Synthetic iron–sulfur clusters of general formulation [FemSqLl]z with core atoms Fe and S and terminal ligands L constitute a family of molecular clusters with remarkably diverse geometrical and electronic structures. Several structure types are also found in proteins. The large majority of research on these clusters has involved elucidation of physical properties. Here, we direct attention to reactivity in the form of cluster conversions in which the FemSq cores of reactants are transformed to new structures, usually of different nuclearity, in overall reactions such as self-assembly and fragment condensation and dissociation. An extensive body of core conversions, many of which have not been recognized as such, are presented including those in biological systems. All structural core types are depicted, and all core conversions are diagrammatically summarized. Clusters containing the cubane-type Fe4S4 core play a central role in conversion chemistry. The core conversion concept tends to reinforce the description of iron–sulfur cores as modular units subject to various covalent bond interactions that lead to different structures.

Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two complexes [MoIVO(bdt)2]2– and [MoVIO2(bdt)2]2– (bdt = benzene-1,2-dithiolate(2−)) that relate to the reduced and oxidized forms of sulfite oxidase (SO). These are compared with those of previously studied dimethyl sulfoxide reductase (DMSOr) models. DFT calculations supported by the data are extended to evaluate the reaction coordinate for oxo transfer to a phosphite ester substrate. Three possible transition states are found with the one at lowest energy, stabilized by a P–S interaction, in good agreement with experimental kinetics data. Comparison of both oxo transfer reactions shows that in DMSOr, where the oxo is transferred from the substrate to the metal ion, the oxo transfer induces electron transfer, while in SO, where the oxo transfer is from the metal site to the substrate, the electron transfer initiates oxo transfer. This difference in reactivity is related to the difference in frontier molecular orbitals (FMO) of the metal–oxo and substrate–oxo bonds. Finally, these experimentally related calculations are extended to oxo transfer by sulfite oxidase. The presence of only one dithiolene at the enzyme active site selectively activates the equatorial oxo for transfer, and allows facile structural reorganization during turnover.

Syntheses of five types of tungsten–iron–sulfur/selenium clusters, namely, incomplete cubanes, single cubanes, edge-bridged double cubanes (EBDCs), PN-type clusters, and double-cuboidal clusters, have been devised using the concept of template-assisted assembly. The template reactant is six-coordinate [(Tp*)WVIS3]1– [Tp* = tris(3,5-dimethylpyrazolyl)hydroborate(1−)], which in the assembly systems organizes Fe2+/3+ and sulfide/selenide into cuboidal [(Tp*)WFe2S3] or cubane [(Tp*)WFe3S3Q] (Q = S, Se) units. With appropriate terminal iron ligation, these units are capable of independent existence or may be transformed into higher-nuclearity species. Selenide is used as a surrogate for sulfide in cluster assembly in order to determine by X-ray structures the position occupied by an external chalcogenide nucleophile or an internal chalcogenide atom in the product clusters. Specific incorporation of selenide is demonstrated by the formation of [WFe3S3Se]2+/3+ cubane cores. Reductive dimerization of the cubane leads to the EBDC core [W2Fe6S6Se2]2+ containing μ4-Se sites. Reaction of these species with HSe– affords the PN-type cores [W2Fe6S6Se3]1+, in which selenide occupies μ6-Se and μ2-Se sites. The reaction of [(Tp*)WS3]1–, FeCl2, and Na2Se yields the double-cuboidal [W2Fe4S6Se3]2+/0 core with μ2-Se and μ4-Se bridges. It is highly probable that in analogous sulfide-only assembly systems, external and internal sulfide reactants occupy corresponding positions in the cluster products. The results further demonstrate the viability of template-assisted cluster synthesis inasmuch as the reduced (Tp*)WS3 unit is present in all of the clusters. Structures, zero-field Mössbauer data, and redox potentials are presented for each cluster type.

Reaction coordinates for oxo transfer from the substrates Me3NO, Me2SO, and Me3PO to the biologically relevant Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)2]− where mdt = 1,2-dimethyl-ethene-1,2-dithiolate(2-), and from Me2SO to the analogous W(IV) complex, have been calculated using density functional theory. In each case, the reaction first proceeds through a transition state (TS1) to an intermediate with substrate weakly bound, followed by a second transition state (TS2) around which breaking of the substrate X–O bond begins. By analyzing the energetic contributions to each barrier, it is shown that the nature of the substrate and metal determines which transition state controls the rate-determining step of the reaction.

The formation and solution properties, including stability in mixed aqueous–Me2SO media, have been investigated for an [Fe4S4]2+ cluster derived from β-cyclodextrin (CD) dithiolate. Clusters of the type [Fe4S4(SAr)4]2– (Ar = Ph, C6H4-3-F) are generated in Me2SO by redox reactions of [Fe4S4(SEt)4]2– with 2 equiv of ArSSAr. An analogous reaction with the intramolecular disulfide of 6A,6D-(3-NHCOC6H4-1-SH)2-6A,6D-dideoxy-β-cyclodextrin (14), whose synthesis is described, affords a completely substituted cluster formulated as [Fe4S4{β-CD-(1,3-NHCOC6H4S)2}2]2– (15). Ligand binding is indicated by a circular dichroism spectrum and also by UV–visible and isotropically shifted 1H NMR spectra and redox behavior convincingly similar to [Fe4S4(SPh)4]2–. One formulation of 15 is a single cluster to which two dithiolates are bound, each in bidentate coordination. With there being no proven precedent for this binding mode, we show that the cluster [Fe4S4(S2-m-xyl)2]2– is a single cubane whose m-xylyldithiolate ligands are bound in a bidentate arrangement. This same structure type was proposed for a cluster formulated as [Fe4S4{β-CD-(1,3-SC6H4S)2}2]2– (16; Kuroda et al. J. Am. Chem. Soc.1988, 110, 4049–4050) and reported to be water-stable. Clusters 15 and 16 are derived from similar ligands differing only in the spacer group between the thiolate binding site and the CD platform. In our search for clusters stable in aqueous or organic–aqueous mixed solvents that are potential candidates for the reconstitution of scaffold proteins implicated in cluster biogenesis, 15 is the most stable cluster that we have thus far encountered under anaerobic conditions in the absence of added ligand.

The planar NNN-pincer complexes [MII(pyN2Me2)(OH)]1– (MII = Ni, Cu) fix CO2 in η1-OCO2H complexes; results for the copper system are described. MnII, FeII, CoII, and ZnII behave differently, forming [MII(pyN2Me2)2]2– with N4O2 coordination. Incorporation of the NiII pincer into binucleating macrocycle 2 containing a triamino MII locus connected by two 1,3-biphenylene groups affords proximal NiII and MII sites for investigation of the synthesis, structure, and reactivity of Ni–X–M bridge units. This ligand structure is taken as a reference for variations in MII atoms and binding sites and bridges X = OH– and CN– to produce additional members of the macrocyclic family with improved properties. Macrocycle 2 with a 22-membered ring is shown to bind MII = Mn, Fe, and Cu with hydroxo bridges. Introduction of the 4-BuiO group (macrocycle 3) improves the solubility of neutral complexes such as those with NiII–OH–CuII and NiII–CN–FeII bridges. Syntheses of macrocycle 5 with a 7-Me-[12]aneSN3 and macrocycle 6 with a 1,8-Me2-[14]aneN4 MII binding site are described together with hydoxo-bridged Ni/Cu and cyano-bridged Ni/Fe complexes. This work was motivated by the presence of a Ni···(HO)–Fe bridge grouping in a reactive state of carbon monoxide dehydrogenase. Attempted decrease in Ni–(OH)–M distances (3.70–3.87 Å) to smaller values observed in the enzyme by use of macrocycle 4 having 1,2-biphenylene connectors led to a mononuclear octahedral NiII complex. Bridge structural units are summarized, and the structures of 14 macrocyclic complexes including 8 with bridges are described.

The similarities and differences in the fundamental coordination chemistry of molybdenum and tungsten mainly in physiological oxidation states MIV–VI are examined in relation to the properties of enzyme sites that catalyze oxygen atom transfer reactions. The comparative aspects of dithiolene complexes, which as synthetic analogues simulate structural and electronic features of these sites, are emphasized. Analogue reaction systems of enzymes are summarized. The mechanism of reduction of the biological substrate Me2SO in one such system as elucidated with density functional calculations is presented as a case study.

The planar complexes [NiII(pyN2R2)(OH)]−, containing a terminal hydroxo group, are readily prepared from N,N′-(2,6-C6H3R2)-2,6-pyridinedicarboxamidate(2-) tridentate pincer ligands (R4N)(OH), and Ni(OTf)2. These complexes react cleanly and completely with carbon dioxide in DMF solution in a process of CO2 fixation with formation of the bicarbonate product complexes [NiII(pyN2R2)(HCO3)]− having η1-OCO2H ligation. Fixation reactions follow second-order kinetics (rate = k2′[NiII–OH][CO2]) with negative activation entropies (−17 to −28 eu). Reactions were monitored by growth and decay of metal-to-ligand charge-transfer (MLCT) bands at 350–450 nm. The rate order R = Me > macro > Et > Pri > Bui > Ph at 298 K (macro = macrocylic pincer ligand) reflects increasing steric hindrance at the reactive site. The inherent highly reactive nature of these complexes follows from k2′ ≈ 106 M–1 s–1 for the R = Me system that is attenuated by only 100-fold in the R = Ph complex. A reaction mechanism is proposed based on computation of the enthalpic reaction profile for the R = Pri system by DFT methods. The R = Et, Pri, and Bui systems display biphasic kinetics in which the initial fast process is followed by a slower first order process currently of uncertain origin.

The cluster [(Tp)MoFe3S4(PEt3)3]1+ containing the cubane-type [MoFe3(μ3-S)4]2+ reduced core undergoes facile ligand substitution reactions at the iron sites leading to an extensive set of mono- and disubstituted species [(Tp)MoFe3S4(PEt3)3–nLn]1–n with L = halide, N3–, PhS–, PhSe–, R3SiO–, and R3SiS– and n = 1 and 2. Structures of 10 members of the set are reported. For two representative clusters, Curie behavior at 2–20 K indicates a spin-quintet ground state. Zero-field Mössbauer spectra consist of two doublets in a 2:1 intensity ratio. 57Fe isomer shifts are consistent with the mean oxidation state Fe32.33+ arising from electron delocalization of the mixed-valence oxidation state description [Mo3+Fe3+Fe2+2]. Reaction of [(Tp)MoFe3S4(PEt3)2Cl] with (Me3Si)2S affords [(Tp)MoFe3S4(PEt3)2(SSiMe3)], a likely first intermediate in the formation of the tricluster compound {[(Tp)MoFe3S4(PEt)2]3S}(BPh4) from the reaction of [(Tp)MoFe3S4(PEt3)3](BPh4) and NaSSiMe3 in tetrahydrofuran (THF). The tricluster consists of three cluster units bound to a central μ3-S atom in a species of overall C3 symmetry. Relatively few clusters in the [MoFe3S4]2+ oxidation state have been prepared compared to the abundance of clusters in the oxidized [MoFe3S4]3+ state. This work is the first comprehensive study of the [MoFe3S4]2+ state, one conspicuous feature of which is its ability to bind hard and soft σ-donors and strong to weak π-acid ligands. (Tp = tris(pyrazolyl)hydroborate(1-))

An extensive series of heterometal-iron-sulfur single cubane-type clusters with core oxidation levels [MFe3S3Q]3+,2+ (M = Mo, W; Q = S, Se) has been prepared by means of a new method of cluster self-assembly. The procedure utilizes the assembly system [(tBu3tach)MVIS3]/FeCl2/Na2Q/NaSR in acetonitrile/THF and affords product clusters in 30–50% yield. The trisulfido precursor acts as a template, binding FeII under reducing conditions and supplying the MS3 unit of the product. The system leads to specific incorporation of a μ3-chalcogenide from an external source (Na2Q) and affords the products [(tBu3tach)MFe3S3QL3]0/1– (L = Cl–, RS–), among which are the first MFe3S3Se clusters prepared. Some 16 clusters have been prepared, 13 of which have been characterized by X-ray structure determinations including the incomplete cubane [(tBu3tach)MoFe2S3Cl2(μ2-SPh)], a possible trapped intermediate in the assembly process. Comparisons of structural and electronic features of clusters differing only in atom Q at one cubane vertex are provided. In comparative pairs of complexes differing only in Q, placement of one selenide atom in the core increases core volumes by about 2% over the Q = S case, sets the order Q = Se > S in Fe-Q bond lengths and Q = S > Se in Fe-Q-Fe bond angles, causes small positive shifts in redox potentials, and has an essentially nil effect on 57Fe isomer shifts. Iron mean oxidation states and charge distributions are assigned to most clusters from isomer shifts. (tBu3tach = 1,3,5-tert-butyl-1,3,5-triazacyclohexane)

