Oxomolybdenum Complexes Research Articles

Size and Electronic Effects on Redox Potentials and Electrocatalytic Processes Performed by Corrole-Chelated Metal Complexes

The future of our planet critically depends on the introduction of new or improved strategies for developing non-polluting energy production. Leading approaches are the electrocatalytic production of hydrogen gas, development of fuel cells relying on efficient reduction of oxygen and either gaseous or liquid fuels, as well as efficient reduction of carbon dioxide. Metal complexes chelated by N4 macrocycles serve as excellent catalysts in many of these processes, performed at both homogeneous and heterogeneous conditions. Our contributions to those aspect focused on introducing metallocorroles as catalysts for reduction of protons, oxygen and carbon dioxide, as well as for water oxidation [1-5]. The common motif in these and other publications was to take advantage of the relatively easy functionalization of the macrocyclic periphery for the tuning of properties and reactivity. A significant game changer was the recently reported synthetic access to corroles with meso-CF3 substituents and even to the parent corrole with no substituents whatsoever [6]. These developments allow for a focus on the size effect, which according to both hypothesis and fast accumulating results has a very strong effect (“smaller is better”) on the performance of electrodes modified by the corresponding metal complexes [7-9].References Sinha, N. Fridman, Y. Diskin-Posner, L. J. W. Shimon and Z. Gross “Superstructured Metallocorroles for Electrochemical CO2 Reduction”, Chem. Commun. 2019, 55, 11912-11915.Sinha, A. Mahammed, N. Fridman and Z. Gross “Water Oxidation Catalysis by Mono- and Binuclear Iron Corroles” ACS Catal. 2020, 10,3764-3772.Sudhakar, A. Mahammed, Q.-C. Chen, N. Fridman, B. Tumanskii and Z. Gross “Copper Complexes of CF3‐Substituted Corroles for Affecting Redox Potentials and Electrocatalysis” ACS Appl. Energy Mater. 2020, 3, 2828-2836.Yadav, I. Nigel-Etinger, A. Kumar, A. Mizrahi, A. Mahammed, N. Fridman, S. Lipstman,I. Goldberg, and Z. Gross “Hydrogen Evolution Catalysis, by Terminal Molybdenum-Oxo Complexes”, Iscience 2021, 24, 102924.-C. Chen, S. Fite, N. Fridman, B. Tumanskii, A. Mahammed, and Z. Gross “Hydrogen Evolution Catalyzed by Corrole-Chelated Nickel Complexes, Characterized in all Catalysis-relevant Oxidation States”, ACS Catalysis 2022, 12, 4310−4317.Kumar, P. Yadav, M. Majdoub, I. Saltsman, N. Fridman, S. Kumar, A. Kumar,A. Mahammed and Z. Gross “Corroles: The Hitherto Elusive Parent Macrocycle and its Metal Complexes” Angew. Chem. 2021, 60, 25097-25103. Mahammed and Z. Gross “Milestones and Most Recent Advances in Corrole’s Science and Technology”, J. Am. Chem. Soc. 2023, 145, 12429–12445Raslin, J. Douglin, A. Kumar, M. Fernandez-Dela-Mora, D. Dekel and Z. Gross “Size and electronic effects on the performance of (corrolato)cobalt-modified electrodes for ORR catalysis”. Inorg. Chem. 2023, 62, 14147–14151.Kumar, S. Fite, A. Raslin, S. Kumar, A. Mizrahi, A. Mahammed, and Z. Gross “Beneficial Effects on the Cobalt-Catalyzed Hydrogen Evolution Reaction Induced by Corrole Chelation”. ACS Catal. 2023, 13, 13344-13353. Figure 1

Protein binding studies and antioxidant activities of two mononuclear dioxomolybdenum(VI) complexes with 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole

Two yellow air-stable mononuclear dioxomolybdenum(VI) complexes, MoO2Cl2L (1) and MoO2Br2L (2) [where L = N,N donor ligand 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole], have been synthesized from their parent oxomolybdenum(V) complexes in dichloromethane. The binding affinity and binding mode of the molybdenum complexes with bovine serum albumin (BSA) have been explored by absorption and fluorescence titrations and time resolved fluorescence measurements. The number of binding sites for the complexes is nearly equal to unity, indicating there is only one binding site per BSA molecule. Molecular docking studies are also carried out to elucidate the binding mechanism, indicating that the complexes could quench the intrinsic fluorescence of BSA in a static quenching manner. Fluorescence resonance energy transfer study reveals that energy transfer takes place from Tryptophan residue of BSA to the synthesized complexes. The anti-oxidative properties of the complexes are examined by ABTS radical scavenging activities and DPPH radical quenching. The complexes exhibit different activities due to the influence of halogen substitution to the metal ion. MoO2Br2L (2) exhibits better activities than MoO2Cl2L (1) according to their EC50 values.

Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes.

Porphyrin complexes are well-known in O2 and CO2 reduction, but their application to N2 reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N2 reduction to ammonia, verified by 15N2 labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N-H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol-1). We place these results in the context of other work on homogeneous N2 reduction catalysis.

Dioxomolybdenum (VI) and oxomolybdenum (IV) complexes with N, O, and S bidentate ligands, syntheses, spectral characterization, and DFT studies

Two dioxomolybdenum (VI) complexes with the chemical formula [MoO2(acac)(HPY)], [MoO2(DTO)(HPY)], and oxomolybdenum (IV) complexes [MoO(acac)(HPY)], [MoO(DTO)(HPY)] have been prepared and characterized by different spectral techniques such as (FTIR, UV-Vis., Mass, 1H NMR) spectra, magnetic susceptibility, and theoretical studies. The ligands used in this study were acetylacetone, 2-hydrazinopyridine, and dithiooximid. The spectroscopic data and the theoretical calculations suggested distorted octahedral structures for the dioxomolybdenum(VI) complexes. The dioxomolybdenum(VI) complexes were diamagnetic. The oxomolybdenum(IV) complexes are paramagnetic and have distorted square pyramidal structures. Theoretical calculations of the free ligands and the prepared complexes have been done by using DFT calculations using (G 09 W) software. The complexes were very stable and their energies ranged from (−708.85 to −921.99 a.u.) whereas the free (HPY) and (DTO) ligands were (−359.06 and −984.54 a.u.), respectively. The prepared complexes are polar (8.11–10.80 Debye) for Mo(VI), and (6.63–13.72 Debye) for Mo(IV). The HOMO orbital energies of the Mo(VI) complexes are (−0.229, and −0.377 a.u.), respectively whereas for the Mo(IV) complexes are (−0.192, −0.318 a.u.), respectively while for the (HPY) and (DTO) ligands are (−0.216, −0.262 a.u.). The LUMO orbitals energies of the Mo(VI) complexes are (−0.124, and −0.247 a.u.) and for the Mo(IV) are (−0.093, −0.208 a.u.), respectively.

Hydrogen evolution catalysis by terminal molybdenum-oxo complexes

SummaryStable complexes with terminal triply bound metal-oxygen bonds are usually not considered as valuable catalysts for the hydrogen evolution reaction (HER). We now report the preparation of three conceptually different (oxo)molybdenum(V) corroles for testing if proton-assisted 2-electron reduction will lead to hyper-reactive molybdenum(III) capable of converting protons to hydrogen gas. The upto 670 mV differences in the [(oxo)Mo(IV)]-/[(oxo)Mo(III)]−2 redox potentials of the dissolved complexes came into effect by the catalytic onset potential for proton reduction thereby, significantly earlier than their reduction process in the absence of acids, but the two more promising complexes were not stable at practical conditions. Under heterogeneous conditions, the smallest and most electron-withdrawing catalyst did excel by all relevant criteria, including a 97% Faradaic efficiency for catalyzing HER from acidic water. This suggests complexes based on molybdenum, the only sustainable heavy transition metal, as catalysts for other yet unexplored green-energy-relevant processes.

The reactions of MoOCl4 with neutral group 15 and 16 ligands and a re-investigation of some N-donor ligand complexes of MoOCl3

