dependent equilibria, as demon-
strated by mutual interconversions and electronic absorption data in pyridine and py/CH3COOH solutions. In non-
pyridine solutions, Zn-containing species 5 and 6 rearrange to the crystallographically characterized species [Zn-
(O2CCH3)2(py)]n (7) and [Zn3(O2CCH3)6(py)2] (8). Compound 7 is a one-dimensional solid featuring a chain of Zn
sites linked by a bidentate acetate group while additionally coordinated by a chelating acetate. Compound 8 is
isostructural to 2. Further perturbations of the described structures are apparent in ionic iron-containing species,
such as the pseudo-seven-coordinate iron in [Ph3PdNdPPh3][FeII(O2CCH3)3(py)] (9), which is obtained from the
reaction of 3 with [PPN][O2CCH3], and the water-coordinated iron in [FeII(H2O)4(trans-py)2][O2CCH3]2 (10), which
reveals an extensive two-dimensional network of hydrogen-bonding interactions. The pyridine-free species [FeII (O2-
3
CCH3)6(OS(CD3)2)2]n (11) is isolable upon extensive incubation of 3 in (CD3)2SO. Compound 11 exhibits a remarkable
one-dimensional structure, featuring four different types of acetate groups. Catalytic oxidations of adamantane, iso-
pentane, benzene, toluene, cis-stilbene, and pyridine mediated by the system 1 (or 2-4)/Zn/O2 in py/AcOH (10:1)
afford product profiles which are not fully compatible with the reported outcome of analogous oxidations by hydroxyl
radicals or biologically relevant high-valent iron-oxo species alone. The intermolecular deuterium kinetic isotope
effect for the oxidation of adamantane to adamantanone is small (kH/kD ) 2.01(12)) by comparison to values obtained
for oxidation of hydrocarbons by biological oxygenases. Employment of hydrogen peroxide, t-BuOOH, or peracetic
acid as potential oxo donors does not provide viable shunt pathways in the catalytic oxygenation of adamantane.
The nature of active oxidant in GifIV-type oxidation is discussed in light of these structural and functional findings.
Introduction
and/or a high-valent iron-oxo intermediate is generated upon
dioxygen activation. Of particular interest in this context is the
currently emerging4 dinuclear FeIII/IV-(µ-O)2-FeIV core and its
precursor FeIII-(µ-O2)-FeIII moiety that may prove to be as
versatile active oxidants in non-heme diiron systems as the
(1) (a) Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759-805. (b)
Que, L., Jr. In Bioinorganic Catalysis; Reedijk, J., Ed.; Marcel Dekker:
New York, 1993; pp 347-393. (c) Wilkins, R. G. Chem. Soc. ReV. 1992,
21, 171-178. (d) Vincent, J. B.; Olivier-Lilley, G. L.; Averill, B. A. Chem.
ReV. 1990, 90, 1447-1467. (e) Que, L., Jr.; True, A. E. Prog. Inorg. Chem.
1990, 38, 97-200. (f) Kurtz, D. M., Jr. Chem. ReV. 1990, 90, 585-606.
(g) Lippard, S. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 344-361.
A substantial number of non-heme, iron-containing biological
sites1 have recently emerged as required or circumstantial centers
in dioxygen transport and activation. Both mononuclear2 and
dinuclear3 iron centers have been firmly recognized to reside
in a protein cavity in which a putative ferric (alkyl/hydro)peroxo
† Boston University.
‡ Harvard University.
§ Universita` degli Studi di Firenze.
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
S0002-7863(97)00562-3 CCC: $14.00 © 1997 American Chemical Society

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7031
putative FeIVdO/residue+• unit has been in heme iron chem-
istry.5 In principle, potent oxidants of this kind can execute
substrate oxygenation/desaturation6 or generate a biologically
beneficial7 (and occasionally detrimental)8 long-lived radical
upon interaction with biological material. Reactive oxygen
species (superoxide, singlet oxygen, alkoxyl/hydroxyl radicals,
nitric oxide)9 have also been implicated in degenerative biologi-
cal oxidations10 via mechanisms involving, inter alia, mediation
by inadvertently present iron (or copper) ions.
One inspirational example of hydroxylating activity is the
remarkable oxidation of methane to methanol mediated by the
well-documented diiron site of the hydroxylase component
of soluble methane monooxygenase (sMMO). The crystal
structures6a,b (2.2 Å (4.0 °C) and 1.7 Å (-160 °C) resolution)
of the “as isolated” (Hox) and dithionite-reduced (Hred) hydroxy-
lase protein from Methulococcus capsulatus (Figure 1), as well
as the recent structure (2.0 Å (-18 °C)) of the Hox protein from
Methylosinus trichosporium OB3b,3,4b have revealed the pres-
ence of a hydroxo-bridged diiron center in Hox (Fe‚‚‚Fe ) 3.4/
3.1 Å at 2.2/1.7 Å resolution) and a hydroxo-devoid diiron site
in Hred (Fe‚‚‚Fe ) 3.28 Å). The lack of hydroxyl bridge in
Hred is coordinatively compensated by a unique carboxylate shift
(Glu 243) from a terminal (Hox) to a monodentate-bridging as
well as chelating position. In addition, the bridging water
molecule in Hox (1.7 Å resolution structure; a buffer-derived
bidentate acetate is occupying this position in the 2.2 Å
resolution structure) assumes a nearly terminal position, with
Figure 1. Structures of Hox (2.2 Å (4.0 °C) and 1.7 Å (-160 °C)
resolution) and Hred of sMMO from M. capsulatus (after Rosenzweig,
A. C.; et al. Chem. Biol. 1995, 2, 409-418).6a
Concomitantly, the terminal water molecule in Hox is almost
dissociated in Hred. These atomic translocations upon reduction
lead to a core structure that is nearly five-coordinate at the iron
sites, although the semivacant coordination positions are at
orthogonal orientations.
Synthetic analogues of dinuclear iron sites11 have been
instrumental in elucidating structural aspects pertaining to the
enzymatic center of sMMO, but instances of functional behavior
are scarce.12 The latter category includes hydrocarbon-oxidizing
systems such as [Fe2O(TPA)2(H2O)2]4+/t-BuOOH(H2O2),13,14
[Fe2O(L)4Xn]m+/t-BuOOH (L ) 2,2′-bipy, 4,4′-Me2-2,2′-bipy,
1,10-phen; X ) H2O, Cl, OAc, CF3CO2),15 [Fe2O(OAc)2(bipy)2-
Cl2]/t-BuOOH(H2O2),16 [Fe2O(OAc)(tmima)2]3+/H2O2,16a [Fe2O-
(H2O)2-(tmima)2]4+/H2O2,17 [Fe2O(HB(pz)3)2(X)2]/Zn/O2 (X )
OAc, hfacac),18 and [Fe2O(salen)2]/2-mercaptoethanol/O2.19
Although only a handful of these systems employ dioxygen,
some of those featuring alternative oxo donors have been
established to operate via a dioxygen-dependent pathway.13a
only a long-range interaction with the other iron atom in Hred
.
(2) (a) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607-2624. (b)
Kappock, T. J.; Caradonna, J. P. Chem. ReV. 1996, 96, 2659-2756.
(3) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625-2657.
(4) (a) Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996, 29, 190-196. (b)
Shu, L.; Nesheim, J. C.; Kauffmann, K.; Mu¨nck, E.; Lipscomb, J. D.; Que,
L., Jr. Science 1997, 275, 515-518. (c) Liu, K. E.; Valentine, A. M.; Wang,
D.; Huynh, B. H.; Edmondson, D. E.; Salifoglou, A.; Lippard, S. J. J. Am.
Chem. Soc. 1995, 117, 10174-10185.
(5) (a) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem.
ReV. 1996, 96, 2841-2887. (b) Cytochrome P-450: Structure, Mechanism,
and Biochemistry; Montellano, P. O., Ed.; Plenum Press: New York, 1986.
(6) sMMO: (a) Rosenzweig, A. C.; Nordlund, P.; Takahara, P. M.;
Frederick, C. A.; Lippard, S. J. Chem. Biol. 1995, 2, 409-418. (b)
Rosenzweig, A. C.; Frederick, C. A.; Lippard, S. J.; Nordlund, P. Nature
1993, 366, 537-543. Toluene 2- and 4-Monooxygenases: (c) Newman,
L. M.; Wackett, L. P. Biochemistry 1995, 34, 14066-14076. (d) Pikus, J.
D.; Studts, J. M.; Achim, C.; Kauffmann, K. E.; Mu¨nck, E.; Steffan, R. J.;
McClay, K.; Fox, B. G. Biochemistry 1996, 35, 9106-9119. Stearoyl-acyl
carrier protein ∆9 desaturase: (e) Fox, B. G.; Shanklin, J.; Somerville, C.;
Mu¨nck, E. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 2486-2490. Rybreryth-
rin: (f) deMare´, F.; Kurtz, D. M., Jr.; Nordlund, P. Nature Struct. Biol.
1996, 3, 539-546. (g) Gupta, N.; Bonomi, F.; Kurtz, D. M., Jr.; Ravi, N.;
Wang, D. L.; Huynh, B. H. Biochemistry 1995, 34, 3310-3318. Phenol
hydroxylase: (h) Nordlund, I.; Powlowski, J.; Shingler, V. J. Bacteriol.
1990, 172, 6826-6833. Xylene monooxygenase and alkane hydroxylase:
(i) Shanklin, J.; Whittle, E.; Fox, B. G. Biochemistry 1994, 33, 12787-
12794. Ferroxidase center of ferritin: (j) Harrison, P. M.; Arosio, P. Biochim.
Biophys. Acta 1996, 1275, 161-203.
(7) Ribonucleotide reductase: (a) Logan, D. T.; Su, X.-D.; Åberg, A.;
Regnstro¨m, K.; Hadju, J.; Eklund, H.; Nordlund, P. Structure 1996, 4, 1053-
1064. (b) Nordlund, P.; Eklund, H. J. Mol. Biol. 1993, 232, 123-164. (c)
Nordlund, P.; Sjo¨berg, B.-M.; Eklund, H. Nature 1990, 345, 593-598. (d)
Ravi N.; Bollinger, J. M., Jr.; Huynh, B. H.; Edmondson, D. E.; Stubbe, J.
J. Am. Chem. Soc. 1994, 116, 8007-8014. (e) Bollinger, J. M., Jr.; Tong,
W. H.; Ravi, N.; Huynh, B. H.; Edmondson, D. E.; Stubbe, J. J. Am. Chem.
Soc. 1994, 116, 8015-8023.
(8) (a) Pogozelski, W. K.; McNeese, T. J.; Tullius, T. D. J. Am. Chem.
Soc. 1995, 117, 6428-6433. (b) Cheng, C.-C.; Goll, J. G.; Neyhart, G. A.;
Welch, T. W.; Singh, P.; Thorp, H. H. J. Am. Chem. Soc. 1995, 117, 2970-
2980. (c) Stadtman, E. R. Science 1992, 257, 1220-1224. (d) Minotti, G.;
Aust, S. D. Lipids 1992, 27, 219-226.
(9) (a) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New
York, 1991; pp 120-187. (b) Stamler, J. S.; Singel, D. J.; Loscalzo, J.
Science 1992, 258, 1898-1902.
(10) (a) Halliwell, B.; Gutteridge, J. M. C. Free Radicals in Biology
and Medicine; Clarendon Press: Oxford, U.K., 1989; pp 22-85. (b) von
Sonntag, C. The Chemical Basis of Radiation Biology; Taylor & Francis:
London, 1987; pp 394-428. (c) Garrison, W. M. Chem. ReV. 1987, 87,
381-398.
(11) Herold, S.; Lippard, S. J. J. Am. Chem. Soc. 1997, 119, 145-156
and references therein.
(12) (a) Stassinopoulos, A.; Mukerjee, S.; Caradonna, J. P. In Mechanistic
Bioinorganig Chemistry; Thorp, H. H., Pecoraro, V. L., Eds.; American
Chemical Society: Washington, DC, 1995; pp 83-120. (b) Caradonna, J.
P.; Stassinopoulos, A. AdV. Inorg. Biochem. 1994, 9, 245-315.
(13) (a) Kim, J.; Harrison, R. G.; Kim, C.; Que, L., Jr. J. Am. Chem.
Soc. 1996, 118, 4373-4379. (b) Kim, J.; Larka, E.; Wilkinson, E. C.; Que,
L., Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 2048-2051. (c) Kojima, T.;
Leising, R. A.; Yan, S.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 11328-
11335. (d) Leising, R. A.; Kim, J.; Perez, M. A.; Que, L., Jr. J. Am. Chem.
Soc. 1993, 115, 9524-9530.
(14) Abbreviations used: TPA, tris(2-pyridylmethyl)amine; tmima, tris-
((1-methylimidazol-2-yl)methyl)amine; HB(pz)3, hydrotris(pyrazolyl)borate;
hfacac, hexafluoroacetylacetone; salen, N,N′-bis(salicylidene)ethylenedi-
amine; EDTA, ethylenediamine-N,N,N′,N′-tetraacetate; ATR, attenuated total
internal reflectance; PPN, bis(triphenylphosphine)iminium; Me3-TACN,
1,4,7-trimethyl-1,4,7-triazacyclononane; ICP, inductively coupled plasma;
iPrOx, bis[2-((4S)-(1-methylethyl)-1,3-oxazolinyl)]methane; PheMe3Eda,
N,N,N′-trimethyl-N′-[4,4-dimethyl-4-(3,5-di-tert-butyl-4-hydroxyphenyl)bu-
tyl]ethylenediamine; BIPhMe, bis(1-methyl-2-imidazolyl)phenylmethoxy-
methane; BIPhOH, bis(1-methyl-2-imidazolyl)phenylhydroxymethane; BID-
PhEH, 1,1-bis(1-methyl-2-imidazolyl)-1-(3,5-di-tert-butyl-4-hydroxyphe-
nyl)ethane; CumOOH, cumene hydroperoxide; PAH, picolinic acid.
(15) (a) Me´nage, S.; Wilkinson, E. C.; Que, L., Jr.; Fontecave, M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 203-205. (b) Me´nage, S.; Vincent, J. M.;
Lambeaux, C.; Chottard, G.; Grand, A.; Fontecave, M. Inorg. Chem. 1993,
32, 4766-4773.
(16) (a) Fish, R. H.; Konings, M. S.; Oberhausen, K. J.; Fong, R. H.;
Yu, W. M.; Christou, G.; Vincent, J. B.; Coggin, D. K.; Buchanan, R. M.
Inorg. Chem. 1991, 30, 3002-3006. (b) Vincent, J. B.; Huffman, J. C.;
Christou, G.; Li, Q.; Nanny, M. A.; Hendrickson, D. N.; Fong, R. H.; Fish,
R. H. J. Am. Chem. Soc. 1988, 110, 6898-6900.
(17) Buchanan, R. M.; Chen, S.; Richardson, J. F.; Bressan, M.; Forti,
L.; Morvillo, A.; Fish, R. H. Inorg. Chem. 1994, 33, 3208-3209.
(18) (a) Kitajima, N.; Ito, M.; Fukui, H.; Moro-oka, Y. J. Chem. Soc.,
Chem. Commun. 1991, 102-104. (b) Kitajima, N.; Fukui, H.; Moro-oka,
Y. J. Chem. Soc., Chem. Commun. 1988, 485-486.
(19) Tabushi, I.; Nakajima, T.; Seto, K. Tetrahedron Lett. 1980, 2, 2565-
2568.

7032 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
However, the nature of the active oxidant(s) involved has
remained largely unspecified or is a matter of debate.
H2O2 combinations in the presence of substrate and excess
chloride have indicated that the former system deviates toward
generation of alkyl radicals (trapped as alkyl chlorides) while
the latter exclusively affords ketones, thus showing no signs of
alkyl radical formation.
On the other hand, iron species that have received less
attention as structurally competent mimics have nonetheless been
long implicated in celebrated hydrocarbon-oxidizing systems,
such as the century-old Fenton reagent (FeII/H2O2, aqueous (pH
2))20 and the more biologically relevant, dioxygen-dependent
Udenfriend (FeII/EDTA/ascorbate/O2)21 and Gif22 systems. Gif
chemistry has achieved notoriety owing to its superior product
yield and selectivity as well as its potential biomimetic value.
The catalysts involved in Gif solutions23 operate primarily in
pyridine/acetic acid (py/AcOH) and consist of an FeII/III salt,
dioxygen or simply air, and a coreducing agent (Fe, Zn,
electrochemical cathode) or the equivalent FeIII/H2O2 and FeIII/
t-BuOOH (60 °C) combinations. Additives such as picolinic
acid (PAH) can substantially enhance the rate of the reaction.24
Typical reagents employing PAH include combinations such
as FeCl3/PAH/H2O2 (1:4:4) in pyridine or py/AcOH (10:1) and
FeII(PA)2/H2O2 (1:20) in py/AcOH (2:1). Copper-based Gif
catalysts have also been reported.25
The proposed mechanistic pathways purported to account for
the suggested selectivity of Gif systems vis-a`-vis hydrocarbon
substrates (sec > tert > prim) deviate from current mechanistic
considerations as applied to P-4505,26 or sMMO4,27 biological
sites. These mechanisms further reflect the continuing debate
with respect to the nature of the active oxidant involved, namely
whether it is metal-based or simply a reactive oxygen species
(hydroxyl/alkoxyl radical). The mode of C-H activation by
the putative oxidant (concerted or radical) has remained equally
controversial.
The original Barton mechanism,22 pertaining to the homo-
geneous FeIII/H2O2 system, includes the formation of an iron
oxenoid species (assuming a mononuclear site), as well as a
number of intriguing but rather unusual steps (eqs 1-6;
illustrated for cyclohexane) such as formation of organometallic
intermediates and insertion of O2 into Fe-C bonds. The latter
has been suggested to account for the observation that substrate
oxygenation is dioxygen dependent. The major products are
ketones and a small amount of secondary alcohols, both
suggested to arise from a common metal-alkylperoxy moiety
by spontaneous decomposition and action of reducing agents,
respectively. A key finding in this context is the detection of
cyclohexylhydroperoxide23 as an intermediate in the oxidation
of cyclohexane, slowly decomposing to cyclohexanone and
cyclohexanol. The involvement of sec-alkyl radicals has been
excluded on the basis of the absence of scavenger-trapped
products, especially in the form of coupled sec-alkylpyridines.28
Competitive experiments29 employing the FeII/H2O2 and FeIII/
FeIII + H2O2 f FeVdO + H2O
(1)
(2)
FeVdO + c-C6H12 f FeV(OH)(c-C6H11)
FeV(OH)(c-C6H11) + H2O2 f
FeIII(OH)(c-C6H11) + O2 + 2H+ (3)
FeIII(OH)(c-C6H11) + O2 f FeIII(OH)(OO-c-C6H11)
(4)
FeIII(OH)(OO-c-C6H11) f FeIII + 2OH- + c-C6H10(O) (5)
FeIII(OH)(OO-c-C6H11) + reducing agents f c-C6H11OH
(6)
More recent interpretations29 of the FeCl3/PAH/H2O2 system,
favor generation of a diferric FeIII-(µ-O2)-FeIII unit that is
subsequently converted to the oxenoid FeIII-(µ-O2)-FeVdO
moiety by reaction with 1 equiv of H2O2. Alternative active
oxidants such as [Fe(OOH)(O2)(PA)2(pyH+)] have been pro-
posed30 by virtue of product-profile similarities observed
between “oxygenated Fenton” (FeII(PA)2/H2O2 (1:20) in py/
AcOH (2:1)) and Gif systems, and corresponding discrepancies
found between these metal-based systems and genuine hydroxyl
radical oxidations.20b However, none of the alleged intermedi-
ates have been rigorously detected.
In contrast, the more traditional radical version of this
mechanism has been proposed by Minisci31 to account at least
for Gif chemistry employing t-BuOOH. Alkoxyl radicals have
been postulated as being the active oxidant, generated by FeII-
dependent decomposition of t-BuOOH. The required Minisci
steps (eqs 7-15) predict a ketone:alcohol ratio of g1. Product
selectivity may be influenced by the solvent-dependent competi-
tive rates of eqs 9-10. Corroborative evidence for the operation
of the radical mechanism has been provided by Ingold and co-
workers32 for Gif-type chemistry involving a variety of alkyl-
hydroperoxides. These and related studies33 have been instru-
mental in demonstrating the ubiquitous nature of alkoxyl and
alkylperoxyl radicals in metal-mediated oxidations by t-
BuOOH.34 Analogous interpretations31b,35 pertaining to catalytic
oxidations mediated by P-450-type mimics coupled to alkyl-
hydroperoxides have been similarly cautious in regards to the
(20) (a) Walling, C. Acc. Chem. Res. 1975, 8, 125-131. (b) Sawyer, D.
T.; Kang, C.; Llobet, A.; Redman, C. J. Am. Chem. Soc. 1993, 115, 5817-
5818.
(21) Udenfriend, S.; Clark, C. T.; Axelrod, J.; Brodie, B. B. J. Biol. Chem.
1954, 208, 731-739.
(22) Barton, D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504-512.
(23) Barton, D. H. R.; Be´vie`re, S. D.; Chavasiri, W.; Csuhai, E.; Doller,
D.; Liu, W.-G. J. Am. Chem. Soc. 1992, 114, 2147-2156.
(24) (a) Balavoine, G.; Barton, D. H. R.; Boivin, J.; Gref, A. Tetrahedron
Lett. 1990, 3, 659-662. (b) Tung, H.-C.; Kang, C.; Sawyer, D. T. J. Am.
Chem. Soc. 1992, 114, 3445-3455.
(25) (a) Barton, D. H. R.; Be´vie`re, S. D.; Chavasiri, W.; Csuhai, E.;
Doller, D. Tetrahedron 1992, 48, 2895-2910. (b) Barton, D. H. R.; Csuhai,
E.; Doller, D. Tetrahedron Lett. 1992, 33, 4389-4392.
(26) (a) McMurry, T. J.; Groves, J. T. In ref 5b, pp 1-28. (b) Groves,
J. T. In Metal Ion ActiVation of Dioxygen; Spiro, T. G., Ed.; Wiley: New
York, 1980; pp 125-162. (c) Groves, J. T. J. Chem. Educ. 1985, 62, 928-
931. (d) Newcomb, M.; Le Tadic-Biadatti, M.-H.; Chestney, D. L.; Roberts,
E. S.; Hollenberg, P. F. J. Am. Chem. Soc. 1995, 117, 12085-12091.
(27) (a) Liu, K. E.; Johnson, C. C.; Newcomb, M.; Lippard, S. J. J. Am.
Chem. Soc. 1993, 115, 939-947. (b) Choi, S.-Y., Eaton, P. E.; Hollenberg,
P. F.; Liu, K. E.; Lippard, S. J.; Newcomb, M.; Putt, D. A.; Upadhyaya, S.
P.; Xiong, Y. J. Am. Chem. Soc. 1996, 118, 6547-6555.
(28) Barton, D. H. R.; Halley, F.; Ozbalik, N.; Schmitt, M.; Young, E.;
Balavoine, G. J. Am. Chem. Soc. 1989, 111, 7144-7149.
(29) Barton, D. H. R.; Hu, B.; Taylor, D. K.; Rojas Wahl, R. U. J. Chem.
Soc., Perkin Trans. 2 1996, 1031-1041.
(30) Sawyer, D. T.; Sobkowiak, A.; Matsushita, T. Acc. Chem. Res. 1996,
29, 409-416.
(31) (a) Minisci, F.; Fontana, F.; Araneo, S.; Recupero, F. J. Chem. Soc.,
Chem. Commun. 1994, 1823-1824. (b) Minisci, F.; Fontana, F.; Araneo,
S.; Recupero, F.; Banfi, S.; Quici, S. J. Am. Chem. Soc. 1995, 117, 226-
232. (c) Minisci, F.; Fontana, F. Tetrahedron Lett. 1994, 35, 1427-1430.
(d) Minisci, F.; Fontana, F.; Araneo, S.; Recupero, F.; Zhao, L. Synlett
1996, 119-125.
(32) Snelgrove, D. W.; MacFaul, P. A.; Ingold, K. U.; Wayner, D. D.
M. Tetrahedron Lett. 1996, 37, 823-826.
(33) (a) Arends, I. W. C. E.; Ingold, K. U.; Wayner, D. D. M. J. Am.
Chem. Soc. 1995, 117, 4710-4711. (b) MacFaul, P. A.; Arends, I. W. C.
E.; Ingold, K. U.; Wayner, D. D. M. J. Chem. Soc., Perkin Trans. 2 1997,
135-145.
(34) (a) Nguyen, C.; Guajardo, R. J.; Mascharak, P. K. Inorg. Chem.
1996, 35, 6273-6281. (b) Rabion, A.; Chen, S.; Wang, J.; Buchanan, R.
M.; Seris, J.-L.; Fish, R. H. J. Am. Chem. Soc. 1995, 117, 12356-12357.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7033
active oxidant involved.
sMMO27a oxidations, although results obtained27a,41 with sMMO
from M. trichosporium suggest longer life spans for the
generated alkyl radicals; and (iv) deuterium primary kinetic
isotope effect (KIE) values tend to be low (e2.5)42 in Gif-type
oxidations by comparison to intrinsic KIE values for hydroxy-
lations catalyzed by P-450 isozymes/mimics (KIE ) 10-14;5a
more modest and disperse values have also been measured43)
and sMMO27a (KIE values larger than 50 have been reported44
for methane with M. trichosporium, although ethane exhibits a
value of 4).
FeII + t-BuOOH f FeIII + OH- + t-BuO•
FeIII + t-BuOOH f FeII + H+ + t-BuOO•
(7)
(8)
(9)
•
t-BuO• + c-C6H12 f t-BuOH + c-C6H11
t-BuO• + t-BuOOH f t-BuOH + t-BuOO•
2t-BuOO• f 2t-BuO• + O2
(10)
(11)
To gain a more intimate view of the role of metal sites in
Gif-type chemistry as well as explore possible connectivities
with structural/functional aspects pertaining to sMMO, we have
turned our attention to GifIV-type systems (FeII/III/O2/Zn/substrate
in py/AcOH) with emphasis on the basic iron acetate [Fe3O(O2-
CCH3)6(py)3]‚py (1) as starting material and catalyst. Several
lines of evidence make the choice of this mixed-valent species
attractive: (i) the metal ligation (Fe-O ) 1.9 Å, Fe‚‚‚Fe )
3.4 Å)45 is representative of corresponding moieties and metrical
parameters in Hox of sMMO;6a (ii) the ferrous site in 1 can be
c-C6H11• + O2 f c-C6H11OO•
(12)
2c-C6H11OO• f c-C6H11OOOO-c-C6H11 f
c-C6H10(O) + c-C6H11OH + O2 (13)
c-C6H11OO• + t-BuOO• f c-C6H11OOOO-t-Bu f
c-C6H10(O) + t-BuOH + O2 (14)
readily replaced45 by a non-redox ion such as Mg2+ or Zn2+
,
c-C6H11• + t-BuOO• f c-C6H11OO-t-Bu
(15)
practically reducing the species to a dinuclear iron compound;
(iii) compound 1 has been isolated46 from catalytic mixtures
and successfully used47 as a source of iron in Gif-type oxidation
of hydrocarbons (GifIV); and (iv) the all-ferric [Fe3O(O2CCH3)6-
(py)3]+ is reported48 to mediate epoxidation of olefins by
molecular oxygen.
Alternative metal-based active oxidants, that have been
suggested by Barton29 to form alkyl radicals, include the couple
FeII-OOH/FeIVdO, generated by the competitive FeII/H2O2
combination under dioxygen-deficient conditions. The alkyl
radical is proposed to be the product of homolytic cleavage
within an HO-FeIV-R unit.
The following are pertinent questions that need to be
progressively addressed in this study: (i) What are the
characteristics of the reduced iron sites that may be accountable
for dioxygen binding and activation during catalytic turnovers?
(ii) What are the products of autoxidation of these reduced
species upon exposure to dioxygen? (iii) Are there any
discernible structural similarities that may allow us to draw
legitimate analogies between iron centers present in Gif-type
solutions and at the active site of sMMO? (iv) How does the
product profile of substrate oxidations mediated by the 1/Zn/
O2 system, and variations thereof, compare in terms of yield,
regio(chemo)specificity, and KIE values to other Fenton- and
Gif-type systems as well as to sMMO and P-450 hydroxylase
activity? (v) Is there any evidence that the isolable “catalysts”
are kinetically competent to mediate substrate oxidation and can
intermediates of dioxygen activation be detected and related to
substrate turnover?
The relationship between Gif chemistry and sMMO activity
relies on potential structural similarities due to the choice of
solvents (py/AcOH) in Gif catalysis and the nature of amino
acid residues of the diiron site in sMMO. However, functional
analogies are much more tentative and need to be carefully
examined. Despite the fact that the peculiar Gif-type selectivity
parallels the one encountered in the activity of sMMO for a
limited number of substrates,36,37 the following discrepancies
exist: (i) Gif catalysts mediate mostly ketonization of secondary
carbons38 while sMMO features hydroxylation of primary
carbons;39 (ii) oxidation of olefins leads largely to ketonization
in Gif chemistry,22 as opposed to mostly stereospecific epoxi-
dation by sMMO37b and P-4505a systems; (iii) radical-clock
experiments indicate40 the presence of diffusively free (long-
lived) alkyl radicals in t-BuOOH/H2O2-based Gif chemistry, but
only short-lived (<100 fs) substrate radicals under P-45026d or
Answers to questions i-iv are investigated in this article.
Question v needs to be addressed on the basis of the results of
a comprehensive kinetic study coupled with a systematic search
for the detection of intermediates. Such work is currently in
(35) (a) Mansuy, D.; Bartoli, J.-F.; Momenteau, M. Tetrahedron Lett.
1982, 23, 2781-2784. (b) Mansuy, D.; Bartoli, J.-F.; Chottard, J.-C.; Lange,
M. Angew. Chem., Int. Ed. Engl. 1980, 19, 909-910. (c) Ledon, H. C. R.
Hebd. Seances Acad. Sci., Ser. C 1979, 288, 29-31. (d) Baccouche, M.;
Ernst, J.; Fuhrhop, J.-H.; Schlo¨zer, R.; Arzoumanian, H. J. Chem. Soc.,
Chem. Commun. 1977, 821-822. (e) Meunier, B. Chem. ReV. 1992, 92,
1411-1456.
(36) (a) Barton, D. H. R.; Boivin, J.; Motherwell, W. B.; Ozbalik, N.;
Schwartzentruber, K. M.; Jankowski, K. New J. Chem. 1986, 10, 387-
398. (b) Sheu, C.; Sobkowiak, A.; Zhang, L.; Ozbalik, N.; Barton, D. H.
R.; Sawyer, D. T. J. Am. Chem. Soc. 1989, 111, 8030-8032. (c) Sheu, C.;
Richert, S. A.; Cofre´, P.; Ross, B., Jr.; Sobkowiak, A.; Zhang, L.; Sawyer,
D. T.; Kanofsky, J. R. J. Am. Chem. Soc. 1990, 112, 1936-1942.
(37) (a) Froland, W. A.; Andersson, K. K.; Lee, S.-K.; Liu, Y.; Lipscomb,
J. D. J. Biol. Chem. 1992, 267, 17588-17597. (b) Green, J.; Dalton, H. J.
Biol. Chem. 1989, 264, 17698-17703.
(38) However, in the presence of reducing agents, Gif systems can operate
as hydroxylation catalysts without significant competition by the presumably
more easily oxidized reducing additive (this has been termed the “Gif
paradox”); see ref 23.
(39) Apparently, the predilection toward hydroxylation of primary sites
is not native to the hydroxylase component of sMMO but is significantly
influenced by the presence of the regulatory protein B. For a discussion,
see ref 37a.
(41) Ruzicka, F.; Huang, D.-S.; Donnelly, M. I.; Frey, P. A. Biochemistry
1990, 29, 1696-1700.
(42) Barton, D. H. R.; Doller, D.; Geletii, Y. V. Tetrahedron Lett. 1991,
32, 3811-3814.
(43) Sorokin, A.; Robert, A.; Meunier, B. J. Am. Chem. Soc. 1993, 115,
7293-7299 and references therein.
(44) (a) Nesheim, J. C.; Lipscomb, J. D. Biochemistry 1996, 35, 5,
10240-10247. (b) Nesheim, J. C.; Lipscomb, J. D. J. Inorg. Biochem. 1995,
59, 369.
(45) Cannon, R. D.; White, R. P. Prog. Inorg. Chem. 1988, 36, 195-
298. (b) Woehler, S. E.; Wittebort, R. J.; Oh, S. M.; Kambara, T.;
Hendrickson, D. N.; Inniss, D.; Strouse, C. E. J. Am. Chem. Soc. 1987,
109, 1063-1072.
(46) Barton , D. H. R.; Gastiger, M. J.; Motherwell, W. B. J. Chem.
Soc., Chem. Commun. 1983, 731-733.
(47) Barton, D. H. R.; Boivin, J.; Gastiger, M.; Morzycki, J.; Hay-
Motherwell, R. S.; Motherwell, W. B.; Ozbalik, N.; Schwartzentruber, K.
M. J. Chem. Soc., Perkin Trans. 1 1986, 947-955.
(48) Ito, S.; Inoue, K.; Mastumoto, M. J. Am. Chem. Soc. 1982, 104,
6450-6452.
(40) Newcomb, M.; Simakov, P. A.; Park, S.-U. Tetrahedron Lett. 1996,
37, 819-822.

7034 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
3
2-H-py), 9.01 (s, br, 3-H-py), 8.15 (t, JHH ) 7.6 Hz, 4-H-py). IR
(KBr): νOCO 1606, 1596, 1445, 1419 cm-1. Near-IR/ATR (solid 3):
λmax ≈ 860, 970 nm. UV-vis (py): λmax ) 396 (ꢀM ) 2107) nm. 57Fe
Mo¨ssbauer (4.2 K): δ 1.22 mm/s (∆EQ ) 3.23 mm/s, Γ ) 0.18 mm/
s). Anal. Calcd for C24H26N4Fe1O4: C, 58.79; H, 5.34; N, 11.43.
Found: C, 58.68; H, 5.31; N, 11.37.
progress and will be reported in due course. Aspects of the
present study have been published in a preliminary report.49
Experimental Section
Preparation of Compounds. All operations were performed under
a pure dinitrogen or argon atmosphere using Schlenk techniques on an
inert gas/vacuum manifold or in a drybox (O2, H2O < 1 ppm). Hexane,
petroleum ether, and toluene were distilled over Na, and THF and
diethyl ether over Na/Ph2CO. Acetonitrile and methylene chloride were
distilled over CaH2. Ethanol and methanol were distilled over the
corresponding magnesium alkoxide, and acetone over drierite. Anhy-
drous pyridine and dimethyl sulfoxide (water < 0.005%) and double-
distilled acetic acid (metallic impurities in ppt) were purchased from
Aldrich. All solvents, with the exception of methanol, were degassed
by three freeze-pump-thaw cycles. Methanol and water were
degassed by bubbling nitrogen or argon for 0.5 h. Compounds
[Fe3O(O2CCH3)6(py)3]‚py (1)50 and [Fe2ZnO(O2CCH3)6(py)3]‚py (1′′)51
were prepared according to literature procedures. Other starting
materials were purchased from Aldrich and are of the highest available
purities.
