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The H-cluster of [Fe–Fe] hydrogenase consists of a [4Fe]H subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe]H subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCH2NHCH2S azadithiolate (adt) to catalyze proton reduction or hydrogen oxidation. Three Fe–S "maturase" proteins, HydE, HydF, and HydG, are responsible for the biosynthesis of the [2Fe]H subcluster and its incorporation into the hydrogenase enzyme to form this catalytically active H-cluster. We have proposed that HydG is a bifunctional enzyme that uses S-adenosylmethione (SAM) bound to a [4Fe–4S] cluster to lyse tyrosine via a transient 5′-deoxyadenosyl radical to produce CO and CN ligands to a unique cysteine-chelated Fe(II) that is linked to a second [4Fe–4S] cluster via the cysteine sulfur. In this "synthon model", after two cycles of tyrosine lysis, the product of HydG is completed: a [Fe(CN)(CO)2(cysteinate)]− organometallic unit that is vectored directly into the synthesis of the [2Fe]H sub-cluster. However our HydG-centric synthon model is not universally accepted, so further validation is important. In this Frontiers article, we discuss recent results using a synthetic "Syn-B" complex that donates [Fe(CN)(CO)2(cysteinate)]− units that match our proposed HydG product. Can Syn-B activate hydrogenase in the absence of HydG and its tyrosine substrate? If so, since Syn-B can be synthesized with specific magnetic nuclear isotopes and with chemical substitutions, its use could allow its enzymatic conversions on the route to the H-cluster to be monitored and modeled in fresh detail. [ABSTRACT FROM AUTHOR]

Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe2(μ-SR)2(CO)x centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe2(μ-SR)2(CO)6 involves the reaction of thiols with Fe2(CO)9 and Fe3(CO)12. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe2(μ-S2)(CO)6, Fe2(μ-SH)2(CO)6, and Li2Fe2(μ-S)2(CO)6. This approach is especially useful for azadithiolates Fe2[(μ-SCH2)2NR](CO)6. Elaborate complexes arise via addition of the FeSH group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe2(μ-SR)2(CO)6 was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe(II)(SR)2 + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe3(CO)12, S8, and a variety of unsaturated substrates results in C-H activation, affording otherwise inaccessible derivatives. Thiones and related C=S-containing reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C-H activation. [ABSTRACT FROM AUTHOR]

The effects of simple thiolates vs chelating dithiolates on M-M bonding, redox potentials, and synthetic outcomes have been probed experimentally and computationally. Nickelocene (Cp2Ni) has long been known to react with simple thiols to give diamagnetic Cp2Ni2(SR)2 with planar Ni2S2 cores and long Ni- - -Ni distances. Ethane- and propanedithiol (edtH2 and pdtH2, respectively) instead give di-, tri-, and pentanickel complexes, with nonplanar Ni2S2 cores. The 36e Cp2Ni2(pdt) (1pdt) adopts a symmetrical butterfly Ni2S2 structure. Variable-temperature NMR spectra indicate that 1pdt possesses a thermally accessible triplet state (ΔG = 2.65(5) kcal/mol) in equilibrium with a diamagnetic ground state. DFT calculations indicate that the singlet-triplet gap is highly sensitive to the nonplanarity of the Ni2S2 core. The calculations further reveal that only the high-spin form of 1pdt features Ni-Ni bonding, which is unprecedented. Cp2Ni3(pdt)2 (2pdt), which derives from 1pdt, crystallized as cis and trans isomers, both with a central Ni(pdt)2 2- unit that is S,S-chelated to two CpNi+ centers. Reaction of Cp2Ni with 1,2-ethanedithiol (H2edt) and 1,2-benzenedithiol (bdtH2) exclusively gave the trinickel species 2edt and 2bdt, which are structurally analogous to cis-2pdt. Solutions of 2edt are unstable, depositing crystals of Cp2Ni5(edt)4 (3edt). Cyclic voltammetric studies show that the Ni2 species oxidize readily to give the mixed-valence cations [1pdt]+ and [1edt]+. Crystallographic and EPR analyses indicate that these cations are delocalized mixed-valence Ni(II)-Ni(III) species. Oxidation of Cp2Ni2(SEt)2, which features a planar Ni2S2 core, afforded a mixed-valence cation, showing the pyramidal Ni2S2 core observed in [1pdt]0/+ and [1edt]0/+. Although not obtained from the Cp2Ni/H2edt reaction, the neutral complex 1edt was obtained by reduction of [1edt]+. Variable-temperature NMR measurements and DFT calculations indicate that the triplet is further stabilized in this highly pyramidalized species. [ABSTRACT FROM AUTHOR]

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe2(SR)2H2 species is provided by the complex [(ferm-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H]0). Spectroscopic and computational studies indicate that [H1H]0 contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe2(pdt)(CO)2(dppv)2]+ ([Hl(CO)]+), which undergoes substitution to afford [(μ-H)Fe2(pdt)(CO)(NCMe)-(dppv)2]+ ([Hl(NCMe)] + ). Upon treatment of [Hl(NCMe)]+ with borohydride salts, the MeCN ligand is displaced to afford [H1H]0. DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s-1 at -40 °C. The compound reacts with D2 to afford [D1D]0, but not mixed isotopomers such as [H1D]0. The dihydride undergoes oxidation with Fc+ under CO to give [l(CO)]+ and H2. Protonation in MeCN solution gives [Hl(NCMe)]+ and H2. Carbonylation converts [H1H]0 into [l(CO)]0. [ABSTRACT FROM AUTHOR]

