Dioxygen-initiated oxidation of heteroatomic substrates incorporated into ancillary pyridine ligands of carboxylate-rich diiron(II) complexes

Article abstract of DOI:10.1021/ic051476s

Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands. Pyridine ligands incor

Full text of DOI:10.1021/ic051476s

Inorg. Chem. 2006, 45, 837−848
Dioxygen-Initiated Oxidation of Heteroatomic Substrates Incorporated
into Ancillary Pyridine Ligands of Carboxylate-Rich Diiron(II) Complexes
Emily C. Carson and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received August 29, 2005
Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane
monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands.
Pyridine ligands incorporating a thiol, sulfide, sulfoxide, or phosphine moiety were allowed to react with the
preassembled diiron(II) complex [Fe2(
di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R
crystallographically. Triply and doubly bridged compounds [Fe2(
[Fe2(
-O2CArTol)2(O2CArTol)2(2-MeS(O)py)2] (5) resulted when 2-methylthiopyridine (2-MeSpy) and 2-pyridylmethyl-
µ
-O2CArR)2(O2CArR)2(THF)2], where -O2CArR is a sterically hindered 2,6-
Tol or 4-FPh). The resulting diiron(II) complexes were characterized
-O2CArTol)3(O2CArTol)(2-MeSpy)] (4) and
)
µ
µ
-
sulfoxide (2-MeS(O)py), respectively, were employed. Another triply bridged diiron(II) complex, [Fe2(
µ
-O2CAr4 FPh)3-
(O2CAr4 FPh)(2-Ph2Ppy)] (3), was obtained containing 2-diphenylphosphinopyridine (2-Ph2Ppy). The use of
2-mercaptopyridine (2-HSpy) produced the mononuclear complex [Fe(O2CArTol)2(2-HSpy)2] (6a). Together with that
-
of previously reported [Fe2(µ µ
-O2CArTol)3(O2CArTol)(2-PhSpy)] (2) and [Fe2( -O2CArTol)3(O2CArTol)(2-Ph2Ppy)] (1), the
dioxygen reactivity of these iron(II) complexes was investigated. A dioxygen-dependent intermediate (6b) formed
upon exposure of 6a to O2, the electronic structure of which was probed by various spectroscopic methods. Exposure
of 4 and 5 to dioxygen revealed both sulfide and sulfoxide oxidation. Oxidation of 3 in CH2Cl2 yields [Fe2(
µ
-OH)2-
-
-
(µ
µ
-O2CAr4 FPh)(O2CAr4 FPh)3(OH2)(2-Ph2P(O)py)] (8), which contains the biologically relevant
{
Fe2(
µ
-OH)2-
3
(
-O2CR)
}
+ core. This reaction is sensitive to the choice of carboxylate ligands, however, since the p-tolyl analogue
1 yielded a hexanuclear species, 7, upon oxidation.
Introduction
lase component (MMOH) of sMMO, is responsible for the
binding and reductive activation of dioxygen.7-10 In the
reduced state (MMOHred), the diiron(II) centers are bridged
by two carboxylates and each has an additional monodentate
carboxylate ligand. One of the latter is involved in hydrogen
bonding with a coordinated water molecule. In the oxidized,
resting state of the enzyme (MMOHox), the diiron(III) center
is bridged by two hydroxide ions and a single carboxylate
group. The other three carboxylate residues are monodentate,
one of which is hydrogen bonded to a coordinated water
molecule. Representations of MMOHred and MMOHox are
provided in Chart 1.
Dioxygen activation and O-atom transfer reactions pro-
moted by iron-containing metalloenzymes are of considerable
interest both for ecological and industrial applications.1,2
Soluble methane monooxygenase (sMMO) is one such
enzyme system that is used by methanotropic bacteria to
catalyze the selective conversion of methane to methanol.3
In addition to saturated hydrocarbons, olefins, methanethiol,
amines, and sulfides can act as substrates.3-6 A carboxylate-
bridged non-heme diiron active site, housed in the hydroxy-
* To whom correspondence should be addressed. E-mail: lippard@
mit.edu.
(1) Erwin, D. P.; Erickson, I. K.; Delwiche, M. E.; Colwell, F. S.; Strap,
J. L.; Crawford, R. L. Appl. EnViron. Microbiol. 2005, 71, 2016-
2025.
(6) Fuse, H.; Ohta, M.; Takimura, O.; Murakami, K.; Inoue, H.; Yamaoka,
Y.; Oclarit, J. M.; Omori, T. Biosci. Biotechnol. Biochem. 1998, 62,
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Lippard, S. J. Angew. Chem., Int. Ed. 2001, 40, 2782-2807.
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ReV. 2003, 103, 2385-2419.
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10.1021/ic051476s CCC: $33.50
Published on Web 12/24/2005
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 2, 2006 837

Carson and Lippard
Chart 1
benzylic hydroxylation followed by dealkylation. The strat-
egy has been extended to include benzyl- or ethyl-derivatized
pyridine ligands, which react to form the corresponding
alcohol, ketone, or both.27b,28
In addition to C-H activation, the carboxylate-rich diiron
center in MMOH is capable of oxidizing alkyl sulfides and
thiols.3,6 We therefore extended our studies to examine car-
boxylate-bridged diiron(II) complexes with pyridine ligands
incorporating thiol, sulfide, sulfoxide, and phosphine as teth-
ered substrates. Upon exposure to dioxygen, these complexes
react to form the corresponding sulfoxide, sulfone, and
phosphine oxide. The results of this study, a portion of which
has been previously communicated,28 are reported here.
Chart 2
Experimental Section
The preparation and study of structural, spectroscopic, and
functional models for the active sites in metalloproteins have
provided insight into the mechanisms of their oxidation
chemistry, enhancing and extending our understanding of
the natural systems.11 A variety of symmetric diiron com-
plexes carrying N-donor and assorted O-donor ligands have
been reported as mimics for the diiron sites in MMOH and
related enzymes.12-14 Complexes with two carboxylate lig-
ands per iron atom are necessary to provide an accurate repre-
sentation of the MMOH active site. Self-assembly of discrete
dinuclear centers with such carboxylate ligands is challenged
by their tendency to form oligomers.15-20 Terphenyl-based
carboxylate ligands facilitate the assembly of discrete di-
nuclear complexes and mimic to some degree the hydro-
phobic pocket present at the protein active sites (Chart 2).21-23
In addition to four carboxylate ligands, two nitrogen
donors are required to replicate the stoichiometry of the
diiron core at the MMOH active site. One approach to
emulate enzyme functionally is to tether a substrate to the
N-donor ligand, increasing its chances of reacting with a
dioxygen-activated dimetallic core. The introduction of
dioxygen into solutions of the diiron(II) complexes
[Fe2(µ-O2CArTol)2(O2CArTol)2(N,N-Bn2en)2],24,25 where-O2CArTol
is 2,6-di(p-tolyl)-benzoate and N,N-Bn2en is N,N-dibenzyl-
ethylenediamine, [Fe2(µ-O2CArTol)4(BAp-OMe)2],26 where
BAp-OMe is p-methoxybenzylamine, and [Fe2(µ-O2CArTol)4-
(NH2(CH2)2SBn)2],27a led to the evolution of benzaldehyde.
This chemistry is formally the result of intramolecular
General Considerations. All reagents were obtained from
commercial suppliers and used as received, unless otherwise noted.
Dichloromethane and pentane were saturated with nitrogen and
purified by passage through activated Al2O3 columns under argon.29
Dioxygen (99.994%, BOC Gases) was dried by passing the gas
stream through a column of Drierite. The synthesis and character-
ization of [Fe2(µ-O2CArTol)2(O2CArTol)2(THF)2], [Fe2(µ-O2CArTol)3-
(O2CArTol)(2-Ph2Ppy)] (1), and [Fe2(µ-O2CArTol)3(O2CArTol)-
(2-PhSpy)] (2) were reported previously.21,28 2-Phenylthiopyridine
(2-PhSpy), 2-phenylpyridylsulfoxide (2-PhS(O)py), 2-methyl-
thiopyridine (2-MeSpy), 2-methylpyridylsulfoxide (2-MeS(O)py),
and 2-methylpyridylsulfone (2-MeS(O)2py) were prepared by modi-
1
fied literature procedures, and their purity was confirmed by H
NMR spectroscopy.30 2-Pyridyldiphenylphosphine oxide (2-Ph2P(O)-
py) was synthesized according to a published procedure,31 and the
purity was checked by 1H and 31P NMR spectroscopy. Air-sensitive
manipulations were carried out under nitrogen in an Mbraun drybox.
All samples were pulverized and dried under vacuum at 60 °C to
remove solvent prior to determining their elemental composition.
[Fe2(µ-O2CAr4-FPh)3(O2CAr4-FPh)(2-Ph2Ppy)] (3). 2-Ph2Ppy
(28.5 mg, 0.108 mmol) was added to a CH2Cl2 (4 mL) solution of
[Fe2(µ-O2CAr4-FPh)2(O2CAr4-FPh)2(THF)2] (150 mg, 0.100 mmol)
to form an orange solution that was stirred for 20 min. Diffusion
of pentane into the reaction mixture yielded orange block crystals
(151 mg, 93%) of 3. FT-IR (KBr, cm-1): 3062 (w), 2954 (w),
2863 (w), 1891 (w), 1604 (s), 1560 (m), 1510 (s), 1455 (s), 1406
(s), 1380 (s), 1299 (w), 1221 (s), 1158 (s), 1093 (m), 1049 (w),
1015 (m), 841 (m), 833 (m), 806 (s), 791 (m), 767 (m), 747 (w),
701 (m), 582 (w), 554 (s), 528 (s), 502 (w), 463 (w), 417 (w).
Anal. Calcd for C93H58NFe2O8F8P: C, 69.29; H, 3.63; N, 0.87; P,
1.92. Found: C, 68.92; H, 3.70; N, 1.11; P, 1.68.
(11) Kryatov, S. V.; Rybak-Akimova, E. V. Chem. ReV. 2005, 105, 2175-
2226 and references therein.
(12) Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996, 29, 190-196.
(13) Que, L., Jr. J. Chem. Soc., Dalton Trans. 1997, 3933-3940.
(14) Du Bois, J.; Mizoguchi, T. J.; Lippard, S. J. Coord. Chem. ReV. 2000,
200-202, 443-485.
[Fe2(µ-O2CArTol)3(O2CArTol)(2-MeSpy)] (4). In a CH2Cl2 (4
mL) solution, [Fe2(µ-O2CArTol)2(O2CArTol)2(THF)2] (114 mg, 0.782
mmol) was allowed to react with 2-MeSpy (21.6 mg, 0.173 mmol)
for 15 min. Removal of the solvent under reduced pressure left a
residue that was extracted into 2 mL of dichloroethane. Diffusion
of pentane vapor into the bright orange-yellow solution yielded
yellow block crystals (0.106 mg, 94%) of 4 suitable for X-ray
crystallography. FT-IR (KBr, cm-1): 3050 (w), 3023 (w), 2918
(15) Lippard, S. J. Chem. Br. 1986, 22, 221-228.
(16) Lippard, S. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 344-361.
(17) Lee, D.; Sorace, L.; Caneschi, A.; Lippard, S. J. Inorg. Chem. 2001,
40, 6774-6781.
(18) Mandal, S. K.; Young, V. G., Jr.; Que, L., Jr. Inorg. Chem. 2000, 39,
1831-1833.
(19) Herold, S.; Lippard, S. J. J. Am. Chem. Soc. 1997, 119, 145-156.
(20) Goldberg, D. P.; Telser, J.; Bastos, C. M.; Lippard, S. J. Inorg. Chem.
1995, 34, 3011-3024.
(27) (a) Carson, E. C.; Lippard, S. J. J. Inorg. Biochem. In press. (b) Carson,
E. C.; Lippard, S. J. Inorg. Chem. 2006, 2, 828-836.
(28) Carson, E. C.; Lippard, S. J. J. Am. Chem. Soc. 2004, 126, 3412-
3413.
(21) Lee, D.; Lippard, S. J. Inorg. Chem. 2002, 41, 2704-2719.
(22) Tolman, W. B.; Que, L., Jr. J. Chem. Soc., Dalton Trans. 2002, 653-
660.
(29) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(30) Furukawa, N.; Ogawa, S.; Matsumura, K.; Fujihara, H. J. Org. Chem.
1991, 56, 6341-6348.
(23) Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987-1012.
(24) Lee, D.; Lippard, S. J. J. Am. Chem. Soc. 2001, 123, 4611-4612.
(25) Lee, D.; Lippard, S. J. Inorg. Chem. 2002, 41, 827-837.
(26) Yoon, S.; Lippard, S. J. Inorg. Chem. 2003, 42, 8606-8608.
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838 Inorganic Chemistry, Vol. 45, No. 2, 2006

Carboxylate-Rich Diiron(II) Complexes
(w), 2860 (w), 1601 (s), 1576 (m), 1559 (m), 1513 (s), 1453 (s),
1406 (s), 1382 (s), 1304 (w), 1283 (w), 1185 (w), 1149 (w), 1109
(w), 1071 (w), 1019 (m), 970 (w), 832 (w), 817 (s), 799 (s), 765
(s), 737 (m), 702 (s), 641 (m), 608 (w), 583 (m), 557 (w), 523 (m),
465 (m), 410 (w). Anal. Calcd for C90H75NO8SFe2‚0.5C2H4Cl2: C,
73.27; H, 5.20; N, 0.94. Found: C, 73.29; H, 5.45; N, 1.12.
[Fe2(µ-O2CArTol)2(O2CArTol)2(2-MeS(O)py)2] (5). A bright red-
orange solution was produced upon the addition of 2-MeS(O)py
(28.2 mg, 0.200 mmol) to [Fe2(µ-O2CArTol)2(O2CArTol)2(THF)2]
(98.9 mg, 0.0677 mmol) in 6 mL of CH2Cl2, and a red powder
(91.8 mg, 85%) was isolated after diffusion of pentane into the
reaction mixture. Red block crystals of 5 suitable for X-ray
diffraction studies were obtained by diffusing pentane vapor into a
chlorobenzene solution of 5. FT-IR (KBr, cm-1): 3053 (w), 3022
(w), 2917 (w), 2862 (w), 1588 (s), 1562 (s), 1540 (s), 1514 (s),
1454 (s), 1410 (s), 1376 (s), 1304 (w), 1263 (w), 1186 (w), 1147
(w), 1109 (w), 1088 (w), 1020 (m), 1008 (s), 967, 818 (s), 800 (s),
785 (s), 765 (s), 737 (m), 711 (m), 703 (m), 637 (m), 608 (w), 584
(m), 542 (w), 520 (m), 450 (w), 411 (w). Anal. Calcd for C96H82N2-
Fe2O10S2: C, 72.09; H, 5.17; N, 1.75. Found: C, 71.71; H, 4.85;
N, 1.81.
[Fe(O2CArTol)2(2-HSpy)2] (6a). A solution of [Fe2(µ-O2C-
ArTol)2(O2CArTol)2(THF)2] (174 mg, 0.119 mmol) in CH2Cl2 (4 mL)
was combined with 2-mercaptopyridine (2-HSpy) (62.9 mg, 0.566
mmol) and stirred for 15 min. Vapor diffusion of pentane into this
orange solution produced orange blocks of 6a (199 mg, 95%)
suitable for X-ray crystallography. FT-IR (KBr, cm-1): 3195 (w),
3114 (w), 3053 (w), 2916 (w), 2862 (w), 1591 (s), 1539 (s), 1514
(s), 1446 (s), 1410 (s), 1375 (s), 1269 (w), 1130 (s), 1085 (w),
1020 (w), 995 (m), 916 (w), 847 (w), 818 (m), 801 (s), 786 (m),
755 (s), 726 (m), 707 (m), 619 (w), 584 (w), 539 (w), 521 (m),
486 (w), 442 (m). Anal. Calcd for C52H44N2FeO4S2: C, 70.90; H,
5.03; N, 3.18. Found: C, 71.81; H, 4.91; N, 3.32.
[Fe6(µ4-O)2(µ-OH)6(µ-O2CArTol)4Cl4(2-Ph2P(O)py)2] (7). An
orange 6.0 mM CH2Cl2 solution of 1 (95.3 mg, 0.0603 mmol) was
bubbled with O2 for 10 min and stirred for an additional 50 min
under an atmosphere of dioxygen. The solvent was removed in
vacuo, and the residue was extracted into 1.5 mL of CH2Cl2. Vapor
diffusion of pentane into the solution yielded orange crystalline
clusters of 7, one of which was used for X-ray diffraction studies.
Although the structure of 7 could be determined, bulk samples could
not be obtained.
[Fe2(µ-OH)2(µ-O2CAr4-FPh)(O2CAr4-FPh)3(OH2)(2-Ph2P(O)-
py)] (8). A bright yellow 5.80 mM toluene solution of 3 (190 mg,
0.117 mmol) was bubbled with dry dioxygen for 10 min and stirred
for an additional 50 min under an atmosphere of O2. The solvent
was then reduced under vacuum leaving a residue that was extracted
into 1.5 mL of CH2Cl2. Bright yellow block crystals of 8 (49.3
mg, 25%) suitable for X-ray diffraction studies were isolated by
diffusing pentane into the solution. FT-IR (KBr, cm-1): 3572 (m),
3432 (br), 3057 (w), 1735 (w), 1604 (s), 1511 (s), 1455(s), 1407
(m), 1345 (s), 1222 (s), 1159 (s), 1136 (m), 1095 (w), 1009 (w),
839 (s), 808 (s), 791 (s), 771 (m), 736 (w), 698 (s), 555 (s), 536
(s), 460 (w), 414 (w). Anal. Calcd for C93H62NFe2O12F8P: C, 66.48;
H, 3.72; N, 0.83; P, 1.84. Found: C, 66.23; H, 3.69; N, 0.76; P,
1.58.
to an external standard, H3PO4, for 31P experiments. All spectra
were recorded at ambient probe temperature, 293 K.
X-ray Crystallographic Studies. Intensity data were collected
on a Bruker (formerly Siemens) APEX CCD diffractometer with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å),
controlled by a Pentium-based PC running the SMART software
package.32 Single crystals were mounted on the tips of glass fibers,
coated with paratone-N oil, and cooled to -100 °C under a stream
of N2 maintained by a KRYO-FLEX low-temperature apparatus.
Data collection and reduction protocols are described elsewhere.33
The structures were solved by direct methods and refined on F2 by
using the SHELXTL-97 software34 incorporated in the SHELXTL
software package.35 Empirical absorption corrections were applied
with SADABS,36 part of the SHELXTL program package, and the
structures were checked for higher symmetry using PLATON.37
All non-hydrogen atoms were located, and their positions were
refined with anisotropic thermal parameters by least-squares cycles
and Fourier syntheses. In general, hydrogen atoms were assigned
to idealized positions and given thermal parameters equivalent to
either 1.5 (methyl hydrogen atoms) or 1.2 (all other hydrogen atoms)
times the thermal parameter of the carbon atom to which they were
attached. The structure of 3 has one dichloroethane molecule in
the lattice in which one of the chlorine atoms is disordered over
two positions and was refined with a 70/30 occupancy. Four carbon
atoms in one of the p-fluorophenyl rings were disordered over two
positions and were refined at 50% occupancy. In the structure of
4, one of the -O2CArTol ligands was disordered and was modeled
over two positions with 50% occupancy. A pentane and 1.5
dichloroethane molecules are present in the unit cell of 4. The
pentane molecule is severely disordered, and each carbon atom was
modeled over three positions using identical anisotropic displace-
ment parameters. The half dichloroethane molecule lies on a center
of symmetry; the chlorine atom is disordered and was refined with
the atoms distributed equally over two positions. Compound 5 has
a chlorobenzene molecule in the crystal lattice. The structure of 7
has four CH2Cl2 molecules in the lattice. The structure of 8 has
three CH2Cl2 molecules in the lattice. The structures of 3-8 are
shown in Figures 1-3 and S1-S6 (Supporting Information). Data
collection and experimental details are summarized in Table S1;
those for 1 and 2 were reported previously.28 Selected bond lengths
and angles for 1-8 are provided in Tables 1, 2, and S2-S4.
Mo1ssbauer Spectroscopy. Mo¨ssbauer spectra were recorded on
an MSI spectrometer (WEB Research Co.) with a 57Co source in a
Rh matrix maintained at room temperature in the MIT DCIF. Solid
samples of 6 and 8 were prepared by suspending ∼0.024 mmol of
the yellow powdered material in Apeizon N grease and coating
the mixture on the lid of a nylon sample holder. Data were collected
at 4.2 K, and the isomer shift (δ) values are reported with respect
to the natural iron foil that was used for velocity calibration at room
temperature. The spectra were fit to Lorentzian lines using the
WMOSS plot and fit program.38
Oxidation Product Analyses. Oxidation reactions were per-
formed by exposing CH2Cl2 solutions of the diiron(II) complex to
(32) SMART, version 5.626; Bruker AXS: Madison, WI, 2000.
(33) Kuzelka, J.; Mukhopadhyay, S.; Spingler, B.; Lippard, S. J. Inorg.
Chem. 2003, 42, 6447-6457.
(34) Sheldrick, G. M. SHELXTL97-2: Program for Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(35) SHELXTL, version 5.10; Bruker AXS: Madison, WI, 1998.
(36) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(37) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 1998.
Physical Measurements. FT-IR spectra were recorded on a
Thermo Nicolet Avatar 360 spectrometer with OMNIC software.
1H NMR data and 31P NMR spectra were recorded on Varian 300
and Varian 500 spectrometers, respectively; both spectrometers are
housed in the Massachusetts Institute of Technology Department
of Chemistry Instrument Facility (MIT DCIF). Chemical shifts were
referenced to the residual solvent peaks for proton experiments and
(38) Kent, T. A. WMOSS, version 2.5; WEB Research Co.: Minneapolis,
MN, 1998.
Inorganic Chemistry, Vol. 45, No. 2, 2006 839