[FeII2(O2CCH3)4(py)3]n (4). Compound 3 (300 mg, 0.61 mmol) was
dissolved in 4.0 mL of pyridine. Diethyl ether (40 mL) was carefully
added to the pyridine solution, with occasional shaking of the flask to
disperse local clouding, until a small amount of a white solid, which
would not easily redissolve, was apparent. The solution was filtered
and diethyl ether was allowed to diffuse slowly into the filtrate at 10
°C. The product crystallized over a period of 1 week as large yellow
polyhedra (115 mg, 64%) suitable for X-ray analysis. Rigorous
exclusion of dioxygen is essential, for the product is exceedingly air-
sensitive. Compound 4 was also obtained pure, but in a feathery form,
from pyridine (4.0 mL)/acetic acid (0.8 mL) by diffusion of diethyl
ether at 10 °C or from pyridine (3.0 mL)/acetic acid (1.5 mL) followed
by diffusion of diethyl ether at -20 °C. 1H NMR ((CD3)2SO, 1.23 ×
10-2 M): δ 74.37 (s, br, CH3CO2), 12.62 (s, br, 2-H-py), 8.59 (s, br,
3-H-py), 8.04 (t, 3JHH ) 7.6 Hz, 4-H-py,). IR (KBr): νOCO 1617, 1600,
1446, 1415 cm-1. Near-IR/ATR (solid 4): λmax ≈ 850, 1100 nm. UV-
vis (py, >1.0 × 10-3 M): λmax ) 424 (sh), 396 (ꢀM ) 2107, 3) nm.
57Fe Mo¨ssbauer (4.2 K): δ 1.27 mm/s (∆EQ ) 2.92 mm/s, Γ ) 0.20
mm/s). Anal. Calcd for C23H27N3Fe2O8: C, 47.21; H, 4.65; N, 7.18.
Found: C, 47.87; H, 4.97; N, 7.45.
[Zn(O2CCH3)2(py)2] (5). Diethyl ether was slowly diffused to a
solution of Zn(O2CCH3)2‚2H2O (439 mg, 2.0 mmol) in pyridine (20
mL) at -20 °C. Large colorless crystals of the product (crystal
system: tetragonal) were deposited (574 mg, 84%). The same
stoichiometric product was also obtained from py/AcOH solutions (10:
1, 5:1) upon diffusion of diethyl ether. However, 5 obtained from py/
AcOH(5:1)/Et2O crystallizes in the triclinic crystal system (see also
isolation of the product in reactivity studies). 1H NMR ((CD3)2SO):
8.58 (d, 2H, JHH ) 4.4 Hz, 2-H-py), 7.83 (tt, 1H, JHH ) 7.6 Hz,
4-H-py), 7.43 (m, 2H, 3-H-py), 1.80 (s, 3H, CH3CO2). Anal. Calcd
for C14H16N2ZnO4: C, 49.22; H, 4.72; N, 8.20. Found: C, 48.59; H,
4.60; N, 7.98.
[Zn2FeII(O2CCH3)6(py)2] (2). Zn powder (1.32 g, 0.02 mol) was
added to a brown-black solution of [Fe3O(O2CCH3)6(py)3]‚py (1) (342
mg, 0.40 mmol) in 4.5 mL of acetonitrile and 0.45 mL of acetic acid.
The reaction mixture was stirred for 0.5 h, and the excess Zn was filtered
off. The light yellow supernatant afforded the pure product 2 (630
mg, 75% based on Fe) as light-yellow plates upon cooling (-20 °C).
The same product can be obtained pure from the following reaction
solvents: acetonitrile (3.0 mL)/acetic acid (1.5 mL); dichloromethane
(4.5 mL)/acetic acid (0.45 mL); and pyridine (3.0 mL)/acetic acid (1.5
mL). In the last case, the pure product was obtained by slow diffusion
of diethyl ether into the yellow-green supernatant at -20 °C (vide infra
for an alternative method of preparation). 1H NMR ((CD3)2SO, 0.98
× 10-2 M): δ 12.01 (s, br, 2-H-py), 8.54 (s, br, 3-H-py), 8.06 (t, 3JHH
) 7.6 Hz, 4-H-py). IR (KBr): νOCO 1643, 1586, 1428, 1311 cm-1
.
3
3
Near-IR/ATR (solid 2): λmax ≈ 980, 1260 nm. UV-vis (CH2Cl2):
λmax ) 346 (ꢀM ) 209) nm. 57Fe Mo¨ssbauer (4.2 K): δ 1.38 mm/s
(∆EQ ) 2.96 mm/s, Γ ) 0.26 mm/s). Anal. Calcd for C22H28N2Fe1O12-
Zn2: C, 37.80; H, 4.04; N, 4.01; Fe, 7.99; Zn, 18.71. Found: C, 38.28;
H, 4.12; N, 4.15; Fe, 8.18; Zn, 18.37.
[Zn2(O2CCH3)4(py)2] (6). Diethyl ether was slowly diffused to a
solution of 5 (342 mg, 1.0 mmol) in 10 mL of pyridine and 5 mL of
acetic acid at -20 °C. Large colorless plates of the product (crystal
system: monoclinic) were deposited (200 mg, 76%) (see also isolation
of the product in reactivity studies). 1H NMR ((CD3)2SO): 8.58 (br,
2H, 2-H-py), 7.81 (tt, 1H, 3JHH ) 7.8 Hz, 4-H-py), 7.41 (m, 2H, 3-H-
py), 1.79 (s, 6H, CH3CO2). Anal. Calcd for C18H22N2Zn2O8: C, 41.17;
H, 4.22; N, 5.33. Found: C, 41.04; H, 4.13; N, 5.17.
[FeII(O2CCH3)2(py)4] (3). Zn powder (1.32 g, 0.02 mol) was added
to a brown-black solution of 1 (342 mg, 0.40 mmol) in 4.5 mL of
pyridine and 0.45 mL of acetic acid. The reaction mixture was stirred
for 0.5 h, and the excess Zn was filtered off. Upon cooling (-20 °C),
the yellow-green filtrate afforded the pure product as yellow-green
polyhedra (489 mg, 83%). The same product was obtained pure from
pyridine (4.0 mL)/acetic acid (0.8 mL) or neat pyridine (5.0 mL). In
the latter case, the reaction was stirred for 6 h and the yellow-brown
filtrate afforded 3 (459 mg, 78%) at -20 °C. Slow diffusion of diethyl
ether into the supernatant precipitated a brown-black amorphous solid
(70 mg) of unknown composition. The latter retains Fe(III) according
to Mo¨ssbaur data. Preactivation of the Zn dust (washing with dilute
HCl, acetic acid, and pyridine) eliminates the brown-black byproduct
in the reduction (30 min) of 1 in pyridine.
[Zn(O2CCH3)2(py)]n (7). A solution of 5 (342 mg, 1.0 mmol) in
ethanol (20 mL) or dichloromethane (20 mL) was evaporated to a small
volume until colorless plates were apparent. The product was redis-
solved in situ upon mild heating and allowed to crystallize slowly (189
mg, 72%) at -20 °C. 1H NMR (CD3CD2OD): 8.57 (d, 2H, JHH
)
3
3
6.4 Hz, 2-H-py), 7.91 (tt, 1H, JHH ) 7.8 Hz, 4-H-py), 7.49 (m, 2H,
3-H-py), 1.95 (s, 6H, CH3CO2). Anal. Calcd for C9H11NZnO4: C,
41.17; H, 4.22; N, 5.33. Found: C, 41.10; H, 4.10; N, 5.26.
Compound 3 was also obtained pure in the following manners. (i)
A brown-black solution of 1 (342 mg, 0.40 mmol) in 5.0 mL of pyridine
or pyridine (4.5 mL)/acetic acid (0.45 mL) was pressurized with
dihydrogen (30 psig) over Pd (10 mg) for 2 h. After filtration of the
catalyst, the clear yellow-green solution afforded 3 in quantitative yield
upon cooling (-20 °C). (ii) Iron powder (1.3 g, 0.023 mol) was added
to a brown-black solution of 1 (342 mg, 0.40 mmol) in 5.0 mL of
pyridine or pyridine (4.5 mL)/acetic acid (0.45 mL). The reaction
mixture was allowed to stir for 6 h (py) or 0.5 h (py/AcOH), followed
by filtration of the excess Fe. Upon cooling (-20 °C), the bright
yellow-green filtrate afforded 3 (471 mg, 80%). (iii) Iron powder (1.3
g, 0.023 mol) stirred for 0.5 h in pyridine (4.5 mL)/acetic acid (0.45
mL) afforded 3 (185 mg, 1.6%) from the filtrate at -20 °C. 1H NMR
((CD3)2SO, 1.40 × 10-2 M): δ 72.43 (s, br, CH3CO2), 13.98 (s, br,
[Zn3(O2CCH3)6(py)2] (8). A solution of 5 (342 mg, 1.0 mmol) in
acetonitrile (20 mL) was evaporated until colorless crystals were
apparent. The product was redissolved in situ upon mild heating and
allowed to crystallize slowly (203 mg, 86%) at -20 °C. The product
was also obtained by recrystallization of 6 (525 mg, 1.0 mmol) in
3
dichloromethane (5 mL). 1H NMR (CD2Cl2): 8.63 (dd, 2H, JHH
)
3
6.0 Hz, 2-H-py), 7.87 (tt, 1H, JHH ) 7.6 Hz, 4-H-py), 7.45 (t, 2H,
3JHH ) 6.8 Hz, 3-H-py), 2.06 (s, 9H, CH3CO2). Anal. Calcd for
C22H28N2O12Zn3: C, 37.29; H, 3.98; N, 3.95. Found: C, 37.32; H,
3.89; N, 4.06.
[PPN][FeII(O2CCH3)3(py)] (9). Compound 3 (300 mg, 0.61 mmol)
and [PPN][O2CCH3] (360 mg, 0.60 mmol) were dissolved in 5.0 mL
of pyridine upon stirring. The yellow-orange solution was filtered and
diethyl ether was allowed to diffuse slowly into the filtrate at 20 °C.
The yellow product crystallized as long needles (237 mg, 46%). 1H
NMR ((CD3)2SO): δ 67.64 (br, CH3CO2), 9.56 (s, br, 2-H-py), 7.85
(t, 3JHH ) 7.6 Hz, 4-H-py), 7.69 (t, 3JHH ) 7.0 Hz, 3-H-py), 7.53-7.60
(m, PPh3). IR (KBr): νOCO 1604, 1590, 1440 cm-1. UV-vis (py):
λmax ) 396 (3), 342 (ꢀM ) 192) nm. 57Fe Mo¨ssbauer (4.0 K): δ 1.21
(49) Singh, B.; Long, J. R.; Papaefthymiou, G. C.; Stavropoulos, P. J.
Am. Chem. Soc. 1996, 118, 5824-5825.
(50) Sorai, M.; Kaji, K.; Hendrickson, D. N.; Oh, S. M. J. Am. Chem.
Soc. 1986, 108, 702-708.
(51) Blake, A. B.; Yavari, A.; Hatfield, W. E.; Sethulekshmi, C. N. J.
Chem. Soc., Dalton Trans. 1985, 2509-2520.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7035
mm/s (∆EQ ) 2.97 mm/s, Γ ) 0.25 mm/s). Anal. Calcd for
C47H44N1Fe1O6P2: C, 66.36; H, 5.21; N, 3.29; P, 7.28. Found: C,
66.15; H, 5.08; N, 3.15; P, 7.11.
Calcd for C32H38N4Fe2O13Zn1: C, 44.50; H, 4.43; N, 6.49; Fe, 12.93;
Zn, 7.57. Found: C, 44.32; H, 4.48; N, 6.41; Fe, 12.84; Zn, 7.51.
Equilibrium-Dependent Transformations. Interconversions among
species 2-5 were observed under the following conditions.
[FeII(H2O)4(py)2][O2CCH3]2 (10). Compound 3 (300 mg, 0.61
mmol) was dissolved in 5.0 mL of pyridine upon stirring. To the
yellow-green solution was added 0.5 mL of water, and diethyl ether
was allowed to diffuse slowly into the filtrate at 20 °C. The product
deposited as light-yellow crystals (122 mg, 49%). 1H NMR ((CD3)2-
SO): δ 54.79 (br, CH3CO2), 13.20 (s, 4H, 2-H-py), 8.90 (m, 4H, 3-H-
py), 8.11 (t, 2H, 3JHH ) 8.0 Hz, 4-H-py), 3.57 (s, 8H, H2O). IR (KBr):
[Zn2FeII(O2CCH3)6(py)2] (2) in Pyridine. Compound 2 (350 mg,
0.50 mmol) was dissolved in 4.0 mL of pyridine to yield a yellow-
green solution, from which large yellow-green crystals of [FeII(O2-
CCH3)2(py)4] (3) (226 mg, 92%) were obtained upon cooling (-20
°C). Diffusion of diethyl ether into the supernatant afforded colorless
plates (crystal system: tetragonal) of [Zn(O2CCH3)2(py)2] (5) (325 mg,
95%). The same result is obtained in py/AcOH (10:1).
νOCO 1617, 1600, 1420 cm-1. UV-vis (py): λmax ) 396 (3) (ꢀM
)
1958) nm. 57Fe Mo¨ssbauer (4.2 K): δ 1.28 mm/s (∆EQ ) 3.00 mm/s,
Γ ) 0.15 mm/s). Anal. Calcd for C14H24N2Fe1O8: C, 41.60; H, 5.98;
N, 6.93. Found: C, 41.67; H, 5.84; N, 6.30.
Regeneration of [Zn2FeII(O2CCH3)6(py)2] (2) from Compounds
3 and 5. Compound 2 (321 mg, 92%) was obtained by mixing [FeII(O2-
CCH3)2(py)4] (3) (245 mg, 0.50 mmol) and [Zn(O2CCH3)2(py)2] (5)
(342 mg, 1.0 mmol) in pyridine (5.0 mL) or pyridine (3.0 mL)/acetic
acid (1.5 mL), followed by careful addition of a layer of diethyl ether
(50 mL) and allowing the solution to stand at 10 °C. Dichloromethane
(5.0 mL), acetonitrile (5.0 mL), and dimethyl sulfoxide (5.0 mL) were
also used as reaction solvents, affording essentially quantitative yields
of 2.
[FeII3(O2CCH3)6(OS(CH3)2)2]n (11). Compound 3 (10 mg, 0.02
mmol) which was allowed to stand in dimethyl sulfoxide-d6 (0.5 mL)
for 48 h, deposited colorless crystals of the deuterated title compound.
On a preparative scale, compound 11 can be synthesized by dissolving
[FeII(O2CCH3)2] (100 mg, 0.57 mmol) in dimethyl sulfoxide (40 mL),
followed by filtration and diffusion of diethyl ether into the filtrate
(20 °C) to yield colorless plates of the product. 1H NMR ((CD3)2SO,
7.37 × 10-3 M): δ 62.22 (br, CH3CO2), 2.53 (br, (CH3)2SO). IR
Catalytic Oxidations. A typical oxidation of substrate (adamantane)
mediated by iron-containing species (for instance, compound 1) was
performed as follows. Compound 1 (30.0 mg, 35.1 µmol) was dissolved
under anaerobic conditions in 27 mL of pyridine and 2.7 mL of acetic
acid followed by adamantane (100 mg, 0.73 mmol). Zn powder (1.3
g, 99.998%) was added to this solution, and the mixture was stirred
for 18 h under pure dioxygen (delivered to the head space of the oxygen-
saturated solution under static conditions) or simply open to air. The
internal standard (hexamethylbenzene) was added to the mixture, and
an aliquot (2 mL) was withdrawn for GC or GC-MS analysis. After
addition of water (3 mL), the aliquot was extracted with diethyl ether
(3 × 5 mL). The combined diethyl ether layers were washed with
water and dried over magnesium sulfate. The samples were analyzed
by a Perkin-Elmer SIGMA 2000 or a Hewlett-Packard 5890 Series II
Capillary GC employing Supelco SPB-1, SPB-50, and SUPELCO-
WAX-10 (adamantane oxidation) capillary columns (30 m (length),
0.32 mm (i.d.), 0.25 µm (df)), a flame ionization detector, and a Hewlett-
Packard 3395 or 3396A integrator. Products were identified by their
respective retention times versus authentic samples and by mass
determination on a Finnigan MAT-90 MS coupled to a Varian 3400
GC.
(KBr): νOCO 1507, 1437, 1419 cm-1. UV-vis ((CH3)2SO): λmax
270 (ꢀM ) 167) nm. 57Fe Mo¨ssbauer (4.2 K): δ 1.40 mm/s (∆EQ1
)
)
3.04 mm/s, Γ1 ) 0.20 mm/s, 66%); 1.38 mm/s (∆EQ2 ) 2.94 mm/s, Γ2
) 0.22 mm/s, 34%). Anal. Calcd for C16H30Fe3O14S2: C, 28.34; H,
4.46; S, 9.46. Found: C, 28.43; H, 4.71; S, 9.37.
Reaction of Ferrous Species with Dioxygen. The following
compounds were isolated during investigations directed toward the
elucidation of the reactivity of ferrous species with dioxygen.
[Fe3O(O2CCH3)6(py)3]‚py (1). Dioxygen was admitted to a solution
of compound 3 (300 mg, 0.61 mmol) in pyridine (20 mL) at -20 °C
for 4 h. The solution was filtered, and the brown filtrate was reduced
under vacuum to 10 mL. Diffusion of diethyl ether into the filtrate at
-20 °C afforded brown-black blocks of the product (150 mg, 86%).
Pyridine/acetic acid (10:1 v/v) was also used as the reaction solvent
with identical results. The product was identified by crystallographic
1
unit cell parameters and H NMR by comparison with an authentic
sample and literature citations.50
[Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]‚py (1′). [Zn2FeII(O2CCH3)6(py)2]
(2) (350 mg, 0.50 mmol) was dissolved in 27 mL of pyridine and 2.7
mL of acetic acid and allowed to react with 1 atm of pure dioxygen or
simply air for 2 h. A brown-black solution was rapidly generatedsa
short induction period was observed with pure dioxygensfrom which,
after reduction of the solvent to 10 mL (solvent composition after
evaporation: py:AcOH 2:1 v/v) and diffusion of diethyl ether at -20
°C, the brown-black crystalline product 1′ was obtained (185 mg, 95%
based on Fe). 1H NMR ((CD3)2SO): δ 31.37 (s, br, CH3CO2), 13.03
(s, br, CH3CO2), 8.58 (s, br, 2-H-py), 7.78 (t, 3JHH ) 8.0 Hz, 4-H-py),
In the catalytic oxidation of benzene, toluene, and cis-stilbene, the
molar ratio of substrate over catalyst was also kept at 20:1. In contrast,
a large excess of isopentane (17.19 mmol) was employed. Static
delivery of dioxygen in a closed system was performed in the case of
the volatile isopentane, benzene, and toluene. Quantification of
hydroxypyridines, produced in the catalytic oxidation of pyridine, was
performed by 1H NMR in (CD3)2CO/D2O following a reported
protocol.29
7.40 (s, br, 3-H-py). IR (KBr): νOCO 1631, 1614, 1447, 1412 cm-1
.
