Formation of Ammonia in the Reactions of a Tungsten Dinitrogen with Ruthenium Dihydrogen Complexes under Mild Reaction Conditions¹

Abstract

Treatment of cis-[W(N2)2(PMe2Ph)4] (5) with an equilibrium mixture of trans-[RuCl(eta 2-H2)(dppp)2]X (3) with pKa = 4.4 and [RuCl(dppp)2]X (4) [X = PF6, BF4, or OTf; dppp = 1,3-bis(diphenylphosphino)propane] containing 10 equiv of the Ru atom based on tungsten in benzene-dichloroethane at 55 degrees C for 24 h under 1 atm of H2 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 further NH3 in 36-45% yields was released after base distillation. Detailed studies on the reaction of 5 with numerous Ru(eta 2-H2) complexes showed that the yield of NH3 produced critically depended upon the pKa value of the employed Ru(eta 2-H2) complexes. When 5 was treated with 10 equiv of trans-[RuCl(eta 2-H2)(dppe)2]X (8) with pKa = 6.0 [X = PF6, BF4, or OTf; dppe = 1,2-bis(diphenylphosphino)ethane] under 1 atm of H2, NH3 was formed in higher yields (up to 79% total yield) compared with the reaction with an equilibrium mixture of 3 and 4. If the pKa value of a Ru(eta 2-H2) complex was increased up to about 10, the yield of NH3 was remarkably decreased. In these reactions, heterolytic cleavage of H2 seems to occur at the Ru center via nucleophilic attack of the coordinated N2 on the coordinated H2 where a proton (H+) is used for the protonation of the coordinated N2 and a hydride (H-) remains at the Ru atom. Treatment of 5, trans-[W(N2)2(PMePh2)4] (14), or trans-[M(N2)2(dppe)2] [M = Mo (1), W (2)] with Ru(eta 2-H2) complexes at room temperature led to isolation of intermediate 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 by X-ray analysis. Further ruthenium-assisted protonation of hydrazido(2-) intermediates such as 19 with H2 at 55 degrees C 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 N2 are provided by the zerovalent tungsten.

A simple synthesis of trans-RuCl(CCR)(dppe)2 complexes and representative molecular structures

“…The conversion of cis-1 to the active five-coordinate species [RuCl(dppe) 2 ] + ([2] + ) takes place readily upon reaction with alkali metal salts including NaPF 6 [3] and KPF 6 [11], and salts of [2] + can be isolated from reaction of cis-1 with NaPF 6 [42,65], NaOTf or NaBPh 4 [66]. The conversion of trans-1 to salts of [2] + has been implicated under similar conditions, although the reaction is considerably slower [10,65].…”

Activation of Molecular Nitrogen: Coordination, Cleavage and Functionalization of N2 Mediated By Metal Complexes

“…Yields of ammonia depend heavily on the geometry and electronics of the tungsten complex and the acidity of the ruthenium hydride used, and the reaction is by no means catalytic; the second hydride remains on ruthenium and is not transferred to nitrogen. [92,93] The reaction of related tungsten complexes with sulfido-bridged dimolybdenum complexes under H 2 has been recently reported. [94] The bis(dinitrogen) complex trans-W(N 2 ) 2 (dppe) 2 4 is used with a large excess of the sulfido reagent and excess H 2 , low yields of ammonia can be detected.…”

Car–Parrinello Molecular Dynamics Study of the Initial Dinitrogen Reduction Step in Sellmann‐Type Nitrogenase Model Complexes

“…Apparently, protonation (path 2) of the sulfur donors should be considered as an essential and most promising initial step during the reduction of the dinuclear N 2 complex. [9,10,[16][17][18][19][20][21][22][23][24][25][26][27][28][29] It is the purpose of this study to investigate this first protonation reduction step in order to obtain a microscopic picture of a possible mechanism of a two-electron reduction of protonated Sellmann-type complexes. A detailed understanding of the reaction mechanism should yield suggestions for a particular ligand sphere for sulfur-rich metal complexes designed to stimulate nitrogen activation.…”