Rhodium Complexes in P-H Bond Activation Reactions

The feasibility of oxidative addition of the P−H bond of PHPh₂ to a series of rhodium complexes to give mononuclear hydrido‐phosphanido complexes has been analyzed. Three main scenarios have been found depending on the nature of the L ligand added to [Rh(Tp)(C₂H₄)(PHPh₂)] (Tp= hydridotris(pyrazolyl)borate): i) clean and quantitative reactions to terminal hydrido‐phosphanido complexes [RhTp(H)(PPh₂)(L)] (L=PMe₃, PMe₂Ph and PHPh₂), ii) equilibria between RhI and RhIII species: [RhTp(H)(PPh₂)(L)]⇄[RhTp(PHPh₂)(L)] (L=PMePh₂, PPh₃) and iii) a simple ethylene replacement to give the rhodium(I) complexes [Rh(κ²‐Tp)(L)(PHPh₂)] (L=NHCs‐type ligands). The position of the P−H oxidative addition–reductive elimination equilibrium is mainly determined by sterics influencing the entropy contribution of the reaction. When ethylene was used as a ligand, the unique rhodaphosphacyclobutane complex [Rh(Tp)(η¹‐Et)(κ^C,P‐CH₂CH₂PPh₂)] was obtained. DFT calculations revealed that the reaction proceeds through the rate limiting oxidative addition of the P−H bond, followed by a low‐barrier sequence of reaction steps involving ethylene insertion into the Rh−H and Rh−P bonds. In addition, oxidative addition of the P−H bond in OPHPh₂ to [Rh(Tp)(C₂H₄)(PHPh₂)] gave the related hydride complex [RhTp(H)(PHPh₂)(POPh₂)], but ethyl complexes resulted from hydride insertion into the Rh−ethylene bond in the reaction with [Rh(Tp)(C₂H₄)₂].