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, 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-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 superstoichiometric 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
multinuclear NMR, vibrational, and ultraviolet–visible (UV–vis)
spectroscopy, as well as single-crystal X-ray and elemental analysis.