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(OCH3)(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) (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-center dot)(1-) is the one-electron reduced p 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)](3+) (S = 1/2), [Ni-II(L-Ox)](2+) (S = 0), [Ni-II(L-center dot)](1+) (S = 1/2), [Ni-II(L-Red)](0) (S = 0). The structure of [NiII(L-Red)](0) has been determined by X-ray crystallography. The electrochemistry of [Ni-II(L-Ox)](PF6)(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](0) (S = 1). Upon further reduction the coordinated Cl- ligands dissociate and [Ni-II(L-center dot)](1+) and [Ni-II(L-Red)](0) are successively generated. Similarly, in the presence of good pi-acceptor ligands such as CO or P(OCH3)(3) the following five-coordinate, square base pyramidal species are found to be stable: [Ni-I(L-Ox)(X)](1+) (S = 1/2), [Ni-I(L-center dot)(X)](0) (S = 0, 1) (X = CO, P(OCH3)(3)). As shown by EPR spectroscopy in the work of J. Lewis and M. Schroder, 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-center dot)(X)](0) are singlet or triplet diradicals comprising a central nickel(I) ion and a p radical anion (L-center dot)(1-). Interestingly, six-coordinate species trans-[Ni(L)(X)(2)](n) (n = 2+, 1+, 0) are computationally not stable in the gas phase or in solution. No experimental evidence has been found for their existence.