Mössbauer, electron paramagnetic resonance, and theoretical studies of a carbene-based all-ferrous Fe4S4 cluster: electronic origin and structural identification of the unique spectroscopic site

Catalytic Activity of the Archetype from Group 4 of the FTR-like Ferredoxin:Thioredoxin Reductase Family Is Regulated by Unique S = 7/2 and S = 1/2 [4Fe-4S] Clusters

Thioredoxin reductases (TrxR) activate thioredoxins (Trx) that regulate the activity of diverse target proteins essential to prokaryotic and eukaryotic life. However, very little is understood of TrxR/Trx systems and redox control in methanogenic microbes from the domain Archaea (methanogens), for which genomes are abundant with annotations for ferredoxin:thioredoxin reductases [Fdx/thioredoxin reductase (FTR)] from group 4 of the widespread FTR-like family. Only two from the FTR-like family are characterized: the plant-type FTR from group 1 and FDR from group 6. Herein, the group 4 archetype (AFTR) from Methanosarcina acetivorans was characterized to advance understanding of the family and TrxR/Trx systems in methanogens. The modeled structure of AFTR, together with EPR and Mössbauer spectroscopies, supports a catalytic mechanism similar to plant-type FTR and FDR, albeit with important exceptions. EPR spectroscopy of reduced AFTR identified a transient [4Fe-4S]1+ cluster exhibiting a mixture of S = 7/2 and typical S = 1/2 signals, although rare for proteins containing [4Fe-4S] clusters, it is most likely the on-pathway intermediate in the disulfide reduction. Furthermore, an active site histidine equivalent to residues essential for the activity of plant-type FTR and FDR was found dispensable for AFTR. Finally, a unique thioredoxin system was reconstituted from AFTR, ferredoxin, and Trx2 from M. acetivorans, for which specialized target proteins were identified that are essential for growth and other diverse metabolisms.

An Iron-Sulfur Cluster with a Highly Pyramidalized Three-Coordinate Iron Center and a Negligible Affinity for Dinitrogen

Attempts to generate open coordination sites for N2 binding at synthetic Fe-S clusters often instead result in cluster oligomerization. Recently, it was shown for Mo-Fe-S clusters that such oligomerization reactions can be prevented through the use of sterically protective supporting ligands, thereby enabling N2 complex formation. Here, this strategy is extended to Fe-only Fe-S clusters. One-electron reduction of (IMes)3Fe4S4Cl (IMes = 1,3-dimesitylimidazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes)6Fe8S8, which loses two IMes ligands to form the face-bridged double-cubane, (IMes)4Fe8S8. The finding that the three supporting IMes ligands do not confer sufficient protection to curtail cluster oligomerization prompted the design of a new N-heterocyclic carbene, SIArMe,iPr (1,3-bis(3,5-diisopropyl-2,6-dimethylphenyl)-2-imidazolidinylidene; abbreviated as SIAr), that features bulky groups strategically placed in remote positions. When the reduction of (SIAr)3Fe4S4Cl or [(SIAr)3Fe4S4(THF)]+ is conducted in the presence of SIAr, the formation of (SIAr)4Fe8S8 is indeed suppressed, permitting characterization of the reduced [Fe4S4]0 product. Surprisingly, rather than being an N2 complex, the product is simply (SIAr)3Fe4S4: a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr)3Fe4S4 does not coordinate N2 to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr)3Fe4S4(CO). This finding demonstates that the binding pocket at the unique Fe is not too small for N2; instead, the exceptionally weak affinity for N2 can be attributed to weak Fe-N2 bonding. The differences in the N2 coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.

Enzymatic Fischer-Tropsch-Type Reactions

The Fischer-Tropsch (FT) process converts a mixture of CO and H2 into liquid hydrocarbons as a major component of the gas-to-liquid technology for the production of synthetic fuels. Contrary to the energy-demanding chemical FT process, the enzymatic FT-type reactions catalyzed by nitrogenase enzymes, their metalloclusters, and synthetic mimics utilize H+ and e- as the reducing equivalents to reduce CO, CO2, and CN- into hydrocarbons under ambient conditions. The C1 chemistry exemplified by these FT-type reactions is underscored by the structural and electronic properties of the nitrogenase-associated metallocenters, and recent studies have pointed to the potential relevance of this reactivity to nitrogenase mechanism, prebiotic chemistry, and biotechnological applications. This review will provide an overview of the features of nitrogenase enzymes and associated metalloclusters, followed by a detailed discussion of the activities of various nitrogenase-derived FT systems and plausible mechanisms of the enzymatic FT reactions, highlighting the versatility of this unique reactivity while providing perspectives onto its mechanistic, evolutionary, and biotechnological implications.

