Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes.

The synthesis of P,N-phosphaalkene ligands, py-CH═PMes* ( 1 , py = 2-pyridyl, Mes* = 2,4,6- t Bu-C 6 H 2 ) and the novel quin-CH═PMes* ( 2 , quin = 2-quinolinyl) is described. The reaction with [Rh(μ-Cl)cod] 2 produces Rh(I) bis(phosphaalkene) chlorido complexes 3 and 4 with distorted trigonal bipyramidal coordination environments. Complexes 3 and 4 show a pronounced metal-to-ligand charge transfer (MLCT) from Rh into the ligand P═C π* orbitals. Upon heating, quinoline-based complex 4 undergoes twofold C-H bond activation at the o - t Bu groups of the Mes* substituents to yield the cationic bis(phosphaindane) Rh(I) complex 5 , which could not be observed for the pyridine-based analogue 3 . Using sub- or superstoichiometric amounts of AgOTf the C-H bond activation at an o - t Bu group of one or at both Mes* was detected, respectively. Density functional theory (DFT) studies suggest an oxidative proton shift pathway as an alternative to a previously reported high-barrier oxidative addition at Rh(I). The Rh(I) mono- and bis(phosphaindane) triflate complexes 6 and 7 , respectively, undergo deprotonation at the benzylic CH 2 group of the phosphaindane unit in the presence of KO t Bu to furnish neutral, distorted square-planar Rh(I) complexes 8 and 9 , respectively, with one of the P,N ligands being dearomatized. All complexes were fully characterized, including multinuclear NMR, vibrational, and ultraviolet-visible (UV-vis) spectroscopy, as well as single-crystal X-ray and elemental analysis.