Carson and Lippard
Figure 1. ORTEP diagrams of [Fe2(µ-O2CAr4-FPh)3(O2CAr4-FPh)(2-Ph2Ppy)] (3), [Fe2(µ-O2CArTol)3(O2CArTol)(2-MeSpy)] (4), [Fe2(µ-O2CArTol)2(O2CArTol)2-
(2-MeS(O)py)2] (5), and [Fe(O2CArTol)2(2-HSpy)2] (6a) showing 50% probability thermal ellipsoids for all non-hydrogen atoms. The aromatic rings of
carboxylate ligands are omitted for clarity.
Figure 2. ORTEP drawing of [Fe6(µ4-O)2(µ-OH)6(µ-O2CArTol)4Cl4-
(2-Ph2P(O)py)2] (7) illustrating 50% probability thermal ellipsoids for all
non-hydrogen atoms. The aromatic rings of the -O2CArTol ligands are
Figure 3. ORTEP drawing of [Fe2(µ-OH)2(µ-O2CAr4-FPh)(O2CAr4-FPh)3-
omitted for clarity.
(OH2)(2-Ph2P(O)py)] (8) illustrating 50% probability thermal ellipsoids for
all non-hydrogen atoms. The aromatic rings of the -O2CAr4-FPh ligands
are omitted for clarity.
dioxygen at room temperature over a period of time and, in the
case of 1 and 3, also in the presence of added 2-Ph2Ppy. During
workup, Chelex was added to the reaction solutions to remove the
iron salts. The resulting slurry was stirred for 2 h, after which the
Chelex was removed by filtration. The resin was washed with
CH2Cl2 twice more, and the filtrates were combined. The solvent
was removed in vacuo to give the N-donor species for analysis.
The products were identified by NMR spectroscopy and GC-MS
by comparing their NMR spectra, retention times, and mass spectral
patterns to those of authentic samples. The amount of oxidized
ligand obtained was quantitated by 31P NMR spectroscopy, using
triphenylphosphine oxide as an internal standard for 1 and 3, and
by GC-MS, using 1,2-dichlorobenzene as an internal standard for
2, 4, and 5. All samples were prepared in an anaerobic glovebox
prior to bubbling with dried dioxygen. Control experiments
established that, in the absence of the diiron(II) complexes, neither
dioxygen saturated CH2Cl2 nor the workup procedure induces
significant ligand oxidation.
GC-MS Analyses. Analyses were carried out on a Hewlett-
Packard HP-5890 gas chromatograph connected to a HP-5971 mass
analyzer. An Alltech Econo-cap EC-1 capillary column of dimen-
sions 30 m × 0.53 mm × 1.2 µm was employed with the following
840 Inorganic Chemistry, Vol. 45, No. 2, 2006

