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

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H ₃ L ² 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(L ² )] 1 in CH ₂ Cl ₂ 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(L ² )] ^z ( z = 0, 1+ and 1-). Oxidation of 1 with AgSbF ₆ affords [Ni(L ² )](SbF ₆ ) (2) and reduction of 1 with cobaltocene yields [Co(η ⁵ -C ₅ H ₅ ) ₂ ][Ni(L ² )] (3). The molecular structures of 1·CH ₃ CN, 2·0.5CH ₂ Cl ₂ and 3·C ₆ H ₆ have been determined by X-ray crystallography at 100 K. Characterization by ¹ H NMR, X-band EPR ( g _av = 2.006 (solid); 2.008 (CH ₂ Cl ₂ -C ₆ H ₅ CH ₃ glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [Ni ^II {(L ² )˙ ^2- }], [Ni ^II {(L ² ) ^- }] ⁺ /1 ⁺ and [Ni ^II {(L ² ) ^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.