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Nitrogenase employs a sophisticated electron transfer system and a Mo-Fe-S-C cofactor, designated the M-cluster [(cit)MoFe7 S9 C]), to reduce atmospheric N2 to bioaccessible NH3 . Previously, we have shown that the cofactor-free form of nitrogenase can be repurposed as a protein scaffold for the incorporation of a synthetic Fe-S cluster [Fe6 S9 (SEt)2 ]4- . Here, we demonstrate the utility of an asymmetric Mo-Fe-S cluster [Cp*MoFe5 S9 (SH)]3- as an alternative artificial cofactor upon incorporation into the cofactor-free nitrogenase scaffold. The resultant semi-artificial enzyme catalytically reduces C2 H2 to C2 H4 , and CN- into short-chain hydrocarbons, yet it is clearly distinct in activity from its [Fe6 S9 (SEt)2 ]4- -reconstituted counterpart, pointing to the possibility to employ molecular design and cluster synthesis strategies to further develop semi-artificial or artificial systems with desired catalytic activities.

The M-cluster is the [(homocitrate)MoFe7S9C] active site of nitrogenase that is derived from an 8Fe core assembled viacoupling and rearrangement of two [Fe4S4] clusters concomitant with the insertion of an interstitial carbon and a 'ninth sulfur'. Combining synthetic [Fe4S4] clusters with an assembly protein template, here we show that sulfite can give rise to the ninth sulfur that is incorporated in the catalytically important belt region of the cofactor after the radical S-adenosyl-L-methionine-dependent carbide insertion and the concurrent 8Fe-core rearrangement have already taken place. Based on the differential reactivity of the formed cluster species, we also propose a new [Fe8S8C] cluster intermediate, the L*-cluster, which is similar to the [Fe8S9C] L-cluster, but lacks the ninth sulfur from sulfite. This work provides a semi-synthetic tool for protein reconstitution that could be widely applicable for the functional analysis of other FeS systems.

Nitrogenase catalyzes substrate reduction at its cofactor center ([(Cit)MoFe7S9C](n-); designated M-cluster). Here, we report the formation of an artificial, nitrogenase-mimicking enzyme upon insertion of a synthetic model complex ([Fe6S9(SEt)2](4-); designated Fe6(RHH)) into the catalytic component of nitrogenase (designated NifDK(apo)). Two Fe6(RHH) clusters were inserted into NifDK(apo), rendering the conformation of the resultant protein (designated NifDK(Fe)) similar to the one upon insertion of native M-clusters. NifDK(Fe) can work together with the reductase component of nitrogenase to reduce C2H2 in an ATP-dependent reaction. It can also act as an enzyme on its own in the presence of Eu(II)DTPA, displaying a strong activity in C2H2 reduction while demonstrating an ability to reduce CN(-) to C1-C3 hydrocarbons in an ATP-independent manner. The successful outcome of this work provides the proof of concept and underlying principles for continued search of novel enzymatic activities based on this approach.

A cubic metal‐sulfur cluster containing three Mo ions and a Pd ion, [CpSiEt33Mo3S4Pd]Cl (Mo3Pd, CpSiEt3=C5Me4SiEt3), was synthesized by the incorporation of the Pd ion into a Mo3S4 cluster [CpSiEt33Mo3S4] (Mo3). Mo3Pd was characterized by 1H NMR, UV‐vis, X‐ray crystallography, and cyclic voltammetry measurements. The electrochemical measurements demonstrated reversible one‐ and two‐electron reduction processes for Mo3Pd, which suggested potential catalytic activity for two‐electron substrate reductions such as hydrogen evolution reaction. Controlled potential electrolysis in the presence of Mo3Pd and trifluoroethanol in THF solvent displayed H2 formation with a constant current over 60 min. The amount of generated H2 by Mo3Pd was two times higher than Mo3, indicating the catalytic activity facilitated by the Pd center. The mechanism of the catalytic cycle was determined by density functional theory. [ABSTRACT FROM AUTHOR]

5‐Diethylboryl‐2,3′‐bipyridine (1), which is inaccessible by conventional batch methods, was synthesized by using a flow microreactor. Compound 1 was obtained as an equilibrium mixture of a cyclic trimer and a cyclic tetramer in solution, the latter of which was crystallized in benzene by vapor diffusion of hexane at 7 °C. The dynamic nature of this system was confirmed by solvent‐ and concentration‐dependent experiments. Notably, the dynamics was verified by using flow NMR spectroscopy, which revealed that the time required to reach equilibrium was influenced by the solvent ratio (<18 s, 24–28 s, and 34–42 s in 2 : 1, 1 : 1, and 1 : 2 mixtures of [D6]acetone and C6D6, respectively). Compound 1 and 3‐[4′‐(diethylboryl)phenyl]pyridine (2) exhibited different self‐assembly behavior in solution and crystals. Density functional theory calculations suggested that this difference was largely due to enhanced planarity between two consecutive aromatic rings. [ABSTRACT FROM AUTHOR]

