Understanding C–H bond activation on a diruthenium(I) platform

Abstract

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 [Ru₂(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···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°C), however, a nonmetalated complex is isolated that reveals axial Ru···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]···C–H interactions (preagostic/agostic). An electrophilic mechanism is proposed for C–H bond cleavage that involves a C(p_π)–H → σ* [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.