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-tBu-C6H2) and the novel quin-CH==PMes* (2, quill = 2-quinolinyl) is described. The reaction with [Rh(mu-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 pi* orbitals. Upon heating, quinoline-based complex 4 undergoes twofold C-H bond activation at the o-tBu 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 superstoichio- metric amounts of AgOTf the C-H bond activation at an o-tBu 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 CH2 group of the phosphaindane unit in the presence of KOtBu 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 multinudear NMR, vibrational, and ultraviolet-visible (UV-vis) spectroscopy, as well as single-crystal X-ray and elemental analysis.