PNNL

Abstract

The series of complexes Ni(PtBu2NR2)2, [Ni(PtBu2NR2)2]BF4, [HNi(PtBu2NPh2)2]BF4, and [Co(PtBu2NPh2)2]BF4 (PtBu2NR2 = 1,5-dialkyl-3,7-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane; alkyl (R) = phenyl, benzyl) have been synthesized and characterized. Spectroscopic, electrochemical, and X-Ray diffraction studies indicate these complexes are stable as a result of the tetrahedral arrangement of the two diphosphine ligands. Electrochemical oxidation of [HNi(PtBu2NPh2)2]BF4 results in rapid proton transfer from nickel at a rate faster that can be observed on the CV timescale. Double protonation of Ni(PtBu2NBz2)2 forms the endo-endo, endo exo, and exo-exo isomers of [Ni(PtBu2NHBz2NBz2)2](BF4)2, which were found to be more stable towards loss of H2 than previously observed for similar complexes. The presence of Ni0•••HN bonds at the endo-protonation sites of [Ni(PtBu2NHBz2NBz2)2](BF4)2 results in significant differences in the Ni(I/0) oxidation potentials of each of the isomers. The differences in E1/2(I/0) values correspond to bond free energies of 7.4 and 3.7 kcal/mol for the first and second Ni0•••HN bonds of the endo-exo and endo-endo isomers, respectively. Computational studies of related model complexes reproduce these Ni0•••HN bonds within 1-2 kcal/mol. 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.