A lucky break: A combination of steric pressure and electron asymmetry has provided the first example of a diiron dithiolate that is both rotated and basic. The present work establishes the feasibility of a hydride free rotated structure for the Hred state of the enzyme (see figure). [ABSTRACT FROM AUTHOR]

Described are experiments demonstrating incorporation of cyanide cofactors and hydride substrate into [NiFe]-hydrogenase (H2ase) active site models. Complexes of the type (CO)2(CN)2Fe(pdt)Ni(dxpe) (dxpe = dppe, 1; dxpe = dcpe, 2) bind the Lewis acid B(C6F5)3 (BArF3) to give the adducts (CO)2(CNBArF3)2Fe(pdt)Ni(dxpe), (1(BArF3)2, 2(BArF3)2). Upon decarbonylation using amine oxides, these adducts react with H2 to give hydrido derivatives [(CO)(CNBArF3)2Fe(H)(pdt)Ni(dxpe)]− (dxpe = dppe, [H3(BArF3)2]−; dxpe = dcpe, [H4(BArF3)2]−). Crystallographic analysis shows that Et4N[H3(BArF3)2] generally resembles the active site of the enzyme in the reduced, hydride-containing states (Ni–C/R). The Fe–H···Ni center is unsymmetrical with rFe–H = 1.51(3) Å and rNi–H = 1.71(3) Å. Both crystallographic and 19F NMR analyses show that the CNBArF3– ligands occupy basal and apical sites. Unlike cationic Ni–Fe hydrides, [H3(BArF3)2]− and [H4(BArF3)2]− oxidize at mild potentials, near the Fc+/0 couple. Electrochemical measurements indicate that in the presence of base, [H3(BArF3)2]− catalyzes the oxidation of H2. NMR evidence indicates dihydrogen bonding between these anionic hydrides and R3NH+ salts, which is relevant to the mechanism of hydrogenogenesis. In the case of Et4N[H3(BArF3)2], strong acids such as HCl induce H2 release to give the chloride Et4N[(CO)(CNBArF3)2Fe(Cl)(pdt)Ni(dppe)]. [ABSTRACT FROM AUTHOR]

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe2(xdt)(CO)2(PMe3)4, where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc+/0. Crystallographic analyses confirm that 1 and 2 adopt C2-symmetric structures (Fe-Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [μ-H1]+, establishing the non-intermediacy of the terminal hydride ([t-H1]+). At higher temperatures, protonation afforded mainly [t-H1]+. The temperature dependence of the ratio [t-H1]+/[μ-H1]+ indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature 31P{1H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe2(Hpdt)(CO)2(PMe3)4]+ ([S-H2]+). This intermediate converts to [t-H2]+ and [μ-H2]+ by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2]+) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3]+, which isomerizes to [t-H3′]+, wherein all PMe3 ligands are basal. [ABSTRACT FROM AUTHOR]

New mixed-valence iron--nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)Ni(pdt)Fe(CO)3]BF4 ([1]BF4, where dppe = Ph2PCH2CH2PPh2 and pdt2- = --SCH2CH2CH2S--) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)2L]BF4 incorporating L = PHCy2 ([la]BF4), PPh-(NEt2)2 ([lb]BF4), P(NMe2)3 ([1e]BF4), P(r-Pr)3 ([ld]BF4), and PCy3 ([le]BF4). The related precursor [(dcpe)Ni(pdt)Fe(CO)3]BF4 ([2]BF4, where dcpe = Cy2PCH2CH2PCy2) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)-Fe(CO)2L]BF4 for L = PPh2(2-pyridyl) ([2a]BF4), PPh3 ([2b]BF4) and PCy3 ([2c]BF4). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF4 and [2c]BF4 showed that they adopt "rotated" FeI centers, in which PCy3 occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as NiIIFe1 (S = ½>) systems according to electron paramagnetic resonance spectroscopy, although with attenuated 31P hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBEO exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [1e]+ is mosdy localized in a FeI-centered d(z2) orbital, orthogonal to the Fe--P bond. The PCy3 complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopicalry incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS2Fe core of the [NiFe]-hydrogenase active site is reproduced, the "hybrid models" incorporate key features of the two major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)Ni(pdt)Fe(CO)2L]+/2+. The resulting unsaturated 32e- dications represent the closest approach to modeling the highly electrophilic Ni--SIa state. In the case of L - PPh2 (2-pyridyl), chelation of this ligand accompanies the second oxidation. [ABSTRACT FROM AUTHOR]

Treatment of Fe2(pdt)(CO)4(dppv)(1) with aryldiazonium salts affords the 34 e–adducts[Fe2(pdt)(μ-N2Ar)(CO)4(dppv)]+(pdt2–= 1,3-propanedithiolate, dppv = cis-C2H2(PPh2)2). Under some conditions, the same reaction gave substantial amountsof [1]+, the product of electron transfer.Consistent with the influence of electron transfer in the reactionsof some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of[Me3S2]+and Fe2(pdt)(CO)4(dppbz) was found to give [Fe2(pdt)(CO)4(dppbz)]+as well as Me2S and Me2S2(dppbz = 1,2-bis(diphenylphosphino)benzene). [ABSTRACT FROM AUTHOR]

The diiron hydride [(μ-H)Fe2(pdt)(CO)4(dppv)]+ ([H2]+, dppv = cis-1,2-C2H2(PPh2)2) is shown to be an effective photocatalyst for the H2 evolution reaction (HER). These experiments establish the role of hydrides in photocatalysis by biomimetic diiron complexes. Trends in redox potentials suggests that other unsymmetrically substituted diiron hydrides are promising catalysts. Unlike previous catalysts for photo-HER, [H2]+ functions without sensitizers: irradiation of [H2]+ in the presence of triflic acid (HOTf) efficiently affords H2. Instead of sacrificial electron donors, ferrocenes can be used as recyclable electron donors for the photocatalyzed HER, resulting in 4 turnovers. [ABSTRACT FROM AUTHOR]