Kinetic Isotope Effect Measurements. The intermolecular deu-
terium kinetic isotope effect was measured for adamantane by conduct-
ing catalytic oxidations (as previously described, but maintained at
25.0(1) °C with the aid of a Brinkmann RM6 thermostat) on a mixture
of adamantane/adamantane-d16 (1:1). KIE values were calculated for
each product (average of three trials) from the ratio of protio/deuterio
product yield, measured by GC analysis and GC-MS calculation of
the relative intensities of the respective molecular ions selected over
the fully resolved GC peaks. Corrections for the exact adamantane/
adamantane-d16 ratio used, as determined by GC-MS, were applied.
X-ray Structure Determinations. Structures were determined for
the compounds listed in Tables 1 and 2. Single crystals were picked
from the reaction products, sealed in capillaries under an inert
atmosphere and/or coated with Paratone-N oil, and transferred to a
Siemens SMART (9) or a Nicolet P3 (all others) diffractometer. Lattice
parameters were obtained from a least-squares analysis of more than
30 carefully centered reflections. None of the crystals showed
significant decay during data collection. The raw intensity data were
converted (including corrections for scan speed, background, and
Lorentz and polarization effects) to structure factor amplitudes and their
esd’s. An empirical absorption correction based on the observed
variation in intensity of azimuthal (Ψ) scans was applied to the data
sets for compounds 7, 9, and 11.
UV-vis (py): λmax 470 (ꢀM ) 431), 352 (ꢀM ) 3170), 320 (ꢀM ) 3150)
nm. 57Fe Mo¨ssbauer (20 K): δ1 0.53 mm/s (∆E ) 1.15 mm/s, Γ1
Q1
) 0.21 mm/s, 90%), δ2 1.25 mm/s (∆EQ2 ) 2.11 mm/s, Γ2 ) 0.20
mm/s, 10%). Anal. Calcd for C32H38N4Fe2.2O13Zn0.78: C, 44.61; H,
4.45; N, 6.50; Fe, 14.39; Zn, 5.92. Found (analyses on two indepen-
dently prepared samples were obtained): C, 44.57 (44.26); H, 4.38
(4.46); N, 6.45 (6.27); Fe, 14.38 (14.13); Zn, 5.80 (6.08).
Diffusion of diethyl ether into the remaining supernatant of 1′
afforded colorless polyhedra (crystal system: orthorhombic) of [Zn2(O2-
CCH3)4(py)2] (6) (197 mg, 75% based on Zn) (vide supra for analytical
details).
[Fe2ZnO(O2CCH3)6(py)3]‚py (1′′). Zn powder (1.32 g, 0.02 mol)
was added to a solution of compound 1 (342 mg, 0.40 mmol) in pyridine
(20 mL), and the reaction mixture was vigorously stirred for 6 h. To
the yellow-brown filtrate (which affords compound 3 if allowed to cool
at -20 °C) was admitted dioxygen for 2 h at ambient temperature.
The product was obtained as large red-green dichroic crystals (220
mg, 42% based on Fe in 1) upon diffusion of diethyl ether at -20 °C
into the resulting brown-black solution after oxygenation. The product
was characterized by analytical and 1H NMR data by comparison with
an authentic sample and literature citations, but it crystallized in a
monoclinic rather than the reported51 rhombohedral space group. Anal.

7036 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
Table 1. Crystallographic Dataa for [Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]‚py (1′), [Fe2ZnO(O2CCH3)6(py)3]‚py (1′′), [Zn2FeII(O2CCH3)6(py)2] (2),
[FeII(O2CCH3)2(py)4] (3), [FeII2(O2CCH3)4(py)3]n (4), and [Zn(O2CCH3)2(py)2] (5)
1′
1′′
2
3
4
5
5
formula
formula wt 861.66
cryst syst trigonal
space group R32
C32H38Fe2.22N4O13Zn0.78 C32H38Fe2N4O13Zn C22H28FeN2O12Zn2 C24H26FeN4O4 C23H27Fe2N3O8 C14H16N2O4Zn C14H16N2O4Zn
863.76
monoclinic
C2
699.10
monoclinic
P21/n
490.34
585.18
341.66
341.66
triclinic
P1h
orthorhombic orthorhombic tetragonal
Pccn
4
Pna21
4
P43212
4
Z
3
2
2
2
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
T, K
color
17.556(4)
12.553(2)
17.574(3)
10.825(2)
10.255(2)
10.756(2)
12.727(3)
8.857(7)
16.401(9)
16.653(10)
16.018(4)
9.687(3)
18.107(6)
8.613(2)
8.370(2)
9.021(2)
10.998(2)
93.57 (1)
102.32(1)
95.38(1)
804.8(3)
10.932(5)
22.534(5)
124.00(2)
95.56(3)
2918(2)
223
red-black
1979.7(5)
295
red-green dichroic light yellow
1.513
1397.2(5)
223
2419(3)
223
yellow-green yellow
2810(1)
295
1671.6(6)
colorless
1.358
colorless
1.410
d
calc, g/cm3 1.469
1.662
1.346
1.383
µ, mm-1
1.329
1.398
2.277
0.660
1.079
1.484
1.541
Rb (Rwc or
4.16 (4.61)c
4.13 (4.21)c
3.34 (3.62)c
3.91 (3.83)c
3.88 (8.87)d
3.62 (3.91)d
3.25 (8.70)d
d
wR2 ), %
2
a Obtained with graphite monochromated Mo KR (λ ) 0.710 73 Å) radiation. b R ) ∑||Fo| - |Fc||/∑|Fo|. c Rw ) {∑[w(|Fo| - |Fc|)2/{∑[w|Fo| ]}1/2
.
2
2
d wR2 ) {∑[w(Fo - Fc2)2/{∑[w(Fo )2]}1/2
.
Table 2. Crystallographic Dataa for [Zn2(O2CCH3)4(py)2] (6), [Zn(O2CCH3)2(py)]n (7), [Zn3(O2CCH3)6(py)2] (8), [PPN][FeII(O2CCH3)3(py)]
(9), [FeII(H2O)4(trans-py)2][O2CCH3]2 (10), and [FeII3(O2CCH3)6((OS(CD3)2)2]n (11)
6
6
7
8
9
10
11
formula
formula wt
cryst syst
space group Pbca
Z
C18H22N2O8Zn2 C18H22N2O8Zn2 C9H11NO4Zn C22H28N2O12Zn3 C47H44FeN2O6P2 C14H24FeN2O8 C16H18D12Fe3O14S2
525.12
525.12
monoclinic
C2/c
262.58
triclinic
P1h
708.64
monoclinic
P21/n
850.67
monoclinic
P21/n
404.20
triclinic
P1h
690.15
monoclinic
C2
orthorhombic
4
4
8
2
4
1
2
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
color
13.182(3)
8.601(2)
19.485(6)
9.940(1)
17.259(2)
12.620(2)
8.626(2)
8.868(3)
28.871(8)
90.17(2)
90.02(2)
99.02(2)
2181(1)
colorless
1.599
10.306(2)
10.837(3)
12.647(4)
9.1187(3)
14.6294(4)
32.8906(5)
7.999(2)
8.002(2)
8.401(2)
69.79(2)
69.69(2)
74.94(2)
467.3(2)
yellow
1.436
12.679(2)
12.483(2)
8.518(1)
97.26(1)
95.16(2)
92.56(0)
94.76(1)
2209(1)
colorless
1.579
2147.7(5)
colorless
1.624
1406.7(6)
colorless
1.673
4383.3(2)
yellow
1.289
1343.5(4)
colorless
1.68
d
calc, g/cm3
µ, mm-1
R (Rw or
2.216
2.280
2.245
2.599
0.467
0.849
1.82
4.71 (4.63)c
5.17 (12.57)d
5.42 (5.96)c
3.51 (3.81)c
6.06 (10.62)c
4.37 (11.00)c
3.75 (7.37)c
wR2),b %
a Obtained at 295 with graphite monochromated Mo KR (λ ) 0.710 73 Å) radiation. b For definitions cf. Table 1. c Rw. d wR2.
Space group assignments were based on systematic absences, E
statistics, and successful refinement of the structures. Structures were
solved by direct methods with the aid of successive difference Fourier
maps and refined using either the SHELXTL PLUS 4.21/V or the
SHELXTL 5.0 software package. Thermal parameters for all non-
hydrogen atoms were refined anisotropically. In the structures of
compounds 1′′, 5 (tetragonal), 6 (orthorhombic), 7, and 8, hydrogen
atoms were fixed at ideal locations 0.96 Å from the bonded carbon
contributions as estimated from Pascal constants52 (ødia ) -353 × 10-6
emu mol-1 for 4 and -312 × 10-6 emu mol-1 for 11).
Other Physical Measurements. 1H and 13C NMR spectra were
recorded on Varian XL-400 and JEOL GSX-270 NMR spectrometers.
The isotropically shifted peaks for the iron-containing compounds were
assigned on the basis of chemical shift, integration, and selective
deuteration. FT-IR spectra were obtained on a Perkin-Elmer 1800 FT-
IR spectrometer. FT-near-IR spectra were recorded on a Bruker Vector
22 FT-near-IR spectrometer (Ge or InAs detectors) equipped with a
fiber optic sample probe for near-IR/ATR spectra of solids. UV-vis
spectra were obtained on a Hewlett-Packard 8452A diode array
spectrophotometer. Mo¨ssbauer data were recorded with a conventional
Mo¨ssbauer spectrometer (57Co source in a Rh matrix) operating at the
constant acceleration mode and calibrated using a thin iron foil. Isomer
shifts are reported relative to metallic iron at room temperature. SEM
microprobe analysis was performed on a Kevex (Delta Series) energy
dispersive X-ray spectrometer. Electrospray mass spectra were recorded
using a Platform II MS (Micromass Instruments, Danvers, MA).
Samples were introduced from py/AcOH (10:1) solutions at a flow
rate of 5 µL/min from a syringe pump (Harvard Apparatus). The
electrospray probe capillary was maintained at a potential of 3.0 kV,
and the orifice to skimmer potential (“cone voltage”) was varied from
15-30 V. Spectra were collected in the multichannel acquisition mode.
EI and CI mass spectra were obtained on a Finnigan MAT-90 mass
spectrometer. Microanalyses were done by H. Kolbe, Mikroanalytis-
ches Laboratorium, Mu¨lheim an der Ruhr, Germany.
atom and given a uniform value of Uiso. In the structure of 9, all
hydrogen atoms were located from difference Fourier maps and refined
with isotropic thermal parameters. For all other structures, hydrogen
atoms were assigned to ideal positions and refined using a riding model
with an isotropic thermal parameter 1.2 times that of the attached carbon
atom (1.5 times for methyl hydrogens). In the structure of 1′′, the iron
and zinc atoms could not be differentiated and were evenly distributed
over the two metal sites; the carbon and nitrogen atoms in its pyridine
solvate molecule were treated similarly. Hydrogen atoms were not
included for this disordered pyridine. Crystallographic data are listed
in Tables 1 and 2. Further details of the structure determinations are
deposited as Supporting Information.
Magnetic Measurements. Magnetic susceptibilities for the one-
dimensional chain compounds 4 and 11 were obtained on a Quantum
Design AC MRMS-5S SQUID magnetosusceptometer equipped with
a 5.5 T magnet. Data were collected in the temperature range 2-300
K (total of 95 temperature points) at a field strength of 0.2 T and were
corrected for the magnetization of the sample holder and for diamagnetic
(52) O′Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203-283.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7037
Scheme 1
obtained at -20 °C. From the supernatant, a small amount of
a brown-black film precipitates upon diffusion of Et2O. At-
tempts to crystallize this material from organic solvents have
been unsuccessful. Mo¨ssbauer data suggest that this species
retains Fe(III), the amount of which is not diminished upon
longer reaction times. However, employment of preactivated
Zn dust (washed with HCl, CH3COOH, and pyridine) enables
complete and rapid (0.5 h) reduction to yellow-green solutions
from which the aforementioned Fe(II) species are obtained.
Pyridine-Dependent Equilibria. The pattern of isolation
of the different species, which at first appeared erratic, suggests
that multiple equilibria may be operative. These potentially
include monomer/dimer equilibria for both iron and zinc species
independently, as well as heterometallic cross-coupling equilibria
(eqs 16-18).
2n[FeII(O2CCH3)2(py)4] (3) S
[FeII (O2CCH3)4(py)3]n (4) + (5n)py (16)
2
[FeII(O2CCH3)2(py)4] (3) + 2[Zn(O2CCH3)2(py)2] (5) S
[Zn2FeII(O2CCH3)6(py)2] (2) + 6py (17)
Results and Discussion
2[Zn(O2CCH3)2(py)2] (5) S
Reduction of Basic Iron/Zinc Acetates. With reference to
Scheme 1, reduction of brown-black [Fe3O(O2CCH3)6(py)3]‚-
py (1)50 or dichroic red-green [Fe2ZnO(O2CCH3)6(py)3]‚py
(1′′)51 with excess Zn in CH3CN/AcOH (10:1 or 2:1 v/v) or
CH2Cl2/AcOH (10:1), followed by Zn filtration, results in the
precipitation of light yellow crystals of the air-sensitive
compound [Zn2FeII(O2CCH3)6(py)2] (2), upon cooling (-20 °C).
The analogous reduction (0.5 h) of 1 (or 1′′) with Zn in py/
AcOH (10:1 or 5:1) affords yellow-green crystals of [FeII(O2-
CCH3)2(py)4] (3)53 directly from the reaction mixture at -20
°C. Evident in this reaction is the slow evolution of dihydrogen
owing to the reaction of Zn with acetic acid. Prolonged
interaction of 1 (or 1′′) with Zn is avoided, since products of
solubilized Zn(OAc)2 (vide infra) are accumulating and finally
depositing from the reaction mixture. If diffusion of diethyl
ether (Et2O) into the py/AcOH filtrates of the reduction of 1
(or 1′′) with Zn is attempted at 10 °C, yellow crystals of
[Zn2(O2CCH3)4(py)2] (6) + 2py (18)
In agreement with this interpretation, dilution of pyridine
solutions with AcOH/Et2O or use of nonpyridine solvents in
isolation procedures promotes an equilibrium shift in the
direction 3 f 4 and 3(4) + 5 f 2. Further confirmation of the
proposed equilibria is provided by the following control
experiments which also serve as preparative methods. Com-
pound 4 is obtained pure in the form of feathery crystals from
saturated solutions of 3 in py/AcOH (5:1, 2:1) upon direct
addition or diffusion of Et2O. However, crystals of 4 suitable
for X-ray analysis (vide infra) could only be prepared from
concentrated solutions of 3 in neat pyridine upon fast addition
of Et2O (in order to avoid precipitation of 3) and subsequent
slow diffusion of Et2O at 10 °C. Storage of the crystallization
apparatus at lower temperatures favors precipitation of 3.
Supersaturated solutions of 3 in pyridine at room-temperature
deposit 4 as a yellow powder, although the latter is frequently
contaminated with 3. Conversely, pure 4 when dissolved in
pyridine or py/AcOH generates intense yellow-green solutions,
from which 3 precipitates upon cooling (-20 °C).
In Gif-type solutions, equilibrium 17 lies predominantly on
the left side. When dissolved in pyridine or py/AcOH (10:1,
5:1), light-yellow 2 affords intensely colored yellow-green
solutions, from which essentially quantitative amounts of 3 and
5 are isolated upon cooling at -20 °C. Conversely, compound
3 and a 2-fold excess of 5 dissolved in pyridine or py/AcOH
deposit light-yellow crystals of 2 upon addition of excess Et2O.
In a similar fashion, 2 is recovered quantitatively from solutions
containing both 3 and 5 in CH3CN, CH2Cl2, or DMSO.
Corroborative evidence for the pyridine-dependent equilibria
among species 2-4 is provided by UV-vis spectra. Compound
3 demonstrates a characteristic charge-transfer absorption (λmax
396 nm (ꢀM ) 2107) in py; λmax 388 nm (ꢀM ) 2084) in py/
AcOH (10:1); λmax 368 nm (ꢀM ) 1900) in py/AcOH (2:1)) in
the concentration range 10-5-10-4 M (Beer’s law observed;
Figure 2 (top)). At higher concentrations (>1.0 × 10-3 M in
py; >0.88 × 10-3 M in py/AcOH (10:1); >0.8 × 10-3 M in
py/AcOH (2:1)), a new broad absorption appears (λmax 424 nm
in py; λmax 406 nm in py/AcOH (10:1); λmax 400 nm in py/
AcOH (2:1)) and Beer’s law is no longer obeyed. At such high
concentrations, the UV-vis criterion alone is not reliable
[FeII (O2CCH3)4(py)3]n (4) are preferentially obtained. Depend-
2
ing on the constitution of the reaction mixture, these can be
contaminated with 3 (py/AcOH 10:1), 2 (py/AcOH 5:1), and
occasionally large colorless crystals of [Zn(O2CCH3)2(py)2]
(5).54 Direct addition, rather than diffusion, of a large excess
of Et2O to the py/AcOH reaction filtrates favors precipitation
of 2. Pure 2 is also consistently obtained by diffusion of Et2O
into the filtrate of the reduction of 1 with Zn in py/AcOH (2:
1). Colorless [Zn2(O2CCH3)4(py)2] (6) is additionally isolated
from py/AcOH (2:1) upon prolonged (>2 h) reduction of 1 (or
1′′) with Zn.
The inefficiency of catalytic Gif-type systems in pure pyridine
prompted us to examine the reduction of 1 with excess Zn in
the absence of AcOH. Prolonged (6 h) reduction results in a
yellow-brown solution, from which compound 3 (80%) is
(53) This species has been previously reported but aspects of its synthesis
and/or characterization have remained unclear: (a) Catterick, J.; Thornton,
P.; Fitzsimmons, B. W. J. Chem. Soc., Dalton Trans. 1977, 1420-1425.
