Dissecting the intimate mechanism of cyanation of {2Fe3S} complexes related to the active site of all-iron hydrogenases by DFT analysis of energetics, transition states, intermediates and products in the carbonyl substitution pathway

A bridging carbonyl intermediate with key structural elements of the diiron sub-site of all-iron hydrogenase has been experimentally observed in the CN/CO substitution pathway of the {2Fe3S} carbonyl precursor, [Fe(2)(CO)(5){MeSCH(2)C(Me)(CH(2)S)(2)}]. Herein we have used density functional theory (DFT) to dissect the overall substitution pathway in terms of the energetics and the structures of transition states, intermediates and products. We show that the formation of bridging CO transitions states is explicitly involved in the intimate mechanism of dicyanation. The enhanced rate of monocyanation of {2Fe3S} over the {2Fe2S} species [Fe(2)(CO)(6){CH(2)(CH(2)S)(2)}] is found to rest with the ability of the thioether ligand to both stabilise a mu-CO transition state and act as a good leaving group. In contrast, the second cyanation step of the {2Fe3S} species is kinetically slower than for the {2Fe2S} monocyanide because the Fe2 atom is deactivated by coordination of the electron-donating thioether group. In addition, hindered rotation and the reaction coordinate of the approaching CN(-) group, are other factors which explain reactivity differences in {2Fe2S} and {2Fe3S} systems. The intermediate species formed in the second cyanation step of {2Fe3S} species is a mu-CO species, confirming the structural assignment made on the basis of FT-IR data (S. J. George, Z. Cui, M. Razavet, C. J. Pickett, Chem. Eur. J. 2002, 8, 4037-4046). In support of this we find that computed and experimental IR frequencies of structurally characterised {2Fe3S} species and those of the bridging carbonyl intermediate are in excellent agreement. In a wider context, the study may provide some insight into the reactivity of dinuclear systems in which neighbouring group on-off coordination plays a role in substitution pathways.     

Ligand K-edge X-ray absorption spectroscopy and DFT calculations on [Fe3S4]0,+ clusters: delocalization, redox, and effect of the protein environment

Ligand K-edge XAS of an [Fe3S4]0 model complex is reported. The pre-edge can be resolved into contributions from the mu(2)S(sulfide), mu(3)S(sulfide), and S(thiolate) ligands. The average ligand-metal bond covalencies obtained from these pre-edges are further distributed between Fe(3+) and Fe(2.5+) components using DFT calculations. The bridging ligand covalency in the [Fe2S2]+ subsite of the [Fe3S4]0 cluster is found to be significantly lower than its value in a reduced [Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2]+ site (-146 cm(-1) vs -360 cm(-1), respectively). This decrease in J, along with estimates of the double exchange parameter B and vibronic coupling parameter lambda2/k(-), leads to an S = 2 delocalized ground state in the [Fe3S4]0 cluster. The S K-edge XAS of the protein ferredoxin II (Fd II) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same oxidation state, which correlates with the number of H-bonding interactions to specific sulfur ligands present in the active site. The changes in ligand-metal bond covalencies upon redox compared with DFT calculations indicate that the redox reaction involves a two-electron change (one-electron ionization plus a spin change of a second electron) with significant electronic relaxation. The presence of the redox inactive Fe(3+) center is found to decrease the barrier of the redox process in the [Fe3S4] cluster due to its strong antiferromagnetic coupling with the redox active Fe2S2 subsite.     

