Multiple-timescale photoreactivity of a model compound related to the active site of [FeFe]-hydrogenase

Ultraviolet (UV) photolysis of (mu-S(CH 2) 3S)Fe 2(CO) 6 ( 1), a model compound of the Fe-hydrogenase enzyme system, has been carried out. When ultrafast UV-pump infrared (IR)-probe spectroscopy, steady-state Fourier transform IR spectroscopic methods, and density functional theory simulations are employed, it has been determined that irradiation of 1 in an alkane solution at 350 nm leads to the formation of two isomers of the 16-electron complex (mu-S(CH 2) 3S)Fe 2(CO) 5 within 50 ps with evidence of a weakly associated solvent adduct complex. 1 is subsequently recovered on timescales covering several minutes. These studies constitute the first attempt to study the photochemistry and reactivity of these enzyme active site models in solution following carbonyl ligand photolysis.     

Modeling [Fe-Fe] hydrogenase: evidence for bridging carbonyl and distal iron coordination vacancy in an electrocatalytically competent proton reduction by an iron thiolate assembly that operates through Fe(0)-Fe(II) levels

IR spectroelectrochemistry of Fe4{Me(CH2S)3}2(CO)8 (4Fe6S) in the nu(CO) region shows that the neutral and anion forms have all their CO groups terminally bound to the Fe atoms; however, for the dianion there is a switch of the coordination mode of at least one of the CO groups. The available structural and nu(CO) spectra are closely reproduced by density-functional theory calculations. The calculated structure of 4Fe6S2- closely mirrors that of the diiron subsite of the [Fe-Fe] hydrogenase H cluster with a bridging CO group and an open coordination site on the outer Fe atom of pairs of dithiolate-bridged Fe0FeII subunits connected by two bridging thiolates. Geometry optimization based on the all-terminal CO isomer of 4Fe6S2- does not give a stable structure but reveals a second-order saddle point ca. 11.53 kcal mol(-1) higher in energy than the CO-bridged form. Spectroelectrochemical studies of electrocatalytic proton reduction by 4Fe6S show that slow turnover from the primary reduction process (E1/2'=-0.71 V vs Ag/AgCl) involves rate-limiting protonation of 4Fe6S- followed by reduction to H:4Fe6S-. Rapid electrocatalytic proton reduction is obtained at potentials sufficient to access 4Fe6S2-, where the rate of dihydrogen elimination from the FeIIFeII core of 4Fe6S is ca. 500 times faster than that from the FeIFeI core of Fe2(mu-S(CH2)3S)(CO)6. The dramatically increased rate of electrocatalysis obtained from 4Fe6S over all previously identified model compounds appears to be related to the features uniquely common between it and the H-cluster, namely, that turnover involves the same formal redox states of the diiron unit (FeIFeII and Fe0FeII), the presence of an open site on the outer Fe atom of the Fe0FeII unit, and the thiolate-bridge to a second one-electron redox unit.     

Time-resolved vibrational spectroscopy of [FeFe]-hydrogenase model compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe-Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) and Fe(2)(μ-S(2)C(2)H(4))(CO)(6) is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) were studied with density functional theory (DFT) calculations. For Fe(2)(μ-S(2)C(3)H(6))(CO)(6) excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe-Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with a semibridging CO is reminiscent of the geometry of the Fe(2)S(2) subcluster of the active site observed in Fe-Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis.     

Modeling the active sites in metalloenzymes. 3. Density functional calculations on models for [Fe]-hydrogenase: structures and vibrational frequencies of the observed redox forms and the reaction mechanism at the Diiron Active Center.