(b) Hardt, H.-D.; Mo¨ller, W. Z. Anorg. Allg. Chem. 1961, 313, 57-69. (c)
Sheu, C.; Richert, S. A.; Cofre´, Ross, B., Jr.; Sobkowiak, A.; Sawyer, D.
T.; Kanofsky, J. R. J. Am. Chem. Soc. 1990, 112, 1936-1942.
(54) Solvation adducts of stoichiometry Zn(OAc)2‚2py along with some
crystallographic data have been reported: (a) Bernard, M. A.; Busnot, A.;
Busnot, F. Bull. Soc. Chim. Fr. 1970, 2867-2870. (b) Bernard, M. A.;
Busnot, A.; Busnot, F. Bull. Soc. Chim. Fr. 1971, 1278-1282. (c) Bernard,
M. A.; Busnot, F. Bull. Soc. Chim. Fr. 1972, 3045-3051. (d) Semenenko,
K. N.; Kurdyumov, G. M. Vestn. Mosk. UniV., Ser. Mat., Mekh., Astron.,
Fiz., Khim. 1958, 13, 207-209.

7038 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
known. 1H NMR spectra of 3 in pyridine or py/ACOH are not
very informative, even at low temperatures, owing to the rapid
exchange of pyridine/acetate moieties among the various iron-
containing species and the solvent matrix. In CD2Cl2 (as well
as in other non-pyridine solvents), chemical shifts for pyridine/
acetate groups of 3/4 are concentration-dependent, the degree
of isotropic shift diminishing with decreasing concentration. It
is noteworthy that 3 is crystallized from saturated solutions of
3 in CH2Cl2, while diffusion of Et2O into CH2Cl2 affords a
microcrystalline product that is analytically identical to com-
pound 4. Antiferromagnetically coupled dinuclear and higher
nuclearity species are expected to demonstrate isotropically
shifted NMR spectra of reduced range by comparison to their
mononuclear counterparts.
The slow reaction of Zn with AcOH results in the formation
of solubilized zinc acetate products which participate in
equilibria in parallel to, as well as along with iron species.
Compound 5 is consistently obtained along with other iron-
containing species from pyridine or py/AcOH (10:1, 5:1) Gif-
type solutions. It can also be crystallized pure from similar
solutions of Zn(O2CCH3)2‚2H2O, albeit in two distinct crystal
systems: tetragonal in pyridine or py/AcOH (10:1) and triclinic
from py/AcOH (5:1). The aforementioned dissociation of 2 in
pyridine or py/AcOH (10:1) to generate 3 and 5 also affords
crystals of 5 in the tetragonal crystal system. However, solutions
of 5 in py/AcOH (2:1) yield exclusively [Zn2(O2CCH3)4(py)2]
(6) upon crystallization. Compound 6 usually crystallizes in
the monoclinic system, although on a single occasion it has also
been crystallized from py/AcOH (2:1) in the orthorhombic
crystal system, as one of the products of the oxidation of 2 (vide
infra). Variable-concentration and -temperature 1H NMR
spectra of 5 in pyridine or py/AcOH are not sufficiently
diagnostic, most likely due to rapid ligand exchange even at
low temperatures. However, CI-MS data reveal that the
molecular ion of 5 (m/z ) 341.7) and fragments of 6 are solely
detected from py/AcOH solutions, the latter being more
pronounced for samples in py/AcOH (2:1). It thus appears that,
in Gif-type solutions, zinc is present in the form of 5 and 6.
Higher nuclearity species have been detected and isolated only
from non-pyridine solvents. The one-dimensional chain com-
pound [Zn(O2CCH3)2(py)]n (7) is obtained from solutions of 5
in EtOH or CH2Cl2, while the trinuclear [Zn3(O2CCH3)6(py)2]
(8) is deposited from solutions of 5 in CH3CN or 6 in CH2Cl2.
As in the case of 4, the nature of 7 in solution is not precisely
known, but interestingly, this species is isomeric to 6.
Other Reducing Agents. Although Zn dust is the most
practical source of reducing equivalents, compounds 3 and 4
can also be isolated by treatment of 1 with excess iron dust in
pyridine or py/AcOH (10:1) for 30 min and from the reaction
of 1 with H2(30 psig)/Pd in pyridine or py/AcOH (10:1) for 1
h. Both reactions, after removal of insoluble components, afford
bright yellow-green solutions from which high yields of
compounds 3/4 are crystallized as previously described. A
control experiment confirmed that these species are also
generated by simply stirring Fe in py/AcOH (10:1). Iron has
been previously used as a reducing agent in the early GifIII (Fe/
O2, 40 °C) catalytic version,56 yielding results nearly identical
to those of GifIV systems. Apparently, the two systems are
equivalent with respect to their solvated iron-containing species.
Related Ferrous Species. We have also explored the
possibility of anionic ferrous species being formed in Gif-type
solutions owing to the nature of the solvent matrix (py/AcOH)
involved. The potential role of water in modifying the structural
elements of the observed ferrous sites has also drawn our
Figure 2. (top) Optical spectrum of 3 (4.73 × 10-4 M) in pyridine.
The inset shows the absorbance at 396 nm as a function of concentra-
tion. The arrow pinpoints to a solution of 2 (1.51 × 10-4 M) in pyridine.
(bottom) Optical spectrum of 3 (1.86 × 10-3 M) in pyridine. The top
inset shows the near-IR spectra of solid 2 (dotted line), 3 (dashed line),
and 4 (solid line). The bottom inset shows the near-IR spectrum of a
solution of 3 (1.02 × 10-2 M) in py/AcOH (10:1).
evidence for the generation of new species in solution.
However, in the near-IR region, solutions of 3 (10-3-10-1 M)
or equimolar filtrates of the reduction of 1 by Zn powder in py
or py/AcOH (10:1, 5:1, 2:1) demonstrate broad d-d transition
bands (λmax ≈ 850 (ꢀM ≈ 8), 1180 (ꢀM ≈ 8) nm) which are
most closely related to those exhibited in n-IR/ATR spectra of
solid 4 (Figure 2, bottom insets). We also note that the
electronic spectrum of 2 (10-5-10-3 M) in pyridine or py/AcOH
(10:1, 5:1, 2:1) is identical with that exhibited by 3/4 and
accounts for greater than 90% conversion of 2 to 3/4 (Figure 2,
top inset). These results suggest that addition of Et2O or
employment of nonpyridine solvents is essential for the genera-
tion of 2.
The precise nature of species 4 in pyridine or py/AcOH
solutions is not easily discernible. It is unlikely that infinite
chains of 4 will be present in solution, although the degree of
aggregation can be substantial in the polar py/AcOH matrix.55
However, electrospray MS data obtained at various initial
concentrations of 3 in py/AcOH (10:1) have only yielded
fragments assigned to [Fe(OAc)(py)]+, [Fe(OAc)(py)2]+, [Fe2-
(OAc)3(py)]+, [Fe2(OAc)3(py)2]+, [Fe3(OAc)5(py)]+, and [Fe3-
(OAc)5(py)2]+. The trinuclear fragments become apparent at
higher initial concentrations ([3] g 0.88 × 10-3 M), although
the effective concentration of 3 in the droplet (travelling against
a countercurrent of inert gas) from which the ions desorb is not
(56) (a) Barton, D. H. R.; Gastiger, M. J.; Motherwell, W. B. J. Chem.
Soc., Chem. Commun. 1983, 41-43. (b) Barton, D. H. R.; Motherwell, R.
S. H.; Motherwell, W. B. Tetrahedron Lett. 1983, 1979-1982.
(55) Golubev, N. S.; Smirnov, S. N.; Gindin, V. A.; Denisov, G. S.;
Benedict, H.; Limbach, H.-H. J. Am. Chem. Soc. 1994, 116, 12055-12056.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7039
attention, since addition of water to py/AcOH (10:1) up to an
optimum amount of 6.6% w/w is known36a to enhance the
selectivity of Gif chemistry.
colorless orthorhombic crystals of [Zn2(O2CCH3)4(py)2] (6).
Compound 1′ is readily distinguished from 1 by virtue of its
1
isotropically shifted H NMR, 57Fe Mo¨ssbauer effect (10% of
Fe(II)), and precise Fe/Zn composition derived from ICP
microanalysis. Scanning electron microprobe analysis per-
formed on 10 single crystals of 1′ confirms that each individual
crystal contains zinc (Fe:Zn ) 74:26). These results along with
X-ray diffraction analysis and literature precedence59 establish
that compound 1′ is neither a collection of crystals of 1 and 1′′
nor an agglomeration of distinct phases of 1 and 1′′ in a single
crystal of 1. As suggested by the method of preparation of 1′′,51
a minimum of a 5-fold excess of solvated Zn(II) over Fe(II) is
necessary for complete occupancy of the divalent site of the
µ-oxo structure by zinc after oxidation.
Admittance of dioxygen to the filtrate of the prolonged
reduction (6 h) of 1 by Zn in pyridine deposits dichroic red-
green crystals of monoclinic 1′′. Interestingly, compound 1′′,
prepared by the original method,51 crystallizes in the rhombo-
hedral crystal system. The isolation of 1′′ indicates that, in
catalytic oxidations in which excess Zn is in contact with py/
AcOH (10:1) for 18 h, compound 1′′ may be more involved as
the product of autoxidation than the all-iron species 1.
Solid State Structures. (a) Iron Species in Gif Solutions.
Among the various stoichiometries reported, only species 1/1′/
1′′ and 3/4 can be legitimately claimed to be present in pyridine
or py/AcOH solutions, with the aforementioned reservations in
regards to the precise nature of 4 in these solutions taken into
account. Compound 1′ (Figure 3, top) crystallizes in space
group R32, featuring trinuclear metal µ-oxo clusters with
crystallographically imposed D3 symmetry. The occupancy of
each metal site was fixed at Fe 0.37 and Zn 0.13, based on
analytical data. Disorder of the trinuclear unit at 223 K does
not allow the different metal sites to be distinguished. Each
M-O(1) vector (1.914(1) Å) lies along a C2 axis, while O(1)
resides on a perpendicular C3 axis. As previously revealed in
similar structures,45 the solvated pyridine is sandwiched along
the C3 axis between two stacked trinuclear clusters and is
severely disordered. In contrast, the structure of compound 1′′
(Figure 3, bottom), synthesized by the in situ oxidation of
reduced (6 h) solutions of 1 by Zn in pyridine (vide supra),
displays the more rarely observed monoclinic crystal system
that, lacking a C3 axis, distinguishes between two metal sites
by virtue of a crystallographically imposed C2 axis. This axis
penetrates atoms O(1), M(2), N(2), and C(14). M(1) and M(2)
represent metal sites of occupancy Zn(1/3)/Fe(2/3) and Zn(1/
6)/Fe(1/3), respectively. The M(1)-O(1) vectors (1.929(3) Å)
are slightly longer than the unique M(2)-O(1) vector (1.900-
(6) Å) but not sufficiently different to allow exclusive assign-
ment of M(1) and M(2) sites to Fe(III) and Zn(II), respectively.
The structure of mononuclear 3 (Figure 4) reveals a distorted
octahedral Fe environment with an imposed C2 axis residing in
the equatorial plane defined by the four Fe-N bonds and
bisecting the symmetry-related N(1)-Fe-N(1A) (89.1(2)°) and
N(2)-Fe-N(2A) (92.2(2)°) angles. Pyridine rings are arranged
in a propeller fashion around the equatorial plane. Relevant to
the observed distortion are the Fe-N(1) and Fe-N(2) bond
distances as well as the O(1)-Fe-N(1) and O(1)-Fe-N(2)
bond angles which slightly differ from each other. The most
visible sign of the distortion is the departure from linearity of
the angle between the two trans acetate groups along the
propeller axis (O(1)-Fe-O(1A) ) 168.5(2)°).
Addition of [Ph3PdNdPPh3][O2CCH3] (1 equiv) to 3 in
pyridine affords yellow needles of [PPN][FeII(O2CCH3)3(py)]
(9) upon diffusion of Et2O at room temperature. In contrast, a
mixture of predominantly 3 and a minor amount of 9 are
deposited directly from concentrated pyridine solution at -20
°C. Solutions of 3 in py/H2O (10:1) or py/AcOH/H2O (10:1:
0.7) afford yellow crystals of [FeII(H2O)4(trans-py)2][O2CCH3]2
(10) upon slow diffusion of Et2O. Compound 10 reverses
quantitatively to 3 in neat pyridine as evidenced by isolation of
3 (-20 °C). UV-vis spectra of 10 in pyridine or py/H2O
exhibit features identical to those observed for 3 ([10] < 1.0 ×
10-3 M), indicating complete dissociation of the water adduct
in favor of the pyridine-ligated species.
The lability of pyridine is also demonstrated thermogravi-
metrically in its facile thermal (50 °C) dissociation from 3 under
vacuum (0.01 Torr), leading to the successive loss of two (1.5
h), three (2.5 h), and finally four (6 h) pyridine equivalents to
yield Fe(OAc)2. The purity and identity of the intermediate
products cannot be easily elucidated, since employment of non-
pyridine solvents to dissolve these species further contributes
to pyridine dissociation. This is dramatically shown in the case
of the prolonged incubation (2 d) of 3 in DMSO-d6 that leads
to precipitation of colorless, air-sensitive crystals of [FeII (O2-
3
CCH3)6(OS(CD3)2)2]n (11). Compound 11 is also obtained from
solutions of Fe(O2CCH3)2 in DMSO.
Reaction of Ferrous Species with Dioxygen. Upon expo-
sure of concentrated pyridine or py/AcOH (10:1) solutions of
3 and 4 to pure dioxygen or air, compound 1 can be isolated in
essentially quantitative yields. However, preliminary UV-vis
studies have indicated that oxidation of solutions of 3 with dry
oxygen at concentrations below 0.88 × 10-3 M (in py/AcOH
(10:1)) and 1.0 × 10-3 M (in neat pyridine) generates an
intermediate (λmax ) 338 (py/AcOH), 344 (py) nm) that only
partially and slowly converts to 1. This intermediate can be
observed even after 24 h. A broad but rather weak absorption
centered at 658 nm is also detected in dilute py/AcOH (10:1)
solutions (for instance, [3] ) 0.65 × 10-4 M). In addition to
absorbances assigned to 1 (λ ) 358, 416, 488, 580 nm in py/
AcOH (10:1)), a feature at 474 nm grows in, associated with a
decay product. Interestingly, similar absorbances at 474 and
670 (broad transient) nm have been recently detected57 in the
oxidation of [Fe2(OH)(Me3TACN)2(OAc)2]+ and attributed to
diiron µ-oxo and µ-peroxo species, respectively. Efforts to
isolate and characterize the observed intermediates are currently
underway. In contrast, at concentrations of 3 above the
threshold values, the onset of a much more rapid and complete
generation of 1 is progressively observed under similar condi-
tions with no well-resolved intermediates being detected by
conventional UV-vis techniques. For instance, oxidation of
solutions of 3 (1.3 × 10-3 M) in py/AcOH (10:1) generates
essentially quantitative amounts of 1 within 1 h. At these higher
concentrations, the anticipated generation of 4 in py/AcOH
solutions may drastically influence the rate and/or nature of
dioxygen activation. A detailed kinetic analysis of the formation
and decay of intermediates is currently in progress by means
of cryostopped-flow mixing of reactants and rapid diode array
detection.58
The reaction of 2 with dioxygen in py/AcOH (10:1) affords
red-black crystals of [Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]‚py (1′).
The supernatant (py/AcOH (2:1) after solvent reduction) yields
The structure of 4 (Figure 5) reveals a one-dimensional
polymer chain comprised of asymmetric diferrous units. In the
unit cell, the chains run parallel to the 21 screw axis (along the
(57) Feig, A. L.; Masschelein, A.; Bakac, A.; Lippard, S. J. J. Am. Chem.
Soc. 1997, 119, 334-342.
(58) Kaderli, S.; Singh, B.; Stavropoulos, P.; Zuberbu¨hler, A. D. Work
in progress.
(59) Jang, H. G.; Kaji, K.; Sorai, M.; Wittebort, R. J.; Geib, S. J.;
Rheingold, A. L.; Hendrickson, D. N. Inorg. Chem. 1990, 29, 3547-3556.

7040 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
Figure 5. Structure of the local dinuclear geometry (top) and one-
dimensional chain (bottom) of [FeII (O2CCH3)4(py)3]n (4) showing 30%
2
probability ellipsoids and the atom labeling scheme. Selected interatomic
distances (Å) and angles (deg): Fe(1)-O(7) 2.073(4), Fe(1)-O(3)
2.076(4), Fe(1)-O(1) 2.157(4), Fe(1)-O(5) 2.179(4), Fe(1)-N(1)
2.228(4), Fe(1)-N(2) 2.247(4), Fe(2)-O(2) 2.033(4), Fe(2)-O(4)
2.117(4), Fe(2)-O(5) 2.212(3), Fe(2)-N(3) 2.259(5), Fe(2)-O(6)
2.273(5), O(7)-Fe(1)-O(1) 92.9(2), O(3)-Fe(1)-O(1) 90.6(2), O(1)-
Fe(1)-O(5) 92.6(2), O(5)-Fe(1)-N(1) 178.61(14), O(1)-Fe(1)-N(2)
175.4(2), O(2)-Fe(2)-O(5) 96.00(14), O(8A)-Fe(2)-O(5) 142.76-
(14), O(2)-Fe(2)-O(6) 152.9(2), O(4)-Fe(2)-O(6) 93.7(2), O(5)-
Fe(2)-O(6) 57.4(2), N(3)-Fe(2)-O(6) 88.7(2).