A [4Fe-4S] cluster dimer bridged by bis(2,2':6',2'-terpyridine-4'-thiolato)iron(II)

The use of 2,2':6',2'-terpyridine-4'-thiol (tpySH) was explored as a bridging ligand for the formation of stable assemblies containing both [4Fe-4S] clusters and single metal ions. Reaction of tpySH (2 equiv) with (NH4)2Fe(SO4)(2).6H2O generated the homoleptic complex [Fe(tpySH)2](2+), which was isolated as its PF6(-) salt. The compound could be fully deprotonated to yield neutral [Fe(tpyS)2], and the absorption spectrum is highly dependent on the protonation state. Reaction of [Fe(tpySH)2](PF6)2 with the new 3:1 site-differentiated cluster (n-Bu4N)2[Fe4S4(TriS)(SEt)] yielded the first metal-bridged [4Fe-4S] cluster dimer, (n-Bu4N)2[{Fe4S4(TriS)(mu-Stpy)}2Fe]. Electrochemical studies indicate that the [4Fe-4S] clusters in the dimer act as independent redox units, while UV-vis spectroscopy provides strong evidence for a thioquinonoid electron distribution in the bridging tpyS(-) ligand. TpySH thus acts as a directional bridging ligand between [4Fe-4S] clusters and single metal ions, thereby opening the way to the synthesis of larger, more complex assemblies.     

Hybrid materials containing organometallic cations and 3-D anionic metal dicyanamide networks of type [Cp*2M][M'(dca)3

A new series of hybrid materials of type [Cp*2M][M'(dca)3] has been prepared by cation templation and structurally characterised (M = Fe(III), Co(III); M'= Mn(II), Fe(II), Co(II), Ni(II), Cd(II); dca-= N(CN)2-). The crystallographic analysis of [Cp*2Fe][Cd(dca)(3)] showed that the [Cd(dca)3]- anionic framework is of a symmetrical 3-D alpha-polonium type, containing octahedral Cd nodes and micro (1,5)-dca bridging ligands. The [Cp*2Fe]+ cations occupy the cube-like cavities within the framework. The cationic and anionic-framework sublattices remain magnetically independent and display susceptibilities, over the range 300 to 2 K, of a Curie-Weiss nature obtained by adding a S= 1/2 (Cp*2Fe+) or a S= 0 (Cp*2Co+) contribution to those of the weakly antiferromagnetically coupled frameworks of M'. These hybrid species do not show any intrinsic long-range magnetic order. The present [Cp*2Fe]M'(dca)3] series display the characteristic, unusually shaped [Cp*2Fe]+ Mossbauer line, in the range 295-5 K, assigned (below 101 K) as the sum of a narrow and a broad line. Relaxation effects were evident. The [Fe(dca)3]- compound showed superimposed low-spin Fe(III) and high-spin Fe(II) lines, the latter giving relaxation broadening effects.     

Initial members of the family of molecular mid-valent high-nuclearity iron nitrides: [Fe4N2X10]4- and [Fe10N8X12]5- (X = Cl-, Br-)

Current theoretical and experimental evidence points toward X = N as the identity of the interstitial atom in the [MoFe7S9X] core of the iron-molybdenum cofactor cluster of nitrogenase. This atom functions with mu6 bridging multiplicity to six iron atoms and, if it is nitrogen as nitride, raises a question as to the existence of a family of molecular iron nitrides of higher nuclearity than known dinuclear Fe(III,IV) species with linear [Fe-N-Fe]5+,4+ bridges. This matter has been initially examined by variation of reactant stoichiometry in the self-assembly systems [FeX4]1-/(Me3Sn)3N (X = Cl-, Br-) in acetonitrile. A 2:1 mol ratio affords [Fe4N2Cl10]4- (1), isolated as the Et4N+ salt (72%). This cluster has idealized C2h symmetry with a planar antiferromagnetically coupled [Fe(III)4(mu3-N)2]6+ core containing an Fe2N2 rhombus to which are attached two FeCl3 units. DFT calculations have been performed to determine the dominant magnetic exchange pathway. An 11:8 mol ratio leads to [Fe10N8Cl12]5- (3) as the Et4N+ salt (37%). The cluster possesses idealized D2h symmetry and is built of 15 edge- and vertex-shared rhomboids involving two mu3-N and six mu4-N bridging atoms, and incorporates two of the core units of 1. Four FeN2Cl2 and four FeN3Cl sites are tetrahedral and two FeN5 sites are trigonal pyramidal. The cluster is mixed-valence (9Fe(III) + Fe(IV)); a discrete Fe(IV) site was not detected by crystallography or M?ssbauer spectroscopy. The corresponding clusters [Fe4N2Br10]4- and [Fe10N8Br12]5- are isostructural with 1 and 3, respectively. Future research is directed toward defining the scope of the family of molecular iron nitrides.     

