Publications by authors named "Nina Vankova"

23 Publications

  • Page 1 of 1

Strong Binding of Noble Gases to [BX]: A Theoretical Study.

J Phys Chem A 2021 Jun 26;125(22):4760-4765. Epub 2021 May 26.

Institute of Resource Ecology, Research Site Leipzig, Helmholtz-Zentrum Dresden-Rossendorf, 04318 Leipzig, Germany.

We systematically explore the stability and properties of [BXNG] adducts resulting from the binding of noble gas atoms to anionic [BX] clusters in the gas phase of mass spectrometers. [BX] can be obtained by stripping one X off the icosahedral -dodecaborate dianion [BX]. We study the binding of the noble gas atoms He, Ne, Ar, Kr, and Xe to [BX] with substituents X = F, Cl, Br, I, and CN. While He cannot be captured by these clusters and Ne only binds at low temperatures, the complexes with the heavier noble gas atoms Ar, Kr, and Xe show appreciable complexation energies and exceed 1 eV at room temperature in the case of [B(CN)Xe]. The predicted B-NG equilibrium distance in the complexes with Ar, Kr, and Xe is only 0.10-0.25 Å longer than the sum of the covalent radii of the two corresponding atoms, and a significant charge transfer from the noble gas atom to the icosahedral B cage is observed.
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http://dx.doi.org/10.1021/acs.jpca.1c01909DOI Listing
June 2021

Chirality Remote Control in Nanoporous Materials by Circularly Polarized Light.

J Am Chem Soc 2021 May 29;143(18):7059-7068. Epub 2021 Apr 29.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

The ability to dynamically control chirality remains a grand challenge in chemistry. Although many molecules possess chiral isomers, lacking their isolation, for instance during photoisomerization, results in racemic mixtures with suppressed enantiospecific chiral properties. Here, we present a nanoporous solid in which chirality and enantioselective enrichment is induced by circularly polarized light (CPL). The material is based on photoswitchable fluorinated azobenzenes attached to the scaffold of a crystalline metal-organic framework (MOF). The azobenzene undergoes -to--photoisomerization upon irradiation with green light and reverts back to upon violet light. While each moiety in conformation is chiral, we show the isomer also possesses a nonplanar, chiral conformation. During photoisomerization with unpolarized light, no enantiomeric enrichment is observed and both isomers, - and - as well as and -, respectively, are formed in identical quantities. In contrast, CPL causes chiral photoresolution, resulting in an optically active material. Right-CPL selectively excites - and - enantiomers, producing a MOF with enriched -enantiomers, and . The induction of optical activity is reversible and only depends on the light-handedness. As shown by first-principle DFT calculations, while both, and , are stabilized in nonplanar, chiral conformations in the MOF, the isomer adopts a planar, achiral form in solution, as verified experimentally. This shows that the chiral photoresolution is enabled by the linker reticulation in the MOF. Our study demonstrates the induction of chirality and optical activity in solid materials by CPL and opens new opportunities for chiral resolution and information storage with CPL.
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http://dx.doi.org/10.1021/jacs.1c01693DOI Listing
May 2021

Dissolving uptake-hindering surface defects in metal-organic frameworks.

Chem Sci 2019 Jan 10;10(1):153-160. Epub 2018 Oct 10.

Institute of Functional Interfaces (IFG) , Karlsruhe Institute of Technology (KIT) , Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen , Germany . Email:

Metal-organic frameworks (MOFs) have unique properties which make them perfectly suited for various adsorption and separation applications; however, their uses and efficiencies are often hindered by their limited stability. When most MOFs are exposed to water or humid air, the MOF structure, in particular at the surface, is destroyed, creating surface defects. These surface defects are surface barriers which tremendously hinder the uptake and release of guest molecules and, thus, massively decrease the performance in any application of MOFs. Here, the destruction by exposure to water vapor is investigated by using well-defined MOF films of type HKUST-1 as a model system for uptake experiments with different-sized probe molecules as well as for spectroscopic investigations, complemented by density functional theory calculations of the defect structure. In addition to the characterization of the surface defects, it is found that the pristine MOF structure can be regenerated. We show that the surface defects can be dissolved by exposure to the synthesis solvent, here ethanol, enabling fast uptake and release of guest molecules. These findings show that the storage of MOF materials in a synthesis solvent results in healing of surface defects and enables ideal performance of MOF materials.
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http://dx.doi.org/10.1039/c8sc03735cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6328000PMC
January 2019

Cavitation energies can outperform dispersion interactions.

