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.