Dioxomolybdenum(VI) Complexes with Acylpyrazolonate Ligands: Synthesis, Structures, and Catalytic Properties

Controlled and sequential single-electron reduction of the uranyl dication

A flexible tripodal pyrrole-imine ligand (H3L) has been used to facilitate the controlled and sequential single-electron reductions of the uranyl dication from the U(VI) oxidation state to U(V) and further to U(IV), processes that are important to understanding the reduction of uranyl and its environmental remediation. The uranyl(VI) complexes UO2(HL)(sol) (sol = THF, py) were straightforwardly accessed by the transamination reaction of H3L with UO2{N(SiMe3)2}2(THF)2 and adopt 'hangman' structures in which one of the pyrrole-imine arms is pendant. While deprotonation of this arm by LiN(SiMe3)2 causes no change in uranyl oxidation state, single-electron reduction of uranyl(VI) to uranyl(V) occurred on addition of two equivalents of KN(SiMe3)2 to UO2(HL)(sol). The potassium cations of this new [UVO2(K2L)]2 dimer were substituted by transmetalation with the appropriate metal chloride salt, forming the new uranyl(V) tetra-heterometallic complexes, [UVO2Zn(L)(py)2]2 and [UVO2Ln(Cl)(L)(py)2]2 (Ln = Y, Sm, Dy). The dimeric uranyl(V)-yttrium complex underwent further reduction and chloride abstraction to form the tetrametallic U(IV) complex [UIVO2YIII(py)]2, so highlighting the adaptability of this ligand to stabilise a variety of different uranium oxidation states.

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.

Titania-supported molybdenum oxide combined with Au nanoparticles as hydrogen-driven deoxydehydration catalyst of diol compounds

A heterogenous catalyst for deoxydehydration (DODH) reaction was developed using less expensive Mo than Re as the active center. Combination of Mo with anatase-rich TiO2 and Au as the support...

Transition metal-catalyzed deoxydehydration: missing pieces of the puzzle

Deoxydehydration (DODH) is a transformation that converts a vicinal diol into an olefin with the help of a sacrificial reductant.

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.

Bis(2,4-dioxo-pentan-3-ido-κ2 O,O')dioxidomolyb-denum(VI): a redetermination

The title compound, [Mo(C5H7O2)2O2] or cis-[MoO2(acac)2] (acac is acetyl-acetonate), contains a molybdenum(VI) atom coordinated by two acetyl-acetonate ligands and two doubly bonded oxido ligands in a distorted octa-hedral shape. The mol-ecule is chiral and the asymmetric unit contains two independent mol-ecules (one Δ, one Λ). Extensive C-H⋯O contacts are present throughout the structure. Data were collected at 100 K, providing higher precision of unit-cell parameters and atomic positions than previous determinations [Kamenar et al. (1973 ▸). Cryst. Struct. Commun. 2, 41-44.; Krasochka et al. (1975). Zh. Strukt. Khim. 16, 696-698].

Direct Deoxydehydration of Cyclic trans-Diol Substrates: An Experimental and Computational Study of the Reaction Mechanism of Vanadium(V)-based Catalysis*

The deoxydehydration of carbohydrates represents a key target to leverage renewable biomass resources chemically. Using a vanadium(V)-based catalyst, it was possible to directly deoxydehydrate cyclic trans-diol substrates. Accompanying mechanistic characterisation of this process by density functional calculations pointed to an energetically tractable route for deoxydehydration of cyclic trans-diol substrates involving stepwise cleavage of the diol C-O bonds via the triplet state; experimentally, this was supported by light dependence of the reaction. Calculations also indicated that cyclic cis-diols and a linear diol substrate could additionally proceed by a concerted singlet DODH mechanism. This work potentially opens a new and cost-effective way to efficiently convert carbohydrates of trans-diol stereochemistry into alkenes.

N-Donor Ligand Supported “ReO2+”: A Pre-Catalyst for the Deoxydehydration of Diols and Polyols

A selected number of tetradentate N2Py2 ligand-supported ReO2+ complexes and a monodentate pyridine-supported ReO2+ complex have been investigated as catalysts for the deoxydehydration (DODH) of diols and polyols. In situ 1H NMR experiments showed that these N-donor ligand-supported ReO2+ complexes are only the pre-catalyst of the DODH reaction. Treatment of (N2Py2) ReO2+ with an excess amount of water generates an active species for DODH catalysis; use of the Re-product of this reaction shows a much shorter induction period compared to the pristine complex. No ligand is coordinated to the “water-treated” complex indicating that the real catalyst is formed after ligand dissociation. IR analysis suggested this catalyst to be a rhenium-oxide/hydroxide oligomer. The monodentate pyridine ligand is much easier to dissociate from the metal center than a tetradentate N2Py2 ligand, which makes the Py4ReO2+-initiated DODH reaction more efficient. For the Py4ReO2+-initiated DODH of diols and biomass-based polyols, both PPh3 and 3-pentanol could be used as a reductant. Excellent olefin yields are achieved.

Influence of the pendant arm in deoxydehydration catalyzed by dioxomolybdenum complexes supported by amine bisphenolate ligands

Molybdenum complexes devoid of a strongly coordinating pendant arm result in enhanced catalytic activity.

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.

