Search

The biosynthesis of the active site of the [FeFe]-hydrogenases (HydA1), the H-cluster, is of interest because these enzymes are highly efficient catalysts for the oxidation and production of H2. The biosynthesis of the [2Fe]H subcluster of the H-cluster proceeds from simple precursors, which are processed by three maturases: HydG, HydE, and HydF. Previous studies established that HydG produces an Fe(CO)2(CN) adduct of cysteine, which is the substrate for HydE. In this work, we show that by using the synthetic cluster [Fe2(μ-SH)2(CN)2(CO)4]2- active HydA1 can be biosynthesized without maturases HydG and HydE.

[FeFe]-hydrogenases use a unique organometallic complex, termed the H cluster, to reversibly convert H2 into protons and low-potential electrons. It can be best described as a [Fe4S4] cluster coupled to a unique [2Fe]H center where the reaction actually takes place. The latter corresponds to two iron atoms, each of which is bound by one CN- ligand and one CO ligand. The two iron atoms are connected by a unique azadithiolate molecule (-S-CH2-NH-CH2-S-) and an additional bridging CO. This [2Fe]H center is built stepwise thanks to the well-orchestrated action of maturating enzymes that belong to the Hyd machinery. Among them, HydG converts l-tyrosine into CO and CN- to produce a unique l-cysteine-Fe(CO)2CN species termed complex-B. Very recently, HydE was shown to perform radical-based chemistry using synthetic complex-B as a substrate. Here we report the high-resolution crystal structure that establishes the identity of the complex-B-bound HydE. By triggering the reaction prior to crystallization, we trapped a new five-coordinate Fe species, supporting the proposal that HydE performs complex modifications of complex-B to produce a monomeric "SFe(CO)2CN" precursor to the [2Fe]H center. Substrate access, product release, and intermediate transfer are also discussed.

The H-cluster of [FeFe]-hydrogenase consists of a [4Fe-4S]H-subcluster linked by a cysteinyl bridge to a unique organometallic [2Fe]H-subcluster assigned as the site of interconversion between protons and molecular hydrogen. This [2Fe]H-subcluster is assembled by a set of Fe-S maturase enzymes HydG, HydE and HydF. Here we show that the HydG product [FeII(Cys)(CO)2(CN)] synthon is the substrate of the radical SAM enzyme HydE, with the generated 5'-deoxyadenosyl radical attacking the cysteine S to form a C5'-S bond concomitant with reduction of the central low-spin Fe(II) to the Fe(I) oxidation state. This leads to the cleavage of the cysteine C3-S bond, producing a mononuclear [FeI(CO)2(CN)S] species that serves as the precursor to the dinuclear Fe(I)Fe(I) center of the [2Fe]H-subcluster. This work unveils the role played by HydE in the enzymatic assembly of the H-cluster and expands the scope of radical SAM enzyme chemistry.

The structural chemistry of cation-containing defect cyanometalate boxes is elucidated. A promising motif for cation receptors is represented by the defect box as these cations are more solvent-accessible and kinetically labile.

A range of derivatives of the type [Fe2(SR)(Sub 2)(CN)(Sub x)(CO)(Sub 4)] (Super 2-) can be prepared by CN-for -CO substitution reactions. These syntheses occur for alkyl- and arylthiolato derivatives and for alkyldithiolates. The products are stable and have been fully characterized by X-ray diffraction for three derivatives.

A DFT study into the N-H...S hydrogen bond in (TACN)2Fe2S6 and in model systems, was undertaken.

Research was conducted to examine the addition of alkenes to ReS4-. The well-behaved reactions were due mainly to the fact that they were cleanly reversible and that single adducts were generated. The reversible binding of alkene is extraordinary for an inorganic complex, particularly when the alkene is not directly bound to the metal. Results demonstrate that inorganic sulfides are fully capable of binding alkenes.

A soluble cubic cyanometalate framework that binds metal cations particularly Cs+ has been synthesized. The box-like cages (M(Cp*Rh(CN)3)4(Mo(CO)3)4)(super 3-) result from the reaction of (Cp*Rh(CN)3)(super -) with (eta(super 6)-C(sub 6)H(sub 3)Me(sub 3))Mo(CO)(sub 3) in the presence of K+ and Cs+. They were isolated as their Et4N+ salts with M = K+ and Cs+. Crystallographic analysis reveal that the M8(mu-CN)12 cages have alkali metal cations. They 12 external CO and 4 external C4Me5 ligands.

The redox behaviour of the S-S bond is significant in several areas of chemistry. Organic disulfides are of interest as an alternative to inorganic sulfur in the cathodes, and they have a similar redox cycle to that involving alkali metal polysulfides. Several new binary carbon sulfide anions and molecules have been prepared, including the tricyclic species C(sub6)S(sub8), which is prepared via thermal decomposition of C(sub3)S(sub5)H(sub2).

