Center for Molecular Modeling - M. Vandichel

Inserting CO2 into Terminal Alkynes via Bis-(NHC)-Metal Complexes

sven — Thu, 19 Jan 2017 09:55:07 +0000

H. Diaz-Velasquez, Z.-X. Wu, M. Vandichel, F. Verpoort

Catalysis Letters

147 (2), 463-471

2017

Abstract

The direct interaction between CO2 and terminal alkynes in the presence of bis-(NHC)-metal catalysts at ambient conditions was studied. Two Cu and Ag-based bis-N-heterocyclic carbene Transition Metal catalysts were synthesized. The (NHC)2-Ag complex showed a better catalytic performance towards the carboxylation of terminal alkynes in comparison with the copper analogue even for the conversion of acetylene gas. The optimized conditions for the carboxylation are: the use of Cs2CO3 as additive, one atmosphere CO2 and room temperature using 1% mol catalyst. Mechanistic insight into the reaction mechanism is obtained by means of state-of-the-art first principles calculations.

DOI

http://dx.doi.org/10.1007/s10562-016-1920-5

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art%3A10.1007%2Fs10562-016-1920-5.pdf

A robust molecular catalyst generated in-situ for photo- and electrochemical water oxidation

wim — Wed, 11 Jan 2017 14:43:33 +0000

H.A. Younus, N. Ahmad, A.H. Chughtai, M. Vandichel, M. Busch, K. Van Hecke, M. Yusubov, S. Song, F. Verpoort

ChemSusChem

10 (5), 862-875

2017

Abstract

Water splitting is the key step towards artificial photosystems for solar energy conversion and storage in the form of chemical bonding. The oxidation of water is the bottle-neck of this process that hampers its practical utility and hence efficient, robust, and also easy to make catalytic systems based on cheap and earth abundant materials are of exceptional importance. Here, we present an in-situ generated cobalt catalyst [Co(II)(TCA)2(H2O)2] (TCA = 1-Mesityl-1,2,3-1H-triazole-4-carboxylate) efficiently conducting photochemical water oxidation at near-neutral conditions. The catalyst is highly stable under photolytic conditions. During electrochemical water oxidation, the catalytic system assembled a catalyst film, which proved not to be cobalt oxide/hydroxide as normally expected, but instead and for the first time, generated a molecular cobalt complex incorporating the organic ligand bound to cobalt ions. The catalyst film exhibited low overpotential for electrocatalytic water oxidation (360 mV) and high oxygen evolution peak current density of 9 mA cm−2 and 2.7 mA cm−2 on GC and ITO electrodes at only 1.49 and 1.39 V (vs. NHE), respectively, under neutral conditions. Our finding, exemplified on the in-situ generated cobalt complex, might be applicable to other molecular systems and suggests that the formation of catalytic film in electrochemical water oxidation experiments is not always an indication of the catalyst decomposition and formation of nanoparticles.

DOI

http://dx.doi.org/10.1002/cssc.201601477

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Younus_et_al-2016-ChemSusChem.pdf

Biocompatible Zr-based nanoscale MOFs coated with modified poly (epsilon-caprolactone) as anticancer drug carriers

wim — Tue, 23 Aug 2016 12:05:16 +0000

M. Filippousi, S. Turner, K. Leus, P.I. Siafaka, E.D. Tseligka, M. Vandichel, S.G. Nanaki, I.S. Vizirianakis, D.N. Bikiaris, P. Van der Voort, G. Van Tendeloo

International Journal of Pharmaceutics

509, 1-2, 208-218

2016

Abstract

Nanoscale Zr-based metal organic frameworks (MOFs) UiO-66 and UiO-67 were studied as potential anticancer drug delivery vehicles. Two model drugs were used, hydrophobic paclitaxel and hydrophilic cisplatin, and were adsorbed onto/into the nano MOFs (NMOFs). The drug loaded MOFs were further encapsulated inside a modified poly(epsilon-caprolactone) with D-alpha-tocopheryl polyethylene glycol succinate polymeric matrix, in the form of microparticles, in order to prepare sustained release formulations and to reduce the drug toxicity. The drugs physical state and release rate was studied at 37 degrees C using Simulated Body Fluid. It was found that the drug release depends on the interaction between the MOFs and the drugs while the controlled release rates can be attributed to the microencapsulated formulations. The in vitro antitumor activity was assessed using HSC-3 (human oral squamous carcinoma; head and neck) and U-87 MG (human glioblastoma grade IV; astrocytoma) cancer cells. Cytotoxicity studies for both cell lines showed that the polymer coated, drug loaded MOFs exhibited better anticancer activity compared to free paclitaxel and cisplatin solutions at different concentrations. (C) 2016 Elsevier B.V. All rights reserved.

