Understanding C-H Bond Activation on a Diruthenium(I) Platform

Activation of the C-H bond at the axial site of a [Ru-I-Ru-I] platform has been achieved. Room-temperature treatment of 2-(R-phenyl)-1,8-naphthyridine (R = H, F, OMe) with [Ru-2(CO)(4)(CH3CN)(6)][BF4](2) in CH2Cl2 affords the corresponding diruthenium(I) complexes, which carry two ligands, one of which is orthometalated and the second ligand engages an axial site via a Ru center dot center dot center dot C-H interaction. Reaction with 2-(2-N-methylpyrrolyl)-1,8-naphthyridine under identical conditions affords another orthometalated/nonmetalated (om/nm) complex. At low temperature (4 degrees C), however, a nonmetalated complex is isolated that reveals axial Ru center dot center dot center dot C-H interactions involving both ligands at sites trans to the Ru-Ru bond. A nonmetalated (nm/nm) complex was characterized for 2-pyrrolyl-1,8-naphthyridine at room temperature. Orthometalation of both ligands on a single [Ru-Ru] platform could not be accomplished even at elevated temperature. X-ray metrical parameters clearly distinguish between the orthometalated and nonmetalated ligands. NMR investigation reveals the identity of each proton and sheds light on the nature of [Ru-Ru]center dot center dot center dot C-H interactions (preagostic/agostic). An electrophilic mechanism is proposed for C-H bond cleavage that involves a C(p(pi))-H -> sigma* [Ru-Ru] interaction, resulting in a Wheland-type intermediate. The heteroatom stabilization is credited to the isolation of nonmetalated complexes for pyrrolyl C-H, whereas lack of such stabilization for phenyl C-H causes rapid proton elimination, giving rise to orthometalation. NPA charge analysis suggests that the first orthometalation makes the [Ru-Ru] core sufficiently electron rich, which does not allow significant interaction with the other axial C-H bond, making the second metalation very difficult.