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With the goal of generating anionic analogues to MN2S2⋅Mn(CO)3Br we introduced metallodithiolate ligands, MN2S22− prepared from the Cys‐X‐Cys biomimetic, ema4− ligand (ema=N,N′‐ethylenebis(mercaptoacetamide); M=NiII, [VIV≡O]2+ and FeIII) to Mn(CO)5Br. An unexpected, remarkably stable dimanganese product, (H2N2(CH2C=O(μ‐S))2)[Mn(CO)3]2 resulted from loss of M originally residing in the N2S24− pocket, replaced by protonation at the amido nitrogens, generating H2ema2−. Accordingly, the ema ligand has switched its coordination mode from an N2S24− cavity holding a single metal, to a binucleating H2ema2− with bridging sulfurs and carboxamide oxygens within Mn‐μ‐S‐CH2‐C‐O, 5‐membered rings. In situ metal‐templating by zinc ions gives quantitative yields of the Mn2 product. By computational studies we compared the conformations of "linear" ema4− to ema4− frozen in the "tight‐loop" around single metals, and to the "looser" fold possible for H2ema2− that is the optimal arrangement for binucleation. XRD molecular structures show extensive H‐bonding at the amido‐nitrogen protons in the solid state. [ABSTRACT FROM AUTHOR]

The nitrosylated diiron complexes, Fe2(NO)3, of this study are interpreted as a mono‐nitrosyl Fe(NO) unit, MNIU, within an N2S2 ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N2S2⋅Fe(NO)2+ complex, 1+, of Enemark–Feltham electronic notation {Fe(NO)}7‐{Fe(NO)2}9, is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)}7‐{Fe(NO)2}10, 10, and {Fe(NO)}8‐{Fe(NO)2}10, 1−, were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S=1/2 , complex, 10, finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe‐N‐O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe‐N‐O angle to 171° in 1−. Compensating shifts in FeMNIU distances from the N2S2 plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1− is a triplet‐state {Fe(NO)}8 with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment. The remarkable stability of the [Fe(NO)N2S2‐Fe(NO)2] species that permits isolation and structural characterization in its three redox levels relates to the delocalization of charge on both Fe(NO) and the Fe(NO)2 support. Subtle shifts of Fe(NO) from the N2S2 base accommodates a rare, non‐heme {Fe(NO)}8 triplet state and ca. linear. [ABSTRACT FROM AUTHOR]

The reaction of Fe(CO)2(NO)2and Ni(N2S2) (N2S2= N,N′-Bis(2-mercaptoethyl)-1,4-diazacycloheptane) by a single CO replacement yields [Ni(N2S2)]Fe(NO)2(CO), while an excess of Fe(CO)2(NO)2leads to triply bridging thiolate sulphurs in a cluster of core composition Ni2S4Fe3, lacking one Fe(NO)2unit to complete the adamantane-like structure. This structural type was earlier identified in a CuICl aggregate of MII(N2S2) (MII= Ni, Cu), in which complete MII2S4CuI4core structures were obtained as the major, and, in the case of CuII(N2S2), the incomplete CuII2S4CuI3as a minor, product. The full Ni2S4Fe4cluster has not yet been realized for Fe = Fe(NO)2. Computational analysis of the NiFe-heterobimetallic complex addresses structural issues including a ∠Ni–S–Fe of 90° in the bimetallic complex. [ABSTRACT FROM AUTHOR]

