Chemical activity of iron in [2Fe-2S]-protein centers and FeS2(100) surfaces

Iron atoms bonded to sulfur play an important role in proteins, heterogeneous catalysts, and gas sensors. First-principles density functional calculations were used to investigate the structure and chemical activity of a unique [2Fe-2S] center in the split-Soret cytochrome c (Ssc) from Desulfovibrio desulfuricans. In agreement with a previously proposed structural model [Abreu et al., J. Biol. Inorg. Chem. 2003, 8, 360], it is found that the [2Fe-2S] cluster is located in a surface pocket of the Ssc and bonded to only three cysteines. The [2Fe-2S] center in the Ssc is nonplanar and somewhat distorted with respect to canonical [2Fe-2S] centers seen in proteins where the iron-sulfur unit is bonded to four cysteines. In the Ssc, the lack of one Fe-cysteine bond is partially compensated by the separation between the cysteines that minimizes electrostatic repulsion among these ligands. The unique structure of the [2Fe-2S] center in the Ssc makes the center more chemically active than canonical [2Fe-2S] centers in proteins, (RS)(4)[2Fe-2S] inorganic complexes, and an FeS2(100) surface. A [2Fe-2S] center in the Ssc interacts efficiently with electron acceptors (O2, NO, CO) and poorly with a Lewis base such as H2O. The interaction with molecular oxygen is so strong that eventually oxidatively destroys the [2Fe-2S] unit. The bonding energy of the ligands to the [2Fe-2S] centers and FeS2(100) surface increases following the sequence: H2O < CO < NO < O2. The higher the electron affinity of the ligand, the larger its bonding energy. A relatively large positive charge on the Fe cations in FeS2(100) makes this sulfide surface less reactive toward O2, CO, and NO than the [2Fe-2S] centers in proteins and inorganic complexes.     

Precursors to clusters with the topology of the P(N) cluster of nitrogenase: edge-bridged double cubane clusters [(Tp)2Mo2Fe6S8L4]z: synthesis, structures, and electron transfer series

Members of the cluster set [(Tp)2Mo2Fe6S8L4]z contain the core unit M2Fe6(mu3-S)6(mu4-S)2 in which two MoFe3S4 cubanes are coupled by two Fe-(mu4-S) interactions to form a centrosymmetric edge-bridged double cubane cluster. Some of these clusters are synthetic precursors to [(Tp)2Mo2Fe6S9L2]3-, which possess the same core topology as the P(N) cluster of nitrogenase. In this work, the existence of a three-member electron-transfer series of single cubanes [(Tp)MoFe3S4L3](z) (z = 3-, 2-, 1-) and a four-member series of double cubanes [(Tp)2Mo2Fe6S8L4]z (z = 4-, 3-, 2-, 1-) with L = F-, Cl-, N3, PhS- is demonstrated by electrochemical methods, cluster synthesis, and X-ray structure determinations. The potential of the [4-/3-] couple is extremely low (<-1.5 V vs SCE in acetonitrile) such that the 4- state cannot be maintained in solution under normal anaerobic conditions. The chloride double cubane redox series was examined in detail. The members [(Tp)2Mo2Fe6S8Cl4]4-,3-,2- were isolated and structurally characterized. The redox series includes the reversible steps [4-/3-] and [3-/2-]. Under oxidizing conditions, [(Tp)2Mo2Fe6S8Cl4]2- cleaves with the formation of single cubane [(Tp)MoFe3S4Cl3]1-. The quasireversible [2-/1-] couple is observed at more positive potentials than those of the single cubane redox step. Structure comparison of nine double cubanes suggests that significant dimensional changes pursuant to redox reactions are mainly confined to the Fe2(mu4-S)2 bridge rhomb. The synthesis and structure of [(Tp)2Mo2Fe6S9F2.H2O]3-, a new topological analogue of the P(N) cluster of nitrogenase, is described. (Tp = hydrotris(pyrazolyl)borate(1-)).     

M?ssbauer characterization of the iron-sulfur clusters in Desulfovibrio vulgaris hydrogenase.