铁铂硫原子簇化合物的合成与结构研究 V: [MoFe3S4]类立方烷簇合物的转化反应和[Et4N]3[Mo2Fe6S8(SC6H4CH3-m)3Cl6]的晶体结构

具[MoFe2S4]类立方烷结构单元的双类立方烷化合物[Et4N]4[Mo2Fe7S8(SR)12](1a,R=Ph; 1b, R=tolyl-m)或单类立方烷化合物[MoFe3S4(dteR2)5](2a, R=Me; 2b, R=Et)与酰氯在乙腈中反应, 分别得到不含Fe桥的双类立方烷化合物(Et4N)3[Mo2Fe6S8(SR)3Cl6](3a, R=Ph; 3b, R=toly-m)与[MoFe3S4]骨架支解后的Fe(dteR2)2Cl(4a, R=Me; 4b, R=Et)。说明在相同反应条件下, [MoFe3S4]单元在1中比在2中稳定, 本文首次将1型与3型结构通过一步化学反应连系起来。3型化合物的产生得到X射线衍射测定及^1H NMR谱的证实。本文报道3b的单晶结构及3的^!H NMR数据, 3b属六方晶系, P63/m, a=1.6827(3), c=1.5951(16)nm; V=3.91158nm^3; Dc=1.491g/cm^3;Z=2; F(000)=1780; 偏离因子R=0.048, 化合物2与酰氯反应产生4, 由红外及紫外可见光谱证实。     

Diruthenium dithiolato cyanides: basic reactivity studies and a post hoc examination of nature's choice of Fe versus Ru for hydrogenogenesis

The reaction of Ru2(S2C3H6)(CO)6 (1) with 2 equiv of Et4NCN yielded (Et4N)2[Ru2(S2C3H6)(CN)2(CO)4], (Et4N)2[3], which was shown crystallographically to consist of a face-sharing bioctahedron with the cyanide ligands in the axial positions, trans to the Ru-Ru bond. Competition experiments showed that 1 underwent cyanation >100x more rapidly than the analogous Fe2(S2C3H6)(CO)6. Furthermore, Ru2(S2C3H6)(CO)6 underwent dicyanation faster than [Ru2(S2C3H6)(CN)(CO)5]-, implicating a highly electrophilic intermediate [Ru2(S2C3H6)(mu-CO)(CN)(CO)5]-. Ru2(S2C3H6)(CO)6 (1) is noticeably more basic than the diiron compound, as demonstrated by the generation of [Ru2(S2C3H6)(mu-H)(CO)6]+, [1H]+. In contrast to 1, the complex [1H]+ is unstable in MeCN solution and converts to [Ru2(S2C3H6)(mu-H)(CO)5(MeCN)]+. (Et4N)2[3] was shown to protonate with HOAc (pKa = 22.3, MeCN) and, slowly, with MeOH and H2O. Dicyanide [3]2- is stable toward excess acid, unlike the diiron complex; it slowly forms the coordination polymer [Ru2(S2C3H6)(mu-H)(CN)(CNH)(CO)4]n, which can be deprotonated with Et3N to regenerate [H3]-. Electrochemical experiments demonstrate that [3H]- catalyzes proton reduction at -1.8 V vs Ag/AgCl. In contrast to [3]2-, the CO ligands in [3H]- undergo displacement. For example, PMe3 and [3H]- react to produce [Ru2(S2C3H6)(mu-H)(CN)2(CO)3(PMe3)]-. Oxidation of (Et4N)2[3] with 1 equiv of Cp2Fe+ gave a mixture of [Ru2(S2C3H6)(mu-CO)(CN)3(CO)3]- and [Ru2(S2C3H6)(CN)(CO)5]-, via a proposed [Ru2]2(mu-CN) intermediate. Overall, the ruthenium analogues of the diiron dithiolates exhibit reactivity highly reminiscent of the diiron species, but the products are more robust and the catalytic properties appear to be less promising.     

