Ferromagnetic Coupling by Spin Polarization in a Trinuclear Copper(II) Metallacyclophane with a Triangular Cage-Like Structure

A series of trinuclear copper(II) complexes of general formula A6[Cu3L2]·nH2O [L = benzene-1,3,5-tris(oxamate); A = Li+ (n = 8), 1a; Na+ (n = 11.5), 1b; and K+ (n = 8.5), 1c] have been synthesized, and they have been structurally and magnetically characterized. X-ray diffraction on single crystals of 1c shows the presence of three square-planar copper(II)−bis(oxamato) moieties which are connected by a double benzene-1,3,5-triyl skeleton to give a unique metallacyclophane-type triangular cage. The copper basal planes are virtually orthogonal to the two benzene rings, which adopt an almost perfect face-to-face alignment. Complexes 1a−c exhibit a quartet (S = 3/2) ground spin state resulting from the moderate ferromagnetic coupling (J values in the range of +7.3 to +16.5 cm−1) between the three CuII ions across the two benzene-1,3,5-tris(amidate) bridges [H = −J(S1·S2 + S2·S3 + S3·S1) with S1 = S2 = S3 = SCu = 1/2]. Density functional theory calculations on the S = 3/2 CuII3 ground spin state of 1c support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction, as evidenced by the sign alternation of the spin density in the 1,3,5-substituted benzene spacers.