Dioxo Complexes Research Articles

Dioxomolybdenum(VI) and -Tungsten(VI) Amino Bisphenolates as Epoxidation Catalysts

Low-cost metallate salts Na2MO4·2H2O (M = molybdenum, tungsten) react with a tridentate amine bisphenol bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)methylamine (H2ONOtBu) under ambient conditions in acidic methanol solutions. The reactions lead to the formation of isostructural dioxo complexes [MO2(ONOtBu)(MeOH)]·MeOH in convenient yields. Spectral data as well as X-ray analyses reveal these complexes to be isostructural. Both compounds were tested as catalysts for epoxidation of olefins using cis-cyclooctene, cyclohexene, norbornene and styrene as substrates and tert-butyl hydroperoxide and hydrogen peroxide as oxidants. The molybdenum complex catalyses selectively the oxidation of cis-cyclooctene and norbornene to corresponding epoxides, whereas oxidation of cyclohexene and styrene lead low yields as the epoxidations were associated with the formation of other oxidation products. Corresponding tungsten complex shows lower activity for epoxidation of norbornene and practically no activity for other olefins. Both complexes can also catalyse the conversion of benzoin to benzil using dimethyl sulphoxide as an oxidant, while the molybdenum complex shows higher activity.

Computationally designed zirconium organometallic catalyst for direct epoxidation of alkenes without allylic H atoms: aromatic linkage eliminates formation of inert octahedral complexes

We used density functional theory to computationally design a Zr organometallic catalyst for selectively oxidizing substrates using molecular oxygen as oxidant without coreductant. Each selective oxidation cycle involves four general steps: (a) a peroxo or weakly adsorbed O2 group releases an O atom to substrate to form substrate oxide and an oxo group, (b) an oxygen molecule adds to the oxo group to generate an η2-ozone group, (c) the η2-ozone group rearranges to form an η3-ozone group, and (d) the η3-ozone group releases an O atom to substrate to form substrate oxide and regenerate the peroxo or weakly adsorbed O2 group. This catalyst could potentially be synthesized via the condensation reaction Zr(N(R)R′)4 + 2 C6H4–1,6-(N(C6H3–2′,6′-(CH(CH3)2)2)OH)2 → Zr(C6H4–1,6-(N(C6H3–2′,6′-(CH(CH3)2)2)O)2)2 [aka Zr_Benzol catalyst] + 4 N(R)(R′)H where R and R′ are CH3, CH2CH3, or other alkyl groups. For direct ethylene epoxidation, the computed enthalpic energetic span (i.e., effective activation energy for the entire catalytic cycle) is 27.1 kcal/mol, which is one of the lowest values for catalysts studied to date. We study reaction mechanisms and the stability of different catalyst forms as a function of the oxygen atom chemical potential. Notably, an aromatic linkage in each ligand prevents this catalyst from deactivating to form an inactive octahedral-like structure that contains the same atoms as the dioxo complex, Zr(Ligand)2(O)2. Due to a side reaction that can transfer an allylic H atom from alkene to catalyst, this catalyst is useful for directly epoxidizing alkenes such as ethylene that do not contain allylic H atoms. To better understand the reaction chemistry, we computed net atomic charges and bond orders for the two catalytically relevant reaction cycles. These results quantify electron transfer and bond forming and breaking during the catalytic process.

Dioxygen Activation by Non-Adiabatic Oxidative Addition to a Single Metal Center.

A chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex [Tp(tBu,Me)Cr(V)(O)2] (Tp(tBu,Me) = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [Tp(tBu,Me)Cr(III)(S2)]. The transformation of the putative peroxo intermediate [Tp(tBu,Me)Cr(III)(O2)] (S = 3/2) into [Tp(tBu,Me)Cr(V)(O)2] (S = 1/2) is spin-forbidden. The minimum-energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(Tp(tBu,Me)Cr)2(μ-O)2] exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.

Dioxygen Activation by Non‐Adiabatic Oxidative Addition to a Single Metal Center

AbstractA chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex [TptBu,MeCrV(O)2] (TptBu,Me=hydrotris(3‐tert‐butyl‐5‐methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [TptBu,MeCrIII(S2)]. The transformation of the putative peroxo intermediate [TptBu,MeCrIII(O2)] (S=3/2) into [TptBu,MeCrV(O)2] (S=1/2) is spin‐forbidden. The minimum‐energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(TptBu,MeCr)2(μ‐O)2] exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.

