Di‑, Tri‑, and Tetranuclear Nickel(II) Complexes with Oximato Bridges: Magnetism and Catecholase-like Activity of Two Tetranuclear Complexes Possessing Rhombic Topology

• Published in: Inorganic chemistry Vol. 52; no. 20; pp. 11744 - 11757
• Main Authors: Das, Lakshmi Kanta, Biswas, Apurba, Kinyon, Jared S, Dalal, Naresh S, Zhou, Haidong, Ghosh, Ashutosh
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 21.10.2013
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic401020m

Oxime-based tridentate Schiff base ligands 3-[2-(diethylamino)ethylimino]butan-2-one oxime (HL1) and 3-[3-(dimethylamino)propylimino]butan-2-one oxime (HL2) produced the dinuclear complex [Ni2L1 2](ClO4)2 (1) and trinuclear complex [Ni3(HL2)3(μ3-O)]­(ClO4)4·CH3CN (2), respectively, upon reaction with Ni(ClO4)2·6H2O. However, in a slightly alkaline medium, both of the ligands underwent hydrolysis and resulted in tetranuclear complexes [{Ni(deen)(H2O)}2(μ3-OH)2{Ni2(moda)4}](ClO4)2·2CH3CN (3) and [{Ni(dmpn)(CH3CN)2}2(μ3-OH)2{Ni2(moda)4}](ClO4)2·CH3CN (4), where deen = 2-(diethylamino)ethylamine, dmpn = 3-(dimethylamino)-1-propylamine, and modaH = diacetyl monoxime. All four complexes have been structurally characterized. Complex 1 is a centrosymmetric dimer where the square planar nickel(II) atoms are joined solely by the oximato bridges. In complex 2, three square planar nickel atoms form a triangular core through a central oxido (μ3-O) and peripheral oximato bridges. Tetranuclear complexes 3 and 4 consist of four distorted octahedral nickel(II) ions held together in a rhombic chair arrangement by two central μ3-OH and four peripheral oximato bridges. Magnetic susceptibility measurements indicated that dinuclear 1 and trinuclear 2 exhibited diamagnetic behavior, while tetranuclear complexes 3 and 4 were found to have dominant antiferromagnetic intramolecular coupling with concomitant ferromagnetic interactions. Despite its singlet ground state, both 3 and 4 serve as useful examples of Kahn’s model for competing spin interactions. High-frequency EPR studies were also attempted, but no signal was detected, likely due to the large energy gap between the ground and first excited state. Complexes 3 and 4 exhibited excellent catecholase-like activity in the aerial oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone, whereas 1 and 2 did not show such catalytic activity. Kinetic data analyses of this oxidation reaction in acetonitrile revealed that the catalytic activity of 3 (k cat = 278.4 h–1) was slightly lower than that of 4 (k cat = 300.0 h–1). X-band EPR spectroscopy indicated that the reaction proceeded through the formation of iminoxyl-type radicals.