New Bidentate Trans-Chelating N-Heterocyclic Carbene Ligands for Palladium

A family of imidazolium salts of the type [BnN(CH(2)CH(2)CH(2)(R)Im)(2)]center dot 2[Cl] (Bn = CH2Ph; (R)Im = 1-methylimidazole (1a), 1-tert-butylimidazole (1b), 1-benzylimidazole (1c), 1-methylbenzimidazole (1d)), which contain a tertiary amine linking two imidazolium groups, has been synthesized. These imidazolium salts can be deprotonated with Ag2O to generate the Ag carbene complexes [{BnN(CH(2)CH(2)CH(2)(R)Im)(2)}Ag]center dot[AgCl2] ((R)Im = 1-methylimidazole (2a), 1-tert-butylimidazole (2b), 1-benzylirnidazole (2c), 1-methylbenzimidazole (2d)). In the solid state 2d exists as an unusual tetramer, which consists of an [Ag2Cl4](2-) core bridging two Ag(NHC) cations. Subsequent reaction of the Ag complexes with PdCl2(MeCN)(2) generates Pd species of the type {BnN(CH(2)CH(2)CH(2)(R)Im)(2)}PdCl2 ((R)Im = 1-methylimidazole (3a), 1-tert-butylimidazole (3b), 1-benzylimidazole (3c), 1-methylbenzimidazole (3d)), which is a rare example of a family of Pd complexes that contain a bidentate trans-chelating N-heterocyclic carbene ligand. Compounds 3a and 3c were crystallographically characterized by X-ray crystallography and contain unusual 12-membered metallacycles. DFT calculations suggest that the preference for trans binding of the ligand is related to conformational effects of the linker. Compound 3b reacts with excess MeI to form {BnN(CH(2)CH(2)CH(2)(tBu)Im)(2)}PdI2 (5b), a reaction in which we believe a Pd(IV) intermediate is generated. Compound 5b was crystallographically characterized. Compounds 3a-d are all active catalysts for the Heck reaction, and 3a can also catalyze the Suzuki reaction.