The role of pendant amines in the breaking and forming of molecular hydrogen catalyzed by nickel complexes

We present the results of a comprehensive theoretical investigation of the role of pendant amine ligands in the oxidation of H ₂ and formation of H ₂ by [Ni(P ^R ₂N ^R' ₂) ₂] ²⁺ electrocatalysts (P ^R ₂N ^R' ₂ is the 1,5-R'-3,7-R derivative of 1,5-diaza-3,7- diphosphacyclooctane, in which R and R' are aryl or alkyl groups). We focus our analysis on the thermal steps of the catalytic cycle, as they are known to be rate-determining for both H ₂ oxidation and production. We find that the presence of pendant amine functional groups greatly facilitates the heterolytic H ₂ bond cleavage, resulting in a protonated amine and a Ni hydride. Only one single 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 H ₂ evolution a very facile process. 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.