PNNL

Abstract

We present the results of a comprehensive theoretical investigation of the role of pendant amine ligands in the oxidation of H2 and formation of H2 by [Ni(P2RN2R’)2]2+ electrocatalysts (where P2RN2R’ is the 1,5-R’-3,7-R derivative of 1,5-diaza-3,7-diphosphacyclooctane with R and R’ are aryl or alkyl groups). We focus our analysis on the thermolchemical part of the catalytic cycle, as it is known to be rate determining for both H2 oxidation and production. We find that the presence of pendant amine functional groups greatly facilitates the heterolytic H2 bond cleavage resulting in a protonated amine functional group and a Ni hydride. Only one single well-positioned pendant amine is required to serve this function. The pendant amine can also effectively shuttle protons to the active site making the redistribution of protons and the H2 evolution a very facile processes. An important requirement for the overall catalytic process is the positioning of at least one amine in close proximity to the metal center. Indeed, only protonation of the pendant amines on the metal center side (endo position) leads to catalytically active intermediates, whereas protonation on the opposite side of the metal center (exo position) leads to a variety of isomers, which are detrimental to catalysis. This material is based upon work supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under FWP56073.