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, epsilon-caprolactone and beta-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 center dot center dot center dot 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 degrees C, and a small heterotactic enrichment was observed for the latter. Compounds 3 and 4 provided the best control of the polymerization of beta-BL in toluene at 70 degrees C in toluene, and compound 3 provided the best control of the polymerization of epsilon-CL in toluene at 70 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.