Lewis vs. Brønsted-basicities of diiron dithiolates: spectroscopic detection of the "rotated structure" and remarkable effects of ethane- vs. propanedithiolate

C-Cl Bond Activation at Rotated vs Unrotated Dinuclear Site Related to [FeFe]-Hydrogenases

The novel dinuclear complex related to the [FeFe]-hydrogenases active site, [Fe2(μ-pdt)(κ2-dmpe)2(CO)2] (1), is highly reactive toward chlorinated compounds CHxCl4-x (x = 1, 2) affording selectively terminal or bridging chloro diiron isomers through a C-Cl bond activation. DFT calculations suggest a cooperative mechanism involving a formal concerted regioselective chloronium transfer depending on the unrotated or rotated conformation of two isomers of 1.

Geometrical influence on the non-biomimetic heterolytic splitting of H2 by bio-inspired [FeFe]-hydrogenase complexes: a rare example of inverted frustrated Lewis pair based reactivity

Despite the high levels of interest in the synthesis of bio-inspired [FeFe]-hydrogenase complexes, H2 oxidation, which is one specific aspect of hydrogenase enzymatic activity, is not observed for most reported complexes. To attempt H-H bond cleavage, two disubstituted diiron dithiolate complexes in the form of [Fe2(μ-pdt)L2(CO)4] (L: PMe3, dmpe) have been used to play the non-biomimetic role of a Lewis base, with frustrated Lewis pairs (FLPs) formed in the presence of B(C6F5)3 Lewis acid. These unprecedented FLPs, based on the bimetallic Lewis base partner, allow the heterolytic splitting of the H2 molecule, forming a protonated diiron cation and hydrido-borate anion. The substitution, symmetrical or asymmetrical, of two phosphine ligands at the diiron dithiolate core induces a strong difference in the H2 bond cleavage abilities, with the FLP based on the first complex being more efficient than the second. DFT investigations examined the different mechanistic pathways involving each accessible isomer and rationalized the experimental findings. One of the main DFT results highlights that the iron site acting as a Lewis base for the asymmetrical complex is the {Fe(CO)3} subunit, which is less electron-rich than the {FeL(CO)2} site of the symmetrical complex, diminishing the reactivity towards H2. Calculations relating to the different mechanistic pathways revealed the presence of a terminal hydride intermediate at the apical site of a rotated {Fe(CO)3} site, which is experimentally observed, and a semi-bridging hydride intermediate from H2 activation at the Fe-Fe site; these are responsible for a favourable back-reaction, reducing the conversion yield observed in the case of the asymmetrical complex. The use of two equivalents of Lewis acid allows for more complete and faster H2 bond cleavage due to the encapsulation of the hydrido-borate species by a second borane, favouring the reactivity of each FLP, in agreement with DFT calculations.

Di-, tri- and tetraphosphine-substituted Fe/Se carbonyls: Synthesis, Characterization and electrochemical properties

The active sites of [FeFe]-hydrogenase promoted by Fe/E (E=S, Se) clusters have attracted considerable interest due to their significance for understanding the interconversion of hydrogen with protons and electrons. As...

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

Phosphine-substituted Fe–Te clusters related to the active site of [FeFe]-H2ases

Mono-, di-, and tetranuclear phosphine-substituted Fe/Te clusters 1–6 were described.

Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [t-HFe₂(Me₂pdt)(CO)₂(dppv)₂]⁺ ([t-H1]⁺) (Me₂pdt^2- = Me₂C(CH₂S^-)₂; dppv = cis-1,2-C₂H₂(PPh₂)₂) and related derivatives were prepared with ⁵⁷Fe enrichment for characterization by NMR, FT-IR, and NRVS. The experimental results were rationalized using DFT molecular modeling and spectral simulations. The spectroscopic analysis was aimed at supporting assignments of Fe-H vibrational spectra as they relate to recent measurements on [FeFe]-hydrogenase enzymes. The combination of bulky Me₂pdt^2- and dppv ligands stabilizes the terminal hydride with respect to its isomerization to the 5-16 kcal/mol more stable bridging hydride ([μ-H1]⁺) with t_1/2(313.3 K) = 19.3 min. In agreement with the nOe experiments, the calculations predict that one methyl group in [t-H1]⁺ interacts with the hydride with a computed CH···HFe distance of 1.7 Å. Although [t-H⁵⁷1]⁺ exhibits multiple NRVS features in the 720-800 cm^-1 region containing the bending Fe-H modes, the deuterated [t-D⁵⁷1]⁺ sample exhibits a unique Fe-D/CO band at ∼600 cm^-1. In contrast, the NRVS spectra for [μ-H⁵⁷1]⁺ exhibit weaker bands near 670-700 cm^-1 produced by the Fe-H-Fe wagging modes coupled to Me₂pdt^2- and dppv motions.

