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 H3L2 in its deprotonated form, at three redox levels, open-shell semiquinonate(1-) pi 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 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(L-2)](z) (z = 0, 1+ and 1-). Oxidation of 1 with AgSbF6 affords (Ni(L-2)](SbF6) (2) and reduction of 1 with cobaltocene yields (Co(eta(5)-C5H5)(2)](Ni(L-2)) (3). The molecular structures of 1 center dot CH3CN, 2 center dot 0.5CH(2)O(2) and 3 center dot C6H6 have been determined by X-ray crystallography at 100 K. Characterization by H-1 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{(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 pi-pi 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.