Ligand Attachment Chemistry in the Preparation of PCsp3P and PCsp2P Complexes of Rhodium

The attachment of electron-rich PCP pincer ligands bis­(2-(dialkylphosphino)­phenyl)­methane (alkyl = isopropyl, tert-butyl) to rhodium via reactions with [(COE)2Rh­(μ-X)]2 (X = Cl, OSO2CF3) through C–H bond activations is reported. The first C–H activation to produce PCsp3P derivatives is facile and favors products wherein the remaining benzylic C–H and the Rh–H hydrogens are trans-disposed across the new Rh–C bond. For the less bulky isopropyl-substituted ligand, chlorido- or triflato-bridged dinuclear products are favored, while for the tert-butyl-adorned ligand, monomers are formed. The favoring of both trans C–H/Rh–H and dinuclear systems hampers the second C–H activation, necessary to form the (more desirable) PCcarbeneP derivatives. Through spectroscopic and structural investigations, the factors that influence the ligand attachment chemistry through successive C–H activations in these ligands are discussed.