Cu-Pd-Cu and Cu-Pt-Cu linear frameworks: synthesis, magnetic properties, and theoretical analysis of two mixed-metal complexes of dipyridylamide (dpa), isostructural, and isoelectronic with [Cu3(dpa)4Cl2]+.

The synthesis and crystal structure of two heteronuclear compounds stabilized by four dipyridylamide (dpa) ligands is reported. Cu2Pd(dpa)4Cl2 (1) and Cu2Pt(dpa)4Cl2 (2) exhibit an approximate D4 symmetry and a linear metal framework. They are structurally similar to the homotrinuclear complexes M3(dpa)4L2 already characterized with various transition metals (M=Cr, Co, Ni, Cu, Rh, Ru). With 26 metal valence electrons, they are also isoelectronic to the oxidized form of the tricopper complex [Cu3(dpa)4Cl2]+ (3), previously characterized and investigated by Berry et al.10 The magnetic properties and the EPR spectra of 1 and 2 are reported. The results for 1 are interpreted in terms of a weak antiferromagnetic interaction (2J=-7.45 cm(-1) within the framework of the Heisenberg Hamiltonian H=-2JAB ŝAŝB) between the Cu(II) magnetic centers. For 2, the antiferromagnetic interaction sharply decreases to <1 cm(-1). These properties are at variance with those of (3), for which a relatively strong antiferromagnetic interaction (2J=-34 cm(-1)) had been reported. DFT/UB3LYP calculations reproduce the decrease of the magnetic interaction from 3 to 1 and assign it to the role of the nonmagnetic metal in the transference of the superexchange coupling. However, the vanishing of the magnetic interaction in 2 could not be reproduced at this level of theory and is tentatively assigned to spin-orbit coupling.