[Ru(CN)(CO)]: Building Block for Multimetallic Cages.

A cluster-ligand is disclosed in the form of [Ru(CN)(CO)] ([]). Produced by simple reaction of [Ru(CO)] with cyanide, [] serves as a precursor to a series of μ-CN cages. When treated with [Ru(CO)], it readily forms the prism [Ru(μ-CN)(CO)].

The H-cluster of [FeFe] Hydrogenases: Its Enzymatic Synthesis and Parallel Inorganic Semisynthesis.

ConspectusNature's prototypical hydrogen-forming catalysts─hydrogenases─have attracted much attention because they catalyze hydrogen evolution at near zero overpotential and ambient conditions. Beyond any possible applications in the energy sphere, the hydrogenases feature complicated active sites, which implies novel biosynthetic pathways. In terms of the variety of cofactors, the [FeFe]-hydrogenase is among the most complex.

Final Stages in the Biosynthesis of the [FeFe]-Hydrogenase Active Site.

The paper aims to elucidate the final stages in the biosynthesis of the [2Fe] active site of the [FeFe]-hydrogenases. The recently hypothesized intermediate [Fe(SCHNH)(CN)(CO)] ([1]) was prepared by a multistep route from [Fe(S)(CN)(CO)]. The following synthetic intermediates were characterized in order: [Fe(SCHNHFmoc)(CNBEt)(CO)], [Fe(SCHNHFmoc)(CN)-(CO)], and [Fe(SCHNHFmoc)(CN)(CO)], where Fmoc is fluorenylmethoxycarbonyl).

Hybrids of [FeFe]- and [NiFe]-Hase Active Site Models.

Complexes of the type (diphosphine)Ni(-SR)Fe(CO) are investigated with azadithiolate (adt, HN(CHS)) as the dithiolate. The resulting complexes are hybrid models for the active sites of the [NiFe]- and [FeFe]-hydrogenases. The key complex (dppv)Ni(-adt)Fe(CO) was prepared from the complex Ni[(SCH)NCbz](dppv), which contains a Cbz-protected adt ligand (Cbz = C()OCHPh, dppv = -1,2-(PhP)CH).

Characterizing the Biosynthesis of the [Fe(II)(CN)(CO)(cysteinate)] Organometallic Product of the Radical-SAM Enzyme HydG by EPR and Mössbauer Spectroscopy.

[FeFe]-hydrogenases employ a catalytic H-cluster, consisting of a [4Fe-4S] cluster linked to a [2Fe] subcluster with CO, CN ligands, and an azadithiolate bridge, which mediates the rapid redox interconversion of H and H. In the biosynthesis of this H-cluster active site, the radical -adenosyl-l-methionine (radical SAM, RS) enzyme HydG plays the crucial role of generating an organometallic [Fe(II)(CN)(CO)(cysteinate)] product that is en route to forming the H-cluster. Here, we report direct observation of this diamagnetic organometallic Fe(II) complex through Mössbauer spectroscopy, revealing an isomer shift of δ = 0.

Synthesis, Spectroscopy, and Structure of [FeRu(μ-dithiolate)(CN)(CO)].

The salt [K(18-crown-6)][Ru(CN)(CO)] ([K(18-crown-6)][]) was generated by the reaction of Ru(CH)(CO) with [K(18-crown-6)]CN. An initial thermal reaction gives [Ru(CN)(CO)], which, upon ultraviolet (UV) irradiation, reacts with a second equiv of CN. Protonation of [] gave [HRu(CN)(CO)] ([H]), which was isolated as a single isomer with mutually trans cyanide ligands.

Fully Refined Semisynthesis of the [FeFe] Hydrogenase H-Cluster.

[FeFe] hydrogenases contain a 6-Fe cofactor that serves as the active site for efficient redox interconversion between H and protons. The biosynthesis of the so-called H-cluster involves unusual enzymatic reactions that synthesize organometallic Fe complexes containing azadithiolate, CO, and CN ligands. We have previously demonstrated that specific synthetic [Fe(CO)(CN)] complexes can be used to functionally replace proposed Fe intermediates in the maturation reaction.

Synthesis and Dynamics of Ferrous Polychalcogenides [Fe(E)(CN)(CO)] (E = S, Se, or Te).

