Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

Chemical and redox non-innocence in low-valent molybdenum β diketonate complexes: novel pathways for CO2 and CS2 activation

The investigation of fundamental properties of low-valent molybdenum complexes bearing anionic ligands is crucial for elucidating the molybdenum's role in critical enzymatic systems involved in the transformation of small molecules, including the nitrogenase's iron molybdenum cofactor, FeMoco. The β-diketonate ligands in [Mo(acac)3] (acac = acetylacetonate), one of the earliest low-valent Mo complexes reported, provide a robust anionic platform to stabilize Mo in its +III oxidation state. This complex played a key role in demonstrating the potential of low-valent molybdenum for small molecule activation, serving as the starting material for the preparation of the first reported molybdenum dinitrogen complex. Surprisingly however, given this fact and the widespread use of β-diketonate ligands in coordination chemistry, only a very limited number of low-valent Mo β-diketonate complexes have been reported. To address this gap, we explored the redox behavior of homoleptic molybdenum tris-β-diketonate complexes, employing a tertiary butyl substituted diketonate ligand (dipivaloylmethanate, tBudiket) to isolate and fully characterize the corresponding Mo complexes across three consecutive oxidation states (+IV, +III, +II). We observed marked reactivity of the most reduced congener with heterocumulenes CE2 (E = O, S), yet with very distinct outcomes. Specifically, CO2 stoichiometrically carboxylates one of the β-diketonate ligands, while in the presence of excess CS2, catalytic reductive dimerization to tetrathiooxalate occurs. Through the isolation and characterization of reaction products and intermediates, we demonstrate that the observed reactivity results from the chemical non-innocence of the β-diketonate ligands, which facilitates the formation of a common ligand-bound intermediate, [Mo( tBudiket)2( tBudiket·CE2)]1- (E = O, S). The stability of this proposed intermediate dictates the specific reduction products observed, highlighting the relevance of the chemically non-innocent nature of β-diketonate ligands.

Ligand-to-metal charge transfer facilitates photocatalytic oxygen atom transfer (OAT) with cis-dioxo molybdenum(vi)-Schiff base complexes

Systems incorporating the cis-Mo(O)2 motif catalyse a range of important thermal homogeneous and heterogeneous oxygen atom transfer (OAT) reactions spanning biological oxidations to platform chemical synthesis. Analogous light-driven processes could offer a more sustainable approach. The cis-Mo(O)2 complexes reported here photocatalyse OAT under visible light irradiation, and operate via a non-emissive excited state with substantial ligand-to-metal charge-transfer (LMCT) character, in which a Mo[double bond, length as m-dash]O π*-orbital is populated via transfer of electron density from a chromophoric salicylidene-aminophenol (SAP) ligand. SAP ligands can be prepared from affordable commercially-available precursors. The respective cis-Mo(O)2-SAP catalysts are air stable, function in the presence of water, and do not require additional photosensitisers or redox mediators. Benchmark OAT between phosphines and sulfoxides shows that electron withdrawing groups (e.g. C(O)OMe, CF3) are necessary for photocatalytic activity. The photocatalytic system described here is mechanistically distinct from both thermally catalysed OAT by the cis-Mo(O)2 motif, as well as typical photoredox systems that operate by outer sphere electron transfer mediated by long-lived emissive states. Both photoactivated and thermally activated OAT steps are coupled to establish a catalytic cycle, offering new opportunities for the development of photocatalytic atom transfer based on readily-available, high-valent metals, such as molybdenum.

