C–H Bond Activation by Cationic Iridium(III) NHC Complexes: A Combined Experimental and Computational Study

The cationic complexes [Cp*Ir­(NHC)­Me­(solv)]+[MeB­(C6F5)3]− were
prepared
and studied as models for methane oxy-functionalization catalysts
(Cp* = η5-C5Me5; NHC = 1,3,4,5-tetramethylimidazol-2-ylidene
(MeIMe, 3a), 1,3-dimethylimidazol-2-ylidene
(IMe, 3b), 1,3-dimethylbenzimidazol-2-ylidene
(BIMe, 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(PMe3)­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)­Me2 are described
(NHC = MeIMe (4a), IMe (4b), BIMe (4c)). When 4a–c were allowed to react with B­(C6F5)3 in CH2Cl2, the methyl abstraction products
[Cp*Ir­(NHC)­Me­(solv)]+[MeB­(C6F5)3] (3a–c) were produced. Complexes 3a–c reacted
with arenes to form the aryl complexes [Cp*Ir­(NHC)­Ar­(solv)]+[MeB­(C6F5)3] and
methane (Ar = C6H5 (7), C6H4F (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.