Paramagnetic Bridging Hydrides of Relevance to Catalytic Hydrogen Evolution at Metallosulfur Centers

Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO

Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various Mn^II PMHs have been proposed as intermediates in catalysis, but isolated Mn^II PMHs are limited to dimeric high-spin Mn^II structures with bridging hydrides. In this paper, a series of the first low-spin monomeric Mn^II PMH complexes are generated by chemical oxidation of their Mn^I analogues. This series is of the type trans-[MnH(L)(dmpe)₂]^+/0 where the trans ligand L is PMe₃, C₂H₄, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the Mn^II hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe₃, the complex is the first example of an isolated monomeric Mn^II hydride complex. In contrast, when L is C₂H₄ or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)₃]⁺, accompanied by ethane and ethylene, whereas the latter eliminated H₂, generating [Mn(MeCN)(CO)(dmpe)₂]⁺ or a mixture of products including [Mn(κ¹-PF₆)(CO)(dmpe)₂], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe₃)(dmpe)₂]⁺ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm^-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. Mn^II-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe₃) to 47 kcal/mol (L = CO).

Biomimetics of [FeFe]-hydrogenases incorporating redox-active ligands: Synthesis, redox and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe4S4 surrogates

[FeFe]-ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H2 studied, including 1,1’-bis(diphenylphosphino)ferrocene (dppf) complexes Fe2(CO)4(-dppf)(-S(CH2)nS) (n = 2, edt; n...

Lewis acid protection turns cyanide containing [FeFe]-hydrogenase mimics into proton reduction catalysts

Sustainable sources of hydrogen are a vital component of the envisioned energy transition. Understanding and mimicking the [FeFe]-hydrogenase provides a route to achieving this goal. In this study we re-visit a molecular mimic of the hydrogenase, the propyl dithiolate bridged complex [Fe₂(μ-pdt)(CO)₄(CN)₂]²⁻, in which the cyanide ligands are tuned via Lewis acid interactions. This system provides a rare example of a cyanide containing [FeFe]-hydrogenase mimic capable of catalytic proton reduction, as demonstrated by cyclic voltammetry. EPR, FTIR, UV-vis and X-ray absorption spectroscopy are employed to characterize the species produced by protonation, and reduction or oxidation of the complex. The results reveal that biologically relevant iron-oxidation states can be generated, potentially including short-lived mixed valent Fe(i)Fe(ii) species. We propose that catalysis is initiated by protonation of the diiron complex and the resulting di-ferrous bridging hydride species can subsequently follow two different pathways to promote H₂ gas formation depending on the applied reduction potential.

Mimicking the hydrogen-bonding interactions of the [FeFe]-hydrogenase active-site using Lewis acids transforms an otherwise unstable cyanide containing hydrogenase mimic into a proton reduction catalyst.

Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase

Binding of N₂ by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e^- and 4 H⁺ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P₆ArC)Fe₂(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e^- transfer which bear some resemblance to a pair of E_n and E_n+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe₂(μ-H)}₂ state of FeMoco (E₄(4H)), the data and analysis presented herein suggest that the hydride ligands in E₄(4H) bridge isovalent (most probably Fe^III) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P₆ArC)Fe₂(μ-H)]⁺¹ and [(P₆ArC)Fe₂(μ-H)]^-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e^- reduction), a conclusion further supported by X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt2 groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt2 group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et2NP-PNEt2-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl2PNEt2 followed by reductive P-P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe2(CO)9 gave the diiron hexacarbonyl complex 3, in which the P-P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt2 group was forced to face the Fe-Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt2 group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et2 group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1'-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt2 group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides

Geometrically flexible 9-borabicyclo[3.3.1]nonyl units within the secondary coordination sphere enable isolation of high-spin Fe(ii)-dihydrides stabilized by boron-hydride interactions and a rare example of an isolable S = 3/2 reduction product. The borane-capped Fe(ii)-dihydride: (1) rapidly deprotonates E-H (E = N, O, P, S) bonds to afford borane-stabilized Fe adducts and (2) releases H2 upon exposure to π-acids. The Lewis acids provide an avenue for redox-leveling in analogy to the near constant operating potential for N2 reduction in nitrogenase.

Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe2(CO)3(μ-dithiolate)(μ,κ1,κ2-triphos)

A series of Fe₂(triphos)(CO)₃(μ-dithiolate) complexes have been prepared and studied as models of the diiron centre in [FeFe]-hydrogenases.

