Electrocatalytic Hydrogen Production by [Ni(7PPh 2NH)2]2+: Removing the Distinction Between Endo- and Exo-Protonation Sites

• Published in: ACS catalysis 2015-04, Vol.5 (4), p.2116-2123
• Main Authors: Brown, Houston J. S, Wiese, Stefan, Roberts, John A. S, Bullock, R. Morris, Helm, Monte L
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/cs502132y

A new Ni­(II) complex, [Ni­(7PPh 2NH)2H]3+ (7PPh 2NH = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7PPh 2NH ligand features an NH, ensuring properly positioned protonated amine groups (N–H+) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H2 production in the presence of organic acids (pK a range 10–13 in CH3CN), with turnover frequencies ranging from 160 to 780 s–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(PPh 2NR′ 2)2]2+ (PPh 2NR′ 2 = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7PPh 2NR′ )2]2+ complexes, catalytic turnover frequencies for H2 production by [Ni­(7PPh 2NH)2]2+ do not show catalytic rate enhancement upon the addition of H2O. This finding supports the assertion that [Ni­(7PPh 2NH)2]2+ eliminates the distinction between the endo- and exo-protonation isomers.