The reaction of molybdenum(VI) oxide tetrachloride, MoOCl4, with neutral ligands including MeCN, Ph3PO, SMe2, MeS(CH2)3SMe, SeMe2, PMe3, 2,2′-bipyridyl and 1,10-phenanthroline, results in reduction to form oxomolybdenum(V) complexes, identified by microanalysis, IR and UV/visible spectroscopy and via X-ray crystal structure analyses of several examples. Unexpectedly, the product formed with SeMe2 was [SeMe3]2[Mo2O2Cl6(μ-Cl)2] and with SMe2, [SMe2Cl]2[Mo2O2Cl6(μ-Cl)2]. X-ray crystal structures are reported for mer-[MoOCl3(MeCN)2], [{MoOCl2(MeCN)}2(µ-Cl)2], [MoOCl3{MeS(CH2)3SMe}], [SeMe3][MoOCl4(H2O)], [SMe2Cl]2[Mo2O2Cl6(μ-Cl)2] and [PMe3H][MoOCl4(PMe3)]. The reaction of MoOCl4 with [Et4N]Cl forms the dinuclear Mo(V) species, [Et4N]2[Mo2O2Cl6(μ-Cl)2].The red mer-[MoOCl3(diimine)] (diimine = 2,2′-bipyridyl, 1,10-phenathroline) complexes were prepared from [MoOCl3(thf)2] and the diimine and fully characterised by spectroscopy and by X-ray crystallography. The long-known ‘green isomers’ of [MoOCl3(diimine)] obtained from MoOCl4 and the diimines (and in other ways), are suggested to be the red isomers co-crystallised with another lower oxidation state molybdenum complex.

The density polarization reveals directions of electron displacements due to the substituent effect: Analysis performed on a metal-organic Mo-Oxo catalyst.

Some Mo-oxo complexes bearing pyridine rings have the capability for dihydrogen production from water. However, energy barrier and overall energy vary depending on the effect exerted by several substituent groups located at different positions around one or more pyridine rings which are ligands of these compounds. Based on the Karunadasa and coworkers investigation where the para-position was experimentally tested in compounds derivatised from the 2,6-bis[1,1-bis(2-pyridil)ethyl]-pyridine oxo-molybdenum complex synthesized (Karunadasa et al., Nature, 2010, 464, 1329), we tested the combined effect of electron-withdrawing and electron-donating groups simulated as perturbations represented by point-charges. Then, we used the density polarization concept, δρ(r), a local reactivity descriptor corresponding to the partially integrated linear response function, χ(r, r') (a non-local reactivity descriptor), which is able to reveal different displacements of π-electrons on molecular structures. We perturbed the para-positions in the pentadentate ligand 2,6-bis[1,1-bis(2-pyridil)ethyl]-pyridine in the Mo-based complex by means of point-charges. They were located in three different configurations of the organic ligand (trans, geminal, and cis) which could help to explain energy barriers and overall energy of reactions catalyzed by this type of Mo-complexes. Our results indicate that the trans configuration of point-charges induces the most amount of fraction of electron shifted on the complex. A Mo-based complex bearing the same trans configuration for electron-withdrawing and electron-donating substituent groups (cyano and amino, respectively), leads to a kinetically more favorable H2 release than the cis or geminal configuration of the substituent groups aforementioned.

Synthesis, Characterization, and Application of Oxo-Molybdenum(V)-Corrolato Complexes in Epoxidation Reactions.

Sharpless et al. have described, while performing the molybdenum-catalyzed epoxidation reaction of olefins using alkyl hydroperoxides, that the molybdenum-oxo moiety is an active catalytic species. Thus, continuous efforts have been made to synthesize molybdenum-oxo complexes of different ligand environments. While plenty of such works on molybdenum porphyrins are reported in the literature, related molybdenum corroles are very less reported. The synthesis and characterization of two new oxo-molybdenum(V)-corrolato complexes are described herein. Both the complexes have been fully characterized by several spectroscopic techniques in conjunction with single-crystal X-ray diffraction analysis. The efficacy of the oxo-molybdenum(V)-corrolato complexes for the catalytic epoxidation reaction of olefins with the help of hydroperoxides has also been explored. The catalytic application of oxo-molybdenum(V)-corrolato complexes in the epoxidation reaction has not been reported earlier. A mechanism has been proposed to explain the experimental findings.

Synthesis, Structure, and Bonding of d3 Molybdenum-Oxo Complexes.

Reduction of d2 metal-oxo ions of the form [MO(PP)2 Cl]+ (M=Mo, W; PP=chelating diphosphine) produces d3 MO(PP)2 Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)2 Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)2 Cl indicate the d3 compounds are low spin with a 2 [(dxy )2 (π*(MoO))1 ] configuration. X-ray crystallographic and vibrational-spectroscopic studies on d2 and d3 [MoO(dppe)2 Cl]0/+ establish that the d3 compound possesses a reduced M-O bond order and significantly longer Mo-O bond, accounting for its greater reactivity. These results indicate that the oxo-centered reactivity of d3 complexes may be controlled through ligand variation.

