Synthesis and X-ray characterization of [RhCl(C2H4)(PiPr3)]2. Multinuclear NMR and DFT investigation of its solid-state and solution reaction with dihydrogen. Ethene and propene hydrogenation by the solid Rh-hydrides

Abstract

[RhCl(C₂H₄)(PiPr₃)]₂ has been synthesised and structurally characterised. It shows an unusual planar Rh₂Cl₂ system, with the two phosphine and the two ethene molecules disposed trans to each other. It easily reacts with dihydrogen in solution and in the solid state to afford ethane and a dimeric complex of formula [RhCl(H)₂P]₂. The latter exhibits a high fluxionality both in solution and in the solid state making impossible the identification of a single limiting structure even at 183 K. 25 different hydride structures are possible, four of which have very low interconversion barriers (1–3 kcal mol⁻¹) and cannot be blocked even at 183 K. A multinuclear NMR study and HECTOR experiments have allowed to identify the most probable structures that interconvert at 198 K. The solid hydrides are able to hydrogenate quite selectively ethene and propene at room temperature. A TON higher than 5600 and a TOF of 97 h⁻¹ was observed with ethene (for propene the values were 5200 and 79, respectively). Toluene was hydrogenated to methylcyclohexane at 323 K under an atmospheric pressure of H₂ with a TOF h⁻¹ equal to 15.