Elemental chalcogens react with [Fe(CN)(CO)] to give the following ferrous derivatives: [K(18-crown-6)][Fe(S)(CN)(CO)], [K(18-crown-6)][Fe(S)(CN)(CO)], [K(18-crown-6)][Fe(Se)(CN)(CO)], [K(18-crown-6)][Fe(Te)(CN)(CO)], and (NEt)[Fe(Te)(CN)(CO)]. While these complex anions crystallized in a single stereochemistry (i.e.

Organometallic Fe(μ-SH)(CO)(CN) Cluster Allows the Biosynthesis of the [FeFe]-Hydrogenase with Only the HydF Maturase.

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 H. The biosynthesis of the [2Fe] 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)(CN) adduct of cysteine, which is the substrate for HydE.

Inhibition of [FeFe]-hydrogenase by formaldehyde: proposed mechanism and reactivity of FeFe alkyl complexes.

The mechanism for inhibition of [FeFe]-hydrogenases by formaldehyde is examined with model complexes. Key findings: (i) CH donated by formaldehyde covalently link Fe and the amine cofactor, blocking the active site and (ii) the resulting Fe-alkyl is a versatile electrophilic alkylating agent. Solutions of Fe[(μ-SCH)NH](CO)(PMe) (1) react with a mixture of HBF and CHO to give three isomers of [Fe[(μ-SCH)NCH](CO)(PMe)] ([2]).

Challenges in the Synthesis of Active Site Mimics for [NiFe]-Hydrogenases.

One of the more active areas in bioorganometallic chemistry is the preparation and reactivity studies of active site mimics of the [NiFe]-hydrogenases. One area of particular recent progress involves reactions that interconvert Ni(-X)Fe centers for X = OH, H, CO, as described by Song et al. Such reactions illustrate new ways to access intermediates related to the Ni-R and Ni-SI states of the enzyme.

Biosynthesis of the [FeFe] hydrogenase H-cluster a synthetic [Fe(II)(CN)(CO)(cysteinate)] complex.

The H-cluster of [Fe-Fe] hydrogenase consists of a [4Fe] subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe] subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCHNHCHS 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] 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 -adenosylmethione (SAM) bound to a [4Fe-4S] cluster to lyse tyrosine 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 the cysteine sulfur.

Surprising Condensation Reactions of the Azadithiolate Cofactor.

Azadithiolate, a cofactor found in all [FeFe]-hydrogenases, is shown to undergo acid-catalyzed rearrangement. Fe [(SCH ) NH](CO) self-condenses to give Fe [(SCH ) N] (CO) . The reaction, which is driven by loss of NH , illustrates the exchange of the amine group.

Homoleptic Rhodium Pyridine Complexes for Catalytic Hydrogen Oxidation.

The homoleptic rhodium pyridine complex [Rh(py)] ([]) is prepared from simple precursors. Lacking good π-acceptor ligands but being sterically protected, [] reversibly oxidizes to colorless [Rh(py)(thf)]. This monomeric = 1/2 Rh(II) complex activates H to give [HRh(py)L], which can also be generated by protonation of [].

Crystal Structure of the [FeFe]-Hydrogenase Maturase HydE Bound to Complex-B.

[FeFe]-hydrogenases use a unique organometallic complex, termed the H cluster, to reversibly convert H into protons and low-potential electrons. It can be best described as a [FeS] cluster coupled to a unique [2Fe] 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.

Vibrational Perturbation of the [FeFe] Hydrogenase H-Cluster Revealed by CH-ADT Labeling.

[FeFe] hydrogenases are highly active catalysts for the interconversion of molecular hydrogen with protons and electrons. Here, we use a combination of isotopic labeling, Fe nuclear resonance vibrational spectroscopy (NRVS), and density functional theory (DFT) calculations to observe and characterize the vibrational modes involving motion of the 2-azapropane-1,3-dithiolate (ADT) ligand bridging the two iron sites in the [2Fe] subcluster. A -CH- ADT labeling in the synthetic diiron precursor of [2Fe] produced isotope effects observed throughout the NRVS spectrum.

CO substitution by PPh in FeS(CO) proceeds via a novel FeS intermediate.

