The interconversion between CO2 and formic acid has received renewed attention as a hydrogen storage system [1]. We recently studied homogeneously catalyzed CO2 hydrogenation and formic acid dehydrogenation in water using bio-inspired iridium catalysts based on the concept of a proton responsive ligand with pendant bases [2]-[4]. Highly efficient catalysts for the interconversion have been achieved using iridium complexes with hydroxyl groups at ortho-positions of a bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). [Cp*Ir(TH4BPM)(H2O)]SO4 (TH4BPM = 4,4',6,6'-tetrahydroxy-2,2'-bipyrimidine) gave an exceptionally high TOF for hydrogenation of CO2 and dehydrogenation of formic acid in water. We have obtained clear evidence from kinetic isotope effects and computational studies of the involvement of a water molecule in the rate-determining step with ortho-hydroxy complexes. In CO2 hydrogenation, heterolysis of H2 was accelerated by proton transfer through the formation of a water bridge.[3] In dehydrogenation of formic acid, the rate limiting step was found to be b-hydrogen elimination from the Ir-formate intermediate owing to a proton relay (i.e., pendent-base effect) which lowers the energy barrier for the subsequent generation of H2.[4] YH and WHW thank Japan-U.S. cooperation project for research and standardization of Clean Energy Technologies for financial support. The work at BNL was carried out under contract DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Geosciences, & Biosciences, Office of Basic Energy Sciences.  [1] Fujita, E., Muckerman, J.T., Himeda, Y., BBA-Bioenergetics 2013 , 1827, 1031-1038.[2] Hull, J.F., Himeda, Y., Wang, W.-H., Hashiguchi, B., Periana, R., Szalda, D.J., Muckerman, J.T., Fujita, E., Nat. Chem. 2012 , 4, 383-388.[3] Wang, W.-H., Muckerman, J.T., Fujita, E., Himeda, Y., ACS Catal. 2013 , 3, 856-860.[4] Wang, W.-H., Xu, S., Manaka, Y., Suna, Y., Kambayashi, H., Muckerman, J.T., Fujita, E., Himeda, Y., submitted for publication.

Highly efficient bio-inspired iridium catalysts for hydrogenation of carbon dioxide and dehydrogenation of formic acid

248th National Meeting (2014)

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