C O M M U N I C A T I O N S
Scheme 2
oxalato) 7a (see Scheme 2). The IR spectrum of 7a shows a broad
and intense vibration centered at 1647 cm-1 (KBr pellet) that shifts
to 1598 cm-1 upon isotopic labeling with 13C-CO2. In one case,
pale pink crystals also formed that were subjected to XRD analysis.
For these crystals, the presence of a µ-oxalato ligand was also
established, but in this case, terminal CO ligands were also present
({[PhBPCH Cy3]Fe(CO)}2(µ-η2:η2-oxalato); 7b). The structure of 7b
2
is provided in the Supporting Information. Studies are now
underway to try to control the selectivity of the CO2 reaction profile
so as to favor the CO2 coupling product/s for further studies.
In summary, THF solutions of 2 provide an effective Fe(I) source
for substrate binding and group transfer reactions. Such solutions
effect the reductive cleavage of CO2 via O-atom transfer to provide
a structurally unique Fe(µ-O)(µ-CO)Fe core. XRD studies reveal
a reductive CO2 coupling process that is also kinetically competent
to generate oxalate. These initial observations establish that Fe(I)
participates in rich CO2 reaction chemistry and motivate continued
studies in this context.
Acknowledgment. We thank Larry M. Henling and Neal
Mankad for crystallographic assistance. C.T.S. is supported by an
NSF graduate fellowship. We are grateful to the NIH (GM070757)
and BP (MC2 program) for financial support.
Supporting Information Available: Detailed experimental pro-
cedures and characterization data for [PhB(CH2P(CH2Cy)2)3]Tl, ligand
precursors, and compounds 1-6. Crystallographic details for 1, 3-6,
7a, and 7b are provided in CIF format. This material is available free
References
(1) For representative stoichiometric reductive CO2 cleavage reactions, see:
(a) Bryan, J. C.; Geib, S. J.; Rheingold, A. L.; Mayer, J. M. J. Am. Chem.
Soc. 1987, 109, 2826. (b) Castro-Rodriguez, I.; Meyer, K. J. Am. Chem.
Soc. 2005, 127, 11242. (c) Fachinetti, G.; Floriani, C.; Chiesi-Villa, A.;
Guastini, C. J. Am. Chem. Soc. 1979, 101, 1767. (d) Procopio, L. J.;
Carroll, P. J.; Berry, D. H. Organometallics 1993, 12, 3087.
(2) Electrocatalytic CO2 reduction can be mediated by later first row metals:
(a) Simo´n-Manso, E.; Kubiak, C. P. Organometallics 2005, 24, 96. (b)
Hammouche, M.; Lexa, D.; Momenteau, M.; Save´ant, J.-M. J. Am. Chem.
Soc. 1991, 113, 8455. (c) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage,
J.-P. J. Am. Chem. Soc. 1986, 108, 7461. (d) Fisher, B.; Eisenberg, R. J.
Am. Chem. Soc. 1980, 102, 7361. (e) Dubois, D. L.; Miedaner, A.;
Haltiwanger, R. C. J. Am. Chem. Soc. 1991, 113, 8753-8764.
(3) For Cu-catalyzed CO2 reduction using diborane reductants, see: Laitar,
D. S.; Muller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127, 17196.
(4) (a) Ragsdale, S. W.; Kumar, M. Chem. ReV. 1996, 96, 2515. (b) Evans,
D. J. Coord. Chem. ReV. 2005, 249, 1582.
benzene/petroleum ether and have been examined by X-ray
crystallography. As shown in Scheme 2, the major product is
{[PhBPCH Cy3]Fe}2(µ-CO,µ-O) (6), indicating a net two-electron
2
reductive cleavage of CO2 to CO and O2-. The connectivity of 6
is very well-established, but of the various sets of crystals that have
been examined by XRD, each has suffered from problematic
disorder, in part due to the floppy methylcyclohexyl substituents.9
An isotropic structure of 6 is therefore depicted in Scheme 2. Its
most striking structural feature pertains to its diiron µ-carbonyl/µ-
oxo core. To our knowledge, a bimetallic µ-oxo/µ-CO structure
type had yet to be reported.10 Complex 6 features a very short Fe-
Fe distance (2.35 Å). CV data in THF show a reversible one-
electron couple at -0.2 V (vs Ag/AgNO3).
Varying the conditions of the CO2 reaction with 2 invariably
leads to the same major product 6. This is true whether 0.5 equiv
of CO2 is employed or a CO2 pressure of 10 atm. Moreover, 6 is
the major product whether the reaction is carried out at -41 °C
(complete in ca. 12 h) or at 22 °C (complete in ca. 15 min). The
iron(I) phosphine adduct 3 also produces 6 as its major product
upon exposure to CO2, albeit much more slowly.
(5) Well-defined reductive coupling reactions of CO2 to generate oxalate are
uncommon. See, for example: Evans, W. J.; Seibel, C. A.; Ziller, J. W.
Inorg. Chem. 1998, 37, 770.
(6) (a) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782. (b)
Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252.
(7) (a) Butts, M. D.; Bergman, R. G. Organometallics 1994, 13, 2668. (b)
Nolan, S. P.; Hoff, C. D.; Stoutland, P. O.; Newman, L. J.; Buchanan, J.
M.; Bergman, R. G.; Yang, G. K.; Peters, K. G. J. Am. Chem. Soc. 1987,
109, 3143.
(8) For related Fe(III) imides, see: (a) Brown, S. D.; Betley, T. A.; Peters, J.
C. J. Am. Chem. Soc. 2003, 125, 322. (b) Brown, S. D.; Peters, J. C. J.
Am. Chem. Soc. 2005, 127, 1913. (c) Thomas, C. M.; Mankad, N. P.;
Peters, J. C. J. Am. Chem. Soc. 2006, 128, 4956. Also see 6a.
(9) Triclinic, monoclinic, and tetragonal crystals of 6 were examined; each
crystal form was problematic. The triclinic crystals diffracted poorly and
contained large areas of highly disordered solvent (see Supporting
Information). In the monoclinic crystals, 6 was very disordered in addition
to the disordered solvent molecules. The tetragonal crystal form yielded
no atomic information.
During the course of these studies, we have consistently observed
a substantial secondary paramagnetic product by NMR spectroscopy
(ca. 15-25% depending on exact conditions). By fractional
crystallization of crude product mixtures, we have been able to pick
out red-brown crystals for XRD analysis of the primary side
(10) There are a few examples of dimetal µ-sulfide/µ-CO core structures. See,
for example: (a) Kubiak, C. P.; Woodcock, C.; Eisenberg, R. Inorg. Chem.
1980, 19, 2733. (b) Balch, A. L.; Catalano, V. J.; Olmstead, M. M. Inorg.
Chem. 1990, 29, 1638.
product to establish its identity as {[PhBPCH Cy3]Fe}2(µ-η2:η2-
JA065524Z
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