Mixed-Valent Polynuclear Cobalt Complexes Incorporating Tetradentate Phenoxyamine Ligands

The new potentially tetradentate ligand precursor 2-[(bis(2-hydroxyethyl)amino)methyl]-4,6-bis-tert-butylphenol (H3L) has been synthesized and characterized. The reaction of H3L with cobalt(II) acetate has afforded the novel mixed-valent tetra- and pentanuclear cobalt complexes [(Co2Co2III)-Co-II(OAc)(2)(L)(2)(HL)] (1) and [(CoCo4III)-Co-II(OAc)(2)(L)(4)] (2). Single-crystal X-ray diffraction studies of these complexes indicate different coordination geometries for the divalent cobalt centres in each complex, with distorted trigonal bipyramidal and distorted tetrahedral coordination evident in 1 and 2, respectively. The variable temperature magnetic susceptibility data for complex 1 reveal behaviour dominated by antiferromagnetic coupling between the two cobalt(II) centres. Their approximate trigonal bipyramidal coordination geometries give rise to a (4)A(2)' ground term, allowing a spin-only treatment assuming local spin quantum numbers of S-i = 3/2. Fitting the data to the Heisenberg exchange Hamiltonian ((H) over cap (ex) = -2JS(1).S-2) results in J = -6.9(1) cm(-1) and g = 2.30(5).