Some enzymes function by coupling substrate turnover with electron transfer from a redox cofactor such as ferredoxin. In the [FeFe]-hydrogenases, nature's fastest catalysts for the production and oxidation of H2, the one-electron redox by a ferredoxin complements the one-electron redox by the diiron active site. In this Article, we replicate the function of the ferredoxins with the redox-active ligand Cp*Fe(C5Me4CH2PEt2) (FcP*). FcP* oxidizes at mild potentials, in contrast to most ferrocene-based ligands, which suggests that it might be a useful mimic of ferredoxin cofactors. The specific model is Fe2[(SCH2)2NBn](CO)3(FcP*)(dppv) (1), which contains the three functional components of the active site: a reactive diiron centre, an amine as a proton relay and, for the first time, a one-electron redox module. By virtue of the synthetic redox cofactor, [1]2+ exhibits unique reactivity towards hydrogen and CO. In the presence of excess oxidant and base, H2 oxidation by [1]2+ is catalytic. [ABSTRACT FROM AUTHOR]

Mild oxidants such as [Fe(C5Me5)2]+ accelerate the activation of H2 by [Fe2[(SCH2)2NBn](CO)3-(dppv) (PMe3)]+ ([1]+), despite the fact that the ferrocenium cation is incapable of oxidizing [1]+. The reaction is first-order in [1]+ and [H2] but independent of the E1/2 and concentration of the oxidant. The analogous reaction occurs with D2 and proceeds with an inverse kinetic isotope effect of 0.75(8). The activation of H2 is further enhanced with the tetracarbonyl [Fe2[(SCH2)2NBn] (CO)4(dppn)]+([2]+), the first crystallographically characterized model for the H2, state of the active site containing an amine cofactor. These studies point to rate- determining binding of H2 followed by proton-coupled electron transfer. Relative to that by [1]+ the rate of H2 activation by [2]+/Fc+ is enhanced by a factor of 104 at 25 °C. [ABSTRACT FROM AUTHOR]

This paper summarizes studies on the redox behavior of synthetic models for the [FeFe]-hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H2, which contrasts with the high activity of these enzymes for H2 oxidation. The redox and acid-base properties of the model complexes [Fe2[(SCH2)2NR](CO)3(dppv)(PMe3)]+ ([2]+ for R = H and [2′]+ for R = CH2C6H5, dppv = cis-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H2 followed by deprotonation are (i) endothermic for the mixed valence (FeIIFeII) state and (ii) exothermic for the diferrous (FeIIFeII) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid-base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [2′]+ induces disproportionation to a 1:1 mixture of the ammonium [H2′]+ (FeIFeI) and the dication [2′]2+ (FeIIFeII). This effect is consistent with substantial enhancement of the basicity of the amine in the FeIFeI state vs the FeIIFeI state. The FeIFeI ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H2 activation by the [FeFe]-hydrogenases. [ABSTRACT FROM AUTHOR]

The [NiFe]-hydrogenase model complex NiFe(pdt)(dppe)(CO)3 (1) (pdt = 1 ,3-propanedithiolate) has been efficiently synthesized and found to be robust. This neutral complex sustains protonation to give the first nickel-iron hydride [1 HJBF4. One CO ligand in [1 H]BF4 is readily substituted by organophosphorus ligands to afford the substituted derivatives [HNiFe(pdt)(dppe)(PR3)(CO)2]BF4, where PPR3 = P(OPh)3 ([2H]BF4); PPh3 ([3H}BF4); PPh2Py ([4H]BF4, where Py = 2-pyridyl). Variable temperature NMR measurements show that the neutral and protonated derivatives are dynamic on the NMR time scale, which partially symmetrizes the phosphine complex. The proposed stereodynamics involve twisting of the Ni(dppe) center, not rotation at the Fe(CO)2(PPR3) center. In MeCN solution, 3, which can be prepared by deprotonation of [3H]BF4 with NaOMe, is about 104 stronger base than is 1. X-ray crystallographic analysis of [3H]BF4 revealed a highly unsymmetrical bridging hydride, the Fe-H bond being 0.40 Å shorter than the Ni-H distance. Complexes [2H]8F4, [3H]BF4, and [4H]BF4 undergo reductions near -1.46 V vs Fc0/+. For [2H]BF4, this reduction process is reversible, and we assign it as a one-electron process. In the presence of trifluoroacetic acid, proton reduction catalysis coincides with this reductive event. The dependence of ic/ip on the concentration of the acid indicates that H2 evolution entails protonation of a reduced hydride. For [2H]+, [3H]+, and [4H]+, the acid-independent rate constants are 50-75 s-1. For [2H]+ and [3H]+, the overpotentials for H2 evolution are estimated to be 430 mV, whereas the overpotential for the N-protonated pyridinium complex [4H2]2+ is estimated to be 260 mV. The mechanism of H2 evolution is proposed to follow an ECEC sequence, where E and C correspond to one-electron reductions and protonations, respectively. On the basis of their values for its PKa and redox potentials, the room temperature values of ΔGH• and ΔGH- are estimated as respectively as 57 and 79 kcal/mol for [1H]+. [ABSTRACT FROM AUTHOR]

