Search

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 [Fe II (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 [Fe I (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 preparation and spectroscopic characterization of a CO-inhibited [FeFe] hydrogenase with a selectively (57)Fe-labeled binuclear subsite is described. The precursor [(57)Fe2(adt)(CN)2(CO)4](2-) was synthesized from the (57)Fe metal, S8, CO, (NEt4)CN, NH4Cl, and CH2O. (Et4N)2[(57)Fe2(adt)(CN)2(CO)4] was then used for the maturation of the [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii, to yield the enzyme selectively labeled at the [2Fe]H subcluster. Complementary (57)Fe enrichment of the [4Fe-4S]H cluster was realized by reconstitution with (57)FeCl3 and Na2S. The Hox-CO state of [2(57)Fe]H and [4(57)Fe-4S]H HydA1 was characterized by Mössbauer, HYSCORE, ENDOR, and nuclear resonance vibrational spectroscopy.

The [FeFe] hydrogenases (H2ases) catalyze the redox reaction that interconverts protons and H2. This area of biocatalysis has attracted attention because the metal-based chemistry is unusual, and the reactions have practical implications. The active site consists of a [4Fe-4S] cluster bridged to a [Fe2(μ-dithiolate)(CN)2(CO)3](z) center (z = 1- and 2-). The dithiolate cofactor is [HN(CH2S)2](2-), called the azadithiolate ([adt(H)](2-)). Although many derivatives of Fe2(SR)2(CO)6-xLx are electrocatalysts for the hydrogen evolution reaction (HER), most operate by slow nonbiomimetic pathways. Biomimetic hydrogenogenesis is thought to involve intermediates, wherein the hydride substrate is adjacent to the amine of the adt(H), being bonded to only one Fe center. Formation of terminal hydride complexes is favored when the diiron carbonyl models contain azadithiolate. Although unstable in the free state, the adt cofactor is stable once it is affixed to the Fe2 center. It can be prepared by alkylation of Fe2(SH)2(CO)6 with formaldehyde in the presence of ammonia (to give adt(H) derivatives) or amines (to give adt(R) derivatives). Weak acids protonate Fe2(adt(R))(CO)2(PR3)4 to give terminal hydrido (term-H) complexes. In contrast, protonation of the related 1,3-propanedithiolate (pdt(2-)) complexes Fe2(pdt)(CO)2(PR3)4 requires strong acids. The amine in the azadithiolate is a kinetically fast base, relaying protons to and from the iron, which is a kinetically slow base. The crystal structure of the doubly protonated model [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) confirms the presence of both ammonium and terminal hydrido centers, which interact through a dihydrogen bond (dppv = cis-C2H2(PPh2)2). DFT calculations indicate that this H---H interaction is sensitive to the counterions and is strengthened upon reduction of the diiron center. For the monoprotonated models, the hydride [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) exists in equilibrium with the ammonium tautomer [Fe2(Hadt(H))(CO)2(dppv)2](+). Both [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) and [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) are highly active electrocatalysts for HER. Catalysis is initiated by reduction of the diferrous center, which induces coupling of the protic ammonium center and the hydride ligand. In contrast, the propanedithiolate [(term-H)Fe2(pdt)(CO)2(dppv)2](+) is a poor electrocatalyst for HER. Oxidation of H2 has been demonstrated, starting with models for the oxidized state ("Hox"), for example, [Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Featuring a distorted Fe(II)Fe(I) center, this Hox model reacts slowly with high pressures of H2 to give [(μ-H)Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Highlighting the role of the proton relay, the propanedithiolate [Fe2(pdt)(CO)3(dppv)(PMe3)](+) is unreactive toward H2. The Hox-model + H2 reaction is accelerated in the presence of ferrocenium salts, which simulate the role of the attached [4Fe-4S] cluster. The redox-complemented complex [Fe2(adt(Bn))(CO)3(dppv)(FcP*)](n+) catalyzes both proton reduction and hydrogen oxidation (FcP* = (C5Me5)Fe(C5Me4CH2PEt2)).

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe(2)(xdt)(CO)(2)(PMe(3))(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 C(2)-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 (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe(2)(Hpdt)(CO)(2)(PMe(3))(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 PMe(3) ligands are basal.

