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Abstract: The active site of the [FeFe]‐hydrogenases features a binuclear [2Fe]H sub‐cluster that contains a unique bridging amine moiety close to an exposed iron center. Heterolytic splitting of H2 results in the formation of a transient terminal hydride at this iron site, which, however is difficult to stabilize. We show that the hydride intermediate forms immediately when [2Fe]H is replaced with [2Ru]H analogues through artificial maturation. Outside the protein, the [2Ru]H analogues form bridging hydrides, which rearrange to terminal hydrides after insertion into the apo‐protein. H/D exchange of the hydride only occurs for [2Ru]H analogues containing the bridging amine moiety. [ABSTRACT FROM AUTHOR]

The reaction of Fe2(pdt)(CO)6 with two equivalents of Ph2PC6H4NH2 (PNH2) affords the amido hydride HFe2(pdt)(CO)2(PNH2)(PNH) {[H1H]0, pdt2- = CH2(CH2S-)2}. Isolated intermediates in this conversion include Fe2(pdt)(CO)5-(κ¹-PNH2) and Fe2(pdt)(CO)4(κ²-PNH2). X-ray crystallographic analysis of [H1H]0 shows that the chelating amino/amido-phosphine ligands occupy trans-dibasal positions. The 31P NMR spectrum indicates that [H1H]0 undergoes rapid proton exchange between the amido and amine centers. No exchange was observed for the hydride. Protonation of [H1H]0 gives [HFe2(pdt)(CO)2(PNH2)2]+ ([H21H]+), which contains two equivalent amino-phosphine ligands. Single-crystal X-ray crystallographic analysis of [H21H]+ also reveals hydrogen bonds between the exo amine protons with a THF molecule and BF4. Deprotonation of [H1H]0 with potassium tert-butoxide gave [HFe2(pdt)(CO)2(PNH)2]- ([1H]-), which was characterized spectroscopically. The complex has time-averaged C2 symmetry with two amido-phosphine ligands. FTIR spectroscopic measurements show that vCO shifts by approximately 20 cm-1 in the series [1H]-, [H1H]0, and [H21H]+. These shifts are comparable to those seen for the S-protonation of the (NC)2(CO)Fe-(μ-Scys)2Ni(Scys)2 site in the [NiFe]-hydrogenases. [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]

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]

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]

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]

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]

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.

Decades of biophysical study on the hydrogenase (H2ase) enzymes have yielded sufficient information to guide the synthesis of analogs of their active sites. Three families of enzymes serve as inspiration for this work: the [FeFe]-H2ases, [NiFe]-H2ases, and [Fe]-H2ases, all of which feature iron centers bound to both CO and thiolate. Artificial H2ases affect the oxidation of H2 and the reverse reaction, the reduction of protons. These reactions occur via the intermediacy of metal hydrides. The inclusion of amine bases within the catalysts is an important design feature that is emulated in related bioinspired catalysts. Continuing challenges are the low reactivity of H2 toward biomimetic H2ases. [Copyright &y& Elsevier]

Using the thermally stable salts of [Fe2(SR)2(Co)3(PMe3)(dppv)]-BArF4, we found that the azadithiolates [Fe2(adtR)(CO)3(PMe3)-(dppv)]+ react with high pressures of H2 to give the hydride [Fe2(μ-H)(adtR)(CO)3(dppv)(PMe3)]BArF4. The related oxadithiolate and propanedithiolate complexes are unreactive toward H2-Molecular hydrogen is proposed to undergo heterolysis assisted by the amine followed by isomerization of an initially formed terminal hydride. Use of H2 and D2O gave the deuteride as well as the hydride, implicating protic intermediates. [ABSTRACT FROM AUTHOR]