The photoreaction of Fe(CO)5 and cyanide salts in MeCN solution affords the dianion [Fe(CN)2(CO)3]2-, conveniently isolated as [K(18-crown-6)]2[Fe(CN)2(CO)3]. Solutions of [Fe(CN)2(CO)3]2 oxidize irreversibly at -600 mV (vs Ag/AgCl) to give primarily [Fe(CN)3(CO)3]-. Protonation of the dianion affords the hydride [K(18-crown-6)][HFe(CN)2(CO)3] with a PKa ≈ 17 (MeCN). The ferrous hydride exhibits enhanced electrophilicity vs its dianionic precursor, which resists substitution. Treatment of [K(18-crown-6)][Fe(CN)2(CO)3] with tertiary phosphines and phosphites gives isomeric mixtures of [HFe(CN)2(CO)2L]- (L = P(OPh)3 and PPh3). Carbonyl substitution on [1H(CO)2]- by P(OPh)3 is first-order in both the phosphite and iron (k = 0.18 M-1 s-1 at 22 °C) with ΔH‡ = 51.6 kJ mol-1 and ΔS‡ = -83.0 J K-1 mol-1. These ligands are displaced under an atmosphere of CO. With cis-Ph2PCH=CHPPh2 (dppv), we obtained the monocarbonyl, [HFe(CN)2(CO)(dppv)]-, a highly basic hydride (PKa > 23.3) that rearranges in solution to a single isomer. Treatment of [K(18-crown-6)][HFe(CN)2(CO)3] with Et4NCN resulted in rapid deprotonation to give [Fe(CN)2(CO)3]2- and HCN. The tricyano hydride [HFe(CN)3(CO)2]2- is prepared by the reaction of [HFe(CN)2(CO)2(PPh3)]- and (K(18-crown-6)]CN. Similar to the phosphine and phosphite derivatives, [HFe(CN)3(CO)2]2 exists as a mixture of all three possible isomers. Protonation of the hydrides [HFe(CN)2(CO)(dppv)]- and [HFe(CN)3(CO)2]- in acetonitrile solutions releases H2 and gives the corresponding acetonitrile complexes [K(18-crown-6)][Fe(CN)3(NCMe)(CO)2] and Fe(CN)2(NCMe)(CO)(dppv). Alkylation of [K(18-crown-6)]2[Fe(CN)2(CO)3] with MeOTf gives the thermally unstable [MeFe(CN)2(CO)3]-, which was characterized spectroscopically at -40 °C. Reaction of dppv with [MeFe(CN)2(CO)3]- gives the acetyl complex, [Fe(CN)2(COMe)(CO)(dppv)]-. Whereas [Fe(CN)2(CO)3]2- undergoes protonation and methylation at Fe, acid chlorides give the iron(0) N-acylisocyanides [Fe(CN)(CO)3(CNCOR)]- (R = Ph, CH3). The solid state structures of [K(18-crown-6)][HFe(CN)2(CO)(dppv)], Fe(CN)2(NCMe)(CO)(dppv), and [K(1 8-crown-6)]2[HFe(CN)3(CO)2] were confirmed crystallographically. In all three cases, the cyanide ligands are cis to the hydride or acetonitrile ligands. [ABSTRACT FROM AUTHOR]

Abstract: The hydrogenase model [Fe2(S2C3H6)(CN)2(CO)4]2− was employed as a molecular tecton for the construction of supramolecular aggregates. IR spectroscopy indicated that cyanide bridged aggregates are formed when [Fe2(S2C3H6)(CN)2(CO)4]2− was treated with Lewis acids such as Zn(tetraphenylporphyrinate), [Cu(NCMe)(2,2′-bipyridine)]PF6 and [Cu(NCMe)4]PF6. Condensation of [Fe2(S2C3H6)(CN)2(CO)4]2− with the tritopic Lewis acid [Cp∗Rh]2+ afforded the novel expanded tetrahedron cage, {[Fe2(S2C3H6)(CN)2(CO)4]6[Cp∗Rh]4}4−. The tetrahedron cage was characterized crystallographically as the PPN salt. [Copyright &y& Elsevier]

Solutions of Cp*lrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)-) (1 H(H)) with O2 generate Cp*lr(TsDPEN) (1) and 1 equiv of H2O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O2]), resulting in an overall rate constant of 0.024 ± 0.013 M-2 s-1. Isotopic labeling revealed that the rate of the reaction of 1 H(H) + O2 was strongly affected by deuteration at the hydride position (kHH2/kDH2 = 6.0 ± 1.3) but insensitive to deuteration of the amine (kHH2/kHD2 = 1.2 ± 0.2); these values are more disparate than for conventional transfer hydrogenation (Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001). The temperature dependence of the reaction rate indicated ΔH* = 82.2 kJ/mol, ΔS* = 13.2 J/mol·K, and a reaction barrier of 85.0 kJ/mol. A CH2CI2 solution under 0.30 atm of H2 and 0.13 atm of O2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)2BArF4 (BArF4 = B(C6H3-3,5-(CF3)2)4). The formation of water from H2 was verified by 2H NMR for the reaction of D2 + O2. Solutions of 1 slowly catalyze the oxidation of amyl alcohol to pentanal; using 1 ,4-benzoquinone as a cocatalyst, the conversion was faster. Complex 1 also catalyzes the reaction of O2 with RNH2BH3 (A = H, t-Bu), resulting in the formation of water and H2. The deactivation of the catalyst 1 in its reactions with O2 was traced to degradation of the Cp* ligand to a fulvene derivative. This pathway is not observed in the presence of amine-boranes, which were shown to reduce fulvenes back to Cp*. This work suggests the potential of transfer hydrogenation catalysts in reactions involving O2. [ABSTRACT FROM AUTHOR]