Energy-Efficient Hydrogen Evolution by Fe-S Electrocatalysts: Mechanistic Investigations

The intrinsic catalytic property of a Fe-S complex toward H₂ evolution was investigated in a wide range of acids. The title complex exhibited catalytic events at -1.16 and -1.57 V (vs Fc⁺/Fc) in the presence of trifluoromethanesulfonic acid (HOTf) and trifluoroacetic acid (TFA), respectively. The processes corresponded to the single reduction of the Fe-hydride-S-proton and Fe-hydride species, respectively. When anilinium acid was used, the catalysis occurred at -1.16 V, identical with the working potential of the HOTf catalysis, although the employment of anilinium acid was only capable of achieving the Fe-hydride state on the basis of the spectral and calculated results. The thermodynamics and kinetics of individual steps of the catalysis were analyzed by density functional theory (DFT) calculations and electroanalytical simulations. The stepwise CCE or CE (C, chemical; E, electrochemical) mechanism was operative from the HOTf or TFA source, respectively. In contrast, the involvement of anilinium acid most likely initiated a proton-coupled electron transfer (PCET) pathway that avoided the disfavored intermediate after the initial protonation. Via the PCET pathway, the heterogeneous electron transfer rate was increased and the overpotential was decreased by 0.4 V in comparison with the stepwise pathways. The results showed that the PCET-involved catalysis exhibited substantial kinetic and thermodynamic advantages in comparison to the stepwise pathway; thus, an efficient catalytic system for proton reduction was established.

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]-Hydrogenase

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu^.), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu^. undergoes the same chemical reactions as H^. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu^. into three models of the [FeFe]‐hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

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.

Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

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](0); 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](0) 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.

EPR detection and characterisation of a paramagnetic Mo(iii) dihydride intermediate involved in electrocatalytic hydrogen evolution

EPR spectroscopy and theoretical data show that the slow heterogeneous electron-transfer kinetics associated with the reduction of an 18-electron Mo(IV) acetato dihydride are a consequence of an η(2)-η(1) rearrangement of the carboxylate ligand which gives a unique paramagnetic 17-electron Mo(III) dihydride.

Isolation and structural characterization of a mainly ligand-based dimetallic radical

The isolation and crystal structure of a mainly ligand-based dimetallic radical of ruthenium together with induced sp³ C–H bond activation were described.

Direct observation of key catalytic intermediates in a photoinduced proton reduction cycle with a diiron carbonyl complex

The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe2(bdt)(CO)6](-), a key intermediate in photocatalytic H2 formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe2(bdt)(CO)6](-) is readily distinguished from the two-electron reduced catalyst [Fe2(bdt)(CO)6](2-) that is obtained inevitably in the electrochemical reduction of [Fe2(bdt)(CO)6]. For the disproportionation rate constant of [Fe2(bdt)(CO)6](-), an upper limit on the order of 10(7) M(-1) s(-1) was estimated, which precludes a major role of [Fe2(bdt)(CO)6](2-) in photoinduced proton reduction cycles. Structurally [Fe2(bdt)(CO)6](-) is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pK(a) ≤ 12.7 protonate [Fe2(bdt)(CO)6](-) with bimolecular rate constants of 4 × 10(6), 7 × 10(6), and 2 × 10(8) M(-1) s(-1) (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe2(bdt)(CO)6H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe2(bdt)(CO)6], with very similar carbonyl stretching frequencies.

A 10(6)-fold enhancement in N2-binding affinity of an Fe2(μ-H)2 core upon reduction to a mixed-valence Fe(II)Fe(I) state

Transient hydride ligands bridging two or more iron centers purportedly accumulate on the iron-molybdenum cofactor (FeMoco) of nitrogenase, and their role in the reduction of N2 to NH3 is unknown. One role of these ligands may be to facilitate N2 coordination at an iron site of FeMoco. Herein, we consider this hypothesis and describe the preparation of a series of diiron complexes supported by two bridging hydride ligands. These compounds bind either one or two molecules of N2 depending on the redox state of the Fe2(μ-H)2 unit. An unusual example of a mixed-valent Fe(II)(μ-H)2Fe(I) is described that displays a 10(6)-fold enhancement of N2 binding affinity over its oxidized congener, quantified by spectroscopic and electrochemical techniques. Furthermore, these compounds show promise as functional models of nitrogenase as substantial amounts of NH3 are produced upon exposure to proton and electron equivalents. The Fe(μ-H)Fe(N2) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.

[FeFe] hydrogenase: protonation of {2Fe3S} systems and formation of super-reduced hydride states

The synthesis and crystallographic characterization of a complex possessing a well-defined {2Fe3S(μ-H)} core gives access to a paramagnetic bridging hydride with retention of the core geometry. Chemistry of this 35-electron species within the confines of a thin-layer FTIR spectro-electrochemistry cell provides evidence for a unprecedented super-reduced Fe(I)(μ-H)Fe(I) intermediate.