A complete biomimetic iron-sulfur cubane redox series

Synthetic iron-sulfur cubanes are models for biological cofactors, which are essential to delineate oxidation states in the more complex enzymatic systems. However, a complete series of [Fe₄S₄]ⁿ complexes spanning all redox states accessible by 1-electron transformations of the individual iron atoms (n = 0-4+) has never been prepared, deterring the methodical comparison of structure and spectroscopic signature. Here, we demonstrate that the use of a bulky arylthiolate ligand promoting the encapsulation of alkali-metal cations in the vicinity of the cubane enables the synthesis of such a series. Characterization by EPR, ⁵⁷Fe Mössbauer spectroscopy, UV-visible electronic absorption, variable-temperature X-ray diffraction analysis, and cyclic voltammetry reveals key trends for the geometry of the Fe₄S₄ core as well as for the Mössbauer isomer shift, which both correlate systematically with oxidation state. Furthermore, we confirm the S = 4 electronic ground state of the most reduced member of the series, [Fe₄S₄]⁰, and provide electrochemical evidence that it is accessible within 0.82 V from the [Fe₄S₄]²⁺ state, highlighting its relevance as a mimic of the nitrogenase iron protein cluster.

Reversible Alkyl-Group Migration between Iron and Sulfur in [Fe4S4] Clusters

Synthetic [Fe4S4] clusters with Fe-R groups (R = alkyl/benzyl) are shown to release organic radicals on an [Fe4S4]3+-R/[Fe4S4]2+ redox couple, the same that has been proposed for a radical-generating intermediate in the superfamily of radical S-adenosyl-l-methionine (SAM) enzymes. In attempts to trap the immediate precursor to radical generation, a species in which the alkyl group has migrated from Fe to S is instead isolated. This S-alkylated cluster is a structurally faithful model of intermediates proposed in a variety of functionally diverse S transferase enzymes and features an "[Fe4S4]+-like" core that exists as a physical mixture of S = 1/2 and 7/2 states. The latter corresponds to an unusual, valence-localized electronic structure as indicated by distortions in its geometric structure and supported by computational analysis. Fe-to-S alkyl group migration is (electro)chemically reversible, and the preference for Fe vs S alkylation is dictated by the redox state of the cluster. These findings link the organoiron and organosulfur chemistry of Fe-S clusters and are discussed in the context of metalloenzymes that are proposed to make and break Fe-S and/or C-S bonds during catalysis.

Evidence for Low-Valent Electronic Configurations in Iron-Sulfur Clusters

Although biological iron-sulfur (Fe-S) clusters perform some of the most difficult redox reactions in nature, they are thought to be composed exclusively of Fe²⁺ and Fe³⁺ ions, as well as mixed-valent pairs with average oxidation states of Fe^2.5+. We herein show that Fe-S clusters formally composed of these valences can access a wider range of electronic configurations─in particular, those featuring low-valent Fe¹⁺ centers. We demonstrate that CO binding to a synthetic [Fe₄S₄]⁰ cluster supported by N-heterocyclic carbene ligands induces the generation of Fe¹⁺ centers via intracluster electron transfer, wherein a neighboring pair of Fe²⁺ sites reduces the CO-bound site to a low-valent Fe¹⁺ state. Similarly, CO binding to an [Fe₄S₄]⁺ cluster induces electron delocalization with a neighboring Fe site to form a mixed-valent Fe^1.5+Fe^2.5+ pair in which the CO-bound site adopts partial low-valent character. These low-valent configurations engender remarkable C-O bond activation without having to traverse highly negative and physiologically inaccessible [Fe₄S₄]⁰/[Fe₄S₄]^- redox couples.

Resolution of Low-Energy States in Spin-Exchange Transition-Metal Clusters: Case Study of Singlet States in [Fe(III)4S4] Cubanes

Polynuclear transition-metal (PNTM) clusters owe their catalytic activity to numerous energetically low-lying spin states and stable oxidation states. The characterization of their electronic structure represents one of the greatest challenges of modern chemistry. We propose a theoretical framework that enables the resolution of targeted electronic states with ease and apply it to two [Fe(III)₄S₄] cubanes. Through direct access to their many-body wave functions, we identify important correlation mechanisms and their interplay with the geometrical distortions observed in these clusters, which are core properties in understanding their catalytic activity. The simulated magnetic coupling constants predicted by our strategy allow us to make qualitative connections between spin interactions and geometrical distortions, demonstrating its predictive power. Moreover, despite its simplicity, the strategy provides magnetic coupling constants in good agreement with the available experimental ones. The complexes are intrinsically frustrated anti-ferromagnets, and the obtained spin structures together with the geometrical distortions represent two possible ways to release spin frustration (spin-driven Jahn–Teller distortion). Our paradigm provides a simple, yet rigorous, route to uncover the electronic structure of PNTM clusters and may be applied to a wide variety of such clusters.

Electron Transfer in Nitrogenase

Nitrogenase is the only enzyme capable of reducing N2 to NH3. This challenging reaction requires the coordinated transfer of multiple electrons from the reductase, Fe-protein, to the catalytic component, MoFe-protein, in an ATP-dependent fashion. In the last two decades, there have been significant advances in our understanding of how nitrogenase orchestrates electron transfer (ET) from the Fe-protein to the catalytic site of MoFe-protein and how energy from ATP hydrolysis transduces the ET processes. In this review, we summarize these advances, with focus on the structural and thermodynamic redox properties of nitrogenase component proteins and their complexes, as well as on new insights regarding the mechanism of ET reactions during catalysis and how they are coupled to ATP hydrolysis. We also discuss recently developed chemical, photochemical, and electrochemical methods for uncoupling substrate reduction from ATP hydrolysis, which may provide new avenues for studying the catalytic mechanism of nitrogenase.

Reactivity, Mechanism, and Assembly of the Alternative Nitrogenases

Biological nitrogen fixation is catalyzed by the enzyme nitrogenase, which facilitates the cleavage of the relatively inert triple bond of N2. Nitrogenase is most commonly associated with the molybdenum-iron cofactor called FeMoco or the M-cluster, and it has been the subject of extensive structural and spectroscopic characterization over the past 60 years. In the late 1980s and early 1990s, two "alternative nitrogenase" systems were discovered, isolated, and found to incorporate V or Fe in place of Mo. These systems are regulated by separate gene clusters; however, there is a high degree of structural and functional similarity between each nitrogenase. Limited studies with the V- and Fe-nitrogenases initially demonstrated that these enzymes were analogously active as the Mo-nitrogenase, but more recent investigations have found capabilities that are unique to the alternative systems. In this review, we will discuss the reactivity, biosynthetic, and mechanistic proposals for the alternative nitrogenases as well as their electronic and structural properties in comparison to the well-characterized Mo-dependent system. Studies over the past 10 years have been particularly fruitful, though key aspects about V- and Fe-nitrogenases remain unexplored.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships

We report the synthesis, characterization, and reactivity of [LFe₃O(^RArIm)₃Fe][OTf]₂, the first Hammett series of a site-differentiated cluster. The cluster reduction potentials and CO stretching frequencies shift as expected on the basis of the electronic properties of the ligand: electron-donating substituents result in more reducing clusters and weaker C-O bonds. However, unusual trends in the energetics of their two sequential CO binding events with the substituent σ_p parameters are observed. Specifically, introduction of electron-donating substituents suppresses the first CO binding event (ΔΔH by as much as 7.9 kcal mol^-1) but enhances the second (ΔΔH by as much as 1.9 kcal mol^-1). X-ray crystallography, including multiple-wavelength anomalous diffraction, Mössbauer spectroscopy, and SQUID magnetometry, reveal that these substituent effects result from changes in the energetic penalty associated with electronic redistribution within the cluster, which occurs during the CO binding event.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

The Fe Protein: An Unsung Hero of Nitrogenase

Although the nitrogen-fixing enzyme nitrogenase critically requires both a reductase component (Fe protein) and a catalytic component, considerably more work has focused on the latter species. Properties of the catalytic component, which contains two highly complex metallocofactors and catalyzes the reduction of N2 into ammonia, understandably making it the “star” of nitrogenase. However, as its obligate redox partner, the Fe protein is a workhorse with multiple supporting roles in both cofactor maturation and catalysis. In particular, the nitrogenase Fe protein utilizes nucleotide binding and hydrolysis in concert with electron transfer to accomplish several tasks of critical importance. Aside from the ATP-coupled transfer of electrons to the catalytic component during substrate reduction, the Fe protein also functions in a maturase and insertase capacity to facilitate the biosynthesis of the two-catalytic component metallocofactors: fusion of the [Fe8S7] P-cluster and insertion of Mo and homocitrate to form the matured [(homocitrate)MoFe7S9C] M-cluster. These and key structural-functional relationships of the indispensable Fe protein and its complex with the catalytic component will be covered in this review.

