Magnetic Properties of End-to-End Azide-Bridged Tetranuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complexes with Reduced Schiff Base Blocking Ligands and DFT Study

Two tetranuclear mixed-valence cobalt(III/II) complexes having the general formula [(mu(1,3)-N-3){Co-II(L-n)(mu-O2CC6H4NO2)Co-III(N-3)}(2)]PF6 (where H2L1 and H2L2 are two reduced Schiff base ligands) have been synthesized and characterized. The structures of both complexes show cobalt(II) and cobalt(III) centers with a distorted octahedral geometry with cobalt(III) and cobalt(II) centers located at the inner N2O2 and outer O-4 cavities of the reduced Schiff base ligands, respectively. The oxidation states of both cobalt centers have been confirmed by bond valence sum (BVS) calculations. The magnetic properties show that both compounds behave as cobalt(II) dimers connected through an end-to-end azido bridging ligand and show moderate antiferromagnetic Co(II)-Co(II) couplings of -11.0 and - 14.4 cm(-1) for 1 and 2, respectively, as also corroborated by DFT calculations, J(theo) = -13.07 cm(-1) for 1 and -12.49 cm(-1) for 2. The calculated spin densities of both complexes at the cobalt(II) centers are -2.75 and +2.75, respectively, clearly supporting that they are the magnetic centers.