Diiron Dithiolate Hydrides Complemented with Proton-Responsive Phosphine-Amine Ligands

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, pdt^2- = 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 υ_CO 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.^[1].

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.

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.

Terminal vs bridging hydrides of diiron dithiolates: protonation of Fe2(dithiolate)(CO)2(PMe3)4

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.

Favorable Protonation of the (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-terminal)](+) Hydrogenase Model Complex Over Its Bridging μ-H Counterpart: A Spectroscopic and DFT Study

The mechanism of hydrogen production in [FeFe] hydrogenase remains elusive. However, a species featuring a terminal hydride bound to the distal Fe is thought to be the key intermediate leading to hydrogen production. In this study, density functional theory (DFT) calculations on the terminal (H-term) and bridging (μ-H) hydride isomers of (μ-edt)-[Fe(2)(PMe(3))(4)(CO)(2)H](+) are presented in order to understand the factors affecting their propensity for protonation. Relative to H-term, μ-H is 12.7 kcal/mol more stable, which contributes to its decreased reactivity towards an acid. Potential energy surface (PES) calculations for the reaction of the H-term isomer with 4-nitropyridinium, a proton source, further reveal a lower activation energy barrier (14.5 kcal/mol) for H-term than for μ-H (29 kcal/mol). Besides these energetic considerations, the H-term isomer displays a key molecular orbital (MO <139>) that has a relatively strong hydride (1s) contribution (23%), which is not present in the μ-H isomer. This indicates a potential orbital control of the reaction of the hydride complexes with acid. The lower activation energy barrier and this key MO together control the overall catalytic activity of (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-term)](+). Lastly, Raman and IR spectroscopy were performed in order to probe the ν(Fe-H) stretching mode of the two isomers and their deuterated counterparts. A ν(Fe-H) stretching mode was observed for the μ-H complex at 1220 cm(-1). However, the corresponding mode is not observed for the less stable H-term isomer.

Vibrational analysis of the model complex (mu-edt)[Fe(CO)(3)](2) and comparison to iron-only hydrogenase: the activation scale of hydrogenase model systems

Research on simple [FeFe] hydrogenase model systems of type (mu-S(2)R)[Fe(CO)(3)](2) (R = C(2)H(4) (edt), C(3)H(6) (pdt)) which have been shown to function as robust electrocatalysts for proton reduction, provides a reference to understand the electronic and vibrational properties of the active site of [FeFe] hydrogenases and of more sophisticated model systems. In this study, the solution and solid state Raman spectra of (mu-edt)[Fe(CO)(3)](2) and of the corresponding (13)CO-labeled complex are presented and analyzed in detail, with focus on the nu(C=O) and nu(Fe-CO)/delta(Fe-C=O) vibrational regions. These regions are specifically important as vibrations involving CO ligands serve as probes for the "electron richness" of low-valent transition metal centers and the geometric structures of the complexes. The obtained vibrational spectra have been completely assigned in terms of the nu(C=O), nu(Fe-CO), and delta(Fe-C=O) modes, and the force constants of the important C=O and Fe-CO bonds have been determined using our Quantum Chemistry Centered Normal Coordinate Analysis (QCC-NCA). In the 400-650 cm(-1) region, fifteen mixed nu(Fe-CO)/delta(Fe-C=O) modes have been identified. The most prominent Raman peaks at 454, 456, and 483 cm(-1) correspond to a combination of nu(Fe-CO) stretching and delta(Fe-C=O) linear bending modes. The less intense peaks at 416 cm(-1) and 419 cm(-1) correspond to pure delta(Fe-C=O) linear bends. In the nu(C=O) region, the nu(C=O) normal modes at lower energy (1968 and 1964 cm(-1)) are almost pure equatorial (eq) nu(C=O)(eq) stretching vibrations, whereas the remaining four nu(C=O) normal modes show dominant (C=O)(eq) (2070 and 1961 cm(-1)) and (C=O)(ax) (2005 and 1979 cm(-1); ax = axial) contributions. Importantly, an inverse correlation between the f(C=O)(ax/eq) and f(Fe-CO)(ax/eq) force constants is obtained, in agreement with the idea that the Fe(I)-CO bond in these types of complexes is dominated by pi-backdonation. Compared to the reduced form of [FeFe] hydrogenase (H(red)), the nu(C=O) vibrational frequencies of (mu-edt)[Fe(CO)(3)](2) are higher in energy, indicating that the dinuclear iron core in (mu-edt)[Fe(CO)(3)](2) is less electron rich compared to H(red) in the actual enzyme. Finally, quantum yields for the photodecomposition of (mu-edt)[Fe(CO)(3)](2) have been determined.