Cp4Fe4(CO)4 is primarily produced from the thermolysis of a xylene solution of Cp2Fe2(CO)4 and PPh3, together with smaller amounts of (C5H4Ph)Cp3Fe4(CO)4 and Cp3Fe3(CO)3(PPh2). This was gleaned from a study of the functionalization of performed Cp-metal clusters. The main finding is that Cp4Fe4(CO)4 can be functionalized using various methods, such as alkylation and arylation by using organolithium reagents and deprotonation with lithium diisopropylamide and treatment with electrophiles to afford (C5H4X)Cp3Fe4(CO)4.

Two dicationic clusters are prepared and purified by ion exchange chromatography, and crystallographic studies are employed to characterize these compounds. Variable temperature 1H NMR study reveals the fluxional nature of the molecule, and it is due to mobile metal-metal bond. Internal Ir(super IV)/Ir(super III) self-exchange is responsible for this mobility. Cyclovoltammetry is employed to analyze redox properties of the clusters.

A study was done on the reduction and protonation of eta4-thiophene ligands in (C6R6)Ru(C4R4S)2+. It was shown that the hydrogenolysis of thiophenes by sulfided metal catalysts proceed by thiophene coordination, electron transfer to the heterocycle, proton transfer to the ring and then C-S cleavage. These findings serve as the basis for further studies on the reactivity of partially hydrogenated thiophene ligands.

A study was done on base hydrolysis of (C5Me5)Rh(C4Me4S)2+ and the relationship of the reaction intermediate to the isomers of (C5Me5)Rh(C4Me4SO). It was shown that hydrolysis of the methylthiophene complex involves the use of OH- as the nucleophile. Thiophene accepts up to two equivalents of the hydroxide resulting in the net addition of O2- to the heterocycle. Further studies have demonstrated that nucleophilic attack at thiophene can occur at sulfur and nucleophile additions at sulfur can occur through initial attack at sulfur.

The complex ((C5Me5)Rh)3(eta4,eta1-C4Me4S)2 (1) was obtained from the thermal decomposition of (C5Me5)Rh(eta4-C4Me4S) and characterized by elemental analysis and 1H and 13C NMR spectroscopy. Single-crystal X-ray diffraction showed the structure of ((C5Me4Et)Rh(eta4, eta1-C4Me4S))2Rh(C5Me4Et) to consist of a pair of (C5Me4Et)Rh(eta4-C4Me4S) ligands binded by sulfur atoms to a central (C5Me4Et)Rh unit. The decomposition of 1 followed a first-order kinetics.

Several studies are conducted to explain the redox behaviour exhibited by the synthetic models of the [FeFe]-hydrogenase with the amine cofactor and its N-benzyl derivative. The present azadithiolate cofactor is shown to play an extremely significant role in these models.

Phosphine-modified thioester derivatives have served as efficient precursors to phosphine-stabilized ferrous acyl thiolato carbonyls, which has replicated key structural features of the active site of the hydrogenase Hmd. Tricarbonyl Fe(SPh)([Ph.sub.2]P[C.sub.6][H.sub.4]CO)[(CO).sub.3] has reversibly protonated to give a thermally labile derivative, IR measurements of which have shown that the acyl and thiolate ligands are not protonated in Hmd.

The [NiFe]-hydrogenase model complex NiFe(pdt)(dppe)[(CO).sub.3] (pdt = 1,3-propanedithiolate) is prepared. The mechanism of [H.sub.2] evolution has followed an ECEC sequence, where E and C have corresponded to one-electron reductions and protonations.

The iron-nickel dithiolato hydride is characterized crystallographically. The hydride has undergone rapid deprotonation and is an electrocatalyst for hydrogen evolution from trifluoroacetic acid.

The acid-catalyzed hydrogenation of ketones by amido-amine chelates of Ru and Ir are examined by focusing on the hydrogen activation step. The addition of [H.sub.2] to the catalyst Cp*Ir (TsDPEN-H) (TsDPEN = racemic [H.sub.2]NCHPhCHPhN[Ts.sup.-]) is more favorable than for corresponding (cymene)Ru derivatives.

The functional role of oxa- and azadithiolates as proton relays is analyzed. The studies have shown that although both azadithiolate and oxadithiolate display relay-like behavior, only the azadithiolate has enabled hydride formation from weak acids, which is relevant to catalysis at low overpotentials.

Several experiments and studies are conducted to demonstrate the role of a non-innocent ligand in the redox-switched oxidation of dihydrogen. The results prove that redox is an extremely essential part of the hydrogenases.

The article discusses the homogeneous catalytic reduction of dioxygen molecules using various transfer hydrogenation catalysts. The reduction reaction is found to be highly temperature-dependent.

