Reactivity of Proton Sources with a Nickel Hydride Complex in Acetonitrile: Implications for the Study of Fuel-Forming Catalysts

• Published in: Inorganic chemistry 2016-05, Vol.55 (10), p.5079-5087
• Main Authors: Rountree, Eric S, Dempsey, Jillian L
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.6b00885

The reactivity of the nickel hydride complex [HNi­(P2 PhN2 Ph)2]+ (P2 PhN2 Ph = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane) with a variety of acids to form hydrogen in acetonitrile was evaluated using stopped-flow spectroscopy in order to gain a better understanding of how the proton source influences the reaction kinetics when evaluating fuel-forming catalysts in acetonitrile. This reaction is initiated by the rate-determining step in the catalytic cycle for the hydrogen-evolving catalyst [Ni­(P2 PhN2 Ph)2]2+. Proton sources were evaluated with respect to pK a, homoconjugation, dimerization, heteroconjugation, and aggregation (for water). The effects of water and conjugate base were also studied. A linear free energy relationship between rate constant and pK a was revealed; rate constants increased with the magnitude of the homoconjugation constant for acids prone to homoconjugation, and second-order reactivity was observed for trifluoroacetic and trichloroacetic acid, suggesting dimerization. Upon the addition of water, an increase in the observed rate constant was seen, in line with the formation of hydronium. When added to trifluoroacetic acid, water was shown to cause a decrease in the observed rate constant, suggesting that water inhibits acid dimerization. Collectively, these findings highlight that the selection of proton sources for the study of molecular electrocatalysts in acetonitrile must account for more than acid pK a.