Ternary Mononuclear and Ferromagnetically Coupled Dinuclear Copper(II) Complexes of 1,10-Phenanthroline and N-Salicylidene-2-methoxyaniline that Show Supramolecular Self-Organization

Abstract

The ternary copper(II) complex [Cu(phen)(L)](ClO₄) (1) and dinuclear copper(II) complex [Cu₂(phen)₃(L)(O₂CMe)](ClO₄)₂ (2) have been prepared and characterized structurally by X-ray crystallography (HL: N-salicylidene-2-methoxyaniline; phen: 1,10-phenanthroline). The crystal structure of 1 displays a square-pyramidal [4+1] coordination geometry in which the basal plane has three nitrogen donor atoms and the phenolate oxygen atom of the Schiff base. The methoxy oxygen atom exhibits axial coordination in the CuN₃O₂ chromophore. Complex 2 has a dinuclear copper(II) core in which the Cu(phen)²²⁺ and Cu(phen)(L)⁺ units are linked by an acetate ion showing equatorial/axial modes of bonding. In the bis(phen) unit, the basal plane has three nitrogen atoms and the acetate oxygen atom with one phen nitrogen atom occupying the axial site. In the mono(phen) unit, the basal plane has three nitrogen atoms and one phenolate oxygen atom. The acetate oxygen atom occupies the axial site. The equatorial/axial bridging mode of the acetate ion makes the dicopper(II) unit weakly ferromagnetic (2J = +22 cm^-1) and the complex in CH₂Cl₂ glass at 77 K shows an axial EPR spectrum [g_|| = 2.23 (A_|| = 150 × 10^-4 cm^-1); g = 2.03] corresponding to a δM_s = ±1 transition and a half-field signal due to a δM_s = ±2 transition. Complex 1 displays an axial EPR spectrum with g_|| > g_⊥ indicating a {d_x²-y²}¹ ground state. The complexes show a d-d band near 650 nm and a charge-transfer band at ca. 415 nm in methanol. The complexes are redox-active and exhibit a Cu^II/Cu^I couple near 0.0 V versus SCE in DMF and CH₂Cl₂/0.1 M TBAP. Complex 1 is catalytically active in the oxidation of ascorbic acid by dioxygen in aqueous methanol. Complex 2 also shows similar catalytic activity, but the core is susceptible to cleavage under the reaction conditions. The complexes show ∏-∏ stacking interactions. Complex 1 forms a one-dimensional chain through intermolecular ∏-∏ stacking interactions involving phen and the phenolate ring of the Schiff base. Complex 2 displays intramolecular and intermolecular ∏-∏ stacking interactions involving phen ligands leading to the formation of a supramspectrum with g_|| > g_⊥ indicating a {d_x²-y²}¹ ground state.