Cationic PCP and PCN NHC Core Pincer-Type Mn(I) Complexes: From Synthesis to Catalysis

The coordination chemistry of NHC core pincer-type PCP and PCN ligands with a Mn(I) center was systematically investigated. The reaction of [Mn(CO)(5)Br] with free carbenes, conveniently prepared from the corresponding imidazolium salts and a strong base, leads to the initial formation of bidentate fac[MnBr(CO)(3) (kappa P-2,C,L)] complexes bearing a pending phosphine or pyridine donor extremity (L), which upon bromide abstraction, afford cationic [Mn(CO)(3)(kappa P-3,C,L)](OTf) derivatives exhibiting a meridional or a facial coordinating geometry of tridentate NHC core ligand depending on its relative flexibility. All bi- and tridentate Mn(I) complexes were characterized by X-ray diffraction analysis. The impact of the tridentate ligand structure on catalytic performance was illustrated in the Mn-catalyzed hydrogenation of acetophenone revealing fac-[Mn(CO)(3)(kappa P-3,C,N)](OTO based on a 5,6-membered metallacyclic architecture to be the most active, thus evidencing the role of the pyridine arm lability in the catalytic cycle.