Journal of the American Chemical Society
COMMUNICATION
(3) (a) Ishida, H.; Tanaka, H.; Tanaka, K.; Tanaka, T. Chem.
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H. D. Inorg. Chem. 1992, 31, 3680.
(4) For example, see: (a) Ratliff, K. S.; Lentz, R. E.; Kubiak, C. P.
Organometallics 1992, 11, 1986. (b) Bruce, M. R. M.; Megehee, E.;
Sullivan, B. P.; Thorp, H. H.; O’Toole, T. R.; Downard, A.; Pugh, J. R.;
Meyer, T. J. Inorg. Chem. 1992, 31, 4864. (c) Raebiger, J. W.; Turner,
J. W.; Noll, B. C.; Curtis, C. J.; Miedaner, A.; Cox, B.; DuBois, D. L.
Organometallics 2006, 25, 3345. (d) Fisher, B.; Eisenberg., R. J. Am.
Chem. Soc. 1980, 102, 7363.
(5) Zanello, P. Structure and electrochemistry of transition metal
carbonyl clusters with interstitial or semi-interstitial atoms: contrast
between nitrides or phosphides and carbides. In Unusual Structures and
Physical Properties in Organometallic Chemistry; Gielen, M., Willem, R.,
Wrackmeter, B., Eds.; Wiley: Chichester, U.K., 2002; pp 2À45.
(6) Darensbourg, D. J.; Rokicki, A.; Darensbourg, M. Y. J. Am. Chem.
Soc. 1981, 103, 3223.
(7) Tachikawa, M.; Muetterties, E. L. J. Am. Chem. Soc. 1980,
102, 4541. Tachikawa, M.; Stein, J.; Muetterties, E. L. J. Am. Chem.
Soc. 1980, 102, 6648.
Figure 3. CVs of 0.34 mM solutions of 1 in MeCN containing 0.1 M
Bu4NClO4 CH3CN. Glassy carbon electrode, 100 mV/s. (a) PhCOOH,
black; with CO2, red. (b) tosic acid, black; with CO2, red.
acid production operate at À1.25 V via ligand-based reduction
events.3c We speculate that the ability of 1À to effect proton and
CO2 reduction at À1.25 V arises from the favorable interaction
between H+ and the 2À charge on the reduced complex 12À; the
majority of known HER catalysts have neutral or positive
charges.20 The presence of the 12 strongly electron-withdrawing
carbonyl groups prevents prohibitively high reduction potentials
for negatively charged 1À. In a similar manner, we could specu-
late that the 2À charge on catalytically active 12À contributes to
the higher reaction rate for formation of (H-1)À than for
protonation of (H-1)À.
In summary, we have demonstrated that the operating poten-
tial of a low-valent iron electrocatalyst can be relatively positive as
a result of multiple carbonyl ligands and the delocalization
afforded by the metalÀmetal-bonded structure of 1À. We have
proposed that that anionic charge on the catalyst facilitates
reduction of protons at low overpotentials (190 mV). Understand-
ing of the relative rates of formation and subsequent reaction of
the (H-1)À intermediate allowed the reduction ability of (H-1)À
to be directed toward CO2 or H+. Future work will build on these
principles of molecular electrocatalyst design to probe electro-
catalytic reactions of small molecules in organic and aqueous media.
(8) Zanello, P.; Laschi, F.; Cinquantini, A.; Pergola, R. D.; Garlaschelli,
L.; Cucco, M.; Demartin, F.;Spalding, T. R. Inorg. Chim. Acta1994, 226, 1.
(9) η is the overpotential.
(10) The stability of complex 1 was confirmed by cyclic voltammetry
and UVÀvis spectroscopy.
(11) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
Solvents; Blackwell Scientific Publications: Oxford, U.K., 1990.
(12) Felton, G. A. N.; Glass, R. S.; Lichtenberger, D. L.; Evans, D. H.
Inorg. Chem. 2006, 45, 9181.
(13) D = 1.6 Â 10À5 cm2 sÀ1 was calculated using the Cottrell
equation as in ref 15 and the following reference: Longmire, M. L.;
Watanabe, M.; Zhang, H.; Wooster, T. T.; Murray, R. Anal. Chem. 1990,
62, 747. See Figure S5 for details.
(14) ic is the catalytic plateau current.
(15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Funda-
mentals and Applications, 2nd ed.; Wiley: New York, 2001. Symbol
definitions: ic = catalytic current, D = diffusion coefficient, n = number of
electrons, F = Faraday’s constant, S = electrode surface area, C°p = catalyst
concentration, k = apparent rate constant, C°s = substrate concentration.
(16) 1 is stable in the presence of 50 mM PhCOOH.
(17) The event at À0.45 V vs SCE was identified as the (H-1)0/À
couple by comparison with an authentic sample of H-1 (Figure S11) .7
(18) ECCE denotes successive electrochemical, chemical, chemical,
and electrochemical elementary steps for the reaction mechanism.
ECEC denotes a mechanism with successive electrochemical, chemical,
electrochemical, and chemical steps.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis and characterization
b
of 1 and electrochemical and NMR measurements. This material
(19) No (H-1)2À/À event was observed. We speculate that oxidation
of (H-1)2À overlaps with oxidation of 12À
.
’ AUTHOR INFORMATION
(20) For example, see: ref 3a, ref 4a, and Beley, M.; Collin, J.-P.;
Rupert, R.; Suavage, J.-P. J. Am. Chem. Soc. 1986, 108, 7461.
Corresponding Author
’ ACKNOWLEDGMENT
This work was supported by UC Davis Startup Funding and
an NSF CAREER Award (CHE-1055417). We thank Drs.
M. Shanmugam and J. Berg and Mr. A. Ferreira for experimental,
GC, and NMR assistance, respectively.
’ REFERENCES
(1) (a) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006,
103, 15729. (b) Rakowski DuBois, M.; DuBois, D. L. Acc. Chem. Res.
2009, 42, 1974.
(2) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem.
Soc. Rev. 2009, 38, 89.
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