Reactions of [RuCl₂(PPh₃)₃] with Nitron and with the “Enders Carbene”: Access to Ruthenium(III) NHC Complexes

The reactions of [RuCl₂(PPh₃)₃] with the “Enders carbene” 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (1) and the “instant carbene” Nitron (2) respectively afforded the Ru^II chelates [RuCl­(3)­(PPh₃)₂] (3 = 3,4-diphenyl-1-o-phenylene-1,2,4-triazol-5-ylidene) and [RuCl­(4)­(PPh₃)₂] (4 = 4-phenyl-3-phenylamino-1-o-phenylene-1,2,4-triazol-5-ylidene) in a process involving the ortho metalation of the 1-Ph group of the respective carbene ligand. It proved possible to synthesize [RuCl­(3)­(PPh₃)₂] more conveniently in higher yield by using the stable carbene precursor 5-methoxy-1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazole (MeO-1-H) instead of the free carbene 1 in the presence of triethylamine to trap the HCl generated by the ortho metalation. Aerobic oxidation of the Ru^II chelates in the presence of chloride ions furnished [RuCl₂(3)­(PPh₃)₂] and [RuCl₂(4)­(PPh₃)₂], which are rare examples of Ru^III NHC complexes. The crystal structures of all four complexes were determined by single-crystal X-ray diffraction studies. In addition, the crystal structure of the hydrochloride of Nitron was also determined. In the Ru^II chelates, the pentacoordinate metal center is in a distorted-square-pyramidal environment with the carbon atom of the ortho-metalated 1-Ph group occupying the apical position. The coordination sphere of the Ru^III chelates is complemented by a second chlorido ligand, which is positioned trans to this carbon atom.