Ni(II) complexes of a new tetradentate NN′NO 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.

Automated summary

Three square planar nickel (II) complexes of a new asymmetric tetradentate redox-active ligand H3L2 in its deprotonated form have been synthesized, and their electronic states and properties have been characterized. The experimental results are rationalized by density functional theory (DFT) calculations. The UV-VIS-NIR spectra and magnetic measurements provide insights into the electronic transitions and magnetic behaviors of the complexes.