Dioxomolybdenum(VI) and ‐tungsten(VI) Complexes with Multidentate Aminobisphenol Ligands as Catalysts for Olefin Epoxidation

AbstractThe synthesis of four molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene (1a) or hydroxyglycolene (1b) substituents is reported. The molybdenum dioxo complexes [MoO2L] (L = 2a, 2b) and tungsten complexes [WO2L] (3a, 3b) were synthesized using [MoO2(acac)2] and [W(eg)3] (eg = 1,2‐ethanediolato, ethylene glycolate), respectively, as precursors. All complexes were characterized by spectroscopic means as well as by single‐crystal X‐ray diffraction analyses. The latter reveal, in all cases, hexacoordinate complexes in which the hydrogen atom of the hydroxy group is involved in hydrogen bonding with one of the metal oxo groups. In the case of the glycol substituent, the ether oxygen atom is coordinated to the metal whereas the hydroxy group remains uncoordinated. The complexes were tested as catalysts in the epoxidation of cyclooctene under eco‐friendly conditions, using an aqueous solution of H2O2 as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions, as substitutes for chlorinated solvents. Molybdenum complexes 2a and 2b showed good catalytic activity using H2O2 without added solvent, and tungsten complexes 3a and 3b showed very high activity in the epoxidation of cyclooctene using H2O2 and DMC as solvents.

New insights into hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds by rhenium(V)-dioxo complexes.

The hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds catalyzed by high-valent rhenium(V)-dioxo complex ReO2I(PPh3)2 (1) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an η(1)-silane rhenium adduct to prompt the heterolytic cleavage of the Si-H bond is the most energetically favorable process for rhenium(V)-dioxo complex 1 catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium(V)-dioxo complex 1 catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the Si-H bond and catalyzing the reduction reactions.

ChemInform Abstract: Molybdenum(VI) Dioxo Complexes for the Epoxidation of Allylic Alcohols and Olefins.

AbstractA newly synthesized molybdenum(VI) dioxo complex is used as a catalyst for the epoxidation of allylic alcohols and olefins.

Stereochemistry of complexes with double and triple metal-ligand bonds: a continuous shape measures analysis.

To each coordination polyhedron we can associate a normalized coordination polyhedron that retains the angular orientation of the central atom-ligand bonds but has all the vertices at the same distance from the center. The use of shape measures of these normalized coordination polyhedra provides a simple and efficient way of discriminating angular and bond distance distortions from an ideal polyhedron. In this paper we explore the applications of such an approach to analyses of several stereochemical problems. Among others, we discuss how to discern the off-center displacement of the metal from metal-ligand bond shortening distortions in families of square planar biscarbene and octahedral dioxo complexes. The normalized polyhedron approach is also shown to be very useful to understand stereochemical trends with the help of shape maps, minimal distortion pathways, and ligand association/dissociation pathways, illustrated by the Berry and anti Berry distortions of triple-bonded [X≡ML4] complexes, the square pyramidal geometries of Mo coordination polyhedra in oxido-reductases, the coordination geometries of actinyl complexes, and the tetrahedricity of heavy atom-substituted carbon centers.

Molybdenum(VI) dioxo complexes for the epoxidation of allylic alcohols and olefins

Several molybdenum(VI) dioxo complexes have been investigated as catalyst precursors for allylic alcohol epoxidation using mainly hydrogen peroxide as oxidant. All catalysts proved to be efficient and selective for the epoxidation of allylic alcohols provided the olefins were rather electron rich. Indeed, electron poor substrates could be converted selectively into the corresponding unsaturated aldehydes. A chiral dioxomolybdenum complex based on an optically pure tridentate Schiff base ligand was synthesized and characterized. Though that complex provided an efficient epoxidation catalyst for allylic alcohols and olefins, no chiral induction was observed. During the X-ray diffraction analysis, the cooling at 100 K led to the appearance of super-lattice reflections on diffraction patterns reflecting an ordering of the structure. Instead of one organometallic species observed at 298 K, three similar complexes of the same molecular structure could be observed in the asymmetric unit at 100 K.

The vibrational spectrum of FeO2(+) isomers--theoretical benchmark and experiment.

Infrared photodissociation is used to record the vibrational spectrum of FeO2 (+)(He)2-4 which shows three bands at 1035, 980, and 506 cm(-1). Quantum chemical multi-reference configuration interaction calculations (MRCISD) of structures and harmonic frequencies show that these bands are due to two different isomers, an inserted dioxo complex with Fe in the +V oxidation state and a side-on superoxo complex with Fe in the +II oxidation state. These two are separated by a substantial barrier, 53 kJ/mol, whereas the third isomer, an end-on complex between Fe(+) and an O2 molecule, is easily converted into the side-on complex. For all three isomers, states of different spin multiplicity have been considered. Our best energies are computed at the MRCISD+Q level, including corrections for complete active space and basis set extension, core-valence correlation, relativistic effects, and zero-point vibrational energy. The average coupled pair functional (ACPF) yields very similar energies. Density functional theory (DFT) differs significantly from our best estimates for this system, with the TPSS functional yielding the best results. The other functionals tested are BP86, PBE, B3LYP, TPSSh, and B2PLYP. Complete active space second order perturbation theory (CASPT2) performs better than DFT, but less good than ACPF.

