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

David L Davies,Kuldip Singh,Neringa Tamosiunaite

doi:10.1039/d1dt02677a

Abstract

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.

Highlights

Carboxylate-assisted cyclometallation is a very well-established reaction both stoichiometrically and in catalysis.[1]
In those cases loss of coordinated acetic acid from the ortho isomer was significantly more endergonic leading to fast reverse proton transfer, which could only be detected by H/D exchange
To examine NHCs as donor ligands we studied cyclometallation of meta-substituted N-phenyl, N′-methyl imidazolium salts L4-R

Summary

Introduction

Carboxylate-assisted cyclometallation is a very well-established reaction both stoichiometrically and in catalysis.[1]. The para-isomers were favoured thermodynamically over the ortho with both metals for R = OMe, CN, and for the more bulky CF3, whilst at shorter reaction times the selectivity was less. Synthesis of L4-OMe. Following the general procedure, a mixture of 1-(3-methoxyphenyl)-1H-imidazole (174 mg, 1.002 mmol), dry DCM (4 mL), methyl trifluoromethanesulfonate (0.125 mL, 181 mg, 1.105 mmol), capped was stirred at rt for 4 h.

Results

Conclusion