Ni(ii) complexes of a new tetradentate NN′N′′O picolinoyl-1,2-phenylenediamide-phenolate redox-active ligand at different redox levels

Abstract

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H₃L² in its deprotonated form, at three redox levels, open-shell semiquinonate(1−) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N′ and N′′ (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L²)] 1 in CH₂Cl₂ established a three-membered electron-transfer series (oxidative response at E_1/2 = 0.57 V and reductive response at −0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L²)]^z (z = 0, 1+ and 1−). Oxidation of 1 with AgSbF₆ affords [Ni(L²)](SbF₆) (2) and reduction of 1 with cobaltocene yields [Co(η⁵-C₅H₅)₂][Ni(L²)] (3). The molecular structures of 1·CH₃CN, 2·0.5CH₂Cl₂ and 3·C₆H₆ have been determined by X-ray crystallography at 100 K. Characterization by ¹H NMR, X-band EPR (g_av = 2.006 (solid); 2.008 (CH₂Cl₂–C₆H₅CH₃ glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [Ni^II{(L²)˙²⁻}], [Ni^II{(L²)⁻}]⁺/1⁺ and [Ni^II{(L²)³⁻}]⁻/1⁻ electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic S_t (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state S_t = 0. Interestingly, the variable-temperature (2–300 K) magnetic measurement reveals that 1 with the S_t = 1/2 ground state attains the antiferromagnetic S_t = 0 state at a very low temperature, due to weak noncovalent interactions via π–π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800–1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions.