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

The series of complexes Ni(P t Bu 2 N R 2 ) 2, [Ni(P t Bu 2 N R 2 ) 2 ]BF 4, [HNi(P t Bu 2 N R 2 ) 2 ]BF 4, and [Co(P t Bu 2 N Ph 2 ) 2 ]BF 4 (P t Bu 2 N R 2 = 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(P t Bu 2 N Ph 2 ) 2 ]BF 4 results in rapid proton transfer from nickel at a rate faster than can be observed on the CV time scale. Double protonation of Ni(P t Bu 2 N Bn 2 ) 2 forms the endo-endo, endo-exo, and exo-exo isomers of [Ni(P t Bu 2 N Bn HN Bn ) 2 ](BF 4 ) 2, which were found to be more stable toward loss of H 2 than previously observed for similar complexes. The presence of Ni 0 ···HN hydrogen bonds at the endo protonation sites of [Ni(P t Bu 2 N Bn HN Bn ) 2 ](BF 4 ) 2 results in significant differences in the Ni(I/0) 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 ···HN hydrogen bonds of the endo-exo and endo-endo isomers, respectively. Computational studies give bond dissociation energies of the Ni 0 ···HN bonds that are within 1–2 kcal/mol of the experimentally determined values.