Rhodium Complexes in P-H Bond Activation Reactions.

Abstract

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)]: i) clean and quantitative reactions to terminal hydrido-phosphanido complexes [RhTp(H)(PPh2)(L)] (L = PMe3, PMe2Ph and PHPh2), ii) equilibria between Rh(I) and Rh(III) species: [RhTp(H)(PPh2)(L)] eq. [RhTp(PHPh2)(L)] (L = PMePh2, PPh3) and iii) a simple ethylene replacement to give the rhodium(I) complexes [Rh(k2-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. If ethylene was used as ligand, the unique rhodaphosphacyclobutane complex [Rh(Tp)(h1-Et)(kC,P-CH2CH2PPh2)] was obtained. DFT-calculations revealed that the reaction proceeds via a 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].