A new family of organo-cyanometalate cages is described that are based on the [[Cb*Co].sup.+] fragment (Cb* = [[eta].sup.4]-[C.sub.4][Me.sub.4]). Crystallographic studies of [Cb*Co[(NCMe).sub.3]]P[F.sub.6] have displayed that the range of N-Co-N bond angles has varied from 97.8 to 89.0, which is compatible with box-like cages.

The 16e amido complexes, [L.sub.4]M-N[R.sub.2], represent a potentially rich source of unusual organometallic Lewis acids. A study demonstrates how Bronsted acids convert 16e amido-amine complexes to novel organometallic Lewis acids that exhibit wide-ranging reactivity.

A systematic study of diferrous dicyano dithiolates is described. The diferrous cyanide features the semibridging Co-ligand, with Fe-muC bond lengths of 2.15 and 1.85 Angstrom.

The preparative foundation for systematic studies on diferrous dithiolates containing mu-CO ligands is laid. Syntheses of mixed ligand differous dithiolate complexes of the general type [Fe2(SR)(sub 2)(CO)(sub 7-x)L(sub x)](super 2+) are developed using isocyanides as illustrative donor ligands.

The reduction of diferrous dithiolates with hydride reagents gives a new approach to an active site model for a vital intermediate. The terminal hydride reacts with Bronsted acids to give H2, in contrast to the nonreactivity of the isomeric mu-H compounds and the new synthetic approach is applicable to other diferrous dithiolates, including those bearing cyanide ligands.

The first example of H2 addition to hydrogenase model and the first dihydrogen complex of a hydrogenase active site model are presented. Experiments demonstrate that diruthenium dithiolates oxidatively add H2 and that the resulting dihydrido species can be protonated to form a dihydrogen complex.

The condensation of iron carbonyl sulfides, formaldehydes and amines to give the proposed azadithiolate cofactor of the Fe-only hydrogenases is detailed. Fe2(SCH)2(CO)6 efficiently condenses with formaldehyde in the presence of primary amines to give the corresponding azadithiolates on addition of diiron complex to a premixed THF solution of paraformaldehyde and the amine at 0 degree centigrade following warming at room temperature.

It has been established that quite basic analogues of the Fe-only hydrogenases can be effective catalysts for the reduction of protons to dihydrogen, starting with protonation at the Fe-Fe bond. The Fe-hydrogenases are believed to bind protons at a single Fe centre, but this catalyst protonates at the metal-metal bond.

Research describing the chemical characteristics of the reactions of dihydrogen activation by homolytic and heterolytic methods is presented. Particular attention is given to descriptions of homolytic activation and to the use of iridium sulfide.

Research investigating the chemical influence that cyanide has on the carbonylation process of iron is presented. Particular attention is given to reactions of carbonylation of iron salts and to determining the role of cyanide in stabilizing low-spin iron.

A structure for the binuclear center of the Fe-only hydrogenases was proposed. The series Fe2(SMe)2-(CO)6-n(CN)n(super n-) were the targets of initial preparative efforts. Solutions of Fe2(SMe)2(CO)6(super 9) in methanol or acetonitrile reacted in minutes with Et4NCN to give exclusively Fe2(SMe)2(CO)4(CN)2(super 2-). The results showed that the electron-rich species Fe2(S2C3H6)2(CN)2(CO)4(super 2-) bore a structural and stoichiometric resemblance to the Fe-only hydrogenases.

Experiments were performed to investigate the synthesis, structure and reactivity of the iron sulfide triamine 1,4,7-triazacyclononane (TACN). An insoluble black species resulted from the sulfidation reaction between FeCl3(TACN) and Na2S.9H20. The utilization of TACN instead of Cp resulted to a metal sulfido complex that facilitated the analysis of the [(TACN)M]nSy series. The reactivity characteristics of molecular iron sulfido complexes may allow the formation of new iron sulfido clusters.

Solvatothermal synthesis techniques have successfully employed preformed C4Sx precursors in the creation of well-defined carbon sulfide polymers. The condensation of (PPh4)2C4Sx (x~5) with MeCN solvent at a temperature of 90 degrees centigrade created one of the polymers. Other research is discussed. Further research would confirm that solvatothermal techniques would be applicable to carbon sulfide polymer synthesis using a range of carbon sulfido precursors.

The complex (MeC5H4)Ru(dppe)SReS3 was found to be reactive to alkenes during investigations on thiometallates. Even though this is expected because unsaturated organic compounds can add to a variety of organometallic sulfido compounds, a series of control experiments revealed that ReS4- itself is highly reactive to a wide range of unsaturated organic substrates. The major finding was that most alkynes and alkenes react with ReS4-.