The periplasmic hydrogenase of Desulfovibrio vulgaris (Hildenbourough) is an all Fe-containing hydrogenase. It contains two ferredoxin type [4Fe-4S] clusters, termed the F clusters, and a catalytic H cluster. Recent X-ray crystallographic studies on two Fe hydrogenases revealed that the H cluster is composed of two sub-clusters, a [4Fe-4S] cluster ([4Fe-4S](H)) and a binuclear Fe cluster ([2Fe](H)), bridged by a cysteine sulfur. The aerobically purified D. vulgaris hydrogenase is stable in air. It is inactive and requires reductive activation. Upon reduction, the enzyme becomes sensitive to O(2), indicating that the reductive activation process is irreversible. Previous EPR investigations showed that upon reoxidation (under argon) the H cluster exhibits a rhombic EPR signal that is not seen in the as-purified enzyme, suggesting a conformational change in association with the reductive activation. For the purpose of gaining more information on the electronic properties of this unique H cluster and to understand further the reductive activation process, variable-temperature and variable-field M?ssbauer spectroscopy has been used to characterize the Fe-S clusters in D. vulgaris hydrogenase poised at different redox states generated during a reductive titration, and in the CO-reacted enzyme. The data were successfully decomposed into spectral components corresponding to the F and H clusters, and characteristic parameters describing the electronic and magnetic properties of the F and H clusters were obtained. Consistent with the X-ray crystallographic results, the spectra of the H cluster can be understood as originating from an exchange coupled [4Fe-4S]-[2Fe] system. In particular, detailed analysis of the data reveals that the reductive activation begins with reduction of the [4Fe-4S](H) cluster from the 2+ to the 1+ state, followed by transfer of the reducing equivalent from the [4Fe-4S](H) subcluster to the binuclear [2Fe](H) subcluster. The results also reveal that binding of exogenous CO to the H cluster affects significantly the exchange coupling between the [4Fe-4S](H) and the [2Fe](H) subclusters. Implication of such a CO binding effect is discussed.     

Reactions of synthetic [2Fe-2S] and [4Fe-4S] clusters with nitric oxide and nitrosothiols

The interaction of nitric oxide (NO) with iron-sulfur cluster proteins results in degradation and breakdown of the cluster to generate dinitrosyl iron complexes (DNICs). In some cases the formation of DNICs from such cluster systems can lead to activation of a regulatory pathway or the loss of enzyme activity. In order to understand the basic chemistry underlying these processes, we have investigated the reactions of NO with synthetic [2Fe-2S] and [4Fe-4S] clusters. Reaction of excess NO(g) with solutions of [Fe2S2(SR)4](2-) (R = Ph, p-tolyl (4-MeC6H4), or 1/2 (CH2)2-o-C6H4) cleanly affords the respective DNIC, [Fe(NO)2(SR)2](-), with concomitant reductive elimination of the bridging sulfide ligands as elemental sulfur. The structure of (Et4N)[Fe(NO)2(S-p-tolyl)2] was verified by X-ray crystallography. Reactions of the [4Fe-4S] clusters, [Fe4S4(SR)4](2-) (R = Ph, CH2Ph, (t)Bu, or 1/2 (CH2)-m-C6H4) proceed in the absence of added thiolate to yield Roussin's black salt, [Fe4S3(NO)7](-). In contrast, (Et4N)2[Fe4S4(SPh)4] reacts with NO(g) in the presence of 4 equiv of (Et4N)(SPh) to yield the expected DNIC. For all reactions, we could reproduce the chemistry effected by NO(g) with the use of trityl-S-nitrosothiol (Ph3CSNO) as the nitric oxide source. These results demonstrate possible pathways for the reaction of iron-sulfur clusters with nitric oxide in biological systems and highlight the importance of thiolate-to-iron ratios in stabilizing DNICs.     

