Exploring structural, magnetic, and catalytic diversity in tetranuclear {Cu4O4} cubane cores and a dinuclear complex derived from closely related ligand systems

Abstract

The coordination compounds featuring a {Cu₄O₄} core, typically bridged by hydroxo or alkoxo groups, are particularly intriguing due to their notable magnetic properties and catalytic activity. In this study, we explored the synthesis and characterization of four new Schiff base ligands and their subsequent complexation with Cu^II salts, which resulted in the formation of three tetranuclear complexes: [Cu₄(L¹)₄]·2H₂O (1), [Cu₄(L²)₂(HL²)₂](Cl)(NO₃)·5H₂O (2), and [Cu₄(L³)₄] (3), as well as one dinuclear complex: [Cu₂(L⁴)₂] (4). These tetranuclear complexes all feature a {Cu₄O₄} core, but with differing coordination environments around the Cu^II centers. For instance, complex 1 exhibits μ₃-phenoxido bridges and a distorted octahedral geometry, while complex 3 features μ₃-alkoxido bridges and a square pyramidal geometry. Complex 2 displays an open cubane core with mixed μ₂-phenoxido and μ₃-alkoxido bridges, with both square planar (CuNO₄) and octahedral (CuNO₅) geometries. In addition, dinuclear complex 4 was synthesized, featuring square planar Cu^II centers linked by μ₂-alkoxido bridges. The magnetic studies revealed that 1 and 2 exhibit strong antiferromagnetic coupling, which is attributed to their larger Cu–O–Cu bond angles, while complex 3 demonstrates moderate ferromagnetic behavior, associated with smaller bond angles. Literature reports indicate that {Cu₄O₄} cubane cores generally show ferromagnetic interactions at bond angles near 104°, but in complex 3, we observed moderate ferromagnetic interactions with a Cu–O–Cu bridging angle of 108.41(9)°, making it one of the highest bond angles observed for ferromagnetic interactions in a {Cu₄O₄} cubane-like system. The planar dinuclear complex 4 exhibits extremely strong antiferromagnetic coupling, which is attributed to the ideal Cu–O–Cu bridging angles and the planar Cu–O–O–Cu core. Additionally, EPR measurements of complex 3 at 4 K reveal a well-isolated S = 2 ground state, separated by a gap of 65.8 cm⁻¹ from the three closest degenerate spin levels (S = 0, 1, and 1). Finally, all four complexes were used as catalysts for the aerobic oxidation of 2-aminophenol, and the mechanism of the oxidation process was elucidated by EPR spectroscopy.