Di-, tri- and tetranuclear nickel(II) complexes with oximato bridges: magnetism and catecholase-like activity of two tetranuclear complexes possessing rhombic topology

Abstract

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 [Ni₂L¹₂](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.