Hemilabile N‑Xylyl‑N′‑methylperimidine Carbene Iridium Complexes as Catalysts for C–H Activation and Dehydrogenative Silylation: Dual Role of N‑Xylyl Moiety for ortho-C–H Bond Activation and Reductive Bond Cleavage

Direct dehydrogenative silylation of pyridyl and iminyl substrates with triethylsilane was achieved using (L)­Ir­(cod)­(X) (1) (L = a perimidine-based carbene ligand, X = OAc and OCOPh) complexes as catalysts under toluene refluxing conditions in the presence of norbornene as a hydrogen scavenger, and the silylated products were obtained in good yields. The isolated bis­(cyclometalated)iridium complexes, (C∧C:)­(C∧N)­IrOAc (2) (C∧C: = a cyclometalated perimidine-carbene ligand and C∧N = a cyclometalated pyridyl- and iminyl-ligated aromatic substrate), were key intermediates, where cyclometalated five-membered metallacycles of substrates such as phenylpyridine were selectively formed before yielding mono-ortho-silylation products. The bis­(cyclometalated)­iridium complex (XyC∧C:)­(C∧N)­IrOAc (2d) (XyC∧C: = a cyclometalated N-xylyl-N′-methylperimidine-carbene ligand and C∧N = a 2-pyridylphenyl ligand), reacted with 2 equiv of Et3SiH to give an iridium hydride complex, (L4)­(C∧N)­Ir­(H)­(SiEt3) (8d) (L4 = N-CH3, N-3,5-(CH3)2C6H3 perimidine), via demetalation of a N-3,5-xylyl ring of the carbene ligand of 2d. The formation of 8d was confirmed by isolating the corresponding chloro complex (L4)­(C∧N)­Ir­(Cl)­(SiEt3) (8d-Cl) by treatment with CCl4. The N-methyl moiety of the carbene ligand coordinated to 8d was cyclometalated in the presence of norbornene at room temperature to afford (MeC∧C:)­(C∧N)­Ir­(SiEt3) (10d) (MeC∧C: = a cyclometalated N-xylyl-N′-methylperimidine-carbene), while at high temperature 8d reacted with norbornene and Et3SiH to afford the silylated product, 2-(2-triethylsilyl)­phenylpyridine (3a) and norbornane. A deuterium labeling experiment using 2d and Et3SiD (excess) revealed the incorporation of deuterium atoms at two ortho-positions of the N-xylyl group (>90%) and at the 3-position of 2-pyridylphenyl ligand (ca. 40%) within 3 h at room temperature, indicating that the cyclometalation/demetalation of the N-xylylperimidine carbene and 2-phenylpyridine ligands were reversible processes. Isolation of these cyclometalated iridium complexes under controlled conditions and D-labeling experiments thus revealed a dual function of the N-aryl group bound to the perimidine-carbene ligand, which acted as both a neutral carbene ligand and a monoanionic ortho-metalated aryl-carbene ligand through reversible C–H bond activation and Ir–C bond cleavage of the N-aryl group during the catalytic cycle.