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.     

Probing the intrinsic electronic structure of the cubane [4Fe-4S] cluster: nature's favorite cluster for electron transfer and storage

The cubane [4Fe-4S] is the most common multinuclear metal center in nature for electron transfer and storage. Using electrospray, we produced a series of gaseous doubly charged cubane-type complexes, [Fe4S4L4]2- (L = -SC2H5, -SH, -Cl, -Br, -I) and the Se-analogues [Fe4Se4L4]2- (L = -SC2H5, -Cl), and probed their electronic structures with photoelectron spectroscopy and density functional calculations. The photoelectron spectral features are similar among all the seven species investigated, revealing a weak threshold feature due to the minority spins on the Fe centers and confirming the low-spin two-layer model for the [4Fe-4S](2+) core and its "inverted level scheme". The measured adiabatic detachment energies, which are sensitive to the terminal ligand substitution, provide the intrinsic oxidation potentials of the [Fe4S4L4]2- complexes. The calculations revealed a simple correlation between the electron donor property of the terminal thiolate as well as the bridging sulfide with the variation of the intrinsic redox potentials. Our data provide intrinsic electronic structure information of the [4Fe-4S] cluster and the molecular basis for understanding the protein and solvent effects on the redox properties of the [4Fe-4S] active sites.     

Diversity of new structural types in polynuclear iron chemistry with a tridentate N,N,O ligand

The syntheses, crystal structures, and magnetochemical characterization of four new iron clusters [Fe7O4(O2CPh)11(dmem)2] (1), [Fe7O4(O2CMe)11(dmem)2] (2), [Fe6O2(OH)4(O2CBut)8(dmem)2] (3), and [Fe3O(O2CBut)2(N3)3(dmem)2] (4) (dmemH=Me2NCH2CH2N(Me)CH2CH2OH)=2-{[2-(dimethylamino)ethyl]methylamino}ethanol) are reported. The reaction of dmemH with [Fe3O(O2CR)6(H2O)3](NO3) (R=Ph (1), Me (2), and But (3)) gave 1, 2, and 3, respectively, whereas 4 was obtained from the reaction of 3 with sodium azide. The complexes all possess rare or novel core topologies. The core of 1 comprises two [Fe4(mu3-O)2]8+ butterfly units sharing a common body Fe atom. The core of 2 consists of a [Fe3O3] ring with each doubly bridging O2- ion becoming mu3 by also bridging to a third, external Fe atom; a seventh Fe atom is attached on the outside of this core via an additional mu3-O2- ion. The core of 3 consists of a [Fe4(mu3-O)2]8+ butterfly unit with an Fe atom attached above and below this by bridging O atoms. Finally, the core of 4 is an isosceles triangle bridged by a mu3-O2- ion with a rare T-shaped geometry and with the azide groups all bound terminally. Variable-temperature, solid-state dc, and ac magnetization studies were carried out on complexes 1-4 in the 5.0-300 K range. Fitting of the obtained magnetization (M) vs field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting) established that 1, 2, and 4 each possess an S=5/2 ground state spin, whereas 3 has an S=5 ground state. As is usually the case, good fits of the magnetization data could be obtained with both positive and negative D values. To obtain more accurate values and to determine the sign of D, high-frequency EPR studies were carried out on single crystals of representative complexes 1.4MeCN and 3.2MeCN, and these gave D=+0.62 cm-1 and |E|>or=0.067 cm-1 for 1.4MeCN and D=-0.25 cm-1 for 3.2MeCN. The magnetic susceptibility data for 4 were fit to the theoretical chiM vs T expression derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, and this revealed the pairwise exchange parameters to be antiferromagnetic with values of Ja=-3.6 cm-1 and Jb=-45.9 cm-1. The combined results demonstrate the ligating flexibility of dmem and its usefulness in the synthesis of a variety of Fex molecular species.     

