Reactivity of the Bis(dihydrogen) Complex [RuH₂(η²-H₂)₂(PCy₃)₂] toward N-Heteroaromatic Compounds. Regioselective Hydrogenation of Acridine to 1,2,3,4,5,6,7,8-Octahydroacridine

The reaction of pyridine (Py), pyrrole (Pyr), or acridine with the bis(dihydrogen) complex [RuH₂(η²-H₂)₂(PCy₃)₂] (1) produces compounds containing heteroaromatic ([RuH₂(η²-H₂)(η¹(N)-C₅H₅N)(PCy₃)₂] (2), [RuH(η⁵-C₄H₄N)(PCy₃)₂]·Pyr (3)) or aromatic rings ([RuH₂(η⁴-C₁₃H₉N)(PCy₃)₂] (5)) coordinated in η¹(N) (2), η⁵(N,C) (3), or η⁴(C,C) (5) modes for Py, Pyr, and acridine, respectively. Complex 3 has been characterized by X-ray crystallography. Its protonation by HBF₄ affords the cationic dihydride complex [RuH₂(η⁵-C₄H₄N)(PCy₃)₂][BF₄] (4). The coordinated Py ligand in 2 and acridine in 5 can readily be displaced by dihydrogen, with regeneration of 1. Regioselective hydrogenation of representative polynuclear heteroaromatic nitrogen compounds is achieved in the presence of 1 under mild reaction conditions (80 °C, 3 bar of H₂). Quinoline (Q) and isoquinoline (iQ) are hydrogenated to 5,6,7,8-tetrahydro derivatives, while acridine is quickly reduced to 1,2,3,4-tetrahydroacridine followed by much slower saturation to 1,2,3,4,5,6,7,8-octahydroacridine (8H-Acr), the nitrogen-containing aromatic ring remaining intact. 8H-Acr has been isolated in analytically pure form and characterized by ¹H and ¹³C NMR as well as by X-ray crystallography. 5 is also active for catalytic acridine hydrogenation and can be regarded as an intermediate in the catalytic cycle. Saturation of a five-membered indole ring proceeds much slower than hydrogenation of six-membered aromatic rings in Q and iQ. Pyridine, pyrrole, and 7,8-benzoquinoline are not hydrogenated under the applied reaction conditions, as a result of the formation of new stable complexes.