Tert-butylamidinate tin(ii) complexes: high activity, single-site initiators for the controlled production of polylactide

Abstract

The tin(II) coordination chemistry of two monoanionic N,N′-bis(2,6-diisopropylphenyl)alkylamidinate ligands is described. Complexation studies with the acetamidinate, [MeC(NAr)₂]⁻, (Ar = 2,6-ⁱPr₂C₆H₃) are complicated by the side formation of the bis(amidinate) tin(II) compound, [MeC(NAr)₂]₂Sn, 1. By contrast, the bulkier tert-butylamidinate, [^tBuC(NAr)₂]⁻, allows tin(II) mono-halide, -alkoxide and -amide complexes to be isolated cleanly in high yields. Thus, the reaction of [^tBuC(NAr)₂]H with ⁿBuLi and subsequent treatment with SnCl₂ generates [^tBuC(NAr)₂]SnCl, 2, in ca. 70% yield. Reactions of 2 with LiOⁱPr, LiNMe₂ and LiNTMS₂ afford [^tBuC(NAr)₂]Sn(OⁱPr), 3, [^tBuC(NAr)₂]Sn(NMe₂), 4, and [^tBuC(NAr)₂]Sn(NTMS₂), 5, respectively. The molecular structures of complexes 1–4 are reported. Complexes 3, 4 and 5 have been investigated as initiators for the ring-opening polymerisation of rac-lactide: 3 and 4 display characteristics of well-controlled polymerisation initiators, but high molecular weight polymer is observed with 5 due to inefficient initiation, a consequence of the steric bulk of the NTMS₂ unit. Polymerisations with 3 and 4 are faster than for the corresponding β-diketiminate tin(II) complexes, consistent with the more open nature of the tin(II) coordination sphere.