Rearrangement of symmetrical dicubane clusters into topological analogues of the P cluster of nitrogenase: nature's choice?

Reaction schemes have been developed that lead to clusters having the topology of the PN cluster of nitrogenase. The single cubane clusters [(Tp)MFe3S4Cl3]z (M = Mo, z = 1-; M = V, z = 2-) react with PEt3 to give [(Tp)MFe3S4(PEt3)3]1+, which are reduced to the neutral edge-bridged double cubanes [(Tp)2M2Fe6S8(PEt3)4] with highly reduced (2[MFe3S4]1+) cores. Reaction of these clusters in acetonitrile with (Et4N)(HS) results in the formation of [(Tp)2Mo2Fe6S9(SH)2]3- and [(Tp)2V2Fe6S9(SH)2]4-. X-ray structures of the Et4N+ salts reveal the bridging pattern M2Fe6(mu2-S)2(mu3-S)6(mu6-S) in which two cuboidal MFe3(mu3-S)3 units share the common bridge atom mu6-S and are externally bridged by two mu2-S atoms. The M sites possess trigonal octahedral, and the Fe sites, distorted tetrahedral coordination. Hydrosulfide ligands and sulfide atoms simulate terminal cysteinate ligation and mu2 bridges, respectively, in the protein-bound cluster Fe8S7(mu2-SCys)2(SCys)4. The synthetic clusters have the same bridging pattern as the PN cluster and approach congruency with it. These clusters are the first molecular topological analogues of a PN cluster. Like the latter, they are substantially reduced (majority of Fe(II)).     

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.     

Fe2(SR)2(mu-CO)(CNMe)6]2+ and analogues: a new class of diiron dithiolates as structural models for the H(ox)Air state of the fe-only hydrogenase

Low-temperature oxidation of Fe(2)(S(2)C(n)H(2n)(CNMe)(6-x)(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type [Fe(2)(S(2)C(n)H(2n)(CO)x(CNMe)(7-x)](2+). The degree of substitution is controlled by the RNC/Fe ratio, as well as the degree of initial substitution at iron, with tricarbonyl derivatives favoring more highly carbonylated products. The structures of the monocarbonyl derivatives [Fe(2)(S(2)C(n)H(2n))(mu-CO)(CNMe)(6)](PF(6))(2) (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 A, although the mu-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of [Fe(2)(S(2)C(3)H(6))(CO)(CNMe)(6)](PF(6))(2) were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 A elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe(2)(S(2)C(2)H(4))(mu-CO)(CO)(CNMe)(5)](PF(6))(2), the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe(2)(S(2)C(2)H(4))(CNMe)(3)(CO)(3) under 1 atm CO gives the deep pink tricarbonyl [Fe(2)(S(2)C(2)H(4))(CO)(3)(CNMe)(4)](PF(6))(2). DFT calculations show that a bridging CO or MeNC establishes a 3-center, 2-electron bond within the two Fe(II) centers, which would otherwise be nonbonding.     

质谱中若干金属配合物及簇合物的碎片重组反应

在质谱实验中,我们发现了与一般单分子分解反应相悖的四种类型碎片产物的重组反应物即(Ⅰ)裂解碎片的自组合产物如S_8~+及不同聚合度的聚硫离子;(Ⅱ)裂解碎片之间的相互组合物如[Fe(S_2CNC_4H_8)_3]~+和[Fe(S_2CNEt_2)_3]~+;(Ⅲ)裂解碎片的阴离子和阳离子部分碎片间的组合物如[Fe_2S_2(NO)_4(CH_3)_2]~+;(Ⅳ)歧化反应产物Cp_3Yb~+和YbL_3~+(Cp=C_5H_5),L=β-二酮)。本文阐述了这种重组反应或离子—分子反应在质谱条件下产生的可能性及其理论根据。     