Structural, Spectroscopic, and Theoretical Investigation of a T-Shaped [Fe3(μ3-O)] Cluster

The synthesis, X-ray crystal and electronic structures of [Fe₃(μ₃-O)(mpmae)₂(OAc)₂ Cl₃], 1, where mpmae-H = 2-(N-methyl-N-((pyridine-2-yl)methyl)amino)ethanol, are described. This cluster comprises three high-spin ferric ions and exhibits a T-shaped site topology. Variable-frequency electron paramagnetic resonance measurements performed on single crystals of 1 demonstrate a total spin S_T = 5/2 ground state, characterized by a small, negative, and nearly axial zero-field splitting tensor D = -0.49 cm^-1, E/D ≈ 0.055. Analysis of magnetic susceptibility, magnetization, and magneto-structural correlations further corroborate the presence of a sextet ground-spin state. The observed ground state originates from the strong anti-ferromagnetic interaction of two iron(III) spins, with J = 115(5) cm^-1, that, in turn, are only weakly coupled to the spin of the third site, with j = 7(1) cm^-1. These exchange interactions lead to a ground state with magnetic properties that are essentially entirely determined by the weakly coupled site. The contributions of the individual spins to the total ground state of the cluster were monitored using variable-field ⁵⁷Fe Mössbauer spectroscopy. Field-dependent spectra reveal that, while one of the iron sites exhibits a large negative internal field, typical of ferric ions, the other two sites exhibit small, but not null, negative and positive internal fields. A theoretical analysis reveals that these small internal fields originate from the mixing of the lowest S_T = 5/2 excited state into the ground state which, in turn, is induced by a minute structural distortion.

A Low-Valent Iron Imido Heterocubane Cluster: Reversible Electron Transfer and Catalysis of Selective C-C Couplings

Enzymes and cofactors with iron-sulfur heterocubane core structures, [Fe4 S4 ], are often found in nature as electron transfer reagents in fundamental catalytic transformations. An artificial heterocubane with a [Fe4 N4 ] core is reported that can reversibly store up to four electrons at very negative potentials. The neutral [Fe4 N4 ] and the singly reduced low-valent [Fe4 N4 ](-) heterocubanes were isolated and fully characterized. The low-valent species bears one unpaired electron, which is localized predominantly at one iron center in the electronic ground state but fluctuates with increasing temperatures. The electrons stored or released by the [Fe4 N4 ]/[Fe4 N4 ](-) redox couple can be used in reductive or oxidative CC couplings and even allow catalytic one-pot reactions, which show a remarkably enhanced selectivity in the presence of the [Fe4 N4 ] heterocubanes.

Protein dynamics and the all-ferrous [Fe4 S4 ] cluster in the nitrogenase iron protein

In nitrogen fixation by Azotobacter vinelandii nitrogenase, the iron protein (FeP) binds to and subsequently transfers electrons to the molybdenum–FeP, which contains the nitrogen fixation site, along with hydrolysis of two ATPs. However, the nature of the reduced state cluster is not completely clear. While reduced FeP is generally thought to contain an [Fe₄S₄]¹⁺ cluster, evidence also exists for an all‐ferrous [Fe₄S₄]⁰ cluster. Since the former indicates a single electron is transferred per two ATPs hydrolyzed while the latter indicates two electrons could be transferred per two ATPs hydrolyzed, an all‐ferrous [Fe₄S₄]⁰ cluster in FeP is potenially two times more efficient. However, the 1+/0 reduction potential has been measured in the protein at both 460 and 790 mV, causing the biological significance to be questioned. Here, “density functional theory plus Poisson Boltzmann” calculations show that cluster movement relative to the protein surface observed in the crystal structures could account for both measured values. In addition, elastic network mode analysis indicates that such movement occurs in low frequency vibrations of the protein, implying protein dynamics might lead to variations in reduction potential. Furthermore, the different reductants used in the conflicting measurements of the reduction potential could be differentially affecting the protein dynamics. Moreover, even if the all‐ferrous cluster is not the biologically relevant cluster, mutagenesis to stabilize the conformation with the more exposed cluster may be useful for bioengineering more efficient enzymes.

