Osmium NHC Complexes from Alcohol-Functionalized Imidazoles and Imidazolium Salts

The hexahydride complex OsH6((PPr3)-Pr-i)(2) (1) reacts with 1-mesitylimidazole, 1-methylimidazole, 1-(2-hydroxy-2-phenylethyl)imidazole, and 1-(2-hydroxypropyl)imidazole to give the N-bound imidazole compounds OsH4-(RIm)((PPr3)-Pr-i)(2) (R = Mes (2), Me (3), CH2CH(OH)Ph (4), CH2CH(OH)CH3 (5)) and H-2. In toluene under reflux the alcohol derivatives 4 and 5 evolve into the C-bound imidazole complexes OsH3{CNHCHCHNCH=C(R)O}((PPr3)-Pr-i)(2) (R = Ph (6), CH3 (7)), bearing an NH wingtip. These NHC-enolate species result from the N-bound to C-bound transformation of the heterocycle and the deprotonation-dehydrogenation of the alcohol substituent. The addition of HBF4 to 6 and 7 affords the NHC-keto derivatives [OsH3{CNHCHCHNCH2C(R)= O} ((PPr3)-Pr-i)(2)]BF4 (R = Ph (8), CH3 (9)). Treatment of 1 with 3-benzyl-1-(2-hydroxy-2-phenylethyl)imidazolium tetrafluoroborate and 3-benzyl-1-(2-hydroxypropyl)imidazolium tetraphenylborate leads to the NHC- keto complexes [OsH3{CN(CH2Ph)CHCHNCH2C(R)=O}((PPr3)-Pr-i)(2)]A. (A = BF4, R = Ph (10); A = BPh4, CH3 (11)), as a consequence of the direct metalation of the heterocycle and the dehydrogenation of the alcohol substituent. The deprotonation of 10 and 11 with (KBuO)-Bu-t gives the NHC-enolate derivatives OsH3{CN(CH2Ph)CHCHNCH=C(R)O}((PPr3)-Pr-i)(2) (R = Ph (12), CH3 (13)). The X-ray diffraction structures of 2, 6, and 10 are also reported.