Bis(α-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-center dot)(2)] (1) were obtained where (L-center dot)(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 CH2Cl2 yields a paramagnetic (S = (1)/(2)), dark violet precipitate of [Ni-I(L-Ox)(2)](PF6) (2) which represents an oxidatively induced reduction of the central nickel ion. From the same reaction but with 2 equiv of [Fc](PF6) in CH2Cl2, light green crystals of [Ni-II(L-Ox)(2)(FPF5)](PF6) (3) (S = 1) were obtained. If the same reaction was carried out in tetrahydrofuran, crystals of [Ni-II(L-Ox)(2)(THF)(FPF5)](PF6)center dot 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)(FPF5)](+) with a weakly coordinated PF6- anion and in 4 the six-coordinate monocation [Ni-II(L-Ox)(2)(THF)(FPF5)](+) 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-center dot)(2)](0), [Ni-I(L-Ox)(2)](+), and [Ni-II(L-Ox)(2)](2+) in comparison with calculations of the corresponding Zn complexes: [Zn-II(L-t(Ox))(2)](2+), [Zn-II(L-t(Ox))(L-t(center dot))](+), [Zn-II(L-t(center dot))(2)](0), and [Zn-II(L-t(center dot))(L-t(Red))](-) where (L-t(Ox))(0) represents the neutral ligand 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene and (L-t(center dot))(1-) and (L-t(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(L-t(Ox))(L-t(center dot))](+) (S = (1)/(2)) are different.