This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe(pdt)Fe(dppe)(CO)H ([1(t-H)]0, Cp*- = Me5C5 -, pdt2- = CH2(CH2S-)2, dppe = Ph2PC2H4PPh2) was prepared by hydride reduction of [Cp*Fe(pdt)Fe(dppe)(CO)(NCMe)]+. As established by X-ray crystallography, [1(t-H)]0 features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [1(t-H)]0 does not isomerize to the bridging hydride [1(μ-H)]0. Oxidation of [1(t-H)]0 gives [1(t-H)]+, which was also characterized crystallographically as its BF4 - salt. Density functional theory (DFT) calculations indicate that [1(t-H)]+ is best described as containing an Cp*FeIII center. In solution, [1(t-H)]+ isomerizes to [1(μ-H)]+, as anticipated by DFT. Reduction of [1(μ-H)]+ by Cp2Co afforded the diferrous bridging hydride [1(μ-H)]0. Electrochemical measurements and DFT calculations indicate that the couples [1(t-H)]+/0 and [1(μ-H)]+/0 differ by 210 mV. Qualitative measurements indicate that [1(t-H)]0 and [1(μ-H)]0 are close in free energy. Protonation of [1(t-H)]0 in MeCN solution affords H2 even with weak acids via hydride transfer. In contrast, protonation of [1(μ-H)]0 yields 0.5 equiv of H2 by a proposed protonation-induced electron transfer process. Isotopic labeling indicates that μ-H/D ligands are inert. [ABSTRACT FROM AUTHOR]

[FeFe]-hydrogenases catalyze the reversible reduction of protons to molecular hydrogen with extremely high efficiency. The active site (-œH-cluster') consists of a [4Fe-4S]H cluster linked through a bridging cysteine to a [2Fe]H subsite coordinated by CN- and CO ligands featuring a dithiol-amine moiety that serves as proton shuttle between the protein proton channel and the catalytic distal iron site (Fed). Although there is broad consensus that an iron-bound terminal hydride species must occur in the catalytic mechanism, such a species has never been directly observed experimentally. Here, we present FTIR and nuclear resonance vibrational spectroscopy (NRVS) experiments in conjunction with density functional theory (DFT) calculations on an [FeFe]-hydrogenase variant lacking the amine proton shuttle which is stabilizing a putative hydride state. The NRVS spectra unequivocally show the bending modes of the terminal Fe-H species fully consistent with widely accepted models of the catalytic cycle. [ABSTRACT FROM AUTHOR]

The article discusses the synthetic models of the [NiFe]-H2ases. It notes that the functional core of this enzyme features a nickel tetrathiolate in which it connects to and Fe(CN)2(CO) center. It mentions the initial approach to biomimetic models wherein an attempt to replace the chloride ligand with hydrides proved unrewarding. It adds that with the three bridging ligands, the structure of the bimetallic complex the ligand set is compatible with the hydrido complex.

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications. [ABSTRACT FROM AUTHOR]

The intermediacy of a reduced nickel--iron hydride in hydrogen evolution catalyzed by Ni--Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1]°; dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1]° initially produces unsym-[H1]+, which converts by a first-order pathway to sym-[H1]+. These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1]+ protonates at sulfur. The S = 1/2 hydride [H1]0 was generated by reduction of [H1]+ with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1]0 is best described as a Ni(I)--Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1]0. Whereas [H1]+ does not evolve H2 upon protonation, treatment of [H1]0 with acids gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1]0 and external acid or from protonation of the Fe--H bond in [H1]0 to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1]0 initially produces [1]+, which is reduced by [H1]0. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit. [ABSTRACT FROM AUTHOR]

Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)3Fe(pdt)-Ni(dppe) (1; pdt2- = -S(CH2)3S-; dppe = Ph2P(CH2)2PPh2) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)2Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy3. Similar to 1, complexes 2 and 3 undergo both protonation and le1 oxidation to afford well-characterized hydrides ([2H]+ and [3H]+) and mixed-valence derivatives ([2] + and [3] +), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)Ni(II) isomer with a square-planar Ni site [ABSTRACT FROM AUTHOR]