A series of heteroleptic 17e cobalt complexes, [CpCoII(PtBu2NPh2)](BF4), [CpC6F5CoII(PtBu2NPh2)](BF4), and [CpC5F4NCoII(PtBu2NPh2)](BF4) (where PtBu2NPh2 = 1,5-diphenyl-3,7-di-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane, CpC6F5 = C5H4(C6F5), and CpC5F4N = C5H4(C5F4N)) were synthesized, and the 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 perfluoropyridyl substituent on the Cp ring. Reduction of these paramagnetic CoII complexes by KC8 led to the diamagnetic 18e complexes CpCoI(PtBu2NPh2), CpC6F5CoI(PtBu2NPh2), and CpC5F4NCoI(PtBu2NPh2), which were also characterized by crystallography. Protonation of these neutral CoI complexes led to the CoIII hydrides [CpCoIII(PtBu2NPh2)H](BF4), [CpC6F5CoIII(PtBu2NPh2)H](BF4), and [CpC5F4NCoIII(PtBu2NPh2)H](BF4), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [CpC5F4NCoII(PtBu2NPh2)]+, is an electrocatalyst for production of H2 using [p-MeOC6H4NH3][BF4] (pKaMeCN = 11.86), with a turnover frequency of 350 s-1 and an overpotential of 0.86 V at Ecat/2. A pKa value of 15.6 was measured in CH3CN for [CpC5F4NCoIII(PtBu2NPh2)H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the Co-H bond.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry