Structural and functional models in molybdenum and tungsten bioinorganic chemistry: description of selected model complexes, present scenario and possible future scopes

Reactivity of metal dioxo complexes

Metal dioxo chemistry and its diverse reactivity are presented with an emphasis on the mechanisms of reactivity. Work from approximately the last decade is surveyed and organized by metal. In particular, the chemistry of cis-dioxo metal complexes is discussed at length. Reactions are grouped by generic type, including addition across a metal oxo bond, oxygen atom transfer, and radical atom transfer reactions. Attention is given to advances in deoxygenation chemistry, oxidation chemistry, and reductive transformations.

Dioxygen Activation by a Bioinspired Tungsten(IV) Complex

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO₂(6-MePyS)₂] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe₃ to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-MePyS)₂(PMe₃)₂] is described. This tungsten(IV) complex partially releases one PMe₃ ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO₂(6-MePyS)₂] and OPMe₃. Therefore, [WO₂(6-MePyS)₂] can be used as a catalyst for the aerobic oxidation of PMe₃, rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.

Replacement of Molybdenum by Tungsten in a Biomimetic Complex Leads to an Increase in Oxygen Atom Transfer Catalytic Activity

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [W^VIO₂L₂] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO₂(PyS)₂] compared to the Mo species, independent of whether PMe₃ or PPh₃ was used as the oxygen acceptor. [W^VIO₂L₂] complexes undergo reduction with an excess of PMe₃, yielding the tungsten(IV) oxido species [WOL₂(PMe₃)₂], while with PPh₃, no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL₂(PMe₃)₂] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV-vis spectroscopy, we observed that tungsten compounds directly react to W^VIO₂ complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo₂O₃L₄]. Density functional theory calculations confirm that the oxygen atom abstraction from W^VIO₂ is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes

The redox non-innocent behavior of the diaryl-azo-oxime ligand L^NOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(L^NO-)Cl₂(PPh₃)₂] 4 and [Ir(L^NO-)₂Cl(PPh₃)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(L^NO-)Cl(CO₂H)(PPh₃)₂] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO₂H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Soft Scorpionate Hydridotris(2-mercapto-1-methylimidazolyl) borate) Tungsten-Oxido and -Sulfido Complexes as Acetylene Hydratase Models

A series of WIV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (TmMe ) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTmMe and the resulting complexes [W(CO)(C2 R2 )(TmMe )Br] (R=H 1, Me 2) oxidized to the target [WE(C2 R2 )(TmMe )Br] (E=O, R=H 4, Me 5; E=S, R=H 6, Me 7) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C2 R2 )(MeCN)(TmMe )](OTf) (E=CO, R=H 10, Me 11; E=O, R=H 12, Me 13; E=S, R=H 14, Me 15). Without MeCN, dinuclear complexes [W2 O(μ-O)(C2 Me2 )2 (TmMe )2 ](OTf)2 (8) and [W2 (μ-S)2 (C2 Me2 )(TmMe )2 ](OTf)2 (9) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C2 Me2 . This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9. All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase.