A [4Fe-4S] cluster dimer bridged by bis(2,2':6',2'-terpyridine-4'-thiolato)iron(II)

The use of 2,2':6',2'-terpyridine-4'-thiol (tpySH) was explored as a bridging ligand for the formation of stable assemblies containing both [4Fe-4S] clusters and single metal ions. Reaction of tpySH (2 equiv) with (NH4)2Fe(SO4)(2).6H2O generated the homoleptic complex [Fe(tpySH)2](2+), which was isolated as its PF6(-) salt. The compound could be fully deprotonated to yield neutral [Fe(tpyS)2], and the absorption spectrum is highly dependent on the protonation state. Reaction of [Fe(tpySH)2](PF6)2 with the new 3:1 site-differentiated cluster (n-Bu4N)2[Fe4S4(TriS)(SEt)] yielded the first metal-bridged [4Fe-4S] cluster dimer, (n-Bu4N)2[{Fe4S4(TriS)(mu-Stpy)}2Fe]. Electrochemical studies indicate that the [4Fe-4S] clusters in the dimer act as independent redox units, while UV-vis spectroscopy provides strong evidence for a thioquinonoid electron distribution in the bridging tpyS(-) ligand. TpySH thus acts as a directional bridging ligand between [4Fe-4S] clusters and single metal ions, thereby opening the way to the synthesis of larger, more complex assemblies.     

Probing the intrinsic electronic structure of the cubane [4Fe-4S] cluster: nature's favorite cluster for electron transfer and storage

The cubane [4Fe-4S] is the most common multinuclear metal center in nature for electron transfer and storage. Using electrospray, we produced a series of gaseous doubly charged cubane-type complexes, [Fe4S4L4]2- (L = -SC2H5, -SH, -Cl, -Br, -I) and the Se-analogues [Fe4Se4L4]2- (L = -SC2H5, -Cl), and probed their electronic structures with photoelectron spectroscopy and density functional calculations. The photoelectron spectral features are similar among all the seven species investigated, revealing a weak threshold feature due to the minority spins on the Fe centers and confirming the low-spin two-layer model for the [4Fe-4S](2+) core and its "inverted level scheme". The measured adiabatic detachment energies, which are sensitive to the terminal ligand substitution, provide the intrinsic oxidation potentials of the [Fe4S4L4]2- complexes. The calculations revealed a simple correlation between the electron donor property of the terminal thiolate as well as the bridging sulfide with the variation of the intrinsic redox potentials. Our data provide intrinsic electronic structure information of the [4Fe-4S] cluster and the molecular basis for understanding the protein and solvent effects on the redox properties of the [4Fe-4S] active sites.     

Carbon monoxide induced decomposition of the active site [Ni-4Fe-5S] cluster of CO dehydrogenase

During the past two years, crystal structures of Cu- and Mo-containing carbon monoxide dehydrogenases (CODHs) and Ni- and Fe-containing CODHs have been reported. The active site of CODHs from anaerobic bacteria (cluster C) is composed of Ni, Fe, and S for which crystallographic studies of the enzymes from Carboxydothermus hydrogenoformans, Rhodospirillum rubrum, and Moorella thermoaceticarevealed structural similarities in the overall protein fold but showed substantial differences in the essential Ni coordination environment. The [Ni-4Fe-5S] cluster C in the fully catalytically competent dithionite-reduced CODH II from C. hydrogenoformans (CODHII(Ch)) at 1.6 A resolution contains a characteristic mu(2)-sulfido ligand between Ni and Fe1, resulting in a square-planar ligand arrangement with four S-ligands at the Ni ion. In contrast, the [Ni-4Fe-4S] clusters C in CO-treated CODH from R. rubrum resolved at 2.8 A and in CO-treated acetyl-CoA synthase/CODH complex from M. thermoacetica at 2.2 and 1.9 A resolution, respectively, do not contain the mu(2)-sulfido ligand between Ni and Fe1 and display dissimilar geometries at the Ni ion. The [Ni-4Fe-4S] cluster is composed of a cubane [Ni-3Fe-4S] cluster linked to a mononuclear Fe site. The described coordination geometries of the Ni ion in the [Ni-4Fe-4S] cluster of R. rubrum and M. thermoacetica deviate from the square-planar ligand geometry in the [Ni-4Fe-5S] cluster C of CODHII(Ch). In addition, the latter was converted into a [Ni-4Fe-4S] cluster under specific conditions. The objective of this study was to elucidate the relationship between the structure of cluster C in CODHII(Ch) and the functionality of the protein. We have determined the CO oxidation activity of CODHII(Ch) under different conditions of crystallization, prepared crystals of the enzyme in the presence of dithiothreitol or dithionite as reducing agents under an atmosphere of N(2) or CO, and solved the corresponding structures at 1.1 to 1.6 A resolutions. Fully active CODHII(Ch) obtained after incubation of the enzyme with dithionite under N(2) revealed the [Ni-4Fe-5S] cluster. Short treatment of the enzyme with CO in the presence of dithiothreitol resulted in a catalytically competent CODHII(Ch) with a CO-reduced [Ni-4Fe-5S] cluster, but a prolonged treatment with CO caused the loss of CO-oxidizing activity and revealed a [Ni-4Fe-4S] cluster, which did not contain a mu(2)-S. These data suggest that the [Ni-4Fe-4S] cluster of CODHII(Ch) is an inactivated decomposition product originating from the [Ni-4Fe-5S] cluster.     

