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Ralf Ludwig

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Yael Diskin-Posner

Jean-Pierre Djukic

UPM - Universiti Putra Malaysia

Sébastien Bontemps

Douglas Heimark

University of Virginia

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Papers by Ralf Ludwig

Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Journal of the American Chemical Society, Jul 20, 2016

Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient... more Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient and clean functional group transformations. In fact, the selective reduction of nitriles, ketones, and aldehydes with molecular hydrogen permits access to a green synthesis of valuable amines and alcohols. Despite more than a century of developments in homogeneous and heterogeneous catalysis, efforts toward the creation of new useful and broadly applicable catalyst systems are ongoing. Recently, Earth-abundant metals have attracted significant interest in this area. In the present study, we describe for the first time specific molecular-defined manganese complexes that allow for the hydrogenation of various polar functional groups. Under optimal conditions, we achieve good functional group tolerance, and industrially important substrates, e.g., for the flavor and fragrance industry, are selectively reduced.

Front Cover: Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy‐Functionalized Pyridinium Cations (ChemPhysChem 18/2021)

ChemPhysChem

Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy‐Functionalized Pyridinium Cations

ChemPhysChem

Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods

Physical Chemistry Chemical Physics

Dispersion interactions in the gas phase ion pairs of ionic liquids determined using purely therm... more

Front Cover: Cyclic Octamer of Hydroxyl‐functionalized Cations with Net Charge Q =+8 e Kinetically Stabilized by a ‘Molecular Island’ of Cooperative Hydrogen Bonds (ChemPhysChem 21/2020)

ChemPhysChem

Clusters of Hydroxyl-Functionalized Cations Stabilized by Cooperative Hydrogen Bonds: The Role of Polarizability and Alkyl Chain Length

Molecules

We explore quantum chemical calculations for studying clusters of hydroxyl-functionalized cations... more We explore quantum chemical calculations for studying clusters of hydroxyl-functionalized cations kinetically stabilized by hydrogen bonding despite strongly repulsive electrostatic forces. In a comprehensive study, we calculate clusters of ammonium, piperidinium, pyrrolidinium, imidazolium, pyridinium, and imidazolium cations, which are prominent constituents of ionic liquids. All cations are decorated with hydroxy-alkyl chains allowing H-bond formation between ions of like charge. The cluster topologies comprise linear and cyclic clusters up to the size of hexamers. The ring structures exhibit cooperative hydrogen bonds opposing the repulsive Coulomb forces and leading to kinetic stability of the clusters. We discuss the importance of hydrogen bonding and dispersion forces for the stability of the differently sized clusters. We find the largest clusters when hydrogen bonding is maximized in cyclic topologies and dispersion interaction is properly taken into account. The kinetic st...

Counting cations involved in cationic clusters of hydroxy-functionalized ionic liquids by means of infrared and solid-state NMR spectroscopy

Physical Chemistry Chemical Physics

Sensitive probe of like-charge attraction: analyzing infrared spectra allows counting the number ... more

Controlling “like–likes–like” charge attraction in hydroxy-functionalized ionic liquids by polarizability of the cations, interaction strength of the anions and varying alkyl chain length

Physical Chemistry Chemical Physics

Like–likes–like charge attraction via hydrogen bonding: three molecular parameters control the ca... more

Dissecting intermolecular interactions in the condensed phase of ibuprofen and related compounds: the specific role and quantification of hydrogen bonding and dispersion forces

Physical Chemistry Chemical Physics

Hydrogen bonding and dispersion interaction in liquid ibuprofen is analyzed by thermodynamic meth... more

$Research paper thumbnail of The double‐faced nature of hydrogen bonding in hydroxyl‐functionalized ionic liquids shown by neutron diffraction and molecular dynamics simulations$

The double‐faced nature of hydrogen bonding in hydroxyl‐functionalized ionic liquids shown by neutron diffraction and molecular dynamics simulations

Angewandte Chemie

Zerlegung der Verdampfungsenthalpien ionischer Flüssigkeiten durch rein experimentelle Methoden: Coulomb‐Wechselwirkung, Wasserstoffbrücken und Dispersionskräfte

