Synthesis and structural characterization of cyclic indium thiolate complexes and their utility as initiators for the ring-opening polymerization of cyclic esters

We have synthesized indium complexes incorporating bifunctional thiolate ligands and examined their utility as initiators for the ring-opening polymerization of rac-lactide and ε-caprolactone. The facile reaction of Me3In with the corresponding bifunctional thiols in diethyl ether or thf resulted in the formation of [MeIn(SCH2C(O)OMe)2]2 (5), [MeIn(SCH2CH2NMe2)2] (6) or [Me2In(SCH2C(O)OMe)] (7). The solid-state structure of 5 is dimeric via short intermolecular In···S interactions, yielding an asymmetric In2S2 core. One pendant and one chelating methyl thioglycolate ligand gives a distorted trigonal bipyramidal S3OC bonding environment for indium. Compound 6 shows a bicyclic monomeric structure with a distorted trigonal bipyramidal S2N2C bonding environment for indium. Compound 5 polymerized bulk rac-lactide rapidly with high conversion, but yielded broad PDIs and low MWs. Solution polymerizations using one equivalent of benzyl alcohol per metal centre were reasonably well controlled at 70 °C, though molecular weights were lower than theoretical values. Compound 6 was also an efficient mediator of bulk rac-lactide polymerization when initiated by benzyl alcohol, reaching >90% conversion in 15 min. Molecular weights were in excellent agreement with the theoretical values and the PDIs were narrow. Solution polymerizations utilizing 6 in conjunction with benzyl alcohol were much slower than the analogous reactions using 5. Compound 5 was less efficient at controlling the ROP of ε-caprolactone versus rac-lactide, while 6 was inactive towards ε-caprolactone under a variety of conditions. This work represents the first study of indium thiolate complexes for the ROP of cyclic esters, and contains rare examples of structurally characterized organoindium bis(thiolate) compounds, the first to be prepared via the hydrocarbon elimination reaction.