Nat Chem 2018 12 8;10(12):1252-1257. Epub 2018 Oct 8.

Department of Life Sciences and Chemistry, Jacobs University Bremen, Bremen, Germany.

The accurate dissection of binding energies into their microscopic components is challenging, especially in solution. Here we study the binding of noble gases (He-Xe) with the macrocyclic receptor cucurbit[5]uril in water by displacement of methane and ethane as H NMR probes. We dissect the hydration free energies of the noble gases into an attractive dispersive component and a repulsive one for formation of a cavity in water. This allows us to identify the contributions to host-guest binding and to conclude that the binding process is driven by differential cavitation energies rather than dispersion interactions. The free energy required to create a cavity to accept the noble gas inside the cucurbit[5]uril is much lower than that to create a similarly sized cavity in bulk water. The recovery of the latter cavitation energy drives the overall process, which has implications for the refinement of gas-storage materials and the understanding of biological receptors.
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http://dx.doi.org/10.1038/s41557-018-0146-0DOI Listing
December 2018

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20 K.

Angew Chem Int Ed Engl 2016 10 7;55(41):12683-7. Epub 2016 Sep 7.

Institut für Funktionelle Grenzflächen, Karlsruher Institut für Technologie, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

We have studied the magnetic properties of the SURMOF-2 series of metal-organic frameworks (MOFs). Contrary to bulk MOF-2 crystals, where Cu(2+) ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu(2+) ions are connected via carboxylate groups in a zipper-like fashion. This unusual coupling of the spin 1/2 ions within the resulting one-dimensional chains is found to stabilize a low-temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy-to-fabricate, material to be described in a detailed fashion.
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http://dx.doi.org/10.1002/anie.201606016DOI Listing
October 2016

Chiral Dodecanuclear Palladium(II) Thio Cluster: Synthesis, Structure, and Formation Mechanism Explored by ESI-MS and DFT Calculations.

Inorg Chem 2016 Aug 25;55(16):7811-3. Epub 2016 Jul 25.

Key Laboratory of Cluster Science, Ministry of Education of China, School of Chemistry, Beijing Institute of Technology , 100081 Beijing, P. R China.

The chiral dodecanuclear palladium(II) thio cluster LaPd12(C3H5NO2S)3(C3H6NO2S)21 (1) was prepared by reacting l-cysteine (l-Cys) with PdCl2 and La2O3 in aqueous solution under carefully controlled conditions. Compound 1 was structurally characterized by single-crystal XRD, TGA, IR, UV-vis, (13)C NMR, and CD spectroscopy. Insight into the dimerization process of 1 was obtained by ESI-MS and DFT calculations.
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http://dx.doi.org/10.1021/acs.inorgchem.6b01005DOI Listing
August 2016

Platinum-Containing Polyoxometalates: syn- and anti-[Pt(II)2(α-PW11O39)2](10-) and Formation of the Metal-Metal-Bonded di-Pt(III) Derivatives.

Chemistry 2016 Apr 3;22(16):5514-9. Epub 2016 Mar 3.

Department of Life Sciences and Chemistry, Jacobs University, P.O. Box 750 561, 28725, Bremen, Germany.

The first examples of dimeric, di-Pt(II) -containing heteropolytungstates are reported. The two isomeric di-platinum(II)-containing 22-tungsto-2-phosphates [anti-Pt(II)2(α-PW11O39)2](10-) (1 a) and [syn-Pt(II)2(α-PW11O39)2](10-) (2 a) were synthesized in aqueous pH 3.5 medium using one-pot procedures. Polyanions 1 a and 2 a contain a core comprising two face-on PtO4 units, with a Pt⋅⋅⋅Pt distance of 2.9-3 Å. Both polyanions were investigated by single-crystal XRD, IR, TGA, UV/Vis, (31) P NMR, ESI-MS, CID-MS/MS, electrochemistry, and DFT. On the basis of DFT and electrochemistry, we demonstrated that the {Pt2(II)} moiety in 1 a and 2 a can undergo fully reversible two-electron oxidation to {Pt2(III)}, accompanied by formation of a single Pt-Pt bond. Hence we have discovered the novel subclass of Pt(III)-containing heteropolytungstates.
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http://dx.doi.org/10.1002/chem.201600555DOI Listing
April 2016

The polyoxo-22-palladate(ii), [Na2PdO12(As(V)O4)15(As(V)O3OH)](25-) .

