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

Activation of the C–H bond at the axial site of
a [RuI–RuI] 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(pπ)–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.