Figure 3. Structures of trigonal [Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]‚py
(1′) (top) and monoclinic [Fe2ZnO(O2CCH3)6(py)3]‚py (1′′) (bottom)
showing 40% and 30% probability ellipsoids, respectively and the atom
labeling schemes. Selected interatomic distances (Å) and angles (deg)
for 1′: M-O(1) 1.914(1), M-O(2) 2.042(5), M-N(1) 2.203(7), O(1)-
M-O(2) 96.2(2), O(1)-M-N(1) 180.0(1), O(2)-M-N(1) 83.8(2);
1′′: M(1)-O(1) 1.929(3), M(1)-O(2) 2.057(6), M(1)-O(4) 2.075(6),
M(1)-N(1) 2.202(9), M(2)-O(1) 1.900(6), M(2)-O(3) 2.072(7),
M(2)-O(5) 2.041(6), M(2)-N(2) 2.211(8), O(1)-M(1)-O(2) 96.2-
(3), O(1)-M(1)-O(4) 95.3(3), O(2)-M(1)-O(4) 89.1(3), O(1)-
M(1)-N(1) 179.7(2), O(1)-M(2)-O(3) 95.3(2), O(1)-M(2)-O(5)
96.2(2), O(3)-M(2)-O(5) 89.8(3), O(1)-M(2)-N(2) 180.0(1), M(1)-
O(1)-M(2) 120.2(2).
more distorted Fe(2) site is surrounded by an O5/N ligating
environment. The two iron atoms of the asymmetric core (Fe-
(1)‚‚‚Fe(2) ) 3.676 Å) are bridged by two bidentate acetate
groups and a unique monodentate acetate bridge which also
serves as a bidentate chelator to Fe(2). The metric parameters
associated with the shift from a bridging monodentate to an
ancillary chelating position (termed “the carboxylate shift”) have
been discussed in the literature,60 the degree of the “shift”
varying with the extent of contact of the distal oxygen atom
with the iron site. The one observed in 4 (Fe(2)-O(6) ) 2.273-
(5) Å) is a rare case of a strong bonding interaction (comparable
to Fe(2)-O(5) ) 2.212(3) Å), instances of which are only found
in the structures of Fe3(O2CPh)6L2 (L ) iPrOx, PheMe3Eda),61
the dimetallic Mn(II)Ca(II) site of concanavalin A,62 and, more
relevantly, the diiron site of the Hred component of sMMO (Fe-
O(Glu243) ) 2.33 Å).6a,b Indeed, the corresponding Fe(2) sites
of 4 and Hred in sMMO demonstrate remarkable metrical
similarities with respect to the arrangement of their respective
O5/N coordination sphere, although the sMMO site is closer to
being five-coordinate. The most pronounced difference is the
existence of a weakly bound semibridging water molecule in
Hred in lieu of a bridging bidentate acetate in 4. The mono-
dentate water bridge may be responsible for the relative
shrinkage of the reported Fe‚‚‚Fe distance (3.28 Å) in Hred versus
the corresponding distance in 4. The coordination sphere of
Figure 4. Structure of the mononuclear [FeII(O2CCH3)2(py)4] (3)
compound showing 50% probability ellipsoids and the atom labeling
scheme. Selected interatomic distances (Å) and angles (deg): Fe-O(1)
2.031(4), Fe-N(1) 2.240(5), Fe-N(2) 2.213(5), O(1)-Fe-N(1) 85.8-
(2), O(1)-Fe-N(2) 94.1(2), N(1)-Fe-N(2) 89.4(2), O(1)-Fe-O(1A)
168.5(2).
(60) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991,
15, 417-430.
(61) (a) Goldberg, D. P.; Telser, J.; Bastos, C. M.; Lippard, S. J. Inorg.
Chem. 1995, 34, 3011-3024. (b) Rardin, R. L.; Poganiuch, P.; Bino, A.;
Goldberg, D. P.; Tolman, W. B.; Liu, S.; Lippard, S. J. J. Am. Chem. Soc.
1992, 114, 5240-5249 and other references therein. (c) Tolman, W. B.;
Liu, S.; Bentsen, J. G.; Lippard, S. J. J. Am. Chem. Soc. 1991, 113, 152-
164.
c axis). Each dinuclear unit is bridged to two adjacent units
via O(8) and O(7A) (Fe(1)‚‚‚Fe(2′) ) 5.275 Å). The essentially
octahedral Fe(1) atom is coordinated by two N(py) donor atoms
and four acetate-derived oxygen atoms, featuring a slight
elongation along the O(1)-Fe(1)-N(2) direction. The much
(62) Hardman, K. D.; Agarwal, R. C.; Freiser, M. J. J. Mol. Biol. 1982,
157, 69-86.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7041
Figure 6. Structure of the centrosymmetric compound [Zn2FeII(O2-
CCH3)6(py)2] (2) showing 40% probability ellipsoids and the atom
labeling scheme. Selected interatomic distances (Å) and angles (deg):
Fe(1)-O(12) 2.115(3), Fe(1)-O(22) 2.099(3), Fe(1)-O(31) 2.156(3),
Zn(1)-O(11) 1.954(3), Zn(1)-O(21) 1.942(3), Zn(1)-O(31) 1.959-
(3), Zn(1)-N(1) 2.017(4), O(12)-Fe(1)-O(22) 94.1(1), O(12)-Fe-
(1)-O(31) 91.5(1), O(11)-Zn(1)-O(21) 112.0(1), O(11)-Zn(1)-
O(31) 105.3(1), O(31)-Zn(1)-N(1) 129.2(1).
Figure 7. Structure of the anionic component of [PPN][FeII(O2CCH3)3-
(py)] (9) showing 30% probability ellipsoids and the atom labeling
scheme. Selected interatomic distances (Å) and angles (deg): Fe-O(5)
2.033(4), Fe-O(3) 2.059(4), Fe-O(1) 2.087(3), Fe-O(2) 2.313(3), Fe-
N(1) 2.160(4), O(5)-Fe-O(3) 90.1(2), O(5)-Fe-O(1) 135.3(2),
O(3)-Fe-O(1) 129.5(2), O(5)-Fe-N(1) 99.3(2), O(3)-Fe-N(1)
104.0(2), O(1)-Fe-N(1) 90.8(2), O(5)-Fe-O(2) 104.04(14), O(3)-
Fe-O(2) 94.96(14), O(1)-Fe-O(2) 59.05(13), N(1)-Fe-O(2) 149.72-
(14).
Fe(1) in 4 differs from the corresponding site in Hred both in
being more nitrogen-rich and in the relevant orientation of the
bridging versus the terminal residues.
(b) Other Ferrous Acetate Structures. The structure of 2
(Figure 6) reveals a linear arrangement of three metal sites,
featuring an essentially octahedral centrosymmetric iron atom
bridged to two tetrahedrally ligated zinc atoms. The bridging
moieties are provided by two bidentate and one monodentate
acetate groups. A slight elongation along the O(31)-Fe(1)-
O(31′) axis (Fe(1)-O(31) ) 2.156(3) Å) of the exclusively
oxygen-coordinated iron site is observed. The tetrahedral
geometry of the outer zinc atoms is distorted, especially with
respect to the position of the bridging oxygen atom of the
monodentate acetate group versus the nitrogen atom of pyridine
(O(31)-Zn(1)-N(1) ) 129.2(1)°). A similar arrangement of
bridging acetate groups has been observed in homonuclear and
heteronuclear M2M′(O2CCR3)6L2 structures (M ) V, Mn, Fe,
Co, Zn; M′ ) V, Mn, Fe, Co, Ni, Zn, Cd, Mg, Ca, Sr; L )
substituted pyridines or bidentate N-donor ligands)61b although
the Zn2Fe combination was unknown. Of particular interest in
this series of compounds are the aforementioned trinuclear iron
species of stoichiometries Fe3(O2CR)6L2 (R ) Me, L )
Figure 8. Structure of the local environment of the cation (left) and
2-D sheet network (right) of centrosymmetric [FeII(H2O)4(trans-
py)2][O2CCH3]2 (10) showing 40% probability ellipsoids and the atom
labeling scheme. Selected interatomic distances (Å) and angles (deg):
Fe-O(1) 2.122(2), Fe-O(2) 2.122(2), Fe-N 2.213(2), O(1)-Fe-O(2)
90.32(10), O(1)-Fe-N 92.83(10), O(2)-Fe-N 92.67(10).
i
BIPhMe, BIPhOH, BIDPhEH; R ) Ph, L ) PrOx, PheMe3-
Eda)61a in which the outer iron atoms are additionally coordi-
nated (weakly or strongly) by the distal oxygen atom of the
monodentately bridging acetate.
The crystal structure of 11 (Figure 9) was obtained from a
crystal grown in solutions of DMSO-d6. The compound is a
one-dimensional solid containing chains comprised of two types
of iron in a 2:1 (Fe(1):Fe(2)) ratio. Two Fe(1) centers are
intimately coupled by one bidentate and two monodentate
acetate bridges (Figure 9, top), although each of the distal
oxygens of the monodentate acetate groups coordinates to an
adjacent Fe(2) site. A crystallographically imposed C2 axis
penetrates atoms C(7) and C(8) of the bidentate acetate group.
In addition, each Fe(1) is coordinated by one DMSO and two
acetate groups. The latter are both bridging to the adjacent
unique Fe(2) site (Figure 9, middle). One of them is a typical
bidentate acetate, while the other is a unique monodentate bridge
which also acts as a chelator to Fe(2). By virtue of a C2 axis
passing through atoms Fe(2), C(5), and C(6), the latter oxygen
atom is also bridging the iron sites of the adjacent Fe(1)2 unit;
thus, this acetate spans the range Fe(1)2‚‚‚Fe(2)‚‚‚Fe(1)2 (Figure
9, bottom).
The structure of compound 9 (Figure 7) reveals a pseudo-
seven-coordinate iron center coordinated by three asymmetri-
cally chelating acetate groups and a unique pyridine moiety.
The asymmetry of the bidentate chelation is evident from the
Fe-O(1) (2.087(3) Å) and Fe-O(2) (2.313(3) Å) distances and
becomes more pronounced with the other two acetates, since
both O(4) and O(6) assume positions of interatomic contact with
the iron center (Fe‚‚‚O(4/6) ) 2.829 Å).
The structure of compound 10 (Figure 8) reveals an octahedral
iron site residing on an inversion center and coordinated by four
water molecules in the equatorial plane and two pyridine
molecules along a perpendicular axis. All hydrogen atoms, with
the exception of those on the Me group of the acetate, were
located in the electron density map and refined isotropically.
Each hydrogen atom belonging to a coordinating water molecule
makes a hydrogen bond with an acetate oxygen atom. The two
noncoordinating acetates associated with the cationic iron center
form hydrogen bonds with cis-water molecules in a bidentate
fashion (O(3)-H(1A) ) 1.92 Å, O(4)-H(2A) ) 1.91 Å;
O(3)‚‚‚O(1) ) 2.680 Å, O(4)‚‚‚O(2) ) 2.680 Å). In addition,
each acetate is hydrogen-bonded to two adjacent cationic units
(O(3)-H(1B) ) 1.98 Å, O(4)-H(2B) ) 2.00 Å), leading to
an overall two-dimensional sheet network structure.
(c) Structures of Zn-Containing Species. Two crystal-
lographic versions of 5 have been obtained. Presumably, they
represent closely spaced energetic minima. The compound
obtained in the tetragonal space group P43212 (Figure 10, top),
reveals a tetrahedral zinc site residing on a crystallographically
imposed C2 axis bisecting the N-Zn-N(A) (110.7(3)°) and
O(1)-Zn-(O1A) (98.0(3)°) angles. Distortions from tetrahedral

7042 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
Figure 10. Structures of tetragonal (top) and triclinic (bottom) [Zn-
(O2CCH3)2(py)2] (5) showing 30% probability ellipsoids and the atom
labeling scheme. Selected interatomic distances (Å) and angles (deg)
for tetragonal 5: Zn-O(1) 1.941(4), Zn-N 2.021(5), O(1)-Zn-N
109.3(2); triclinic 5: Zn-O(1) 1.945(2), Zn-O(3) 1.941(2), Zn-N(1)
2.047(2), Zn-N(2) 2.075(2), O(3)-Zn-O(1) 110.88(9), O(3)-Zn-
N(1) 113.47(8), O(1)-Zn-N(1) 115.16(10), O(3)-Zn-N(2) 112.46-
(9), O(1)-Zn-N(2) 96.45(8), N(1)-Zn-N(2) 107.16(8).
Figure 9. (top) Structure of the local environment of the equivalent
Fe(1) sites (appoximately down the C2 axis) in the one-dimensional
[FeII (O2CCH3)6(OS(CD3)2)2]n (11) showing 40% probability ellipsoids
3
and the atom labeling scheme. (middle) Structure of the bridging
arrangement to the unique Fe(2) site showing 30% probability ellipsoids
and the atom labeling scheme. A C2 axis passes through atoms C(6),
C(5), and Fe(2). (bottom) Overview of the one-dimensional chain
structure of 11. Selected interatomic distances (Å) and angles (deg):
Fe(1)-O(1) 2.042(4), Fe(1)-O(6) 2.057 (4), Fe(1)-O(7) 2.130(4), Fe-
(1)-O(3) 2.134(4), Fe(1)-O(5) 2.223(3), Fe(2)-O(2) 2.060(4), Fe-
(2)-O(4) 2.119(4), Fe(2)-O(5) 2.236(4), O(1)-Fe(1)-O(3) 95.8(2),
O(1)-Fe(1)-O(5) 93.5(2), O(3A)-Fe(1)-O(5) 171.97(13), O(3)-Fe-
(1)-O(5) 93.68(13), O(5)-Fe(1)-O(7) 89.38(14), O(5)-Fe(2)-O(5A)
58.8(2), O(2)-Fe(2)-O(5) 104.5(2), O(4)-Fe(2)-O(5) 85.3(2), O(2)-
Fe(2)-O(4) 103.1(2), O(2A)-Fe(2)-O(5A) 104.5(2).
symmetry are made evident by the latter angle and are likely
due to two long-range Zn‚‚‚O(2) interactions. Compound 5
crystallized in the triclinic space group P1h with a slightly
different geometry (Figure 10, bottom). The two structures
differ primarily in the relative positions of the two long-range
Zn‚‚‚O contacts.
Crystals of compound 6 are also obtained in two crystal-
lographic versions. In its monoclinic form, the dinuclear
molecule (Figure 11) resides on a C2 axis which penetrates
atoms C(7), N(1), Zn(1), Zn(2), N(2), and C(10). In its
orthorhombic form (see Supporting Information), the compound
lies on an inversion center midway along the internuclear axis.
The only noteworthy difference between the two structures lies
in the torsion angles made by the planes of the two axial pyridine
ligands.
Figure 11. Structures of monoclinic [Zn2(O2CCH3)4(py)2] (6) showing
30% probability ellipsoids and the atom labeling scheme. The structure
of orthorhombic 6 is similar. Selected interatomic distances (Å) and
angles (deg) for monoclinic 6: Zn(1)-N(1) 2.037(5), Zn(1)-O(3)
2.041(3), Zn(1)-O(1) 2.042(3), Zn(1)...Zn(2) 2.8921(10), Zn(2)-O(2)
2.027930, Zn(2)-O(4) 2.042(3), Zn(2)-N(2) 2.047(5), N(1)-Zn(1)-
O(3) 101.07(10), N(1)-Zn(1)-O(1) 98.87(10), O(3)-Zn(1)-O(1)
87.4(2).
neighbors by a bidentate acetate and is further coordinated by
a pyridine and an asymmetrically chelating acetate group.
Pyridine rings are oriented nearly perpendicular to each other
at adjacent Zn sites. Most importantly, the chelating acetate
groups are nonequivalent and asymmetric at these Zn centers
(Zn(1)-O(1) ) 2.024(6) Å, Zn(1)-O(2) 2.414(7) Å; Zn(2)-
O(3) ) 2.116(6) Å, Zn(2)-O(4) 2.255(7) Å).
Compound 8 (Figure 13) is isostructural with 2, featuring a
trinuclear zinc molecule residing on an inversion center. The
unique octahedral zinc atom is bridged via two bidentate and
The structure of compound 7 (Figure 12) reveals an extended
one-dimensional solid comprised of an array of five-coordinate
zinc centers. The chain repeat unit is defined by two zinc
centers, Zn(1) and Zn(2). Each zinc atom is bridged to two

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7043
Figure 12. Structure of the one-dimensional [Zn(O2CCH3)2(py)]n (7)
chain showing 30% probability ellipsoids and the atom labeling scheme.
Selected interatomic distances (Å) and angles (deg): Zn(1)-O(1) 2.024-
(6), Zn(1)-O(2) 2.414(7), Zn(1)-O(5) 1.974(5), Zn(1)-O(7) 1.979-
(5), Zn(1)-N(1) 2.065(6), Zn(2)-O(3) 2.116(6), Zn(2)-O(4) 2.255(7),
Zn(2)-O(6) 1.987(5), Zn(2)-O(8) 1.999(5), Zn(2)-N(2) 2.068(6),
O(1)-Zn(1)-O(2) 57.2(2), O(1)-Zn(1)-O(5) 131.7(2), O(2)-Zn(1)-
O(5) 94.1(2), O(5)-Zn(1)-N(1) 103.3(2), O(3)-Zn(2)-O(4) 57.7-
(2), O(3)-Zn(2)-O(6) 119.7(2), O(3)-Zn(2)-O(8) 138.6(2), O(3)-
Zn(2)-N(2) 94.4(2).
Figure 14. Mo¨ssbauer spectra at 4.2 K of polycrystalline 4 (top) and
11 (bottom). Solid lines are theoretical fits corresponding to a single
ferrous site in the case of 4 and two superimposing ferrous sites in a
ratio of 2:1 in the case of 11.
