Versatile Reactivity of the Bis(dihydrogen) Complex RuH₂(H₂)₂(PCy₃)₂ toward Functionalized Olefins: Olefin Coordination versus Hydrogen Transfer via the Stepwise Dehydrogenation of the Phosphine Ligand

Abstract

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 CH2CH(SiEt3) or CH2CHtBu, 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 CH2CHCO2Me or trans-MeO2CCHCHCO2Me 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-MeO2CCHCHCO2Me yielded the alkene-coordinated complex [RuH2(η2-CH3O2CCHCHCO2CH3)(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 MeO2CCHCHCO2Me isomerization.