Computational investigation of [FeFe]-hydrogenase models: characterization of singly and doubly protonated intermediates and mechanistic insights

The [FeFe]-hydrogenase enzymes catalyze hydrogen oxidation and production efficiently with binuclear Fe metal centers. Recently the bioinspired H₂-producing model system Fe₂(adt)(CO)₂(dppv)₂ (adt=azadithiolate and dppv=diphosphine) was synthesized and studied experimentally. In this system, the azadithiolate bridge facilitates the formation of a doubly protonated ammonium-hydride species through a proton relay. Herein computational methods are utilized to examine this system in the various oxidation states and protonation states along proposed mechanistic pathways for H₂ production. The calculated results agree well with the experimental data for the geometries, CO vibrational stretching frequencies, and reduction potentials. The calculations illustrate that the NH···HFe dihydrogen bonding distance in the doubly protonated species is highly sensitive to the effects of ion-pairing between the ammonium and BF₄^– counterions, which are present in the crystal structure, in that the inclusion of BF₄^– counterions leads to a significantly longer dihydrogen bond. The non-hydride Fe center was found to be the site of reduction for terminal hydride species and unsymmetric bridging hydride species, whereas the reduced symmetric bridging hydride species exhibited spin delocalization between the Fe centers. According to both experimental measurements and theoretical calculations of the relative pK_a values, the Fe_d center of the neutral species is more basic than the amine, and the bridging hydride species is more thermodynamically stable than the terminal hydride species. The calculations implicate a possible pathway for H₂ evolution that involves an intermediate with H₂ weakly bonded to one Fe, a short H₂ distance similar to the molecular bond length, the spin density delocalized over the two Fe centers, and a nearly symmetrically bridged CO ligand. Overall, this study illustrates the mechanistic roles of the ammonium-hydride interaction, flexibility of the bridging CO ligand, and intramolecular electron transfer between the Fe centers in the catalytic cycle. Such insights will assist in the design of more effective bioinspired catalysts for H₂ production.

Theoretical calculations in conjunction with supporting experimental data are used to analyze the mechanistic pathway for hydrogen evolution catalyzed by the bioinspired model Fe₂(adt)(CO)₂(dppv)₂. This study elucidates the site of reduction and the pK_a values associated with formation of the singly and doubly protonated species, as well as the roles of the ammonium-hydride interaction, flexibility of the bridging CO ligand, and intramolecular electron transfer between the Fe centers in the catalytic cycle for H₂ production.

Hydrogenase biomimetics: Fe2(CO)4(μ-dppf)(μ-pdt) (dppf = 1,1'-bis(diphenylphosphino)ferrocene) both a proton-reduction and hydrogen oxidation catalyst

Fe2(CO)4(μ-dppf)(μ-pdt) catalyses the conversion of protons and electrons into hydrogen and also the reverse reaction thus mimicing both types of binuclear hydrogenase enzymes.

Borane-protected cyanides as surrogates of H-bonded cyanides in [FeFe]-hydrogenase active site models

Triarylborane Lewis acids bind [Fe2(pdt)(CO)4(CN)2](2-) [1](2-) (pdt(2-) = 1,3-propanedithiolate) and [Fe2(adt)(CO)4(CN)2](2-) [3](2-) (adt(2-) = 1,3-azadithiolate, HN(CH2S(-))2) to give the 2:1 adducts [Fe2(xdt)(CO)4(CNBAr3)2](2-). Attempts to prepare the 1:1 adducts [1(BAr3)](2-) (Ar = Ph, C6F5) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B(C6F4-o-C6F5)3 (BAr(F)*3). By virtue of the N-protection by the borane, salts of [Fe2(pdt)(CO)4(CNBAr3)2](2-) sustain protonation to give hydrides that are stable (in contrast to [H1](-)). The hydrides [H1(BAr3)2](-) are 2.5-5 pKa units more acidic than the parent [H1](-). The adducts [1(BAr3)2](2-) oxidize quasi-reversibly around -0.3 V versus Fc(0/+) in contrast to ca. -0.8 V observed for the [1](2-/-) couple. A simplified synthesis of [1](2-), [3](2-), and [Fe2(pdt)(CO)5(CN)](-) ([2](-)) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.

Synthesis, spectroscopy, and hydrogen/deuterium exchange in high-spin iron(II) hydride complexes

Very few hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin iron(II) hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin iron hydride complexes may be relevant to postulated nitrogenase mechanisms.