Abstract: Studies on diiron dithiolato complexes have proven fruitful for modeling the active site of the [FeFe]-hydrogenases. Here we present a departure from the classical Fe2S2 motif by examining the viability of Fe2N2 butterfly compounds as functional models for the diiron active site of [FeFe]-hydrogenases. Derivatization of Fe2(BC)(CO)6 (1, BC=benzo-[c]-cinnoline) with PMe3 affords Fe2(BC)(CO)4(PMe3)2, which subsequently undergoes protonation at the Fe–Fe bond. The hydride [(μ-H)Fe2(BC)(CO)4(PMe3)2]PF6 was characterized crystallographically as the C 2v isomer. It represents a rare example of a hydrido diiron complex that exists as observable isomers, depending on the location of the phosphine ligands – diapical and apical–basal. This hydride catalyzes the electrochemical reduction of protons. [Copyright &y& Elsevier]

Low-temperature oxidation of Fe2(S2CnH2n)(CNMe)6-x(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type [Fe2(S2CnH2n)(CO)x(CNMe)7-x]2+. The degree of substitution is controlled by the RNC/Fe ratio, as well as the degree of initial substitution at iron, with tricarbonyl derivatives favoring more highly carbonylated products. The structures of the monocarbonyl derivatives [Fe2(S2CnH2n)-(μ-CO)(CNMe)6](PF6)2 (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 Å, although the, μ-CO is unsymmetrical in the propanedithiolate derivative. lsomeric forms of [Fe2(S2C3H6)(CO)(CNMe)6J(PF6)2 were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 Å elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe2(S2C2H4)(μ-CO)(CO)(CNMe)5](pF6)2, the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe2(S2C2H4)(CNMe)3(CO)3 under 1 atm CO gives the deep pink tricarbonyl [Fe2(S2C2H4)(CO)3(CNMe)4(PF6)2 DFT calculations show that a bridging CO or MeNC establishes a 3-center, 2-electron bond within the two Fe(ll) centers, which would otherwise be nonbonding. [ABSTRACT FROM AUTHOR]

The molecular box [CpCo(CN)3]4[Cp*Ru]4 (Co4Ru4) reacts readily with a variety of monocations to form M⊂Co4Ru4+ (M=K+, Cs+, Rb+). Ion competition experiments, monitored by ESI-MS, show that the molecular box binds the smaller K+ more rapidly than Cs+, but that thermodynamically Co4Ru4 prefers the larger ion. The rates of ion-insertion for K+ and Cs+ into Co4Ru4 were found to qualitatively follow second order kinetics with K+, 300 M−1 s−1 and Cs+, 36 M−1 s−1. The ratio kK/kCs qualitatively matched the ESI-MS results from ion competition experiments. The rates of ion-insertion into Co4Ru4 were found to depend on the counter anions. In particular, RbBF4 reacted with Co4Ru4 more slowly than did RbOTf. The slower rates allowed us to establish second order kinetics. 1H NMR studies reveal that the Cp signal for Co4Ru4 is very sensitive to the presence of entering ions, e.g., Rb+, whereas the corresponding Cp signal for Rb⊂Co4Ru4+ was insensitive to the presence of Rb+. The molecular structures of [Co4Ru4] · 6MeCN, [K⊂Co4Ru4]BF4 · 7MeCN, [Cs⊂Co4Ru4]BF4 · 6MeCN and [Tl⊂Co4Ru4]BF4 · 6MeCN, determined by X-ray diffraction, showed that although the compounds crystallized in the same space group I23, a correlation exists between the Ru–N/Co–C bond distances and the size of the interstitial ion. [Copyright &y& Elsevier]

Cites four themes of the research on soluble metal sulfides. Synthesis from polysulfido complexes to functional models for the hydrogenases; Use of exothermic disulfurization of polysulfido complexes to generate clusters and rings; Role of donor solvents on the reaction of metals with sulfur; Exploration of the organic and organometallic chemistry of the tetrathiometalates; Structural model for bio-organometallic centers.

