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 [(μ1,3-N3)­{CoII(Ln)­(μ-O2CC6H4NO2)­CoIII(N3)}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 O4 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, Jtheo = −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.