The reaction of FeS(CO) and PPh affords FeS(CO)(PPh) by an unprecedented mechanism involving the intermediacy of SPPh and FeS(CO)(PPh).

Reactions of [FeC(CO)(S)]: Cluster Growth, Redox, Sulfiding.

Redox reactions, substitutions, and metalations are reported for the iron carbido sulfide [FeC(CO)(S)] ([]). Dianion [] oxidized to [FeC(CO)(S)] ([]) upon treatment with of [Fe(CH)]BF or HBF (H formation) under an atmosphere of CO. Reaction of [] with BuNC gave [FeC(S)(CO)(BuNC)], consisting of FeC(CO) and [Fe(BuNC)] subunits linked by a μ-S.

Computational and Experimental Investigations of the Fe(μ-S)/Fe(μ-S) Equilibrium.

Density functional theory (DFT) calculations on FeS(CO)(PMe) for = 0, 1, and 2 reveal that the most electron-rich derivatives ( = 2) exist as diferrous disulfides lacking an S-S bond. The thermal interconversion of the Fe(S) and Fe(S) valence isomers is symmetry-forbidden. Related electron-rich diiron complexes [FeS(CN)(CO)] of an uncertain structure are implicated in the biosynthesis of [FeFe]-hydrogenases.

Using nature's blueprint to expand catalysis with Earth-abundant metals.

Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis.

Radical SAM Enzyme HydE Generates Adenosylated Fe(I) Intermediates En Route to the [FeFe]-Hydrogenase Catalytic H-Cluster.

The H-cluster of [FeFe]-hydrogenase consists of a [4Fe-4S]-subcluster linked by a cysteinyl bridge to a unique organometallic [2Fe]-subcluster assigned as the site of interconversion between protons and molecular hydrogen. This [2Fe]-subcluster is assembled by a set of Fe-S maturase enzymes HydG, HydE and HydF. Here we show that the HydG product [Fe(Cys)(CO)(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.

Spectroscopic and Computational Evidence that [FeFe] Hydrogenases Operate Exclusively with CO-Bridged Intermediates.

[FeFe] hydrogenases are extremely active H-converting enzymes. Their mechanism remains highly controversial, in particular, the nature of the one-electron and two-electron reduced intermediates called HH and HH. In one model, the HH and HH states contain a semibridging CO, while in the other model, the bridging CO is replaced by a bridging hydride.

Asymmetry in the Ligand Coordination Sphere of the [FeFe] Hydrogenase Active Site Is Reflected in the Magnetic Spin Interactions of the Aza-propanedithiolate Ligand.

[FeFe] hydrogenases are very active enzymes that catalyze the reversible conversion of molecular hydrogen into protons and electrons. Their active site, the H-cluster, contains a unique binuclear iron complex, [2Fe], with CN and CO ligands as well as an aza-propane-dithiolate (ADT) moiety featuring a central amine functionality that mediates proton transfer during catalysis. We present a pulsed C-ENDOR investigation of the H-cluster in which the two methylene carbons of ADT are isotope labeled with C.

The binuclear cluster of [FeFe] hydrogenase is formed with sulfur donated by cysteine of an [Fe(Cys)(CO)(CN)] organometallic precursor.

The enzyme [FeFe]-hydrogenase (HydA1) contains a unique 6-iron cofactor, the H-cluster, that has unusual ligands to an Fe-Fe binuclear subcluster: CN, CO, and an azadithiolate (adt) ligand that provides 2 S bridges between the 2 Fe atoms. In cells, the H-cluster is assembled by a collection of 3 maturases: HydE and HydF, whose roles aren't fully understood, and HydG, which has been shown to construct a [Fe(Cys)(CO)(CN)] organometallic precursor to the binuclear cluster. Here, we report the in vitro assembly of the H-cluster in the absence of HydG, which is functionally replaced by adding a synthetic [Fe(Cys)(CO)(CN)] carrier in the maturation reaction.

Iron Carbide-Sulfide Carbonyl Clusters.

Described is the preparation of the first iron carbide-sulfides. The cluster [FeC(CO)(SO)] ([2]), which is generated quantitatively from [FeC(CO)] ([1]), was O-methylated to give the sulfinite [2Me]. Demethoxylation of [2Me] with BF gave the face-capped octahedral cluster FeC(CO)(SO) (3).