Mechanisms of catalyst poisoning in palladium-catalyzed cyanation of haloarenes. remarkably facile C-N bond activation in the [(Ph3P)4Pd]/[Bu4N]+ CN- system

Reaction paths leading to palladium catalyst deactivation during cyanation of haloarenes (eq 1) have been identified and studied. Each key step of the catalytic loop (Scheme 1) can be disrupted by excess cyanide, including ArX oxidative addition, X/CN exchange, and ArCN reductive elimination. The catalytic reaction is terminated via the facile formation of inactive [(CN)4Pd]2-, [(CN)3PdH]2-, and [(CN)3PdAr]2-. Moisture is particularly harmful to the catalysis because of facile CN- hydrolysis to HCN that is highly reactive toward Pd(0). Depending on conditions, the reaction of [(Ph3P)4Pd] with HCN in the presence of extra CN- can give rise to [(CN)4Pd]2- and/or the remarkably stable new hydride [(CN)3PdH]2- (NMR, X-ray). The X/CN exchange and reductive elimination steps are vulnerable to excess CN- because of facile phosphine displacement leading to stable [(CN)3PdAr]2- that can undergo ArCN reductive elimination only in the absence of extra CN-. When a quaternary ammonium cation such as [Bu4N]+ is used as a phase-transfer agent for the cyanation reaction, C-N bond cleavage in the cation can occur via two different processes. In the presence of trace water, CN- hydrolysis yields HCN that reacts with Pd(0) to give [(CN)3PdH]2-. This also releases highly active OH- that causes Hofmann elimination of [Bu4N]+ to give Bu3N, 1-butene, and water. This decomposition mode is therefore catalytic in H2O. Under anhydrous conditions, the formation of a new species, [(CN)3PdBu]2-, is observed, and experimental studies suggest that electron-rich mixed cyano phosphine Pd(0) species are responsible for this unusual reaction. A combination of experimental (kinetics, labeling) and computational studies demonstrate that in this case C-N activation occurs via an S(N)2-type displacement of amine and rule out alternative 3-center C-N oxidative addition or Hofmann elimination processes.     

Coordination chemistry of N-heterocyclic stannylenes: a combined synthetic and Mössbauer spectroscopy study

The N-heterocyclic stannylenes (NHSns), [(Dipp) N(CH(2))(n)N(Dipp)S n] (Dipp = 2,6- (i)Pr(2)C(6)H(3); n = 2, 1; n = 3, 5) and [((t)Bu) N(CHMe)(2)N((t)Bu)S n] (10) are competent ligands toward a variety of transition metal centers, as seen in the complexes [W(CO)(5)·1] (2), [(OC)(4)Fe(μ-1)(2)Fe(CO)(4)] (3), [(OC)(4)Fe(μ-1)Fe(CO)(4)] (4), [Fe(CO)(4)·5](n) (6, n = 1 or 2), [(OC)(4)Fe(μ-5)Fe(CO)(4)] (7), [Ph(3)PPt(μ-1)(2)PtPPh(3)] (8), [Fe(CO)(4)·10] (11), and [(η(5)-C(5)H(5))(OC)(2)Mn·10] (12). X-ray crystallographic studies show that the NHSns are structurally largely unperturbed binding to the metal, but in contrast to the parent NHCs, NHSns often adopt a bridging position across dinuclear metal units. The balance between terminal and bridging positions for the stannylene is evidently closely balanced as shown by the observation of both monomers and dimers for 6 in the solid state and in solution. (119)Sn and (57)Fe Mo?ssbauer spectroscopy of the complexes shows the tin atoms in such complexes to be consistent with electron deficient Sn(II) centers.     