A series of heptametallic cyanide cages are described; they represent soluble analogues of defect-containing cyanometalate solid-state polymers. Reaction of 0.75 equiv of [Cp[sup *]Ru(NCMe)[sub 3]]PF[sub 6], Et[sub 4]N[Cp[sup *]Rh(CN)[sub 3]], and 0.25 equiv of CsOTf in MeCN solution produced {Cs⊂[Cp[sup *]Rh(CN)[sub 3]][sub 4][Cp[sup *]Ru][sub 3]} (Cs⊂Rh[sub 4]Ru[sub 3]). ¹H and [sup 133]Cs NMR measurements show that Cs⊂Rh[sub 4]Ru[sub 3] exists as a single C[sub s] isomer. In contrast, {Cs⊂[CpCo(CN)[sub 3]][sub 4][Cp[sup *]Ru][sub 3]} (Cs⊂Co[sub 4]Ru[sub 3]), previously lacking crystallographic characterization, adopts both Cs isomers in solution. In situ ESI-MS studies on the synthesis of Cs⊂Rh[sub 4]Ru[sub 3] revealed two Cs-containing intermediates, Cs⊂Rh[sub 2]Ru[sub 2, sup +] (1239 m/z) and Cs⊂Rh[sub 3]Ru[sub 3 , sup +] (1791 m/z), which underscore the participation of Cs[sup +] in the mechanism of cage formation. [sup 133]Cs NMR shifts for the cages correlated with the number of CN groups bound to Cs[sup +]: Cs⊂Co[sub 4]Ru[sub 4, sup +] (δ 1 vs δ 34 for CsOTf), Cs⊂Rh[sub 4]Ru[sub 3] where Cs[sup +] is surrounded by ten CN ligands (δ 91), Cs⊂Co[sub 4]Ru[sub 3], which consists of isomers with 11 and 10 π-bonded CNs (δ 42 and δ 89, respectively). Although {K⊂[Cp[sup *]Rh(CN)[sub 3]][sub 4][Cp[sup *]Ru][sub 3]} could not be prepared, {NH4⊂[Cp[sup *]Rh(CN)[sub 3]][sub 4][Cp[sup *]Ru][sub 3]} (NH[sub 4]⊂Rh[sub 4]Ru[sub 3]) forms readily by NH[sub 4, sup +]-template cage assembly. IR and NMR measurements indicate that NH[sub 4, sup +] binding is weak and that the site symmetry is low. CsOTf quantitatively and rapidly converts NH[sub 4][sub 1]Rh[sub 4]Ru[sub 3] into Cs⊂Rh[sub 4]Ru[sub 3], demonstrating the kinetic advantages of the M[sub 7] cages as ion receptors. Crystallographic characterization of CsCo[sub 4]Ru[sub 3] revealed that it crystallizes in the C[sub s](exo)[sub 1](endo)[sub 2] isomer. In addition to the nine µ-CN ligands, two CN [sub t] ligands are π-bonded to Cs[sup +]. M[sub 1]Rh[sub 4]Ru[sub 3] (M = NH[sub 4], Cs) crystallizes as the second C[sub s] isomer, that is, (exo)[sub 2](endo)[sub 1], wherein only one CN[sub t] ligand interacts with the included cation. The distorted framework of NH[sub 4]⊂Rh[sub 4]Ru[sub 3] reflects the smaller ionic radius of NH[sub 4, sup +]. The protons of NH[sub 4, sup +] were located crystallographically, allowing precise determination of the novel NH[sub 4]···CN interaction. A competition experiment between calix[4]arene-bis(benzocrown-6) and NH[sub 4]⊂Rh[sub 4]Ru[sub 3] reveals NH[sub 4]⊂Rh[sub 4]Ru[sub 3] has a higher affinity for cesium. [ABSTRACT FROM AUTHOR]

Explores the reactivity of [MoS[sub 4]][sup 2-] toward PMe[sub 3] in the presence and absence of proton donors. Catalysis of the formation of H[sub 2] and SPMe[sub 3]; Reaction of MoS[sub 2](PMe[sub 3])[sub 4] with thiols; Crystallographic re-examination of (NH[sub 4])[sub 2]MoS[sub 4].

Investigates a series of (Me3TACN)FeII derivatives with soft coligands, where Me3TACN is N,N',N''-trimethyl-1,4,7-triazacyclononane. Treatment of Me3TACN with FeCl2 to produce (Me3TACN)FeCl2 (1); Ability of compound 1 to act as a versatile precursor reagent; Single-crystal x-ray diffraction; Binding of aqueous solutions of 1 to CO upon the addition of CN-.

Explores the binding of alkenes to the tetrathioperrhenate anion, ReS4-. Reversible formation of alkene adducts; Isolation of norbornene and norbornadiene; Solid-state structures of the adducts derived from norbornene and norbornadiene; Constraint of the chelate backbone; Isomerization of dimethyl maleate to dimethyl fumarate.

Presents a study which investigated the effect of ancillary ligands on the structure and reactivity of zinc polysulfido ligands. Difference between polysulfido ligands from classical chelating ligands; Information on experiments conducted; Results of the study.

Studies the sulfido-persulfido equilibria in sulfur-rich metal clusters. Application of crystallographic and DNMR studies; Establishment of the structures of (C5Me5)3Rh(MeCN)Ru2S4; Reaction of the dication with acetone; Demonstration of a (C5Me5) site exchange; Use of variable temperature nuclear magnetic resonance measurements.

Develops sulfur modified derivatives or thiofullerenes as precursors to more complex molecules or materials. Synthesis and isolation of iron sulfido derivatives; Cyrstallographic characterization of the derivatives; Spectroscopic and electrochemical data for fullerene derivatives.

Investigates thiophene derivatives of oxometal centers with low d-electron counts. Synthesis; Crystallographic studies; Oxidation and hydrolysis; Structure of the non-hydrogen atoms; Electrochemical studies; Spectroscopic data.

Reports on the reactions of zinc powder with solutions of elemental sulfur in various donor solvents. Ligand competition studies on pyridine; Occurrences of ligand exchange; Solubility and desulfurization of the zinc media.

Investigates the mobility of the metal-metal bonds. Analysis of the bond/no-bond interchange within a cluster framework; Use of cubane clusters in the study of metal-metal bonds; Multiple oxidation states of the clusters; Synthesis of dicationic clusters; Crystallographic characterization of the tetrairidium clusters; Structure-activity of the cubane clusters.

Reports on the synthesis and structure of (RC5H4)4Ru4E4 0/2+ (E = S, Se, Te). Cubane clusters (MeC5H4)4Ru4E4; Effects on nonbonding and bonding interactions of chalcogen identity; Influence of steric factors on dynamics; Ru-Ru bonding in clusters; Closeness of potentials for two oxidation waves; Mechanism of movements of Ru-Ru bonds.

Provides information on a study showing that compound C6S6O2 is a tricyclic species with a twisted 1,2-dithiin (unsaturated C4S2 ring) core. Methodology used to conduct the study; Results of the study; Discussion on the results.

