Cobalt(II) and cobalt(III) complexes of tripodal tetradentate diamino-bis(phenolate) ligands: Synthesis, characterization, crystal structures and evaluation in radical polymerization processes

Abstract

The synthesis and characterization of cobalt(II) (3, 4) and acetato cobalt(III) (5) complexes with two tripodal tetradentate diamino-bis(phenolate) ligands (L1 and L2) are reported. The solid-state structures of proligand H2L1 and complexes 4 and 5 were determined by X-ray crystallography. Complex 4 crystallizes as a dimer via the formation of a Co2O2 core. Complex 5 is monomeric with a six-coordinated cobalt(III) center bearing a chelating acetate ligand. In both structures, the diamino-bis(phenolate) ligand (L1 or L2) is acting as a tetradentate dianionic κ4-(O2,N2) chelating ligand. The diamagnetic nature of complex 5 allowed its characterization by multinuclear NMR spectroscopic techniques. Complex 5 is not a competent unimolecular initiator for the radical polymerization of methyl acrylate or methyl methacrylate. Its cobalt(II) analog (3) did not act as an efficient OMRP moderator for the radical polymerization of activated (styrene, tert-butyl acrylate) and non-activated (vinyl acetate) monomers.