Charge distribution in bis-dioxolene radical metal complexes. Synthesis and DFT characterization of dinuclear Co(III) and Cr(III) complexes with a mixed-valent, S=1/2 semiquinone-catecholate ligand

Bis-dioxolene bridged dinuclear metal complexes of general formula M-2(CTH)(2)(diox-diox)(PF6)(n) (n = 2, 3; M = Co(III), Cr(III); CTH = tetraazamacrocycle) have been synthesized using the bis-bidentate ligand 5,5'-di-tert-butyl-3,3',3,4'-tetrahydroxybiphenyl. These complexes were characterized by means of ESR, UV-vis, temperature dependent magnetic susceptibility,and cyclic voltammetry. Our results unambiguously suggest that the tripositive dimetal cations can be described as containing a fully delocalized bis-dioxolene trinegative radical ligand (Cat-Sq) bridging two tripositive metal cations. In this frame the sextet electronic ground state characterizes the Cr-2(CTH)(2)(Cat-SQ)(3+) as a result of the antiferromagnetic coupling of the radical bridging ligand with the two equivalent paramagnetic metal centers. The electronic and geometrical structure: and the magnetic properties of Cat-Sq and of its complexes have been studied with density functional theory.