A victory in the pocket: An international team of chemists and biophysicists have resolved the long‐standing question of the structure of the active site of the [FeFe] hydrogenases by assembling the active enzyme with a version of the active site synthesized in vitro (see scheme; HydF is a scaffold protein, HydA1 is a natural hydrogenase). The protein incorporating the diiron complex 2 showed similar activity to that of the natural enzyme. [ABSTRACT FROM AUTHOR]

Den Sieg in der Tasche: Ein internationales Team aus Chemikern und Biophysikern hat das Rätsel um die Struktur des aktiven Zentrums der [FeFe] ‐ Hydrogenasen gelöst. Den Forschern gelang es, mit einer in vitro synthetisierten Version des aktiven Zentrums (siehe Bild; HydF ist ein Gerüstprotein, HydA1 ist eine natürliche Hydrogenase) ein aktives Enzym aufzubauen. Das Protein mit dem eingebauten Dieisenkomplex 2 ist ähnlich aktiv wie das natürliche Enzym. [ABSTRACT FROM AUTHOR]

Protonation of the symmetrical tetraphosphine complexes Fe2(S2C~H2~)(CO)2(dppv)2 afforded the corresponding terminal hydrides, establishing that even symmetrical diiron(l) dithiolates undergo protonation at terminal sites. The terminal hydride [HFe2(S2C3H6)(CO)2(dppv)2]~ was found to catalyze proton reduction at potentials 200 mV milder than the isomeric bridging hydride, thereby establishing a thermodynamic advantage for catalysis operating via terminal hydride. The azadithiolate protonates to afford, [Fe2[(SCH2)2NH2](CO)2(dppv)2]t [HFe2[(SCH2)2NH](CO)2- (dppv)21t and [HFe2[(SCH2)2NH2](CO)2(dppv)2]2~, depending on conditions. [ABSTRACT FROM AUTHOR]

Cp*M2+ complexes (M = Rh, Ir; Cp* = C5Me5) are described for 6-(carboxymethyl)-4-methyl-2-hydroxypyridine (cmhpH2), an analogue of the guanylylpyridone cofactor in the hydrogenase Hmd. Three findings indicate that Cp*M(Hcmhp)+ stabilizes the binding of hydrogen-bond acceptors to the sixth coordination site: (i) water binds in preference to Cl-, (ii) the adduct Cp*Rh(cmhp)(2- hydroxypyridine) exhibits a very short intramolecular hydrogen bond (ro-o = 2.38 Å; 1H NMR δH 17.2), and (iii) Cp*Ir(cmhpH)Cl efficiently catalyzes the dehydrogenation of PhCH(OH)Me to PhC(O)Me. [ABSTRACT FROM AUTHOR]

The new iron carbonyl cyanide trans-[Fe(CN)[sub 2](CO)[sub 3]][sup 2-], [2][sup 2-], forms in high yield via photosubstitution of Fe(CO)[sub 5] with 2 equiv of Et4NCN. Protonation of [2][sup 2-] generated [HFe(CN)[sub 2](CO)[sub 3]][sup -], [2H][sup -], the first H-Fe-CN-CO species. Further protonation gives dihydrogen. This simple system provides insights into hydrogen evolution by the hydrogenase enzymes, which also feature H-FeCN-CO centers. [ABSTRACT FROM AUTHOR]

Electron-rich polyisocyano derivatives Fe[sub 2](S[sub 2]C[sub n]H[sub 2n])(CO)[sub 6-x](CNMe)[sub x] (x ∼ 4) undergo oxidatively induced (FeCp[sub 2, sup +]) reaction with additional CNMe to give [Fe[sub 2](SR)[sub 2](CNMe)[sub 7]](PF[sub 6])[sub 2], a new class of iron thiolates. Crystallographic characterization established that the 34 e[sup -] dinuclear core resembles the oxidized (H[sub 2]-binding) form of the active sites of the Fe-only hydrogenases, key features being the face-sharing bioctahedral geometry, the μ-CX ligand, and an Fe-Fe separation of 2.61 Å. Oxidation of the phenylthiolate Fe[sub 2](SPh)[sub 2](CO)[sub 2](CNMe)[sub 4] led to mononuclear [Fe(SPh)(ONMe)[sub 5]](PF[sub 6]), which is analogous to [Fe[sub 2](SR)[sub 2](CNMe)[sub 10]](PF[sub 6])[sub 2] formed upon treatment of [Fe[sub 2](S[sub 2]C[sub 3]H[sub 6])(CNMe)[sub 7]](PF[sub 6])[sub 2] with excess CNMe. [ABSTRACT FROM AUTHOR]

The paper aims to elucidate the final stages in the biosynthesis of the [2Fe]H active site of the [FeFe]‐hydrogenases. The recently hypothesized intermediate [Fe2(SCH2NH2)2(CN)2(CO)4]2− ([1]2−) was prepared by a multistep route from [Fe2(S2)(CN)(CO)5]−. The following synthetic intermediates were characterized in order: [Fe2(SCH2NHFmoc)2(CNBEt3)(CO)5]−, [Fe2(SCH2NHFmoc)2(CN)−(CO)5]−, and [Fe2(SCH2NHFmoc)2(CN)2(CO)4]2−, where Fmoc is fluorenylmethoxycarbonyl). Derivatives of these anions include [K(18‐crown‐6)]+, PPh4+ and PPN+ salts as well as the 13CD2‐isotopologues. These Fe2 species exist as a mixture of two isomers attributed to diequatorial (ee) and axial‐equatorial (ae) stereochemistry at sulfur. In vitro experiments demonstrate that [1]2− maturates HydA1 in the presence of HydF and a cocktail of reagents. HydA1 can also be maturated using a highly simplified cocktail, omitting HydF and other proteins. This result is consistent with HydA1 participating in the maturation process and refines the roles of HydF. [ABSTRACT FROM AUTHOR]