Angewandte Chemie

Dissecting the vaporization enthalpies of ionic liquids by exclusively experimental methods: Coulomb interaction, hydrogen bonding and dispersion forces

Angewandte Chemie

, S. P. Verevkin, R. Ludwig et al. show how these different types of interactions contribute to v... more

When hydrogen bonding overcomes Coulomb repulsion: from kinetic to thermodynamic stability of cationic dimers

Physical Chemistry Chemical Physics

“Coulomb explosion” prevented! Quantum chemical calculations of dimers of like-charged molecular ... more “Coulomb explosion” prevented! Quantum chemical calculations of dimers of like-charged molecular ions show that Coulomb repulsion can be overcome by hydrogen bonding and dispersion forces. Quantum-type short-range attraction wins over classical long-range electrostatic repulsion providing the first thermodynamically stable cationic dimer.

Rotational and translational dynamics and their relation to hydrogen bond lifetimes in an ionic liquid by means of NMR relaxation time experiments and molecular dynamics simulation

The Journal of Chemical Physics

Spektroskopischer Nachweis einer attraktiven Kation-Kation- Wechselwirkung in OH-funktionalisierten ionischen Flüssigkeiten: ein H-Brücken-gebundenes kettenförmiges Trimer

Angewandte Chemie

Spectroscopic Evidence for an Attractive Cation-Cation Interaction in Hydroxy-Functionalized Ionic Liquids: A Hydrogen-Bonded Chain-like Trimer

Angewandte Chemie (International ed. in English), Jan 10, 2018

We address the formation of hydrogen bonded domains among the cationic constituents of the ionic ... more We address the formation of hydrogen bonded domains among the cationic constituents of the ionic liquid (IL) 1-(3-hydroxypropyl)pyridinium tetrafluoroborate [HPPy][BF ] by means of cryogenic ion vibrational predissociation spectroscopy of cold (ca. 35 K) gas-phase cluster ions and quantum chemistry. Specifically, analysis of the OH stretching bands reveals a chain-like OH⋅⋅⋅OH⋅⋅⋅OH⋅⋅⋅BF binding motif involving the three cations in the cationic quinary cluster ion (HPPy ) (BF ) . Calculations show that this cooperative H-bond attraction compensates for the repulsive Coulomb forces and results in stable complexes that successfully compete with those in which the OH groups are predominantly attached to the counter anions. Our combined experimental and theoretical approach provides insight into the cooperative effects that lead to the formation of hydrogen bonded domains involving the cationic constituents of ILs.

Like-likes-Like: Cooperative Hydrogen Bonding Overcomes Coulomb Repulsion in Cationic Clusters with Net Charges up to Q=+6e

Chemphyschem : a European journal of chemical physics and physical chemistry, Jan 17, 2018

Quantum chemical calculations have been employed to study kinetically stable cationic clusters, w... more Quantum chemical calculations have been employed to study kinetically stable cationic clusters, wherein the monovalent cations are trapped by hydrogen bonding despite strongly repulsive electrostatic forces. We calculated linear and cyclic clusters of the hydroxy-functionalized cation N-(3-hydroxypropyl) pyridinium, commonly used as cation in ionic liquids. The largest kinetically stable cluster was a cyclic hexamer that very much resembles the structural motifs of molecular clusters, as known for water and alcohols. Surprisingly, strong cooperative hydrogen bonds overcome electrostatic repulsion and result in cationic clusters with a high net charge up to Q=+6e. The structural, spectroscopic, and electronic signatures of the cationic and related molecular clusters of 3-phenyl-1-propanol could be correlated to NBO parameters, supporting the existence of "anti-electrostatic" hydrogen bonds (AEHB), as recently suggested by Weinhold. We also showed that dispersion forces enha...