Characterization of [2Fe–2S]-Cluster-Bridged Protein Complexes and Reaction Intermediates by use of Native Mass Spectrometric Methods

Many iron–sulfur proteins involved in cluster trafficking form [2Fe–2S]-cluster-bridged complexes that are often challenging to characterize because of the inherent instability of the cluster at the interface. Herein, we illustrate the use of fast, online buffer exchange coupled to a native mass spectrometry (OBE nMS) method to characterize [2Fe–2S]-cluster-bridged proteins and their transient cluster-transfer intermediates. The use of this mechanistic and protein-characterization tool is demonstrated with holo glutaredoxin 5 (GLRX5) homodimer and holo GLRX5:BolA-like protein 3 (BOLA3) heterodimer. Using the OBE nMS method, cluster-transfer reactions between the holo-dimers and apo-ferredoxin (FDX2) are monitored, and intermediate [2Fe–2S] species, such as (FDX2:GLRX5:[2Fe–2S]:GSH) and (FDX2:BOLA3:GLRX5:[2Fe–2S]:GSH) are detected. The OBE nMS method is a robust technique for characterizing iron–sulfur-cluster-bridged protein complexes and transient iron–sulfur-cluster transfer intermediates.     

High-nuclearity sulfide-rich molybdenum[bond]iron[bond]sulfur clusters: reevaluation and extension

High-nuclearity Mo[bond]Fe[bond]S clusters are of interest as potential synthetic precursors to the MoFe(7)S(9) cofactor cluster of nitrogenase. In this context, the synthesis and properties of previously reported but sparsely described trinuclear [(edt)(2)M(2)FeS(6)](3-) (M = Mo (2), W (3)) and hexanuclear [(edt)(2)Mo(2)Fe(4)S(9)](4-) (4, edt = ethane-1,2-dithiolate; Zhang, Z.; et al. Kexue Tongbao 1987, 32, 1405) have been reexamined and extended. More accurate structures of 2-4 that confirm earlier findings have been determined. Detailed preparations (not previously available) are given for 2 and 3, whose structures exhibit the C(2) arrangement [[(edt)M(S)(mu(2)-S)(2)](2)Fe(III)](3-) with square pyramidal Mo(V) and tetrahedral Fe(III). Oxidation states follow from (57)Fe M?ssbauer parameters and an S = (3)/(2) ground state from the EPR spectrum. The assembly system 2/3FeCl(3)/3Li(2)S/nNaSEt in methanol/acetonitrile (n = 4) affords (R(4)N)(4)[4] (R = Et, Bu; 70-80%). The structure of 4 contains the [Mo(2)Fe(4)(mu(2)-S)(6)(mu(3)-S)(2)(mu(4)-S)](0) core, with the same bridging pattern as the [Fe(6)S(9)](2-) core of [Fe(6)S(9)(SR)(2)](4-) (1), in overall C(2v) symmetry. Cluster 4 supports a reversible three-member electron transfer series 4-/3-/2- with E(1/2) = -0.76 and -0.30 V in Me(2)SO. Oxidation of (Et(4)N)(4)[4] in DMF with 1 equiv of tropylium ion gives [(edt)(2)Mo(2)Fe(4)S(9)](3-) (5) isolated as (Et(4)N)(3)[5].2DMF (75%). Alternatively, the assembly system (n = 3) gives the oxidized cluster directly as (Bu(4)N)(3)[5] (53%). Treatment of 5 with 1 equiv of [Cp(2)Fe](1+) in DMF did not result in one-electron oxidation but instead produced heptanuclear [(edt)(2)Mo(2)Fe(5)S(11)](3-) (6), isolated as the Bu(4)N(+)salt (38%). Cluster 6 features the previously unknown core Mo(2)Fe(5)(mu(2)-S)(7)(mu(3)-S)(4) in molecular C(2) symmetry. In 4-6, the (edt)MoS(3) sites are distorted trigonal bipramidal and the FeS(4) sites are distorted tetrahedral with all sulfide ligands bridging. M?ssbauer spectroscopic data for 2 and 4-6 are reported; (mean) iron oxidation states increase in the order 4 < 5 approximately 1 < 6 approximately 2. Redox and spectroscopic data attributed earlier to clusters 2 and 4 are largely in disagreement with those determined in this work. The only iron and molybdenum[bond]iron clusters with the same sulfide content as the iron[bond]molybdenum cofactor of nitrogenase are [Fe(6)S(9)(SR)(2)](4-) and [(edt)(2)Mo(2)Fe(4)S(9)](3-)(,4-).     

