Stabilization of Nickel Complexes with Ni0•••H-N Bonding Interactions Using Sterically Demanding Cyclic Diphosphine Ligands

The series of complexes Ni((P2N2R)-N-tBu)(2), [Ni((P2N2R)-N-tBu)(2)]BF4, [HNi((P2N2R)-N-tBu)(2)]BF4, and [Co((P2N2Ph)-N-tBu)(2)]BF4 ((P2N2R)-N-tBu = 1,5-dialkyl-3,7-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane; alkyl = 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((P2N2Ph)-N-tBu)(2)]BF4 results in rapid proton transfer from nickel at a rate faster than can be observed on the CV time scale. Double protonation of Ni((P2N2Bn)-N-tBu)(2) forms the endo-endo, endo-exo, and exo-exo isomers of [Ni((P2NHNBn)-N-tBu-H-Bn)(2)](BF4)(2) , which were found to be more stable toward loss of H-2 than previously observed for similar complexes. The presence of Ni-0 center dot center dot center dot HN hydrogen bonds at the endo protonation sites of [Ni((P2NHNBn)-N-tBu-H-Bn)(2)](BF4)(2) results in significant differences in the Ni(I/O) oxidation potentials of each of the isomers. The differences in E-1/2(I/0) values correspond to bond free energies of 7.4 and 3.7 kcal/mol for the first and second Ni-0 center dot center dot center dot HN hydrogen bonds of the endo-exo and endo-endo isomers, respectively. Computational studies give bond dissociation energies of the Ni-0 center dot center dot center dot HN bonds that are within 1-2 kcal/mol of the experimentally determined values.