On the reduction of basic iron acetate: Isolation of ferrous specis mediating Gif-type oxidation of hydrocarbons

Full text of DOI:10.1021/ja960529p

5
824
J. Am. Chem. Soc. 1996, 118, 5824-5825
On the Reduction of Basic Iron Acetate: Isolation
of Ferrous Species Mediating Gif-Type Oxidation of
Hydrocarbons
Scheme 1
Bharat Singh,1a Jeffrey R. Long,
1b
Georgia C. Papaefthymiou, and Pericles Stavropoulos*,1a
1c
Department of Chemistry, Boston UniVersity
Boston, Massachusetts 02215
Department of Chemistry, HarVard UniVersity
Cambridge, Massachusetts 02138
Francis Bitter Magnet Laboratory
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
ReceiVed February 20, 1996
40%).10 These are occasionally contaminated with 3 (py/AcOH
10:1), 2 (py/AcOH 5:1), and colorless crystals of [Zn(O2CCH3)2-
(py)2] (5). Addition of a large excess of Et2O to the py/AcOH
Structurally unspecified iron species have been implicated
2
3
4
in the hydrocarbon-oxidizing Fenton, Udenfriend, and Gif
5
12
systems. Early versions of the Gif system were based on
dioxygen activation by ferrous species in pyridine/acetic acid
reaction filtrates favors the formation of 2. Clearly, enrichment
in AcOH and/or Et2O shifts the pyridine-dependent equilibria
in the order 3 f 4 f 2. Pure 2 is obtained by diffusion of
Et2O into the filtrate of the reduction of 1 with Zn in py/AcOH
(2:1). Feathery crystals of 4 are obtained pure from concentrated
solutions of 3 in py/AcOH (5:1, 2:1) upon addition of Et2O.
Crystals of 4 suitable for X-ray analysis could only be prepared
from 3 in py/Et2O(excess) at 10 °C. In neat pyridine, the
reduction (6 h) of 1 with Zn yields 3 (80%, -20 °C) and a
small amount of a brown-black film. Attempts to crystallize
(
py/AcOH) solutions. The choice of solvents and the selectivity
6
for the ketonization/hydroxylation of certain substrates suggest
6
c
that structural/functional similarities, but also discrepancies,
may exist between active Gif reagents and the diiron site of the
hydroxylase component of soluble methane monooxygenase
(
sMMO).7 To investigate these assumptions, we have selected
IV 8
the basic iron acetate [Fe3O(O2CCH3)6(py)3]‚py (1) (Gif ) as
a point of departure. Pertinent transformations explored in this
study are summarized in Scheme 1.
9
57
Reduction (30 min) of 0.40 mmol of brown-black 1 with
the latter material, which retains Fe(III) according to Fe
excess Zn in 5.0 mL of CH3CN/AcOH (10:1 or 2:1 v/v) or CH2-
M o¨ ssbauer data, have been unsuccessful. Species 3 and 4 are
also obtained by treatment of 1 with iron dust or with H2 (30
psig)/Pd in py or py/AcOH (10:1). A control experiment
confirmed that compounds 3 and 4 are also generated by stirring
Fe in py/AcOH.
Cl2/AcOH (10:1) results in the precipitation (-20 °C) of light
II
10
yellow crystals of [Zn2Fe (O2CCH3)6(py)2] (2, 65%). The
analogous reduction of 1 in py/AcOH (10:1 or 5:1) affords
II
10,11
yellow-green crystals of [Fe (O2CCH3)2(py)4] (3, 80%).
In
contrast, diffusion of Et2O into the original py/AcOH filtrates
at 10 °C yields yellow crystals of [Fe 2(O2CCH3)4(py)3]n (4,
Upon exposure of their py or py/AcOH solutions to pure
dioxygen or air, both 3 and 4 are quantitatively converted to 1.
The reaction of 2 with dioxygen in py/AcOH (2:1) yields red-
II
(
1) (a) Boston University. (b) Harvard University. (c) Massachusetts
1
0
black crystals of [Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]‚py (1′)
Institute of Technology.
12
(
2) (a) Walling, C. Acc. Chem. Res. 1975, 8, 125-131. (b) Stubbe, J.;
(95%) and colorless crystals of [Zn2(O2CCH3)4(py)2] (6, 75%).
Kozarich, J. W. Chem. ReV. 1987, 87, 1107-1136. (c) Sawyer, D. T.; Kang,
C.; Llobet, A.; Redman, C. J. Am. Chem. Soc. 1993, 115, 5817-5818.
1
57
Compound 1′ is distinguished from 1 by its H NMR, Fe
M o¨ ssbauer data (10% Fe(II)), and Fe/Zn ICP microanalyses.10
(
3) Udenfriend, S.; Clark, C. T.; Axelrod, J.; Brodie, B. B. J. Biol. Chem.
1
954, 208, 731-739.
Electron microprobe analysis confirmed that each individual
(
(
4) Barton, D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504-512.
5) Barton, D. H. R.; B e´ vi e` re, S. D.; Chavasiri, W.; Csuhai, E.; Doller,
13
crystal of 1′ contains zinc (Fe:Zn ) 74:26).