Optimized structures for the redox species of the diiron active site in [Fe]-hydrogenase as observed by FTIR and for species in the catalytic cycle for the reversible H(2) oxidation have been determined by density-functional calculations on the active site model, [(L)(CO)(CN)Fe(mu-PDT)(mu-CO)Fe(CO)(CN)(L')](q)(L = H(2)O, CO, H(2), H(-); PDT = SCH(2)CH(2)CH(2)S, L' = CH(3)S(-), CH(3)SH; q = 0, 1-, 2-, 3-). Analytical DFT frequencies on model complexes (mu-PDT)Fe(2)(CO)(6) and [(mu-PDT)Fe(2)(CO)(4)(CN)(2)](2)(-) are used to calibrate the calculated CN(-) and CO frequencies against the measured FTIR bands in these model compounds. By comparing the predicted CN(-) and CO frequencies from DFT frequency calculations on the active site model with the observed bands of D. vulgaris [Fe]-hydrogenase under various conditions, the oxidation states and structures for the diiron active site are proposed. The fully oxidized, EPR-silent form is an Fe(II)-Fe(II) species. Coordination of H(2)O to the empty site in the enzyme's diiron active center results in an oxidized inactive form (H(2)O)Fe(II)-Fe(II). The calculations show that reduction of this inactive form releases the H(2)O to provide an open coordination site for H(2). The partially oxidized active state, which has an S = (1)/(2) EPR signal, is an Fe(I)-Fe(II) species. Fe(I)-Fe(I) species with and without bridging CO account for the fully reduced, EPR-silent state. For this fully reduced state, the species without the bridging CO is slightly more stable than the structure with the bridging CO. The correlation coefficient between the predicted CN(-) and CO frequencies for the proposed model species and the measured CN(-) and CO frequencies in the enzyme is 0.964. The proposed species are also consistent with the EPR, ENDOR, and M?ssbauer spectroscopies for the enzyme states. Our results preclude the presence of Fe(III)-Fe(II) or Fe(III)-Fe(III) states among those observed by FTIR. A proposed reaction mechanism (catalytic cycle) based on the DFT calculations shows that heterolytic cleavage of H(2) can occur from (eta(2)-H(2))Fe(II)-Fe(II) via a proton transfer to "spectator" ligands. Proton transfer to a CN(-) ligand is thermodynamically favored but kinetically unfavorable over proton transfer to the bridging S of the PDT. Proton migration from a metal hydride to a base (S, CN, or basic protein site) results in a two-electron reduction at the metals and explains in part the active site's dimetal requirement and ligand framework which supports low-oxidation-state metals. The calculations also suggest that species with a protonated Fe-Fe bond could be involved if the protein could accommodate such species.     

The role of CN and CO ligands in the vibrational relaxation dynamics of model compounds of the [FeFe]-hydrogenase enzyme

The vibrational dynamics of (μ-propanedithiolate)Fe(2)(CO)(4)(CN)(2)(2-), a model compound of the active site of the [FeFe]-hydrogenase enzyme, have been examined via ultrafast 2D-IR spectroscopy. The results indicate that the vibrational coupling between the stretching modes of the CO and CN ligands is small and restricted to certain modes but the slow growth of off-diagonal peaks is assigned to population transfer processes occurring between these modes on timescales of 30-40 ps. Analysis of the dynamics in concert with anharmonic density functional theory simulations shows that the presence of CN ligands alters the vibrational relaxation dynamics of the CO modes in comparison to all-carbonyl model systems and suggests that the presence of these ligands in the enzyme may be an important feature in terms of directing the vibrational relaxation mechanism.     

Active-site models for iron hydrogenases: reduction chemistry of dinuclear iron complexes

Reduction of Fe2(mu-S2C3H6)(CO)6 (1) in tetrahydrofuran with 1 equiv of decamethylcobaltocene (Cp*2Co) affords a tetranuclear dianion 2. The IR spectra of samples of 2 in solution and in the solid state exhibit a band at 1736 cm(-1), suggestive of the presence of a bridging carbonyl (CO) ligand. X-ray crystallography confirms that the structure of 2 consists of two Fe2 units bridged by a propanedithiolate moiety formulated as [Fe2(mu-S2C3H6)(CO)5(SCH2CH2CH2-mu-S)Fe2(mu-CO)(CO)6](2-). One of the Fe2 units has a bridging CO ligand and six terminal CO ligands. The second subunit exhibits a bridging propanedithiolate moiety. One CO ligand has been replaced by a terminal thiolate ligand, replicating the basic architecture of Fe-only hydrogenases. The reduction reaction can be reversed by treatment of 2 with 2 equiv of [Cp2Fe][PF6], reforming complex 1 in near-quantitative yield. Complex 2 can also be oxidized by acids such as p-toluenesulfonic acid, regenerating complex 1 and forming H2.     

