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 investiga- tion of the role of pendant amine li- gands in the oxidation of H2 and for- mation of H2 by (NiA R 2N R' 2)2) 2 + elec- trocatalysts (P R 2N R' 2 is the 1,5-R'-3,7-R derivative of 1,5-diaza-3,7-diphosphacy- clooctane, 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-de- termining 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 and a Ni hydride. Only one single positioned pendant amine is re- quired to serve this function. The pend- ant amine can also effectively shuttle protons to the active site, making the redistribution of protons and the H2 evolution a very facile process. An im- portant requirement for the overall cat- alytic process is the positioning of at least one amine in close proximity to the metal center. Indeed, only protona- tion 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.