Effect of Lewis acid on the structure of a diiron dithiolate complex based on the active site of [FeFe]-hydrogenase assessed by density functional theory

The effect of Lewis acid on the structure and H2 productivity of a diiron dithiolate complex was investigated by using density functional theory (DFT) calculations. When a model molecule of [(CH3SH)(CO)2Fe(p)(mu-SCH2NHCH2S)Fe(d)(CO)3] was geometrically optimized, two isomers were found: one is the unrotated structure (1) with no ligand between two Fe atoms and the other is the rotated structure (1*) with one CO ligand between two Fe atoms. The energy of 1* was higher than 1 by 6.4 kcal/mol in a vacuum. DFT calculations also revealed that all Lewis acids bound to the rotated structure more strongly than to the unrotated structure, leading to the stabilization of the rotated structure. In particular, when AlCl3 is used, the rotated structure (1*/AlCl3) is more stable than the unrotated one (1/AlCl3) by 1.2 kcal/mol in a vacuum. The stabilization of the rotated structure arises from both the stronger basicity of the mu-CO ligand than the axial CO ligand and the increase of the bond strength between the mu-CO ligand and Fe(p) atom upon binding of Lewis acid to 1*. Calculation of energy barriers during electrocatalytic H2 production revealed that 1*/AlCl3 could efficiently produce H2via a chemical-electrochemical-chemical-electrochemical mechanism. The analysis of the energy level of the lowest unoccupied molecular orbital showed that 1*/AlCl3 may produce H2 at significantly lower reduction potential as compared with 1*. It is also found that the catalytic activity decreases with increasing polarity of the medium.

New nitrosyl derivatives of diiron dithiolates related to the active site of the [FeFe]-hydrogenases

Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the precursor Fe(2)(S(2)C(n)H(2n))(dppv)(CO)(4) (dppv = cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe(2)(S(2)C(2)H(4))(CO)(3)(dppv)(NO)]BF(4) ([1(CO)(3)]BF(4)), the following compounds were prepared: [1(CO)(2)(PMe(3))]BF(4), [1(CO)(dppv)]BF(4), NEt(4)[1(CO)(CN)(2)], and 1(CO)(CN)(PMe(3)). Some of these substitution reactions occur via the addition of 2 equiv of the nucleophile followed by the dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe(2)(S(2)C(2)H(4))(CO)(3)(dppv)(NO)(PMe(3))(2)]BF(4). This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)(2)(NO) derivative. Compounds [1(CO)(3)]BF(4), [1(CO)(2)(PMe(3))]BF(4), and [1(CO)(dppv)]BF(4) were also prepared by the addition of NO(+) to the di-, tri-, and tetrasubstituted precursors. In these cases, the NO(+) appears to form an initial 36e(-) adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO. IR and EPR spectra of 1(CO)(dppv), generated by low-temperature reduction of [1(CO)(dppv)]BF(4) with Co(C(5)Me(5))(2), indicates that the SOMO is located on the FeNO subunit.

