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Understanding C-H bond activation on a diruthenium(I) platform
, M. Majumdar, M. Sarkar, , J.K. Bera
Published in
Volume: 32
Issue: 1
Pages: 340 - 349
Activation of the C-H bond at the axial site of a [RuI-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. © 2013 American Chemical Society.
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