Redox Trends in Terpyridine Nickel Complexes

A synthesis has been developed that allows the isolation of four- coordinate [(tpy)Ni-Br] (1, tpy = terpyridine) in high yield. Complex 1 has been structurally characterized, and the X-ray data reveal a square-planar geometry, unlike the    known    [(tpy0 )Ni-I]    (tpy0    =    4,40 ,400 -tri-tert-butyl-terpyridine)    but    similar    to [(tpy)Ni-CH3]. In the solid-state, EPR spectroscopy indicates, however, that unlike [(tpy)Ni-CH3], the electronic structure of 1 is a metal-centered, not a ligand- centered radical. Density functional theory (DFT) analyses support this assignment. The preparation of 1 also facilitated the analysis of the redox potentials of a series of terpyridine nickel derivatives. It was found that the overall ligand sphere (one vs two coordinated terpyridine ligands) plays more of a role in determining the redox potentials of these derivatives than do the formal oxidation states of the nickel ions in the solution phase.