Treatment of RuH2(H2)2(PCy3)2 (1) with ethylene leads to the formation of [RuH{η3-C6H8)PCy2}(C2H4)(PCy3)] (2), which contains an η3-cyclohexenyl ring. Upon addition of 3 equiv of an alkene bearing σ-donor substituents such as CH2CH(SiEt3) or CH2CHtBu, 1 transforms in high yield into the trihydridoruthenium(IV) complex [RuH3{(η3-C6H8)PCy2}(PCy3)] (3) and the corresponding alkane is obtained quantitatively. Further hydrogen abstraction from 1 can be achieved by using 5 equiv of alkene, leading to the hydridoruthenium(II) complex [RuH{(η3-C6H8)PCy2}{(η2-C6H9)PCy2}] (4). This is the first complex where C−H activation within two cyclohexyl rings of two tricyclohexylphosphines has occurred. 4 can also be obtained by treatment of 3 with 2 equiv of alkene. The reactions can be reversed by bubbling dihydrogen into a solution of 2−4 yielding 1 at the final stage of hydrogenation. 3 and 4 undergo rapid H−D exchange with the deuterated solvent. Reaction of 1 with alkenes bearing electron-withdrawing substituents such as CH2CHCO2Me or trans-MeO2CCHCHCO2Me allows the formation of the hydrido dihydrogen complexes [RuH(H2)(η2-O2CCH2CH3)(PCy3)2] (5) and [RuH(H2)(η2-O2CCH2CH2CO2Me)(PCy3)2] (6), respectively. An X-ray structure determination on 5 was carried out. Surprisingly, reaction of 1 with cis-MeO2CCHCHCO2Me yielded the alkene-coordinated complex [RuH2(η2-CH3O2CCHCHCO2CH3)(PCy3)2] (7). Variable-temperature NMR studies on 7 showed a barrier of rotation for the alkene of 10.5 kcal/mol. 7 was shown to catalyze cis−trans MeO2CCHCHCO2Me isomerization.

The reaction of pyridine (Py), pyrrole (Pyr), or acridine with the bis(dihydrogen) complex [RuH2(η 2-H2)2(PCy3)2] (1) produces compounds containing heteroaromatic ([RuH2(η 2-H2)(η 1(N)-C5H5N)(PCy3)2] (2), [RuH(η 5-C4H4N)(PCy3)2]·Pyr (3)) or aromatic rings ([RuH2(η 4-C13H9N)(PCy3)2] (5)) coordinated in η 1(N) (2), η 5 (N,C) (3), or η 4(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(η 5-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.