DOI

http://dx.doi.org/10.1016/j.ijpharm.2016.05.048

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16-IntJPharma-509(1-2)208-Filippousi.pdf

Water coordination and dehydration processes in defective UiO-66 type metal organic frameworks

michel — Sat, 09 Jul 2016 19:22:40 +0000

M. Vandichel, J. Hajek, A. Ghysels, A. De Vos, M. Waroquier, V. Van Speybroeck

CrystEngComm

18 (37), 7056-7069

2016

Abstract

The UiO-66 metal organic framework is one of the most thermally and chemically stable hybrid materials reported to date. However, it is also accepted that the material contains structurally embedded defects, which may be engineered to enhance properties towards specific applications such as catalysis, sensing, etc. The synthesis conditions determine to a large extent the level of perfection of the material and additionally the catalytic activity may be enhanced by post-synthesis activation at high temperature under vacuum, in which defect coordinating species (H2O, HCl, monocarboxylic modulators, etc.) evaporate. The molecular level characterization of defects is extremely challenging from both theoretical and experimental points of view. Such experimental endeavor was recently proposed via experimental SXRD measurements, also unraveling the coordination of water on the Zr–O–Zr defect sites [Angew. Chem., Int. Ed., 2015, 54, 11162–11167]. The present work provides a theoretical understanding of defect structures in UiO-66(Zr) by means of periodic density functional theory calculations and ab initio molecular dynamics simulations. A range of defect structures are generated with different numbers of missing linkers. For each of the defects, the free energetic and mechanical stability is discussed and the coordination of water and charge balancing hydroxide ions is studied. For catalysis applications, the material is mostly pretreated to remove water by dehydration reactions. For each of the proposed defect structures, mechanistic pathways for dehydration reactions of the Zr-bricks are determined employing nudged elastic band (NEB) calculations. During the dehydroxylation trajectory, loose hydroxyl groups and terephthalate decoordinations are observed. Furthermore, dehydration reactions are lower activated if terephthalate linkers are missing in the immediate environment of the inorganic brick. The creation of defects and the dehydration processes have a large impact on the mechanical properties of the material, which is evidenced by lower bulk moduli and elastic constants for structures with more defects.

DOI

10.1039/C6CE01027J

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Vandichel-CrystEngComm-2016-SupportingInformation-c6ce01027j4.pdf

Vandichel et al.pdf

Origin of highly active metal–organic framework catalysts: defects? Defects!

mat — Sun, 15 Nov 2015 10:46:45 +0000

J. Canivet, M. Vandichel, D. Farrusseng

Dalton Transactions

45, 4090-4099

2016

Abstract

This article provides a comprehensive review of the nature of catalytic sites in MOFs. In the last decade, a number of striking studies have reported outstanding catalytic activities of MOFs. In all cases, the authors were intrigued as it was unexpected from the ideal structure. We demonstrate here that (surface) defects are at the origin of the catalytic activities for the reported examples. The vacancy of ligands or linkers systematically generates (surface) terminations which can possibly show Lewis and/or Brønsted acido-basic features. The engineering of catalytic sites at the nodes by the creation of defects (on purpose) appears today as a rational approach for the design of active MOFs. Similarly to zeolite post-treatments, post-modifications of MOFs by linker or metal cation exchange appear to be methods of choice. Despite the mild acidity of defective MOFs, we can account for very active MOFs in a number of catalytic applications which show higher performances than zeolites or benchmark catalysts.

