Experimental and Computational Studies on the Acetate-Assisted C-H Activation of N-Aryl Imidazolium Salts at Rhodium and Iridium: A Chloride Additive Changes the Selectivity of C-H Activation.

Combined experimental and computational mechanistic studies of the reactions of unsymmetrical, para-substituted N-aryl imidazolium salts, L2-R1,R2, at [MCl2Cp*]2 (M = Rh, Ir) in the presence of NaOAc are reported. These proceed via intermediate N-heterocyclic carbene complexes that then allow an internal competition between two differently substituted aryl rings toward C-H activation to be monitored. At 348 K in dichloroethane C-H activation of the aryl with the more electron-withdrawing substituents is generally favored. DFT calculations show similar barriers for proton transfer and dissociative HOAc/Cl- ligand substitution, with proton transfer favoring electron-donating substituents, and ligand substitution favoring electron-withdrawing substituents. Microkinetic simulations reproduce the experimental preference implying that the ligand substitution step dominates selectivity. For several substrates, notably L2-F,OMe and L2-F,H, running the C-H activation reactions at 298 K in the presence of added [Et4N]Cl reverses the selectivity. The greater availability of chloride in solution makes an alternative dissociative interchange ligand substitution mechanism accessible, leaving proton transfer as selectivity determining and so favoring electron-donating substituents. Our results highlight the potential importance of the ligand substitution step in the interpretation of substituent effects and demonstrate how a simple additive, [Et4N]Cl, can have a dramatic effect on selectivity by changing the mechanism of ligand substitution.

Steric effects on acetate-assisted cyclometallation of meta-substituted N-phenyl and N-benzyl imidazolium salts at [MCl2Cp*]2 (M = Ir, Rh).

meta-Substituted N-phenyl,N'-methyl and N-benzyl,N'-methyl imidazolium salts undergo acetate-assisted cyclometallation to provide mixtures of ortho and para substituted cyclometallated complexes. The effect of the substituents on the isomer ratios is discussed; steric effects are more important in the 6-membered rings derived from the N-benzyl imidazolium salts than 5-membered rings from the N-phenyl salts. Comparisons are made to steric effects with some other common directing groups.

Steric and electronic effects on acetate-assisted cyclometallation of 2-phenylpyridines at [MCl2Cp*]2 (M = Ir, Rh).

The reactions of substituted 2-phenylpyridines at [MCl2Cp*]2 dimers (M = Ir, Rh) in the presence of NaOAc form cyclometallated complexes Cp*M(Phpyr)Cl. H/D exchange experiments and substrate competition experiments show that kinetic selectivity favours electron donating substituents whilst substrates with electron withdrawing substituents are favoured thermodynamically. Experiments with Ir are mostly irreversible under the conditions used whilst those for Rh are more easily reversible. For meta-substituted phenylpyridines steric effects are important, larger substituents leading to formation of the para-substituted cyclometallated product.