Deoxydehydration of vicinal diols by homogeneous catalysts: a mechanistic overview

Deoxydehydration (DODH) is an important reaction for the upconversion of biomass-derived polyols to commodity chemicals such as alkenes and dienes. DODH can be performed by a variety of early metal-oxo catalysts incorporating Re, Mo and V. The varying reduction methods used in the DODH catalytic cycle impact the product distribution, reaction mechanism and the overall yield of the reaction. This review surveys the reduction methods commonly used in homogeneous DODH catalyst systems and their impacts on yield and reaction conditions.

Crystal structure of bis(μ2-1-(4-chlorophenyl)-3-phenyl-4-thenoyl-1H-pyrazol-5-olato-κ3 O,O′:O′)-bis-(1-(4-chlorophenyl)-3-phenyl-4-thenoyl-1H-pyrazol-5-olato-κ2 O,O)-bis(ethyl acetate-κO)dicadmium(II), C88H64Cl4N8O12S4Cd2

C88H64Cl4N8O12S4Cd2, monoclinic, P21/c (no. 14), a = 13.5247(6) Å, b = 15.3257(7) Å, c = 20.6985(8) Å, β = 101.8630(10)°, V = 4198.7(3) Å3, Z = 2, R gt(F) = 0.0484, wR ref(F 2) = 0.0967, T = 173(2) K.

Deoxydehydration of glycerol in presence of rhenium compounds: reactivity and mechanistic aspects

Re compounds in different oxidation states are activated during a delay time into an active Re alkoxide precipitate catalysing the DODH of glycerol.

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.

Oxidation–reductive coupling of alcohols catalyzed by oxo-vanadium complexes

Oxo-vanadium complexes catalyze the novel oxidation–reductive coupling of benzylic and allylic alcohols.

Linkage Isomerism in Silver Acylpyrazolonato Complexes and Correlation with Their Antibacterial Activity

Novel silver(I) acylpyrazolonato coordination polymers of formula [Ag(Q(R))]n (1-3) have been synthesized by interaction of silver nitrate with HQ(R) in methanol in the presence of an equivalent quantity of KOH (in general HQ(R) = 1-phenyl-3-methyl-4-RC(═O)-5-pyrazolone, in detail HQ(fb), R = -CF2CF2CF3; HQ(cy), R = -cyclo-C6H11; HQ(be), R = -C(H)═C(CH3)2). [Ag(Q(R))]n react with 2-ethylimidazole (2EtimH), 1-methylimidazole (Meim), and triphenylphosphine (PPh3), affording the mononuclear Ag(Q(fb))(EtimH) (4), Ag(Q(cy))(Meim)2 (5), Ag(Q(be))(Meim) (6), and Ag(Q(R))(PPh3)2 (7-9). All complexes have been analytically and spectroscopically characterized, and for some of them the X-ray crystal structure has been resolved. In particular, the single crystal molecular structure determination of Ag(Q(fb))(EtimH) and Ag(Q(be))(PPh3)2 has confirmed the different coordination modes of the HQ(fb) and HQ(be) acylpyrazolone ligands, the former being bound to the silver(I) ion in a monodentate fashion while the latter in the O2-chelating mode. Density functional theory computations suggest new insights about metal-ligand interactions and the observed linkage isomerism. While phosphine-containing complexes Ag(Q(R))(PPh3)2 (7-9) seem not to be able to efficiently inhibit the growth of Escherichia coli and Staphylococcus aureus, the polynuclear complexes [Ag(Q(R))]n (1-3) and the mononuclear Ag(Q(fb))(EtimH) (4), Ag(Q(cy))(Meim)2 (5), and Ag(Q(be))(Meim) (6) show a high and almost steady in time antibacterial activity, comparable to that of AgNO3. This activity is likely related to the degree of saturation of the silver center and to the presence of different ancillary ligands in the diverse typologies of complexes.

Oxo-rhenium catalyzed reductive coupling and deoxygenation of alcohols

Representative benzylic, allylic and α-keto alcohols are deoxygenated to alkanes and/or reductively coupled to alkane dimers by reaction with PPh₃ catalyzed by (PPh₃)₂ReIO₂ (1).

Rhenium-catalyzed deoxydehydration of diols and polyols

The substitution of platform chemicals of fossil origin by biomass-derived analogues requires the development of chemical transformations capable of reducing the very high oxygen content of biomass. One such reaction, which has received increasing attention within the past five years, is the rhenium-catalyzed deoxydehydration (DODH) of a vicinal diol into an alkene; this is a model system for abundant polyols like glycerol and sugar alcohols. The present contribution includes a review of early investigations of stoichiometric reactions involving rhenium, diols, and alkenes followed by a discussion of the various catalytic systems that have been developed with emphasis on the nature of the reductant, the substrate scope, and mechanistic investigations.

Molybdenum-catalyzed deoxydehydration of vicinal diols

The commercially available (NH4 )6 Mo7 O24 and other molybdenum compounds are shown to be viable substitutes for the typically employed rhenium compounds in the catalytic deoxydehydration of aliphatic diols into the corresponding alkenes. The transformation, which represents a model system for the various hydroxyl groups found in biomass-derived carbohydrates, can be conducted in an inert solvent (dodecane), under solvent-free conditions, and in a solvent capable of dissolving biomass-derived polyols (1,5-pentanediol). The reaction is driven by the simultaneous oxidative deformylation of the diol resulting in an overall disproportionation of the substrate.