TY - JOUR
T1 - C-H bond activation by cationic iridium(III) NHC complexes
T2 - A combined experimental and computational study
AU - Meredith, Joseph M.
AU - Robinson, Robert
AU - Goldberg, Karen I.
AU - Kaminsky, Werner
AU - Heinekey, D. Michael
PY - 2012/3/12
Y1 - 2012/3/12
N2 - The cationic complexes [Cp*Ir(NHC)Me(solv)] +[MeB(C 6F 5) 3] - were prepared and studied as models for methane oxy-functionalization catalysts (Cp* = η 5-C 5Me 5; NHC = 1,3,4,5- tetramethylimidazol-2-ylidene (MeI Me, 3a), 1,3-dimethylimidazol-2- ylidene (I Me, 3b), 1,3-dimethylbenzimidazol-2-ylidene (BI Me, 3c); solv = solvent or open site). These complexes were targeted on the basis of the C-H bond activation reactions of the previously reported complexes [Cp*Ir(PMe 3)R] + (R = Me, H) and the general robustness of Ir-NHC complexes under oxidizing conditions. The syntheses of the new iridium(III) complexes Cp*Ir(NHC)Me 2 are described (NHC = MeI Me (4a), I Me (4b), BI Me (4c)). When 4a-c were allowed to react with B(C 6F 5) 3 in CH 2Cl 2, the methyl abstraction products [Cp*Ir(NHC)Me(solv)] +[MeB(C 6F 5) 3] - (3a-c) were produced. Complexes 3a-c reacted with arenes to form the aryl complexes [Cp*Ir(NHC)Ar(solv)] +[MeB(C 6F 5) 3] - and methane (Ar = C 6H 5 (7), C 6H 4F (8)). Complexes 3a-c reacted very slowly with alkanes; the slow reaction rate is attributed to steric congestion due to the NHC ligand. DFT calculations support this hypothesis: the barriers to C-H activation are in qualitative agreement with the empirical reaction rates, and the C-H activation transition state structures show significant steric crowding. Several of these complexes have been analyzed by X-ray diffraction.
AB - The cationic complexes [Cp*Ir(NHC)Me(solv)] +[MeB(C 6F 5) 3] - were prepared and studied as models for methane oxy-functionalization catalysts (Cp* = η 5-C 5Me 5; NHC = 1,3,4,5- tetramethylimidazol-2-ylidene (MeI Me, 3a), 1,3-dimethylimidazol-2- ylidene (I Me, 3b), 1,3-dimethylbenzimidazol-2-ylidene (BI Me, 3c); solv = solvent or open site). These complexes were targeted on the basis of the C-H bond activation reactions of the previously reported complexes [Cp*Ir(PMe 3)R] + (R = Me, H) and the general robustness of Ir-NHC complexes under oxidizing conditions. The syntheses of the new iridium(III) complexes Cp*Ir(NHC)Me 2 are described (NHC = MeI Me (4a), I Me (4b), BI Me (4c)). When 4a-c were allowed to react with B(C 6F 5) 3 in CH 2Cl 2, the methyl abstraction products [Cp*Ir(NHC)Me(solv)] +[MeB(C 6F 5) 3] - (3a-c) were produced. Complexes 3a-c reacted with arenes to form the aryl complexes [Cp*Ir(NHC)Ar(solv)] +[MeB(C 6F 5) 3] - and methane (Ar = C 6H 5 (7), C 6H 4F (8)). Complexes 3a-c reacted very slowly with alkanes; the slow reaction rate is attributed to steric congestion due to the NHC ligand. DFT calculations support this hypothesis: the barriers to C-H activation are in qualitative agreement with the empirical reaction rates, and the C-H activation transition state structures show significant steric crowding. Several of these complexes have been analyzed by X-ray diffraction.
UR - http://www.scopus.com/inward/record.url?scp=84858148131&partnerID=8YFLogxK
U2 - 10.1021/om2012166
DO - 10.1021/om2012166
M3 - Article
AN - SCOPUS:84858148131
SN - 0276-7333
VL - 31
SP - 1879
EP - 1887
JO - Organometallics
JF - Organometallics
IS - 5
ER -