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

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-(i)Pr(2)C(6)H(3)) are complicated by the side formation of the bis(amidinate) tin(ii) compound, MeC(NAr)(2)](2)Sn. By contrast, the bulkier tert-butylamidinate, (t)BuC(NAr)(2)](-), allows tin(ii) mono-halide, -alkoxide and -amide complexes to be isolated cleanly in high yields. Thus, the reaction of (t)BuC(NAr)(2)]H with (n)BuLi and subsequent treatment with SnCl(2) generates (t)BuC(NAr)(2)]SnCl, in ca. 70% yield. Reactions of with LiO(i)Pr, LiNMe(2) and LiNTMS(2) afford (t)BuC(NAr)(2)]Sn(O(i)Pr), (t)BuC(NAr)(2)]Sn(NMe(2)), and (t)BuC(NAr)(2)]Sn(NTMS(2)), respectively. The molecular structures of complexes are reported. Complexes, and have been investigated as initiators for the ring-opening polymerisation of rac-lactide: and display characteristics of well-controlled polymerisation initiators, but high molecular weight polymer is observed with due to inefficient initiation, a consequence of the steric bulk of the NTMS(2) unit. Polymerisations with and are faster than for the corresponding beta-diketiminate tin(ii) complexes, consistent with the more open nature of the tin(ii) coordination sphere.