Rhodium Complexes in P−H Bond Activation Reactions

The feasibility of oxidative addition of the P−H bond of PHPh 2 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)(C 2 H 4 )(PHPh 2 )] (Tp= hydridotris(pyrazolyl)borate): i) clean and quantitative reactions to terminal hydrido‐phosphanido complexes [RhTp(H)(PPh 2 )(L)] (L=PMe 3 , PMe 2 Ph and PHPh 2 ), ii) equilibria between Rh I and Rh III species: [RhTp(H)(PPh 2 )(L)]⇄[RhTp(PHPh 2 )(L)] (L=PMePh 2 , PPh 3 ) and iii) a simple ethylene replacement to give the rhodium(I) complexes [Rh(κ 2 ‐Tp)(L)(PHPh 2 )] (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)(η 1 ‐Et)(κ C,P ‐CH 2 CH 2 PPh 2 )] 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 OPHPh 2 to [Rh(Tp)(C 2 H 4 )(PHPh 2 )] gave the related hydride complex [RhTp(H)(PHPh 2 )(POPh 2 )], but ethyl complexes resulted from hydride insertion into the Rh−ethylene bond in the reaction with [Rh(Tp)(C 2 H 4 ) 2 ].