Cubane-type clusters [Fe4S4(SR*)4]2– containing chiral thiolate ligands with R* = CH(Me)Ph (1), CH2CH(Me)Et (2), and CH2CH(OH)CH2OH (3) have been prepared by ligand substitution in the reaction systems [Fe4S4(SEt)4]/R*SH (1–3, acetonitrile) and [Fe4S4Cl4]2–/NaSR*(3, Me2SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe4S4L4–n(SR*)n]2– (L = SEt, Cl) with n = 1–4. Clusters 1 and 2 were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (n = 1) and seven diastereomers and their enantiomers (n = 2–4). Reactions were monitored by isotropically shifted 1H NMR spectra in acetonitrile or Me2SO. In systems affording 1 and 2 as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et4N)2[1] afforded 1-[SS(RS)(RS)] in which two sites are disordered because of occupancy of R and S ligands. Similarly, (Et4N)2[2] led to 2-[SSSS], a consequence of spontaneous resolution upon crystallization. The clusters 3-[RRRR] and 3-[SSSS] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with n = 1–4 by appearance of four pairs of diastereotopic SCH2 signals in both acetonitrile and Me2SO reaction systems. Identical spectra were obtained with racemic, R-(−), and S-(+) thiols, indicating that ligand–ligand interactions are too weak to allow detection of diastereomers (e.g., [SSSS] vs [SSRR]). The stability of 3 in Me2SO/H2O media is described.

The stability of cubane-type [Fe4S4(SR)4]2− clusters in mixed organic/aqueous solvents was examined as an initial step in the development of stable water-soluble cluster compounds possibly suitable for reconstitution of scaffold proteins in protein biosynthesis. The research involves primarily spectrophotometric assessment of stability in 20–80% Me2SO/aqueous media (v/v), from which it was found that conventional clusters tend to be stable for up to 12 h in 60% Me2SO but are much less stable at higher aqueous content. α-Cyclodextrin mono- and dithioesters and thiols were prepared as ligand precursors for cluster binding, which was demonstrated by spectroscopic methods. A potentially bidentate cyclodextrin dithiolate was found to be relatively effective for cluster stabilization in 40% Me2SO, suggesting (together with earlier results) that other exceptionally large thiolate ligands may promote cluster stability in aqueous media.The solution stabilities of the clusters [Fe4S4(SR)4]2− in Me2SO/water solvent were examined, including new clusters of α-cyclodextrin thiolates.

Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two Mo bis-dithiolene complexes, [Mo(OSi)(bdt)2]1− and [MoO(OSi)(bdt)2]1−, where OSi = [OSiPh2tBu]1− and bdt = benzene-1,2-dithiolate(2−), that model the Mo(IV) and Mo(VI)═O states of the DMSO reductase family of molybdenum enzymes. These results show that the Mo(IV) complex undergoes metal-based oxidation unlike Mo tris-dithiolene complexes, indicating that the dithiolene ligands are behaving innocently. Experimentally validated calculations have been extended to model the oxo transfer reaction coordinate using dimethylsulfoxide (DMSO) as a substrate. The reaction proceeds through a transition state (TS1) to an intermediate with DMSO weakly bound, followed by a subsequent transition state (TS2) which is the largest barrier of the reaction. The factors that control the energies of these transition states, the nature of the oxo transfer process, and the role of the dithiolene ligand are discussed.

Reactions directed at the synthesis of structural analogues of the active site of molybdenum-containing carbon monoxide dehydrogenase have been investigated utilizing [WO2S(bdt)]2− (1) and [WOS2(bdt)]2− (2) and sterically hindered [Cu(R)L] or [Cu(SSiR′3)2]− as reactants. All successful reactions of 2 afford the binuclear WVI/CuI products [WO(bdt)(μ2-S)2Cu(L)]2−/− with L = carbene (3), Ar*S (4), Ar* (7), SSiR3 (R = Ph (5), Pri (6)). Similarly, [W(bdt)(OSiPh3)S2]− leads to [W(bdt)(OSiPh3)(μ2-S)2Cu(SAr*)]− (8). These complexes, with apical oxo and basal dithiolato and sulfido coordination (excluding 8), terminal thiolate ligation at CuI (4−6, 8), and W-(μ2-S)-Cu bridging, bear a structural resemblance to the enzyme site. Differences include two bridges instead of one and the absence of basal oxo/hydroxo ligation. Complex 8 differs from the others by utilizing apical and basal sulfido ligands in bridge formation. Related reaction systems based on 1 gave 4 in small yield or product mixtures in which the desired monobridged complex [WO2(bdt)(μ2-S)Cu(R)]2− was not detected. Mass spectrometric analysis of the reaction system with L = carbene suggests that any monobridged species forms may converted to the dibridged form by disproportionation. In these experiments, the use of WVI preserves the structural integrity of MoVI, whose analogues of 1 and 2 have not been isolated. (Ar* = 2,6-bis(2,4,6-triisopropylphenyl)phenyl, bdt = benzene-1,2-dithiolate(2-)).