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1039/C5DT03522H

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Mechanistic studies of aldol condensations in UiO-66 and UiO-66-NH2 metal organic frameworks

michel — Wed, 24 Jun 2015 09:17:46 +0000

J. Hajek, M. Vandichel, B. Van de Voorde, B. Bueken, D. De Vos, M. Waroquier, V. Van Speybroeck

Journal of Catalysis

331, 1-12

2015

Abstract

A full mechanistic investigation is proposed for the industrially important cross-aldol condensation reaction of heptanal with benzaldehyde on the UiO-66 and the amino-functionalized UiO-66-NH2 metal–organic frameworks to form jasminaldehyde. Several experimental studies indicate that the activity for the aldol condensation reaction can be increased by proper functionalization of the material, e.g. by introducing an additional basic amino site and thus creating a bifunctional acid–base catalyst for the aldol condensation. The precise molecular level origin for this behavior is to date unclear. Herein state-of-the-art Density-Functional Theory (DFT) calculations have been performed to unravel the mechanism of the cross- and self-aldol condensations of benzaldehyde and propanal. To this end free energy calculations have been performed on both extended cluster and periodic models. It is found that the mechanism on both catalysts is essentially the same, although a slightly stronger adsorption of the reactants and slightly lower barriers were found on the amino functionalized material, pointing toward higher initial activities. New experiments were performed to confirm these observations. It is indeed found that the initial activity toward cross-aldol condensation on the amino functionalized material is higher, although after about 40 min of reaction both materials become equally active. Our results furthermore point out that the basic amino groups may promote side reactions such as imine formation, which is induced by water. The study as presented can assist to engineer materials at the molecular level toward the desired products.

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1016/j.jcat.2015.08.015

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Water-soluble NHC-Cu catalysts: applications in click chemistry, bioconjugation and mechanistic analysis

mat — Thu, 18 Dec 2014 16:12:29 +0000

H. Diaz-Velasquez, Y. Ruiz Garcia, M. Vandichel, A. Madder, F. Verpoort

Organic & Biomolecular Chemistry

12, 9350-9356

2014

Abstract

Copper(I)-catalyzed 1,3-dipolar cycloaddition of azides and terminal alkynes (CuAAC), better known as “Click” reaction, has triggered the use of 1,2,3-triazoles in bioconjugation, drug discovery, materials science and combinatorial chemistry. Here we report a new series of water-soluble catalysts based on N-heterocyclic carbenes (NHC)-Cu complexes which are additionally functionalized with a sulfonate group. The complexes show superior activity towards CuAAC reactions and display a high versatility, enabling the production of triazoles with different substitution patterns. Additionally, successful application of these complexes in bioconjugation using unprotected peptides acting as DNA binding domains was achieved for the first time. Mechanistic insight into the reaction mechanism is obtained by means of state-of-the-art first principles calculations.

DOI

http://dx.doi.org/10.1039/C4OB01350F

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CuNHC_good_text.pdf

Au@UiO-66: a base free oxidation catalyst

michel — Wed, 12 Nov 2014 16:36:37 +0000

K. Leus, P. Concepcion, M. Vandichel, M. Meledina, A. Grirrane, D. Esquivel, S. Turner, D. Poelman, M. Waroquier, V. Van Speybroeck, G. Van Tendeloo, H. Garcia, P. Van der Voort

RSC Advances

5 (29), 22334–22342

2015

Abstract

We present the in situ synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66. The resulting Au@UiO-66 materials were characterized by means of N2 sorption, XRPD, UV-Vis, XRF, XPS and TEM analysis. The Au nanoparticles (NP) are homogeneously distributed along the UiO-66 host matrix when using NaBH4 or H2 as reducing agents. The Au@UiO-66 materials were evaluated as catalysts in the oxidation of benzyl alcohol and benzyl amine employing O2 as oxidant. The Au@MOF materials exhibit a very high selectivity towards the ketone (up to 100 %). Regenerability and stability tests demonstrate that the Au@UiO-66 catalyst can be recycled with a negligible loss of Au species and no loss of crystallinity. In situ IR measurements of UiO-66 and Au@UiO-66-NaBH4, before and after treatment with alcohol, showed an increase in IR bands that can be assigned to a combination of physisorbed and chemisorbed alcohol species. This was confirmed by velocity power spectra obtained from the molecular dynamics simulations. Active peroxo and oxo species on Au could be visualized with Raman analysis.