Admitting
dioxygen to the filtrate of the reduction (6 h) of 1 by Zn in py
D.; Liu, W.-G. J. Am. Chem. Soc. 1992, 114, 2147-2156.
14
(
6) (a) Barton, D. H. R.; Boivin, J.; Motherwell, W. B.; Ozbalik, N.;
generates [Fe2ZnO(O2CCH3)6(py)3]‚py (1′′). Analogous treat-
ment of 1′′ with Zn dust yields results nearly identical to those
described for 1.
Schwartzentruber, K. M.; Jankowski, K. New J. Chem. 1986, 10, 387-
3
98. (b) Green, J.; Dalton, H. J. Biol. Chem. 1989, 264, 17698-17703. (c)
Froland, W. A.; Andersson, K. K.; Lee, S.-K.; Liu, Y.; Lipscomb, J. D. J.
Biol. Chem. 1992, 267, 17588-17597.
Species 2, 3, 4, and 5 are related by equilibria. When
dissolved in py or py/AcOH, 2 affords quantitative amounts of
(
7) (a) Liu, K. E.; Valentine, A. M.; Wang, D.; Huynh, B. H.;
Edmondson, D. E.; Salifoglou, A.; Lippard, S. J. J. Am. Chem. Soc. 1995,
3
and 5 at -20 °C. Conversely, 0.5 mmol of 3 and 1.0 mmol
1
3
17, 10174-10185. (b) Lipscomb, J. D. Annu. ReV. Microbiol. 1994, 48,
of 5 in py or py/AcOH deposit pure 2 upon addition of Et2O.
Absorption spectra of 2 in py or py/AcOH indicate a greater
than 90% conversion to 3/4. The UV-vis spectrum of 3 in py
71-399.
(
8) 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.
(
λmax 396 nm (ꢀM ) 2107); Beer’s law observed ([3] < 1.0 ×
(
9) Sorai, M.; Kaji, K.; Hendrickson, D. N.; Oh, S. M. J. Am. Chem.
-3
Soc. 1986, 108, 702-708.
10 M)) exhibits a new broad absorption (λmax 424 nm) at
higher concentrations. In the near-IR region, concentrated
solutions of 3 (10 M) or filtrates of the reduction of 1 by Zn
dust in py or py/AcOH demonstrate broad d-d transition bands
(
10) Analytical, spectroscopic and detailed crystallographic data have
been deposited as supporting information. Crystal data for 1′, R32, a )
-
1
3
1
0
1
0
1
0
7.556(4) Å, c ) 10.932(5) Å, V ) 2918(2) Å , T ) 223 K, Z ) 3, R )
.0416, Rw ) 0.0461; 2, P21/n, a ) 10.255(2) Å, b ) 10.756(2) Å, c )
3
2.727(3) Å, â ) 95.56(3)°,V ) 1397.2(5) Å , T ) 223 K, Z ) 2, R )
.0334, Rw ) 0.0362; 3, Pccn, a ) 8.857(7) Å, b ) 16.401(9) Å, c )
(12) Zn(O2CCH3)2‚2H2O consistently crystallizes as [Zn(O2CCH3)2(py)2]
(5) from py or py/AcOH (10:1, 5:1) and as [Zn2(O2CCH3)4(py)2] (6) from
py/AcOH (2:1). Structural details will be published elsewhere.
(13) This result along with preliminary powder X-ray diffraction analysis
of 1, 1′ and 1′′ render it unlikely that domains of 1 and 1′′ are present in
the structure of 1′. Also see: 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.
3
6.653(10) Å, V ) 2419(3) Å , T ) 223 K, Z ) 4, R ) 0.0391, Rw )
.0383; 4, Pna21, a ) 16.018(4) Å, b ) 9.687(3) Å, c ) 18.107(6) Å, V
3
)
2810(1) Å , T ) 295 K, Z ) 4, R ) 0.0388, wR2 ) 0.0887.
(
11) 3 has been previously synthesized from Fe(II) precursors, but details
of its synthesis and/or characterization were unclear: (a) Catterick, J.;
Thornton, P.; Fitzsimmons, B. W. J. Chem. Soc., Dalton Trans. 1977, 1420-
1
6
425. (b) Hardt, H.-D.; M o¨ ller, W. Z. Anorg. Allg. Chem. 1961, 313, 57-
9. (c) Sheu, C.; Richert, S. A.; Cofr e´ , Ross, B., Jr.; Sobkowiak, A.; Sawyer,
(14) Blake, A. B.; Yavari, A.; Hatfield, W. E.; Sethulekshmi, C. N. J.
Chem. Soc., Dalton Trans. 1985, 2509-2520.