An extensive series of 3:1 site-differentiated cubane-type clusters [Fe4S4(PPri3)3L] (L = Cl−, Br−, I−, RO−, RS−, RSe−) has been prepared in 40−80% yield by two methods: ligand substitution of [Fe4S4(PPri3)4]1+ in tetrahydrofuran (THF)/acetonitrile by reaction with monoanions, and reductive cleavage of ligand substrates (RSSR, RSeSeR, I2) by the all-ferrous clusters [Fe8S8(PPri3)6]/[Fe16S16(PPri3)8] in THF. These neutral clusters are stable and do not undergo ligand redistribution reactions involving charged species in benzene and THF solutions. X-ray structural studies confirm the cubane stereochemistry but with substantial and variable distortions of the [Fe4S4]1+ core from idealized cubic core geometry. Based on Fe−S bond lengths, seven clusters were found to have compressed tetragonal distortions (4 short and 8 long bonds), and the remaining seven display other types of distortions with different combinations of long, short, and intermediate bond lengths. These results further emphasize the facile deformabililty of this core oxidation state previously observed in [Fe4S4(SR)4]3− clusters. The Fe2.25+ mean oxidation state was demonstrated from 57Fe isomer shifts, and the appearance of two quadrupole doublets arises from the spin-coupled |9/2,4,1/2⟩ state. The S = 1/2 ground state was further supported by electron paramagnetic resonance spectra and magnetic susceptibility data.

The recent demonstration that the carbene cluster [Fe4S4(Pri2NHCMe2)4] (9) is an accurate structural and electronic analogue of the fully reduced cluster of the iron protein of Azotobacter vinelandii nitrogenase, including a common S = 4 ground state, raises the issue of the existence and magnetism of other [M4S4L4]z clusters, none of which are known with transition metals other than iron. The system CoCl2/Pri3P/(Me3Si)2S/THF assembles [Co4S4(PPri3)4] (3), which is converted to [Co4S4(Pri2NHCMe2)4] (5) upon reaction with carbene. The clusters support the redox series [3]1−/0/1+ and [5]0/1+/2+; monocations (4, 6) have been isolated by chemical oxidation. Redox potentials and substitution reactions indicate that the carbene is the more effective electron donor to tetrahedral FeII and CoII sites. Clusters 3−6 have the same overall cubane-type geometry as 9. Neutral clusters 3 and 5 have an S = 3 ground state. As with the S = 4 state of 9 with local spins SFe = 2, the septet spin state can be described in terms of the coupling of three parallel and one antiparallel spins SCo = 3/2. The octanuclear clusters [Co8S8(PPri3)6]0,1+ were isolated as minor byproducts of the formation and chemical oxidation of 3. The clusters exhibit a rhomb-bridged noncubane (RBNC) structure, whereas clusters with the Fe8S8 core possess edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M8S8 core in the form of topological isomers whose stability may depend on valence electron count. A conceptual model for the RBNC ↔ EBDC interconversion is presented. (Pri2NHCMe2 = C11H20N2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).

A series of two-coordinate thiolate complexes [Cu(SAr*)L] was synthesized as possible reactants in forming analogues of the active site of Mo/Cu-containing carbon monoxide dehydrogenase. Complexes with L = PPh3 (1), 2,6-lutidine (2), and the N-heterocyclic carbene Pri2NHCMe2 (3) have been prepared by the reaction of [CuCl(PPh3)3] (1) or [CuBr(SMe2)] (2, 3) with ligand L and the exceptionally sterically encumbered ligand Ar*S = 2,6-bis(2,4,6-triisopropylphenyl)benzenethiolate(1-). The reaction of [CuBr(SMe2)] with the thiolate in the absence of added L afforded trinuclear [Cu3(SAr*)2Br] (7). The carbene complex (3) undergoes Cu−C bond insertion with sulfur to form the thiourea complex [Cu(SAr*)(Pri2Me2ImS)] (4). The complexes [Cu(Ar*)L] with L = tetrahydrothiophene (5) and 2,6-lutidine (6) were obtained by reaction of Ar*Li(OEt2) with CuBr/L. These species did not undergo clean Cu−C bond insertion with sulfur transfer agents; the disulfide Ar*SSCH2Ph (9) was isolated from the reaction of 6 with (PhCH2S)2S. The structures of all complexes and 9 were determined. Whereas 5 and 6 are strictly two-coordinate with linear C−Cu−L angles, 1−4 are quasi-two-coordinate because of weak 3d-C(pπ) interactions with a phenyl group, leading to nonlinear structures (S−Cu−L = 135−164 °).

