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

The series of complexes Ni­(PtBu2NR2)2, [Ni­(PtBu2NR2)2]­BF4, [HNi­(PtBu2NR2)2]­BF4, and [Co­(PtBu2NPh2)2]­BF4 (PtBu2NR2 = 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(PtBu2NPh2)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(PtBu2NBn2)2 forms the endo-endo, endo-exo, and exo-exo isomers of [Ni­(PtBu2NBnHNBn)2]­(BF4)2, which were found to be more stable toward loss of H2 than previously observed for similar complexes. The presence of Ni0···HN hydrogen bonds at the endo protonation sites of [Ni­(PtBu2NBnHNBn)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 hydrogen bonds of the endo-exo and endo-endo isomers, respectively. Computational studies give bond dissociation energies of the Ni0···HN bonds that are within 1–2 kcal/mol of the experimentally determined values.