Low-spin pseudotetrahedral iron(I) sites in Fe₂(μ-S) complexes

Fe(I) centers in iron-sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron-sulfur clusters. A series of three diiron [(L3Fe)2(μ-S)] complexes that were isolated and characterized in the low-valent oxidation states Fe(II)-S-Fe(II), Fe(II)-S-Fe(I), and Fe(I)-S-Fe(I) is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe2(μ-S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low-spin S=1/2 states upon reduction from Fe(II) to Fe(I). The possibility of accessing low-spin, pseudotetrahedral Fe(I) sites compatible with S(2-) as a ligand was previously unknown.

Characterization of [4Fe-4S] cluster vibrations and structure in nitrogenase Fe protein at three oxidation levels via combined NRVS, EXAFS, and DFT analyses

Azotobacter vinelandii nitrogenase Fe protein (Av2) provides a rare opportunity to investigate a [4Fe-4S] cluster at three oxidation levels in the same protein environment. Here, we report the structural and vibrational changes of this cluster upon reduction using a combination of NRVS and EXAFS spectroscopies and DFT calculations. Key to this work is the synergy between these three techniques as each generates highly complementary information and their analytical methodologies are interdependent. Importantly, the spectroscopic samples contained no glassing agents. NRVS and DFT reveal a systematic 10-30 cm(-1) decrease in Fe-S stretching frequencies with each added electron. The "oxidized" [4Fe-4S](2+) state spectrum is consistent with and extends previous resonance Raman spectra. For the "reduced" [4Fe-4S](1+) state in Fe protein, and for any "all-ferrous" [4Fe-4S](0) cluster, these NRVS spectra are the first available vibrational data. NRVS simulations also allow estimation of the vibrational disorder for Fe-S and Fe-Fe distances, constraining the EXAFS analysis and allowing structural disorder to be estimated. For oxidized Av2, EXAFS and DFT indicate nearly equal Fe-Fe distances, while addition of one electron decreases the cluster symmetry. However, addition of the second electron to form the all-ferrous state induces significant structural change. EXAFS data recorded to k = 21 Å(-1) indicates a 1:1 ratio of Fe-Fe interactions at 2.56 Å and 2.75 Å, a result consistent with DFT. Broken symmetry (BS) DFT rationalizes the interplay between redox state and the Fe-S and Fe-Fe distances as predominantly spin-dependent behavior inherent to the [4Fe-4S] cluster and perturbed by the Av2 protein environment.

Trigonal Mn3 and Co3 clusters supported by weak-field ligands: a structural, spectroscopic, magnetic, and computational investigation into the correlation of molecular and electronic structure

Transamination of divalent transition metal starting materials (M(2)(N(SiMe(3))(2))(4), M = Mn, Co) with hexadentate ligand platforms (R)LH(6) ((R)LH(6) = MeC(CH(2)NPh-o-NR)(3) where R = H, Ph, Mes (Mes = Mesityl)) or (H,Cy)LH(6) = 1,3,5-C(6)H(9)(NHPh-o-NH(2))(3) with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type ((R)L)Mn(3)(py)(3) and ((R)L)Co(3)(PMe(2)R')(3) (R' = Me, Ph). While the sterically less encumbered ligand varieties, (H)L or (Ph)L, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand ((Mes)L) engenders local tetrahedral coordination. Both the neutral Mn(3) and Co(3) clusters feature S = (1)/(2) ground states, as determined by direct current (dc) magnetometry, (1)H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn(3) clusters, the long internuclear Mn-Mn separations suggest minimal direct metal-metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = -16.8 to -42 cm(-1). Conversely, the short Co-Co interatomic distances suggest a significant degree of direct metal-metal orbital overlap, akin to the related Fe(3) clusters. With the Co(3) series, the S = (1)/(2) ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co(3) clusters affords diamagnetic cationic clusters of the type [((R)L)Co(3)(PMe(2)R)(3)](+). Density functional theory (DFT) calculations on the neutral (S = (1)/(2)) and cationic (S = 0) Co(3) clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.