It is well established that the cysteinate-coordinated [Fe4S4] cluster of the iron protein of nitrogenase from Azotobacter vinelandii (Av2) can attain the all-ferrous core oxidation state. Mössbauer and electron paramagnetic resonance (EPR) studies have shown that the all-ferrous cluster has a ground-state spin S = 4 and an effective 3:1 site symmetry in the spin structure and 57Fe quadrupole interactions. Recently, Deng and Holm reported the synthesis of [Fe4S4(Pri2NHCMe2)4],2 (1; Pri2NHCMe2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physicochemical studies. Mössbauer and EPR studies of 1, reported here, reveal that the electronic structure of this complex is strikingly similar to that of the protein-bound cluster, suggesting that the ground-state spin and the 3:1 site ratio are consequences of spontaneous distortions of the cluster core. To gain insight into the origin of the peculiar ground state of the all-ferrous clusters in 1 and Av2, we have studied a theoretical model that is based on a Heisenberg−Dirac−van Vleck Hamiltonian whose exchange-coupling constants are a function of the Fe−Fe distances. By combining the exchange energies with the elastic deformation energies in the harmonic approximation, we obtain for a T2 distortion a minimum with spin S = 4 and a C3v core structure in which one iron is unique and three irons are equivalent. This minimum has all of the spectroscopic and structural characteristics of the all-ferrous clusters of 1 and Av2. Our analysis maps the unique spectroscopic iron site to a specific site in the X-ray structure of the [Fe4S4]0 core both in complex 1 and in Av2.

Pyridine-2,6-dimethanethiolate and pyridine-2,6-dithiocarboxylate form sparingly soluble NiII pincer complexes formulated as [Ni(pdmt)]2 and [Ni(pdtc)]2, respectively, with two Ni-(μ2-S)-Ni bridges. In acetonitrile reaction systems, the latter undergoes the facile bridge cleavage reactions [Ni(pdtc)]2 + 2L0,− → 2[Ni(pdtc)L]0,− with an extensive set of nucleophiles to afford planar mononuclear products with L− = halide, CN, Me3SiO−, RS− and L0 = Et3P and a N-heterocyclic carbene. [Ni(pdmt)]2 is considerably less reactive toward bridge disruption. Cleavage products support several reactions of interest leading to other mononuclear species and to di- and trinuclear complexes. [Ni(pdtc)(OSiMe3)]1− deprotonates acetonitrile and acetone to form [Ni(pdtc)(CH2R)]1− (R = CN, COMe). Reaction of [Ni(pdtc)SEt]1− with FeII yields the thiolate-bridged dimer {[Ni(pdtc)]2(SEt)}1−. Refluxing an acetonitrile solution of [Ni(pdtc)SH]1− in air results in formation of trinuclear [Ni(pdtc)]3S]2− containing the rare unsupported Ni3(μ3-S) bridge core. Reaction of [Ni(pdtc)CN]1− with [Fe(Me6tren)(OTf)]1+ forms the complex [Ni(pdtc)CNFe(Me6tren)]1+, the only example of a single Ni−C≡N−Fe bridge within a molecule. Structures of the various types of reaction products are presented. This work demonstrates the potential utility of bridge cleavage of polynuclear NiII thiolates, an extensive family of compounds, to produce mononuclear products.

The compound Fe[N(SiMe3)2]2 is shown to be a useful precursor to dinuclear and trinuclear iron−sulfur−silylamido complexes by reaction with thiols or thiols and sulfur in tetrahydrofuran (THF) or toluene. Reaction with 1 equiv of p-tolylthiol affords [Fe2(μ2-S-p-tol)2(N(SiMe3)2)2(THF)2] (1); with 0.5 equiv of adamantane-1-thiol, [Fe2(μ2-S-1-Ad)(μ2-N(SiMe3)2)(N(SiMe3)2)2] (2) is formed. The clusters [Fe3(μ3-Q)(μ2-SR)3(N(SiMe3)2)3] are available by three methods: (i) self-assembly in the systems Fe[N(SiMe3)2]2/RSH/S or Se [Q = S, R = p-tol (3) and 1-Ad (5)]; (ii) reaction of 1 with Q = S or Se to yield 3 or [Fe3Se(S-p-tol)3(N(SiMe3)2)3] (4); (iii) reaction of 2 with 1-AdSH and S to give 5. Structures of 1−5 are presented. Complexes 1 and 2 contain planar Fe2S2 and Fe2SN rhombs. Clusters 3−5 contain a mixed-valence Fe3Q(SR)3 core with trigonal (cuboidal) geometry. Of known iron−sulfur clusters, these most closely resemble previously reported [Fe3S(S−R−S)3]2− stabilized by bidentate thiolate ligands. Complexes 1−5, together with a small set of recently described clusters of nuclearities 2, 4, and 8, constitute a new class of iron−sulfur−silylamido clusters. Complexes 3−5 constitute a new structure type of mixed-valence iron−sulfur clusters.