Specific features of the structure of monomeric octahedral d 0-rhenium(VII) oxo complexes

The specific structural features of mononuclear octahedral oxo complexes of rhenium(VII) are considered. The crystal structures of mono- and dioxo complexes d0-Re(VII) with one or two multiply bonded (=N,=C) organic ligands, di- and trioxo compounds, and pseudooctahedral trioxo complexes ReO3+ with cyclopentadienyl and its derivatives are analyzed.

Bis(oxalato)dioxovanadate(V) and Bis(oxalato)oxoperoxo-vanadate(V) Complexes: Spectroscopic Characterization and Biological Activity

Two structurally related vanadium(V) complexes, K3[VO2(C2O4)2] · 3H2O and K3[VO(O2)(C2O4)2] · 1/2H2O, were thoroughly characterized by infrared, Raman, and electronic spectroscopies. The effect of both complexes on the viability of the human MG-63 osteosarcoma cells was tested using the MTT assay. The monoperoxo complex shows a very strong antiproliferative activity (at 100-μM concentration, this complex diminished the cell viability ca. 80 %), whereas the dioxo complex was inactive.

Quantum Chemical Studies on Dioxygen Activation and Methane Hydroxylation by Diiron and Dicopper Species as well as Related Metal–Oxo Species

Abstract Quantum chemical studies by the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metal–oxo species are reviewed. The activation of the O–O bond of dioxygen at the mono- and dimetal sites of metalloenzymes is an essential process in the initial catalytic stages of naturally occurring oxygenation reactions. A way of orbital thinking about dioxygen activation at diiron and dicopper enzymes is developed at the extended Hückel level of theory. Two different reaction pathways that lead from dimetal peroxo complexes with µ-η1:η1-O2 and µ-η2:η2-O2 modes to the corresponding dioxo complexes are discussed in detail. In considering the mechanism of methane hydroxylation, special attention to FeO+-mediated methane hydroxylation that occurs under ion cyclotron resonance (ICR) conditions is given. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ions ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+ are systematically analyzed on the basis of density functional theory (DFT) calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process. The spin inversion from the high-spin state to the low-spin state effectively decreases the barrier height of C–H activation. Another feature in the reaction is that no radical species is involved in the course of the hydroxylation because the methyl species formed as a result of the C–H bond cleavage can directly coordinate to the metal active site. The coupling of the OH and CH3 ligands occurs to produce methanol at the metal active center. The reaction profiles obtained from DFT calculations are similar to those of the Gif chemistry proposed by D. H. R. Barton, in particular the involvement of the HO–M–CH3 species as an intermediate in the hydroxylation of methane. The nonradical mechanism is extended to methane hydroxylation by the diiron and dicopper species of methane monooxygenase (MMO). This mechanism is possible when the metal active center is coordinatively unsaturated to have a space for the coordination of the OH and CH3 groups as ligands. Although compelling discussion to support the involvement of oxygen- and carbon-centered radicals is provided, the nonradical mechanism still seems to be applicable for methane hydroxylation from the viewpoint of reaction selectivity. Kinetic isotope effects (KIEs) in the C–H activation process by the bare FeO+ complex and diiron and dicopper enzyme models are compared with respect to the radical and nonradical mechanisms by using transition state theory.

ChemInform Abstract: Specific Features of Technetium Mononuclear Octahedral Oxo Complexes: A Review

AbstractReview: 87 refs.

Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates

High-valent intermediates of binuclear nonheme iron enzymes are structurally unknown despite their importance for understanding enzyme reactivity. Nuclear resonance vibrational spectroscopy combined with density functional theory calculations has been applied to structurally well-characterized high-valent mono- and di-oxo bridged binuclear Fe model complexes. Low-frequency vibrational modes of these high-valent diiron complexes involving Fe motion have been observed and assigned. These are independent of Fe oxidation state and show a strong dependence on spin state. It is important to note that they are sensitive to the nature of the Fe2 core bridges and provide the basis for interpreting parallel nuclear resonance vibrational spectroscopy data on the high-valent oxo intermediates in the binuclear nonheme iron enzymes.