Biosynthesis of the catalytic H-cluster of [FeFe] hydrogenase: the roles of the Fe–S maturase proteins HydE,HydF, and HydG

[FeFe] hydrogenases carry out the redox interconversion of protons and molecular hydrogen (2H⁺ + 2e⁻ ⇌ H₂) at a complex Fe–S active site known as the H-cluster. The H-cluster consists of a [4Fe–4S] subcluster, denoted here as [4Fe]_H, linked via a cysteine sulfur to an interesting organometallic [2Fe]_H subcluster thought to be the subsite where the catalysis occurs. This [2Fe]_H subcluster consists of two Fe atoms, linked with a bridging CO and a bridging SCH₂NHCH₂S azadithiolate (adt), with additional terminal CO and CN ligands bound to each Fe. Synthesizing such a complex organometallic unit is a fascinating problem in biochemistry, complicated by the toxic nature of both the CO and CN⁻ species and the relative fragility of the azadithiolate bridge. It has been known for a number of years that this complex biosynthesis is carried out by a set of three essential Fe–S proteins, HydE, HydF, and HydG. HydF is a GTPase, while HydE and HydG are both members of the large family of radical S-adenosylmethionine (rSAM) enzymes. In this perspective we describe the history of research and discovery concerning these three Fe–S “maturase” proteins and describe recent evidence for a sequential biosynthetic pathway beginning with the synthesis of a mononuclear organometallic [Fe(ii)(CO)₂CN(cysteine)] complex by the rSAM enzyme HydG and its subsequent activation by the second rSAM enzyme HydE to form a highly reactive Fe(i)(CO)₂(CN)S species. In our model a pair of these Fe(i)(CO)₂(CN)S units condense to form the [Fe(CO)₂(CN)S]₂ diamond core of the [2Fe]_H cluster, requiring only the installation of the central CH₂NHCH₂ portion of the azadithiolate bridge, whose atoms are all sourced from the amino acid serine. This final step likely occurs with an interplay of HydE and HydF, the details of which yet remain to be elucidated.

Fe–S cluster enzymes HydG, HydE, and HydF provide sequential assembly of the catalytic H-cluster of [FeFe] hydrogenase.     

Characterization of [2Fe–2S]‐Cluster‐Bridged Protein Complexes and Reaction Intermediates by use of Native Mass Spectrometric Methods

Many iron–sulfur proteins involved in cluster trafficking form [2Fe–2S]‐cluster‐bridged complexes that are often challenging to characterize because of the inherent instability of the cluster at the interface. Herein, we illustrate the use of fast, online buffer exchange coupled to a native mass spectrometry (OBE nMS) method to characterize [2Fe–2S]‐cluster‐bridged proteins and their transient cluster‐transfer intermediates. The use of this mechanistic and protein‐characterization tool is demonstrated with holo glutaredoxin 5 (GLRX5) homodimer and holo GLRX5:BolA‐like protein 3 (BOLA3) heterodimer. Using the OBE nMS method, cluster‐transfer reactions between the holo‐dimers and apo‐ferredoxin (FDX2) are monitored, and intermediate [2Fe–2S] species, such as (FDX2:GLRX5:[2Fe–2S]:GSH) and (FDX2:BOLA3:GLRX5:[2Fe–2S]:GSH) are detected. The OBE nMS method is a robust technique for characterizing iron–sulfur‐cluster‐bridged protein complexes and transient iron–sulfur‐cluster transfer intermediates.