Catalytic behavior of a polynuclear Mg-Mo complex and nitrogenase active site (FeMoco) isolated from the enzyme in reactions with C2H2, N2, and CO: A comparative study

Abstract

In order to identify common and distinctive features in the catalytic behavior of natural and artificial nitrogen fixation clusters, the kinetics of the catalytic reduction of C 2 H 2 in the presence of Mg-Mo cluster (1) was investigated and compared with the kinetics of acetylene reduction catalyzed by the cluster FeMoco (2) isolated from the enzyme nitrogenase we studied previously. The reactions were conducted in the presence of Zn/Hg and Eu/Hg as reducing agents and PhSH and C 6 F 5 SH as proton donors, i.e., under the same conditions as had been used in the case of 2. Both polynuclear Mg-Mo complex and the europium amalgam reduced FeMoco have multiple interdependent binding sites for substrates and/or inhibitors. Carbon monoxide inhibits the acetylene reduction much less efficiently in systems with clus ter 1 than in systems with cluster 2, although the type of inhibition is mixed in both systems: CO binds to multiple sites of the cluster and affects both C 2 H 2 complexation to the reduced cluster and decomposition of the catalyst-substrate complex to give the products. Unlike isolated FeMoco, the Mg-Mo cluster efficiently catalyzes the reduction of molecular nitro gen. The reaction is greatly inhibited by acetylene, while no inhibiting effect of N 2 is observed in acetylene reduction, as was found earlier for a system with the natural cluster as the catalyst.Previously, 1 a nitrogen reducing system based on poly nuclear molybdenum complexes was discovered. Cur rently, this is the only known non enzyme system capable of catalytic reduction of N 2 at atmospheric pressure and room temperature at rates comparable with nitro genase. The reaction is carried out in methanol with a minor water additive, which apparently serves as the pro ton donor in ammonia and/or hydrazine formation. Sodium amalgam, europium amalgam or a cathode with a specified potential equal to the Na/Hg potential can serve as reducing agents. 2 The molecular structure of the Mg containing molybdenum anionic cluster {[Mg 2 Mo 8 O 22 (MeO) 6 (MeOH) 4 ] 2-[Mg(MeOH) 6 ] 2+ }• •6MeOH (1), which forms the active site of the system upon reduction, has been determined 3 by X ray diffrac tion (Fig. 1).The available data 1-4 indicate that this compound is reduced with retention of the cluster core, although, strictly speaking, the structure of the complex active with respect to N 2 is unknown. The molecular mechanism of N 2 catalytic reduction with participation of this cluster is also unknown.The active site of the natural nitrogen fixing enzyme nitrogenase incorporates an octanuclear heterobimetallic cluster FeMoco ((µ 6 N)MoFe 7 S 9 •homocitrate (2)) in which nitrogen is bound and then reduced to ammonia. 5 The chemical mechanism of the multielectron reduction of nitrogenase substrates catalyzed by the cluster is un known.In order to elucidate the function of the cofactor and the contribution made by the whole protein matrix and by the amino acids located most closely to FeMoco to the nitrogen reduction under mild conditions, we studied the catalyt...

Modeling the nitrogenase FeMo cofactor with high‐spin Fe8S9X+ (XN, C) clusters. Is the first step for N2 reduction to NH3 a concerted dihydrogen transfer?

“…Besides N 2 , several other molecules are known to bind to the FeMo cofactor, and some are reduced as well 80–94. For example acetylene is reduced to ethylene but not to ethane, while carbon monoxide binds to the FeMo cofactor but is not reduced.…”

“…It is known that nitrogenase can reduce acetylene to ethylene 95. The first PR step to form 4PRN‐HCCH2 and the second PR step to form 4PRN‐H2CCH2 have CH bond energies of 76.0 and 77.5 kcal/mol, respectively.…”

“…Carbon monoxide is known to be an inhibitor of nitrogen reduction by nitrogenase 94. The ligand binds to a single Fe site at low concentrations and adopts a bridging site at high concentrations.…”

Density functional theoretical study of the electronic structure and vibrational spectra of a polynuclear [Mg2(MeOH)4Mo8O22(OMe)6]2− complex

“…At the same time, the active center of nitrogenase, a Fe-Mo cofactor, in the isolated form is capable of only reversible N 2 molecule binding. 2,3 Therefore, to understand the role of the surroundings, information on the structure of the synthetic active center is of special importance. 3 However, the experimental determination of its structure is difficult to perform.…”

“…2,3 Therefore, to understand the role of the surroundings, information on the structure of the synthetic active center is of special importance. 3 However, the experimental determination of its structure is difficult to perform. Only the structure of a catalyst precursor, mixed-valence cluster [Mg 2 (MeOH) 4 4 It is reasonable to use theoretical modeling for understanding the structure of 1 in the reduced state.…”