Dynamical heterogeneities in ionic liquids as revealed from deuteron NMR

Chemical communications (Cambridge, England), Jan 28, 2018

The heterogeneity in dynamics has important consequences for understanding the viscosity, diffusi... more The heterogeneity in dynamics has important consequences for understanding the viscosity, diffusion, ionic mobility, and the rates of chemical reactions in technology relevant systems such as polymers, metallic glasses, aqueous solutions, and inorganic materials. Herein, we study the spatial and dynamic heterogeneities in ionic liquids by means of solid state NMR spectroscopy. In theH spectra of the protic ionic liquid [TEA][OTf] we observe anisotropic and isotropic signals at the same time. The spectra measured below the melting temperature at 306 K could be simulated by a superposition of the solid and liquid line shapes, which provided the transition enthalpies between the rigid and mobile fractions. Consequently, we measured the spin-lattice relaxation times Tfor the anisotropic and the isotropic signals for the temperature range between 203 and 436 K. Both dispersion curves could be fitted to models including rotational correlation times, activation barriers and rate constants....

The influence of like-charge attraction on the structure and dynamics of ionic liquids: NMR chemical shifts, quadrupole coupling constants, rotational correlation times and failure of Stokes-Einstein-Debye

Physical chemistry chemical physics : PCCP, Jan 7, 2018

Ion pairing is one of the most fundamental atomic interactions in chemistry and biology. In contr... more Ion pairing is one of the most fundamental atomic interactions in chemistry and biology. In contrast, pairing between like-charged ions remains an elusive concept. So far, this phenomenon was observed only for large-scaled structures, assemblies, stabilizing frameworks, or in aqueous solution wherein like-charge attraction is supported by mediating water molecules. Recently, we reported the formation of cationic clusters in pure ionic liquids (ILs) which all include hydroxyl groups (OH) for possible hydrogen bonding. In such structures like-charge repulsion is overcome by cooperative hydrogen bonds. The vibrational bands in the OH-stretch region of the infrared spectra can be clearly assigned to H-bonded ion pairs (c-a) or to H-bonded cationic clusters (c-c). The equilibrium between both types of ionic clusters can be controlled by using the same cation but differently strong interacting anions. In the present work, we study the influence of the cationic cluster formation on structu...

Controlling the kinetic and thermodynamic stability of cationic clusters by the addition of molecules or counterions

Phys. Chem. Chem. Phys.

We discuss the stability of cationic clusters when adding molecules or counterions, and predict t... more

Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Journal of the American Chemical Society, Jul 20, 2016

Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient... more Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient and clean functional group transformations. In fact, the selective reduction of nitriles, ketones, and aldehydes with molecular hydrogen permits access to a green synthesis of valuable amines and alcohols. Despite more than a century of developments in homogeneous and heterogeneous catalysis, efforts toward the creation of new useful and broadly applicable catalyst systems are ongoing. Recently, Earth-abundant metals have attracted significant interest in this area. In the present study, we describe for the first time specific molecular-defined manganese complexes that allow for the hydrogenation of various polar functional groups. Under optimal conditions, we achieve good functional group tolerance, and industrially important substrates, e.g., for the flavor and fragrance industry, are selectively reduced.

Front Cover: Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy‐Functionalized Pyridinium Cations (ChemPhysChem 18/2021)

ChemPhysChem

Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy‐Functionalized Pyridinium Cations

ChemPhysChem

Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods

Physical Chemistry Chemical Physics

Dispersion interactions in the gas phase ion pairs of ionic liquids determined using purely therm... more

Front Cover: Cyclic Octamer of Hydroxyl‐functionalized Cations with Net Charge Q =+8 e Kinetically Stabilized by a ‘Molecular Island’ of Cooperative Hydrogen Bonds (ChemPhysChem 21/2020)

ChemPhysChem

Clusters of Hydroxyl-Functionalized Cations Stabilized by Cooperative Hydrogen Bonds: The Role of Polarizability and Alkyl Chain Length

Molecules

We explore quantum chemical calculations for studying clusters of hydroxyl-functionalized cations... more We explore quantum chemical calculations for studying clusters of hydroxyl-functionalized cations kinetically stabilized by hydrogen bonding despite strongly repulsive electrostatic forces. In a comprehensive study, we calculate clusters of ammonium, piperidinium, pyrrolidinium, imidazolium, pyridinium, and imidazolium cations, which are prominent constituents of ionic liquids. All cations are decorated with hydroxy-alkyl chains allowing H-bond formation between ions of like charge. The cluster topologies comprise linear and cyclic clusters up to the size of hexamers. The ring structures exhibit cooperative hydrogen bonds opposing the repulsive Coulomb forces and leading to kinetic stability of the clusters. We discuss the importance of hydrogen bonding and dispersion forces for the stability of the differently sized clusters. We find the largest clusters when hydrogen bonding is maximized in cyclic topologies and dispersion interaction is properly taken into account. The kinetic st...

