Syntheses of ketonated disulfide-bridged diruthenium complexes via C-H bond activation and C-S bond formation

The alpha-C-H bonds of 3-methyl-2-butanone, 3-pentadone, and 2-methyl-3-pen tanone were activated on the sulfur center of the disulfide-bridged rutheni um dinuclear complex [{RuCl(P(OCH3)(3))(2)}(2)(mu-S-2)(mu-Cl)(2)] (1) in th e presence of AgX (X = PF6, SbF6) with concomitant formation of C-S bonds t o give the corresponding ketonated complexes [{Ru(CH3CN)(2)(P(OCH3)(3))(2)} (mu-(SSCHRCOR2)-C-1){Ru(CH3CN)(3)(P(OCH3)(3))(2)}]X-3 ([5](PF6)(3), R-1 = H , R-2 = CH(CH3)(2), X = PF6; [6](PF6)(3), R-1 = CH3, R-2 = CH2CH3, X = PF6; [7](SbF6)(3), R-1 = CH3, R-2 = CH(CH3)(2), X = SbF6). For unsymmetric keto nes, the primary or the secondary carbon of the alpha-C-H bond, rather than the tertiary carbon, is preferentially bound to one of the two bridging su lfur atoms. The alpha-C-H bond of the cyclic ketone cyclohexanone was cleav ed to give the complex [{Ru(CH3CN)(2)(P(OCH3)(3))(2)}(mu-SS-1-cyclohexanon- 2-yl){Ru(CH3CN)(3)(P(OCH3)(3))(2)}](SbF6)(3) ([8](SbF6)(3)). And the reacti ons of acetophenone and p-methoxyacetophenone, respectively, with the chlor ide-free complex [( Ru(CH3CN)(3)(P(OCH3)(3))(2)}(2)(mu-S-2)](4+) (3) gave [ {Ru(CH3CN)(2)(P(OCH3)(3))(2)}(mu-SSCH2COAr){Ru(CH3CN)(3)(P(OCH3)(3))(2)}](C F3SO3)(3) ([9](CF3SO3)(3), Ar = Ph; [10](CF3SO3)(3), Ar = p-CH3OC6H4) The r elative reactivities of a primary and a secondary C-H bond were clearly obs erved in the reaction of butanone with complex 3, which gave a mixture of t wo complexes, i.e., [{Ru(CH3CN)(2)(P(OCH3)(3))(2)}(mu-SSCH2COCH2-CH3{Ru(CH3 CN)(3)(P(OCH3)(3))(2)}](CF3SO3)(3)) {Ru(CH3CN)(3)(P(OCH3)(3))(2)}](CF3SO3)( 3) ([11](CF3SO3)(3)) and [{Ru(CH3CN)(2)(P(OCH3)(3))(2)}(mu-SSCH2COCH2-CH3){ Ru(CH3CN)(3)(P(OCH3)(3))(2)}](CF3SO3)(3) ([12](CF3SO3)(3)), in a molar rati o of 1:1.8. Complex 12 was converted to 11 at room temperature if the react ion time was prolonged. The relative reactivities of the alpha-C-H bends of the ketones were deduced to be in the order 2 degrees > 1 degrees > 3 degr ees, on the basis of the consideration of contributions from both electroni c and steric effects. Additionally, the C-S bonds in the ketonated complexe s were found to be cleaved easily by protonation at room temperature. The mechanism for the formation of the ketonated disulfide-bridged rutheniu m dinuclear complexes is as follows: initial coordination of the oxygen ato m of the carbonyl group to the ruthenium center, followed by addition of an cc-C-H bond to the disulfide bridging ligand, having S=S double-bond chara cter, to form a C-S-S-H moiety, and finally completion of the reaction by d eprotonation of the S-H bond.