Proton Delivery and Removal in [Ni(P2RN2R′)]2 Hydrogen Production and Oxidation Catalysts

To examine the role of proton delivery and removal in the electrocatalytic oxidation and production of hydrogen by Ni((P2N2R)-N-R')(2))(2+) (where (P2N22R')-N-R is 1,5-R'-3,7-R-1,5-diaza-3,7-diphosphacyclooctane); we report experimental and theoretical studies of the intermolecular proton exchange reactions underlying the isomerization of [Ni((P2N2H)-N-Cy-H-Bn)(2)](2+)(Cy - cyclohexyl, Bn = benzyl) species formed during the oxidation of H-2 by [Ni-II((P2N2Bn)-N-Cy)(2)](2+) or the protonation of [Ni-0((P2N2Bn)-N-Cy)(2)]. Three... protonated isomers are formed-(endo/endo, endo/exo, or exo/exo) which differ in position Of the to nickel. The endo/endo isomer is most productive isomer dne to the two protons being sufficiently close to the nickel proceed readily to the transition state to form/cleave H-2. Therefore, the rate of the (endo/endo, endo/exo, or exo/exo), isomers to generate the endoterido isomer can have an important impact on catalytic rates. We have found that the rate of isomenzation is limited by proton removal from, or delivery,to, the complex. In particular, the endo position is more-stencally. : hindered than the exo position, therefore, protonation exo to the metal is kinetically favOred over endoprotonation which leads to less catalytically productive pathway. In hydrogen oxidation, deprotonation of the sterically hindered endo position in external base may lead to slow catalytic turnover For hydrogen production catalysts, the limited accessibility of the endb.position: can result in the preferential formation of the exo protonated isomerS, which may undergo one or more isomerization.steps to generate the catalytically productive end protonafed isoMer. The results of these studies highlight the importance of precise proton delivery, and the Mechanistic details described herein will be used to -future catalyst design.