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

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 [Ni2L21](ClO4)(2) (1) and trinuclear complex [Ni-3(HL2)(3)(mu(3)-O)] (ClO4)(4)center dot CH3CN (2), respectively, upon reaction with Ni(ClO4)(2)center dot 6H(2)O. However, in a slightly alkaline medium, both of the ligands underwent hydrolysis and resulted in tetranuclear complexes [{Ni(deen)(H2O)}(2)(mu(3)-OH)(2){Ni-2(moda)(4)}](ClO4)(2)center dot 2CH(3)CN (3) and [(Ni(dmpn)(CH3CN)(2)}(2)(mu(3)-OH)(2){Ni-2(moda)(4)}](ClO4)(2)center dot 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(11) atoms are joined solely by the oximato bridges. In complex 2, three square planar nickel atoms form a triangular core through a central oxido (mu(3)-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 mu(3)-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.