Synthesis, structures and electrochemical properties of nitro- and amino-functionalized diiron azadithiolates as active site models of Fe-only hydrogenases

Complex [[(mu-SCH2)2N(4-NO2C6H4)]Fe2(CO)6] (4) was prepared by the reaction of the dianionic intermediate [(mu-S)2Fe2(CO)6](2-) and N,N-bis(chloromethyl)-4-nitroaniline as a biomimetic model of the active site of Fe-only hydrogenase. The reduction of 4 by Pd-C/H2 under a neutral condition afforded complex [[(mu-SCH2)2N(4-NH2C6H4)]Fe2(CO)6] (5) in 67 % yield. Both complexes were characterized by IR, 1H and 13C NMR spectroscopy and MS spectrometry. The molecular structure of 4, as determined by X-ray analysis, has a butterfly 2Fe2S core and the aryl group on the bridged-N atom slants to the Fe(2) site. Cyclic voltammograms of 4 and 5 were studied to evaluate their redox properties. It was found that complex 4 catalyzed electrochemical proton reduction in the presence of acetic acid. A plausible mechanism of the electrocatalytic proton reduction is discussed.     

The first example of macrocycles containing butterfly transition metal cluster cores via novel tandem reactions

A novel type of double butterfly, two mu-CO-containing dianions {[(mu-CO)Fe2(CO)6]2[mu-SCH2(CH2OCH2)nCH2S-mu]}2- (m1, n = 2, 3), has been synthesized from dithiol HSCH2(CH2OCH2)nCH2SH (n = 2, 3), Fe3(CO)12, and Et3N in THF at room temperature. While dianions m1 react in situ with CS2 followed by treatment with dihalide 1,4-(BrCH2)2C6H4 or 1,4-I(CH2)4I to give macrocyclic clusters [mu-SCH2(CH2OCH2)nCH2S-mu](mu-CS2ZCS2-mu)[Fe2(CO)6]2 (1a, n = 2, Z = 1,4-(CH2)2C6H4; 1b, n = 3, Z = (CH2)4), reactions of dianions m1 with (mu-S2)Fe2(CO)6 followed by treatment with dihalide 1,4-I(CH2)4I afford macrocyclic clusters [mu-SCH2(CH2OCH2)nCH2S-mu]{[Fe2(CO)6]2(mu4-S)}2[mu-S(CH2)4S-mu] (2a, n = 2; 2b, n = 3). The crystal structures of 1a and 2b are described.     

Carbon Monoxide and Cyanide Ligands in a Classical Organometallic Complex Model for Fe-Only Hydrogenase

The Fe(I) organometallic complex [(μ-SCH(2)CH(2)CH(2)S)Fe(2)(CO)(6)] provides a structural model for the cyano-carbonyl diiron site of Fe-only hydrogenase as characterized by X-ray crystallography (the picture shows the structure (black) of the model overlaid with that of the Fe-Fe dimetallic site in the hydrogenase isolated from Desulfovibrio desulfuricans). Cyanide substitution of CO occurs readily and provides spectroscopic references for the active site.     

Reactions of thioethers with Mn(2)(CO)(7)(mu-S(2)) proceed with CO displacement and insertion of the sulfur atom into the Mn-Mn bond

The reaction of Mn(2)(CO)(7)(mu-S(2)) (2) with SMe(2) yielded the new complexes Mn(2)(CO)(6)(mu-S(2))(mu-SMe(2)) (3) and Mn(4)(CO)(14)(SMe(2))(mu(3)-S(2))(mu(4)-S(2)) (4) in 18 and 41% yields, respectively. The reaction of 2 with the cyclic thioether thietane SCH(2)CH(2)CH(2) yielded the new complexes Mn(2)(CO)(6)(mu-S(2))(mu-SCH(2)CH(2)CH(2)) (5) and Mn(4)(CO)(14)(SCH(2)CH(2)CH(2))(mu(3)-S(2))(mu(4)-S(2)) (6) in 12 and 52% yields, respectively, and the reaction of 2 with 1,4,9-trithiacyclododecane (12S3) yielded Mn(2)(CO)(6)(mu-12S3)(mu-S(2)) (7) and Mn(4)(CO)(14)(12S3)(mu(3)-S(2))(mu(4)-S(2)) (8) in 8 and 24% yields, respectively. Compounds 3 and 5-7 were characterized crystallographically. Compounds 3, 5, and 7 have similar structures in which the thioether ligand has replaced the bridging carbonyl ligand of 2 and its sulfur atom has been inserted into the manganese-manganese bond. The two manganese atoms are not mutually bonded, and two Mn(CO)(3) groups are held together through the bridging disulfido ligand and the bridging sulfur atom of the thioether ligand. Compound 6 contains a Mn(4)(mu(3)-S(2))(mu(4)-S(2)) moiety without metal-metal bonds. On the basis of spectroscopic data, compounds 4 and 8 are believed to have similar structures.     

