Electronic structure of the members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) and [NiL(X)](n) (X = Cl, CO, P(OCH(3))(3)) species containing a tetradentate, redox-noninnocent, Schiff base macrocyclic ligand L: an experimental and density functional theoretical study.

The electronic structure of the four members of the electron transfer series NiL (z = 3+, 2+, 1+, 0) have been established experimentally (EPR spectroscopy and X-ray crystallography) and by density functional theoretical (DFT) calculations using the B3LYP functional in conjunction with a conductor-like screening model (COSMO) for acetonitrile solvent effects. L represents a generic designation of the tetradentate macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]-heptadeca-1(17),2,11,13,15-pentane where the true oxidation level is not specified; (L(Ox))(0) represents its neutral form, (L )(1-) is the one-electron reduced pi radical anion, and (L(Red))(2-) is the singlet (or triplet) diradical dianion of this ligand. It is shown that the above series consists of square planar Ni(III)(L(Ox)) (S = 1/2), Ni(II)(L(Ox)) (S = 0), Ni(II)(L ) (S = 1/2), Ni(II)(L(Red)) (S = 0). The structure of Ni(II)(L(Red)) has been determined by X-ray crystallography. The electrochemistry of Ni(II)(L(Ox))(2) in the presence of hard chloride anions shows the presence of trans-Ni(III)(L(Ox))Cl(2), the EPR spectrum of which has been recorded and calculated, and of trans-Ni(II)(L(Ox))Cl(2) (S = 1). Upon further reduction the coordinated Cl(-) ligands dissociate and Ni(II)(L ) and Ni(II)(L(Red)) are successively generated. Similarly, in the presence of good pi-acceptor ligands such as CO or P(OCH(3))(3) the following five-coordinate, square base pyramidal species are found to be stable: Ni(I)(L(Ox))(X) (S = 1/2), Ni(I)(L )(X) (S = 0, 1) (X = CO, P(OCH(3))(3)). As shown by EPR spectroscopy in the work of J. Lewis and M. Schröder, J. Chem. Soc., Dalton Trans., 1982, 1085, the monocations consist of a central nickel(i) ion (d(9), S(Ni) = 1/2). These spectra have been faithfully reproduced by the calculations. The neutral complexes Ni(I)(L )(X) are singlet or triplet diradicals comprising a central nickel(i) ion and a pi radical anion (L )(1-). Interestingly, six-coordinate species trans-Ni(L)(X)(2) (n = 2+, 1+, 0) are computationally not stable in the gas phase or in solution. No experimental evidence has been found for their existence.