Figure 13. Structure of the centrosymmetric [Zn3(O2CCH3)6(py)2] (8)
compound showing 30% probability ellipsoids and the atom labeling
scheme. Selected interatomic distances (Å) and angles (deg): Zn(1)-
O(1) 2.130(3), Zn(1)-O(3) 2.049(3), Zn(1)-O(5) 2.102(3), Zn(2)-
O(1) 1.95193), Zn(2)-O(4) 1.946(3), Zn(2)-O(6) 1.951(3), Zn(2)-
N(1) 2.016(3), O(1)-Zn(1)-O(3) 89.2(1), O(3)-Zn(1)-O(5) 93.2(1),
O(4)-Zn(2)-O(6) 111.2(1), O(1)-Zn(2)-N(1) 130.7(1), Zn(1)-
O(1)-Zn(2) 104.7(1).
one monodentate acetate groups to the outer tetrahedral zinc
sites. Metric parameters are virtually identical with those
observed in 2. Structures similar to 6 and 8 have been reported
for compounds of stoichiometry Zn2(carboxylate)4(base)2 and
Zn3(carboxylate)6(base)2, respectively.63
Mo1ssbauer Spectroscopy and Magnetization Studies. All
iron-containing compounds presented in this investigation have
been studied by 57Fe Mo¨ssbauer spectroscopy. The zero-field
spectra of the ferrous pyridine/acetate compounds (2-4 and
9-11) exhibit quadrupole doublets at isomer shifts and quad-
rupole splitting parameters characteristic of high-spin ferrous
sites (S ) 2). The isomer shift values for the neutral species
2-4 and 11 indicate a trend that is consistent with increasing
δ values as the amount of oxygen ligation increases versus
nitrogen ligation. This trend has been previously suggested4c
for Fe(III) species. Even in the case of the asymmetric diiron
site of 4, the zero-field Mo¨ssbauer spectrum in the solid state
at 4.2 K (Figure 14, top) demonstrates a unique quadrupole
doublet (δ ) 1.27 mm/s, ∆EQ ) 2.92 mm/s, Γ ) 0.20 mm/s),
thus the two iron sites remain indistinguishable even at low
temperature.64 In contrast, the Mo¨ssbauer spectrum of 11 shows
Figure 15. Plots of molar susceptibility (øM) per metal ion versus
temperature (deg) and effective moment versus temperature (∆) for
compound 4. The solid line is the best fit obtained as described in the
text by employing the parameters deposited as Supporting Information.
a substantially broadened and rather asymmetric quadrupole
doublet (Figure 14, bottom). The spectrum can be successfully
fitted by virtue of two different ferrous sites at a ratio of 2:1 (δ
) 1.40 mm/s, ∆EQ ) 3.04 mm/s, Γ ) 0.20 mm/s, 66%; δ )
1.38 mm/s, ∆EQ ) 2.94 mm/s, Γ ) 0.22 mm/s, 34%).
The measured magnetic susceptibility per iron ion of 4 as a
function of temperature is shown in Figure 15. The µeff value
of 4 at room temperature is 5.0 µB, close to the value expected
for uncoupled Fe(II) ions. The molar susceptibility, øM,
increases with decreasing temperature reaching a local maximum
at 10 K (øM ) 0.1012 emu mol-1). At lower temperatures, øM
again increases, probably due to the presence of a paramagnetic
impurity.
(63) (a) Clegg, W.; Hunt, P. A.; Straughan, B. P.; Mendiola, M. A. J.
Chem. Soc., Dalton Trans. 1989, 1127-1131. (b) Clegg, W.; Little, I. R.;
Straughan, B. P. Inorg. Chem. 1988, 27, 1916-1923. (c) Clegg, W.; Little,
I. R.; Straughan, B. P. J. Chem. Soc., Dalton Trans. 1986, 1283-1288. (d)
Clegg, W.; Little, I. R.; Straughan, B. P. J. Chem. Soc., Chem. Commun.
1985, 73-74.
(64) For a similar case applying to the closely related species (Et4N)[Fe2(O2-
CCH3)5(H2O)(py)2], see: Coucouvanis, D.; Reynolds, R. A., III; Dunham,
W. R. J. Am. Chem. Soc. 1995, 117, 7570-7571.

7044 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
As mentioned earlier, the structure of 4 shows two largely
different Fe‚‚‚Fe distances (3.676 and 5.275 Å), and therefore,
alternation of exchange coupling constants is anticipated. The
shorter bridge, comprised of two bidentate and one monodentate
as well as chelating acetate groups, is expected to be more
efficient than the longer one in transmitting the magnetic
interaction. The geometry between the closely spaced Fe ions
is similar to that found in other metal ion dimers61,65 and is
known to mediate a small magnetic exchange between ferrous
sites.61 In contrast, the single bidentate acetato group, bridging
the longer Fe‚‚‚Fe separation, is expected to be very ineffective
in transmitting exchange interactions. On this basis, magnetic
data were first analyzed by using a simple dinuclear model,
that is, setting the coupling constant along the longer bridge to
zero. The magnetic susceptibility data were fitted to the
analytical expression derived from the isotropic Heisenberg-
Dirac-van Vleck model using the spin Hamiltonian 19 which
consists of three terms describing the isotropic exchange, axial
zero-field local anisotropy and Zeeman perturbation.
6 metal sites; a six-ion ring is the upper limit for S ) 2 ions,
due to the computational effort involved.
The fit with the six-metal model (g ) 2.059(3), J ) -2.011-
(7) cm-1) was less satisfactory by comparison to the one
obtained with the dinuclear model, confirming the suggestion
that the two magnetic coupling constants, along the shorter and
longer Fe‚‚‚Fe distances, must be substantially different from
each other. Unfortunately, analytical formulas for alternating
chains of S ) 2 ions are not currently available.
The µeff value of 11 at room temperature is 5.3 µB per ferrous
ion, in agreement with independent spins. On decreasing
temperature, µeff remains essentially constant down to ca. 10
K, rapidly decreasing beyond that point (see Supporting
Information). The susceptibility øM continuously increases with
decreasing temperature, but no maximum is observed, suggest-
ing that the exchange coupling, if any, is close to zero.
Since the structure of 11 reveals similar Fe‚‚‚Fe separation
distances between neighboring ferrous sites (3.267, 3.543 Å),
attempts were made to fit the magnetic data with the homoge-
neous chain model, even though the bridging geometry is
different in the two cases. The parameters thus obtained are g
) 2.18(1) and J ) -0.22(1) cm-1, but the fit (see Supporting
Information) was quite poor for the data at lower temperatures.
To our knowledge, no other models are available for a magnetic
system of this complexity, the fitting being even more difficult
due to the weakness of the magnetic interaction. No further
attempts were undertaken to improve on the fitting procedure.
2
Hˆ ) -JS ‚S + D (S 2 - Si(Si + 1)/3) + µBGiSiH (19)
∑
1
2
i
iz
i)1
The best fit of molar susceptibility data is shown in Figure 15
as a solid line and corresponds to g ) 2.077, J ) -3.36 cm-1
,
and D ) -1.441 cm-1
. The paramagnetic impurity was
evaluated as 4.45 wt % of Fe(III) (S ) 5/2), assuming oxidative
decomposition of this exceedingly air-sensitive compound. The
J value so obtained is in good agreement with values determined
for similar, antiferromagnetically coupled compounds (-20 e
Catalytic Oxidations. Oxidation of adamantane mediated
by 2-4 under Gif-type conditions affords the following products
(unless otherwise specified, percentages are based on mol % of
substrate conversion): 2-adamantanone (2, 10.7%; 3, 10.7%;
4, 10.1%), 2-adamantanol (2, 1.2%; 3, 1.3%; 4, 1.3%), 1-ada-
mantanol (2, 0.8%; 3, 0.9%; 4, 0.7%), 4-(1-adamantyl)pyridine
(2, 6.0%; 3, 5.9%; 4, 6.0%), and 2-(1-adamantyl)pyridine (2,
3.8%; 3, 4.0%; 4, 3.2%). Traces of 4-(2-adamantyl)pyridine
and 2-(2-adamantyl)pyridine are also observed. Oxidation of
2-adamantanol mediated by 1 affords only 6.9% of 2-adaman-
tanone under identical conditions. Oxidation of adamantane by
1/Zn/O2 in CH3CN/AcOH (10:1) yields low conversions of
1-adamantanol (2.8%), 2-adamantanol (1.2%), and 2-adaman-
tanone (0.8%). Acceptable mass balances are larger than 95%
in all cases.
These values are comparable to those previously reported36a
for 1 and are further indicative of the equivalence of species
1-4 in catalytic solutions; this fact was anticipated due to the
observed pyridine-dependent interconversions. The normalized
3°/2° preference on a per-hydrogen basis gives an average value
of 2.65 for Gif-type oxidations. The selectivity for the tertiary
position (3°/2° ) 4.2) increases for oxidations in CH3CN/AcOH,
at the expense of lower yields achieved. These values can be
compared with values of 11-48 for oxidation of adamantane
by P-450 mimics (depending on porphyrin substitution) coupled
to PhIO,69 20 for generic radical reactions of adamantane,70 10
for radical reactions (t-BuO•) of adamantane in pyridine29 (albeit
2.7 for chlorination with t-BuOCl),31b 10 for oxidation of
adamantane by Fe2O(2,2′-bipy)4(H2O)2(ClO4)4/t-BuOOH in
CH3CN,15b 9.5 for oxidation of adamantane by [FeCl2-
(TPA)](ClO4)/t-BuOOH (or CumOOH) in CH3CN/C6H6 (1:1),13c
3-10 for oxidation of adamantane by Mn-porphyrin/KHSO5
J e 0 cm-1 61,65
and in particular with small values (J ) -1.0
)
cm-1) reported for the similarly bridged species Fe3(O2CPh)6L2
(L ) iPrOx, PheMe3Eda).61 The D value is also close to those
reported for mononuclear zero-field splitting at individual sites
of dinuclear ferrous systems.66 Attempts were also made to
analyze the data by means of a homogeneous chain model.67
The spin Hamiltonian (eq 20) used to describe the magnetic
interaction between adjacent ions in a regular magnetic chain
and the analytical expression68 (eq 21) employed to fit the data
are as follows:
∞
Hˆ ) -J S S
(20)
(21)
∑
i
i+1
i)1
Tr2 + ATr + B
Tr3 + CTr2 + DTr + E
ør )
2
where ør ) 3ø|J|/NµB g2S(S + 1) and Tr ) kT/|J| are the reduced
susceptibility and temperature, respectively. The analytical
expression is obtained by an extrapolation procedure on data
calculated for closed chains (rings) of increasing size from 3 to
(65) (a) Mashuta, M. S.; Webb, R. J.; McCusker, J. K.; Schmitt, E. A.;
Oberhausen, K. J.; Richardson, J. F.; Buchanan, R. M.; Hendrickson, D.
N. J. Am. Chem. Soc. 1992, 114, 3815-3827. (b) Nie, H.; Aubin, S. M. J.;
Mashuta, M. S.; Wu, C.-C.; Richardson, J. F.; Hendrickson, D. N.;
Buchanan, R. M. Inorg. Chem. 1995, 34, 2382-2388.
(66) Reem, R. C.; Solomon, E. I. J. Am. Chem. Soc. 1987, 109, 1216-
1226.
(67) (a) Miller, J. S., Ed. In Extended Linear Chain Compounds;
Plenum: New York, 1983; Vol. 3. (b) Kahn, O. Molecular Magnetism;
VCH: New York, 1993; pp 251-286. (c) Bonner, J. C. In Magneto-
Structural Correlations in Exchange Coupled Systems; Willett, R. D.,
Gatteschi, D., Kahn, O., Eds.; D. Reidel: Dordrecht, The Netherlands, 1985;
pp 157-205.
(68) (a) Bo´rras-Almenar, J. J. Doctoral Dissertation, University of
Valencia, 1992. (b) The numerical parameters used in the analytical
expression (A ) 1.35365, B ) -0.317459, C ) 4.20982, D ) 24.7699, E
) -4.96588) were computed by the group in Florence: Gatteschi, D.
Unpublished results.
(69) Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 6243-
6248.
(70) (a) Murahashi, S.-I.; Oda, Y.; Naota, T. J. Am. Chem. Soc. 1992,
114, 7913-7914. (b) Fukunishi, K.; Tabushi, I. Synthesis 1988, 826-827.
(c) Fossey, J.; Lefort, D.; Massoudi, M.; Nedelec, J.-Y.; Sorba, J. Can. J.
Chem. 1985, 63, 678-680.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7045
(or magnesium monoperphthalate) catalytic systems,71 4.0 for
oxidation of adamantane by Fe(PA)2/H2O2 (1:16) in py/AcOH
(2:1),36c 3.5 for oxidation of adamantane by Fe2O(OAc)2Cl2-
(2,2′-bipy)2 (or Fe2O(OAc)(tmima)2(ClO4)3/H2O2 in CH3CN,16
3 for oxidation of adamantane (1-ol, 50%; 2-ol, 50%) by
sMMO,37b 2.2 for oxidation of adamantane by Fe2O-
(HB(pz)3)2(OAc)2/Zn/O2 in CH2Cl2/AcOH (400/1),18 and ∼2
for generic oxidation of alkanes by HO•.72
Oxidation of isopentane (17.19 mmol) by the 1/Zn/O2
combination yields the following oxo products: 2-methyl-2-
butanol (0.05 mmol), 3-methyl-2-butanol (0.01 mmol), 3-meth-
yl-2-butanone (0.17 mmol), 3-methyl-1-butanol (0.001 mmol),
isovaleraldehyde (0.04 mmol), 2-methyl-1-butanol (0.003 mmol),
and 2-methylbutyraldehyde (0.08 mmol). The normalized 3°/
2°/1° preference of 3.6/6.5/1.0 indicates a net preponderance
for the oxygenation of the secondary position and a small, but
not negligible, activation of the primary C-H bonds. Oxidation
of 2-methyl-1-butanol and 3-methyl-1-butanol under similar
conditions affords only traces of the corresponding aldehydes.
The oxidation of isopentane by sMMO from M. trichosporium
OB3b has been carefully studied37a to reveal exclusive formation
of isopentanols. The product distribution indicates that there
is a net preference for the production of tertiary alcohol in the
absence of regulatory component B, but that primary alcohols
are strongly favored in oxidations with the fully reconstituted
enzyme.
The intermolecular deuterium KIE values (kH/kD) obtained
by oxidation of a mixture of adamantane/adamantane-d16 (1:1)
under Gif conditions at 298 K are as follows for each product:
2.01(12) (2-adamantanone), 0.92(4) (2-adamantanol), 1.18(3)
(1-adamantanol), 1.18(6) (4-(1-adamantyl)pyridine), and 1.25-
(2) (2-(1-adamantyl)pyridine). These values are comparable to
those reported73 for the oxidation of cyclohexane to cyclohex-
anone (KIE ) 2.3) by GifIV-type systems, but substantially lower
than intramolecular KIE values obtained from P-450-5a,41 or
sMMO-catalyzed oxidations,27a,42 although similar to intermo-
lecular KIE values (e2) associated with P-450 oxidations.5a
Typically, the KIE value for oxidations by HO• is 1.72 However,
values close to unity, as some of those obtained in the present
competitive intermolecular experiments are, may be the outcome
of a rate-determining step not related to the C-H(D) bond-
cleavage process. This can be true, for instance, for 2-ada-
mantanol assuming that it is produced by rate-determining
reduction of a common M-OOC(H)R2 precursor via O-O
cleavage. Importantly, the KIE value for cyclohexanol has been
shown73 to substantially increase in the presence of PPh3 in the
catalytic mixture. Hydrogen abstraction to yield the ketone is
expected to be facile, hence KIE values for the ketone is largely
influenced by the initial C-H cleavage.
Oxidation of adamantane by 1/H2O2 (20 equiv) in py/AcOH
(10:1) under N2 affords only traces of 2-(1-adamantyl)pyridine
(<0.2%). At 150 equiv of H2O2, minor amounts of products
are observed: 0.4% (2-adamantanone), 0.3% (2-adamantanol),
0.4% (1-adamantanol), and 0.2% (2-(1-adamantyl)pyridine) (3°/
2° ) 2.6). The reactions are accompanied by evolution of O2
(monitored by virtue of an oxygen-sensitive Clark electrode),
apparantly generated by catalase-type decomposition of H2O2.
Similar catalytic oxidations with H2O2 under O2 afford minor
amounts of oxo products: 2-adamantanone (0.5%) and 1-ada-
mantanol (0.6%) (3°/2° ) 3.6). Identical results were obtained
by employing the 3/H2O2 combination. The analogous oxidation
with 1/t-BuOOH (dry, 20 equiv) affords low yields of 4-(1-
adamantyl)pyridine (4.0%) and 2-(1-adamantyl)pyridine (4.7%)
anaerobically and minor amounts of oxo products under O2:
2-adamantanone (0.2%), 2-adamantanol (0.15%), and 1-ada-
mantanol (0.5%) (3°/2° ) 4.6). Catalytic oxidation with
peracetic acid affords only traces of products. Apparently, none
of these oxo donors provide viable alternatives to the Zn/O2
system. The catalase-type reaction has been noted by the Barton
group in conjunction with catalytic reactions mediated by the
FeCl3/H2O2 system in the absence of PAH in neat pyridine29,74
and by the present 1/H2O2 combination in py/AcOH (10:1) in
oxidations of cyclohexane.75
Oxidation of benzene by the 1/Zn/O2 system in py/AcOH
(10:1) affords very low yields of phenol (0.3%) and no biphenyl
(major product72 of oxidation by genuine HO• along with
phenol). Oxidation of toluene by the same system provides very
low conversions to the following products: benzaldehyde
(0.7%), benzyl alcohol (0.6%), benzoic acid (0.4%), o-cresol
(0.2%), m-cresol (0.1%), p-cresol (0.1%), bibenzyl (trace).
Noteworthy is the fact that genuine hydroxyl radicals add to
the phenyl ring almost exclusively (97%; k ) 3.0 × 109 M-1
s-1).72 Oxidation of cis-stilbene provides mostly benzaldehyde
(29.6%) and small amounts of trans-stilbene oxide (2.4%) and
deoxybenzoin (0.2%). This finding may argue against the
presence of high-valent iron-oxo species similar in function
to those postulated in biological oxygenases. However, metal-
77
oxo entities more akin to [MnO4]- 76 and RuO4 in terms of
reactivity can potentially cleave CdC bonds oxidatively.
