Rethinking the Lewis Acidity of Cationic Gallium and Indium Catalysts

Indium compounds are commonly considered weaker Lewis acids than their gallium counterparts. We explore the difference in Lewis acidity of the cationic dialkylgallium [GaR2(THF)(2)](+) and indium [InR2(THF)(n)](+) species and show that the gallium compounds are more active for the cationic polymerization of epoxide and epsilon-caprolactone. We rationalize this difference in reactivity by quantifying the relative Lewis acidity of the gallium and indium complexes. Experimental probes such as the Gutmann-Beckett and the fluorescent Lewis adduct method show that the gallium complexes are somewhat more Lewis acidic than the indium analogues, but not sufficiently to explain the major difference in reactivity. Computational studies suggest that, in solution, indium complexes exist in an equilibrium between [InR2(THF)(2)](+) and [InR2(THF)(3)](+) while the gallium species are stable as [GaR2(THF)(2)](+). Global Electrophilicity Index calculations for all possible THF coordinate compounds of alkylgallium and alkylindium show that indium species are consistently more Lewis acidic than their gallium counterparts with the same number of coordinated THF molecules. We propose that the slow reactivity of indium species is not due to weaker Lewis acidity but rather is due to their ability to accommodate additional donor substrates.