Neutral bis(1,4-diaza-1,3-butadiene)nickel complexes and their corresponding monocations: molecular and electronic structures. A combined experimental and density functional theoretical study.

The reaction of 2 equivalents of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene, ((1)L(Ox))(0), or 2-methyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene, ((2)L(Ox))(0), in diethyl ether or n-hexane with 1 equivalent of Ni(cdt) where (cdt)(0) is the ligand cyclododecatriene affords dark red, diamagnetic precipitates of [Ni(II)((1)L )(2)] () and of [Ni(II)((2)L )(2)] (). The ligands ((1)L )(1-) and ((2)L )(1-) are the one-electron reduced, monoanionic pi radicals of the above neutral 1,4-diaza-1,3-butadienes. and have been structurally characterized by X-ray crystallography; both possess a distorted tetrahedral geometry where the dihedral angle theta between the two metallacycles Ni-N-C-C-N is 47.9 degrees and 53 degrees , respectively, (theta = 0 degrees for square planar and 90 degrees for a regular tetrahedral geometry). The reaction of and with 1 equivalent of ferrocenium hexafluorophosphate gives the paramagnetic (S(t) = 1/2) complexes and , respectively: [Ni(I)((1)L(Ox))(2)](PF(6)) (), [Ni(I)((2)L(Ox))(2)](PF(6)) (). Their EPR spectra indicate the presence of a central Ni(i) ion (d(9); S(Ni) = 1/2). Thus, the one-electron oxidation of and by [Fc]PF(6) induces an intramolecular one-electron reduction of the central Ni ion and a concomitant one-electron oxidation of the second pi radical (L )(1-)--> (L(Ox))(0) + e. Broken symmetry DFT calculations (B3LYP) corroborate the correctness of the electronic structure descriptions of . The reaction of ((1)L(Ox))(0) with NiI(2) (1 : 1) in tetrahydrofuran yields tetrahedral [Ni(II)((1)L(Ox))I(2)] () with an S(t) = 1 ground state.