Dalton Trans 2016 Feb 13;45(6):2394-8. Epub 2016 Jan 13.

Department of Life Sciences and Chemistry, Jacobs University, P.O. Box 750561, 28725 Bremen, Germany.

The polyoxo-22-palladate [Na2PdO12(As(V)O4)15(As(V)O3OH)](25-) (1), which represents a novel polyoxo-noble-metalate structural type, was synthesized by reaction of Pd(2+) and AsO4(3-) ions in aqueous solution. Polyanion 1 comprises two {NaPd11} units linked by two arsenate bridges, and hence represents the first example of a defect, monolacunary {Pd11} polyoxopalladate nanocube with arsenate capping groups. The title polyanion was characterized in the solid state as well as by theoretical calculations.
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http://dx.doi.org/10.1039/c5dt04526fDOI Listing
February 2016

A kinetic study on the reduction of CO2 by frustrated Lewis pairs: from understanding to rational design.

Phys Chem Chem Phys 2016 Feb 11;18(5):3567-74. Epub 2016 Jan 11.

Department of Physics and Earth Science, Jacobs University Bremen, Campus Ring 1, 28759, Bremen, Germany.

Carbon dioxide (CO2) is known as one of the major reasons for global warming. On the other hand, CO2 is considered as an abundant carbon source. Therefore, transformation of CO2 into target chemicals nowadays is of great interest. Recently, a concept of so-called "frustrated Lewis pairs" (FLPs) has been proposed. Such FLPs show unusual reactivity, such as hydrogen activation and the reduction of CO2. In this study, by means of density functional theory (DFT) and ab initio calculations, we conduct a kinetic survey on the reduction of CO2 by a series of FLPs. We investigate the relationship between the electronic structures and kinetic properties. The kinetic properties include: (1) reaction energy barriers, (2) the structural properties of the associated transition states (TSs), and (3) the natural charge population in these TSs. Our results indicate that there is a systematic relationship between the electronic structures and the kinetic properties, and, as a rule of thumb, similar activation barriers for both individual reactions are needed for best performance. The derived relationship can be used not only to rationalize the published experimental results, but also to assist the future design of more efficient Lewis acid-base pairs as metal-free catalysts for the reduction of CO2.
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http://dx.doi.org/10.1039/c5cp06925dDOI Listing
February 2016

Are intramolecular frustrated Lewis pairs also intramolecular catalysts? A theoretical study on H2 activation.

Phys Chem Chem Phys 2015 Apr;17(16):10687-98

Department of Physics & Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.

We investigate computationally a series of intramolecular frustrated Lewis pairs (FLPs), with the general formula Mes2PCHRCH2B(C6F5)2, that are known from the literature to either activate molecular hydrogen (FLPs with R = H (1) or Me (4)), or remain inert (FLPs with R = Ph (2) or SiMe3 (3)). The prototypical system Mes2PCH2CH2B(C6F5)2 (1) has been described in the literature (Grimme et al., Angew. Chem., Int. Ed., 2010; Rokob et al., J. Am. Chem. Soc., 2013) as an intramolecular reactant that triggers the reaction with H2 in a bimolecular concerted fashion. In the current study, we show that the concept of intramolecular H2 activation by linked FLPs is not able to explain the inertness of the derivative compounds 2 and 3 towards H2. To cope with this, we propose an alternative intermolecular mechanism for the investigated reaction, assuming stacking of two open-chain FLP conformers, and formation of a dimeric reactant with two Lewis acid–base domains, that can split up to two hydrogen molecules. Using quantum-chemical methods, we compute the reaction profiles describing these alternative mechanisms, and compare the derived predictions with earlier reported experimental results. We show that only the concept of intermolecular H2 activation could explain both the activity of the FLPs having small substituents in the bridging molecular region, and the inertness of the FLPs with a bulkier substitution, in a consistent way. Importantly, the intermolecular H2 activation driven by intramolecular FLPs indicates the key role of steric factors and noncovalent interactions for the design of metal-free systems that can efficiently split H2, and possibly serve as metal-free hydrogenation catalysts.
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http://dx.doi.org/10.1039/c5cp00368gDOI Listing
April 2015

The mixed gold-palladium polyoxo-noble-metalate [NaAu(III)4Pd(II)8O8(AsO4)8](11-).