Oxidation of pyridine (33.4 mmol) in the absence of other
substrates by the system 1/Zn/O2 in py/AcOH (10:1) yields
2-HOpy (0.11 mmol), 3-HOpy (0.07 mmol) and 4-HOpy (0.01
mmol), as quantified by 1H NMR. In addition, 2,2′-bipy (0.01
mmol) and 2,4′-bipy (0.05 mmol) have been detected and
evaluated by GC. Reportedly,29 pyridine (33 mL) is oxidized
by FeCl3 (1 mmol)/PAH (4 mmol)/H2O2 (4 mmol) to afford
2-HOpy (1.04 mmol) and 3-HOpy (0.30 mmol) but only traces
of 4-HOpy. The system Fe(PA)2(10 mM)/H2O2(10 mM) in py/
AcOH (2:1) yields 3-HOpy as the primary product.20b In
contrast, we note that HO• addition to pyridine in pulse radiolysis
experiments affords 2-HOpy• and 4-HOpy• (2:1).72,78 In addition
to the corresponding hydroxypyridines, these initial radicals can
yield bipyridines via a dimerization/dehydration process.72
Finally, hydroxyl radicals, generated by photolysis of N-hy-
droxy-2-thiopyridinone in py/AcOH, have been reported to yield
hydroxylation products 2-HOpy/3-HOpy/4-HOpy in a ratio of
13.0/1.0/5.9.79
The Nature of Active Oxidant(s) in Gif-Type Chemistry.
The most unresolved issue in conjunction with Gif-type oxida-
tions concerns the active oxidant involved. In principle, for
O2- or H2O2-based oxidations, this could be a high-valent iron-
oxo unit and/or hydroxyl radicals. Structural evidence presented
in this investigation reinforces the argument that there exist
(71) Hoffmann, P.; Robert, A.; Meunier, B. Bull. Soc. Chim. Fr. 1992,
129, 85-97.
(72) (a) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B.
J. Phys. Chem. Ref. Data 1988, 17, 513-886. (b) Baker, R. R.; Baldwin,
R. R.; Walker, R. W. Trans. Faraday Soc. 1970, 66, 2812-2826. (c) Russell,
G. A. In The chemistry of alkanes and cycloalkanes; Patai, S., Rappoport,
Z., Eds.; Wiley: New York, 1992; pp 966-983. (d) Kochi, J. K. In Free
Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. II, pp 689-
690.
(76) (a) Fatiadi, A. J. Synthesis 1987, 85-127. (b) Gardner, K. A.; Mayer,
J. M. Science 1995, 269, 1849-1851.
(77) (a) Lee, D. G.; van den Engh, M. In Oxidation in Organic Chemistry,
Vol. 5; Trahanovsky, W. S., Ed.; Academic Press: New York, 1973; Part
B, Chapter IV, pp 177-227. (b) Torii, S.; Inokuchi, T.; Kondo, K. J. Org.
Chem. 1985, 50, 4980-4982. (c) Carlsen, P. H. J.; Katsuki, T.; Martin, V.
S.; Sharpless, K. B. J. Org. Chem. 1981, 46, 3936-3938.
(73) Barton, D. H. R.; Doller, D.; Geletii, Y. V. Tetrahedron Lett. 1991,
32, 3811-3814.
(74) Barton, D. H. R.; Hu, B.; Taylor, D. K.; Rojas Wahl, R. U.
Tetrahedron Lett. 1996, 37, 1133-1136.
(78) Cercek, B.; Ebert, M. Trans. Faraday Soc. 1967, 63, 1687-1698.
(79) Barton, D. H. R.; Be´vie`re, S. D.; Chavasiri, W.; Doller, D.; Liu,
W.-G.; Reibenspies, J. H. New J. Chem. 1992, 16, 1019-1029.
(75) Barton, D. H. R. Personal communication.

7046 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Singh et al.
ferrous sites in Gif-type solutions which bear intriguing similari-
ties to the diiron site in Hred of sMMO. As demonstrated in
the structural features of 4, the documented lability of pyridine
favors oxygen-rich ligation by acetato moieties which, in turn,
may better precondition the diferrous site for dioxygen activa-
tion. Indeed, on a qualitative basis compound 4 reacts with
dioxygen much more readily than species 3. The relative
importance of oxygen- versus nitrogen-rich ligation has been
noted in conjunction with structural and functional features of
the diiron center in hemerythrin80 (oxygen carrier in a nitrogen-
rich environment) and sMMO6a,b (oxygen activator in an
oxygen-rich environment). Interestingly, products of autoxi-
dation include “as isolated” diferric species such as [Fe2ZnO-
(O2CCH3)6(py)3]‚py (1′′) which exhibit more than one analogies
with the corresponding diiron sites in Hox of sMMO. Of greater
significance, however, would be to characterize the precursor
intermediates generated by the interaction of the ferrous sites
with dioxygen as suggested by preliminary UV-vis spectra.
The importance of structural parameters to guide further
analysis of metal-centered events notwithstanding, functional
characteristics are more relevant to the mode of action of the
active oxidant involved and potentially revealing with respect
to its nature. The product profiles and intermolecular KIE
values obtained from Gif-type oxidations in the present study
share aspects of, but are not completely consistent with,
oxidations performed by genuine hydroxyl radicals (generated
by pulse radiolysis)72 or high-valent iron-oxo species, as the
latter have been known from studies involving P-450 isozymes/
mimics or sMMO. It is worth noting, however, that only in
does not initiate a viable shunt pathway may also speak against
a dominant role for HO• in the present system. The same result
can also be interpreted as indicating absence of ferric peroxo
units, generated by the interaction of ferrous sites with dioxygen.
However, hydrogen peroxide and dioxygen are most likely
acting at substantially different iron centers. Indeed, the µ-oxo
compound 1 may be better predisposed to mediate catalase-
type activity, as judged by structural similarities with the metal
site of manganese-containing catalases84 and iron-based catalase
mimics.85 Interestingly, the all-ferric [Fe3O(O2CCH3)6(H2O)3]+
has been recently shown86 to generate the trisperoxo compound
[Fe6(O)2(O2)3(O2CCH3)9]- upon interaction with H2O2.
Similarly, the potent oxidant in Gif solutions does not display
typical characteristics of high-valent iron-oxo species, as those
are commonly invoked in explaining the activity of P-450 and
sMMO. However, the absence of model systems bearing
genuine (per)ferryl units and the only recently emerging4
physicochemical characterization of the iron-oxo moiety in the
biological oxygenases poses limitations to the generality of the
comparative assessment. Nevertheless, the present system
neither mediates epoxidation of olefinic substrates nor does it
exhibit analogous chemo- and regioselectivity toward aliphatic
hydrocarbons by comparison to the biological oxygenases.
Admittedly, the influence of the protein environment in
modulating chemo/regioselection may superimpose on the
intrinsic ability of the putative iron-oxo moiety to bring about
a given transformation. The small 3°/2° and KIE values
calculated in the present study cannot be easily reconciled with
an iron-oxo unit abstracting a hydrogen atom from a C-H
bond via a more or less linear transition state, unless its potency
is such that resembles the indiscriminate activity of hydroxyl
radicals. A side-on C-H approach to a putative FedO moiety
would be more consistent with the observed values, but the
paucity of primary carbons remains problematic in both cases.
the case of sMMO has a reasonably well-characterized FeIV
-
2
(µ-O)2 unit been detected,4b,c while the elusive FeIVdO/residue+•
moiety of P-450 is believed to have a long-lived analogue in
the form of the isolable peroxidase compound I.81
It can be argued that the enhanced preference for the
activation of the secondary positions of adamantane by com-
parison to typical radical reactions is nevertheless compatible
with the action of the most indiscriminate radicals, including
HO• (3°/2° ≈ 2-2.5).72 On the other hand, isopentane and other
substrates (methylcyclohexane,47 methylcyclopentane,47 trans-
decalin,47 3-ethylpentane,82 1,4-trans-dimethylcyclohexane83)
show a net preference for the oxygenation of the secondary
position, a result that needs to be confirmed with a wider
selection of substrates. However, the following observations
make the presence of free hydroxyl radicals as the sole actiVe
oxidant improbable: (i) alcohols (2-adamantanol, isopentanols)
are oxidized to the corresponding aldehydes and ketones only
to a limited extent; indeed, alcohols are inferior substrates by
comparison to the parent alkanes; (ii) toluene is mostly oxidized
at the alkyl substituent rather than via addition to the aromatic
ring, albeit at low conversions; (iii) products of pyridine
oxidation by the present system are not consistent with those
obtained from hydroxyl radical oxidations; and (iv) reportedly,22
primary positions remain largely unaffected under Gif-type
conditions, although the present results with isopentane call for
a wider evaluation of the oxygenation at primary sites. The
low conversions of benzene to phenol cannot be fully assessed,
but do not exclude the presence of low fluxes of hydroxyl
radicals, unless the active oxidant is capable of activating the
aromatic ring. The fact that hydrogen peroxide, a reagent that
may have a higher propensity to generate hydroxyl radicals,
Of great interest in this context will be the exploration of the
conditions that influence the formation and govern the lifetime
of substrate radicals, potentially generated in oxidations medi-
ated by the 1/Zn/O2 system.87 With the exception of a limited
number of substrates (adamantane being one of them), Barton
has concluded29 (on the basis of extensive radical-scavenging
studies applied to the FeCl3/PAH/H2O2 oxidizing system) that
Gif-type oxidations do not involve alkyl radicals (concerted
C-H activation), unless a circumstantial Fenton-type FeII/H2O2
system preponderates. In a study comparing the same Gif
system (as well as the one employing t-BuOOH) with P-450
and sMMO, Newcomb has suggested38 that Gif-type oxidation
of diagnostic radical-clock substrates indicates the presence of
diffusively free alkyl radicals, as opposed to short-lived (<100
fs) substrate radicals generated by the action of the biological
oxygenases. Although these studies do not directly reveal the
nature of the active oxidant responsible for hydrogen abstraction,
they are nonetheless suggestive of a fundamental difference in
the mode of C-H activation by Gif-type systems and their
biological counterparts. It remains to be seen whether this
difference is solely due to the intrinsic characteristics of the
(84) (a) Varynin, V. V.; Vagin, A. A.; Melik-Adamyan, V. R.; Grebenko,
A. I.; Khangulov, S. V.; Popov, A. N.; Andrianova, M. E.; Vainshtein, B.
K. Dokl. Akad. Nauk. SSSR 1986, 288, 877-880. (b) Pecoraro, V. L.;
Gelasco, A.; Baldwin, M. J. In Mechanistic Bioinorganic Chemistry; Thorp,
H. H., Pecoraro, V. L., Eds.; American Chemical Society: Washington,
DC, 1995; pp 265-301. (c) Penner-Hahn, J. E. In Manganese Redox
Enzymes; Pecoraro, V. L., Ed.; VCH Publishers: New York, 1992; pp 29-
45. (d) Wieghardt, K. Angew. Chem., Int. Ed. Engl. 1989, 28, 1153-1172.
(85) Mauerer, B.; Crane, J.; Schuler, J.; Wieghardt, K.; Nuber, B. Angew.
Chem., Int. Ed. Engl. 1993, 32, 289-291.
(80) (a) Stenkamp, R. E. Chem. ReV. 1994, 94, 715-726. (b) Wilkins,
P. C.; Wilkins, R. G. Coord. Chem. ReV. 1987, 79, 195-214.
(81) (a) Sundaramoorthy, M.; Terner, J.; Poulos, T. L. Structure (London)
1995, 3, 1367-1377. (b) Ortiz de Montellano, P. R. Annu. ReV. Pharmacol.
Toxicol. 1992, 32, 89-107.
(82) Barton, D. H. R.; Doller, D.; Ozbalik, N.; Balavoine, G.; Gref, A.
Tetrahedron Lett. 1990, 31, 353-356.
(83) Barton, D. H. R.; Chavasiri, W. Tetrahedron 1997, 53, 2997-3004.
(86) Shweky, I.; Pence, L. E.; Papaefthymiou, G. C.; Sessoli, R.; Yun,
J. W.; Bino, A.; Lippard, S. J. J. Am. Chem. Soc. 1997, 119, 1037-1042.
(87) Work in progress in collaboration with M. Newcomb.

Oxidation of Hydrocarbons under Gif-Type Conditions
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7047
respective oxo donor/C-H acceptor couples or whether the
protein envelope has some major role to play in the latter cases.
Alternative active oxidants that need to be further considered
are disguised forms of the two extreme candidates. These may
include metal- or residue-bound hydroxyl radicals, as for
instance in FeII/III-O‚‚‚OH30 or HO•pyN/pyN•OH88 and their
dioxygen adducts thereof, as well as metal-(per)oxo units
bearing configurations more suitable for ketonization processes
(for instance, cis-FeO2 moieties).89
A dominant FeII/O2- interactionsas opposed to an FeII/H2O2
alternativeshas been suggested to be operational in GifIV
systems on the basis of control experiments29,36a and electro-
chemical studies.90 Although the intermediacy of superoxo
moieties is undisputable in dioxygen activation, their generation
is more likely to occur in the facile reduction of O2 at Fe(II)
sites rather than as part of the sluggish reaction of dioxygen
with Zn. In contrast, Zn can easily reduce Fe(III) centers to
their Fe(II) counterparts. That the FeII/O2 interaction in the
present system is important for catalytic turnovers has also been
suggested both by the observed suppression of all product yields
on increasing dioxygen pressure (most likely due to overstabi-
lization of the ferric components) and by the complete cessation
of catalytic activity with the trifluoroacetate analogue system
[Fe3O(O2CCF3)6(H2O)3]/Zn/O2 in py/CF3COOH (10:1), owing
to the stability of FeII(O2CCF3)2(py)4 with respect to dioxygen.91
Interestingly, the latter system becomes productive with excess
H2O2 replacing the Zn/O2 combination. These observations
indicate that the iron environment influences the outcome of
catalysis and may play a prominent role, rather than merely
acting as a decomposition center of organoperoxides.
(OAc)6(py)3]‚py, the divalent site M(II) being occupied by a
statistical mixture of Fe and Zn. Depending on the initial
concentration of the ferrous component, intermediates are
observed that need to be further characterized. Otherwise, these
consistent chemical redox cycles (mediated by a variety of
reducing agents and dioxygen) may be responsible for sustaining
the catalytic conversions of Gif chemistry.
(3) Analogies with structural elements of the diiron site in
Hred or Hox of sMMO are apparent in [FeII (O2CCH3)4(py)3]n
2
(4) and [Fe2ZnIIO(OAc)6(py)3]‚py (1′′), respectively. However,
the nuclearity of the present system upon chemical redox cycles
is not conserved.
(4) Product profiles and KIE values obtained from catalytic
oxidation of a number of substrates reveal features that are not
fully consistent with corresponding characteristics assigned to
HO•-based oxidations or to sMMO/P-450 oxygenase activity
due to high-valent iron-oxo moieties. Shunt pathways involv-
ing H2O2 lead to oxygen evolution rather than substrate
oxygenation.
Future studies directed toward exploring the nature and
kinetics of intermediates generated in both metal- and substrate-
centered events will address specifically whether a novel oxidant
is involved or hydroxyl radicals and/or (per)ferryl units are
present in disguise.
Acknowledgment. We thank Dr. M. J. Scott for experi-
mental assistance and Prof. R. H. Holm for use of the X-ray
diffractometer. We are grateful to Dr. Georgia C. Papaefthymiou
for recording and fitting the Mo¨ssbauer spectra. We also thank
Dr. Andrew Tyler for assisting in obtaining electrospray mass
spectra. The Harvard University Department of Chemistry and
Chemical Biology Mass Spectrometry facility is supported by
grants from NSF (CHE-9020043) and NIH (S10-RR08458). The
present work was generously supported by grants from the
donors of the Petroleum Research Fund, administered by the
ACS (ACS-PRF-29383-G3 to P.S.) and the U.S. Environmental
Protection Agency (R823377-01-1 to P.S.) and by an Alzhe-
imer’s Association Investigator-Initiated grant (IIRG-95-087 to
P.S.).
Conclusions
The following are the principal findings of this study:
(1) Reduction of basic iron or iron/zinc acetates under Gif-
type conditions generates a host of mononuclear, oligonuclear,
and polymeric chain ferrous and zinc species which are
implicated in pyridine-dependent equilibria; the [FeII(O2CCH3)2-
(py)4] (3)/[FeII (O2CCH3)4(py)3]n (4) interconversion dominates
2
in py/AcOH solutions, although the exact nature of 4 in solution
is not presently known.
(2) Upon exposure to dioxygen or air, the Fe(II)/Zn(II) sites
Supporting Information Available: Mo¨ssbauer spectrum
(4.2 K) of a polycrystalline sample of 1′ with theoretical fit,
plot of molar susceptibility (øM) per ferrous ion and effective
magnetic moment (µeff, with best fit) as a function of temperature
for compound 11, tables containing numerical values of
measured and calculated molar susceptibility and effective
magnetic moment versus temperature for 4 and 11, and tables
containing listings of crystal and data collection parameters,
atomic coordinates and isotropic thermal parameters, interatomic
distances, bond angles, and anisotropic displacement parameters
for 1′, 1′′, and 2-11 (95 pages). See any current masthead
page for ordering and Internet access instructions.
eventually regenerate the basic acetate structure [Fe2MIIO-
(88) An early proposal by Shilov favored generation of electrophilic
pyNO+: Geletii, Y. V.; Lavrushko, V. V.; Shilov, A. E. Dokl. Akad. Nauk.
SSSR 1986, 288, 139-143.
(89) Ferrate(V,VI)-like species cannot be excluded, especially for oxida-
tions with H2O2, although oxidation of primary and secondary alcohols is
expected to occur readily. See: (a) Rush, J. D.; Bielski, B. H. J. Inorg.
Chem. 1989, 28, 3947-3951. (b) Sharma, V. K.; Bielski, B. H. J. Inorg.
Chem. 1991, 30, 4306-4310.
(90) (a) Barton, D. H. R.; Sobkowiak, A. New J. Chem. 1996, 20, 929-
932. (b) Balavoine, G.; Barton, D. H. R.; Boivin, J.; Gref, A.; Le Coupanec,
P.; Ozbalik, N.; Pestana, J. A. X.; Rivie`re, H. Tetrahedron 1988, 44, 1091-
1106. (c) Balavoine, G.; Barton, D. H. R.; Boivin, J.; Gref, A.; Hallery, I.;
Ozbalik, N.; Pestana, J. A.; Rivie`re, H. New J. Chem. 1990, 14, 175-183.
(91) Tapper, A.; Long, J. R.; Stavropoulos, P. Manuscript in preparation.
JA970562R