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-(Pr2C6H3)-Pr-i) are complicated by the side formation of the bis( amidinate) tin( II) compound, [ MeC( NAr)(2)](2)Sn, 1. By contrast, the bulkier tert-butylamidinate, [(BuC)-Bu-t(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 [(BuC)-Bu-t( NAr)(2)] SnCl, 2, in ca. 70% yield. Reactions of 2 with (LiOPr)-Pr-i, LiNMe2 and LiNTMS2 afford [(BuC)-Bu-t(NAr)(2)] Sn(OiPr), 3, [(BuC)-Bu-t(NAr)(2)]Sn(NMe2), 4, and [(BuC)-Bu-t(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 beta-diketiminate tin(II) complexes, consistent with the more open nature of the tin( II) coordination sphere.