The ferrous dithiolato carbonyl Fe(S(2)C(3)H(6))(CO)(2)(dppe) (1) condenses with NiCl(2)(dppe) to give [FeNi(pdt)(mu-Cl)(CO)(dppe)(2)]BF(4) ([2Cl](+)). The corresponding reaction of the ditosylate Ni(OTs)(2)(dppe) gave the dication [(CO)(2)(dppe)Fe(pdt)Ni(dppe)](OTs)(2) ([2(CO)](OTs)(2)) (pdt = 1,3-propanedithiolate; dppe = 1,2-C(2)H(4)(PPh(2))(2); OTs(-) = CH(3)C(6)H(4)-4-SO(3)(-)). Reduction of the bimetallic dicarbonyl with borohydride salts afforded impure, thermally stable hydride, [(CO)(dppe)Fe(pdt)(mu-H)Ni(dppe)](+) ([2H](+)). A reliable route to NiFe(SR)(2)H species entailed protonation of (CO)(3)Fe(pdt)Ni(dppe) to give [(CO)(3)Fe(pdt)(mu-H)Ni(dppe)](+) ([3H](+)). The iron-nickel dithiolato hydride, [3H]BF(4), was characterized crystallographically: as anticipated by biophysical studies, the hydride ligand is bridging, the Fe center is octahedral, and the Ni center is pentacoordinate. Solutions of [3H]BF(4) undergo substitution by dppe to give [2H](+). The hydride undergoes rapid deprotonation and is an electrocatalyst for hydrogen evolution from trifluoroacetic acid. Oxidation of 3 gives a mixed valence species [3](+), a potential model for the Ni-L state.

The new complexes Fe2(S2CnH2n)(CO)2(dppv)2 (n = 2, 3; dppv = cis-1,2-C2H2(PPh2)2) form adducts with AlBr3 and B(C6F5)3, which adopt the "rotated structure" proposed for the active site of the Fe-only hydrogenases--the propanedithiolate is significantly more Lewis basic due to nonbonded interactions between the dithiolate strap and the ligands on Fe.

The new iron carbonyl cyanide trans-[Fe(CN)(2)(CO)(3)](2)(-), [2](2)(-), forms in high yield via photosubstitution of Fe(CO)(5) with 2 equiv of Et(4)NCN. Protonation of [2](2)(-) generated [HFe(CN)(2)(CO)(3)](-), [2H](-), 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-Fe-CN-CO centers.

Research has been conducted on mixed ligand species [Fe (sub)2 -(S (sub)2 C (sub)3 H (sub)6)(CN)(CO) (sub)4 (PMe (sub)3)] (super)- which catalyzes proton electroreduction to dihydrogen. The description of this compound and its derivatives and analogues as they relate to the proton reduction process, analyzed theoretically and electrochemically, is presented.

Research has been conducted on the carbon monoxide, cyanide and ferrous derivatives. Results indicate that the reaction of these derivatives has led to the isolation of the new products characterized via the use of single-crystal X-ray diffraction.

Research has been conducted on iron carbonyl cyanide trans-[Fe(CN) (sub)2 (CO) (sub)3] (super)2- complex. The authors report that this complex forms via photosubstitution of Fe(CO )sub)5 with Et (sub)4 NCN, and that the protonation of the complexes produces [HFe(CN) (sub)2 (CO) (sub)3] (super)-.

The reactions of [Fe3(CO)12] with the dithiadiazacyclooctanes [SCH2N(R)CH2]2 (R = Me, Bn) afford the diiron complexes [Fe2{SCH2N(R)CH2}(CO)6] ( 1Me, 1Bu). The methyl derivative 1Me was characterized crystallographically [Fe-Fe = 2.5702(5) Å]. Its low symmetry was verified by variable-temperature 13C NMR spectroscopy, which revealed that the turnstile rotation of the {S(CH2)Fe(CO)3} and {S(NMe)Fe(CO)3} centers are subject to very different energy barriers. Although 1Me resists protonation, it readily undergoes substitution by tertiary phosphines, first at the {S(CH2)Fe(CO)3} center, as verified crystallographically for [Fe2{SCH2N(Me)CH2}(CO)5(PPh3)]. Substitution by the chelating diphosphine Ph2PCH2CH2PPh2 (dppe) gave [Fe2{SCH2N(Me)CH2}(CO)4(dppe)] by substitution at both the {S(CH2)Fe(CO)3} and {S(NMe)Fe(CO)3} sites. [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]

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-.