Metal carbonyl derivatives of sulfur-containing quinones and hydroquinones: synthesis, structures, and electrochemical properties

The reaction of CpMoMn(mu-S(2))(CO)(5), 1, with 1,4-benzoquinone in the presence of irradiation with visible light yielded the quinonedithiolato complex CpMoMn(CO)(5)(mu-S(2)C(6)H(2)O(2)), 2. The new complex CpMoMn(CO)(5)(mu-S(2)C(6)Cl(2)O(2)) (4) was synthesized similarly from 1 and 2,3-dichloro-1,4-benzoquinone. Compounds 2 and 4 were reduced with hydrogen to yield the hydroquinone complexes CpMoMn(CO)(5)[mu-S(2)C(6)H(2)(OH)(2)], 3, and CpMoMn(CO)(5)[mu-S(2)C(6)Cl(2)(OH)(2)], 5. UV-vis irradiation of solutions of Fe(2)(CO)(6)(mu-S(2)) and 1,4-benzoquinone yielded the hydroquinone complex Fe(2)(CO)(6)[mu-S(2)C(6)H(2)(OH)(2)], 6. Compound 6 was oxidized to the quinone complex Fe(2)(CO)(6)(mu-S(2)C(6)H(2)O(2)), 7, by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Substitution of the CO ligands on 6 by PPh(3) yielded the derivatives Fe(2)(CO)(5)(PPh(3))[mu-S(2)C(6)H(2)(OH)(2)], 8, and Fe(2)(CO)(4)(PPh(3))(2)[mu-S(2)C(6)H(2)(OH)(2)], 9. The electrochemical properties of 3, 5, 6, 8, and 9 were measured by cyclic voltammetry. The molecular structure of each of the new compounds 2-9 was established by single-crystal X-ray diffraction analyses.     

Vibrational relaxation of CH3I in the gas phase and in solution

Transient electronic absorption measurements reveal the vibrational relaxation dynamics of CH(3)I following excitation of the C-H stretch overtone in the gas phase and in liquid solutions. The isolated molecule relaxes through two stages of intramolecular vibrational relaxation (IVR), a fast component that occurs in a few picoseconds and a slow component that takes place in about 400 ps. In contrast, a single 5-7 ps component of IVR precedes intermolecular energy transfer (IET) to the solvent, which dissipates energy from the molecule in 50 ps, 44 ps, and 16 ps for 1 M solutions of CH(3)I in CCl(4), CDCl(3), and (CD(3))(2)CO, respectively. The vibrational state structure suggests a model for the relaxation dynamics in which a fast component of IVR populates the states that are most strongly coupled to the initially excited C-H stretch overtone, regardless of the environment, and the remaining, weakly coupled states result in a secondary relaxation only in the absence of IET.     

Activation of alkenes and H2 by [Fe]-H2ase model complexes

The established ability of the Fe(II) bridging hydride species (micro-H)(micro-pdt)[Fe(CO)2(PMe3)]2+, 1-H+, to take-up and heterolytically activate dihydrogen, resulting in H/D scrambling of H2/D2 and H2/D2O mixtures (Zhao et al. Inorg. Chem. 2002, 41, 3917) has prompted a study of simultaneous alkene/H2 activation by such [Fe]H2ase model complexes. That the required photolysis produced an open site was substantiated by substitution of CO in 1-H+ by CH3CN with formation of structurally characterized [(micro-H)(micro-pdt)[Fe(CO)2(PMe3)][Fe(CO)(CH3CN)(PMe3)]]+[PF6]-. Under similar photolytic conditions, H/D exchange reactions between D2 and terminal alkenes (ethylene, propene and 1-butene), but not bulkier alkenes such as 2-butene or cyclohexene, were catalyzed by 1-H+ and the edt (SCH2CH2S) analogue, 2-H+. Substantial regioselectivity for H/D exchange at the internal vinylic hydrogen was observed. The extent to which the olefins were deuterium enriched vs deuterated was catalyst dependent. The stabilizing effect of the binuclear chelating ligands, SCH2CH2CH2S, pdt, and SCH2CH2S, edt, is required for the activity of binuclear catalysts, as the mono-dentate micro-SEt analogue decomposed to inactive products under the photolytic conditions of the catalysis. Reactions of 1 and 2 with EtOSO2CF3 yielded the S-alkylated products, [(micro-SCH2CH2CH2SEt)[Fe(CO)2(PMe3)]2]+[SO3CF3]- (1-Et+), and 2-Et+, rather than micro-C2H5 analogues to the micro-H of 1-H+. The stability and lack of reactivity toward H2 of 1-Et+ and 2-Et+, indicates they are not on the reaction path of the olefin/D2 H/D exchange process. A mechanism with olefin binding to an open site created by CO loss and formation of an Fe-(CH2CHDR) intermediate is indicated. A likely role of a binuclear chelate effect is implicated for the unique S-XXX-S cofactor in the active site of [Fe]H2ase.     

