Shuttling of Nickel Oxidation States in N4S2 Coordination Geometry versus Donor Strength of Tridentate N2S Donor Ligands

• Published in: Inorganic chemistry 2012-07, Vol.51 (14), p.7625-7635
• Main Authors: Chatterjee, Sudip K, Roy, Suprakash, Barman, Suman Kumar, Maji, Ram Chandra, Olmstead, Marilyn M, Patra, Apurba K
• Format: Article
• Language: English
• Subjects: Ligands; Models, Molecular; Molecular Structure; Nickel - chemistry; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Oxidation-Reduction
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic300606g

Seven bis-NiII complexes of a N2S donor set ligand have been synthesized and examined for their ability to stabilize Ni0, NiI, NiII and NiIII oxidation states. Compounds 1–5 consist of modifications of the pyridine ring of the tridentate Schiff base ligand, 2-pyridyl-N-(2′-methylthiophenyl)methyleneimine (XL1), 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 XL1, the Ni–Npy distances for 1∼5 < 3 < 4 increase and UV–vis λmax values corresponding to the 3A2g(F)→3T2g(F) transition show an increasing trend 1∼5 < 2 < 3 < 4. Cyclic voltammetry of 1–5 reveals two quasi-reversible reduction waves that correspond to NiII→NiI and NiI→Ni0 reduction. The E 1/2 for the NiII/NiI 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 (Δ E p = 60 mV) that corresponds to NiII→NiIII. The electrochemically generated NiIII species, [(L3)2NiIII]+ is stable, showing a new UV–vis band at 470 nm. EPR measurements have also been carried out.