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1039/C4RA16800C

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Active site engineering in UiO-66 type metal-organic frameworks by intentional creation of defects: a theoretical rationalization

mat — Tue, 30 Sep 2014 15:08:29 +0000

M. Vandichel, J. Hajek, F. Vermoortele, D. De Vos, M. Waroquier, V. Van Speybroeck

CrystEngComm

17 (2), 395-406

2015

Abstract

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 can be drastically increased in the absence of part of the linkers, as it removes the full coordination of the framework metal ions and leads to open sites. As a result, metal centers become more accessible and thus more active for Lewis acid catalysed reactions. Addition of modulators (MDL) to the synthesis mixture can create more linker deficiencies (Vermoortele et al., J Am Chem Soc, 2013, 135, 11465) and can lead to a significant increase of the catalytic activity due to the creation of a larger number of open sites. In this paper, we rationalize the function of the modulators under real synthesis conditions by the construction of free energy diagrams. The UiO-66 type materials form a very appropriate test case as the effect of addition of modulators hydrochloric acid (HCl) and trifluoroacetate (TFA) has been intensively investigated experimentally for the synthesis process and post-synthetic thermal activation. In synthesis conditions, direct removal of BDC linkers requires a high free energy but replacement of such linker by one or more TFA species might occur especially at high TFA:BDC ratios in the reaction mixture. The presence of HCl furthermore enhances the creation of defect structures. Post-synthesis activation procedures at higher temperatures lead to a substantial removal of the species coordinated to the Zr bricks creating open metal sites. A mechanistic pathway is presented for the dehydroxylation process of the hexanuclear Zr cluster. For the citronellal cyclization, we show that the presence of some residual TFA in the structure may lead to faster reactions in complete agreement with experiment. Hirshfeld-e partial charges for the Zr ions have been computed to investigate their sensitivity to substituent effects; a strong correlation with the experimental Hammett parameters, and with the rates of the citronellal cyclization is found. The theoretical rationalization may serve as a basis for detailed active site engineering studies.

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1039/C4CE01672F

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c4ce01672f1.pdf

Active site engineering in UiO-66 type Metal Organic Frameworks by intentional creation of defects : A theoretical rationalization

michel — Tue, 12 Aug 2014 19:16:17 +0000

M. Vandichel, J. Hajek, F. Vermoortele, M. Waroquier, D. De Vos, V. Van Speybroeck

CrystEngComm

First published online 15 Sep 2014

2014

Published while none of the authors were employed at the CMM

Abstract

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 can be drastically increased in the absence of part of the linkers, as it removes the full coordination of the framework metal ions and leads to open sites. As a result, metal centers become more accessible and thus more active for Lewis acid catalysed reactions. Addition of modulators (MDL) to the synthesis mixture can create more linker deficiencies (Vermoortele et al., J Am Chem Soc, 2013, 135, 11465) and can lead to a significant increase of the catalytic activity due to the creation of a larger number of open sites. In this paper, we rationalize the function of the modulators under real synthesis conditions by the construction of free energy diagrams. The UiO-66 type materials form a very appropriate test case as the effect of addition of modulators HCl and trifluoroacetate (TFA) has been intensively investigated experimentally for the synthesis process and post-synthetic thermal activation. In synthesis conditions, direct removal of BDC linkers requires a high free energy but replacement of such linker by one or more TFA species might occur especially at high TFA:BDC ratios in the reaction mixture. The presence of HCl furthermore enhances the creation of defect structures. Post-synthesis activation procedures at higher temperatures lead to a substantial removal of the species coordinated to the Zr bricks creating open metal sites. A mechanistic pathway is presented for the dehydroxylation process of the hexanuclear Zr cluster. For the citronellal cyclization, we show that the presence of some residual TFA in the structure may lead to faster reactions in complete agreement with experiment. Hirshfeld-e partial charges for the Zr ions have been computed to investigate their sensitivity to substituent effects; a strong correlation with the experimental Hammett parameters, and with the rates of the citronellal cyclization is found. The theoretical rationalization may serve as a basis for detailed active site engineering studies.

DOI

http://dx.doi.org/10.1039/C4CE01672F