Generation of high-spin iron(I) in a protein environment using cryoreduction

High-spin Fe(1+) sites are potentially important in iron-sulfur proteins but are rare in synthetic compounds and unknown in metalloproteins. Here, we demonstrate a spectroscopically characterized example of high-spin non-heme Fe(1+) in a protein environment. Cryoreduction of Fe(2+)-substituted azurin at 77 K with (60)Co γ radiation generates a new species with a S = (3)/2 (high-spin) Fe(1+) center having D > 0 and E/D ~ 0.25. This transient species is stable in a glycerol-water glass only up to ~170 K. A combination of electron paramagnetic resonance and Mössbauer spectroscopies provides a powerful means of identifying a transient high-spin Fe(1+) site in a protein scaffold.

Mixed-valence [Fe(I)Fe(II)] hydrogenase active site model complexes stabilized by a bidentate carborane bis-phosphine ligand

A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe(2)(dt)(CO)(4)(BC)] 1-3 (dt = dithiolate, pdt = propyl-1,3-dt (1), bdt = benzene-1,2-dt (2), edt = ethyl-1,2-dt (3); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized. While the electrochemical reductions of 1-3 are largely invariant to the different nature of their dt bridges, the oxidations differ by more than 120 mV in between the series. Remarkably, all three compounds are reversibly oxidized, with complex 1 that contains the most electron-donating pdt ligand at the mildest potential of -0.09 V vs. Fc/Fc(+). The one-electron oxidized state 1(ox) is stable for several minutes and was spectroscopically characterized by FTIR and EPR. EPR spectroscopy provided evidence that in the mixed-valence [Fe(I)Fe(II)] state most of the spin density is located on the iron with the BC-ligand. This is monitored through the strong (31)P hyperfine coupling of the phenyl groups of the BC ligand, while further delocalization into the o-carborane unit is negligible.

Synthetic models for the active site of the [FeFe]-hydrogenase: catalytic proton reduction and the structure of the doubly protonated intermediate

This report compares biomimetic hydrogen evolution reaction catalysts with and without the amine cofactor (adt(NH)): Fe(2)(adt(NH))(CO)(2)(dppv)(2) (1(NH)) and Fe(2)(pdt)(CO)(2)(dppv)(2) (2) [(adt(NH))(2-) = HN(CH(2)S)(2)(2-), pdt(2-) = 1,3-(CH(2))(3)S(2)(2-), and dppv = cis-C(2)H(2)(PPh(2))(2)]. These compounds are spectroscopically, structurally, and stereodynamically very similar but exhibit very different catalytic properties. Protonation of 1(NH) and 2 gives three isomeric hydrides each, beginning with the kinetically favored terminal hydride, which converts sequentially to sym and unsym isomers of the bridging hydrides. In the case of 1(NH), the corresponding ammonium hydrides are also observed. In the case of the terminal amine hydride [t-H1(NH)]BF(4), the ammonium/amine hydride equilibrium is sensitive to counteranions and solvent. The species [t-H1(NH(2))](BF(4))(2) represents the first example of a crystallographically characterized terminal hydride produced by protonation. The NH---HFe distance of 1.88(7) Å indicates dihydrogen-bonding. The bridging hydrides [μ-H1(NH)](+) and [μ-H2](+) reduce near -1.8 V, about 150 mV more negative than the reductions of the terminal hydride [t-H1(NH)](+) and [t-H2](+) at -1.65 V. Reductions of the amine hydrides [t-H1(NH)](+) and [t-H1(NH(2))](2+) are irreversible. For the pdt analogue, the [t-H2](+/0) couple is unaffected by weak acids (pK(a)(MeCN) = 15.3) but exhibits catalysis with HBF(4)·Et(2)O, albeit with a turnover frequency (TOF) around 4 s(-1) and an overpotential greater than 1 V. The voltammetry of [t-H1(NH)](+) is strongly affected by relatively weak acids and proceeds at 5000 s(-1) with an overpotential of 0.7 V. The ammonium hydride [t-H1(NH(2))](2+) is a faster catalyst, with an estimated TOF of 58 000 s(-1) and an overpotential of 0.5 V.

Modeling the signatures of hydrides in metalloenzymes: ENDOR analysis of a Di-iron Fe(μ-NH)(μ-H)Fe core

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L(3)Fe(μ-NH)(μ-H)FeL(3) (L(3) = [PhB(CH(2)PPh(2))(3)](-)) complex, 3, is a novel S = (1)/(2) type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. (1,2)H and (14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, -T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [-T, -T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum-iron cofactor of nitrogenase contains two [Fe-H(-)-Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E(4)). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E(4). This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.