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands

Four dioxidomolybdenum(VI) complexes of the general structure [MoO₂L₂] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, 1), pyridine-2-thiolate (PyS, 2), 4-methylpyridine-2-thiolate (4-MePyS, 3) and 6-methylpyridine-2-thiolate (6-MePyS, 4) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis (2-4). Complexes 1-4 were reacted with PPh₃ and PMe₃, respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh₃ leads to symmetric molybdenum(V) dimers of the general structure [Mo₂O₃L₄] (6-9). Kinetic studies showed that the OAT from [MoO₂L₂] to PPh₃ is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes 1-3 with PMe₃ gives stable molybdenum(IV) complexes of the structure [MoOL₂(PMe₃)₂] (10-12), while reduction of [MoO₂(6-MePyS)₂] (4) yields [MoO(6-MePyS)₂(PMe₃)] (13) with only one PMe₃ coordinated to the metal center. The activity of complexes 1-4 in catalytic OAT reactions involving Me₂SO and Ph₂SO as oxygen donors and PPh₃ as an oxygen acceptor has been investigated to assess the influence of the varied ligand frameworks on the OAT reaction rates. It was found that [MoO₂(PymS)₂] (1) and [MoO₂(6-MePyS)₂] (4) are similarly efficient catalysts, while complexes 2 and 3 are only moderately active. In the catalytic oxidation of PMe₃ with Me₂SO, complex 4 is the only efficient catalyst. Complexes 1-4 were also found to catalytically reduce NO₃^- with PPh₃, although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo(NO)(PymS)₃] (14), which was identified by single-crystal X-ray diffraction analysis. Computed ΔG^⧧ values for the very first step of the OAT were found to be lower for complexes 1 and 4 than for 2 and 3, explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.

Anion Binding and Oxidative Modification at the Molybdenum Cofactor of Formate Dehydrogenase from Rhodobacter capsulatus Studied by X-ray Absorption Spectroscopy

Formate dehydrogenase (FDH) enzymes are versatile catalysts for CO₂ conversion. The FDH from Rhodobacter capsulatus contains a molybdenum cofactor with the dithiolene functions of two pyranopterin guanine dinucleotide molecules, a conserved cysteine, and a sulfido group bound at Mo(VI). In this study, we focused on metal oxidation state and coordination changes in response to exposure to O₂, inhibitory anions, and redox agents using X-ray absorption spectroscopy (XAS) at the Mo K-edge. Differences in the oxidative modification of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor relative to samples prepared aerobically without inhibitor, such as variations in the relative numbers of sulfido (Mo═S) and oxo (Mo═O) bonds, were observed in the presence of azide (N₃^-) or cyanate (OCN^-). Azide provided best protection against O₂, resulting in a quantitatively sulfurated cofactor with a displaced cysteine ligand and optimized formate oxidation activity. Replacement of the cysteine ligand by a formate (HCO₂^-) ligand at the molybdenum in active enzyme is compatible with our XAS data. Cyanide (CN^-) inactivated the enzyme by replacing the sulfido ligand at Mo(VI) with an oxo ligand. Evidence that the sulfido group may become protonated upon molybdenum reduction was obtained. Our results emphasize the role of coordination flexibility at the molybdenum center during inhibitory and catalytic processes of FDH enzymes.

Deoxydehydration of polyols catalyzed by a molybdenum dioxo-complex supported by a dianionic ONO pincer ligand

Deoxydehydration (DODH) is the net reduction of diols and polyols to alkenes or dienes and water. Molybdenum cis-dioxo bis-phenolate ONO complexes were synthesized and have been shown to be active for DODH. Catalysts were screened for activity at 150-190 °C, and appreciable yields of up to 59% were obtained. PPh₃, Na₂SO₃, Zn, C, 3-octanol and 2-propanol were screened as reductants. Additionally, the reactivities of a variety of diols were screened. With (R,R)-(+)-hydrobenzoin as substrate, DODH occurs via a mechanism where reduction of the Mo catalyst is a result of diol oxidation to form two equivalents of aldehyde. These reactions result in complete conversion and near quantitative yields of trans-stilbene and benzaldehyde.

