Versatile reactivity of the bis(dihydrogen) complex RuH2(H-2)(2)(PCy(3))(2) toward functionalized olefins: Olefin coordination versus hydrogen transfer via the stepwise dehydrogenation of the phosphine ligand

Treatment of RuH2(H-2)(2)(PCy(3))(2) (1) with ethylene leads to the formation of [RuH{eta(3)-C6H8)-PCy(2)}(C2H4)(PCy(3))] (2), which contains an eta(3)-cyclohexenyl ring. Upon addition of 3 equiv of an alkene bearing sigma-donor substituents such as CH2=CH(SiEt(3)) or CH2=CH(t)Bu, 1 transforms in high yield into the trihydridoruthenium(IV) complex [RuH3{(eta(3)-C6H8)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{(eta(3)-C6H8)PCy(2)}{eta(2)-C6H9)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 CH2=CHCO2-Me or trans-MeO(2)CCH=CHCO(2)Me allows the formation of the hydride dihydrogen complexes [RUH(H-2)(eta(2)-O2CCH2CH3)(PCy(3))(2)] (5) and [RuH(H-2)(eta(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 [RuH2(eta(2)-CH3O2-CCH=CHCO2CH3)(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.