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

Abstract

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˙)¹⁻ is the one-electron reduced π 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˙)]¹⁺ (S = 1/2), [Ni^II(L^Red)]⁰ (S = 0). The structure of [Ni^II(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)Cl₂]⁺, the EPR spectrum of which has been recorded and calculated, and of trans-[Ni^II(L^Ox)Cl₂]⁰ (S = 1). Upon further reduction the coordinated Cl⁻ ligands dissociate and [Ni^II(L˙)]¹⁺ and [Ni^II(L^Red)]⁰ are successively generated. Similarly, in the presence of good π-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˙)(X)]⁰ (S = 0, 1) (X = CO, P(OCH₃)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 (d⁹, S_Ni = 1/2). These spectra have been faithfully reproduced by the calculations. The neutral complexes [Ni^I(L˙)(X)]⁰ are singlet or triplet diradicals comprising a central nickel(I) ion and a π radical anion (L˙)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.