Both vanadium and molybdenum cofactor clusters are found in nitrogenase. In biomimetic research, many fewer heterometal MFe3S4 cubane-type clusters have been synthesized with M = V than with M = Mo because of the well-established structural relationship of the latter to the molybdenum coordination unit in the enzyme. In this work, a series of single cubane and edge-bridged double cubane clusters containing the cores [VFe3(μ3-S)4]2+ and [V2Fe6(μ3-S)6(μ4-S)2]2+ have been prepared by ligand substitution of the phosphine clusters [(Tp)VFe3S4(PEt3)3]1+ and [(Tp)2V2Fe6S8(PEt3)4]. The single cubanes [(Tp)VFe3S4L3]2− and double cubanes [(Tp)2V2Fe6S8L4]4− (L = F−, N3−, CN−, PhS−) are shown by X-ray structures to have trigonal symmetry and centrosymmetry, respectively. Single cubanes form the three-member electron transfer series [(Tp)VFe3S4L3]3−,2−,1−. The ligand dependence of redox potentials and electron distribution in cluster cores as sensed by 57Fe isomer shifts (δ) have been determined. Comparison of these results with those previously determined for the analogous molybdenum clusters (Pesavento, Berlinguette, and Holm Inorg. Chem. 2007, 46, 510) allows detection of the influence of heterometal M on the properties. At constant M and variable L, redox potentials are lowest for π-donor ligands and largest for cyanide and relate approximately with decreasing ferrous character in clusters with constant charge z = 2−. At constant L and z and variable M, EV > EMo and δavV < δavMo, demonstrating that M = Mo clusters are more readily oxidized and suggesting a qualitative relation between lower potentials (greater ease of oxidation) and ferrous character.

The computational prediction of gas phase enthalpy (neutral substrates) and aqueous free energy (anion substrates) changes has been evaluated for the oxygen atom transfer reaction X + 1/2O2 → XO. Several density functionals (SVWN, BP86, B3LYP) at double- and triple-ζ levels were surveyed, along with one composite ab initio method (G3(MP2)). Results are presented for extensive main group test sets for which experimental thermochemistry is available. In addition, several minimal reaction couples of the type [MIVOL2]/[MVIO2L2] (M = Mo, W) have been examined. Overall, the results suggest a computational approach to the energetics of oxo transfer is feasible, potentially affording an expanded oxo transfer reactivity scale.The quantum chemical calculation of oxo transfer reaction energies in gas and aqueous phases has been evaluated by density functional and composite ab initio methods. Computational accuracy is surveyed for reactions involving neutral and anionic main group (2p–5p) species, as well as select neutral molybdenum and tungsten complexes.

Molybdenum- or tungsten-containing enzymes catalyze oxygen atom transfer reactions involved in carbon, sulfur, or nitrogen metabolism. It has been observed that reduction potentials and oxygen atom transfer rates are different for W relative to Mo enzymes and the isostructural Mo/W complexes. Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations on [MoVO(bdt)2]− and [WVO(bdt)2]−, where bdt = benzene-1,2-dithiolate(2-), have been used to determine that the energies of the half-filled redox-active orbital, and thus the reduction potentials and MO bond strengths, are different for these complexes due to relativistic effects in the W sites.

The scope of formation and structures of tungsten–iron–sulfur clusters has been explored using reactions based on [(Tp*)WS3]1− (1) as the ultimate precursor. The reaction system 1/FeCl2/NaSEt/S affords the cubane cluster [(Tp*)WFe3S4Cl3]1− (2), which with NaSEt is converted to [(Tp*)WFe3S4(SEt)3]1− (3).Clusters 2 and 3 contain the cubane [WFe3(μ3-S)4]3+ core.Complex 1 with FeCl2/NaSEt forms [(Tp*)WFe2S3Cl2(SEt)]1− (4) with the cuboidal [WFe2(μ2-S)2(μ3-S)(μ2-SR)]2+ core.Treatment of 2 with excess Et3P yields the edge-bridged double [(Tp*)2W2Fe6S8(PEt3)4] (5) with the [W2Fe6(μ3-S)6(μ4-S)2] core. Reaction of 2 with excess Et3P/BH4-/HS- leads a mixture of products, from which [(Tp*)2W2Fe5S9Na(SH)(MeCN)]3−(6) was identified.This cluster, as closely related [(Tp)2Mo2Fe6S9(SH)2]3−, exhibits a core topology [W2Fe5Na(μ2-S)2(μ3-S)6(μ6-S)] very similar to the PN cluster of nitrogenase. All reactions were carried out in acetonitrile. The structures of 2–6 were established crystallographically as Et4N+ salts. In the cubane series, substitution of tungsten for molybdenum decreases the [MFe3S4]3+/2+ redox potential by ca. 0.20 V but has a negligible effect on electron distribution. This work expands the small set of previously known weak-field W–Fe–S clusters, demonstrates the existence of tungsten-containing edge-bridged double cubanes and clusters with the PN core topology, and introduces a new cuboidal core structure as found in 4 (Tp = hydrotris(pyrazolyl)borate, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate).A series of tungsten–iron–sulfur clusters have been prepared. The reaction of [(Tp*)WS3]− with FeCl2/NaSEt/S affords cubane type [(Tp*)WFe3S4Cl3]−, which can be converted to [(Tp*)WFe3S4(SEt)3]− by NaSEt. A cuboidal structure [(Tp*)WFe2S3Cl2(SEt)]− is prepared and characterized. An edge-bridged tungsten–iron–sulfur double cubane [(Tp*)2W2Fe6S8(PEt3)4] is also prepared and characterized. In addition, [(Tp*)2W2Fe5NaS9(SH)(MeCN)]3− is found to show the topology of the PN cluster of nitrogenase.

