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

Preparative and Catalytic Properties of MoVI Mononuclear and Metallosupramolecular Coordination Assemblies Bearing Hydrazonato Ligands

A series of polynuclear, dinuclear, and mononuclear Mo(VI) complexes were synthesized with the hydrazonato ligands derived from 5-methoxysalicylaldehyde and the corresponding hydrazides (isonicotinic hydrazide (H2L1), nicotinic hydrazide (H2L2), 2-aminobenzhydrazide (H2L3), or 4-aminobenzhydrazide (H2L4)). The metallosupramolecular compounds obtained from non-coordinating solvents, [MoO2(L1,2)]n (1 and 2) and [MoO2(L3,4)]2 (3 and 4), formed infinite structures and metallacycles, respectively. By blocking two coordination sites with cis-dioxo ligands, the molybdenum centers have three coordination sites occupied by the ONO donor atoms from the rigid hydrazone ligands and one by the N atom of pyridyl or amine-functionalized ligand subcomponents from the neighboring Mo building units. The reaction in methanol afforded the mononuclear analogs [MoO2(L1-4)(MeOH)] (1a-4a) with additional monodentate MeOH ligands. All isolated complexes were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP) as an oxidant in water. The impact of the structure and ligand lability on the catalytic efficiency in homogeneous cyclooctene epoxidation was elucidated based on theoretical considerations. Thus, dinuclear assemblies exhibited better catalytic activity than mononuclear or polynuclear complexes.

New bis[MoO2] and [MoO(O2)] compounds: An artificial enzyme with peroxidase activity against o-phenylenediamine and dopamine

In this study, two binuclear dioxido- and oxidoperoxido molybdenum (VI) complexes, [{Mo^VIO₂}₂(L)(H₂O)₂] 1 and [{Mo^VIO(O₂)}₂(L)(H₂O)₂] 2, were synthesized. Complex 1 was obtained through a 1:2 reaction of ligand I with MoO₂(acac)₂, while complex 2 was synthesized in situ by reacting MoO₃ with H₂O₂ in a 1:2 ratio. The structures and characteristics of the complexes were examined employing several techniques such as elemental (CHN) analysis, spectroscopy (FT-IR, UV-Vis, ¹H, and ¹³CNMR), and thermal study (TGA). SC-XRD analysis of complex 1a revealed that the molybdenum central atom adopts an octahedral geometry and is bonded to phenolic oxygen, enolate oxygen, and azomethine nitrogen atoms. Powder X-ray diffraction was used to determine the purity of the bulk material, and the results were compared to single crystal data. Computational calculations were performed using density functional theory (DFT) at the B3LYP/6-31G(d, p) level of theory for the ligand and the LANL2DZ level of theory for the complexes, yielding geometry optimized structures that were then employed in frequency and NMR-calculations. These theoretical findings were compared to the experimental results and showed a good correlation. Furthermore, the complexes exhibited peroxidase-like activity in the presence of hydrogen peroxide as evidenced by the oxidation of o-phenylenediamine and dopamine.

Oxido-Molybdenum(V) Corroles as Robust Catalysts for Oxidative Bromination and Selective Epoxidation Reactions in Aqueous Media under Mild Conditions

Two new meso-substituted oxido-molybdenum corroles were synthesized and characterized by various spectroscopic techniques. In the thermogram, MoO[TTC] (1) exhibited excellent thermal stability up to 491 °C while MoO[TNPC] (2) exhibited good stability up to 318 °C. The oxidation states of the molybdenum(V) were verified by electron paramagnetic resonance (EPR) spectroscopy and exhibited an axial compression with d_xy¹ configuration. Oxido-molybdenum(V) complexes were utilized for the selective epoxidation of various olefins with high TOF values (2066-3287 h^-1) in good yields in a CH₃CN/H₂O (3:2, v/v) mixture in the presence of hydrogen peroxide as a green oxidant and NaHCO₃ as a promoter. The oxidative bromination catalytic activity of oxido-molybdenum(V) complexes in an aqueous medium has been reported for the first time. Surprisingly, MoO[TNPC] (2) biomimics of the vanadium bromoperoxidase (VBPO) enzyme activity exhibited remarkably high TOF values (36 988-61 646 h^-1) for the selective oxidative bromination of p-cresol and other phenol derivatives. Catalyst MoO[TNPC] (2) exhibited higher TOF values and better catalytic activity than catalyst MoO[TTC] (1) due to the presence of electron-withdrawing nitro groups evident from cyclic voltammetric studies.

Impedance Spectroscopy as a Powerful Tool for Researching Molybdenum-Based Materials with Schiff Base Hydrazones

Molybdenum coordination complexes are widely applied due to their biological and pharmacological potential, as well as their performance in different catalytic processes. Parent dioxidomolybdenum Schiff base complexes were prepared via the reaction of [MoO₂(acac)₂] with a hydrazone Schiff-base tetradentate ligand. A new hydrazone-Schiff base (H₂L^{1 and 2}) and its corresponding mononuclear and polynuclear dioxidomolybdenum(VI) complex were synthesized and characterized by spectroscopic methods and elemental analyses, and their thermal behavior was investigated by thermogravimetry. The crystal and molecular structures of H₂L² ligands and the complexes [MoO₂(L¹)(H₂O)], [MoO₂(L²)(H₂O)], [MoO₂(L¹)(MeOH)]∙MeOH, [MoO₂(L¹)(EtOH)]∙EtOH, [MoO₂(L¹)(2-PrOH)]∙2-PrOH, and [MoO₂(L¹)]_n were determined by single-crystal X-ray diffraction. Using the in situ impedance spectroscopy method (IS), the structural transformations of chosen complexes were followed, and their electrical properties were examined in a wide range of temperatures and frequencies.

Efficient Molybdenum Hydrazonato Epoxidation Catalysts Operating under Green Chemistry Conditions: Water vs. Decane Competition

Molybdenum compounds containing benzaldehyde-based hydrazones were obtained. The reaction in MeOH resulted with monomeric Mo complexes, [MoO2(L)(MeOH)], while the reaction in dichloromethane (DCM) provided oligomeric complexes, [MoO2(L)]n. The solid-state structures of the obtained compounds were investigated through Infrared Spectroscopy - Attenuated Total Reflection (IR-ATR), Thermogravimetric analysis (TGA), and via X-ray diffraction. The prepared molybdenum species were employed as cyclooctene epoxidation catalysts. TBHP (tert-butylhydroperoxide) in water and TBHP in decane were employed and compared as oxidants, with 0.25 mol% [Mo]. The catalyst activity and selectivity towards epoxide is >90% for all the reactions. The results have been linked to theoretical calculations, showing the importance of the first step, i.e., the transformation of [MoO2(L)(MeOH)] into the pentacoordinate [MoO2(L)].

Discrete and polymeric ensembles based on dinuclear molybdenum(vi) building blocks with adaptive carbohydrazide ligands: from the design to catalytic epoxidation

Discrete and polymeric ensembles based on dimolybdenum(vi) units with adaptive carbohydrazide ligands are described. The polymeric complexes are efficient catalysts for cyclooctene epoxidation under eco-friendly conditions.

Recent Advances in Facile Liquid Phase Epoxidation of Light Olefins over Heterogeneous Molybdenum Catalysts

Molybdenum complexes are versatile and efficient for liquid phase olefin epoxidation reactions. Rational design of catalysts is critical to achieve high atom efficiency during epoxidation processes. Although liquid phase epoxidation has been a popular topic for decades, three key issues, (a) rational control of morphology of molybdenum nanoparticles, (b) manipulating metal-support interaction and (c) altering electronic configuration at molybdenum center remains unsolved in this area. Therefore, in this paper, we have critically revised recent research progress on heterogeneous molybdenum catalysts for facile liquid phase olefin epoxidation in terms of catalyst synthesis, surface characterization, catalytic performance and structure-function relationship. Furthermore, plausible reaction mechanisms will be systematically discussed with the aim to provide insights into fundamental understanding on novel epoxidation chemistry.