Enhancement of Diarylamine Antioxidant Activity by Molybdenum Dithiocarbamates

Molybdenum dithiocarbamates (MDTCs) are indispensable lubricant additives. Although their role as antiwear agents is well established, they have also been attributed antioxidant properties that are not understood. MDTCs do not inhibit autoxidation, but they markedly enhance the capacity of diphenylamines (DPAs)─ubiquitous radical-trapping antioxidants (RTAs)─to do so. We find this synergy to be evident not only at elevated temperatures (160 °C in n-hexadecane) but also at moderate temperatures, where autoxidations can be continuously monitored and kinetics more easily interpreted (100 °C in squalane). Interestingly, the synergy disappeared in an unsaturated hydrocarbon (n-hexadec-1-ene), where the RTA activity of the DPA is known to result from the diarylnitroxide derived therefrom. Autoxidations of squalane carried out in the presence of the diarylnitroxide─wherein it is a poor inhibitor─were much better inhibited in the presence of MDTC, suggesting that it converts the nitroxide to (a) more competent RTA(s). Indeed, preparative experiments revealed two species: DPA and a DPA dimer into which a single oxygen atom had been incorporated. This conversion is accelerated by the oxidation of MDTC to a dioxo molybdenum species. A mechanism is proposed to account for these observations, and the implications of our findings and their interpretation are discussed.

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDI^R)Cl₂] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl₃(THF)₂ or WOCl₃(THF)₂ and LiBDI^R. Reactivity studies show that the BDI^Dipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt₃). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

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.

Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity

Two oxoiron(IV) isomers (R 2a and R 2b) of general formula [FeIV (O)(R PyNMe3 )(CH3 CN)]2+ are obtained by reaction of their iron(II) precursor with NBu4 IO4 . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between R 2a and R 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that R 2a reacts one order of magnitude faster than R 2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in R 2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.

Soybean Oil Epoxidation Catalyzed by a Functionalized Metal–Organic Framework with Active Dioxo-Molybdenum (VI) Centers

In this work, a functionalized gallium metal–organic framework with active dioxo-molybdenum (VI) centers was evaluated as a catalyst in the epoxidation of soybean oil using tert-butyl-hydroperoxide as an oxidizing agent. The influence of the reaction time, temperature, and concentration of the oxidizing agent was studied, and it was demonstrated that the highest epoxide selectivity was obtained at 110 °C after 4 h of reaction (29% conversion and 91% selectivity) using a soybean oil/oxidizing agent ratio of 1/2. The stability of the metal–organic framework was confirmed by infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy EDS. The stability tests demonstrated that the catalyst could be reused in the catalytic process for the recovery of vegetable oils.

Graphical Abstract

Deoxygenation reactions in organic synthesis catalyzed by dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes have been applied as efficient, inexpensive and benign catalysts to deoxygenation reactions of a diverse number of compounds in the last two decades. Dioxomolybdenum complexes have demonstrated wide applicability to the deoxygenation of sulfoxides into sulfides and reduction of N-O bonds. Even the challenging nitro functional group was efficiently deoxygenated, affording amines or diverse heterocycles after reductive cyclization reactions. More recently, carbon-based substrates like epoxides, alcohols and ketones have been successfully deoxygenated. Also, dioxomolybdenum complexes accomplished deoxydehydration (DODH) reactions of biomass-derived vicinal 1,2-diols, affording valuable alkenes. The choice of the catalytic systems and reductant is decisive to achieve the desired transformation. Commonly found reducing agents involved phosphorous-based compounds, silanes, molecular hydrogen, or even glycols and other alcohols.

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.

Dioxygen Activation with Molybdenum Complexes Bearing Amide-Functionalized Iminophenolate Ligands

Two novel iminophenolate ligands with amidopropyl side chains (HL2 and HL3) on the imine functionality have been synthesized in order to prepare dioxidomolybdenum(VI) complexes of the general structure [MoO₂L₂] featuring pendant internal hydrogen bond donors. For reasons of comparison, a previously published complex featuring n-butyl side chains (L1) was included in the investigation. Three complexes (1–3) obtained using these ligands (HL1–HL3) were able to activate dioxygen in an in situ approach: The intermediate molybdenum(IV) species [MoO(PMe₃)L₂] is first generated by treatment with an excess of PMe₃. Subsequent reaction with dioxygen leads to oxido peroxido complexes of the structure [MoO(O₂)L₂]. For the complex employing the ligand with the n-butyl side chain, the isolation of the oxidomolybdenum(IV) phosphino complex [MoO(PMe₃)(L1)₂] (4) was successful, whereas the respective Mo(IV) species employing the ligands with the amidopropyl side chains were found to be not stable enough to be isolated. The three oxido peroxido complexes of the structure [MoO(O₂)L₂] (9–11) were systematically compared to assess the influence of internal hydrogen bonds on the geometry as well as the catalytic activity in aerobic oxidation. All complexes were characterized by spectroscopic means. Furthermore, molecular structures were determined by single-crystal X-ray diffraction analyses of HL3, 1–3, 9–11 together with three polynuclear products {[MoO(L2)₂]₂(µ-O)} (7), {[MoO(L2)]₄(µ-O)₆} (8) and [C₉H₁₃N₂O]₄[Mo₈O₂₆]·6OPMe₃ (12) which were obtained during the synthesis of reduced complexes of the type [MoO(PMe₃)L₂] (4–6).

