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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 [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

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]

The synthesis of 3-(dimethylboryl)pyridine and the effect of substituents at the boron atom on the stability of the oligomer is investigated. The result that both type 1- and 2- nucleophilic substitution mechanisms operate together during the reaction is verified by kinetic studies and calculations with model compounds.

The all-ferric [Fe4S4]4+ cluster [Fe4S4{N(SiMe3)2}4] 1 and its one-electron reduced form [1]- serve as convenient precursors for the synthesis of 3:1-site differentiated [Fe4S4] clusters and high-potential iron-sulfur protein (HiPIP) model clusters. The reaction of 1 with four equivalents (equiv) of the bulky thiol HSDmp (Dmp = 2,6-(mesityl)2C6H3, mesityl = 2,4,6-Me3C6H2) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of [Fe4S4(SDmp)3(THF)3] 2. Cluster 2 contains an octahedral iron atom with three THF ligands, and its Fe(S)3(O)3 coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of [1]- with four equiv of HSDmp gave [Fe4S4(SDmp)4]- 3, which models the oxidized form of HiPIP. The THF ligands in 2 can be replaced by tetramethyl-imidazole (Me4Im) to give [Fe4S4(SDmp)3(Me4Im)] 4 modeling the [Fe4S4(Cys)3(His)] cluster in hydrogenases, and its one-electron reduced form [4]- was synthesized from the reaction of 3 with Me4Im. The reversible redox couple between 3 and [3]- was observed at E1/2 = -820 mV vs. Ag/Ag+, and the corresponding reversible couple for 4 and [4]- is positively shifted by +440 mV. The cyclic voltammogram of 3 also exhibited a reversible oxidation couple, which indicates generation of the all-ferric [Fe4S4]4+ cluster, [Fe4S4(SDmp)4]. [ABSTRACT FROM AUTHOR]

The all-ferric [Fe4S4]4+ cluster [Fe4S4{N(SiMe3)2}4] 1 and its one-electron reduced form [1]- serve as convenient precursors for the synthesis of 3:1-site differentiated [Fe4S4] clusters and high-potential iron-sulfur protein (HiPIP) model clusters. The reaction of 1 with four equivalents (equiv) of the bulky thiol HSDmp (Dmp = 2,6-(mesityl)2C6H3, mesityl = 2,4,6-Me3C6H2) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of [Fe4S4(SDmp)3(THF)3] 2. Cluster 2 contains an octahedral iron atom with three THF ligands, and its Fe(S)3(O)3 coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of [1]- with four equiv of HSDmp gave [Fe4S4(SDmp)4]- 3, which models the oxidized form of HiPIP. The THF ligands in 2 can be replaced by tetramethyl-imidazole (Me4Im) to give [Fe4S4(SDmp)3(Me4Im)] 4 modeling the [Fe4S4(Cys)3(His)] cluster in hydrogenases, and its one-electron reduced form [4]- was synthesized from the reaction of 3 with Me4Im. The reversible redox couple between 3 and [3]- was observed at E1/2 = -820 mV vs. Ag/Ag+, and the corresponding reversible couple for 4 and [4]- is positively shifted by +440 mV. The cyclic voltammogram of 3 also exhibited a reversible oxidation couple, which indicates generation of the all-ferric [Fe4S4]4+ cluster, [Fe4S4(SDmp)4]. [ABSTRACT FROM AUTHOR]

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, (CO)3 Fe(μStBu)3Ni(SDmp), 1 (Dmp= C6H3-2;6-(mesityl)2), was synthesized from the reaction of the tetranuclear Fe-Ni-Ni-Fe complex [(CO)3Fe(μ-StBu)3Ni)2 (p-Br)2. 2 with NaSDmp at -40°C. The nickel site of complex 1 was found to add CO or CNtBU at -40°C to give (CO)3Fe(StBu)(μStBu)2Ni(CO) (SDmp), 3. or (CO)3 Fe(StBu) (μStBu)2Ni(CNtBu) (SDmp), 4, respectively. One of the CO bands of 3. appearing at 2055cm-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 1 is reversible. while the CNtBu adduct 4 is more robust. [ABSTRACT FROM AUTHOR]

