Binuclear Nickel and Copper Complexes with Bridging 2,5-Diamino-1,4-benzoquinonediimines: Synthesis, Structures, and Catalytic Olefin Polymerization

A series of binuclear, divalent nickel and copper acetylacetonato complexes of the type [M(acac){µ-C6H2(NAr)4}M(acac)] (M = Ni, Cu) have been synthesized by reaction of the corresponding M(acac)2 precursor with various bulky steric hindrace π-acceptor N-substituted 2,5-diamino-1,4-benzoquinonediimines C6H2(NHAr)2(=NAr)2 (1a, Ar = 4-C6H4Me; 1b, Ar = 2-C6H4Me; 1c, Ar = 2,6-C6H3Me2), which are metalated and become bridging ligands. The ligands and complexes were determined by IR and UV–visible spectra and element analysis. Cyclic voltammetric behavior of complexes 2c and 3c has been tested. The molecular structures of the ligand 1c and the complexes [Ni(acac){µ-C6H2(N(4-methyl-Ph))4}Ni(acac)]) (2a), [Ni(acac){µ-C6H2(N(2,6-dimethyl-Ph))4}Ni(acac)]) (2c), and [Cu(acac){µ-6H2(N(2,6-dimethyl-Ph))4}Cu(acac)]) (3c) have been determined by X-ray diffraction. The coordination geometry around the metal ions of the Ni and Cu complexes is square-planar, and a complete electronic delocalization of the quinonoid π-system occurs between the metal centers over the two NC–CCN halves of the ligand. In the presence of MAO as cocatalyst, all the Ni complexes exhibited high activities both for addition polymerization of norbornene and for methyl methacrylate (MMA) polymerization, which produce syndiotactic-rich poly(methyl methacrylate) (PMMA) with broad molecular weight distribution; however, the Cu complexes show moderate activities for norbornene polymerization and are inactive for MMA polymerization.