Interaction of Metal Oxido Compounds with B(C6 F5 )3

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C₆ F₅ )₃ . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.

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.

Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states – balance between coordinated α-hydroxy and α-alkoxy groups

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K₂{Mo^IV₃O₄(im)₃[Mo^VIO₃(Hcit)]₂}·3im·4H₂O (1), (Him)₂{Mo^IV₃SO₃(im)₃[Mo^VIO₃(Hcit)]₂}·im·6H₂O (2), molybdenum(v) bipyridine homocitrate trans-[(Mo^VO)₂O(H₂homocit)₂(bpy)₂]·4H₂O (3) and molybdenum(vi) citrate (Et₄N)[Mo^VIO₂Cl(H₂cit)]·H₂O (4) (H₄cit = citric acid, H₄homocit = homocitric acid, im = imidazole and bpy = 2,2′-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [Mo^VIO₃(Hcit)]³⁻ anions and incomplete cubane units [Mo^IV₃O₄]⁴⁺ ([Mo^IV₃SO₃]⁴⁺) with monodentate imidazoles, respectively, where tridentate citrates coordinate with α-hydroxy, α-carboxy and β-carboxy groups, forming pentanuclear skeleton structures. The molybdenum atoms in 1 and 2 show unusual +4 and +6 valences based on charge balances, theoretical bond valence calculations and Mo XPS spectrum. The coordinated citrates in 1 and 2 are protonated with α-hydroxy groups, while 3 and 4 with higher oxidation states of +5 and +6 are deprotonated with α-alkoxy group even under strong acidic condition, respectively. This shows the relationship between the oxidation state and protonation of the α-alkoxy group in citrate or homocitrate, which is related to the protonation state of homocitrate in FeMo-cofactor of nitrogenase. The homocitrate in 3 chelates to molybdenum(v) with bidentate α-alkoxy and monodentate α-carboxy groups. Molybdenum(vi) citrate 4 is only protonated with coordinated and uncoordinated β-carboxy groups. The solution behaviours of 1 and 2 are discussed based on ¹H and ¹³C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.

Oxo and thiomolybdenum(iv/vi) citrates, molybdenum(v) homocitrate and molybdenum(vi) citrate were obtained, showing the influence of coordinated α-hydroxy and α-alkoxy groups with different oxidation states.

Mechanism of the Molybdenum-Mediated Cadogan Reaction

Oxygen atom transfer reactions are receiving increasing attention because they bring about paramount transformations in the current biomass processing industry. Significant efforts have therefore been made lately in the development of efficient and scalable methods to deoxygenate organic compounds. One recent alternative involves the modification of the Cadogan reaction in which a Mo(VI) core catalyzes the reduction of o-nitrostyrene derivatives to indoles in the presence of PPh₃. We have used density functional theory calculations to perform a comprehensive mechanistic study on this transformation, in which we find two clearly defined stages: an associative path from the nitro to the nitroso compound, characterized by the reduction of the catalyst in the first step, and a peculiar mechanism involving oxazaphosphiridine and nitrene intermediates leading to an indole product, where the metal catalyst does not participate.

Heterolytic Si-H Bond Cleavage at a Molybdenum-Oxido-Based Lewis Pair

The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C6 F5 )3 gave access to the Lewis adduct [Mo{OB(C6 F5 )3 }(NtBu)L2 ] featuring reversible B-O bonding in solution. The resulting frustrated Lewis pair (FLP)-like reactivity is reflected by the compound's ability to heterolytically cleave Si-H bonds, leading to a clean formation of the novel cationic MoVI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR3 )(NtBu)L2 ][HB(C6 F5 )3 ]. These compounds possess properties highly unusual for molybdenum d0 species such as an intensive, charge-transfer-based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single-crystal X-ray diffraction analyses of 2 and 4 b, a derivative of 3 b featuring the [FB(C6 F5 )3 ]- anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3 a and 3 b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B-H bond of the borohydride anion.

