Synthesis, Properties and Catalysis of Transition Metal Complexes with Bulky Pyridine-Carbene Ligand

Coordination of pyridine-imidazole-2-ylidene ligands (pyN^C-R) [R = Me, Mes], processing bulky substituents, toward rhodium (I) and iridium (I) metal ions has been investigated. The complexes [(C-pyN^C-R)M(COD)Cl] (8, M=Rh; 9, M=Ir) were prepared via trans- metallation from corresponding silver carbene complexes. Upon chloride abstraction, the chelation of pyN^C-R becomes feasible and results the formation of [(C,N-pyN^C-R)M(COD)]BF4 (11a, M= Rh, R= Mes; 11b, M= Rh, R = Me; 12a, M= Ir, R= Mes; 12b, M= Ir, R = Me). Furthermore, the palladium (II) complex [(C-pyN^C-Mes)Pd(C3H5)Cl] (10) was prepared by a similar method. Treatment of complex 10 with AgBF4 afforded a chelation species [(C,N-pyN^C-Mes)Pd(C3H5)]BF4 (13).
The coordinated COD of 3 and 4 can be replaced by carbon monoxide to yield the corresponding dicarbonyl species [(C,N-pyN^C-R) M(CO)2]BF4 (14, M= Rh; 15, M= Ir). Ligand substitution of 11a by phosphines and azide causes the de-complexation of pyridinyl-nitrogen donor to generate the corresponding carbene rhodium complexes [(C-pyN^C-Mes)Rh(COD)PR3] (17a, R = Ph, 17b, R = Et) and [(C-pyN^C-R)Rh(COD)N3] (19), respectively, indicating the labile nature of steric-hindered pyridinyl-nitrogen donor. Three equivalent of trimethylphosphite, a π-acid donor, can replace both COD and pyridinyl-nitrogen donor to form complex [(C-pyN^C-R)Rh{P(OMe)3}3] BF4 (18, R = Mes), and excess of bisphosphines replace all ligands to give [Rh(P^P)2]BF4. Palladium complexe 13 decomposes via the reaction with KOtBu to form Pd nanoparticles. The π-allyl donor was decomposed into propene when complex 13 was treated with concentrated HCl.
Complexes 11a and 17a are active catalysts for the hydroformylation of styrene derivatives with a chemoselectivity favored the branched ones. Substituted anilines and phenols containing ortho-ethynyl groups undergo cyclization in the presence of carbene iridium complexes to yield the indole or benzofuran, respectively. Hydrogen transfer reductions of nitroarenes catalyzed by iridium complexes have been investigated. Complex 12b was found to be the best catalyst for reduction of nitroarenes. Meanwhile, the diazo-compounds can be obtained as the exclusive product by manipulation through changing the concentration of base and substrates. Suzuki-Miyaura and Mizoroki-Heck reactions catalyzed by complex 13 were also examined. The activity of 13 appears to be less active as compared to those of diaminocarbene Pd(II) complexes reported in the literatures.