Abnormal and Normal N-Heterocyclic Carbene Osmium Polyhydride Complexes Obtained by Direct Metalation of Imidazolium Salts

The complex OsH6(PiPr3)2 (1) is a basic precursor which promotes the direct metalation of imidazolium salts. Thus, it reacts with 1-mesityl-3-methylimidazolium tetraphenylborate and 1-mesityl-3-ethylimidazolium tetraphenylborate to give the abnormal NHC derivatives [OsH5(1-mesityl-3-methylimidazol-4-ylidene)(PiPr3)2]BPh4 (2) and [OsH5(1-mesityl-3-ethylimidazol-4-ylidene)(PiPr3)2]BPh4 (3), respectively. Treatment of 2 and 3 with NaH produces the deprotonation of the metal center and the formation of the tetrahydride derivatives OsH4(1-mesityl-3-methylimidazol-4-ylidene)(PiPr3)2 (4) and OsH4(1-mesityl-3-ethylimidazol-4-ylidene)(PiPr3)2 (5). In contrast to the case for the mesityl-substituted imidazolium salts, the reaction with 1-benzyl-3-methylimidazolium tetraphenylborate leads to the normal NHC complex [OsH5(1-benzyl-3-methylimidazol-2-ylidene)(PiPr3)2]BPh4 (6). The deprotonation of 6 with NaH affords OsH4(1-benzyl-3-methylimidazol-2-ylidene)(PiPr3)2 (7). Complexes 2 and 4 have been characterized by X-ray diffraction analysis.