Chemistry 2014 Jul 4;20(28):8556-60. Epub 2014 Jun 4.

Jacobs University, School of Engineering and Science, P.O. Box 750561, 28725 Bremen (Germany), Fax: (+49) 421-200-3229; Current address: Peter-Grünberg-Institut, PGI-6, Forschungszentrum Jülich, 52425 Jülich (Germany), On leave from Nikolaev Institute of Inorganic Chemistry, Prospekt Lavrentyeva 3, 630090 Novosibirsk (Russia).

The first fully inorganic, discrete gold-palladium-oxo complex [NaAu(III) 4 Pd(II) 8 O8 (AsO4 )8 ](11-) has been synthesized in aqueous medium. The combination of single-crystal XRD, elemental analysis, mass spectrometry, and DFT calculations allowed establishing the structure and composition of the novel polyanion, and UV/Vis studies suggest that it is stable in neutral aqueous media.
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http://dx.doi.org/10.1002/chem.201403032DOI Listing
July 2014

Extension of the Universal Force Field to Metal-Organic Frameworks.

J Chem Theory Comput 2014 Feb 31;10(2):880-91. Epub 2014 Jan 31.

School of Engineering and Science, Jacobs University Bremen Campus Ring 1, 28759 Bremen, Germany.

The Universal Force Field (UFF) (Rappé et al., J. Am. Chem. Soc. 1992) provides a general approach to molecular mechanics for molecules and materials composed of elements throughout the periodic table. Though the method is tunable by the specification of bond orders and the introduction of effective charges, the presently available list of atom types is insufficient to treat various systems containing transition metals, including metal-organic frameworks (MOFs). As MOFs are composite materials built of a combination of individually stable building blocks, a plethora of MOF structures are possible, and the prediction of their structure with a low-cost method is important. We have extended the UFF parameter set to include transition metal elements Zn, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, Sc, and Al, as they occur in MOFs, and have proposed additional O parameters that provide reliable structures of the metal oxide clusters of the connectors. We have benchmarked the performance of the MOF extension to UFF (UFF4MOF) with respect to experimentally available data and to DFT calculations. The parameters are available in various well-spread programs, including GULP, deMonNano, and ADF, and all information is provided to include them in other molecular mechanics codes.
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http://dx.doi.org/10.1021/ct400952tDOI Listing
February 2014

On the mechanism of hydrogen activation by frustrated Lewis pairs.

Chemistry 2013 Dec 8;19(51):17413-24. Epub 2013 Nov 8.

School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759, Bremen (Germany).

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H2, have been subjected to DFT and (SCS-)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H2 and hydrogenation to the ion-pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H2 , 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H2 by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H2. The derived predictions could assist the future design of Lewis acid-base systems with desired properties and applicability as metal-free hydrogenation catalysts.
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http://dx.doi.org/10.1002/chem.201302727DOI Listing
December 2013

Chemical speciation of metal complexes from chemical shift calculations: the interaction of 2-amino-N-hydroxypropanamide with V(V) in aqueous solution.

J Phys Chem B 2013 Oct 11;117(39):11670-80. Epub 2013 Sep 11.

Grupo de Pesquisa em Química Inorgânica Teórica-GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais , 31.270-901 Belo Horizonte, MG, Brazil.

The chemical speciation of 2-amino-N-hydroxypropanamide (β-alaninohydroxamic acid, HL) and vanadium (V) in aqueous solution has been investigated through calculations of the thermodynamic properties and the (51)V nuclear magnetic resonance (NMR) chemical shifts of the species formed at equilibrium. The results have been compared directly with the experimental (51)V NMR data. The (51)V NMR chemical shifts have been calculated by using a density functional theory (DFT) approach accounting for relativistic corrections and solvent effects. All tautomers of the 1:1 and 1:2 VO2(+)/β-ala complexes with different degrees of protonation have been calculated and thermodynamic and structural properties are presented for the most stable species. The system is better modeled as tautomeric equilibria, and species lying down in the range of 10 kcal·mol(-1) cannot be neglected at the BP/TZ2P/COSMO approach. In fact, the metal complex speciation in aqueous solution should not be investigated based solely on the thermodynamic analysis, but together with spectroscopic calculations such as NMR.
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http://dx.doi.org/10.1021/jp403744yDOI Listing
October 2013

On the gas-phase dimerization of negatively charged closo-dodecaborates: a theoretical study.

