Syntheses of Ketonated Disulfide-Bridged Diruthenium Complexes via C−H Bond Activation and C−S Bond Formation

The α-C−H bonds of 3-methyl-2-butanone, 3-pentanone, and 2-methyl-3-pentanone were activated on the sulfur
center of the disulfide-bridged ruthenium dinuclear complex [{RuCl(P(OCH3)3)2}2(μ-S2)(μ-Cl)2] (1) in the presence
of AgX (X = PF6, SbF6) with concomitant formation of C−S bonds to give the corresponding ketonated complexes
[{Ru(CH3CN)2(P(OCH3)3)2}(μ-SSCHR1COR2){Ru(CH3CN)3(P(OCH3)3)2}]X3 ([5](PF6)3, R1 = H, R2 = CH(CH3)2,
X = PF6; [6](PF6)3, R1 = CH3, R2 = CH2CH3, X = PF6; [7](SbF6)3, R1 = CH3, R2 = CH(CH3)2, X = SbF6). For
unsymmetric ketones, the primary or the secondary carbon of the α-C−H bond, rather than the tertiary carbon,
is preferentially bound to one of the two bridging sulfur atoms. The α-C−H bond of the cyclic ketone cyclohexanone
was cleaved to give the complex [{Ru(CH3CN)2(P(OCH3)3)2}(μ-SS-1-cyclohexanon-2-yl){Ru(CH3CN)3(P(OCH3)3)2}](SbF6)3 ([8](SbF6)3). And the reactions of acetophenone and p-methoxyacetophenone, respectively,
with the chloride-free complex [{Ru(CH3CN)3(P(OCH3)3)2}2(μ-S2)]4+ (3) gave [{Ru(CH3CN)2(P(OCH3)3)2}(μ-SSCH2COAr){Ru(CH3CN)3(P(OCH3)3)2}](CF3SO3)3 ([9](CF3SO3)3, Ar = Ph; [10](CF3SO3)3, Ar = p-CH3OC6H4).
The relative reactivities of a primary and a secondary C−H bond were clearly observed in the reaction of butanone
with complex 3, which gave a mixture of two complexes, i.e., [{Ru(CH3CN)2(P(OCH3)3)2}(μ-SSCH2COCH2CH3){Ru(CH3CN)3(P(OCH3)3)2}](CF3SO3)3 ([11](CF3SO3)3) and [{Ru(CH3CN)2(P(OCH3)3)2}(μ-SSCHCH3COCH3){Ru(CH3CN)3(P(OCH3)3)2}](CF3SO3)3 ([12](CF3SO3)3), in a molar ratio of 1:1.8. Complex 12 was converted
to 11 at room temperature if the reaction time was prolonged. The relative reactivities of the α-C−H bonds of the
ketones were deduced to be in the order 2° > 1° > 3°, on the basis of the consideration of contributions from
both electronic and steric effects. Additionally, the C−S bonds in the ketonated complexes were found to be
cleaved easily by protonation at room temperature. The mechanism for the formation of the ketonated disulfide-bridged ruthenium dinuclear complexes is as follows:  initial coordination of the oxygen atom of the carbonyl
group to the ruthenium center, followed by addition of an α-C−H bond to the disulfide bridging ligand, having
SS double-bond character, to form a C−S−S−H moiety, and finally completion of the reaction by deprotonation
of the S−H bond.