Formation of ammonia in the reactions of a tungsten dinitrogen with ruthenium dihydrogen complexes under mild reaction conditions

Treatment of cis-[W(N-2)(2)(PMe2Ph)(4)] (5) with an equilibrium mixture of trans-[RuCl(eta (2)-H-2)(dppp)(2)]X (3) with pK(a) = 4.4 and [RuCl(dppp)(2) ]X (4) [X = PF6, BF4, or OTf; dppp = 1,3-bis(diphenylphosphino)propane] con taining 10 equiv of the Ru atom based on tungsten in benzene-dichloroethane at 55 degreesC for 24 h under 1 atm of H-2 gave NH3 in 45-55% total yields based on tungsten, together with the formation of trans-[RuHCl(dppp)(2)] ( 6). Free NH3 in 9-16% yields was observed in the reaction mixture, and furt her NH3 in 36-45% yields was released after base distillation. Detailed stu dies on the reaction of 5 with numerous Ru(eta (2)-H-2) complexes showed th at the yield of NH3 produced critically depended upon the pK(a) value of th e employed Ru(eta (2)-H-2) complexes. When 5 was treated with 10 equiv of t rans-[RuCl(eta (2)-H-2)(dppe)(2)]X (8) with pK(a) = 6.0 [X = PF6, BF4, or O Tf; dppe = 1,2-bis(diphenylphosphino)ethane] under 1 atm of H-2, NH3 was fo rmed in higher yields (up to 79% total yield) compared with the reaction wi th an equilibrium mixture of 3 and 4. If the pK(a) value of a Ru(eta (2)-H- 2) complex was increased up to about 10, the yield of NH3 was remarkably de creased. In these reactions, heterolytic cleavage of H-2 seems to occur at the Ru center via nucleophilic attack of the coordinated N-2 on the coordin ated H-2 where a proton (H+) is used for the protonation of the coordinated N-2 and a hydride (H-) remains at the Ru atom. Treatment of 5, trans-[W(N- 2)(2)(PMePh2)(4)] (14), or trans-[M(N-2)(2)(dppe)(2)] [M = Mo (1), W (2)] w ith Ru(eta (2)-H-2) complexes at room temperature led to isolation of inter mediate hydrazido(2-) complexes such as trans-[W(OTf)(NNH2)(PMe2Ph)(4)]OTf (19), trans-[W(OTf)(NNH2)(PMePh2)(4)]OTf (20), and trans-[WX(NNH2)(dppe)(2) ](+) [X = OTf (15), F (16)]. The molecular structure of 19 was determined b y X-ray analysis. Further ruthenium-assisted protonation of hydrazido(2-) i ntermediates such as 19 with H-2 at 55 degreesC was considered to result in the formation of NH3. concurrent with the generation of W(VI) species. All of the electrons required for the reduction of Na are provided by the zero valent tungsten.