D. T.; Kanofsky, J. R. J. Am. Chem. Soc. 1990, 112, 1936-1942.
S0002-7863(96)00529-X CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 24, 1996 5825
trans positions along the propeller axis (O(1)-Fe-O(1′) )
1
68.5(2)°). The structure of 4 (Figure 2) reveals a one-
dimensional chain comprised of asymmetric diferrous units. The
octahedral Fe(1) atom is bridged by three acetate groups to a
more distorted Fe(2) site (Fe(1)‚‚‚Fe(2) ) 3.676 Å). The
distortion arises due to the bidentate chelation of the unique
monodentate acetate bridge to Fe(2) (Fe(2)-O(6) ) 2.273(5)
Å). Each dinuclear unit is bridged to two adjacent units via
O(8) and O(7A) (Fe(1)‚‚‚Fe(2′) ) 5.275 Å).
The zero-field Fe M o¨ ssbauer spectrum of 4 in the solid state
at 4.2 K does not distinguish between the two iron sites (δ )
1
1
5
57
Figure 1. ORTEP diagram of 2 showing 40% probability ellipsoids
and atom-labeling schemes.
.27 mm/s, ∆EQ ) 2.92 mm/s, Γ ) 0.20 mm/s).16 The
corr
temperature dependence of the magnetic moment of 4 (µeff
)
7.07 µB (300 K), 2.58 µB (2 K)) is suggestive of antiferro-
17
magnetically coupled ferrous sites within the dinuclear unit.
Catalytic oxidation of adamantane mediated by 2-4 under
Gif conditions (adamantane (100 mg)/catalyst (10 mg)/Zn (1.3
g)/air) in py (27 mL)/AcOH (2.7 mL) for 18 h yields similar
6
a
results to those previously reported for 1. The adamantane-
based products, analyzed by GC and GC-MS, include 2-ada-
mantanone (2, 11.7%; 3, 11.3%; 4, 10.1%), 2-adamantanol (2,
1
0
.4%; 3, 1.3%; 4, 1.3%), 1-adamantanol (2, 0.5%; 3, 0.6%; 4,
.7%), 4-(1-adamantyl)pyridine (2, 5.4%; 3, 5.8%; 4, 6.0%),
and 2-(1-adamantyl)pyridine (2, 3.2%; 3, 3.3%; 4, 3.2%). The
preference for secondary over tertiary carbons (C2/C3) is in the
range 1.2-1.4.
This study provides an inorganic template to place Gif
chemistry on a rational basis. Structure 4 relates favorably to
1
8
the asymmetric diferrous site of sMMO.
However, any
genuine relation to the active site of sMMO needs to be drawn
on the basis of the geometric features of the active oxidant
involved. This point, along with the question of whether the
present ferrous compounds are kinetically competent to mediate
oxidation of hydrocarbons, will be the subject of future
investigations.
Acknowledgment. We thank Dr. M. J. Scott for assisting in the
structure determination of 2 and Prof. R. H. Holm for use of the X-ray
diffractometer. This work was generously supported by grants from
the Petroleum Research Fund (ACS-PRF-29383-G3), the U.S. Envi-
ronmental Protection Agency (R823377-01-0), and the Alzheimer’s
Association (IIRG-95-087).
Figure 2. ORTEP diagrams of 3 (top) and 4 (bottom) showing 50%
and 30% probability ellipsoids, respectively, and atom-labeling schemes.
Supporting Information Available: Text describing analytical and
spectroscopic data and tables containing listings of crystal and data
collection parameters, atomic coordinates and isotropic thermal pa-
rameters, interatomic distances, bond angles, and anisotropic displace-
ment parameters for 1′, 2, 3, and 4 (27 pages). Ordering information
is given on any current masthead page.
(
λmax ≈ 850 (ꢀM ≈ 8), 1180 (ꢀM ≈ 8) nm) which are almost
identical to those exhibited in near-IR/ATR (ATR ) attenuated
total internal reflectance) spectra of solid 4.
Compound 1′ (supporting information) crystallizes in space
group R32 featuring trinuclear µ-oxo units with crystallographi-
cally imposed D3 symmetry (at 223 K cluster disorder does not
allow the different metal sites to be distinguished). The structure
of 2 (Figure 1) features a linear arrangement of the metal centers,
with the centrosymmetric iron atom bridged, via two bidentate
and one monodentate acetate groups, to the tetrahedrally ligated
zinc atoms. The structure of 3 (Figure 2) reveals a distorted
octahedral Fe environment with an imposed C2 axis bisecting
the equatorial plane defined by the four Fe-N bonds. Pyridine
rings are arranged in a propeller fashion, while acetates assume
JA960529P
(
15) (a) Goldberg, D. P.; Telser, J.; Bastos, C. M.; Lippard, S. J. Inorg.
Chem. 1995, 34, 3011-3024. (b) Rardin, R. L.; Tolman, W. B.; Lippard,
S. J. New J. Chem. 1991, 15, 417-430.
(
16) For a similar result on a closely related compound, see: Coucou-
vanis, D.; Reynolds, R. A., III; Dunham, W. R. J. Am. Chem. Soc. 1995,
117, 7570-7571.
(17) Evaluation of the exchange coupling constant(s) J for the one-
dimensional solid 4 is in progress.
18) Rosenzweig, A. C.; Nordlund, P.; Takahara, P. M.; Frederick, C.
A.; Lippard, S. J. Chem. Biol. 1995, 2, 409-418.
(