Iron-iron hydrogenases ([FeFe]H2ases) are exceptional natural catalysts for the reduction of protons to dihydrogen. Future biotechnological applications based on these enzymes require a precise understanding of their structures and properties. Although the [FeFe]H2ases have been characterized by single-crystal X-ray crystallography and a range of spectroscopic techniques, ambiguities remain regarding the details of the molecular structures of the spectroscopically observed forms. We use density functional theory (DFT) computations on small-molecule computational models of the [FeFe]H2ase active site to address this problem. Specifically, a series of structural candidates are geometry optimized and their infrared (IR) spectra are simulated using the computed C-O and C-N stretching frequencies and infrared intensities. Structural assignments are made by comparing these spectra to the experimentally determined IR spectra for each form. The Hred form is assigned as a mixture of an Fe'F& form with an open site on the distal iron center and either a Fe'Fe' form in which the distal cyanide has been protonated or a Fe°Fe" form with a bridging hydride ligand. The H0~ form is assigned as a valence-localized F&Fe° redox level with an open site at the distal iron. The ~ form is assigned as an Fe"Fe" redox level with OH- or OOH bound to the distal iron center that may or may not have an oxygen atom bound to one of the sulfur atoms of the dithiolate linker. Comparisons of the computed IR spectra of the 12C0 and 13C0 inhibited form with the experimental IR spectra show that exogenous CO binds terminally to the distal iron center. [ABSTRACT FROM AUTHOR]

Simple synthetic di-iron dithiolate complexes provide good models of the composition of the active site of the iron-iron hydrogenase enzymes. However, the formally FeIFeI complexes synthesized to date fail to reproduce the precise orientation of the diatomic ligands about the iron centers that is observed in the molecular structure of the reduced form of the enzyme active site. This structural difference is often used to explain the fact that the synthetic di-iron complexes are generally poor catalysts when compared to the enzyme. Herein, density functional theory computations are used for the rational design of synthetic complexes as structural models of the reduced form of the enzyme active site. These computations suggest several possible synthetic targets. The synthesis of complexes containing five-atom S-to-S linkers of the form S(CH2)2X(CH2)2S (X = CH2, NH, or O) or pendant functionalities attached to the three-carbon framework is one method. Another approach is the synthesis of asymmetrically substituted complexes, in which one iron center has strongly electron donating ligands and the adjacent iron center has strongly electron accepting ligands. The combination of a sterically demanding S-to-S linker and asymmetric substitution of the CO ligands is predicted to be a particularly effective synthetic target. [ABSTRACT FROM AUTHOR]

Hydrogen uptake in hydrogenase enzymes can be assayed by H/D exchange reactivity in H[sub 2]/D[sub 2]O or H[sub 2]/D[sub 2]/H[sub 2]O mixtures. Diiron(I) complexes that serve as structural models for the active site of iron hydrogenase are not active in such isotope scrambling but serve as precursors to Fe[sup II]Fe[sup II] complexes that are functional models of [Fe]H[sub 2]ase. Using the same experimental protocol as used previously for {(μ-H)(µ-pdt)[Fe(CO)[sub 2](PMe[sub 3])][sub 2, sup +]}, 1-H[sup +] (Zhao et al. J. Am. Chem. Soc. 2001, 123, 9710), we now report the results of studies of {(μ-SMe)(μ-pdt)[Fe(CO)[sub 2](PMe[sub 3])][sub 2, sup +]}, l-SMe[sup +], toward H/D exchange. Th1 1-SMe[sup +] complex can take up H[sub 2] and catalyze the H/D exchange reaction in D[sub 2]/H[sub 2]O mixtures under photolytic, CO-loss conditions. Unlike 1-H[sup +], it does not catalyze H[sub 2]/D[sub 2] scrambling under anhydrous conditions. The molecular structure of 1-SMe[sup +] involves an elongated Fe···Fe separation, 3.11 Å, relative to 2.58 Å in 1-H[sup +]. It is proposed that the strong SMe[sup -] bridging ligand results in catalytic activity localized on a single Fe[sup II] center, a scenario that is also a prominent possibility for the enzyme active site. The single requirement is an open site on Fe[sup II] available for binding of D[sub 2] (or H[sub 2]), followed by deprotonation by the external base H[sub 2]O (or D[sub 2]O). [ABSTRACT FROM AUTHOR]

The nitrosylated diiron complexes, Fe2(NO)3, of this study are interpreted as a mono‐nitrosyl Fe(NO) unit, MNIU, within an N2S2 ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N2S2⋅Fe(NO)2+ complex, 1+, of Enemark–Feltham electronic notation {Fe(NO)}7‐{Fe(NO)2}9, is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)}7‐{Fe(NO)2}10, 10, and {Fe(NO)}8‐{Fe(NO)2}10, 1−, were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S=1/2 , complex, 10, finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe‐N‐O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe‐N‐O angle to 171° in 1−. Compensating shifts in FeMNIU distances from the N2S2 plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1− is a triplet‐state {Fe(NO)}8 with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment. The remarkable stability of the [Fe(NO)N2S2‐Fe(NO)2] species that permits isolation and structural characterization in its three redox levels relates to the delocalization of charge on both Fe(NO) and the Fe(NO)2 support. Subtle shifts of Fe(NO) from the N2S2 base accommodates a rare, non‐heme {Fe(NO)}8 triplet state and ca. linear. [ABSTRACT FROM AUTHOR]

Metallodithiolate ligands are used to design heterobimetallic complexes by adduct formation through S-based reactivity. Such adducts of dinitrosyl iron were synthesized with two metalloligands, namely, Ni(bme-daco) and V≡O(bme-daco) (bme-daco = bismercaptoethane diazacyclooctane), and, for comparison, an N-heterocyclic carbene, namely, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (Imes), by cleavage of the (μ-I)2[Fe(NO)2]2 dimer of electronic configuration {Fe(NO)2}9 (Enemark-Feltham notation). With Fe(NO)2I as Lewis acid acceptor, 1:1 adducts resulted for both the IMes·Fe(NO)2I, complex 2, and V≡O(bme-daco)·Fe(NO)2I, complex 4. The NiN2S2 demonstrated binding capability at both thiolates, with two Fe(NO)2I addenda positioned transoid across the NiN2S2 square plane, Ni(bme-daco)·2(Fe(NO)2I), complex 3. Enhanced binding ability was realized for the dianionic vanadyl dithiolate complex, [Et4N]2[V≡O(ema)], (ema = N,N'-ethylenebis(2-mercaptoacetamide)), which, unlike the neutral (V≡O)N2S2, demonstrated reactivity with the labile tungsten carbonyl complex, cis-W(CO)4(pip)2, (pip = piperidine), yielding [Et4N]2[V≡O(ema)W(CO)4], complex 1, whose τ(CO) IR values indicated the dianionic vanadyl metalloligand to be of similar donor ability to the neutral NiN2S2 ligands. The solid-state molecular structures of 1-4 were determined by X-ray diffraction analyses. Electron paramagnetic resonance (EPR) measurements characterize the {Fe(NO)2}9 complexes in solution, illustrating superhyperfine coupling via the 127I to the unpaired electron on iron for complex 2. The EPR characterizations of 3 [Ni(bme-daco)·2(Fe(NO)2I)] and 4 [V≡O(bme-daco)·Fe(NO)2I] indicate these complexes are EPR silent, likely due to strong coupling between paramagnetic centers. Within samples of complex 4, individual paramagnetic centers with localized superhyperfine coupling from the 51V and 127I are observed in a 3:1 ratio, respectively. However, spin quantitation reveals that these species represent a minor fraction (<10%) of the total complex and thus likely represent disassociated paramagnetic sites. Computational studies corroborated the EPR assignments as well as the experimentally observed stability/instability of the heterobimetallic DNIC complexes. [ABSTRACT FROM AUTHOR]

