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 C(2)-chiral bis(oxazoline) ligand ((R(1),R(2))BOX, with R(1) = (4S)-tBu, R(2) = H (a); R(1) = (4S)-Ph, R(2) = H (b); R(1) = (4R)-Ph, R(2) = (5S)-Ph (c)), has been synthesised and structurally characterised. ((R(1),R(2))BOX)H ligands react with ZnEt(2) in toluene to give the heteroleptic three-coordinate compounds of ((R(1),R(2))BOX)ZnEt, 1a, 1b and 1c in high yield. However, when the addition of (BOX)H ligands (a-b) over ZnEt(2) 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,Ph)BOX)Zn(OMe), 4c. The reaction of acetylacetone (acacH) with compounds 1a-c leads straightforwardly to the more stable four-coordinate compounds corresponding to ((R(1),R(2))BOX)Zn(η(2)-acac), 5a-c. The potential of these compounds as initiators for the copolymerisation of epoxides with CO(2) was investigated.