Structural redox control in a 7Fe ferredoxin isolated from Desulfovibrio alaskensis

The redox behaviour of a ferredoxin (Fd) from Desulfovibrio alaskensis was characterized by electrochemistry. The protein was isolated and purified, and showed to be a tetramer containing one [3Fe-4S] and one [4Fe-4S] centre. This ferredoxin has high homology with FdI from Desulfovibrio vulgaris Miyazaki and Hildenborough and FdIII from Desulfovibrio africanus. From differential pulse voltammetry the following signals were identified: [3Fe-4S](+1/0) (E(0')=-158±5mV); [4Fe-4S](+2/+1) (E(0')=-474±5mV) and [3Fe-4S](0/-2) (E(0')=-660±5mV). The effect of pH on these signals showed that the reduced [3Fe-4S](0) cluster has a pK'(red)(')=5.1±0.1, the [4Fe-4S](+2/+1) centre is pH independent, and the [3Fe-4S](0/-2) reduction is accompanied by the binding of two protons. The ability of the [3Fe-4S](0) cluster to be converted into a new [4Fe-4S] cluster was proven. The redox potential of the original [4Fe-4S] centre showed to be dependent on the formation of the new [4Fe-4S] centre, which results in a positive shift (ca. 70mV) of the redox potential of the original centre. Being most [Fe-S] proteins involved in electron transport processes, the electrochemical characterization of their clusters is essential to understand their biological function. Complementary EPR studies were performed.     

过渡金属硫簇的几种簇骼转化反应

报道了过渡金属硫簇化合物的几种簇骼转化反应，即三核链状簇的转化；双核配合物的组合；Ｍｏ２Ｆｅ７Ｓ８和Ｍｏ２Ｆｅ６Ｓ８两种双立方烷的关联；Ｆｅ４Ｓ４立方烷簇向篮状簇的转化；簇降解以及簇骼原子置换反应等。探讨了配体及氧化还原条件对簇骼转化反应的影响。     

Mechanistic insight into the nitrosylation of the [4Fe-4S] cluster of WhiB-like proteins

The reactivity of protein bound iron-sulfur clusters with nitric oxide (NO) is well documented, but little is known about the actual mechanism of cluster nitrosylation. Here, we report studies of members of the Wbl family of [4Fe-4S] containing proteins, which play key roles in regulating developmental processes in actinomycetes, including Streptomyces and Mycobacteria, and have been shown to be NO responsive. Streptomyces coelicolor WhiD and Mycobacterium tuberculosis WhiB1 react extremely rapidly with NO in a multiphasic reaction involving, remarkably, 8 NO molecules per [4Fe-4S] cluster. The reaction is 10(4)-fold faster than that observed with O(2) and is by far the most rapid iron-sulfur cluster nitrosylation reaction reported to date. An overall stoichiometry of [Fe(4)S(4)(Cys)(4)](2-) + 8NO → 2[Fe(I)(2)(NO)(4)(Cys)(2)](0) + S(2-) + 3S(0) has been established by determination of the sulfur products and their oxidation states. Kinetic analysis leads to a four-step mechanism that accounts for the observed NO dependence. DFT calculations suggest the possibility that the nitrosylation product is a novel cluster [Fe(I)(4)(NO)(8)(Cys)(4)](0) derived by dimerization of a pair of Roussin's red ester (RRE) complexes.     

