Bis(phosphinite)-Supported Palladium Catalysts for Homogeneous Dehydrogenation of Formic Acid

Abstract

Palladium-catalyzed dehydrogenation of formic acid under homogeneous conditions is rare. In most cases, decomposition of palladium complexes is observed under the reaction conditions. In this study, bis(phosphinite)-supported palladium(II) complexes, {2,6-(R2PO)2C6H3}PdX [R = iPr, tBu; X = H (1a–b), OCHO (2a–b)], were examined for catalytic formic acid dehydrogenation (FAD). Their derivative complexes, {2,6-(R2PO)2C6H3}PdCH3 (3a–b), were found to be precatalysts. During catalysis, complex degradation was observed; however, by optimizing the reaction conditions (80 °C, ethanol), a homogeneous and additive-free procedure for FAD was achieved. Alcohols and dihydrogen were identified as the possible sources of catalyst decomposition. Mechanistic investigations reveal that the protonation of {2,6-(R2PO)2C6H3}PdH (1a–b) is both kinetically and thermodynamically favored, which drives FAD while suppressing catalyst degradation pathways. It was found that the sterically crowded {2,6-(tBu2PO)2C6H3}PdCH3 (3b) is a more effective catalyst compared to {2,6-(iPr2PO)2C6H3}PdCH3 (3a). Experimental data show that CO2 elimination, the rate-limiting step of the catalytic cycle, occurs more rapidly from {2,6-(tBu2PO)2C6H3}PdOCHO (2b) than from {2,6-(iPr2PO)2C6H3}PdOCHO (2a), thereby accelerating the overall reaction rate.