Ligand redox controlled amine dehydrogenation and imine hemilability in singlet diradical azo-aromatic Ni(ii) complexes: characterization of the electron transfer series of azo-imine complexes of Ni(ii)

Abstract

Herein, using azo-amine (H₂L) and azo-imine (L^1–3) ligands, singlet diradical Ni(II) complexes [1] and [2] were synthesized from Ni(0)(COD)₂ in THF. In separate reactions, homoleptic Ni^II complexes, [3a]²⁺–[3c]²⁺, were synthesized from [Ni^II(H₂O)₆](ClO₄)₂ and L^1–3, respectively. All these complexes were characterized thoroughly. The X-ray structures of [1] and [2] showed that the amine side arm in [1] and the imine side arm in [2] are de-coordinated. The d_N–N lengths in these two complexes were found to be ∼1.32 Å, which corresponds to the one-electron reduced azo-bond length. These complexes, [1] and [2], showed ¹H NMR signals characteristic of diamagnetic compounds. These studies, along with DFT calculations, indicated that the unpaired spins on ligands coupled antiferromagnetically with the two unpaired spins on Ni^II to result in s = 0 ground states. Complex [1] showed ligand-based redox-induced dehydrogenation of the distal amine side arm to result in L¹. Complexes [3a]²⁺–[3c]²⁺ have d_N–N lengths of ∼1.27 Å and d_C–N lengths of ∼1.28 Å. In cyclic voltammetry, complex [3a]²⁺ showed four well-resolved reversible reductive waves at 0.5 V to −1.6 V in dichloromethane. The first two waves became irreversible when they were measured in acetonitrile solution. The electron transfer series of [3a]²⁺ was further characterized by spectro-electrochemistry, EPR, and DFT calculations. These showed that all the reductions were associated with the ligand. It was further probed by redox events in complexes [3b]²⁺ and [3c]²⁺. While the electron donor –OMe group on the phenyl ring of the azo moiety in [3b]²⁺ showed a prominent cathodic shift of the potentials, the –F substitution on the phenyl group on the imine side arm of [3c]²⁺ has almost no effect. It has to be noted here that the oxidation of [2] by two electrons returns it back to complex [3a]²⁺. Reduction of [3a]²⁺ by two electrons also resulted in complex [2], indicating reversible ligand redox-induced hemilability of the imine moiety between [3a]²⁺ and [2]. Moreover, characterization of the electron transfer series of [3a]²⁺ and [2] has shown superior redox non-innocent behaviour and coordination ability of the azo-pyridine moiety in nickel(II) complexes over the imino-pyridine moiety of the ligand.