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

• Synthesis of six new methylindium dithiolate complexes incorporating a series of tetradentate dithiolate ligands.
• Structural characterization of four and preliminary data for two compounds, revealing two new strained bonding environments for indium.
• Computational studies to account for observed structures (i.e. monomer vs. dimer vs. trimer), and to quantify degrees of strain.
• Exploration of reactivity through ROP studies of cyclic esters and analysis with respect to metal bonding environments.

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.

Figure optionsDownload as PowerPoint slide