[Cu{(L)˙}] (HL: thioether-appended o-aminophenol ligand) monocation triggers change in donor site from NO to NOS and valence-tautomerism.

Using a potentially tridentate o-aminophenol-based redox-active ligand HL (2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) in its deprotonated form, [Cu(L)] has been synthesized and crystallized as [Cu(L)]·CHCl (1·CHCl). A cyclic voltammetry experiment (in CHCl; V vs. SCE (saturated calomel electrode)) on 1·CHCl exhibits two oxidative (E = 0.20 V (peak-to-peak separation, ΔE = 100 mV) and E = 0.90 V (ΔE = 140 mV)) and two reductive (E = -0.52 V (ΔE = 110 mV) and E = -0.92 V (ΔE = 120 mV)) responses. Upon oxidation using a stoichiometric amount of Fe(η-CH), 1·CHCl yielded Cu(L) (2). Structural analysis (100 K) reveals that 1·CHCl is a four-coordinate bis(iminosemiquinonato)copper(ii) complex (CuNO coordination), and that the thioethers remain uncoordinated. The twisted geometry of 1 (distorted tetrahedral) results in considerable changes in the electronic structure, compared to well-known square-planar analogues. Crystallographic analysis of 2 both at 100 K and at 293 K reveals that it is effectively a four-coordinate complex with a CuNOS coordination; however, a substantial interaction with the other phenolate O is observed. The metal-ligand bond distances and metric parameters associated with the o-aminophenolate rings indicate a valence-tautomeric (VT) equilibrium involving monocationic (iminosemiquinonato)(iminoquinone)copper(ii) and bis(iminoquinone)copper(i). Complex 1·CHCl is a three-spin system and a magnetic study (4-300 K) established that it has a S = 1/2 ground-state, owing to the strong antiferromagnetic coupling between the unpaired spin of the copper(ii) and the iminosemiquinonate(1-) π-radical anion. Electron paramagnetic resonance (EPR) spectral studies corroborate this result. Complex 2 is diamagnetic and the existence of VT in 2 was probed using variable-temperature (248-328 K) H NMR and EPR (100-298 K) spectral measurements and X-ray photoelectron spectroscopic studies at 298 K. Remarkably, modification of the well-studied 2-anilino-4,6-di-tert-butylphenol by incorporation of a benzylthioether arm leads to the occurrence of VT in 2. The electronic structure of 1·CHCl and 2 has been assigned using density functional theory (DFT) calculations at the B3LYP-D3 level of theory. Time-dependent (TD)-DFT calculations have been performed to elucidate the origin of the observed UV-VIS-NIR absorptions.