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Title: Cobalt Complexes Containing Pendant Amines in the Second Coordination Sphere as Electrocatalysts for H2 Production

A series of heteroleptic 17e- cobalt complexes, [CpCoII(PtBu2NPh2)](BF4), [CpC6F5CoII(PtBu2NPh2)](BF4), [CpC5F4NCoII(PtBu2NPh2)](BF4), [where P2tBuN2Ph = 1,5-diphenyl-3,7-di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane, CpC6F5 = C5H4(C6F5), and CpC5F4N = C5H4(C5F4N)] were synthesized, and structures of all three were determined by X-ray crystallography. Electrochemical studies showed that the CoIII/II couple of [CpC5F4NCoII(PtBu2NPh2)]+ appears 250 mV positive of the CoIII/II couple of [CpCoII(PtBu2NPh2)] as a result of the strongly electron-withdrawing perfluorpyridyl substituent on the Cp ring. Reduction of these paramagnetic CoII complexes by KC8 led to the diamagnetic 18e- complexes CpICo(PtBu2NPh2), CpC6F5CoI(PtBu2NPh2), CpC5F4NCoI(PtBu2NPh2), which were also characterized by crystallography. Protonation of these neutral CoI complexes led to the cobalt hydrides [CpCoIII(PtBu2NPh2)H](BF4), [CpC6F5CoIII(PtBu2NPh2)H](BF4), and [CpC5F4NCoIII(PtBu2NPh2)H](BF4). The cobalt hydride with the most electron-withdrawing Cp ligand, [CpC5F4NCoIII(PtBu2NPh2)H]+ is an electrocatalyst for production of H2 using 4-MeOC6H4NH3BF4 (pKaMeCN = 11.86) with a turnover frequency of 350 s-1 and an overpotential of 0.75 V. Experimental measurement of thermochemical data provided further insights into the thermodynamics of H2 elimination. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
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Journal Article
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Journal Name: Organometallics, 33(20):5820-5833
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Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
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Country of Publication:
United States
hydrogen; catalysis; electrocatalysis; proton relay