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

Abstract

The reaction of 2 equivalents of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene, (¹L^Ox)⁰, or 2-methyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene, (²L^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(¹L˙)₂] (1) and of [Ni^II(²L˙)₂] (3). The ligands (¹L˙)¹⁻ and (²L˙)¹⁻ are the one-electron reduced, monoanionic π radicals of the above neutral 1,4-diaza-1,3-butadienes. 1 and 3 have been structurally characterized by X-ray crystallography; both possess a distorted tetrahedral geometry where the dihedral angle θ between the two metallacycles Ni–N–C–C–N is 47.9° and 53°, respectively, (θ = 0° for square planar and 90° for a regular tetrahedral geometry). The reaction of 1 and 3 with 1 equivalent of ferrocenium hexafluorophosphate gives the paramagnetic (S_t = 1/2) complexes 2 and 4, respectively: [Ni^I(¹L^Ox)₂](PF₆) (2), [Ni^I(²L^Ox)₂](PF₆) (4). Their EPR spectra indicate the presence of a central Ni(I) ion (d⁹; S_Ni = 1/2). Thus, the one-electron oxidation of 1 and 3 by [Fc]PF₆ induces an intramolecular one-electron reduction of the central Ni ion and a concomitant one-electron oxidation of the second π radical (L˙)¹⁻ → (L^Ox)⁰ + e. Broken symmetry DFT calculations (B3LYP) corroborate the correctness of the electronic structure descriptions of 1–4. The reaction of (¹L^Ox)⁰ with NiI₂ (1 : 1) in tetrahydrofuran yields tetrahedral [Ni^II(¹L^Ox)I₂] (5) with an S_t = 1 ground state.