Nitrosyl derivatives of diiron(I) dithiolates mimic the structure and Lewis acidity of the [FeFe]-hydrogenase active site

This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment of Fe2(S2C(n)H(2n))(CO)(6-x)(PMe3)x compounds (n = 2, 3; x = 1, 2, 3) with NOBF4 gave the derivatives [Fe2(S2C(n)H(2n))(CO)(5-x)(PMe3)x(NO)]BF4, which are electronically unsymmetrical because of the presence of a single NO(+) ligand. Whereas the monophosphine derivative is largely undistorted, the bis(PMe3) derivatives are distorted such that the CO ligand on the Fe(CO)(PMe3)(NO)(+) subunit is semibridging. Two isomers of [Fe2(S2C3H6)(CO)3(PMe3)2(NO)]BF4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich Fe(CO)2PMe3 and electrophilic Fe(CO)(PMe3)(NO)(+) subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (DeltaH = -35 kJ/mol, DeltaS = -139 J mol(-1) K(-1)) to give the symmetrical adduct [Fe2(S2C3H6)(mu-NO)(CO)4(PMe3)2]BF4. In contrast to Fe2(S2C3H6)(CO)4(PMe3)2, the bis(PMe3) nitrosyl complexes readily undergo CO substitution to give the (PMe3)3 derivatives. The nitrosyl complexes reduce at potentials that are approximately 1 V milder than their carbonyl counterparts. Results of density functional theory calculations, specifically natural bond orbital analysis, reinforce the electronic resemblance of the nitrosyl complexes to the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild potentials. The results show that the novel structural and chemical features associated with mixed-valence diiron dithiolates (the so-called H(ox) models) can be replicated in the absence of mixed-valency by the introduction of electronic asymmetry.

Redox and structural properties of mixed-valence models for the active site of the [FeFe]-hydrogenase: progress and challenges

The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these low-molecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2(S 2C 3H 6)(CO) 3(L)(dppv) (L = CO, PMe 3, P i-Pr 3) oxidize at potentials approximately 180 mV milder than the related ethanedithiolates ( Angew. Chem., Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. Electron Paramagnetic Resonance (EPR) spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe 2(S 2C 3H 6)(CO) 4(dppv)]BF 4 and [Fe 2(S 2C 3H 6)(CO) 3(PMe 3)(dppv)]BF 4, as seen previously for the ethanedithiolate [Fe 2(S 2C 2H 4)(CO) 3(PMe 3)(dppv)]BF 4. For [Fe 2(S 2C n H 2 n )(CO) 3(P i-Pr 3)(dppv)]BF 4; however, the spin is localized on the Fe(CO) 2(P i-Pr 3) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and density-functional theory (DFT) calculations, suggest that the Fe(CO) 2(P i-Pr 3) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe 2(S 2C 2H 4)(CN)(CO) 3(dppv)] (-) resulted in 2e oxidation of 0.5 equiv to give the mu-cyano derivative [Fe (I) 2(S 2C 2H 4)(CO) 3(dppv)](mu-CN)[Fe (II) 2(S 2C 2H 4)(mu-CO)(CO) 2(CN)(dppv)], which was characterized spectroscopically.

Diiron dithiolato carbonyls related to the H(ox)CO state of [FeFe]-hydrogenase

Oxidation of the electron-rich (E(1/2) = -175 vs Ag/AgCl) ethanedithiolato complex Fe2(S2C2H4)(CO)2(dppv)2 (1) under a CO atmosphere yielded [Fe2(S2C2H4)(mu-CO)(CO)2(dppv)2](+) ([1(CO)](+)), a model for the H(ox)(CO) state of the [FeFe]-hydrogenases. This complex exists as two isomers: a kinetically favored unsymmetrical derivative, unsym-[1(CO)](+), and a thermodynamically favored isomer, sym-[1(CO)](+), wherein both diphosphines span apical and basal sites. Crystallographic characterization of sym-[1(CO)](+) confirmed a C2-symmetric structure with a bridging CO ligand and an elongated Fe-Fe bond of 2.7012(14) A, as predicted previously. Oxidation of sym-[1(CO)](+) and unsym-[1(CO)](+) again by 1e(-) oxidation afforded the respective diamagnetic diferrous derivatives where the relative stabilities of the sym and unsym isomers are reversed. DFT calculations indicate that the stabilities of sym and unsym isomers are affected differently by the oxidation state of the diiron unit: the mutually trans CO ligands in the sym isomer are more destabilizing in the mixed-valence state than in the diferrous state. EPR analysis of mixed-valence complexes revealed that, for [1](+), the unpaired spin is localized on a single iron center, whereas for unsym/sym-[1(CO)](+), the unpaired spin was delocalized over both iron centers, as indicated by the magnitude of the hyperfine coupling to the phosphine ligands trans to the Fe-Fe vector. Oxidation of 1 by 2 equiv of acetylferrocenium afforded the dication [1](2+), which, on the basis of low-temperature IR spectrum, is structurally similar to [1](+). Treatment of [1](2+) with CO gives unsym-[1(CO)](2+).

