Coordination sphere flexibility of active-site models for Fe-only hydrogenase: studies in intra- and intermolecular diatomic ligand exchange

Bio-inspired hydrogenase models: mixed-valence triion complexes as proton reduction catalysts

Mixed-valence triiron complexes Fe(3)(CO)(7-x)(PPh(3))(x)(μ-edt)(2) (x = 0-2) have been prepared and are shown to act as proton reduction catalysts. Catalysis takes place via an ECEC mechanism with a reduced overpotential of ca. 0.45 V for Fe(3)(CO)(7)(μ-edt)(2) as compared to the corresponding diiron complex.

Sulfonated diiron complexes as water-soluble models of the [Fe-Fe]-hydrogenase enzyme active site

A series of diiron complexes developed as fundamental models of the two-iron subsite in the [FeFe]-hydrogenase enzyme active site show water-solubility by virtue of a sulfonate group incorporated into the -SCH(2)NRCH(2)S- dithiolate unit that bridges two Fe(I)(CO)(2)L moieties. The sulfanilic acid group imparts even greater water solubility in the presence of β-cyclodextrin, β-CyD, for which NMR studies suggest aryl-sulfonate inclusion into the cyclodextrin cavity as earlier demonstrated in the X-ray crystal structure of 1Na·2 β-CyD clathrate, where 1Na = Na(+)(μ-SCH(2)N(C(6)H(4)SO(3)(-))CH(2)S-)[Fe(CO)(3)](2), (Singleton et al., J. Am. Chem. Soc.2010, 132, 8870). Electrochemical analysis of the complexes for potential as electrocatalysts for proton reduction to H(2) finds the presence of β-CyD to diminish response, possibly reflecting inhibition of structural rearrangements required of the diiron unit for a facile catalytic cycle. Advantages of the aryl sulfonate approach include entry into a variety of water-soluble derivatives from the well-known (μ-SRS)[Fe(CO)(3)](2) parent biomimetic, that are stable in O(2)-free aqueous solutions.

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.

Isomerization of the hydride complexes [HFe2(SR)2(PR3)(x)(CO)(6-x)]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases

The stepwise formation of bridging (mu-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(pdt)(CO)(4)(dppv) (2) followed an isomerization pathway similar to that for [1H](+), except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)(3) or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.

Sulfur oxygenates of biomimetics of the diiron subsite of the [FeFe]-hydrogenase active site: properties and oxygen damage repair possibilities

This study explores the site specificity (sulfur vs the Fe-Fe bond) of oxygenation of diiron (Fe(I)Fe(I) and Fe(II)Fe(II)) organometallics that model the 2-iron subsite in the active site of [FeFe]-hydrogenase: (mu-pdt)[Fe(CO)(2)L][Fe(CO)(2)L'] (L = L' = CO (1); L = PPh(3), L' = CO (2); L = L' = PMe(3) (4)) and (mu-pdt)(mu-H)[Fe(CO)(2)PMe(3)](2) (5). DFT computations find that the Fe-Fe bond in the Fe(I)Fe(I) diiron models is thermodynamically favored to produce the mu-oxo or oxidative addition product, Fe(II)-O-Fe(II); nevertheless, the sulfur-based HOMO-1 accounts for the experimentally observed mono- and bis-O-atom adducts at sulfur, i.e., (mu-pst)[Fe(CO)(2)L][Fe(CO)(2)L'] (pst = -S(CH(2))(3)S(O)-, 1,3-propanesulfenatothiolate; L = L' = CO (1-O); L = PPh(3), L' = CO (2-O); L = L' = PMe(3) (4-O)) and (mu-pds)[Fe(CO)(2)L][Fe(CO)(2)L'] (pds = -(O)S(CH(2))(3)S(O)-, 1,3-propanedisulfenato; L = PPh(3), L' = CO (2-O(2))). The Fe(II)(mu-H)Fe(II) diiron model (5), for which the HOMO is largely of sulfur character, exclusively yields S-oxygenation. The depressing effect of such bridging ligand modification on the dynamic NMR properties arising from rotation of the Fe(CO)(3) correlates with higher barriers to the CO/PMe(3) exchange of (mu-pst)[Fe(CO)(3)](2) as compared to (mu-pdt)[Fe(CO)(3)](2). Five molecular structures are confirmed by X-ray diffraction: 1-O, 2-O, 2-O(2), 4-O, and 6. Deoxygenation with reclamation of the mu-pdt parent complex occurs in a proton/electron-coupled process. The possible biological relevance of oxygenation and deoxygenation studies is discussed.

Dithiolate-bridged Fe-Ni-Fe trinuclear complexes consisting of Fe(CO)(3-n)(CN)(n) (n = 0, 1) components relevant to the active site of [NiFe] hydrogenase

A dithiolate-bridged Fe-Ni-Fe trinuclear carbonyl complex [(CO)(3)Fe(mu-ndt)Ni(mu-ndt)Fe(CO)(3)] (1, ndt = norbornane-exo-2,3-dithiolate) has been synthesized from the reaction of [Fe(CO)(4)I(2)] and Li(2)[Ni(ndt)(2)]. This reaction was found to occur with concomitant formation of a tetranuclear cluster [Ni(3)(mu-ndt)(4)FeI] (2). Treatment of 1 with Na[N(SiMe(3))(2)] transforms some of the CO ligands into CN(-), and the monocyanide complex (PPh(4))[(CO)(2)(CN)Fe(mu-ndt)Ni(mu-ndt)Fe(CO)(3)] (3) and the dicyanide complex (PPh(4))(2)[(CO)(2)(CN)Fe(mu-ndt)Ni(mu-ndt)Fe(CO)(2)(CN)] (4) were isolated. X-ray structural analyses of the trinuclear complexes revealed a Fe-Ni-Fe array in which the metal centers are connected by the ndt sulfur bridges and direct Fe-Ni bonds. Hydrogen bonding between the CN ligand in 3 and cocrystallized ethanol was found in the solid-state structure. The monocyanide complex 3 and dicyanide complex 4 reacted with acids such as HOTf or HCl generating insoluble materials, whereas complex 1 did not react.

Carbon Monoxide and Cyanide Ligands in the Active Site of [FeFe]-Hydrogenases

The [FeFe]-hydrogenases, although share common features when compared to other metal containing hydrogenases, clearly have independent evolutionary origins. Examples of [FeFe]-hydrogenases have been characterized in detail by biochemical and spectroscopic approaches and the high resolution structures of two examples have been determined. The active site H-cluster is a complex bridged metal assembly in which a [4Fe-4S] cubane is bridged to a 2Fe subcluster with unique non-protein ligands including carbon monoxide, cyanide, and a five carbon dithiolate. Carbon monoxide and cyanide ligands as a component of a native active metal center is a property unique to the metal containing hydrogenases and there has been considerable attention to the characterization of the H-cluster at the level of electronic structure and mechanism as well as to defining the biological means to synthesize such a unique metal cluster. The chapter describes the structural architecture of [FeFe]-hydrogenases and key spectroscopic observations that have afforded the field with a fundamental basis for understanding the relationship between structure and reactivity of the H-cluster. In addition, the results and ideas concerning the topic of H-cluster biosynthesis as an emerging and fascinating area of research, effectively reinforcing the potential linkage between iron-sulfur biochemistry to the role of iron-sulfur minerals in prebiotic chemistry and the origin of life.

Theoretical studies on the redox potentials of Fe dinuclear complexes as models for hydrogenase

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.

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.