Bioinspired Tungsten Complexes Employing a Thioether Scorpionate Ligand

The synthesis and characterization of a series of novel tungsten complexes employing the bioinspired, sulfur-rich scorpionate ligand [PhTt] (phenyltris((methylthio)methyl)borate) are reported. Starting from the previously published tungsten precursor [WBr₂(CO)₃(NCMe)₂], a salt metathesis reaction with 1 equiv of Cs[PhTt] led to the desired complex [WBr(CO)₃(PhTt)] (1), making it the first tungsten complex employing a poly(thioether)borate ligand. Surprisingly, the reaction of [WBr₂(CO)₃(NCMe)₂] with an excess of the ligand gave complex [W(CO)₂(η²-CH₂SMe)(PhTt)] (2) with a bidentate (methylthio)methanide ligand as the major product. Thereby, phenyldi((methylthio)methyl)borane is formed, which was isolated and characterized by NMR spectroscopy. The bromido ligand in [WBr(CO)₃(PhTt)] was further substituted by the S,N-bidentate methimazole in order to make the first coordination sphere more sulfur-rich forming [W(CO)₂(mt)(PhTt)] (3). Alkyne tungsten complexes employing the sulfur-rich scorpionate ligand were accessible by reaction of [WBr₂(CO)(C₂R₂)₂(NCMe)] (R = Me, Ph) with Cs[PhTt] forming [WBr(CO)(C₂R₂)₂(PhTt- S, S')] (R = Me 4, Ph 5), with the potentially tridentate ligand coordinated only via two sulfur atoms. In the case of 4, the higher flexibility of the bidentate coordination leads to the formation of two isomers with respect to the six-membered ring formed by the tungsten center and the two coordinated sulfur atoms of the ligand. All complexes 1-5 were characterized by single-crystal X-ray diffraction analysis.

Activation and Photoinduced Release of Alkynes on a Biomimetic Tungsten Center: The Photochemical Behavior of the W-S-Phoz System

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio-inspired S,N-donor ligand 2-(4',4'-dimethyloxazoline-2'-yl)thiophenolate (S-Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis-carbonyl precursor [W(CO)2 (S-Phoz)2 ] was used for the complexes [W(CO)(C2 R2 )(S-Phoz)2 ] (R=H, 1 a; Me, 1 b; Ph, 1 c). Oxidation with pyridine-N-oxide gave the corresponding W-oxo species [WO(C2 R2 )(S-Phoz)2 ] (R=H, 2 a; Me, 2 b; Ph, 2 c). All W-oxo-alkyne complexes (2 a, b, c) were found to be capable of alkyne release upon light irradiation to afford five-coordinate [WO(S-Phoz)2 ] (3). The photoinduced release of the alkyne ligand was studied in detail by in situ 1 H NMR measurements, which revealed correlation of the photodissociation rate constant (2 b>2 a>2 c) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S-Phoz)2 ] (3) with pyridine-N-oxide yielded [WO2 (S-Phoz)2 ] (4), which shows highly fluxional behavior in solution. Variable-temperature 1 H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W4 O4 (μ-O)6 (S-Phoz)4 ] (5) and dinuclear [{WO(μ-O)(S-Phoz)}2 ] (6) over time. The latter two were identified by single-crystal X-ray diffraction analyses.

Remote Charge Effects on the Oxygen-Atom-Transfer Reactivity and Their Relationship to Molybdenum Enzymes

We report the syntheses, crystal structures, and characterization of the novel cis-dioxomolybdenum(VI) complexes [Tpm*Mo^VIO₂Cl](MoO₂Cl₃) (1) and [Tpm*Mo^VIO₂Cl](ClO₄) (2), which are supported by the charge-neutral tris(3,5-dimethyl-1-pyrazolyl)methane (Tpm*) ligand. A comparison between isostructural [Tpm*Mo^VIO₂Cl]⁺ and Tp*Mo^VIO₂Cl [Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate] reveals the effects of one unit of overall charge difference on their spectroscopic and electrochemical properties, geometric and electronic structures, and O-atom-transfer (OAT) reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the lowest unoccupied molecular orbital (LUMO) energy of cationic [Tpm*MoO₂Cl]⁺ relative to Tp*MoO₂Cl. Despite their virtually identical geometric structures revealed by crystal structures, the Mo^VI/Mo^V redox potential of 2 is increased by 350 mV relative to that of Tp*Mo^VIO₂Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO₂Cl]⁺ relative to that of Tp*MoO₂Cl, resulting in a more favorable resonant interaction between the molydenum complex LUMO and the highest occupied molecular orbital (HOMO) of the PPh₃ substrate. This leads to a greater thermodynamic driving force, an earlier transition state, and a lowered activation barrier for the orbitally controlled first step of the OAT reaction in the Tpm* system relative to the Tp* system. An Eyring plot analysis shows that this initial step yields an O≡Mo^IV-OPPh₃ intermediate via an associative transition state, and the reaction is ∼500-fold faster for 2 than for Tp*MoO₂Cl. The second step of the OAT reaction entails solvolysis of the O≡Mo^IV-OPPh₃ intermediate to afford the solvent-substituted Mo^IV product and is 750-fold faster for the Tpm* system at -15 °C compared to the Tp* system. The observed rate enhancement for the second step is ascribed to a switch of the reaction mechanism from a dissociative pathway for the Tp* system to an alternative associative pathway for the Tpm* system. This is due to a more Lewis acidic Mo^IV center in the Tpm* system.

