Strained metal bonding environments in methylindium dithiolates and their reactivity as initiators for the ring-opening polymerization of cyclic esters

We have synthesized indium complexes containing a variety of metal bonding environments through use of polyfunctional dithiolate ligands and examined their reactivity as initiators for the ring-opening polymerization of l-lactide, rac-lactide, ε-caprolactone and β-butyrolactone. The facile reaction of Me3In with the corresponding polyfunctional dithiols in toluene, thf or diethyl ether resulted in the formation of [MeIn(SOOS)]2 (3), MeIn(SNNS) (4), [MeIn(ONS2)]3 (5), MeIn(NNS2) (6), MeIn(NNS2Pr) (7) and MeIn(pyrS)2 (8). The solid-state structures of 3 and 5 each show the corresponding ligand to be tridentate with an uncoordinated ligand O atom. Dimeric (3) and trimeric (5) structures result from short intermolecular In … S interactions. All structures show five coordinate indium centres in distorted trigonal bipyramidal bonding environments, but with various arrangements of donor atoms (eq/ax): SSC/OS (3,5), SNC/NS (4), SSN/NC (6), SSC/NS (7) and SSC/NN (8). DFT studies of model MeIn(SMe)2(NH3)2 systems show the bonding environments in 4 and 6 to be highly strained, while the axial In–Me bond of 6 shows the longest bond distance and lowest vibrational frequency. Compound 5 provided the best control of the polymerization of l-lactide and rac-lactide in THF at 70 °C, and a small heterotactic enrichment was observed for the latter. Compounds 3 and 4 provided the best control of the polymerization of β-BL in toluene at 70 °C in toluene, and compound 3 provided the best control of the polymerization of ε-CL in toluene at 70 °C. In all cases, polymerization rates were low. This work demonstrates a systematic approach to exploring the modification and reactivity of main group metal bonding motifs, which has resulted in identification of two novel “strained” bonding environments for indium.