Abstract: Coordinatively unsaturated rhodium and iridium complexes having a bulky thiolate, [Cp∗M(PMe3)(SDmp)](BArF 4) (1a: M=Rh; 1b: M=Ir; Dmp=2,6-(mesityl)2C6H3, ArF =3,5-(CF3)2C6H3), catalyzed the hydrogenation of benzaldehyde, N-benzylideneaniline, and cyclohexanone, under 1atm of H2 at low temperatures. In these catalytic reactions, the M–H/S–H complexes [Cp∗M(PMe3)(H)(HSDmp)](BArF 4) (2a: M=Rh; 2b: M=Ir) generated via H2 heterolysis by 1a or 1b were suggested to transfer both M–H hydride and S–H proton to substrates. The catalytic reactions were terminated by the dissociation of H-SDmp from the metal centers of 2a and 2b that occurs at ambient temperature under H2 atmosphere. [Copyright &y& Elsevier]

The FeMo-cofactor of nitrogenase, a metal-sulfur cluster that contains eight transition metals, promotes the conversion of dinitrogen into ammonia when stored in the protein. Although various metal-sulfur clusters have been synthesized over the past decades, their use in the activation of N2 has remained challenging, and even the FeMo-cofactor extracted from nitrogenase is not able to reduce N2. Herein, we report the activation of N2 by a metal-sulfur cluster that contains molybdenum and titanium. An N2 moiety bridging two [Mo3S4Ti] cubes is converted into NH3 and N2H4 upon treatment with Brønsted acids in the presence of a reducing agent. Nitrogenase—whose cofactor consists of a metal-sulfur cluster—catalyzes the production of NH3 from N2, but designing metal-sulfur complexes capable of promoting this conversion remains challenging. Here, the authors report on the activation of N2 by a metal-sulfur cluster containing [Mo3S4Ti] cubes, demonstrating NH3 and N2H4 production. [ABSTRACT FROM AUTHOR]

The FeMo-cofactor of nitrogenase, a metal-sulfur cluster that contains eight transition metals, promotes the conversion of dinitrogen into ammonia when stored in the protein. Although various metal-sulfur clusters have been synthesized over the past decades, their use in the activation of N 2 has remained challenging, and even the FeMo-cofactor extracted from nitrogenase is not able to reduce N 2 . Herein, we report the activation of N 2 by a metal-sulfur cluster that contains molybdenum and titanium. An N 2 moiety bridging two [Mo 3 S 4 Ti] cubes is converted into NH 3 and N 2 H 4 upon treatment with Brønsted acids in the presence of a reducing agent.

The all-ferric [Fe(4)S(4)](4+) cluster [Fe(4)S(4){N(SiMe(3))(2)}(4)] 1 and its one-electron reduced form [1](-) serve as convenient precursors for the synthesis of 31-site differentiated [Fe(4)S(4)] clusters and high-potential iron-sulfur protein (HiPIP) model clusters. The reaction of 1 with four equivalents (equiv) of the bulky thiol HSDmp (Dmp = 2,6-(mesityl)(2)C(6)H(3), mesityl = 2,4,6-Me(3)C(6)H(2)) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of [Fe(4)S(4)(SDmp)(3)(THF)(3)] 2. Cluster 2 contains an octahedral iron atom with three THF ligands, and its Fe(S)(3)(O)(3) coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of [1](-) with four equiv of HSDmp gave [Fe(4)S(4)(SDmp)(4)](-) 3, which models the oxidized form of HiPIP. The THF ligands in 2 can be replaced by tetramethyl-imidazole (Me(4)Im) to give [Fe(4)S(4)(SDmp)(3)(Me(4)Im)] 4 modeling the [Fe(4)S(4)(Cys)(3)(His)] cluster in hydrogenases, and its one-electron reduced form [4](-) was synthesized from the reaction of 3 with Me(4)Im. The reversible redox couple between 3 and [3](-) was observed at E(1/2) = -820 mV vs. Ag/Ag(+), and the corresponding reversible couple for 4 and [4](-) is positively shifted by +440 mV. The cyclic voltammogram of 3 also exhibited a reversible oxidation couple, which indicates generation of the all-ferric [Fe(4)S(4)](4+) cluster, [Fe(4)S(4)(SDmp)(4)].