Axial Interaction of the [Ru₂(CO)₄]²⁺Core with the Aryl C−H Bond:  Route to Cyclometalated Compounds Involving a Metal−Metal-Bonded Diruthenium Unit

Room-temperature activation of the aromatic C−H bond by the [Ru 2 (CO) 4 ] 2+ core has been achieved. The reactions of 2-phenyl-1,8-naphthyridine (phNP) and 2-(2,5-dimethyl-3-furyl)-1,8-naphthyridine (Me 2 fuNP) with [Ru 2 (CO) 4 (MeCN) 6 ][BF 4 ] 2 in dichloromethane provide the agostic-cyclometalated compounds [Ru 2 (phNP)(C 6 H 4 -NP)(CO) 4 ][BF 4 ] ( 1 ) and [Ru 2 (Me 2 fuNP)(C 4 OMe 2 -NP)(CO) 4 ][BF 4 ] ( 2 ), respectively. In both compounds, one of the ligands is ortho-metalated, while the second ligand is engaged in an agostic interaction. The ortho metalation is preferred over the potential S coordination for 2-(2-thienyl)-1,8-naphthyridine (thNP), yielding [Ru 2 (thNP)(C 4 H 2 S-NP)(CO) 4 ][BF 4 ] ( 3 ). In acetonitrile, the compound [Ru 2 (thNP) 2 (CO) 4 ][BF 4 ] 2 ( 4 ) is obtained exclusively. The donation of a C−H bonding electron pair to the Ru−Ru σ* LUMO and back-donation from the filled Ru−Ru π* orbital to the C−H σ* orbital cause facile C−H bond cleavage. In contrast, the isoelectronic [Rh 2 ] 4+ provides the agostic compounds [Rh 2 (OAc) 3 (phNP)Cl] ( 5 ) and [Rh 2 (L)(η 1 -L)(OAc) 2 (CH 3 CN) 2 ][BF 4 ] 2 (L = phNP, nplNP (2-(2-naphthyl)-1,8-naphthyridine) for compounds 6 and 7, respectively). The molecular structures of compounds 1 − 3, 5, and 7 have been established by X-ray crystallography.