Synthesis and characterisation of trigonal C2-chiral di- and tetra-substituted bis(oxazoline) alkyl zinc complexes and their reactivity towards protic reagents

Abstract

A series of zinc(II) alkyl complexes stabilised by the C₂-chiral bis(oxazoline) ligand (^R₁,R₂BOX, with R₁ = (4S)-tBu, R₂ = H (a); R₁ = (4S)-Ph, R₂ = H (b); R₁ = (4R)-Ph, R₂ = (5S)-Ph (c)), has been synthesised and structurally characterised. (^R₁,R₂BOX)H ligands react with ZnEt₂ in toluene to give the heteroleptic three-coordinate compounds of (^R₁,R₂BOX)ZnEt, 1a, 1b and 1c in high yield. However, when the addition of (BOX)H ligands (a–b) over ZnEt₂ is “uncontrolled”, the formation of homoleptic four-coordinate compounds are favoured (2a–b), but not for the more sterically crowded ligand (c). The zinc-ethyl derivatives (1a–c) react readily with protic reagents such as acetic acid (HOAc) and methanol (MeOH). For compounds 1a–c a redistribution of ligands is observed leading preferentially to homoleptic compounds, except for the bulkier ligand c providing a three-coordinate complex identified as (^Ph,PhBOX)Zn(OMe), 4c. The reaction of acetylacetone (acacH) with compounds 1a–c leads straightforwardly to the more stable four-coordinate compounds corresponding to (^R₁,R₂BOX)Zn(η²-acac), 5a–c. The potential of these compounds as initiators for the copolymerisation of epoxides with CO₂ was investigated.