Oxygen atom transfer between DMSO and benzoin catalyzed by cis-dioxidomolybdenum(vi) complexes of tetradentate Mannich bases

Dioxidomolybdenum(vi) complexes of tetradentate ONNO donor Mannich base ligands for the catalytic oxygen atom transfer between benzoin and DMSO are reported.

Cooperative bimetallic reactivity of a heterodinuclear molybdenum–copper model of Mo–Cu CODH

Modeling the reactivity of Mo–Cu CODH: Cu(i) brings the substrate close to Mo–oxo and develops electrophilic character in CO carbon.

Acid-facilitated product release from a Mo(IV) center: relevance to oxygen atom transfer reactivity of molybdenum oxotransferases

We report that pyridinium ions (HPyr⁺) accelerate the conversion of [Tp*Mo^IVOCl(OPMe₃)] (1) to [Tp*Mo^IVOCl(NCCH₃)] (2) by 10³-fold, affording 2 in near-quantitative yield; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate. This novel reactivity and the mechanism of this reaction were investigated in detail. The formation of 2 followed pseudo-first-order kinetics, with the observed pseudo-first-order rate constant (k _obs) linearly correlated with [HPyr⁺]. An Eyring plot revealed that this HPyr⁺-facilitated reaction has a small positive value of ∆S ^‡ indicative of a dissociative interchange (I_d) mechanism, different from the slower associative interchange (I_a) mechanism in the absence of HPyr⁺ marked with a negative ∆S ^‡. Interestingly, log(k _obs) was found to be linearly correlated to the acidity of substituted pyridinium ions. This novel reactivity is further investigated using combined DFT and ab initio coupled cluster methods. Different reaction pathways, including I_d, I_a, and possible alternative routes in the absence or presence of HPyr⁺, were considered, and enthalpy and free energies were calculated for each pathway. Our computational results further underscored that the I_d route is energetically favored in the presence of HPyr⁺, in contrast with the preferred I_a-NNO pathway in the absence of HPyr⁺. Our computational results also revealed molecular-level details for the HPyr⁺-facilitated I_d route. Specifically, HPyr⁺ initially becomes hydrogen-bonded to the oxygen atom of the Mo(IV)-OPMe₃ moiety, which lowers the activation barrier for the Mo-OPMe₃ bond cleavage in a rate-limiting step to dissociate the OPMe₃ product. The implications of our results were discussed in the context of molybdoenzymes, particularly the reductive half-reaction of sulfite oxidase.

Mononuclear bis(dithiolene) Mo(iv) and W(iv) complexes with P,P; S,S; O,S and O,O donor ligands: a comparative reactivity study

Comparative redox reactions of eight Mo^IV/W^IVcomplexes with P,P; S,S; S,O and O,O donor ligands are presented.

