Cis-Trans Isomerism and Hydride Thermodynamics Govern H2 Activation and Hydrogenation Activity in Indium(III) Pincer...

Trivalent boron Lewis acids activate H2 for catalytic hydrogenations, yet only one trivalent indium analog is known to do so. Nevertheless, heavier main-group elements also form five-, six-, and seven-coordinate compounds. However, complexes of Ga and In generally fail at hydrogenation catalysis, without clear guiding principles for their design. We show that coordination geometry, hydride thermodynamics, and the counteranion govern catalytic imine hydrogenation at In(III) pincer complexes. Cationic five-coordinate In complexes bearing NNN-, PNN-, and PNP-pincer ligands with [InX4]- anions catalyze this reaction under 120 °C and 15 bar of H2. Catalytic activity correlated with the Gibbs free energy of H2 activation. This endergonic step determined turnover. Rather than classical Lewis acidity descriptors, hydride and fluoride affinity, and Gutmann-Beckett acidity, cis-trans isomerism of key intermediates controlled H2 activation thermodynamics. Indium complexes outperformed lighter group-13 Al and Ga congeners not for their higher Lewis acidity, but for their more favorable hydride formation thermodynamics. The ligands modulated Lewis base binding without overcoming hydride thermodynamic limitations. Finally, the [InX4]- anions may activate the imine substrate toward reduction. Our findings explain the hydrogenation activity of indium Lewis acids, establishing coordination geometry, hydride thermodynamics, and counteranion as key parameters for their rational design.