This report describes examples ofcombined Fe- and O-centered reactivityof Fe(P2O2)(CO)2(1),where P2O2is the diphosphinoglycolate (Ph2PC6H4CHO)22–. This 18e low-spin ferrous dialkoxide undergoes substitution ofCO to give the labile monosubstituted derivatives Fe(P2O2)(CO)(L) (L = PMe3, pyridine, MeCN). Treatmentof Fe(P2O2)(CO)2with Brønstedacids results in stepwise O-protonation, affording rare examples oflow-spin Fe(II) complexes containing alcohol ligands. Substitutionreactions with amides (RC(O)NH2) proceeds with bindingof the carbonyl and formation of an intramolecular hydrogen bond betweenNHand the neighboring alkoxo ligand. This two-sitebinding was confirmed with crystallographic characterization of thethiourea-substituted derivative. Fe(P2O2)(CO)2reacts with Ph2SiH2to give the O-silylatedhydrido complex, which is inactive for hydrosilylation. The monocarbonylderivatives Fe(P2O2)(CO)(L) (L = NCMe, PMe3, acetamide) are precursors to catalysts for the hydrosilylationof benzaldehyde, acetophenone, and styrene. [ABSTRACT FROM AUTHOR]

Reportedare complexes of the formula Fe(dithiolate)(CO)2(diphos)and their use to prepare homo- and heterobimetallic dithiolatoderivatives. The starting iron dithiolates were prepared by a one-potreaction of FeCl2and CO with chelating diphosphines anddithiolates, where dithiolate = S2(CH2)22–(edt2–), S2(CH2)32–(pdt2–), S2(CH2)2(C(CH3)2)2–(Me2pdt2–) and diphos = cis-C2H2(PPh2)2(dppv), C2H4(PPh2)2(dppe), C6H4(PPh2)2(dppbz), C2H4[P(C6H11)2]2(dcpe). The incorporation of 57Fe into such building block complexes commenced with theconversion of 57Fe into 57Fe2I4(iPrOH)4, which thenwas treated with K2pdt, CO, and dppe to give 57Fe(pdt)(CO)2(dppe). NMR and IR analyses show that thesecomplexes exist as mixtures of all-cis and trans-CO isomers, edt2–favoring the former and pdt2–thelatter. Treatment of Fe(dithiolate)(CO)2(diphos) with theFe(0) reagent (benzylideneacetone)Fe(CO)3gave Fe2(dithiolate)(CO)4(diphos), thereby defining a route fromsimple ferrous salts to models for hydrogenase active sites. Extendingthe building block route to heterobimetallic complexes, treatmentof Fe(pdt)(CO)2(dppe) with [(acenaphthene)Mn(CO)3]+gave [(CO)3Mn(pdt)Fe(CO)2(dppe)]+([3d(CO)]+). Reduction of [3d(CO)]+with BH4–gave the Cs-symmetricμ-hydride (CO)3Mn(pdt)(H)Fe(CO)(dppe) (H3d). Complex H3dis reversibly protonated by strong acids,the proposed site of protonation being sulfur. Treatment of Fe(dithiolate)(CO)2(diphos) with CpCoI2(CO) followed by reductionby Cp2Co affords CpCo(dithiolate)Fe(CO)(diphos) (4), which can also be prepared from Fe(dithiolate)(CO)2(diphos) and CpCo(CO)2. Like the electronicallyrelated (CO)3Fe(pdt)Fe(CO)(diphos), these complexes undergoprotonation to afford the μ-hydrido complexes [CpCo(dithiolate)HFe(CO)(diphos)]+. Low-temperature NMR studies indicate that Co is the kineticsite of protonation. [ABSTRACT FROM AUTHOR]

The reaction of Fe2(S2C2H4)(CO)6 with cis-Ph2PCHCHPPh2 (dppv) yields Fe2(S2C2H4)(CO)4(dppv), 1(CO)4, wherein the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO)4, the dppv ligand spans axial and basal coordination sites. In addition to the axial-basal isomer, the 1,3-propanedithiolate and azadithiolate derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial-basal isomer is destabilized by nonbonding interactions between the dppv and the central NH or Ch2 of the larger dithiolates. The Fe(CO)3 subunit in 1 (CO)4 undergoes substitution with PMe3 and cyanide to afford 1 (CO)3(PMe3) and (Et4N)[1(CN)(CO)3], respectively. Kinetic studies show that 1(CO)4 reacts faster with donor ligands than does its parent Fe2(S2C2H4)(CO)6. The rate of reaction of 1(CO)4 with PMe3 was first order in each reactant, k = 3.1 × 10-4 M-1 s-1. The activation parameters for this substitution reaction, ΔH‡ = 5.8(5) kcal/mol and ΔS‡ = -48(2) cal/deg·mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe2(S2C2H4)(CO)6, the enhanced electrophilicity of 1(CO)4 arises from the stabilization of a "rotated" transition state, which is favored by the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1 (CO)3(PMe3) with Cp2FePF6 yielded [Fe2(S2C2H4)(μ-CO)(CO)2(dppv)(PMe3)(NCMe)](PF6)2. Reaction of this compound with PMe3 yielded [Fe2(S2C2H4)-(μ-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2. [ABSTRACT FROM AUTHOR]