Carboxylate-Rich Diiron(II) Complexes
Table 1. Selected Interatomic Distances (Å) and Angles (deg) for
[Fe2(µ-O2CArTol)3(O2CArTol)(2-Ph2Ppy)] (1),
Table 2. Selected Interbond Lengths and Angles for 8a
bond length (Å)
2.9725(11)
interbond angle (deg)
[Fe2(µ-O2CArTol)3(O2CArTol)(2-PhSpy)] (2),
[Fe2(µ-O2CAr4-FPh)3(O2CAr4-FPh)(2-Ph2Ppy)] (3), and
[Fe2(µ-O2CArTol)3(O2CArTol)(2-MeSpy)] (4)a
Fe1‚‚‚Fe2
Fe1-O2-Fe2
Fe2-O3-Fe1
O1-Fe1-N1
O2-Fe1-N1
O3-Fe1-N1
O5-Fe1-N1
O7-Fe1-N1
O1-Fe1-O2
O1-Fe1-O3
O1-Fe1-O5
O1-Fe1-O7
O2-Fe1-O3
O2-Fe1-O5
O2-Fe1-O7
O3-Fe1-O5
O3-Fe1-O7
O5-Fe1-O7
O2-Fe2-O3
O2-Fe2-O4
O2-Fe2-O6
O2-Fe2-O9
O2-Fe2-O11
O3-Fe2-O4
O3-Fe2-O6
O3-Fe2-O9
O3-Fe2-O11
O4-Fe2-O6
O4-Fe2-O9
O4-Fe2-O11
O6-Fe2-O9
O6-Fe2-O11
O9-Fe2-O11
98.10(19)
97.35(18)
80.49(15)
84.76(17)
92.34(17)
172.91(15)
92.17(15)
97.51(16)
172.11(17)
93.26(14)
89.81(14)
78.40(17)
92.81(17)
171.47(17)
93.68(16)
93.80(16)
91.14(14)
77.85(17)
89.52(19)
91.58(16)
91.96(16)
174.61(17)
88.81(17)
93.02(15)
168.94(16)
97.06(15)
178.02(16)
86.77(17)
88.61(17)
91.55(14)
90.43(14)
92.97(14)
Fe1-N1
Fe1-O1
Fe1-O2
Fe1-O3
Fe1-O5
Fe1-O7
Fe2-O2
Fe2-O3
Fe2-O4
Fe2-O6
Fe2-O9
Fe2-O11
2.210(4)
2.023(3)
1.954(4)
1.981(4)
2.003(3)
1.947(3)
1.981(4)
1.977(4)
2.097(4)
2.017(3)
1.974(3)
1.976(3)
1
2
3
4
Fe1‚‚‚Fe2
Fe2‚‚‚X
Fe1-O1
Fe1-O2
Fe1-O3
Fe1-O5
Fe1-O7
Fe2-N1
Fe2-O4
Fe2-O6
Fe2-O8
3.3094(6)
2.8322(7)
3.2712(8)
3.0897(9)
3.4534(8)
2.9551(13)
2.297(3)
2.068(3)
2.000(3)
2.054(3)
2.017(3)
2.127(3)
2.022(3)
1.985(2)
1.998(3)
3.2496(7)
2.9091(10)
2.256(2)
2.059(2)
2.055(2)
2.026(2)
2.046(14)
2.077(3)
1.977(2)
2.034(2)
1.982(12)
2.2162(13)
2.0826(13)
2.0512(13)
2.0342(12)
2.0602(12)
2.1314(16)
1.9892(12)
2.0570(12)
1.9981(13)
2.2013(16)
2.0869(15)
2.0610(16)
2.0266(15)
2.0484(15)
2.1024(18)
1.9767(16)
2.0155(15)
1.9786(15)
O2‚‚‚O10
O3‚‚‚O8
O4‚‚‚O12
2.708(6)
2.803(6)
2.537(6)
O1-Fe1-O2
O1-Fe1-O3
O1-Fe1-O5
O1-Fe1-O7
O2-Fe1-O3
O2-Fe1-O5
O2-Fe1-O7
O3-Fe1-O5
O3-Fe1-O7
O5-Fe1-O7
O4-Fe2-N1
O6-Fe2-N1
O8-Fe2-N1
O4-Fe2-O6
O4-Fe2-O8
O6-Fe2-O8
61.35(5)
61.19(6)
88.64(6)
109.45(6)
92.77(6)
109.83(7)
169.04(6)
114.07(6)
89.61(6)
130.35(6)
95.59(6)
106.40(7)
97.36(7)
99.52(7)
104.73(6)
138.73(7)
103.10(6)
60.07(9)
86.96(10)
166.37(10)
91.98(10)
104.65(11)
106.30(10)
118.47(11)
97.32(11)
129.17(12)
95.25(11)
88.53(12)
122.81(12)
120.56(12)
103.10(11)
122.18(11)
99.91(11)
60.8(15)
87.4(16)
168.1(15)
91.7(16)
109.4(17)
108.4(16)
111.3(18)
92.2(16)
132.5(17)
97.3(16)
110(2)
95.8(18)
100(2)
102.1(17)
102.4(18)
102.4(18)
91.81(5)
167.13(5)
87.81(5)
111.56(5)
107.01(5)
110.11(6)
98.07(5)
131.95(5)
91.65(5)
103.14(6)
91.33(5)
a Numbers in parentheses are estimated standard deviations of the last
significant figure. Atoms are labeled as indicated Figures 3 and S6.
111.28(6)
100.35(5)
139.10(5)
100.29(5)
helium EPR 900 cryostat. X-band EPR samples were prepared by
making a 0.513 mM solution of 6a in CH2Cl2. Aliquots of ∼300
µL were transferred to the EPR tubes in the drybox; the tubes were
septum-sealed. One of the samples was frozen at 77 K. Another
was cooled to 4 °C under N2, treated with O2 for 45 s, and then
frozen at 77 K.
a Numbers in parentheses are estimated standard deviations of the last
significant figure. Atoms are labeled as indicated Figures 1, S2, and S3.
Resonance Raman Spectroscopy. Solutions of 6a in a J-Young
NMR tube were oxygenated with either 16O2 or 18O2 at -78 °C
and subsequently warmed to room temperature for the measurement.
The sample was excited at 647.1 nm using a Coherent Innova 90C
Kr+ ion laser. Plasma lines for the laser were rejected using a holo-
graphic band-pass (Kaiser Optical) to obtain a clean background.
Approximately 16 mW were focused to a ∼100 µm spot. The
resulting Raman light was filtered with a holographic notch filter
(Kaiser Optical Systems) to attenuate the Rayleigh scattered light
and was focused into a 150 µm slit of a single-stage spectrograph
(Acton Spectro Pro SP300i). The spectrograph has a 300 mm focal
length and is equipped with a 1200 grooves/nm grating giving a
linear dispersion of 2.7 nm/mm. The spectral resolution is ap-
proximately 5.8 cm-1. Spectra were recorded on a thermoelectrically
cooled back-illuminated CCD. The cooling of the CCD to -70 °C
significantly reduces the dark current (<0.1 e/p/s).
Prior to each measurement, the Raman system was initially
optimized using frozen CH2Cl2. The exposure time was adjusted
to yield a signal with ∼ 40,000 counts per accumulation, and the
total number of exposures taken was adjusted to yield a 5 min
gathering. Methylene chloride bands at 1156, 897, 704, and 286
cm-1 were used as a wavelength calibration standard. The sample
concentrations were between 2 and 4 mM, and each measurement
was made on more than one freshly prepared sample and in triplicate
to ensure the authenticity of the results. Data were processed by
program to effect all separations: initial temperature ) 100 °C,
initial time ) 4 min, temperature ramp ) 100-210 °C at 20 °C/
min, and final time ) 4.5 min. Quantitations were made by com-
parison of the total ion count of 2-PhSpy, 2-PhS(O)py, 2-MeSpy,
2-MeS(O)py, and 2-MeS(O)2py with that of the 1,2-dichlorobenzene
internal standard. Calibration plots for the detector response were
prepared for authentic samples of 2-PhSpy, 2-PhS(O)py, 2-MeSpy,
2-MeS(O)py, 2-MeS(O)2py, and the 1,2-dichlorobenzene standard
by using stock solutions of known concentrations.
Oxygenation of 6a. In a typical reaction, compound 6a was
dissolved in CH2Cl2 to ∼1 mM and loaded into a vessel fitted with
a rubber septum. The solution was cooled to -10 °C in an ice/
acetone bath. Dioxygen was gently bubbled into the solution,
resulting in a color change from pale orange to dark blue that
signaled formation of 6b.
UV-vis Spectroscopy. UV-vis spectra were recorded on a
Hewlett-Packard 8453 diode array spectrophotometer. Low-tempera-
ture UV-vis experiments were performed with a custom-made
quartz cuvette, 1 cm path length, fused into a vacuum-jacketed dewar.
A solution of 6a, 1.1 mM in CH2Cl2 (6 mL), under N2 was cooled
to -10 °C. Dry O2 was gently purged through the solution for 30
s, and the UV-vis spectra were recorded at various time intervals.
EPR Spectroscopy. EPR spectra were recorded on a Bruker
Model 300 ESP X-band spectrometer operating at 9.42 GHz.
Temperatures were maintained with an Oxford Instruments liquid
Inorganic Chemistry, Vol. 45, No. 2, 2006 841

Carson and Lippard
Scheme 1
Scheme 2
using WinSpec 3.2.1 (Princeton Instruments, Inc.), and the resultant
ASCII files were further manipulated using Microsoft Excel.
Results
Synthesis and Structural Characterization of
Diiron(II) Complexes with 2-Diphenylphosphinopyridine
as the N-donor Ligand: [Fe2(µ-O2CArTol)3(O2CArTol)(2-
Ph2Ppy)] (1) and [Fe2(µ-O2CAr4-FPh)3(O2CAr4-FPh)(2-
Ph2Ppy)] (3). Compounds 1 and 3 were prepared by
displacement of THF from [Fe2(µ-O2CArR)2(O2CArR)2-
(THF)2] with 2-Ph2Ppy, Scheme 1, where R ) Tol and
4-FPh, respectively. The structure of 1 was previously
reported, but for comparison purposes selected bond lengths
and angles are provided in Table 1.28 Figures 1 and S1 show
the structure of 3, and Table 1 lists selected bond lengths
and angles. The solid-state structures of 1 and 3 reveal triply
carboxylate-bridged diiron(II) centers with Fe‚‚‚Fe separa-
tions of 3.3094(6) and 3.4517(9) Å, respectively. A fourth
chelating carboxylate is bound to one of the iron atoms to
complete its coordination sphere. On the other metal center,
the 2-Ph2Ppy coordinates through the pyridine nitrogen atom.
The Fe‚‚‚P distance is longer in 3, 2.9551(13) Å, than in 1,
2.8322(7) Å. The IR spectrum of 3, displayed in Figure S7,
exhibits features associated with aromatic and carboxylate
groups. In addition, the C-F stretches originating from the
-O2CAr4-FPh ligands are visible at 1221 cm-1.39
Synthesis and Structural Characterization of
Diiron(II) Complexes with Pendant Sulfur-Containing
Substrates [Fe2(µ-O2CArTol)3(O2CArTol)(2-PhSpy)] (2),
[Fe2(µ-O2CArTol)3(O2CArTol)(2-MeSpy)] (4), and [Fe2(µ-
O2CArTol)2(O2CArTol)2(2-MeS(O)py)2] (5). Compounds 2
and 4 differ only in the sulfide substituent. The combination
of 2-PhSpy or 2-MeSpy with [Fe2(µ-O2CArTol)2(O2CArTol)2-
(THF)2] led to the isolation of [Fe2(µ-O2CArTol)3(O2CArTol)-
(2-PhSpy)] (2) or [Fe2(µ-O2CArTol)3(O2CArTol)(2-MeSpy)]
(4), respectively (Scheme 2). The structure of 2 was
previously reported, but for comparison purposes selected
bond lengths and angles are provided in Table 1.28 The
structure of 4 is shown in Figures 1 and S2, and selected
bond lengths and angles are provided in Table 1. Both 2
and 4 contain triply carboxylate-bridged diiron(II) centers
with Fe‚‚‚Fe separations of 3.2712(8) and 3.2496(7) Å,
respectively. The Fe‚‚‚S distance is longer in 2, 3.0897(9)
Å, compared to that in 4, 2.9091(10) Å. A fourth bidentate
carboxylate ligand completes the metal coordination sphere
of the iron atom not bound to a pyridine moiety.
Bright red crystals of 5 were obtained from the reaction
of [Fe2(µ-O2CArTol)2(O2CArTol)2(THF)2] with 2-MeS(O)py,
Scheme 2. Figures 1 and S3 show the structure of 5, and
selected bond lengths and angles are given in Table S2. The
(39) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of
Organic Compounds, 6th ed.; John Wiley & Sons: New York, 1998.
842 Inorganic Chemistry, Vol. 45, No. 2, 2006

