Facile synthesis of InIII-salen complexes from indium metal and their use as catalysts for the chemical fixation of carbon dioxide in mild conditions

An alternative, very simple and improved route to obtain indium(III) tetrahaloindate salts of tetrabutylammonium (TBA)[InX4] – 1a (X = Cl), 1b (X = Br) and 1c (X = I) – in high yields (74 – 82%) was developed. Indium(III) tetrahaloindates were then used as starting materials for the synthesis of In(salen)X complexes 2a (X = Cl, 88%), 2b (X = Br, 94%) and 2c (X = I, 88%). Compounds 2a-c were characterized by FT-IR, Raman, and 1H and 13C NMR spectroscopies. The In-salen complexes were able to promote the CO2 cycloaddition reaction with styrene oxide in mild and solvent-free conditions; complex 2a presented the best performance in the initial screening, achieving quantitative conversion (>99%). The parameters of the catalytic system were optimized, and a series of epoxides were tested as substrates for the production of cyclic carbonates with conversions varying from 38% to quantitative. The present study provides a simple method to cheaply and easily prepare indium(III) complexes and employ them to produce value-added chemicals from CO2.