Characterization of the Fe site in iron-sulfur cluster-free hydrogenase (Hmd) and of a model compound via nuclear resonance vibrational spectroscopy (NRVS)

We have used (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to study the iron site in the iron-sulfur cluster-free hydrogenase Hmd from the methanogenic archaeon Methanothermobacter marburgensis. The spectra have been interpreted by comparison with a cis-(CO)2-ligated Fe model compound, Fe(S2C2H4)(CO)2(PMe3)2, as well as by normal mode simulations of plausible active site structures. For this model complex, normal mode analyses both from an optimized Urey-Bradley force field and from complementary density functional theory (DFT) calculations produced consistent results. For Hmd, previous IR spectroscopic studies found strong CO stretching modes at 1944 and 2011 cm(-1), interpreted as evidence for cis-Fe(CO)2 ligation. The NRVS data provide further insight into the dynamics of the Fe site, revealing Fe-CO stretch and Fe-CO bend modes at 494, 562, 590, and 648 cm(-1), consistent with the proposed cis-Fe(CO)2 ligation. The NRVS also reveals a band assigned to Fe-S stretching motion at approximately 311 cm(-1) and another reproducible feature at approximately 380 cm(-1). The (57)Fe partial vibrational densities of states (PVDOS) for Hmd can be reasonably well simulated by a normal mode analysis based on a Urey-Bradley force field for a five-coordinate cis-(CO)2-ligated Fe site with additional cysteine, water, and pyridone cofactor ligands. A "truncated" model without a water ligand can also be used to match the NRVS data. A final interpretation of the Hmd NRVS data, including DFT analysis, awaits a three-dimensional structure for the active site.     

Facile synthesis and functionality-dependent electrochemistry of Fe-only hydrogenase mimics

A series of azadithiolate (adt)-bridged Fe-only hydrogenase model complexes, Fe2(CO)6(mu-adt)C6H4I-4 (1), Fe2(CO)6(mu-adt)C6H4C[triple bond]CR [R = C6H4NO2-4 (2), C6H4CHO-4 (3), C6H4NH2-4 (4), C6H4COOH-4 (5), C6H4COOCH2CH3-4 (6), C6H4F-4 (7), C6H5 (8), C6H4OCH3-4 (9), C6H4N(CH3)2-4 (10)], [Fe2(CO)5(PPh3)(mu-adt)C6H4I-4 (11), and Fe2(CO)5(PPh3)(mu-adt)C6H4C[triple bond]CC6H4NO2-4 (12), have been synthesized in high yields under mild conditions. The linear geometry and rigidity of a triple bond act as an effective bridge to anchor a functionality ranging from electron-donating to electron-accepting, even coordinative groups in the adt model complexes. X-ray crystal analysis of 2, 3, and 6-12 reveals that the model complexes retain the butterfly structure of Fe2S2 model analogues. A rigid phenylacetylene offers excellent control over the distance between the functional group and the active site of Fe2S2 model complexes. The unusual Fe-Fe distance and the angles found in the molecular packing of 6 are originated from the intriguing intermolecular C-H...O and C-H...S interactions. More importantly, electrochemical studies reveal that all of the complexes can catalyze electrochemical reduction of protons to molecular hydrogen, but the reduction potential for the electron-transfer step can be remarkably altered by the functionality R. The electroreductively active nitro group in 2 and 12 displays the enhanced current at a potential substantially less negative than the reduction of [Fe(I)Fe(I)] + e(-) --> [Fe(I)Fe(0)], which is most accessible and becomes the initial step. For complex 3, the second reduction peak for the electron-transfer step involves the contribution from the aldehyde functionality. As the electroreductively inactive groups are incorporated, the reduction process of [Fe(I)Fe(I)] + e(-) --> [Fe(I)Fe(0)] appears first and the second reduction peak for the electron-transfer step from the [Fe(I)Fe(0)] + e(-) --> [Fe(0)Fe(0)] process for 4-10 is clearly observed. Therefore, the order of electron and proton uptake is closely related to the electroreductively active functionality, R. Varying the nature of the functionality R leads to the electron-transfer step changes from the reduction of the electroreductively active R group to the active site of Fe2S2 model complexes subsequently. Accordingly, notwithstanding, acetic acid is too weak to protonate the series of 2-12, different reduction pathways can be followed, and the electrochemically catalyzed behavior may occur at different reduction levels.     

Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states

Picosecond time-resolved photoelectron spectroscopy is used to investigate intramolecular vibrational redistribution (IVR) following excitation of S(1) 18a(1) in p-fluorotoluene (pFT) at an internal energy of 845 cm(-1), where ν(18a) is a ring bending vibrational mode. Characteristic oscillations with periods of 8 ps and 5 ps are observed in the photoelectron signal and attributed to coupling between the initially excited zero-order bright state and two doorway states. Values for the coupling coefficients connecting these three vibrational states have been determined. In addition, an exponential change in photoelectron signal with a lifetime of 17 ps is attributed to weaker couplings with a bath of dark states that play a more significant role during the latter stages of IVR. A tier model has been used to assign the most strongly coupled doorway state to S(1) 17a(1) 6a(2)('), where ν(17a) is a CH out-of-plane vibrational mode and 6a(2)(') is a methyl torsional level. This assignment signifies that a torsion-vibration coupling mechanism mediates the observed dynamics, thus demonstrating the important role played by the methyl torsional mode in accelerating IVR.     

Correlation between computed gas-phase and experimentally determined solution-phase infrared spectra: models of the iron-iron hydrogenase enzyme active site

Gas-phase density functional theory calculations (B3LYP, double zeta plus polarization basis sets) are used to predict the solution-phase infrared spectra for a series of CO- and CN-containing iron complexes. It is shown that simple linear scaling of the computed C--O and C--N stretching frequencies yields accurate predictions of the the experimentally determined nu(CO) and nu(CN) values for a variety of complexes of different charges and in solvents of varying polarity. As examples of the technique, the resulting correlation is used to assign structures to spectroscopically observed but structurally ambiguous species in two different systems. For the (mu-SCH2CH2CH2S)[Fe(CO)3]2 complex in tetrahydrofuran solution, our calculations show that the initial electrochemical reduction process leads to a simple one-electron reduced product with a structure very similar to the (mu-SCH2CH2CH2S)[Fe(CO)3]2 parent complex. For the iron-iron hydrogenase enzyme active site, our computations show that the absence or presence of a water molecule near the distal iron center (the iron center further from the [4Fe4S] cluster and protein backbone) has very little effect on the predicted infrared spectra.     

New disulfido molybdenum-manganese complexes exhibit facile addition of small molecules to the sulfur atoms

The reaction of Mn(2)(CO)(7)(mu-S2) (1) with [CpMo(CO)(3)](2) (Cp = C(5)H(5)) and [Cp*Mo(CO)(3)](2) (Cp* = C(5)(CH(3))(5)) yielded the new mixed-metal disulfide complexes CpMoMn(CO)(5)(mu-S2) (2) and Cp*MoMn(CO)(5)(mu-S2) (3) by a metal-metal exchange reaction. Compounds 2 and 3 both contain a bridging disulfido ligand lying perpendicular to the Mo-Mn bond. The bond distances are Mo-Mn = 2.8421(10) and 2.8914(5) A and S-S = 2.042(2) and 1.9973(10) A for 2 and 3, respectively. A tetranuclear metal side product CpMoMn(3)(CO)(13)(mu3-S)(mu4-S) (4) was also isolated from the reaction of 1 with [CpMo(CO)(3)](2). Compounds 2 and 3 react with CO to yield the dithiocarbonato complexes CpMoMn(CO)(5)[mu-SC(=O)S] (5) and Cp*MoMn(CO)(5)[mu-SC(=O)S] (6) by insertion of CO into the S-S bond. Similarly, tert-butylisocyanide was inserted into the S-S bond of 2 and 3 to yield the complexes CpMoMn(CO)(5)[mu-S(C=NBu(t))S] (7) and Cp*MoMn(CO)(5)[mu-S(C=NBu(t))S] (8), respectively. Ethylene and dimethylacetylene dicarboxylate also inserted into the S-S bond of 2 and 3 at room temperature to yield the ethanedithiolato ligand bridged complexes CpMoMn(CO)(5)(mu-SCH(2)CH(2)S) (9), Cp*MoMn(CO)(5)(mu-SCH(2)CH(2)S) (10), CpMoMn(CO)(5)[mu-SC(CO(2)Me)=C(CO(2)Me)S] (11), and Cp*MoMn(CO)(5)[mu-SC(CO(2)Me)=C(CO(2)Me)S] (12). Allene was found to insert into the S-S bond of 2 by using one of its two double bonds to yield the complex CpMoMn(CO)(5)[mu-SCH(2)C(=CH(2))S] (13). The molecular structures of the new complexes 2-7 and 9-13 were established by single-crystal X-ray diffraction analyses.     