Carboxylate-Rich Diiron(II) Complexes
compound is a doubly carboxylate-bridged diiron(II) species
with an Fe‚‚‚Fe separation of 4.585(3) Å. The coordination
sphere of each iron atom is completed by an additional
bidentate carboxylate ligand and an N-donor 2-MeS(O)py
ligand that is bound through both the N and O atoms,
rendering each iron atom 6-coordinate. A five-membered
ring is formed involving Fe1-N1-C1-S1-O1, with an
Fe1-O1 distance of 2.234(5) Å. A comparison of the IR
spectra of 4 and 5 (Figure S8) shows the vibrations of two
compounds to be nearly identical, with the exception of the
SdO stretch at 1008 cm-1 in 5.39
Isolation of Mononuclear [Fe(O2CArTol)2(2-HSpy)2]
(6a). The combination of one equiv of [Fe2(µ-O2CArTol)2(O2C-
ArTol)2(THF)2] with 4 equiv of 2-HSpy, as indicated in
Scheme 2, resulted in the formation of the mononuclear iron-
(II) complex, [Fe(O2CArTol)2(2-HSpy)2] (6a). Its structure is
shown in Figures 1 and S4, and selected bond lengths and
angles are provided in Table S3. This pseudo-tetrahedral
iron(II) complex is coordinated by two monodentate car-
boxylates and two thioamides bound to the metal center
through the sulfur atoms. The largest of the metal-centered
bond angles is the O-Fe-O angle of 133.35(9)° and the
smallest is the S-Fe-S angle of 92.04(2)°. The other four
angles are closer to the ideal tetrahedral value of 109.5°,
averaging 105.96(6)°. There is hydrogen bonding between
the noncoordinated carboxylate oxygen atoms and the
thioamide protons at distances of 2.767(14) Å for O1‚‚‚N1
and 2.709(18) Å for O3‚‚‚N2. The characteristic S-H
stretching absorption in the range of ∼2600-2550 cm-1 is
absent from the solid-state IR spectrum of 2, depicted in
Figure S9, but the N-H modes are observed at 3196 cm-1.39
This result is consistent with the X-ray structure.
Figure 4. UV-vis spectra of [Fe(O2CArTol)2(2-HSpy)2] (1a) (- - -) and the
intermediate 1b (;) in CH2Cl2 at -10 °C.
Mo1ssbauer Spectroscopic Properties of 3. The Mo¨ss-
bauer spectrum, measured at 4.2 K, of a solid sample of 3 is
pictured in Figure S10. Although the two iron(II) sites differ
in coordination environment, there is no evidence for two
overlapping signals. The single sharp quadrupole doublet (Γ
) 0.37-0.39 mm s-1) reveals that the iron centers are
indistinguishable by Mo¨ssbauer spectroscopy. The quadru-
pole splitting (∆EQ ) 2.66(2) mm s-1) and isomer shift (δ
) 1.22(2) mm s-1) parameters are consistent with those of
high-spin iron(II) complexes having oxygen-rich coordination
environments, indicating that 3 has a high-spin S ) 2 ground
state.23,40
Reactions of 6a with Dioxygen. When a CH2Cl2 or
toluene solution of 6a reacts with dioxygen at -10 °C, a
deep blue intermediate (6b) develops over a period of 10
min. Absorptions in the UV-vis spectrum, presented in
Figure 4, include maxima at 378 (ꢀ ) 3000 M-1 cm-1), 627
(ꢀ ) 800 M-1 cm-1), and >1100 nm (ꢀ ≈ 500 M-1 cm-1).
Both the ferrous starting material, 6a, and the blue intermedi-
ate, 6b, are EPR silent.
Figure 5. Resonance Raman spectra of a CH2Cl2 solution of 1b derived
from the oxygenation of [Fe(O2CArTol)2(2-Hspy)2] (6a) with 16O2 (top
spectrum) and 18O2 (bottom spectrum). The asterisk indicates a solvent band.
Raman spectra of (µ-peroxo)diiron(III) moieties result from
O-O (800-900 cm-1) and Fe-O (450-550 cm-1) stretch-
ing vibrations.23,41,42 Raman spectra were obtained for 6b in
CH2Cl2 at room temperature. Upon excitation at 647.1 nm,
two Raman active bands appear at 843 and 504 cm-1. These
features are absent in the Raman spectra of both 6a and the
decomposition product of 6b. When the blue intermediate
is prepared by reacting 6a with 18O-labeled dioxygen (6b*),
the peak positions shift to 811 and 494 cm-1. A comparison
of the two spectra is shown in Figure 5.
Tethered Ligand Oxidation in 2, 4, and 5. Oxidation of
the substrates appended to the N-donor ligand was investi-
gated by product analysis following the introduction of O2
into solutions of 2, 4, and 5. A summary of the conditions
and the amount of oxidation product recovered is provided
in Table 3. A pale yellow CH2Cl2 solution of 4 reacts with
Resonance Raman spectroscopy revealed that 6b contains
bound oxygen. Characteristic resonances observed in the
(41) Chavez, F. A.; Ho, R. Y. N.; Pink, M.; Young, V. G., Jr.; Kryatov, S.
V.; Rybak-Akimova, E. V.; Andres, H.; Mu¨nck, E.; Que, L., Jr.;
Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41, 149-152.
(42) Costas, M.; Cady, C. W.; Kryatov, S. V.; Ray, M.; Ryan, M. J.; Rybak-
Akimova, E. V.; Que, L., Jr. Inorg. Chem. 2003, 42, 7519-7530.
(40) Mu¨nck, E. In Physical Methods in Bioinorganic Chemistry: Spectros-
copy and Magnetism; Que, L., Jr., Ed.; University Science Books:
Sausalito, CA, 2000; pp 287-319.
Inorganic Chemistry, Vol. 45, No. 2, 2006 843

Carson and Lippard
Table 3. Summary of the Conditions and Amount of Oxidation
Product Isolated for the Reaction of Compounds 2, 4, and 5 with
Dioxygen in CH2Cl2
Table 4. Summary of Conditions and Amount of Oxidation Product
Isolated for the Reaction of 1 and 3 with Additional Equivalents of
Phosphine and Dioxygen
[Fe2]
(mM)
reaction time
(h)
oxidized ligand recovereda
(%)
[Fe2]
(mM)
2-Ph2Ppy
added (equiv)
reaction
time (h)
2-Ph2P(O)py
compound
compound
recovereda (equiv)
2
4
5
8.6
8.2
5.1
1.5
1.5
16
29
58
40
2.5
1.4
0.31
3.2
1
1
1
3
8.44
47.0
51.5
8.55
1
13
17
17
3.98
13.4
17.0
2.95
a Based on [Fe2].
[Fe2]
(mM)
PPh3 added
(equiv)
reaction
time (h)
(O)PPh3 recovered
(equiv)
compound
dioxygen at room temperature and immediately turns golden
brown. Workup of the mixture and analysis by GC-MS
revealed 58% conversion to 2-pyridylmethylsulfoxide (2-
MeS(O)py). The unmodified sulfido ligand was quantitatively
recovered. The peaks were assigned by comparing the
retention times of the M+ ) 125 and 141 signals with those
of authentic samples of 2-methylthiopyridine and 2-methyl-
pyridylsulfoxide. No other products, such as the correspond-
ing pyridyl sulfones, were evident in the GC traces. The
analogous oxidation of 2, analyzed by GC-MS, converts only
29% of the sulfide to the sulfoxide.28
The bright red CH2Cl2 solution of 5 becomes dark red-
brown upon exposure to dioxygen. Analysis of the products
using GC-MS revealed 40% conversion of the 2-MeS(O)py
to the sulfone, based on 5. This assignment was confirmed
by comparison of the M+ ) 157 signal in the GC-MS with
an authentic sample of 2-MeS(O)2py. The remaining material
recovered was identified as unaltered pyridylsulfoxide ligand.
Stoichiometric and Catalytic Phosphine Oxidation of
the Tethered Phosphine Unit in 1 and 3. Oxidation of the
diphenylphosphino group appended to the pyridine ligand
was investigated by product analysis following the introduc-
tion of dioxygen into solutions of 1 and 3. Quantitative
formation of 2-Ph2P(O)py occurs within 30 min of the
exposure of 1 to dioxygen,28 and the same conversion occurs
in the corresponding reaction of 3 in 30 min, with quantitative
recovery of the pyridine ligand. Control experiments estab-
lished that, in the absence of 3, neither O2-saturated CH2Cl2
nor the workup process induced ligand oxidation over the
same time interval (∼1 h). 31P NMR spectroscopy with
Ph3PO as an internal standard was used to analyze the
products of the reaction and monitor the recovery of all
species.
Catalytic formation of 2-Ph2P(O)py ensues when a CH2Cl2
solution of 1 is supplied with additional equivalents of 2-Ph2-
Ppy. The results from several experiments show that 4 equiv
are converted in an hour and slightly more than 13 equiv
are converted over 12 h at a metal complex concentration
of 1.5 mM. A 5-fold dilution of the concentration of 1 leads
to 17 equiv being oxidized in 17 h. When a phosphine that
did not contain a pendant coordinating ligand, such as
triphenylphosphine, is added to the reaction mixture, the
extent of oxidation is >20-fold less than that from the
2-pyridyl-derivatized phosphine ligand under the same
conditions. Catalytic oxidation is not observed to the same
degree in MeCN or C6H6. In both cases, 2 equiv of 2-Ph2-
Ppy are converted to the phosphine oxide overnight. When
a CH2Cl2 solution of 3 is provided with more of the pyridine
ligand, catalytic phosphine oxidation similarly transpires. A
0.42
1
5.62
14
0.70
a Including 1 equiv of 2-Ph2Ppy from [Fe2].
14 h reaction resulted in conversion of 3 out of almost 9
equiv of 2-Ph2Ppy to the phosphine oxide. This result is
comparable to the 13 out of 50 equiv of 2-Ph2Ppy oxidized
under the same conditions by the corresponding O2CArTol
complex, 1. A summary of the reaction conditions used and
the products recovered is provided in Table 4.
-
Isolation and Structural Characterization of Iron(III)
Complexes [Fe6(µ4-O)2(µ-OH)6(µ-O2CArTol)4Cl4(2-Ph2P-
(O)py)2] (7) and [Fe2(µ-OH)2(µ-O2CAr4-FPh)(O2CAr4-FPh)3-
(OH2)(2-Ph2P(O)py)] (8). The hexairon(III) species 7 was
isolated as one of the final products from the reaction of 1
with dioxygen (Scheme 2). The structure is best described
by the diagrams in Figures 2 and S5, and a list of selected
interatomic distances and angles is included in Table S4.
The structure of 7 consists two (µ3-oxo)triiron(III) units that
are related by an inversion center and linked through two
O2-, two OH-, and two -O2CArTol bridging groups. Four
octahedral and two distorted trigonal bipyramidal iron(III)
sites result. A noteworthy feature in the structure of 1 is that
two octahedral and two trigonal bipyramidal iron centers are
coordinated by chloride ions. Their origin is thought to be a
result of solvent oxidation because no comparable material
could be obtained when toluene was used as a solvent for
this reaction. The two outside iron centers are bound to
2-Ph2P(O)py, the oxidized N-donor ligand, through the
oxygen atom of the phosphine oxide.
Pentane diffusion into the yellow-orange CH2Cl2 or toluene
solution generated upon oxygenation of 3 produced [Fe2(µ-
OH)2(µ-O2CAr4-FPh)(O2CAr4-FPh)3(OH2)(2-Ph2P(O)py)] (8)
(Scheme 1) in 25% yield. The structure of 8 is depicted in
Figures 3 and S6, and selected interatomic distances and
angles are listed in Table 2. Two distorted octahedral iron(III)
atoms are bridged by two hydroxides and one
carboxylate ligand. The largest deviations from the ideal 90°
interbond angles are 11.60(2)° and 12.15(2)° for Fe1 and
Fe2, respectively, and occur at the O-Fe-O angle of the
bent four-membered Fe2(µ-OH)2 ring. The Fe1-O2-Fe2
unit has Fe1-O2 and Fe2-O2 distances of 1.954(4) and
1.981(4) Å, respectively, and a 98.10(19)° bridging angle.
The Fe-O bond lengths in the Fe1-O3-Fe2 unit are slightly
longer at 1.981(4) and 1.977(4) Å for Fe1-O3 and Fe2-
O3, respectively, but the bridging angle is smaller, 97.35(18)°.
The longest Fe-O distances are those to the oxygen atoms
of the bridging carboxylate ligand, 2.003(3) and 2.017(3)
Å. The Fe‚‚‚Fe separation of 2.972(1) Å is similar to those
844 Inorganic Chemistry, Vol. 45, No. 2, 2006