Phys Chem Chem Phys 2013 Jul 15;15(25):10358-66. Epub 2013 May 15.

School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.

We have studied the intriguing gas-phase dimerization of the B12In(-) (n = 9, 8) anions to B24I2n(2-) dianions by means of density functional theory calculations. The dimerization of B12I9(-) to B24I18(2-) has been detected experimentally in a previous study (Phys. Chem. Chem. Phys., 2011, 13, 5712) utilizing electrospray ionization ion trap mass spectrometry (ESI-IT-MS), whereas the formation of B24I16(2-) from B12I8(-) is modeled here prior to experiment. Calculations are carried out to determine the molecular and electronic structures, the bonding situation and the stability of the dimers relative to the respective monomers. The dimerization process is found to be thermodynamically favorable, and the stability of the lowest-energy structures is substantiated by molecular dynamics simulations. The calculations imply that the experimentally observed B24I18(2-) and the hypothetical B24I16(2-) species are formed through dimerization of the respective B12In(-) (n = 9, 8) monomers, rather than through loss of two I radicals from B24I2n+2(2-) dimers. Electronic properties such as natural charges, vertical detachment energies (VDEs), frontier molecular orbitals (FMOs), and HOMO-LUMO gaps are computed and analyzed in detail for all monomers and dimers. The analysis shows that the most stable B24I2n(2-) dimers are formed through two 2c-2e B-B and two 3c-2e B-I-B bridges between the parent B12In(-) (n = 9, 8) monomers. These new bridging bonds engage the deiodinated (bare) faces of the two B12 icosahedra, as well as one (per monomer) of the nearest boron neighbors and its iodine substituent.
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http://dx.doi.org/10.1039/c3cp50722jDOI Listing
July 2013

From an icosahedron to a plane: flattening dodecaiodo-dodecaborate by successive stripping of iodine.

Chemistry 2012 Oct 7;18(41):13208-12. Epub 2012 Sep 7.

Institut de Ciència de Materials de Barcelona ICMAB-CSIC, 08193 Bellaterra, Spain.

It has been shown by electrospray ionization-ion-trap mass spectrometry that B(12)I(12)(2-) converts to an intact B(12) cluster as a result of successive stripping of single iodine radicals or ions. Herein, the structure and stability of all intermediate B(12)I(n)(-) species (n=11 to 1) determined by means of first-principles calculations are reported. The initial predominant loss of an iodine radical occurs most probably via the triplet state of B(12)I(12)(2-), and the reaction path for loss of an iodide ion from the singlet state crosses that from the triplet state. Experimentally, the boron clusters resulting from B(12)I(12)(2-) through loss of either iodide or iodine occur at the same excitation energy in the ion trap. It is shown that the icosahedral B(12) unit commonly observed in dodecaborate compounds is destabilized while losing iodine. The boron framework opens to nonicosahedral structures with five to seven iodine atoms left. The temperature of the ions has a considerable influence on the relative stability near the opening of the clusters. The most stable structures with five to seven iodine atoms are neither planar nor icosahedral.
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http://dx.doi.org/10.1002/chem.201200828DOI Listing
October 2012

Noncovalent bifunctional organocatalysts: powerful tools for contiguous quaternary-tertiary stereogenic carbon formation, scope, and origin of enantioselectivity.

Chemistry 2012 Mar 22;18(13):4088-98. Epub 2012 Feb 22.

School of Engineering & Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.

Relying on the assembly of commercially available catalyst building blocks, highly stereocontrolled quaternary carbon (all carbon substituted) formation has been achieved with unmatched substrate diversity. For example, the in situ assembly of a tricomponent catalyst system allows α-branched aldehyde addition to nitroalkene or maleimide electrophiles (Michael products), while addition to an α-iminoester affords Mannich reaction products. Very good yields are observed and for fifteen of the eighteen examples 96-99 % ee is observed. Using racemic α-branched aldehydes, two contiguous (quaternary-tertiary) stereogenic centers can be formed in high diastereo- and enantiomeric excess (eight examples) via an efficient in situ dynamic kinetic resolution, solving a known shortcoming for maleimide electrophiles in particular. The method is of practical value, requiring only 1.2 equiv of the aldehyde, a 5.0 mol % loading of each catalyst component, for example, O-tBu-L-threonine (O-tBu-L-Thr), sulfamide, DMAP or O-tBu-L-Thr, KOH, and room temperature reactions. As a highlight, the first demonstration of ethylisovaleraldehyde (7) addition is disclosed, providing the most congested quaternary stereogenic carbon containing succinimide product (8) known to date. Finally, mechanistic insight, via DFT calculations, support a noncovalent assembly of the catalyst components into a bifunctional catalyst, correctly predict two levels of product stereoselectivity, and suggest the origin of the tricomponent catalyst system's exceptionality: an alternative hydrogen bond motif for the donor-acceptor pair than currently suggested for non-assembled catalysts.
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http://dx.doi.org/10.1002/chem.201103005DOI Listing
March 2012

Polyoxomolybdodiphosphonates: examples incorporating ethylidenepyridines.