Spectroscopic evidence for site specific chemistry at a unique iron site of the [4Fe-4S] cluster in ferredoxin:thioredoxin reductase

Ferredoxin:thioredoxin reductase (FTR) catalyzes the reduction of the disulfide in thioredoxin in two one-electron steps using an active site comprising a [4Fe-4S] in close proximity to a redox active disulfide. M?ssbauer spectroscopy has been used to investigate the ligation and electronic properties of the [4Fe-4S] cluster in as-prepared FTR which has the active-site disulfide intact and in the N-ethylmaleimide (NEM)-modified form which provides a stable analogue of the one-electron-reduced heterodisulfide intermediate and has one of the cysteines of the active-site disulfide alkylated with NEM. The results reveal novel site-specific cluster chemistry involving weak interaction of the active-site disulfide with a unique Fe site of the [4Fe-4S]2+ cluster in the resting enzyme and cleavage of the active-site disulfide with concomitant coordination of one of the cysteines to yield a [4Fe-4S]3+ cluster with a five-coordinate Fe site ligated by two cysteine residues in the NEM-modified enzyme. The results provide molecular-level insight into the catalytic mechanism of FTR and other Fe-S-cluster-containing disulfide reductases, and suggest a possible mechanism for the reductive cleavage of S-adenosylmethionine by the radical SAM family of Fe-S enzymes.     

Evidence from Mössbauer spectroscopy for distinct [2Fe-2S](2+) and [4Fe-4S](2+) cluster binding sites in biotin synthase from Escherichia coli

Biotin synthase is an AdoMet-dependent radical enzyme that catalyzes the insertion of an FeS cluster-derived sulfur atom into dethiobiotin. The dimeric enzyme is purified containing one [2Fe-2S]2+ cluster per monomer, but it is most active when reconstituted with an additional [4Fe-4S]2+ cluster per monomer. Using M?ssbauer spectroscopy coupled with differential reconstitution of each cluster with 57Fe, we show that the reconstituted enzyme has approximately 1:1 [2Fe-2S]2+ and [4Fe-4S]2+ clusters and that the [4Fe-4S]2+ cluster is assembled at an alternate site not previously occupied by the [2Fe-2S]2+ cluster. These data suggest that biotin synthase is evolved to simultaneously accommodate two different clusters with unique roles in catalysis.     

Porphyrin hybrid complex ([2Fe-2S]2TPPS) as a catalyst for determination of hydrogen peroxide.

A new promising mimetic enzyme, a [2Fe-2S] cluster-porphyrin hybrid complex ([2Fe-2S]2TPPS), has been synthesized and applied to the determination of hydrogen peroxide. Under the optimum condition, the calibration graph has a linear range of 8.0 x 10(-8) - 1.0 x 10(6) mol/l H2O2 with a detection limit (3sigma, N = 9) of 5.3 x 10(-9) mol/l.     

Structure and Properties of [Fe(4)S(4){2,6-bis(acylamino)benzenethiolato-S}(4)](2)(-) and [Fe(2)S(2){2,6-bis(acylamino)benzenethiolato-S}(4)](2)(-): Protection of the Fe-S Bond by Double NH.S Hydrogen Bonds