Carboxylate-Rich Diiron(II) Complexes
indicating that the two ferric atoms are indistinguishable by
Mo¨ssbauer spectroscopy.
Bond Valence Sum Analysis of 7 and 8. The bond
valence sum (BVS) is an empirical quantity that has been
used to determine the oxidation state of metal ions in
coordination complexes on the basis of crystallographically
determined metal-ligand bond distances.48 Bond valences,
sij, between cation i and anion j, are calculated according to
eq 1, where r is the observed bond distance and ro and B are
empirically determined parameters. The summation of the
bond valences for a particular metal cation yields the
oxidation state, Vi. Bond valence sum analysis of both 7 and
8 supports the assignment of the six metal centers in 7 and
the two in 8 as iron(III). The calculation of the individual
bond valences as well as their summation is available in
Table S5.
Figure 6. Zero-field Mo¨ssbauer spectrum (experimental data (|), calculated
fit (-)) recorded at 4.2 K for [Fe2(µ-OH)2(µ-O2CAr4-FPh)(O2CAr4-FPh)3-
(2-Ph2P(O)py)(OH2)] (8) in the solid state. See text for derived Mo¨ssbauer
parameters.
reported for the {Fe2(µ-OH)2(µ-O2CR)}3+ core in the resting
state of MMOHox43,44 and two other synthetic analogues.21,24,45
In addition to the bridging ligands, one iron atom (Fe1) is
coordinated by a monodentate carboxylate group. The other
two coordination sites are occupied by O and N atoms from
the oxidized N-donor ligand, 2-Ph2P(O)py, generating a five-
(ro - r)
V ) S )
exp
(1)
∑ ∑
i
ij
(
)
B
j
j
Discussion
In addition to oxidizing methane to methanol, MMOH can
accept a variety of oxidizable substrates including meth-
anethiol.3 We therefore investigated 2-mercaptopyridine as
an ancillary ligand at the oxygen-rich core of the m-terphenyl
carboxylate diiron(II) complexes. MMOH also catalyzes the
conversion of sulfides to sulfoxides, amines to amine oxides,
and olefins to epoxides.3,6 The first of these transformations
inspired the selection of 2-phenylthiopyridine and 2-methyl-
thiopyridine as N-donor ligands. Although as yet unprec-
edented in the natural system, the oxidation of dimethyl
sulfoxide is thermodynamically more favorable than that of
dimethyl sulfide,49 so 2-pyridylmethysulfoxide was installed
at the {Fe2(µ-O2CArTol)2(O2CArTol)2} core. The oxidation of
triphenylphosphine is even more thermodynamically facile,
∆H° being -67 kcal/mol versus -33 kcal/mol for dimethyl
sulfide.49 Although not a known substrate of the native
enzyme, this potentially more reactive species was provided
as a 2-diphenylphosphinopyridine ancillary ligand in the
carboxylate-rich diiron(II) centers to gain further insight into
the chemistry.
membered Fe1-N1-C1-P1-O1 ring. The other iron center
(Fe2) has two monodentate carboxylate ligands and a
coordinated water molecule to complete its coordination
sphere. Notwithstanding the bridging hydroxide groups, the
three monodentate carboxylate ligands provide the shortest
Fe-O distances, 1.947(3), 1.974(3), and 1.976(3) Å. The
Fe-O bond to the terminally bound water molecule is longer
than that to the O atom of 2-Ph2P(O)py, 2.099(4) versus
2.023(3) Å. The Fe-N distance of 2.210(4) Å is ∼0.1 Å
longer than that in the diferrous starting material, 3.
The three terminal carboxylate ligands are involved in
hydrogen bonding interactions with the bridging hydroxides,
O2‚‚‚O10, 2.712(5) Å, and O3‚‚‚O8, 2.801(6) Å, and the
coordinated water molecule, O4‚‚‚O12, 2.537(5) Å. The IR
spectrum of 8, depicted in Figure S7, shows the hydrogen-
bonded and non-hydrogen-bonded hydroxide stretches as a
broad band centered around 3400 cm-1 and a sharper peak
at 3572 cm-1, respectively.39
Mo1ssbauer Spectroscopic Properties of 8. The Mo¨ss-
bauer spectrum, recorded at 4.2 K, of a solid sample of 8 is
presented in Figure 6. A single sharp quadrupole doublet is
observed (δ ) 0.50(2) mm s-1, ∆EQ ) 0.83(2) mm s-1).
The isomer shift (δ) falls in the range typical of the high-
spin Fe(III) complexes.46,47 Although the two iron(III) sites
differ, having O6 and NO5 donor-atom sets, there is no
evidence for two overlapping doublets. The line widths (Γ
) 0.31-0.32 mm s-1) are close to natural line widths,
Despite the differing substituents on the pyridine and
carboxylate ligands, the iron atoms in 1-4 have nearly
identical coordination environments. It was initially thought
that the steric bulk provided by the diphenylphosphino and
phenylthio moieties at the ortho position of the pyridine ring
was responsible for only one pyridine ligand being accom-
modated at the diiron cores in 1-3. This also proved to be
the case in 4 where the heteroatom substituent is a less
sterically demanding methyl group. In all four structures,
the Fe-O bonds of the three bridging carboxylate ligands
are on average 0.05 Å shorter for the iron atom to which
the pyridine moiety is coordinated: 2.0191(12) versus
2.0485(13) Å in 1, 1.9903(15) versus 2.0453(16) Å in 2,
(43) Whittington, D. A.; Lippard, S. J. J. Am. Chem. Soc. 2001, 123, 827-
838.
(44) Elango, N.; Radhakrishnan, R.; Froland, W. A.; Wallar, B. J.; Earhart,
C. A.; Lipscomb, J. D.; Ohlendorf, D. H. Protein Sci. 1997, 6, 556-
568.
(45) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2004, 126, 2666-2667.
(46) Kurtz, D. M., Jr. Chem. ReV. 1990, 90, 585-606.
(47) Liu, T.; Lovell, T.; Han, W.-G.; Noodleman, L. Inorg. Chem. 2003,
42, 5244-5251.
(48) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244-247.
(49) Harlan, E. W.; Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1986,
108, 6992-7000.
Inorganic Chemistry, Vol. 45, No. 2, 2006 845

