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 [Ru2(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 (Me2fuNP) with [Ru2(CO)4(MeCN)6][BF4]2 in dichloromethane provide the agostic-cyclometalated compounds [Ru2(phNP)(C6H4-NP)(CO)4][BF4] (1) and [Ru2(Me2fuNP)(C4OMe2-NP)(CO)4][BF4] (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 [Ru2(thNP)(C4H2S-NP)(CO)4][BF4] (3). In acetonitrile, the compound [Ru2(thNP)2(CO)4][BF4]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 [Rh2]4+ provides the agostic compounds [Rh2(OAc)3(phNP)Cl] (5) and [Rh2(L)(η1-L)(OAc)2(CH3CN)2][BF4]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.