The paper aims to elucidate the final stages in the biosynthesis of the [2Fe]H active site of the [FeFe]‐hydrogenases. The recently hypothesized intermediate [Fe2(SCH2NH2)2(CN)2(CO)4]2− ([1]2−) was prepared by a multistep route from [Fe2(S2)(CN)(CO)5]−. The following synthetic intermediates were characterized in order: [Fe2(SCH2NHFmoc)2(CNBEt3)(CO)5]−, [Fe2(SCH2NHFmoc)2(CN)−(CO)5]−, and [Fe2(SCH2NHFmoc)2(CN)2(CO)4]2−, where Fmoc is fluorenylmethoxycarbonyl). Derivatives of these anions include [K(18‐crown‐6)]+, PPh4+ and PPN+ salts as well as the 13CD2‐isotopologues. These Fe2 species exist as a mixture of two isomers attributed to diequatorial (ee) and axial‐equatorial (ae) stereochemistry at sulfur. In vitro experiments demonstrate that [1]2− maturates HydA1 in the presence of HydF and a cocktail of reagents. HydA1 can also be maturated using a highly simplified cocktail, omitting HydF and other proteins. This result is consistent with HydA1 participating in the maturation process and refines the roles of HydF. [ABSTRACT FROM AUTHOR]

Presents information on the development of reagents based on other metals. Basic properties of ZnS6L2; Photosulfide strength of ZnS6L2; Preparation procedure for 1,2-SeS6; Physical properties of 1,2-SeS6; Rearrangment reaction of 1,2-SeS6.

A cluster‐ligand is disclosed in the form of [Ru3(CN)3(CO)9]3− ([1]3−). Produced by simple reaction of [Ru3(CO)12] with cyanide, [1]3− serves as a precursor to a series of μ‐CN cages. When treated with [Ru3(CO)12], it readily forms the prism [Ru6(μ‐CN)3(CO)18]3−. With 1.5 equiv. of [Cu(MeCN)4]+ [1]3− reacts to give the expanded prism {Cu3[Ru3(μ‐CN)3(CO)9]2}3−, which features three two‐coordinate Cu(I) centers. Sources of Ni2+ and Fe2+ bind two equivalents of [1]3−, giving the double cages {M[Ru3(μ‐CN)3(CO)9]2}4− (M=Ni, Fe) wherein the central metal is octahedral. The 1 : 1 reaction using [Fe(H2O)6]2+ gave the interpenetrated super‐tetrahedrane {Fe4(μ4‐O)[Ru3(μ‐CN)3(CO)9]4}4−. [ABSTRACT FROM AUTHOR]

Redox reactions, substitutions, and metalations are reported for the iron carbido sulfide [Fe6C(CO)14(S)]2– ([1]2–). Dianion [1]2– oxidized to [Fe6C(CO)16(S)]0 ([2]0) upon treatment with of [Fe(C5H5)2]BF4 or HBF4 (H2 formation) under an atmosphere of CO. Reaction of [2]0 with tBuNC gave [Fe6C(S)(CO)13(tBuNC)5], consisting of Fe5C(CO)13 and [Fe(tBuNC)5]2+ subunits linked by a µ3‐S2–. The Fe7CS cluster [Fe7C(CO)17(S)]2– formed upon treatment of (Ph4P)2[1] with Fe(benzylideneacetone)(CO)3. The Fe7 species is an edge‐fused cluster with [Fe6C(CO)10(µ‐CO)4] and Fe(CO)3 subunits joined by µ3‐S and two Fe–Fe bonds. The analogous reaction using Mo(CO)4(norbornadiene) gave [MoFe6C(CO)18(S)]2–. In this cluster, the Mo center is located in the octahedral subunit. Treatment of [1]2– with SO2 afforded [Fe6C(S)(SO2)(CO)13]2–. This cluster features an Fe6C core decorated with µ3‐S and µ2‐SO2 ligands. These experiments were undertaken in an effort to connect organometallic clusters to FeMoco. [ABSTRACT FROM AUTHOR]

The reaction of Fe2S2(CO)6 and PPh3 affords Fe2S2(CO)4(PPh3)2 by an unprecedented mechanism involving the intermediacy of SPPh3 and Fe2S(CO)6(PPh3)2. [ABSTRACT FROM AUTHOR]

Review: 428 refs. [ABSTRACT FROM AUTHOR]

The article discusses the formation of a biomimetic catalyst for the hydrogenation of organic compounds and for hydrogen production. The compound is modeled around nickel-iron hydrogenase. The compound is unique in its nature because it undergoes the reaction with H2 to give the corresponding hydride. The compound is catalytically active and catalyzes the hydrogenation of benzaldehyde to the corresponding alcohol, it features a sulfur-rich core. Sulphur compounds poison most industrial catalysts but nature relies heavily on sulfur-rich environments for catalytic transformations.

The article studies the Lewis acidity of unsaturated iridium amides induced by proton. Brønsted acids convert 16e amido-amine complexes to novel organometallic Lewis acids exhibiting wide-ranging reactivity. Oxidation of 1H(H) with Ph3CPF6 yielded [1H(NCMe)PF6. The oxidation also represents a formal oxidation of dihydrogen, the proton residing on the untosylated amine. Iridium experiences no net change in oxidate state during the conversion.

The article reports that two functional states of active site in Fe-only hydrogenases (Fe H2ases) have been characterized. It suggests that one structural difference between these two functional states is the presence of a symmetrically bridging (Hox) or semibridging (Hred) CO ligand. It argues that reduction of diferrous dithiolates with hydride reagents provides a fresh approach to an active site model for critically important intermediates. The terminal hydride indeed reacts with Brønsted acids to give H2, in contrast to the non reactivity of the isomeric μ-H compounds. The new synthetic methodology could be applicable to other diferrous dithiolates, including those bearing cyanide ligands.