Terminal hydride in [FeFe]-hydrogenase model has lower potential for H2 production than the isomeric bridging hydride

Protonation of the symmetrical tetraphosphine complexes Fe2(S2CnH2n)(CO)2(dppv)2 afforded the corresponding terminal hydrides, establishing that even symmetrical diiron(I) 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](+), [HFe2[(SCH2)2NH](CO)2(dppv)2](+), and [HFe2[(SCH2)2NH2](CO)2(dppv)2](2+), depending on conditions.

Desymmetrized Diiron Azadithiolato Carbonyls: A Step Toward Modeling the Iron-Only Hydrogenases

Condensation of Fe(2)(SH)(2)(CO)(6), acetaldehyde, and (NH(4))(2)CO(3) affords the methyl-substituted azadithiolate Fe(2)[(SCHMe)(2)NH](CO)(6) (1). The complex exists mainly (~95%) as the meso diastereomer, but the d,l diastereoisomers could be detected. DFT calculations predict that the meso isomer would be 2.5 kcal/mol more stable than the d,l isomer due to conventional nonbonding interactions between the methyl groups and the ring hydrogen atoms. Crystallographic analysis of meso-1 confirms that the two methyl groups are equatorial, constraining the diferraazadithiolate bicycle to a conformation that desymmetrizes the diiron center. The lowered symmetry is confirmed by the observation of two (13)C NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO)(3) groups. The pK(a) value of the amine in 1 is 7.89 (all pK(a)'s determined in MeCN solution), which is similar to a redetermined value for Fe(2)[(SCH(2))(2)NH](CO)(6) (2, pK(a) = 7.98) and only slightly less basic than the tertiary amine Fe(2)[(SCH(2))(2)NMe](CO)(6) (pK(a) = 8.14). Substitution of 1 with PMe(3) proceeded via the intermediacy of two isomers of Fe(2)[(SCHMe)(2)NH](CO)(5)(PMe(3)), affording Fe(2)[(SCHMe)(2)NH](CO)(4)(PMe(3))(2) (3). (31)P NMR spectra confirm that the two PMe(3) ligands in 3 are nonequivalent, consistent with the desymmetrizing effect of the dithiolate. The pK(a) value of the amine in 3 was found to be 11.3. Using triphenylphosphine, we prepared Fe(2)[(SCHMe)(2)NH](CO)(5)(PPh(3)) as a single regioisomer.

Computational definition of a mixed valent Fe(II)Fe(I) model of the [FeFe]hydrogenase active site resting state

Density-functional calculations have been used to examine the electronic structure and bonding in the recently reported complex [(PMe(3))(CO)(2)Fe(mu-pdt)(mu-CO)Fe(CO)(IMes)](+) (1(+), IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene). This mixed valent Fe(II)Fe(I) complex features a rotated geometry that places a carbonyl ligand in a semi-bridging position, which makes it an accurate model of the S =(1/2) resting state of the [FeFe]-hydrogenase active site. Calculations indicate that the unpaired electron in this complex lies almost entirely on the rotated iron center, implying that this iron remains in the Fe(I) oxidation state, while the unrotated iron has been oxidized to Fe(II). The frontier molecular orbitals in 1(+) are compared with those in the neutral Fe(I)Fe(I) precursor (PMe(3))(CO)(2)Fe(mu-pdt)(mu-CO)Fe(CO)(IMes) at both its optimized geometry (1) and constrained to a rotated geometry (1(rot)). These theoretical results are used to address the role of the bridging CO ligand in 1(+) and to predict reactivity patterns; they are related back to the intricate biological mechanism of [FeFe]-hydrogenase.