The synthetic cubane-type iron–sulfur clusters [Fe4S4(SR)4] z form a four-member electron transfer series (z = 3–, 2–, 1–, and 0), all members of which except that with z = 0 have been isolated and characterized. They serve as accurate analogues of protein-bound [Fe4S4(SCys)4] z redox centers, which, in terms of core oxidation states, exhibit the redox couples [Fe4S4]3+/2+ and [Fe4S4]2+/1+. Clusters with the all-ferrous core [Fe4S4]0 have never been isolated because of their oxidative sensitivity. Recent work on the Fe protein of Azotobacter vinelandii nitrogenase has demonstrated the formation of the all-ferrous state upon reaction with a strong reductant. Treatment of the cyanide cluster [Fe4S4(CN)4]3– with K[Ph2CO] in acetonitrile/tetrahydrofuran affords the all-ferrous cluster [Fe4S4(CN)4]4–, isolated as the Bu4N+ salt. The x-ray structure demonstrates retention of a cubane-type structure with idealized D 2 d symmetry. The Mössbauer spectrum unambiguously demonstrates the [Fe4S4]0 oxidation state. Bond distances, core volumes, 57Fe isomer shifts, and visible absorption spectra make evident the high degree of structural and electronic similarity with the fully reduced Fe protein. The attribute of cyanide ligation causes positive [Fe4S4]2+/1+ and [Fe4S4]1+/0 redox potential shifts, facilitating the initial isolation of an analogue of the [Fe4S4]0 protein site.

There exist a limited but growing number of biological metal centers whose properties lie conspicuously outside the realm of known inorganic chemistry. The synthetic analogue approach, broadly directed, offers a powerful exploratory tool that can define intrinsic chemical possibilities for these sites while simultaneously expanding the frontiers of fundamental inorganic chemistry. This speculative application of analogue study is exemplified here in the evolution of synthetic efforts inspired by the cluster chemistry of biological nitrogen fixation.

A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-bridged Ni···Fe locus where substrate is bound and transformed in the reversible reaction CO + H2O ⇌ CO2 + 2H+ + 2e−. A similar structure has been sought in this work. Mononuclear planar NiII complexes [Ni(pyN2Me2)L]1− (pyN2Me2 = bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate(2−)) derived from a NNN pincer ligand have been prepared including L = OH− (1) and CN− (7). Complex 1 reacts with ethyl formate and CO2 to form unidentate L = HCO2− (5) and HCO3− (6) products. A binucleating macrocycle was prepared which specifically binds NiII at a NNN pincer site and five-coordinate FeII at a triamine site. The NiII macrocyle forms hydroxo (14) and cyanide complexes (15) analogous to 1 and 7. Reaction of 14 with FeCl2 alone and with ethyl formate and 15 with FeCl2 affords molecules with the NiII−L−FeII bridge unit in which L = μ2:η1-OH− (17) and μ2:η2-HCO2− (18) and −CN− (19). All bridges are nonlinear (17, 140.0°; 18, M−O−C 135.9° (Ni), 120.2° (Fe); 19, Ni−C−N 170.3°, Fe−N−C 141.8°) with Ni···Fe separations of 3.7−4.8 Å. The NiIIFeII complexes, lacking appropriate Ni−Fe−S cluster structures, are not site analogues, but their synthesis and reactivity provide the first demonstration that molecular NiII···FeII sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.