Catalysis of H(2)/D(2) scrambling and other H/D exchange processes by [Fe]-hydrogenase model complexes

Protonation of the [Fe]-hydrogenase model complex (mu-pdt)[Fe(CO)(2)(PMe(3))](2) (pdt = SCH(2)CH(2)CH(2)S) produces a species with a high field (1)H NMR resonance, isolated as the stable [(mu-H)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+)[PF(6)](-) salt. Structural characterization found little difference in the 2Fe2S butterfly cores, with Fe.Fe distances of 2.555(2) and 2.578(1) A for the Fe-Fe bonded neutral species and the bridging hydride species, respectively (Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710). Both are similar to the average Fe.Fe distance found in structures of three Fe-only hydrogenase active site 2Fe2S clusters: 2.6 A. A series of similar complexes (mu-edt)-, (mu-o-xyldt)-, and (mu-SEt)(2)[Fe(CO)(2)(PMe(3))](2) (edt = SCH(2)CH(2)S; o-xyldt = SCH(2)C(6)H(4)CH(2)S), (mu-pdt)[Fe(CO)(2)(PMe(2)Ph)](2), and their protonated derivatives likewise show uniformity in the Fe-Fe bond lengths of the neutral complexes and Fe.Fe distances in the cationic bridging hydrides. The positions of the PMe(3) and PMe(2)Ph ligands are dictated by the orientation of the S-C bonds in the (mu-SRS) or (mu-SR)(2) bridges and the subsequent steric hindrance of R. The Fe(II)(mu-H)Fe(II) complexes were compared for their ability to facilitate H/D exchange reactions, as have been used as assays of H(2)ase activity. In a reaction that is promoted by light but inhibited by CO, the [(mu-H)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+) complex shows H/D exchange activity with D(2), producing [(mu-D)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+) in CH(2)Cl(2) and in acetone, but not in CH(3)CN. In the presence of light, H/D scrambling between D(2)O and H(2) is also promoted by the Fe(II)(mu-H)Fe(II) catalyst. The requirement of an open site suggests that the key step in the reactions involves D(2) or H(2) binding to Fe(II) followed by deprotonation by the internal hydride base, or by external water. As indicated by similar catalytic efficiencies of members of the series, the nature of the bridging thiolates has little influence on the reactions. Comparison to [Fe]H(2)ase enzyme active site redox levels suggests that at least one Fe(II) must be available for H(2) uptake while a reduced or an electron-rich Fe(I)Fe(I) metal-metal bonded redox level is required for proton uptake.     

羰基铁-硫醇-三乙胺反应体系的研究, μ-酰基与μ-烯基铁硫配合物的合成系脱羰反应动力学

本文由反式巴豆酰氯与活性中间物[(μ-CO)(μ-R1S)Fe2(CO)6]ˉEt3N^+反应, 合成了两个新的μ-酰基铁硫配合物(μ-R1S)(μ-CH5CH=CHCO)Fe2(CO)6(R1=Et, Bu^t)和三个新的μ-烯基铁硫配合物(μ-R1S)(μ-CH3CH=CH)Fe2(CO)6(R1=Et, Bu^+, CH2=CHCH2), 此外还研究了μ-酰基配合物脱羰生成μ-烯基配合物的反应动力学, 证明为一级反应, 并求得在一定条件下的速率常数和半衰期。