Synthesis, Structure, and Bonding of d3 Molybdenum–Oxo Complexes

AbstractReduction of d2 metal–oxo ions of the form [MO(PP)2Cl]+ (M=Mo, W; PP=chelating diphosphine) produces d3 MO(PP)2Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)2Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)2Cl indicate the d3 compounds are low spin with a 2[(dxy)2(π*(MoO))1] configuration. X‐ray crystallographic and vibrational‐spectroscopic studies on d2 and d3 [MoO(dppe)2Cl]0/+ establish that the d3 compound possesses a reduced M−O bond order and significantly longer Mo−O bond, accounting for its greater reactivity. These results indicate that the oxo‐centered reactivity of d3 complexes may be controlled through ligand variation.

A biradical oxo-molybdenum complex containing semiquinone and o-aminophenol benzoxazole-based ligands.

We report a new mononuclear molybdenum(iv) complex, MoOLBISLSQ, in which LSQ (2,4-di-tert-butyl o-semibenzoquinone ligand) has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, LBIS, and MoO2(acac)2. The complex was characterized by X-ray crystallography, elemental analysis, IR and UV-vis spectroscopy and magnetic susceptibility measurements. The crystal structure of MoOLBISLSQ revealed a distorted octahedral geometry around the metal centre, surrounded by one O and two N atoms of LBIS and two O atoms of LSQ. The effective magnetic moment (μeff) of MoOLBISLSQ decreased from 2.36 to 0.2 μB in the temperature range of 290 to 2 K, indicating a singlet ground state caused by antiferromagnetic coupling between the metal and ligand centred unpaired electrons. Also, the latter led to the EPR silence of the complex. Cyclic voltammetry (CV) studies indicate both ligand and metal-centered redox processes. MoOLBISLSQ was applied as a catalyst for the oxidative cleavage of cyclohexene to adipic acid and selective oxidation of sulfides to sulfones with aqueous hydrogen peroxide.

Cis- and trans molybdenum oxo complexes of a prochiral tetradentate aminophenolate ligand: Synthesis, characterization and oxotransfer activity

Reaction of [MoO2Cl2(dmso)2] with the tetradentate O2N2 donor ligand papy [H2papy = N-(2-hydroxybenzyl)-N-(2-picolyl)glycine] leads to formation of the dioxomolybdenum(VI) complex [MoO2(papy)] (1) as a mixture of cis and trans isomers. Recrystallization from methanol furnishes solid cis-1, whereas the use of a dichloromethane-hexane mixture allows for the isolation of the trans-1 isomer. Both isomers have been structurally characterized by X-ray crystallography and the energy difference between the isomeric pair has been investigated by electronic structure calculations. Optimization of two configurational isomers in the gas phase predicts the trans isomer to lie 2.5 kcal/mol lower in energy (ΔG) than the cis isomer, which is inconsistent with the solution NMR data in d3-MeCN that exhibit a Keq of ca. 3 at 298 K for the trans ⇌ cis equilibrium. The DFT-computed energy difference is significantly improved (Keq = 5.4) by the inclusion of the MeCN solvent using the polarization continuum model (PCM). Density functional calculations reveal that the isomerization proceeds via a Ray-Dutt twist mechanism with a barrier of 14.5 kcal/mol, which is in accordance with the 1H NMR spectral data and the rapid equilibration of these isomers in solution. The catalytic reactivity of [MoO2(papy)] in the epoxidation of cis-cyclooctene is described, as well as its ability to effect oxo transfer from DMSO to PPh3.

New mononuclear and binuclear oxomolybdenum(V) complexes containing N[sbnd]N chelator: Syntheses, DFT calculations, interaction with BSA protein and in vitro cytotoxic activity

