Syntheses, characterizations and catalytic properties of three zinc complexes and one lithium compound chelated by β-diketiminate ligands

Abstract

The syntheses, characterizations, and catalytic activities of three zinc complexes and one lithium compound based on β-diketiminate ligands are described. The reactions of ZnEt₂ with one equivalent of and two equivalents of ligand HL¹, which bears a large 2,4,6-tricyclopentylphenyl substituent, afforded compounds [(L¹)ZnEt] (1) and [(L¹)₂Zn] (2), respectively. The deprotonation of bis(β-diketiminato) ligand H₂L² by ⁿBuLi generated complex [(L²)Li₂(THF)₂] (3). Compound 3 reacted with anhydrous zinc chloride in a stoichiometric ratio of 1 : 2 to produce [(HL²)Zn₂Cl₃]·Tol (4 Tol). In compound 4 Tol, only one β-diketimine unit was deprotonated, and the two zinc atoms were bridged by a chloride atom. The catalytic activities of 1–4 toward the borylation of aryl iodides with 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bis(1,3,2-dioxa-borolane) (B₂Pin₂) were studied. Compounds 1 and 4 were both active for this coupling reaction, and complex 4 displayed much higher activity than that of 1. The borylation reactions feature a wide substrate scope, high functional group versatility and mild conditions. Compound 3 was not catalytically active toward the borylation reaction, but it showed excellent catalytic performance in the hydroboration of aldehydes and ketones by pinacol borane (HBpin), leading to a series of boric esters with high yield.