Inorg Chem 2011 Nov 17;50(22):11667-75. Epub 2011 Oct 17.

School of Engineering and Science, Jacobs University, P.O. Box 750561, 28725 Bremen, Germany.

We have synthesized and structurally characterized three pyridylethylidene-functionalized diphosphonate-containing polyoxomolybdates, [{Mo(VI)O(3)}(2){Mo(V)(2)O(4)}{HO(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)](6-) (1), [{Mo(VI)(2)O(6)}(2){Mo(V)(2)O(4)}{O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)](8-) (2), and [{Mo(V)(2)O(4)(H(2)O)}(4){O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(4)](12-) (3). Polyanions 1-3 were prepared in a one-pot reaction of the dinuclear, dicationic {Mo(V)(2)O(4)(H(2)O)(6)}(2+) with 1-hydroxo-2-(3-pyridyl)ethylidenediphosphonate (Risedronic acid) in aqueous solution. Polyanions 1 and 2 are mixed-valent Mo(VI/V) species with open tetranuclear and hexanuclear structures, respectively, containing two diphosphonate groups. Polyanion 3 is a cyclic octanuclear structure based on four {Mo(V)(2)O(4)(H(2)O)} units and four diphosphonates. Polyanions 1 and 2 crystallized as guanidinium salts [C(NH(2))(3)](5)H[{Mo(VI)O(3)}(2){Mo(V)(2)O(4)}{HO(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)]·13H(2)O (1a) and [C(NH(2))(3)](6)H(2)[{Mo(VI)(2)O(6)}(2){Mo(V)(2)O(4)}{O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)]·10H(2)O (2a), whereas polyanion 3 crystallized as a mixed sodium-guanidinium salt, Na(8)[C(NH(2))(3)](4)[{Mo(V)(2)O(4)(H(2)O)}(4){O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(4)]·8H(2)O (3a). The compounds were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analyses. The formation of polyanions 1 and 3 is very sensitive to the pH value of the reaction solution, with exclusive formation of 1 above pH 7.4 and 3 below pH 6.6. Detailed solution studies by multinuclear NMR spectrometry were performed to study the equilibrium between these two compounds. Polyanion 2 was insoluble in all common solvents. Detailed computational studies on the solution phases of 1 and 3 indicated the stability of these polyanions in solution, in complete agreement with the experimental findings.
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http://dx.doi.org/10.1021/ic201652hDOI Listing
November 2011

A noble-metalate bowl: the polyoxo-6-vanado(V)-7-palladate(II) [Pd7V6O24(OH)2]6-.

Angew Chem Int Ed Engl 2010 Oct;49(42):7807-11

Jacobs University, School of Engineering and Science, Postfach 750561, 28725 Bremen, Germany.

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http://dx.doi.org/10.1002/anie.201001738DOI Listing
October 2010

Polyoxometalates made of gold: the polyoxoaurate [Au(III)4As(V)4O20]8-.

Angew Chem Int Ed Engl 2010 Mar;49(10):1886-9

Jacobs University, School of Engineering and Science, P.O. Box 750561, 28725 Bremen, Germany.

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http://dx.doi.org/10.1002/anie.200905566DOI Listing
March 2010

Emulsification in turbulent flow 2. Breakage rate constants.

J Colloid Interface Sci 2007 Sep 3;313(2):612-29. Epub 2007 May 3.

Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Ave., 1164 Sofia, Bulgaria.

