Synthesis and Characterization of Nitrile-Functionalized N-Heterocyclic Carbenes and Their Complexes of Silver(I) and Rhodium(I)

A series of unsymmetrical nitrile-functionalized N-heterocyclic carbene precursors were synthesized by N-alkylation and N-arylation of 2-cyanophenylimidazole. These imidazolium salts were used to synthesize N-heterocyclic carbene complexes of silver(I), all of which were characterized by NMR and Fourier transform infrared (FT-IR) spectroscopies and elemental analyses. The compound bis[1-(2-cyanophenyl)-3-methylimidazol-2-ylidene]silver(I) tetrafluoroborate ([Ag(m-CN)2]BF4, 2a) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the preparation of the silver(I) carbene complex of 3-(cyanomethyl)-1-(2-cyanophenyl)imidazolium (1b) with excess silver(I) oxide leads to the formation of the desired bis-carbene complex, and the bis-carbene complex with hydrolyzed cyanomethyl groups on the ligands. The selectivity in hydrolysis of the cyanomethyl group over the cyanophenyl group on the ligand, as evident from the NMR data, suggests that the process must be mediated by silver(I) centers. The use of an N-heterocyclic carbene complex of silver(I) as a carbene transfer reagent was demonstrated by the reaction of 2a with [Rh(cod)Cl]2. The crystal structure of bis[(1-(2-cyanophenyl)-3-methylimidazol-2-ylidene)-(η4-1,5-cycloctadiene)rhodium(I) tetrafluoroborate ([Rh(m-CN)(cod)]2(BF4)2, 3a) revealed the dimeric nature of the complex, of which the nitrile nitrogen and the carbene carbon of the bridging ligand were coordinated to two different rhodium(I) centers, with a slight distortion of the Rh−N≡C bond angles. This is the first dimeric N-functionalized N-heterocyclic carbene complex of rhodium(I) that has been structurally characterized.