Redox Trends in Terpyridine Nickel Complexes

Abstract

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 [(tpy')Ni-I] (tpy' = 4,4',4''-tri-tert-butyl-terpyridine) but similar to [(tpy)Ni-CH(3)]. In the solid-state, EPR spectroscopy indicates, however, that unlike [(tpy)Ni-CH(3)], 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.

Exploring Mechanisms in Ni Terpyridine Catalyzed C–C Cross-Coupling Reactions—A Review

“…Unfortunately, EPR spectra were not reported. For [Ni(tpy)I], quantum chemical calculations show similar electron distribution as that found for the Br derivative [52]. In summary, it can be assumed that the largely superior σ-donor character of the Me or aryl coligands raise the d x 2 −y 2 orbital at the nickel above the lowest π* orbitals of the tpy ligand, and reduction leads to a (d 8 )(π* 1 ) configured species, while for the weaker ligands Cl, Br, and I, a (d 8 )(d x 2 −y 2 1 ) configuration is found [41,52].…”

“…EPR and quantum chemical calculations clearly point to a complex containing monovalent nickel [Ni(I)(tpy)Br] and a neutral tpy ligand, in marked contrast to the methyl and aryl derivatives, as described above [52].…”

“…The low-temperature solid-state powder EPR spectrum of [Ni(tpy)Br] • exhibits an axial signal with g = 2.256 and g ⊥ = 2.091 consistent with a metal-centered d x 2 −y 2 ground state [52]. In DMF solution, an isotropic signal can be observed with g iso = 2.139.…”

“…The complex [Ni(II)(tpy)2] 2+ is reversibly reduced in three one-electron steps, and can also be reversibly oxidized (Scheme 15) [40,41,52,[90][91][92][93]. For the oxidized species a [Ni(III)(tpy)2] 3+ character has recently been assumed from calculations [41]; however, further spectroscopic evidence is not available from this report.…”

“…In DMF solution, an isotropic signal can be observed with g iso = 2.139. Thus, both in the crystaline form and in solution, signals for a radical with substantial metal character are observed for [Ni(tpy)Br] • [52]. In contrast to this, [Ni(tpy)(Me)] • , exhibits an isotropic signal at 298 K with g iso = 2.021, and a rhombic spectrum with g 1 = 2.056, g 2 = 2.021, and g 3 = 1.999 at 77 K in a glassy frozen solution [44].…”