Systematic experimental study of the effects of several factors on the breakage rate constant, k(BR), during emulsification in turbulent flow is performed. These factors are the drop size, interfacial tension, viscosity of the oil phase, and rate of energy dissipation in the flow. As starting oil-water premixes we use emulsions containing monodisperse oil drops, which have been generated by the method of membrane emulsification. By passing these premixes through a narrow-gap homogenizer, working in turbulent regime of emulsification, we study the evolution of the number concentration of the drops with given diameter, as a function of the emulsification time. The experimental data are analyzed by a kinetic scheme, which takes into account the generation of drops of a given size (as a result of breakage of larger drops) and their disappearance (as a result of their own breakage process). The experimental results for k(BR) are compared with theoretical expressions from the literature and their modifications. The results for all systems could be described reasonably well by an explicit expression, which is a product of: (a) the frequency of collisions between drops and turbulent eddies of similar size, and (b) the efficiency of drop breakage, which depends on the energy required for drop deformation. The drop deformation energy contains two contributions, originating from the drop surface extension and from the viscous dissipation inside the breaking drop. In the related subsequent paper, the size distribution of the daughter drops formed in the process of drop breakage is analyzed for the same experimental systems.
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http://dx.doi.org/10.1016/j.jcis.2007.04.064DOI Listing
September 2007

Emulsification in turbulent flow 1. Mean and maximum drop diameters in inertial and viscous regimes.

J Colloid Interface Sci 2007 Aug 30;312(2):363-80. Epub 2007 Mar 30.

Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Ave., 1164 Sofia, Bulgaria.

Systematic experimental study of the effects of several factors on the mean and maximum drop sizes during emulsification in turbulent flow is performed. These factors include: (1) rate of energy dissipation, epsilon; (2) interfacial tension, sigma; (3) viscosity of the oil phase, eta(D); (4) viscosity of the aqueous phase, eta(C); and (5) oil volume fraction, Phi. The emulsions are prepared by using the so-called "narrow-gap homogenizer" working in turbulent regime of emulsification. The experiments are performed at high surfactant concentration to avoid the effect of drop-drop coalescence. For emulsions prepared in the inertial turbulent regime, the mean and the maximum drop sizes increase with the increase of eta(D) and sigma, and with the decrease of epsilon. In contrast, Phi and eta(C) affect only slightly the mean and the maximum drop sizes in this regime of emulsification. These results are described very well by a theoretical expression proposed by Davies [Chem. Eng. Sci. 40 (1985) 839], which accounts for the effects of the drop capillary pressure and the viscous dissipation inside the breaking drops. The polydispersity of the emulsions prepared in the inertial regime of emulsification does not depend significantly on sigma and epsilon. However, the emulsion polydispersity increases significantly with the increase of oil viscosity, eta(D). The experiments showed also that the inertial turbulent regime is inappropriate for emulsification of oils with viscosity above ca. 500 mPa s, if drops of micrometer size are to be obtained. The transition from inertial to viscous turbulent regime of emulsification was accomplished by a moderate increase of the viscosity of the aqueous phase (above 5 mPa s in the studied systems) and/or by increase of the oil volume fraction, Phi>0.6. Remarkably, emulsions with drops of micrometer size are easily formed in the viscous turbulent regime of emulsification, even for oils with viscosity as high as 10,000 mPa s. In this regime, the mean drop size rapidly decreases with the increase of eta(C) and Phi (along with the effects of epsilon, sigma, and eta(D), which are qualitatively similar in the inertial and viscous regimes of emulsification). The experimental results are theoretically described and discussed by using expressions from the literature and their modifications (proposed in the current study).
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http://dx.doi.org/10.1016/j.jcis.2007.03.059DOI Listing
August 2007

Emulsification in turbulent flow: 3. Daughter drop-size distribution.

J Colloid Interface Sci 2007 Jun 14;310(2):570-89. Epub 2007 Feb 14.

Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Ave., 1164 Sofia, Bulgaria.

Systematic set of experiments is performed to clarify the effects of several factors on the size distribution of the daughter drops, which are formed as a result of drop breakage during emulsification in turbulent flow. The effects of oil viscosity, etaD, interfacial tension, sigma, and rate of energy dissipation in the turbulent flow, epsilon, are studied. As starting oil-water premixes we use emulsions containing monodisperse oil drops, which have been generated by membrane emulsification. By passing these premixes through a narrow-gap homogenizer, working in turbulent regime of emulsification, we monitor the changes in the drop-size distribution with the emulsification time. The experimental data are analyzed by using a new numerical procedure, which is based on the assumption (supported by the experimental data) that the probability for formation of daughter drops with diameter smaller than the maximum diameter of the stable drops, d
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http://dx.doi.org/10.1016/j.jcis.2007.01.097DOI Listing
June 2007