Iron-sulfur clusters containing a singly or doubly NH.S hydrogen-bonded arenethiolate ligand, [Fe(4)S(4)(S-2-RCONHC(6)H(4))(4)](2)(-) (R = CH(3), t-Bu, CF(3)), [Fe(4)S(4){S-2,6-(RCONH)(2)C(6)H(3)}(4)](2)(-), [Fe(2)S(2)(S-2-RCONHC(6)H(4))(4)](2)(-) (R = CH(3), t-Bu, CF(3)), and [Fe(2)S(2){S-2,6-(RCONH)(2)C(6)H(3)}(4)](2)(-), were synthesized as models of bacterial [4Fe-4S] and plant-type [2Fe-2S] ferredoxins. The X-ray structures and IR spectra of (PPh(4))(2)[Fe(4)S(4){S-2,6-(CH(3)CONH)(2)C(6)H(3)}(4)].2CH(3)CN and (NEt(4))(2)[Fe(2)S(2){S-2,6-(t-BuCONH)(2)C(6)H(3)}(4)] indicate that the two amide NH groups at the o,o'-positions are directed to the thiolate sulfur atom and form double NH.S hydrogen bonds. The NH.S hydrogen bond contributes to the positive shift of the redox potential of not only (Fe(4)S(4))(+)/(Fe(4)S(4))(2+) but also (Fe(4)S(4))(2+)/(Fe(4)S(4))(3+) in the [4Fe-4S] clusters as well as (Fe(2)S(2))(2+)/(Fe(2)S(2))(3+) in the [2Fe-2S] clusters. The doubly NH.S hydrogen-bonded thiolate ligand effectively prevents the ligand exchange reaction by benzenethiol because the two amide NH groups stabilize the thiolate by protection from dissociation.     

Nitrosylated iron-thiolate-sulfinate complexes with {Fe(NO)}6/7 electronic cores: relevance to the transformation between the active and inactive NO-bound forms of iron-containing nitrile hydratases

The five-coordinated iron-thiolate nitrosyl complexes [(NO)Fe(S,S-C6H3R)2]- (R = H (1), m-CH3 (2)), [(NO)Fe(S,S-C6H2-3,6-Cl2)2]- (3), [(NO)Fe(S,S-C6H3R)2]2- (R = H (10), m-CH3 (11)), and [(NO)Fe(S,S-C6H2-3,6-Cl2)2]2- (12) have been isolated and structurally characterized. Sulfur oxygenation of iron-thiolate nitrosyl complexes 1-3 containing the {Fe(NO)}6 core was triggered by O2 to yield the S-bonded monosulfinate iron species [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]- (R = H (4), m-CH3 (5)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2(2-) (6), respectively. In contrast, attack of O2 on the {Fe(NO)}7 complex 10 led to the formation of complex 1 accompanied by the minor products, [Fe(S,S-C6H4)2]2(2-) and [NO3]- (yield 9%). Reduction of complexes 4-6 by [EtS]- in CH3CN-THF yielded [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]2- (R = H (7), m-CH3 (8)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2- (9) along with (EtS)2 identified by 1H NMR. Compared to complex 10, complexes 7-9 with the less electron-donating sulfinate ligand coordinated to the {Fe(NO)}7 core were oxidized by O2 to yield complexes 4-6. Obviously, the electronic perturbation of the {Fe(NO)}7 core caused by the coordinated sulfinate in complexes 7-9 may serve to regulate the reactivity of complexes 7-9 toward O2. The iron-sulfinate nitrosyl species with the {Fe(NO)}6/7 core exhibit the photolabilization of sulfur-bound [O] moiety. Complexes 1-4-7-10 (or 2-5-8-11 and 3-6-9-12) are interconvertible under sulfur oxygenation, redox processes, and photolysis, respectively.     

Secondary bonding interactions in biomimetic [2Fe-2S] clusters

A series of synthetic [2Fe-2S] complexes with terminal thiophenolate ligands and tethered ether or thioether moieties has been prepared and investigated in order to provide models for the potential interaction of additional donor atoms with the Fe atoms in biological [2Fe-2S] clusters. X-ray crystal structures have been determined for six new complexes that feature appended Et (1(C)), OMe (1(O)), or SMe (1(S)) groups, or with a methylene group (2(C) ), an ether-O (2(O)), or an thioether-S (2(S)) linking two aryl groups. The latter two systems provide a constrained chelate arrangement that induces secondary bonding interactions with the ether-O and thioether-S, which is confirmed by density functional theory (DFT) calculations that also reveal significant spin density on those fifth donor atoms. Structural consequences of the secondary bonding interactions are analyzed in detail, and effects on the spectroscopic and electronic properties are probed by UV-vis, M?ssbauer, and (1)H NMR spectroscopy, as well by SQUID measurements and cyclic voltammetry. The potential relevance of the findings for biological [2Fe-2S] sites is considered.     

