Reversible Chemical and Electrochemical Dehydrogenation/Hydrogenation of Primary Alcohols Catalyzed By Iridium Complexes

Development of efficient electrocatalysts for both dehydrogenation and hydrogenation of organic fuels is critical to the realization of regenerative liquid fuel cells (RLFCs). Alcohols are convenient fuels currently utilized in direct alcohol fuel cells (DAFCs) and are capable of undergoing reversible partial dehydrogenation without CO2 emission in a RLFC.¹ An Ir-diaminodiolefin complex, [Ir(trop₂DACH)][OTf] (trop₂DACH = N,N’-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane) (1) was proposed by Grützmacher et al.² as a catalyst for alcohol oxidation with benzoquinone. Previously we demonstrated that 1 also catalyzes chemical dehydrogenation of 4-methoxybenzyl alcohol to p-anisaldehyde with one-electron oxidants in the presence of a base as well as electrochemical dehydrogenation at a low applied potential with high selectivity and Faradaic efficiency.³ In this presentation we report both chemical and electrochemical dehydrogenation of primary alcohols and hydrogenation of the corresponding aldehydes using  1 or an analogous iridium complex [Ir(trop₂DAD)]OTf (trop₂DAD = N,N’-bis(5H-dibenzo[a,d]cycloheptene-5-yl)-1,4-diazabuta-1,3-diene) (2) as a single homogeneous catalyst. In the chemical processes the same acid/conjugate base pair (substituted phenols/phenolates) is used with an appropriate one electron reductant (e.g. cobaltocene) for hydrogenation or oxidant (e.g. ferrocene) for dehydrogenation. In the electrochemical processes 1 and 2 catalyze dehydrogenation and hydrogenation with high faradaic efficiency and excellent selectivity though hydrogenation produced smaller yields. Further details on the effects of the base, solvent and electrolysis conditions on chemical and electrochemical catalysis will be provided.

               This work is supported as part of the Center for Electrocatalysis, Transport Phenomena, and Materials (CETM) for Innovative Energy Storage, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001055.

1. Kariya, N. F., Fukuoka, A.; Ichikawa, M., Phys. Chem. Chem. Phys. 2006, 8, 1724

2. Königsmann, M.; Donati, N.; Stein, D.; Schönberg, H.; Harmer, J.; Sreekanth, A.; Grützmacher, H., Angew. Chem. Int. Ed. 2007, 46, 3567.

3. Bonitatibus, P. J.; Rainka, M. P.; Peters, A. J.; Simone, D. L.; Doherty, M. D., Chem. Communs 2013, 49, 10581.