Dioxo and oxo-peroxo molybdenum(VI) complexes bearing salicylidene 2-picoloyl hydrazone: Structures and catalytic performances

Molybdenum complexes bearing a salicylidene 2-picoloyl hydrazone ligand have been synthesized and characterized. The salicylidene 2-picoloyl hydrazone ligand (H2sal-phz) was prepared by reaction of picoloyl hydrazine with salicylaldehyde. Reaction of H2sal-phz with MoO2(acac)2 generates the dioxo molybdenum complex MoO2(sal-phz)(CH3OH). Treatment of MoO3 in the presence of H2O2 with H2sal-phz produces the oxo-peroxo molybdenum(VI) complex MoO(O2)(sal-phz)(CH3OH). The new compounds were characterized by 1H NMR and IR spectroscopies, and elemental analysis. The complexes were also characterized by single crystal X-ray diffraction crystallography. The complexes were evaluated for epoxidation catalysis with tert-butyl hydroperoxide (tert-BuOOH). The MoO2(sal-phz)(CH3OH) complex exhibited high catalytic activity for the epoxidation of olefins, whereas the peroxo analogue is inert to reaction with olefins, thus ruling out the peroxide compound as the intermediate responsible in the [MoO2(sal-phz)(CH3OH)]-catalyzed epoxidation with tert-BuOOH.

Dimeric μ-oxo bridged molybdenum(vi) dioxo complexes as catalysts in the epoxidation of internal and terminal alkenes

The preparation of the tridentate phenol based amine ligands HL1–HL4 is achieved via a convenient one-pot synthesis by reductive amination in quantitative yield in an autoclave under 7 bar H2 gas. Reaction of [MoO2(acac)2] and the corresponding ligand HLX (X = 1, 2 and 4) in methanol–H2O results in the formation of orange to red dimeric μ-oxo bridged [{MoO2(LX)}2(μ-O)] (X = 1, 2 and 4) complexes 1–3 in high yield and high purity. Complexes 1–3 are stable towards air and water. Both ligands coordinate via the phenolic O atom, the amine N atom and the third donor atom in the side chain (OMe for 1 and NMe2 for 2 and 3) in a fac mode to the metal center. The molybdenum atoms are linked by a bridging μ-oxo moiety to each other as confirmed by X-ray diffraction analyses of complexes 2 and 3. All complexes have been tested in the epoxidation of several internal and terminal alkenes using TBHP as an oxidant. Depending on the nature of the substrate, the epoxides are obtained in moderate to good yields and high selectivities. In the epoxidation of cyclooctene a TOF = 467 h−1 with complex 1 has been observed, significantly higher compared to other dimeric complexes reported in the literature. In the more challenging epoxidation of styrene, complexes 1 and 2 have proven to be highly selective as only the formation of styrene oxide is observed. The OMe based complex 1 has also proven to be more active than the NMe2 based counterparts 2 and 3. The basic conditions induced by the NMe2 groups in complexes 2 and 3 lower their catalytic activity.

Specific features of technetium mononuclear octahedral oxo complexes: A review

The specific structural features of technetium mononuclear octahedral oxo complexes have been considered. The structures of d 2-Tc(V) mono- and dioxo complexes, d 2-Tc(V) pseudodioxo compounds (Tc(V) mono-oxo complexes with an additional multiply bonded RO− ligand), and d 0-Tc(VII) trioxo compounds are analyzed.

Controlled Deprotection and Reorganization of Uranyl Oxo Groups in a Binuclear Macrocyclic Environment

Switching on uranium(V) reactivity: The silylated uranium(V) dioxo complex [(Me(3)SiOUO)(2)(L)(2)] (A) is inert to oxidation, but after two-electron reduction to [(Me(3)SiOUO)(2)(L)](2-) (1), it can be desilylated to form [OU(μ-O)(2)UO(L)(2)](2-) (2) with reinstated uranyl character. Removal of the silyl group uncovers new redox and oxo rearrangement chemistry for uranium, thus reforming the uranyl motif and involving the U(VI/V) couple in dioxygen reduction.

Molybdenum(VI) Dioxo Complexes Employing Schiff Base Ligands with an Intramolecular Donor for Highly Selective Olefin Epoxidation

Reaction of [MoO(2)(η(2)-tBu(2)pz)(2)] with Schiff base ligands HL(X) (X = 1-5) gave molybdenum(VI) dioxo complexes of the type cis-[MoO(2)(L(X))(2)] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R(2) = OMe or NMe(2)) in the side chain in complexes 1-4 is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes 1 and 3. Complexes 1, 3, and 5 exist as a mixture of two isomers in solution, whereas complexes 2 and 4 with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe(2) groups in complexes 3 and 4 lead to lower activity.