Ligand K-edge X-ray absorption spectroscopy and DFT calculations on [Fe3S4]0,+ clusters: delocalization, redox, and effect of the protein environment

Ligand K-edge XAS of an [Fe3S4]0 model complex is reported. The pre-edge can be resolved into contributions from the mu(2)S(sulfide), mu(3)S(sulfide), and S(thiolate) ligands. The average ligand-metal bond covalencies obtained from these pre-edges are further distributed between Fe(3+) and Fe(2.5+) components using DFT calculations. The bridging ligand covalency in the [Fe2S2]+ subsite of the [Fe3S4]0 cluster is found to be significantly lower than its value in a reduced [Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2]+ site (-146 cm(-1) vs -360 cm(-1), respectively). This decrease in J, along with estimates of the double exchange parameter B and vibronic coupling parameter lambda2/k(-), leads to an S = 2 delocalized ground state in the [Fe3S4]0 cluster. The S K-edge XAS of the protein ferredoxin II (Fd II) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same oxidation state, which correlates with the number of H-bonding interactions to specific sulfur ligands present in the active site. The changes in ligand-metal bond covalencies upon redox compared with DFT calculations indicate that the redox reaction involves a two-electron change (one-electron ionization plus a spin change of a second electron) with significant electronic relaxation. The presence of the redox inactive Fe(3+) center is found to decrease the barrier of the redox process in the [Fe3S4] cluster due to its strong antiferromagnetic coupling with the redox active Fe2S2 subsite.     

Redox chemistry of the acetato-bridged clusters [M3(mu3-O)n(mu-O2CCH3)6(H2O)3]2+ (M = Mo, W, n = 1, 2): reversible redox between mono-micro3-oxo d8 M(III)2M(IV) and d9 M(III)3 forms

A cyclic voltammogram of aqueous 0.1 mol dm(-3) triflic acid solutions of the d6 bioxo-capped M-M bonded cluster [Mo3(mu3-O)2(O2CCH3)6(H2O)3]2+ at a glassy carbon electrode at 25 degrees C gives rise to an irreversible 3e- cathodic wave to a d9 Mo(III)3 species at -0.8 V vs. SCE which on the return scan gives rise to two anodic waves at +0.05 V vs. SCE (E(1/2), 1e- reversible to d8 Mo(III)2Mo(IV)) and +0.48 V vs. SCE (2e- irreversible back to d6 Mo(IV)3). The number of electrons passed at each redox wave has been confirmed by redox titration and controlled potential electrolysis which resulted in 90% recovery of [Mo3(mu3-O)2(O2CCH3)6(H2O)3]2+ following electrochemical re-oxidation at +0.8 V. A corresponding CV study of the d8 monoxo-capped W(III)2W(IV) cluster [W3(mu3-O)(O2CCH3)6(H2O)3]2+ gives rise to a reversible 1e- cathodic process at -0.92 V vs. SCE to give the d9 W(III)3 species [W3(mu3-O)(O2CCH3)6(H2O)3]+; the first authentic example of a W(III) complex with coordinated water ligands. However the cluster is too unstable (O2/water sensitive) to allow isolation. Comparisons with the cv study on [Mo3(mu3-O)2(O2CCH3)6(H2O)3]2+ suggest irreversible reduction of this complex to monoxo-capped [Mo(III)3(mu3-O)(O2CCH3)6(H2O)3]+ followed by reversible oxidation to its d8 counterpart [Mo3(mu3-O)(O2CCH3)6(H2O)3]2+ (Mo(III)2Mo(IV)) and finally irreversible oxidation back to the starting bioxo-capped cluster. Exposing the d9 Mo(III)3 cluster to air (O2) however gives a different final product with evidence of break up of the acetate bridged framework. Corresponding redox processes on d6 [W3(mu3-O)2(O2CCH3)6(H2O)3]2+ are too cathodic to allow similar generation of the monoxo-capped W(III)3 and W(III)2W(IV) clusters at the electrode surface.