Carson and Lippard
2.002(3) versus 2.024(3) Å in 3, and 1.994(2) versus 2.044-
(2) Å in 4. The Fe-O bonds of the terminal carboxylate
ligand are the longest and asymmetric at ∼2.07 and ∼2.25
Å. Such a result reflects the greater electron-releasing
character of a pyridine N donor compared with a bidentate
carboxylate because the lower-coordinate iron atom would
be expected to have the shorter bond lengths, all other things
being equal.
When 2-MeS(O)py rather than 2-MeSpy was employed
as the N-donor ligand, a more open, doubly carboxylate-
bridged structure is adopted for the resulting diiron(II)
complex, 5. Two N donors are now accommodated, com-
pared to only one in the triply bridged complexes, 1-4. The
four Fe-O bond distances range from 1.961(5) to 2.267(5)
Å. The Fe‚‚‚Fe distance of 4.585(3) Å in 5 is >1 Å greater
than those observed in the triply bridged structures 1-4 of
∼3.3 Å and is quite long, reflecting the syn,anti binding mode
of the bridging carboxylates.50
Instead of the carboxylate-bridged dimetallic unit antici-
pated upon displacement of the THF molecules in [Fe2(µ-
O2CArTol)2(O2CArTol)2(THF)2] by 2-mercaptopyridine, a mono-
nuclear species was isolated. The Cambridge Structural
Database reports no other structures of four-coordinate iron
containing only sulfur and oxygen ligands; 6a appears to be
the first. The hydrogen bonding between the carboxylate
oxygen atoms and the thioamide protons most likely
stabilizes this unique mononuclear structure. The iron atom
is coordinated by a thioamido ligand, the tautomer of
2-mercaptopyridine (Scheme S1), in which the nitrogen atom
is protonated and a carbon-sulfur double bond is present.
The C1-S1 and C6-S2 distances of 1.716(3) and 1.717(3)
Å are characteristic of CdS double bonds,51 and the bond
lengths of the pyridine ring similarly reflect this assignment.
A single sharp quadrupole doublet occurs in the Mo¨ssbauer
spectrum of 3, the quadrupole splitting (∆EQ ) 2.66(2) mm
s-1) and isomer shift (δ ) 1.22(2) mm s-1) values of which
are consistent with those of high-spin diiron(II) complexes
in oxygen-rich coordination environments.23 The isomer shift
is among the lowest reported for five-coordinate, and slightly
higher than those observed for four-coordinate, iron(II) sites
(1.0-1.1 mm s-1).52 The quadrupole splitting and isomer
shift parameters obtained for 3 are slightly lower than those
of MMOHred, 3.01 and 1.3 mm s-1, respectively.52 This
disparity is attributed to complex 3 having fewer nitrogen
ligands than the MMOHred active site.
However, both the ferrous starting material, 6a, and the blue
intermediate, 6b, are EPR silent. The UV-vis spectrum of
the latter also resembles those of (µ-peroxo)diiron(III) units
in both synthetic analogue and biological systems.11,23,54-58
Although 6a is a mononuclear compound, oxygenation of
mononuclear iron(II) can result in diiron(II) complexes,
possibly via an {Fe2(O2)} intermediate.45 Theoretically, from
a simple harmonic oscillator calculation, the downshift upon
18O2-labeling (∆18O) should be -48 cm-1 for an O-O stretch
and -23 cm-1 for the Fe-O stretch. For 6b, the ∆18O values
are slightly less (-32 cm-1, -10 cm-1) for the O-O and
Fe-O vibrations, respectively, suggesting coupling with
other modes in the molecule or possibly an Fe-O-O-X
unit.
Oxidation of diphenyl sulfide is slightly more energetically
favorable than oxidation of dimethyl sulfide (∆H° ) -36
vs -33 kcal/mol),59,60 which is inconsistent with the greater
conversion of 2-MeSpy (58%) than that of 2-PhSpy (29%)
to the sulfoxide in 4 and 2, respectively. The higher activity
of 4, despite its carrying the less energetically favorable
substrate, may be a result of increased access to the O2
activated at the diiron center because of the smaller methyl
substituent compared to the phenyl group of 2-PhSpy in 2.
In a study involving ∆9-desaturase, which has a carboxy-
late-bridged diiron site similar to that in MMOH,43,61,62 the
reaction of 10-thiasterarate-ACP with the reconstituted
soluble stearoyl-ACP ∆9-desaturase complex produced the
10-sulfoxide as the sole product.63 The comparable reaction
with 9-thiasearoyl-ACP desaturase, however, gave the 9-sul-
foxide as ∼5% of the total products. It was concluded that
the relative reactivity of the 9- and 10-thia-substituted acyl-
ACPs is controlled by the proximity of two positions to the
O2-activated diiron center in the transition state. The reactiv-
ity of compound 4 supports this view. Exposure of 4 to
dioxygen leads only to the formation of the sulfoxide. No
C-H bond activation of the methyl substituent was observed.
Oxidation of an ethyl moiety tethered to pyridine in
[Fe2(µ-O2CArTol)2(O2CArTol)2(2-Etpy)2] produces only R-meth-
yl-2-pyridinemethanol, a result that also may be a conse-
quence of the proximity to the O2-activated diiron center.27b
Although the two iron atoms in 5 are coordinatively
saturated, the compound readily reacts at room temperature
following exposure to dioxygen. The observed ligand oxida-
tion may require carboxylate shifts or partial dissociation of
the sulfoxide ligand.21 The 40% conversion of the sulfoxide
Clues to the identity of the blue intermediate observed
upon oxygenation of 6a are provided by resonance Raman
and EPR spectroscopic measurements. The electronic ab-
sorption spectrum of 6b is similar to those of paramagnetic
FeIIFeIII cations formed by other {Fe2(µ-O2CArTol)4} systems,
which display intervalence charge-transfer transitions and
have a characteristic g ) 10 signal in their EPR spectra.53
(54) 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.
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Biochemistry 1998, 37, 14664-14671.
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121, 3876-3887.
(57) Lee, S.-K.; Lipscomb, J. D. Biochemistry 1999, 38, 4423-4432.
(58) 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.
(59) Mackle, H.; O’Hare, P. A. G. Tetrahedron 1963, 19, 961-971.
(60) Douglas, T. B. J. Am. Chem. Soc. 1946, 68, 1072-1076.
(61) Logan, D. T.; Su, X.-D.; Åberg, A.; Regnstro¨m, K.; Hajdu, J.; Eklund,
H.; Nordlund, P. Structure 1996, 4, 1053-1064.
(50) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15,
417-430.
(51) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton,
FL, 1979; Vol. 60.
(52) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 8386-8397.
(53) Lee, D.; DuBois, J. L.; Pierce, B.; Hedman, B.; Hodgson, K. O.;
Hendrich, M. P.; Lippard, S. J. Inorg. Chem. 2002, 41, 3172-3182.
(62) Rosenzweig, A. C.; Nordlund, P.; Takahara, P. M.; Frederick, C. A.;
Lippard, S. J. Chem. Biol. 1995, 2, 409-418.
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846 Inorganic Chemistry, Vol. 45, No. 2, 2006

