Reactivity of the Bis(dihydrogen) Complex [RuH<INF>2</INF>(η<SUP>2</SUP>-H<INF>2</INF>)<INF>2</INF>(PCy<INF>3</INF>)<INF>2</INF>] 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 [RuH<INF>2</INF>(η²-H<INF>2</INF>)<INF>2</INF>(PCy<INF>3</INF>)<INF>2</INF>] (<BO>1</BO>) produces compounds containing heteroaromatic ([RuH<INF>2</INF>(η²-H<INF>2</INF>)(η¹(N)-C<INF>5</INF>H<INF>5</INF>N)(PCy<INF>3</INF>)<INF>2</INF>] (<BO>2</BO>), [RuH(η⁵-C<INF>4</INF>H<INF>4</INF>N)(PCy<INF>3</INF>)<INF>2</INF>]·Pyr (<BO>3</BO>)) or aromatic rings ([RuH<INF>2</INF>(η⁴-C<INF>13</INF>H<INF>9</INF>N)(PCy<INF>3</INF>)<INF>2</INF>] (<BO>5</BO>)) coordinated in η¹(N) (<BO>2</BO>), η⁵(N,C) (<BO>3</BO>), or η⁴(C,C) (<BO>5</BO>) modes for Py, Pyr, and acridine, respectively. Complex <BO>3</BO> has been characterized by X-ray crystallography. Its protonation by HBF<INF>4</INF> affords the cationic dihydride complex [RuH<INF>2</INF>(η⁵-C<INF>4</INF>H<INF>4</INF>N)(PCy<INF>3</INF>)<INF>2</INF>][BF<INF>4</INF>] (<BO>4</BO>). The coordinated Py ligand in <BO>2</BO> and acridine in <BO>5</BO> can readily be displaced by dihydrogen, with regeneration of <BO>1</BO>. Regioselective hydrogenation of representative polynuclear heteroaromatic nitrogen compounds is achieved in the presence of <BO>1</BO> under mild reaction conditions (80 °C, 3 bar of H<INF>2</INF>). 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. <BO>5</BO> 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.