Density functional theory study of an all ferrous 4Fe-4S cluster

The all-ferrous, carbene-capped Fe(4)S(4) cluster, synthesized by Deng and Holm (DH complex), has been studied with density functional theory (DFT). The geometry of the complex was optimized for several electronic configurations. The lowest energy was obtained for the broken-symmetry (BS) configuration derived from the ferromagnetic state by reversing the spin projection of one of the high spin (S(i) = 2) irons. The optimized geometry of the latter configuration contains one unique and three equivalent iron sites, which are both structurally and electronically clearly distinguishable. For example, a distinctive feature of the unique iron site is the diagonal Fe···S distance, which is 0.3 Å longer than for the equivalent irons. The calculated (57)Fe hyperfine parameters show the same 1:3 pattern as observed in the Mössbauer spectra and are in good agreement with experiment. BS analysis of the exchange interactions in the optimized geometry for the 1:3, M(S) = 4, BS configuration confirms the prediction of an earlier study that the unique site is coupled to the three equivalent ones by strong antiferromagnetic exchange (J > 0 in J Σ(j<4)Ŝ(4)·Ŝ(j)) and that the latter are mutually coupled by ferromagnetic exchange (J' < 0 in J' Σ(i<j<4)Ŝ(i)·Ŝ(j)). In combination, these exchange couplings stabilize an S = 4 ground state in which the composite spin of the three equivalent sites (S(123) = 6) is antiparallel to the spin (S(4) = 2) of the unique site. Thus, DFT analysis supports the idea that the unprecedented high value of the spin of the DH complex and, by analogy, of the all-ferrous cluster of the Fe-protein of nitrogenase, results from a remarkably strong dependence of the exchange interactions on cluster core geometry. The structure dependence of the exchange-coupling constants in the Fe(II)-(μ(3)-S)(2)-Fe(II) moieties of the all-ferrous clusters is compared with the magneto-structural correlations observed in the data for dinuclear copper complexes. Finally, we discuss two all-ferric clusters in the light of the results for the all-ferrous cluster.

The modular nature of all-ferrous edge-bridged double cubanes

Two all-ferrous, edge-bridged 8Fe-8S clusters, one capped with carbenes (2) and the other with phosphenes (3), have been characterized by (57)Fe Mossbauer spectroscopy. The clusters have diamagnetic ground states that yield spectra consisting of one quadrupole doublet with a large splitting (25% of absorption) and one (3) or two (2) doublets with much smaller splittings (75% of absorption). These patterns closely resemble those observed for all-ferrous 4Fe-4S clusters. Structurally, the 4Fe-4S fragments of 2 and 3 are remarkably similar to all-ferrous 4Fe-4S clusters, sharing with them the characteristic 3:1 pattern of the iron sites, a differentiation that has been shown previously to reflect spontaneous distortions of the cluster core. These spectroscopic and geometric similarities suggest that the diamagnetic ground state of the 8Fe-8S cluster results from antiferromagnetic exchange coupling of two identical 4Fe-4S modules, each carrying spin S(4Fe) = 4. The iron atoms with the largest quadrupole splittings are located at the opposite ends of the body diagonal containing the bridging sulfides.

Cubane-type Co4S4 clusters: synthesis, redox series, and magnetic ground states

The recent demonstration that the carbene cluster [Fe(4)S(4)(Pr(i)(2)NHCMe(2))(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 [M(4)S(4)L(4)](z) clusters, none of which are known with transition metals other than iron. The system CoCl(2)/Pr(i)(3)P/(Me(3)Si)(2)S/THF assembles [Co(4)S(4)(PPr(i)(3))(4)] (3), which is converted to [Co(4)S(4)(Pr(i)(2)NHCMe(2))(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 Fe(II) and Co(II) 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 S(Fe) = 2, the septet spin state can be described in terms of the coupling of three parallel and one antiparallel spins S(Co) = 3/2. The octanuclear clusters [Co(8)S(8)(PPr(i)(3))(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 Fe(8)S(8) core possess edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M(8)S(8) 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. (Pr(i)(2)NHCMe(2) = C(11)H(20)N(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).

Biomimetic chemistry of iron, nickel, molybdenum, and tungsten in sulfur-ligated protein sites

Biomimetic inorganic chemistry has as its primary goal the synthesis of molecules that approach or achieve the structures, oxidation states, and electronic and reactivity features of native metal-containing sites of variant nuclearity. Comparison of properties of accurate analogues and these sites ideally provides insight into the influence of protein structure and environment on intrinsic properties as represented by the analogue. For polynuclear sites in particular, the goal provides a formidable challenge for, with the exception of iron-sulfur clusters, all such site structures have never been achieved and few have even been closely approximated by chemical synthesis. This account describes the current status of the synthetic analogue approach as applied to the mononuclear sites in certain molybdoenzymes and the polynuclear sites in hydrogenases, nitrogenase, and carbon monoxide dehydrogenases.