Heterolytic Cleavage of Dihydrogen Promoted by Sulfido-Bridged Tungsten−Ruthenium Dinuclear Complexes

A series of sulfido-bridged tungsten−ruthenium dinuclear complexes Cp*W(μ-S)3RuX(PPh3)2
(4a; X = Cl, 4b; X = H), Cp*W(O)(μ-S)2RuX(PPh3)2 (5a; X = Cl, 5b; X = H), and Cp*W(NPh)(μ-S)2RuX(PPh3)2 (6a; X = Cl, 6b; X = H) have been synthesized by the reactions of (PPh4)[Cp*W(S)3] (1), (PPh4)[Cp*W(O)(S)2] (2), and (PPh4)[Cp*W(NPh)(S)2] (3), with RuClX(PPh3)3 (X = Cl, H). The heterolytic cleavage
of H2 was found to proceed at room temperature upon treating 5a and 6a with NaBArF4 (ArF = 3, 5-C6H3(CF3)2) under atmospheric pressure of H2, which gave rise to [Cp*W(OH)(μ-S)2RuH(PPh3)2](BArF4) (7a)
and [Cp*W(NHPh)(μ-S)2RuH(PPh3)2](BArF4) (8), respectively. When Cp*W(O)(μ-S)2Ru(PPh3)2H (5b) was
treated with a Brφnstead acid, [H(OEt2)2](BArF4) or HOTf, protonation occurred exclusively at the terminal
oxide to give [Cp*W(OH)(μ-S)2RuH(PPh3)2](X) (7a; X = BArF4, 7b; X = OTf), while the hydride remained
intact. The analogous reaction of Cp*W(μ-S)3Ru(PPh3)2H (4b) led to immediate evolution of H2. Selective
deprotonation of the hydroxyl group of 7a or 7b was induced by NEt3 and 4b, generating Cp*W(O)(μ-S)2Ru(PPh3)2H (5b). Evolution of H2 was also observed for the reactions of 7a or 7b with CH3CN to give
[Cp*W(O)(μ-S)2Ru(CH3CN)(PPh3)2](X) (11a; X = BArF4, 11b; X = OTf). We examined the H/D exchange
reactions of 4b, 5b, and 7a with D2 and CH3OD, and found that facile H/D scrambling over the W−OH and
Ru−H sites occurred for 7a. Based on these experimental results, the mechanism of the heterolytic H2
activation and the reverse H2 evolution reactions are discussed.