From triplesalen to triplesalacen: Synthesis spectroscopic, redox, and magnetic properties of the trinuclear CuII3 triplesalacen complex

The tris(tetradentate) triplesalacen ligand H6talacen and its trinuclear copper complex, namely [(talacen)CuII3], were synthesized and its molecular and electronic structure determined. The ligand is prepared by the triple condensation of 2,4,6-trisacetyl-1,3,5-trihydroxybenzene (1) and excess acacen half-unit (2). The molecular structure of [(talacen)CuII3] exhibits a strong ligand folding around the central CuII-phenolate bonds. This ligand folding is stronger than that observed in the trinuclear copper triplesalen complexes. The electronic absorption spectrum exhibits a prominent intense band at 32 400 cm−1 and d–d transitions around 19 000 cm−1. These spectral features are related to the ligand folding, which opens an OPh(pz)– Cu II ( d x 2 - y 2 ) bonding pathway. Electrochemical studies provide an irreversible oxidation at a relatively low potential of 0.17 V versus Fc+/Fc, which is assigned to a ligand-centered oxidation of the central phloroglucinol backbone. A ferromagnetic coupling of J = 1.20 cm−1 ( H = ∑ i < j - 2 J ij S i S j ) is established in agreement with the spin-polarization mechanism. A critical analysis of the bond distances of the central phloroglucinol unit provides evidence that not only the usual phenolate-imine but also the keto-enamide mesomeric form contribute to the resonance hybrid. This might explain the relatively weak ferromagnetic coupling.