Complexes of Bis-ortho-cyclometalated Bisphosphinoaryl Ruthenium(II) Cations with Neutral Meta-bisphosphinoarene Ligands Containg an Agostic C-H•••Ru Interaction

Abstract

The synthesis, characterization, and reactivity of various ruthenium(II) complexes [Ru(R-PCP)(R'-PCHP)][OTf] are presented. These complexes are highly dissymmetric and show fluxional behavior in solution. They contain one monoanionic, 3-P,C,P'-tridentate-bonded bisphosphinoaryl ligand (R-PCP = [C6H2(CH2PPh2)2-2,6-R-4]-; R = H or Br) and one neutral, trans-P,P'-bidentate-bonded bisphosphinoarene ligand (R'-PCHP = C6H3(CH2PPh2)2-3,5-R'-1). The C-H bond that is ortho to both CH2PPh2 substituents of the bidentate-bonded ligand is interacting with the metal center (i.e., has an agostic contact with the cationic ruthenium center). The location of the agostic proton in the 1H NMR spectrum as well as the three-dimensional structure of these complexes in solution was obtained by means of several two-dimensional NMR techniques (1H-1H COSY, 31P-1H COSY, 13C-1H COSY, 1H NOESY, 1H ROESY, and 31P EXSY). The 1JHC of the agostic contact amounted to 112 Hz, which represents a reduction of 46 Hz in relation with the same coupling for this C-H bond in the free ligand. Furthermore, the molecular geometry of [Ru(H-PCP)(PCHP)][OTf] in the solid state was established by single-crystal X-ray diffraction techniques, and the structural features corroborate with the observations in solution. In the complex [Ru(H-PCP)(Br-PCHP)][OTf], interconversion of the cyclometalated and neutral bisphosphine ligands via exchange of the agostic hydrogen atom from the neutral to the cyclometalated carbon to yield [Ru(Br-PCP)(H-PCHP)][OTf] was observed. A mechanistic proposal based on an aromatic electrophilic substitution is discussed. Moreover, the reaction of [Ru(OSO2CF3)(NCN)(PPh3)] (NCN = [C6H3(CH2NMe2)2-2,6]-) with the neutral bisphosphinoarene resulted in the quantitative formation of the new cyclometalated species [Ru(R-PCP)(R-PCHP)][OTf] with the net loss of the neutral bisaminoarene ligand NCHN and PPh3.