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

Treatment of RuH 2 (H 2 ) 2 (PCy 3 ) 2 ( 1 ) with ethylene leads to the formation of [RuH{η 3 -C 6 H 8 )PCy 2 }(C 2 H 4 )(PCy 3 )] ( 2 ), which contains an η 3 -cyclohexenyl ring. Upon addition of 3 equiv of an alkene bearing σ-donor substituents such as CH 2 CH(SiEt 3 ) or CH 2 CH t Bu, 1 transforms in high yield into the trihydridoruthenium(IV) complex [RuH 3 {(η 3 -C 6 H 8 )PCy 2 }(PCy 3 )] ( 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{(η 3 -C 6 H 8 )PCy 2 }{(η 2 -C 6 H 9 )PCy 2 }] ( 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 CH 2 CHCO 2 Me or trans -MeO 2 CCH CHCO 2 Me allows the formation of the hydrido dihydrogen complexes [RuH(H 2 )(η 2 -O 2 CCH 2 CH 3 )(PCy 3 ) 2 ] ( 5 ) and [RuH(H 2 )(η 2 -O 2 CCH 2 CH 2 CO 2 Me)(PCy 3 ) 2 ] ( 6 ), respectively. An X-ray structure determination on 5 was carried out. Surprisingly, reaction of 1 with cis -MeO 2 CCH CHCO 2 Me yielded the alkene-coordinated complex [RuH 2 (η 2 -CH 3 O 2 CCH CHCO 2 CH 3 )(PCy 3 ) 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 MeO 2 CCH CHCO 2 Me isomerization.