A neutral bidentate ligand 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole (L) has been synthesized by refluxing equimolar proportions of 2-hydrazino benzthiazole and benzoyl acetone in ethanol. The ligand acts in a NN donor fashion and forms stable mononuclear, MoOX3L [L = Ligand, X = Cl (1), Br (2)] and binuclear Mo2O4X2L2 [L = Ligand, X = Cl (3), Br (4)] complexes with molybdenum(V). The ligand and complexes are thoroughly characterized by elemental analyses, IR, UV–Vis spectroscopy, EPR study, magnetic susceptibility, thermogravimetry and cyclic voltammetry. Magnetic moment measurements reveal that the mononuclear complexes are paramagnetic while the binuclear complexes are diamagnetic in nature. EPR studies also confirm the presence of a mononuclear Mo(V) moiety in the complexes. Relevant Density Functional Theory (DFT) calculations have been carried out to determine the structures of the synthesized compounds. The binding mode and mechanism of interaction of the synthesized compounds with bovine serum albumin (BSA) were studied by concentration dependent absorption and fluorescence titration experiments. The ligand and complexes 1–4 are screened for their potential in vitro anticancer activities against three different human cancer cell lines, namely, cervix adenocarcinoma epithelial cells (HeLa), renal carcinoma cells (SK-RC-45) and breast adenocarcinoma cells (MCF-7). The oxomolybdenum(V) complexes are found to exhibit higher anticancer potency towards the cancer cells than the free ligand. Also, structure activity relationship (SAR) studies of this new series of oxomolybdenum(V) complexes indicate that the anticancer activity is to some extent dependent on the electronic effects of the halogen atom coordinated to the molybdenum centre.

2,2′-Bipyridine or 1,10-phenanthroline chelated oxomolybdenum(V) complexes with glycolate, lactate and malate in acidic media

A series of dimeric oxomolybdenum(V) 2,2′-bipyridine or 1,10-phenanthroline α-hydroxycarboxylates trans-[(MoO)2O(glyc)2(bpy)2]·3H2O (1), trans-[(MoO)2O(lact)2(bpy)2]·3H2O (2), trans-[(MoO)2O(Hmal)2(bpy)2] (3), trans-[(MoO)2O(lact)2(phen)2]·4H2O (4) and trans-[(MoO)2O(Hmal)2(phen)2]·4H2O (5) have been synthesized in acidic media for comparisons, where glycolate, lactate and malate in 1–5 chelate to molybdenum(V) with bidentate α-alkoxy and α-carboxy groups respectively. Unlike the molybdenum(IV) lactate in low valence [1], no protonation was observed for α-alkoxy groups even under acidic condition. The two molybdenum atoms in 1 and 3 are paramagnetic independently as examples by EPR spectroscopies. Solid state 13C NMR spectra of 1 and 3 show large downfield shifts for the coordinated glycolate and malate (δα-alkoxy 75.4, 82.8 ppm, Δδ 13.4 ppm). Moreover, a monomeric mixed ligand oxomolybdenum(VI) complex MoO2(glyc)(bpy) (6) was isolated.

Scrutinizing the substituent effect on Mo-based electrocatalysts for molecular hydrogen release through axial-equatorial decomposition: a DFT study.

Based on the experimental precedent discovered by Kuranadasa and coworkers [H. I. Karunadasa et al., Nature, 2010, 464, 1329] to produce dihydrogen from water electrocatalyzed by 2,6-bis[1,1-bis(2-pyridyl)ethyl]-pyridine oxo-molybdenum complexes, we performed an extensive analysis to study the substituent group effect of derivatised compounds coming from the before mentioned Mo-based metal-organic cations in terms of two kinds of substitutions: axial and equatorial at the para-position of pyridine rings; several conceptual tools were used to back up our conclusions. We found that each type of substituent group (electron-withdrawing and electron-donating ones) exerts an independent influence on energetic parameters (energy barrier and overall energy). This opens the chance to search for a synergistic effect by combining these opposite behaviours of these substituents located in the equatorial and axial para-positions of pyridine rings to computationally modulate the aforementioned energetic parameters. This procedure will make easier the proposal of new catalysts to favour either kinetically or thermodynamically or in both ways the production of dihydrogen from water. Additionally, we encompassed a key point: the number of solvent molecules, so that including their presence in further investigations is mandatory.

Mo(VI) and W(VI) complexes as heterogeneous catalysts for degradation of azo dyes

Oxomolybdenum(VI) and polyoxotungstate(VI) complexes, [C2H10N2][(MoO2)4O3(Hcit)2].6H2O (1) and Al2W24O85C96H64Cl1.33Cu5 (2) (H4cit = citric acid) were synthesized and structurally characterized by single crystal X-ray diffraction analysis. Compound 1 is a tetranuclear oxomolybdenum complex and 2 is a Keggin-type polyoxotungstate hybrid containing phen ligand (phen = 1,10-phenanthroline). The obtained materials were further characterized by powder X-ray diffraction (PXRD), inductive coupled plasma optical emission spectroscopy (ICP-OES) and Fourier transformation infrared (FT-IR). The prepared complexes exhibit good activity for the catalytic wet air oxidation (CWAO) and catalytic wet oxidation (CWO) of Bismarck Brown (BB) and Direct Blue 71 (DB) as typical examples of di- and three-azo dye pollutants in water. The degradation products were analyzed by ion chromatography (IC) and the catalysts could be reused without observable loss of their activity.