Imidazolate-containing {Fe(NO)2}9 molecular squares have been synthesized by oxidative CO displacement from the reduced Fe(CO)2(NO)2 precursor. The structures of complex 1 [(imidazole)Fe(NO)2]4, (Ford, Li, et al.; Chem. Commun.2005, 477-479), 2 [(2-isopropylimidazole)Fe(NO)2]4, and 3 [(benzimidazole)Fe(NO)2]4, as determined by X-ray diffraction analysis, find precise square planes of irons with imidazolates bridging the edges and nitrosyl ligands capping the irons at the corners. The orientation of the imidazolate ligands in each of the complexes results in variations of the overall structures, and molecular recognition features in the available cavities of 1 and 3. Computational studies show multiple low energy structural isomers and confirm that the isomers found in the crystallographic structures arise from intermolecular interactions. EPR and IR spectroscopic studies and electrochemical results suggest that the tetramers remain intact in solution in the presence of weakly coordinating (THF) and noncoordinating (CH2Cl2) solvents. Mössbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)2}10 compounds A ((NHC-iPr)2Fe(NO)2), and C ((NHC-iPr)(CO)Fe(NO)2), and oxidized {Fe(NO)2}9 compounds B ([(NHC-iPr)2Fe(NO)2][BF4]), and D ((NHC-iPr)(SPh)Fe(NO)2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene) demonstrate distinct differences of the isomer shifts and quadrupole splittings between the oxidized and reduced forms. The reduced compounds have smaller positive isomer shifts as compared to the oxidized compounds ascribed to the greater π-backbonding to the NO ligands. Mössbauer data for the tetrameric complexes 1-3 demonstrate larger isomer shifts, most comparable to compound D; all four complexes contain cationic {Fe(NO)2}9 units bound to one anionic ligand and one neutral ligand. At room temperature, the paramagnetic, S = ½ per iron, centers are not coupled. [ABSTRACT FROM AUTHOR]

N-heterocyclic carbenes (NHCs) are shown to be reasonable mimics of imidazole ligands in dinitrosyl iron complexes determined through the synthesis and characterization of a series of {Fe(NO)2}10 and {Fe(NO)2}9 (Enemark-Feltham notation) complexes. Monocarbene complexes (NHC-iPr)(CO)Fe(NO)2 (1) and (NHC-Me)(CO)Fe(NO)2 (2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene and NHC-Me = 1,3-dimethylimidazol-2-ylidene) are formed from CO/L exchange with Fe(CO)2(NO)2. An additional equivalent of NHC results in the bis-carbene complexes (NHC-iPr)2Fe(NO)2 (3) and (NHC-Me)2Fe(NO)2 (4), which can be oxidized to form the {Fe(NO)2}9 bis-carbene complexes 3+ and 4+. Treatment of complexes 1 and 2 with [NO]BF4 results in the formation of uncommon trinitrosyl iron complexes, (NHC-iPr)Fe(NO)3+ (5+) and (NHC-Me)Fe(NO)3+ (6+), respectively. Cleavage of the Roussin's Red "ester" (μ-SPh)2[Fe(NO)2]2 with either NHC or imidazole results in the formation of (NHC-iPr)(PhS)Fe(NO)2 (7) and (Imid-iPr)(PhS)Fe(NO)2 (10) (Imid-iPr = 2-isopropylimidazole). The solid-state molecular structures of complexes 1, 2, 3, 4, 5+, and 7 show that they all have pseudotetrahedral geometry. Infrared spectroscopic data suggest that NHCs are slightly better electron donors than imidazoles; electrochemical data are also consistent with what is expected for typical donor/acceptor abilities of the spectator ligands bound to the Fe(NO)2 unit. Although the monoimidazole complex (Imid-iPr)(CO)Fe(NO)2 (8) was observed via IR spectroscopy, attempts to isolate this complex resulted in the formation of a tetrameric {Fe(NO)2}9 species, [(Imid-iPr)Fe(NO)2]4 (9), a molecular square analogous to the unsubstituted imidazole reported by Li and Wang et al. Preliminary NO-transfer studies demonstrate that the {Fe(NO)2}9 bis-carbene complexes can serve as a source of NO to a target complex, whereas the {Fe(NO)2}10 bis-carbenes are unreactive in the presence of a NO-trapping agent. [ABSTRACT FROM AUTHOR]