Zinc bimetallics supported by a xanthene-bridged dinucleating ligand: Synthesis, characterization, and lactide polymerization studies

Abstract

A new, potentially dinucleating xanthene-bridged bis(iminophenolate) ligand L (L = 6,6′-((1E,1′E)-((2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(azanylylidene))bis(methanylylidene))bis(2,4-di-tert-butylphenol)) has been synthesized and its coordination chemistry with zinc precursors featuring alkoxide, chloride, and ethyl leaving groups has been investigated. The reaction of a zinc precursor bearing two bulky alkoxides, Zn(Cl)(μ2-OR)2Li(THF) (OR = di-tert-butyl-phenylmethoxide), formed a mononuclear complex Zn(L) that was isolated as an H-bond adduct with HOR, Zn(L)·HOR. In contrast, the reaction of L (or its lithium salt) with diethylzinc (or zinc chloride) led to the formation of the corresponding dinuclear complexes Zn2(L)(Et)2 and Zn2(L)(μ2-Cl)4Li2(OEt2)2. X-ray crystallography revealed syn-parallel geometry for Zn2(L)(Et)2 (Zn⋯Zn distance of 4.5 Å) and anti-parallel geometry for Zn2(L)(μ2-Cl)4Li2(OEt2)2 (Zn⋯Zn distance of 6.7 Å). Zn2(L)(Et)2 was found to be somewhat unstable, demonstrating decomposition into Zn(L) and ZnEt2; this decomposition can be reversed by the addition of excess ZnEt2. Treatment of Zn2(L)(Et)2 with benzyl alcohol (BnOH) in deuterated benzene, toluene, or dichloromethane resulted in the formation of Zn2(L)(OBn)2, which was characterized by 1H and 13C NMR spectroscopy. Zn2(L)(OBn)2 was found to be active in the ring-opening polymerization of rac-lactide to afford heterotactically inclined PLA.