Interaction of alkynes with palladium POCOP-pincer hydride complexes and its unexpected relation to palladium-catalyzed hydrogenation of alkynes

Abstract

Palladium POCOP-pincer hydride complexes [2,6-(R₂PO)₂C₆H₃]PdH (R = ^tBu, 2a; R = ⁱPr, 2b; R = ^cPe, 2c, ^cPe = cyclopentyl) have been synthesized from [2,6-(R₂PO)₂C₆H₃]PdCl (1a–c) and LiAlH₄ or LiBEt₃H. These hydride complexes react with phenylacetylene to afford H₂, [2,6-(R₂PO)₂C₆H₃]PdCCPh (3a–c) and a small amount of styrene. When the R groups are isopropyl groups, a second palladium species is generated, and it has been identified as an alkenyl complex (E)-[2,6-(ⁱPr₂PO)₂C₆H₃]PdCHCHPh (4b). Mechanistic studies have shown that decomposition of these palladium pincer complexes and related palladium methyl complexes [2,6-(R₂PO)₂C₆H₃]PdCH₃ (5a–c) occurs at room temperature in the presence of H₂ (1 atm or lower), resulting in the leaching of palladium particles. These particles have been shown to catalyze the hydrogenation of phenylacetylene and diphenylacetylene to their alkene and alkane products. A mechanism for the formation of palladium particles has been proposed. The structures of 1a, 1c, 2a, 2c, 3a, 4b and 5b have been studied by X-ray crystallography.