Metal Catalysts for Reductive Processes (HER, CO2RR)

To target catalysts modeled after the diiron center in nature's hydrogenase enzymes, a modular family of ligands based on a naphthalene core was developed. This ligand system allows for considerable tuning of steric and electronic properties for the reactive center of bridged diiron cores, and these principles were demonstrated with the synthesis and study of several new electrocatalysts for proton reduction.1 For Fe2(CO)6(1,2-S2C6H4) as catalyst, experimental and computational studies allowed identification of electrocatalytic proton reduction intermediates and demonstration of a new mechanism involving a two-electron reduction that severs an Fe-S bond to generate a dangling thiolate and an electron rich Fe center. The latter iron center is the initial site of protonation, and this event is followed by protonation at the dangling thiolate to give the thiol thiolate [Fe2H(CO)6(1,2-SHSC6H4)]. This species then undergoes an intramolecular acid-base reaction to form a dihydrogen complex.2 Proton reductions by diaryldithiolene cobalt complexes revealed a number of intermediates that were isolated and structurally characterized. These results, supported by DFT calculations by the Head-Gordon group, suggest a mechanism involving protonations at both Co and S, and then H2 elimination by transfer of the S–H proton to cobalt.3,4 In another aspect of this work, linear trimetallic CoIII/CoII/CoIII cobalt complexes with bridging acyl–alkoxy ligands were found to be electrocatalysts for the proton reduction at a modest onset overpotential.

References

1. "Diiron Proton Reduction Catalysts Possessing Electron-rich and Electron-poor Naphthalene-1,8-dithiolate Ligands." R. J. Wright, C. Lim and T. D. Tilley. Chem. Eur. J. 2009, 15, 8518-8525. DOI: 10.1002/chem.200900989

2. "Isolation, observation, and computational modeling of proposed intermediates in catalytic proton reductions with the hydrogenase mimic Fe2(CO)6S2C6H4." R. J. Wright, W. Zhang, X. Yang, M. Fasulo and T. D. Tilley, Dalton Trans. 2012, 41, 73–82 DOI: 10.1039/C1DT11428J

3. "Mechanism of the Electrocatalytic Reduction of Protons with Diaryldithiolene Cobalt Complexes." C. S. Letko, J. A. Panetier, M. Head-Gordon and T. D. Tilley, J. Am. Chem. Soc. 2014, 136, 9364-9376. DOI: 10.1021/ja5019755

4. "Computational Characterization of Redox Non-Innocence in Cobalt-Bis(diaryldithiolene) Catalyzed Proton Reduction." J. A. Panetier, C. S. Letko, T. D. Tilley and M. Head-Gordon, J. Chem. Theory Comput. 2016, 12, 223–230. DOI: 10.1021/acs.jctc.5b00968