Nitrogen (N2) fixation by nature, which is a crucial process for the supply of bio-available forms of nitrogen, is performed by nitrogenase. This enzyme uses a unique transition-metal–sulfur–carbon cluster as its active-site co-factor ([(R-homocitrate)MoFe7S9C], FeMoco)1,2, and the sulfur-surrounded iron (Fe) atoms have been postulated to capture and reduce N2 (refs. 3–6). Although there are a few examples of synthetic counterparts of the FeMoco, metal–sulfur cluster, which have shown binding of N2 (refs. 7–9), the reduction of N2 by any synthetic metal–sulfur cluster or by the extracted form of FeMoco10 has remained elusive, despite nearly 50 years of research. Here we show that the Fe atoms in our synthetic [Mo3S4Fe] cubes11,12 can capture a N2 molecule and catalyse N2 silylation to form N(SiMe3)3 under treatment with excess sodium and trimethylsilyl chloride. These results exemplify the catalytic silylation of N2 by a synthetic metal–sulfur cluster and demonstrate the N2-reduction capability of Fe atoms in a sulfur-rich environment, which is reminiscent of the ability of FeMoco to bind and activate N2.Iron atoms in a synthetic metal–sulfur cluster can capture nitrogen and catalyse its silylation, demonstrating successful nitrogen reduction by iron atoms in a sulfur-rich environment. [ABSTRACT FROM AUTHOR]

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, [(CO).sub.3]Fe[([mu]-[S.sup.t]Bu).sub.3]Ni(SDmp), 1 (Dmp= [C.sub.6][H.sub.3]-2;6-[(mesityl).sub.2]), was synthesized from the reaction of the tetranudear Fe-Ni-Ni-Fe complex [[[(CO).sub.3]Fe[([mu]-[S.sup.t]Bu).sub.3]Ni].sub.2]([mu]-Br)2, 2 with NaSDmp at -40[degrees]C. The nickel site of complex 1 was found to add CO or CNtBu at -40[degrees]C to give [(CO).sub.3]Fe([S.sup.t]Bu)[([mu]-[S.sup.t]Bu).sub.2]Ni(CO) (SDmp), 3, or [(CO).sub.3]Fe([S.sup.t]Bu)[([mu]-[S.sup.t]Bu).sub.2]Ni([CN.sup.t]Bu)(SDmp), 4, respectively. One of the CO bands of 3, appearing at 2055[cm.sup.-1] in the infrared spectrum, was assigned as the Ni-CO band, and this frequency is comparable to those observed for the CO-inhibited forms of [NiFe] hydrogenase. Like the CO-inhibited forms of [NiFe] hydrogenase, the coordination of CO at the nickel site of I is reversible, while the CNtBu adduct 4 is more robust. iron | nickel | thiolates doi/10.10731pnas.0913399107

The preparation of the mono-[Tm.sup.R] (where Tm.sup.R= hydrotris(mercaptoimidazolyl)borates) complexes of metal halides that are useful starting materials for various transition metal complex of [Tm.sup.R] are reported. It is indicated that [Tm.sup.R.sub.2]M as well as [Tm.sup.R]MCl is formed at the initial stage of the reaction between M[Cl.sub.2] and the [Tm.sup.R] anion.

A description of the synthesis and characterization of sulfo-bridged dinuclear Mo-Cu complexes related to the active site of [MoCu] carbon monoxide dehydrogenase is presented. The dinuclear Mo-Cu complex, [O(sub2)MoS(sub2)Cu(SPh)](super 2-), is prepared from the reaction of [MoO(sub2)S(sub2)](super 2-) with [Cu(SPh)(sub3)](super 2-).

The elimination-addition mechanism for nucleophilic substitution at the vinylic carbon is established, and the reaction can be employed as a mild method for generation of cyclohexynes. The regioselectivity of the nucleophilic addition to the substituted cyclohexynes is explored.

The heterolytic cleavage of H2 is regarded as a key step in the metalloenzymatic reactions of hydrogenases, the actives sites of which consist of the sulfur-bridged Fe/Fe and Fe/Ni dinuclear centers. Based on the experimental results, the mechanism of the heterocyclic H2 activation and the reverse H2 evolution reactions are discussed.