Switchover from NiIIN2O2 to NiIIN2O2S2 coordination triggered by the redox behaviour of a non-innocent 2-aminophenolate ligand

Structural analysis of four-coordinate (S = 0) complex [Ni(L1)2] 1 (H2L1 = 4,6-di-tert-butyl-2-(2-benzylthiophenyl)aminophenol) revealed that it has approximately planar NiN2O2 coordination with dangling thioether arms (Mukherjee A and Mukherjee R 2005 Indian J Chem 50A 484–490). The two coordinated ligands are in 2-iminobenzosemiquinonate(1−) π radical (LISQ)·− redox level. Chemical oxidation of 1 by [Fe(η5-C5H5)2](PF6) in CH2Cl2 in the air led to oxidation of one of the (LISQ)·− ligands affording a six-coordinate Ni(II) complex [Ni(L1)2](PF6) 2 in mer configuration. The two tridentate ligands are in (LISQ)·− and 2-iminobenzoquinone (LIBQ)0 redox level (ligand mixed valency). The Ni–N/O/S distances and ligand backbone metrical parameters led us to assign unambiguously the electronic structure of 11+/2 as [NiII{(LISQ)·−}{(LIBQ)0}]1+. The EPR spectral signal of 2 at 120 K exhibits a very large g anisotropy and the magnetic measurement indicates an S = 3/2 ground state. The potential shift observed in going from 1 to coulometrically-generated 12+ is marginal. DFT calculations at the CAM-B3LYP level of theory rationalizes the electronic structure of 1, 11+ and 12+. Time-Dependent (TD)-DFT calculations throw light on the nature of observed absorptions of 1, 11+ and 12+. One-electron oxidation of four-coordinate [NiII(L1)2] 1 (S = 0) Causes additional coordination by two dangling thioethers affording six-coordinate [NiII(L1)2](PF6) 2 (S = 3/2); characterization of 2 has been done by X-ray structural analysis, MOS values, magnetism (ferromagnetic coupling), absorption and EPR spectra, and DFT and TD-DFT calculations.