Carboxylate-Rich Diiron(II) Complexes
to the corresponding sulfone is, to the best of our knowledge,
unprecedented in small molecule model compounds but is
not surprising from a thermodynamic perspective.11,23,64 The
∆H° for the oxidation of dimethyl sulfide to dimethyl
sulfoxide is -27 kcal/mol compared to -52 kcal/mol for
the oxidation of dimethyl sulfoxide to dimethyl sulfone.49
Despite the fact that compound 5 is more sterically hindered
than 4, once dioxygen can access the diiron center and
become activated, the ligand oxidation reaction will be more
favorable.
The catalytic ligand oxidation exhibited by 1 is difficult
to rationalize. Previously, we reported the catalytic oxidation
of PPh3 in the presence of dioxygen by the diiron(II) complex
[Fe2(µ-O2CArTol)2(Me3TACN)2(MeCN)](OTf)2, where Me3-
TACN ) 1,4,7-trimethyl-1,4,7-triazacyclononane.65 This
phenomenon was only observed when THF was used as the
solvent. Further studies revealed that the corresponding
diiron(III) complex, [Fe2(µ-O)(µ-O2CArTol)2(Me3TACN)2]-
(OTf)2, the only detectable iron-containing species during
the reaction, could itself promote the catalysis.66 Furthermore,
it was determined that phosphine oxidation is coupled to
catalytic oxidation of the THF solvent producing a C-C
bond cleavage product, which occurs until the phosphine is
consumed. For 1, although autoxidation has not been
eliminated, substitution of the added 2-Ph2Ppy with PPh3
dramatically decreased the reactivity, >20-fold, which sug-
gests that the metal center not only initiates the catalysis
but that substrate binding and intramolecular oxygenation
most likely occur. The reactivity of [Fe2(µ-O2CArTol)3(O2C-
ArTol)(2-Ph2Ppy)2] also differs from the previous catalytic
system because the reaction also occurs in MeCN and C6H6,
albeit with lower conversion, which we attribute to the
limited solubility of the starting diiron(II) complex.
A small difference in the remote carboxylate substituents,
-O2CArTol (1) versus -O2CAr4-FPh (3), leads to pronounced
differences in dioxygen reactivity. Previous work revealed
that carboxylate ligands with p-tolyl substituents form more
reactive diiron(II) centers than the p-fluorophenyl analogues
for both 2-benzyl- and 2-ethylpyridine derivatives.27b This
situation is also apparent for the present 2-diphenylphosphi-
nopyridine complexes because 1 catalytically oxidizes five
times the amount of phosphine as is oxidized by 3. The steric
properties of these two terphenyl carboxylate groups are
similar so the decrease in catalytic activity between the
-O2CArTol and -O2CAr4-FPh systems is attributed to elec-
tronic factors. The pKa values of HO2CAr4-FPh and
HO2CArTol are 6.14 and 6.50, respectively,67 but there are
four such ligands, which amplifies the effect.
{Fe2(µ-OH)2(µ-O2CR)}3+ core, a hexanuclear complex, 7,
is recovered from the reaction of 1 with O2. Most notably,
the structure of 7 contains coordinated chloride ions.
Decomposition of the solvent, CH2Cl2, could generate Cl-,
which may drive the formation of hexanuclear compound
7. An analogous hexanuclear complex with coordinated
chloride ligands was isolated following oxygenation of
[Fe2(µ-O2CArTol)4(4-CNpy)2], where 4-CNpy is 4-cyanopy-
ridine.52 Oxidation of CH2Cl2 also occurs in MMOH,3 the
product formed being unstable and decomposing to form-
aldehyde and HCl, so the present results may relate to the
enzyme system. Solvent oxidation by 1 would provide the
chloride ions observed in the final species isolated, 7. Our
observations of increased C-H activation with more electron
releasing ligands at the diiron(II) center suggests that the
MMOH active site may be similarly activated.
The synthesis of complexes with a {Fe2(µ-OH)2-
(µ-O2CR)}3+ core has been challenging because of the
tendency of iron(III) salts to form oligo- or polynuclear
clusters. The first such compound to be isolated, [Fe2(µ-OH)2-
(µ-O2CArTol)(O2CArTol)3(N-Bnen)(N,N-Bn2en)] (9), was gen-
erated by oxygenation of [Fe2(µ-O2CArTol)2(O2CArTol)2(N,N-
Bn2en)2].24,25 This approach mimics the chemistry occurring
at the enzyme active sites. A second route to the {Fe2(µ-
OH)2(µ-O2CR)}3+ core was discovered during oxygenation
of mononuclear [Fe(O2CArTol)2(Hdmpz)2], where Hdmpz is
dimethylpyrazole, which produced [Fe2(µ-OH)2(µ-O2CArTol)-
(O2CArTol)(OH2)(Hdmpz)2] (10).45 Representations of 9 and
10 are provided in Chart S1. In the present work, exposure
of 3 to dioxygen led to 8, which has the {Fe2(µ-OH)2-
(µ-O2CR)}3+ unit and is only the third structural mimic of
the MMOHox core structure. There was no evidence for
solvent oxidation in this reaction. The Fe‚‚‚Fe and
Fe-Ohydroxide distances of 2.972(1) and 1.954(4)-1.981(4)
Å, respectively, in 8 are similar to those of MMOHox, in
which the Fe‚‚‚Fe distance is 2.99 Å, and the Fe-Ohydroxide
distances are 1.7-2.2 Å.44 In the protein, two histidine and
three carboxylate terminal ligands, a bridging carboxylate,
and solvent-derived molecules complete the NO5 donor-
atom sets at each metal. A slightly different ligand combina-
tion is adopted by 8, which has an NO5 set for one iron atom
and an O6 set for the other. One of the O donors is a
terminally bound water molecule, exactly like that observed
in all structurally characterized forms of MMOH to date.43,44
Hydrogen-bonding interactions between axial carboxylate
ligands and bridging hydroxide ligands in 8 also are observed
in MMOHox.43,44
The single sharp quadrupole doublet exhibited in the
Mo¨ssbauer spectrum of 8 differs from the results obtained
for MMOHox from both Methylococcus capsulatus (Bath)
and Methylosinus trichosporium OB3b, both of which have
Mo¨ssbauer spectra indicating the presence of two slightly
inequivalent high-spin iron(III) sites.54,68 The isomer shift
(δ ) 0.50(2) mm s-1) of 8 falls in the range associated with
high-spin Fe(III) complexes46,47 and is analogous to those
The influence of different carboxylates is also manifest
in the results of the oxygenation reactions of 1 and 3, as
measured by the final product isolated. Whereas oxidation
of 3 leads to the formation of a discrete dinuclear
(64) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. ReV. 2005, 105,
2329-2363.
(65) Tshuva, E. Y.; Lee, D.; Bu, W.; Lippard, S. J. J. Am. Chem. Soc.
2002, 124, 2416-2417.
(66) Moreira, R. F.; Tshuva, E. Y.; Lippard, S. J. Inorg. Chem. 2004, 43,
4427-4434.
(68) Fox, B. G.; Hendrich, M. P.; Surerus, K. K.; Andersson, K. K.; Froland,
W. A.; Lipscomb, J. D.; Mu¨nck, E. J. Am. Chem. Soc. 1993, 115,
3688-3701.
(67) Chen, C.-T.; Siegel, J. S. J. Am. Chem. Soc. 1994, 116, 5959-5960.
Inorganic Chemistry, Vol. 45, No. 2, 2006 847

Carson and Lippard
Table 5. Mo¨ssbauer Parameters for 8, 9, 10, and MMOHox
methyl group oxidation occurred for the case of 2-meth-
ylthiopyridine complex. The reactivity of these compounds
is not only affected by the N-donor ligand but also by the
remote substituents on the carboxylate ligands. The more
electron-releasing carboxylate groups showed increased C-H
activation, in agreement with results for related systems
having tethered benzyl- and ethylpyridine ligands.27b When
the less electron-releasing carboxylate group was used,
subsequent solvent oxidation did not occur; instead, the {Fe2-
(µ-OH)2(µ-O2CR)}3+core of MMOHox was recovered having
structural and Mo¨ssbauer spectroscopic properties analogous
to those observed in the enzyme. These observations provide
valuable information to guide future work on more advanced
MMOH synthetic analogues.
coordinate atoms
Fe1
Fe2
δ (mm s-1
)
∆EQ (mm s-1
)
ref
8
9
10
NO5
N2O4
NO5
O6
NO5
O6
0.50(2)
0.48(2)
0.45(2)
0.83(2)
0.61(2)
1.21(2)
a
24
45
MMOHox NO5
NO5
0.51(2), 0.50(2)b 0.79(3), 1.12(3)b 54
0.50, 0.51c
0.87, 1.16c
68
a This work. b M. capsulatus (Bath). c M. trichosporium OB3b.
reported for MMOHox. As indicated in Table 5, the quad-
rupole splitting parameter (∆EQ ) 0.83(2) mm s-1) in 8 lies
between that of the structurally related compounds 9 and
10.24,45 Their Mo¨ssbauer parameters, along with those of 8,
are compared to values of MMOHox in Table 5. The ∆EQ
values obtained for these three compounds are significantly
different, despite similar octahedral coordination geometries
at the iron(II) sites. Most reported hydroxo-bridged diiron(III)
complexes have quadrupole splitting values of ∼0.5 mm
s-1.46 Notably, 8 and 10 have more O-rich coordination
environments than 9, with a H2O (O-donor) ligand instead
of an N-donor ligand on the second iron atom. The {Fe2-
(µ-OH)2(µ-O2CR)}3+ core in MMOHox, which also has one
terminally bound water molecule,43,44 displays the similar
∆EQ values, 0.79(3) and 1.12(3) mm s-1 and 0.87 and 1.16
mm s-1 for M. capsulatus (Bath) and M. trichosporium
OB3b, respectively.54,68 An O-rich metal coordination envi-
ronment may be the source of the similar ∆EQ (∼1 mm s-1)
values in 8, 10, and the diiron sites of MMOHox.
Acknowledgment. This work was supported by Grant
GM-32134 from the National Institute of General Medical
Sciences. Raman spectra were obtained at the George R.
Harrison Spectroscopy Lab Raman Facilities supported by
NIH under the Biomedical Research Technology Program
(P41RR02594) and by NSF Grant 0111370-CHE. We thank
Ms. Mi Hee Lim for assistance with the 18O2-labeling
experiments, Ms. Liz Nolan and Dr. Joe Gardecki for
performing the resonance Raman experiments, Dr. Peter
Mu¨ller for assistance with X-ray crystallography, and Dr.
Sungho Yoon for help in acquiring the Mo¨ssbauer spectra.
Supporting Information Available: Table S1 summarizing the
X-ray crystallographic information for 3-8; Tables S2, S3, and
S4 providing geometric parameters for 5, 6, and 7; Table S5
describing bond valence sum analyses for 7 and 8; Chart S1
picturing 9 and 10; Scheme S1 picturing the tautomer of 2-mer-
captopyridine; Figures S1-S6 showing ORTEP diagrams of 3-8;
Figures S7-S9 containing FT-IR spectra of 3-6 and 8; Figure S10
illustrating Mo¨ssbauer data and fits for 3; and X-ray crystallographic
files in CIF format for 3-8. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusion
Tethered thiol, sulfide, sulfoxide, and phosphine moieties
on pyridine ligands serve as substrates for oxidation at O2-
activated carboxylate-bridged diiron(II) centers. Intramo-
lecular oxidation of the heteroatoms in these groups further
demonstrates that proximity is important for oxidation. Only
the group adjacent to the diiron center is oxidized, since no
IC051476S
848 Inorganic Chemistry, Vol. 45, No. 2, 2006