Inorganic Chemistry, Vol.51, No.14, 7625-7635, 2012
Shuttling of Nickel Oxidation States in N4S2 Coordination Geometry versus Donor Strength of Tridentate N2S Donor Ligands
Seven bis-Ni-II complexes of a N2S donor set ligand have been synthesized and examined for their ability to stabilize Ni-0, Ni-I, Ni-II and Ni-III oxidation states. Compounds 1-5 consist of modifications of the pyridine ring of the tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine ((X)L1), where X = 6-H, 6-Me, 6-p-ClPh, 6-Br, 5-Br; compound 6 is the reduced amine form (L2); compound 7 is the amide analog (L3). The compounds are perchlorate salts except for 7, which is neutral. Complexes 1 and 3-7 have been structurally characterized. Their coordination geometry is distorted octahedral. In the case of 6, the tridentate ligand coordinates in a facial manner, whereas the remaining complexes display meridional coordination. Due to substitution of the pyridine ring of (X)L1, the Ni-N-py distances for 1 similar to 5 < 3 < 4 increase and UV-vis lambda(max) values corresponding to the (3)A(2g)(F) -> T-3(2g)(F) transition show an increasing trend 1 similar to 5 < 2 < 3 < 4. Cyclic voltammetry of 1-5 reveals two quasi-reversible reduction waves that correspond to Ni-II -> Ni-I and NiI -> Ni-0 reduction. The E-1/2 for the Ni-II/Ni-I couple decreases as 1 > 2 > 3 > 4. Replacement of the central imine N donor in 1 by amine 6 or amide 7 N donors reveals that complex 6 in CH3CN exhibits an irreversible reductive response at E-pc = -1.28 V, E-pa = +0.25 V vs saturated calomel electrode (SCE). In contrast, complex 7 shows a reversible oxidation wave at E-1/2 = +0.84 V (Delta E-p = 60 mV) that corresponds to Ni-II -> Ni-III. The electrochemically generated Ni-III species, [(L3)(2)Ni-III](+) is stable, showing a new UV-vis band at 470 nm. EPR measurements have also been carried out.