Bis(alpha-diimine)nickel complexes: molecular and electronic structure of three members of the electron-transfer series [Ni(L)(2)](z)() (z = 0, 1+, 2+) (L = 2-Phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene). A combined experimental and theoretical study.

From the reaction of Ni(COD)(2) (COD = cyclooctadiene) in dry diethylether with 2 equiv of 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene (L(Ox))(0) under an Ar atmosphere, dark red, diamagnetic microcrystals of [Ni(II)(L*)(2)] (1) were obtained where (L*)(1-) represents the pi radical anion of neutral (L(Ox))(0) and (L(Red))(2-) is the closed shell, doubly reduced form of (L(Ox))(0). Oxidation of 1 with 1 equiv of ferrocenium hexafluorophosphate in CH(2)Cl(2) yields a paramagnetic (S = 1/2), dark violet precipitate of Ni(I)(L(Ox))(2) (2) which represents an oxidatively induced reduction of the central nickel ion. From the same reaction but with 2 equiv of Fc in CH(2)Cl(2), light green crystals of Ni(II)(L(Ox))(2)(FPF(5)) (3) (S = 1) were obtained. If the same reaction was carried out in tetrahydrofuran, crystals of Ni(II)(L(Ox))(2)(THF)(FPF(5)) x THF (4) (S = 1) were obtained. Compounds 1, 2, 3, and 4 were structurally characterized by X-ray crystallography: 1 and 2 contain a tetrahedral neutral complex and a tetrahedral monocation, respectively, whereas 3 contains the five-coordinate cation Ni(II)(L(Ox))(2)(FPF(5)) with a weakly coordinated PF(6)(-) anion and in 4 the six-coordinate monocation Ni(II)(L(Ox))(2)(THF)(FPF(5)) is present. The electro- and magnetochemistry of 1-4 has been investigated by cyclic voltammetry and SQUID measurements. UV-vis and EPR spectroscopic data for all compounds are reported. The experimental results have been confirmed by broken symmetry DFT calculations of Ni(II)(L*)(2), Ni(I)(L(Ox))(2), and Ni(II)(L(Ox))(2) in comparison with calculations of the corresponding Zn complexes: Zn(II)((t)L(Ox))(2), Zn(II)((t)L(Ox))((t)L*), Zn(II)((t)L*)(2), and Zn(II)((t)L*)((t)L(Red)) where ((t)L(Ox))(0) represents the neutral ligand 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene and ((t)L*)(1-) and ((t)L(Red))(2-) are the corresponding one- and two-electron reduced forms. It is clearly established that the electronic structures of both paramagnetic monocations Ni(I)(L(Ox))(2) (S = 1/2) and Zn(II)((t)L(Ox))((t)(L*) (S = 1/2) are different.