Oxidoperoxidomolybdenum(vi) complexes with acylpyrazolonate ligands: synthesis, structure and catalytic properties

Oxidoperoxido–molybdenum(vi) complexes Ph₄P[Mo(O)(O₂)₂(Q^R)] and [Mo(O)(O₂)(Q^R)₂] (Q^R = acylpyrazolonate ligands) were synthesised and tested as catalysts in epoxidation, sulphoxidation and epoxide deoxygenation.

cis-Dioxorhenium(V/VI) Complexes Supported by Neutral Tetradentate N4 Ligands. Synthesis, Characterization, and Spectroscopy

A series of cis-dioxorhenium(V) complexes containing chiral tetradentate N₄ ligands, including cis-[Re^V(O)₂(pyxn)]⁺ (1; pyxn = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)cyclohexane-1,2-diamine), cis-[Re^V(O)₂(6-Me₂pyxn)]⁺ (cis-2), cis-[Re^V(O)₂(R,R-pdp)]⁺ (3; R,R-pdp = 1,1'-bis((R,R)-2-pyridinylmethyl)-2,2'-bipyrrolidine), cis-[Re^V(O)₂(R,R-6-Me₂pdp)]⁺ (4), and cis-[Re^V(O)₂(bqcn)]⁺ (5; bqcn = N,N'-dimethyl-N,N'-di(quinolin-8-yl)cyclohexane-1,2-diamine), were synthesized. Their structures were established by X-ray crystallography, showing Re-O distances in the range of 1.740(3)-1.769(8) Å and O-Re-O angles of 121.4(2)-124.8(4)°. Their cyclic voltammograms in MeCN (0.1 M [NBu₄]PF₆) display a reversible Re^VI/V couple at E_1/2 = 0.39-0.49 V vs SCE. In aqueous media, three proton-coupled electron transfer reactions corresponding to Re^VI/V, Re^V/III, and Re^III/II couples were observed at pH 1. The Pourbaix diagrams of 1·OTf, 3·OTf, and 5·OTf have been examined. The electronic absorption spectra of the cis-dioxorhenium(V) complexes show three absorption bands at around 800 nm (600-1730 dm³ mol^-1 cm^-1), 580 nm (1700-5580 dm³ mol^-1 cm^-1), and 462-523 nm (3170-6000 dm³ mol^-1 cm^-1). Reaction of 1 with Lewis acids (or protic acids) gave cis-[Re^V(O)(OH)(pyxn)]²⁺ (1·H⁺), in which the Re-O distances are lengthened to 1.788(5) Å. Complex cis-2 resulted from isomerization of trans-2 at elevated temperature. cis-[Re^VI(O)₂(pyxn)](PF₆)₂ (1'·(PF₆)₂) was obtained by constant-potential electrolysis of 1·PF₆ in MeCN (0.1 M [NBu₄]PF₆) at 0.56 V vs SCE; it displays shorter Re-O distances (1.722(4), 1.726(4) Å) and a smaller O-Re-O angle (114.88(18)°) relative to 1 and shows a d-d transition absorption band at 591 nm (ε = 77 dm³ mol^-1 cm^-1). With a driving force of ca. 75 kcal mol^-1, 1' oxidizes hydrocarbons with weak C-H bonds (75.5-76.3 kcal mol^-1) via hydrogen atom abstraction. DFT and TDDFT calculations on the electronic structures and spectroscopic properties of the cis-dioxorhenium(V/VI) complexes were performed.

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.

Inter-ligand electronic coupling mediated through a dimetal bridge: dependence on metal ions and ancillary ligands

The metal–metal bond orbitals and the ancillary ligands influence inter-ligand charge transfer through the dimetal bridge.

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.

