Fe(I)-mediated reductive cleavage and coupling of CO2: An FeII(μ-O,μ-CO)FeII core

Article abstract of DOI:10.1021/ja065524z

THF solutions of a new iron(I) source, [PhBP^CH₂^Cy₃]Fe ([PhBP^CH₂Cy₃] = [PhBP(CH₂P(CH₂Cy)₂)₃]-), effect the reductive cleavage of CO

Full text of DOI:10.1021/ja065524z

Published on Web 12/16/2006
Fe(I)-Mediated Reductive Cleavage and Coupling of CO2: An
FeII(µ-O,µ-CO)FeII Core
Connie C. Lu, Caroline T. Saouma, Michael W. Day, and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received July 31, 2006; E-mail: jpeters@caltech.edu
Scheme 1
Direct O-atom transfer from CO2 is a difficult transformation to
realize given the molecule’s thermodynamic and kinetic stability.
Highly reducing early transition, lanthanide, and actinide metal
complexes are known that facilitate reductive C-O cleavage of
CO2.1 Later first row ions, while active for CO2 binding, do not
typically display similar cleavage transformations.2,3 Nature, how-
ever, is presumed to exploit low-valent, later first row metal ions
(e.g., Ni, Fe) to mediate CO2 reduction/CO oxidation in the C cluster
of CODH enzymes.4
We describe herein an unusual iron(I) system that reacts readily
with CO2 at ambient temperature to mediate its reductive cleavage.
The dominant cleavage product is a structurally unprecedented
bimetallic µ-carbonyl/µ-oxo core (i.e., Fe(µ-CO)(µ-O)Fe)). Struc-
tural evidence is also available for minor oxalate side products of
the type Fe(µ-η2:η2-oxalato)Fe. This iron system is therefore able
to mediate both the reductive cleavage and coupling of CO2.5
Entry into the CO2 chemistry of present interest was realized
using a new tris(phosphino)borate ligand, [PhB(CH2P(CH2Cy)2)3]-
(abbreviated as [PhBPCH Cy3]), featuring cyclohexylmethyl substit-
2
uents at phosphorus. The yellow iron precursor [PhBPCH Cy3]FeCl
2
and the formation of CHCl3 (∼40%, detected by 1H and 13C NMR)
(1) was obtained in good yield from Tl[PhBPCH Cy3] and FeCl2.
2
upon the addition of 1 equiv of CCl4 in THF-d8.7
XRD, combustion analysis, and a solution magnetic moment
determination establish that 1 is a monomeric, pseudotetrahedral S
) 2 species. When compound 1 is chemically reduced by Na/Hg
in THF under N2, an intense lime-green solution is formed. This
Regardless of its exact structure/s in THF solution, 2 behaves
chemically as a very clean [PhBPCH Cy3]Fe(I) source. For instance,
2
the addition of PMe3 to a THF solution of 2 generates the d7 S )
3/2 complex [PhBPCH Cy3]Fe(PMe3) (3). Also, the addition of 1 equiv
2
observation contrasts that of the Na/Hg reduction of its cousin
of 1-adamantyl azide to 2 triggers oxidative nitrene transfer to
[PhBPiPr3]FeCl under a N2 atmosphere, which gives rise to the red-
provide the S ) 1/2 Fe(III) imide [PhBPCH Cy3]FetNAd (4).8 Both
2
brown dinitrogen-bridged dimer {[PhBPiPr3]Fe}2(µ-N2).6 We have
3 and 4 are formed almost quantitatively and have been structurally
no evidence for N2 uptake upon Na/Hg reduction of the [PhBPCH Cy3]-
2
characterized (Scheme 1). Additionally, reconstitution of a THF
FeCl system under a N2 atmosphere. Combustion analysis data for
solution of 2 into benzene provides, almost quantitatively, the
the isolated reduction product confirm its empirical formula as
dinuclear benzene complex {[PhBPCH Cy3]Fe}2(µ-η3:η3-C6H6) 5.
2
[PhBPCH Cy3]Fe (2) and rules out the presence of nitrogen.
2
Given these observations, it seem likely that the iron center in 2 is
A sample of 2 in THF-d8 exhibits complicated solution NMR
spectra indicative of both paramagnetic and diamagnetic compo-
nents that are likely undergoing rapid exchange. For example, its
31P NMR spectrum features a single broad resonance that shifts
1
reversibly solvated by THF to produce an S ) /2 iron(I) species
such as [PhBPCH Cy3]Fe(THF)2 in THF. Such a species could in
2
1
fact account for the S ) /2 signature of 2 in THF at 4 K, rather
than the cyclometalated hydride A shown in Scheme 1.
from -29 to 4 ppm when the temperature is varied from 60 to
We next explored the reactivity of THF solutions of 2 with CO2
-60 °C. A 1H NMR spectrum of the sample contains broad,
and found that an immediate though subtle color change occurs
temperature-dependent resonances ranging from -7 to 72 ppm and
from lime-green to pine-green upon CO2 exposure. Inspection of
sharp resonances in the diamagnetic region of the spectral window.
1
the reaction solution in situ by H NMR spectroscopy indicates
1
Also, an axial EPR signal indicative of an S ) /2 iron center is
one major diamagnetic product (ca. 75% using 5 equiv of CO2,
five runs). This product can be crystallized in analytically pure form
(65% isolated yield) and exhibits a single peak in the 31P NMR
spectrum at 51.9 ppm and an intense ν(CO) IR stretch at 1730 cm-1
(KBr, C6D6; 1734 cm-1, KBr pellet). This IR stretch represents a
µ-CO ligand. Using 13C-labeled CO2, a 13C NMR resonance for
the µ-CO ligand at 289.8 ppm has been established. The ν(µ-CO)
vibrations shift to 1692 cm-1 (calcd 1691 cm-1) upon isotopic
substitution. Blue-green plate-like crystals of 6 can be grown from
observed in a THF glass of 2 at 4 K. Scheme 1 shows two possible
isomeric structures that would be consistent with these spectral data
and the empirical formula [PhBPCH Cy3]Fe. They include an Fe(III)
2
alkyl hydride wherein one of the cyclohexyl C-H bonds of the
ligand is cyclometalated (A, S ) 1/2) and an antiferromagnetically
coupled dimer of such a structure with the hydride ligands in
bridging positions (B, S ) 0). Direct evidence for the presence of
a metal hydride includes an IR stretch at 2058 cm-1 (KBr pellet)
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J. AM. CHEM. SOC. 2007, 129, 4-5
10.1021/ja065524z CCC: $37.00 © 2007 American Chemical Society

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
of charge via the Internet at http://pubs.acs.org. This material is available
free of charge via the Internet at http://pubs.acs.org.
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,
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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-
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