Structural Features of [{MoO2(Lbi)}2(μ-O)] Based Oxomolybdenum(VI) Complexes with Five-Coordinate Molybdenum(VI)

Structural features of eight binuclear complexes with the general formula [{MoO2(L bi n )}2(μ-O)] (I–VIII) (Lbi is a bidentate chelate ligand, n = 1–8), in which the coordination number of Mo atoms is five, are considered. The parameter τ = (A–B)/60, where A and B are the greatest bond angles among the ten bond angles at the Mo atoms in coordination pentahedra, can be used as a criterion characterizing the coordination polyhedron of the molybdenum atom in complexes I–VIII. The parameter τ is zero for an ideal square pyramid and unity for an ideal trigonal bipyramid.

N-hetercycle dimeric molybdenum(V) complexes with strong interactions and their catalytic degradations of methyl orange

A series of dimeric N-hetercycle oxomolybdenum(V) complexes cis-Mo2O4(SO4)(phen)2·2H2O (1), cis-Mo2O4(SO4)(bpy)2·4H2O (2), cis-Mo2O4Cl2(bpy)2 (3) and cis-Mo2O4(SO4)(im)4 (4) (phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine, im = imidazole) have been synthesized and characterized by the elemental analyses, IR, UV–vis, molecular fluorescence spectra, X-ray structural analyses and theoretical calculations. The two N-hetercycle ligands in 1 or 2 exhibit an approximate plane owing to SO42− steric hindrance with strong interactions (2.215av and 2.186av Å), which are new discrete dinuclear oxomolybdenum(V) complexes. 1–3 have been synthesized in acidic media under hydrothermal conditions and the transformations of oxomolybdenum(V) 2–3 and 1 to cis-Mo2O4Cl2(phen)2 (5) are also outlined respectively. Moreover, the catalytic performances of 1–5 were tested by the degradation of methyl orange. The conjugated N-hetercycle ligands have obvious influences on their catalytic performances.

Control of C-H Bond Activation by Mo-Oxo Complexes: pKa or Bond Dissociation Free Energy (BDFE)?

A density functional theory (DFT) study (BMK/6-31+G(d)) was initiated to investigate the activation of benzylic carbon-hydrogen bonds by a molybdenum-oxo complex with a potentially redox noninnocent supporting ligand-a simple mimic of the active species of the enzyme ethylbenzene dehydrogenase (EBDH)-through deprotonation (C-H bond heterolysis) or hydrogen atom abstraction (C-H bond homolysis) routes. Activation free-energy barriers for neutral and anionic Mo-oxo complexes were high, but lower for anionic complexes than neutral complexes. Interesting trends as a function of substituents were observed that indicated significant Hδ+ character in the transition states (TS), which was further supported by the preference for [2 + 2] addition over HAA for most complexes. Hence, it was hypothesized that C-H activation by these EBDH mimics is controlled more by the pKa than by the bond dissociation free energy of the C-H bond being activated. Therefore, the results suggest promising pathways for designing more efficient and selective catalysts for hydrocarbon oxidation based on EBDH active-site mimics.

Direct Conversion of Carbohydrates into 5‐Ethoxymethylfurfural (EMF) and 5‐Hydroxymethylfurfural (HMF) catalyzed by Oxomolybdenum complexes

AbstractThis work reports the first methodology for one‐pot synthesis of 5‐ethoxymethylfurfural (EMF) and 5‐hydroxymethylfurfural (HMF) from various carbohydrates catalyzed by oxo‐molybdenum complexes. The best results of EMF (53‐60 %) were obtained using a mixture of ethanol/THF (5:2) catalyzed by MoO2Cl2(H2O)2 at 120 °C, after 17 h. This dioxo‐molybdenum complex also promoted the synthesis of EMF from other carbohydrates such as inulin and sucrose. The selective conversion of fructose into HMF was achieved in good yield (75 %) from fructose using MoO2Cl2(H2O)2 in DMSO at 120 °C after 30 minutes. This novel methodology has the advantages of using an inexpensive, environmental friendly and air stable catalyst with an easy preparation.