Light-Induced Activation of a Molybdenum Oxotransferase Model within a Ru(II)-Mo(VI) Dyad

Nature uses molybdenum-containing enzymes to catalyze oxygen atom transfer (OAT) from water to organic substrates. In these enzymes, the two electrons that are released during the reaction are rapidly removed, one at a time, by spatially separated electron transfer units. Inspired by this design, a Ru(II)-Mo(VI) dyad was synthesized and characterized, with the aim of accelerating the rate-determining step in the cis-dioxo molybdenum-catalyzed OAT cycle, the transfer of an oxo ligand to triphenyl phosphine, via a photo-oxidation process. The dyad consists of a photoactive bis(bipyridyl)-phenanthroline ruthenium moiety that is covalently linked to a bioinspired cis-dioxo molybdenum thiosemicarbazone complex. The quantum yield and luminescence lifetimes of the dyad [Ru(bpy)₂(L²)MoO₂(solv)]²⁺ were determined. The major component of the luminescence decay in MeCN solution (τ = 1149 ± 2 ns, 67%) corresponds closely to the lifetime of excited [Ru(bpy)₂(phen-NH₂)]²⁺, while the minor component (τ = 320 ± 1 ns, 31%) matches that of [Ru(bpy)₂(H₂-L²)]²⁺. In addition, the (spectro)electrochemical properties of the system were investigated. Catalytic tests showed that the dyad-catalyzed OAT from dimethyl sulfoxide to triphenyl phosphine proceeds significantly faster upon irradiation with visible light than in the dark. Methylviologen acts as a mediator in the photoredox cycle, but it is regenerated and hence only required in stoichiometric amounts with respect to the catalyst rather than sacrificial amounts. It is proposed that oxidative quenching of the photoexcited Ru unit, followed by intramolecular electron transfer, leads to the production of a reactive one-electron oxidized catalyst, which is not accessible by electrochemical methods. A significant, but less pronounced, rate enhancement was observed when an analogous bimolecular system was tested, indicating that intramolecular electron transfer between the photosensitizer and the catalytic center is more efficient than intermolecular electron transfer between the separate components.

Oxygen activation and catalytic aerobic oxidation by Mo(iv)/(vi) complexes with functionalized iminophenolate ligands

Synthesis of molybdenum(vi) dioxido complexes 1-3, coordinated by one or two functionalized iminophenolate ligands HL1 or HL2, bearing a donor atom side chain or a phenyl substituent, respectively, allowed for systematic investigation of the oxygen atom transfer (OAT) reactivity of such complexes towards phosphanes. Depending on stoichiometry and employed phosphane (PMe3 or PPh3), different molybdenum(iv) and molybdenum(v) complexes 4-7 were obtained. Whereas molybdenum(iv) complexes 4 and 5, bearing a terminal PMe3 ligand, readily reacted with molecular O2 to form oxido peroxido complexes 8 and 9, phosphane free μ-oxido bridged dinuclear molybdenum(v) complexes 6 and 7 proved to be stable towards oxidation with molecular O2 under ambient conditions. Single-crystal X-ray diffraction analyses revealed different isomeric structures in the solid state for dioxido complexes 1 and 2 in comparison with oxido phosphane complex 5, dinuclear oxido μ-oxido complex 6 and oxido peroxido complexes 8 and 9, pointing towards an isomeric rearrangement during OAT. Compounds 1 and 2 were furthermore tested for their ability to catalyze the aerobic oxidation of PMe3 and PPh3. A significant difference in catalytic activity has been observed in the oxidation of PMe3, where complex 1 bearing donor atom functionalized ligands led to higher conversion and selectivity than complex 2 coordinated by phenyl iminophenolate ligands. In the oxidation of PPh3, complex 2 leads to higher conversion compared to 1. In a control experiment, phenyl-based dinuclear μ-oxido complex 7, derived from complex 2, was found to be catalytically active, which suggests a lower energy barrier for disproportionation into [MoO(L)2] and [MoO2(L)2] in comparison with methoxypropylene based compound 6, a prerequisite for subsequent reactivity toward molecular O2.

Templated C-C and C-N Bond Formation Facilitated by a Molybdenum(VI) Metal Center

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C-C and C-N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides.