Structure and properties of bivalent nickel and copper complexes with pyrazine-amide-thioether coordination: stabilization of trivalent nickel

Abstract

Acyclic pyrazine-2-carboxamide and thioether containing hexadentate ligand 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H₂bpzctb), in its deprotonated form, has afforded light brown [Ni^II(bpzctb)] (1) (S = 1) and green [Cu^II(bpzctb)] (2) (S = 1/2) complexes. The crystal structures of 1·CH₃OH and 2·CH₂Cl₂ revealed that in these complexes the ligand coordinates in a hexadentate mode, affording examples of distorted octahedral M^IIN₂(pyrazine)N′₂(amide)S₂(thioether) coordination. Each complex exhibits in CH₂Cl₂ a reversible to quasireversible cyclic voltammetric response, corresponding to the Ni^III/Ni^II (1) and Cu^II/Cu^I (2) redox process. The E_1/2 values reveal that the complexes of bpzctb(2−) are uniformly more anodic by ∼0.2 V than those of the corresponding complexes with the analogous pyridine ligand, 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane (H₂bpctb), attesting that compared to pyridine, pyrazine is a better stabilizer of the Ni(II) or Cu(I) state. Coulometric oxidation of the previously reported complex [Ni^II(bpctb)] and 1 generates [Ni^III(bpctb)]⁺ and [Ni^III(bpzctb)]⁺ species, which exhibit a LMCT transition in the 470–480 nm region and axial EPR spectra corresponding to a tetragonally elongated octahedral geometry. Complex 2 exhibits EPR spectra characteristic of the d_z² ground state.