Molybdenum complexes with a μ-O{MoO2}2 core: their synthesis, crystal structure and application as catalysts for the oxidation of bicyclic alcohols using N-based additives

Binuclear molybdenum(vi) complexes with a μ-O{MoO₂}₂ core as catalysts for the oxidation of bicyclic alcohols (fenchyl alcohol and isoborneol), using 30% H₂O₂ as the oxidant in the presence of NEt₃ as the additive, are reported.

Synthesis, characterization and oxygen atom transfer reactivity of a pair of Mo(iv)O- and Mo(vi)O2-enedithiolate complexes – a look at both ends of the catalytic transformation

A novel pair of mono-oxo and di-oxo bis-dithiolene molybdenum complexes were synthesized, characterized and catalytically investigated as models for a molybdenum dependent oxidoreductase.

Bioinorganic modeling chemistry of carbon monoxide dehydrogenases: description of model complexes, current status and possible future scopes

Carbon monoxide dehydrogenases (CODHs) use CO as their sole source of carbon and energy and are found in both aerobic and anaerobic carboxidotrophic bacteria. Reversible transformation of CO to CO2 is catalyzed by a bimetallic [Mo-(μ2-S)-Cu] system in aerobic and by a highly asymmetric [Ni-Fe-S] cluster in anaerobic CODH active sites. The CODH activity in the microorganisms effects the removal of almost 10(8) tons of CO annually from the lower atmosphere and earth and thus help to maintain a sub-toxic concentration of CO. Despite an appreciable amount of work, the mechanism of CODH activity is not clearly understood yet. Moreover, biomimetic chemistry directed towards the active sites of CODHs faces several synthetic challenges. The synthetic problems associated with the modeling chemistry and strategies adopted to overcome those problems are discussed along with their limitations. A critical analysis of the exciting results delineating the present status of CODH modeling chemistry and its future prospects are presented.

Sulfido and cysteine ligation changes at the molybdenum cofactor during substrate conversion by formate dehydrogenase (FDH) from Rhodobacter capsulatus

Formate dehydrogenase (FDH) enzymes are attractive catalysts for potential carbon dioxide conversion applications. The FDH from Rhodobacter capsulatus (RcFDH) binds a bis-molybdopterin-guanine-dinucleotide (bis-MGD) cofactor, facilitating reversible formate (HCOO(-)) to CO2 oxidation. We characterized the molecular structure of the active site of wildtype RcFDH and protein variants using X-ray absorption spectroscopy (XAS) at the Mo K-edge. This approach has revealed concomitant binding of a sulfido ligand (Mo=S) and a conserved cysteine residue (S(Cys386)) to Mo(VI) in the active oxidized molybdenum cofactor (Moco), retention of such a coordination motif at Mo(V) in a chemically reduced enzyme, and replacement of only the S(Cys386) ligand by an oxygen of formate upon Mo(IV) formation. The lack of a Mo=S bond in RcFDH expressed in the absence of FdsC implies specific metal sulfuration by this bis-MGD binding chaperone. This process still functioned in the Cys386Ser variant, showing no Mo-S(Cys386) ligand, but retaining a Mo=S bond. The C386S variant and the protein expressed without FdsC were inactive in formate oxidation, supporting that both Mo-ligands are essential for catalysis. Low-pH inhibition of RcFDH was attributed to protonation at the conserved His387, supported by the enhanced activity of the His387Met variant at low pH, whereas inactive cofactor species showed sulfido-to-oxo group exchange at the Mo ion. Our results support that the sulfido and S(Cys386) ligands at Mo and a hydrogen-bonded network including His387 are crucial for positioning, deprotonation, and oxidation of formate during the reaction cycle of RcFDH.

Recent developments in the study of molybdoenzyme models

Over the past two decades, a plethora of crystal structures of molybdenum enzymes has appeared in the literature providing a clearer picture of the enzymatic active sites and increasing the challenge to chemists to develop accurate models for those sites. In this minireview we discuss the most recent model studies aimed to reproduce detailed features of the pterin-dithiolene ligand, both as the uncoordinated form and as a chelate coordinated to molybdenum.