Stereoisomers and functional groups in oxidorhenium(v) complexes: effects on catalytic activity

The syntheses of oxidorhenium(v) complexes [ReOCl(L1a-c)₂] (3a-c), equipped with the bidentate, mono-anionic phenol-dimethyloxazoline ligands HL1a-c are described. Ligands HL1b-c contain functional groups on the phenol ring, compared to parent ligand 2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol H1a; namely a methoxy group ortho to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-6-methoxyphenol, H1b), or a nitro group para to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol, H1c). Furthermore, oxidorhenate(v) complexes (NBu₄)[ReOCl₃(L1a-b)] (2a-b) were synthesized for solid state structural comparisons to 3a-b. All novel complexes are fully characterized including NMR, IR and UV-Vis spectroscopy, MS spectrometry, X-ray crystallography, elemental analysis as well as cyclic voltammetry. The influence of functional groups (R = -H, -OMe and -NO₂) on the catalytic activity of 3a-c was investigated in two benchmark catalytic reactions, namely cyclooctene epoxidation and perchlorate reduction. In addition, the previously described oxidorhenium(v) complex [ReOCl(oz)₂] (4), employing the phenol-oxazoline ligand 2-(4,5-dihydro-2-oxazolyl)phenol Hoz, was included in these catalysis studies. Complex 4 is a rare case in oxidorhenium(v) chemistry where two stereoisomers could be separated and fully characterized. With respect to the position of the oxazoline nitrogen atoms on the rhenium atom, these two stereoisomers are referred to as N,N-cis and N,N-trans isomer. A potential correlation between spectroscopic and structural data to catalytic activity was evaluated.

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).

Recent Progress in Application of Molybdenum-Based Catalysts for Epoxidation of Alkenes

Epoxides are important industrial intermediates applied in a variety of industrial processes. During the production of epoxides, catalysts have played an irreplaceable and unique role. In this review, the historic progress of molybdenum-based catalysts in alkene epoxidation are covered and an outlook on future challenge discussed. Efficient catalysts are demonstrated including soluble molybdenum complexes, polyoxometalates catalysts, molybdenum-containing metal organic frameworks, silica supported molybdenum-based catalysts, polymer supported molybdenum-based catalysts, magnetic molybdenum-based catalysts, hierarchical molybdenum-based catalysts, graphene-based molybdenum containing catalysts, photocatalyzed epoxidation catalysts, and some other systems. The effects of different solvents and oxidants are discussed and the mechanisms of epoxidation are summarized. The challenges and perspectives to further enhance the catalytic performances in alkenes epoxidation are presented.

Discrete mononuclear and dinuclear compounds containing a MoO22+ core and 4-aminobenzhydrazone ligands: synthesis, structure and organic-solvent-free epoxidation activity

New aroylhydrazone molybdenum(vi) complexes have been synthesized, characterized and used as (pre)catalysts for epoxidation of cyclooctene using aqueous TBHP as the oxidant.

Scorpionate Catalysts for Coupling CO2 and Epoxides to Cyclic Carbonates: A Rational Design Approach for Organocatalysts

Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO₂ activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO₂ pressure of only 10 bar, and is highly recyclable.

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.

Tetranuclear Ni4 cubane complexes with high χT maxima: magneto-structural analysis

Seven tetranuclear Ni(ii) cubane complexes with unexpectedly high χT maxima coming from the ZFS contribution and with an interesting magneto-structural correlation.

Activation of Molecular Oxygen by a Molybdenum(IV) Imido Compound

Activation of molecular dioxygen at a molybdenum(IV) imido compound led to the isolation and full characterization of a remarkably stable transition-metal imidoperoxido complex.