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.