Counting cations involved in cationic clusters of hydroxy-functionalized ionic liquids by means of infrared and solid-state NMR spectroscopy

Physical Chemistry Chemical Physics

Sensitive probe of like-charge attraction: analyzing infrared spectra allows counting the number ... more

Controlling “like–likes–like” charge attraction in hydroxy-functionalized ionic liquids by polarizability of the cations, interaction strength of the anions and varying alkyl chain length

Physical Chemistry Chemical Physics

Like–likes–like charge attraction via hydrogen bonding: three molecular parameters control the ca... more

Dissecting intermolecular interactions in the condensed phase of ibuprofen and related compounds: the specific role and quantification of hydrogen bonding and dispersion forces

Physical Chemistry Chemical Physics

Hydrogen bonding and dispersion interaction in liquid ibuprofen is analyzed by thermodynamic meth... more

$Research paper thumbnail of The double‐faced nature of hydrogen bonding in hydroxyl‐functionalized ionic liquids shown by neutron diffraction and molecular dynamics simulations$

The double‐faced nature of hydrogen bonding in hydroxyl‐functionalized ionic liquids shown by neutron diffraction and molecular dynamics simulations

Angewandte Chemie

Zerlegung der Verdampfungsenthalpien ionischer Flüssigkeiten durch rein experimentelle Methoden: Coulomb‐Wechselwirkung, Wasserstoffbrücken und Dispersionskräfte

Angewandte Chemie

Dissecting the vaporization enthalpies of ionic liquids by exclusively experimental methods: Coulomb interaction, hydrogen bonding and dispersion forces

Angewandte Chemie

, S. P. Verevkin, R. Ludwig et al. show how these different types of interactions contribute to v... more

When hydrogen bonding overcomes Coulomb repulsion: from kinetic to thermodynamic stability of cationic dimers

Physical Chemistry Chemical Physics

“Coulomb explosion” prevented! Quantum chemical calculations of dimers of like-charged molecular ... more “Coulomb explosion” prevented! Quantum chemical calculations of dimers of like-charged molecular ions show that Coulomb repulsion can be overcome by hydrogen bonding and dispersion forces. Quantum-type short-range attraction wins over classical long-range electrostatic repulsion providing the first thermodynamically stable cationic dimer.

Rotational and translational dynamics and their relation to hydrogen bond lifetimes in an ionic liquid by means of NMR relaxation time experiments and molecular dynamics simulation

The Journal of Chemical Physics

Spektroskopischer Nachweis einer attraktiven Kation-Kation- Wechselwirkung in OH-funktionalisierten ionischen Flüssigkeiten: ein H-Brücken-gebundenes kettenförmiges Trimer

Angewandte Chemie

Spectroscopic Evidence for an Attractive Cation-Cation Interaction in Hydroxy-Functionalized Ionic Liquids: A Hydrogen-Bonded Chain-like Trimer

Angewandte Chemie (International ed. in English), Jan 10, 2018

We address the formation of hydrogen bonded domains among the cationic constituents of the ionic ... more We address the formation of hydrogen bonded domains among the cationic constituents of the ionic liquid (IL) 1-(3-hydroxypropyl)pyridinium tetrafluoroborate [HPPy][BF ] by means of cryogenic ion vibrational predissociation spectroscopy of cold (ca. 35 K) gas-phase cluster ions and quantum chemistry. Specifically, analysis of the OH stretching bands reveals a chain-like OH⋅⋅⋅OH⋅⋅⋅OH⋅⋅⋅BF binding motif involving the three cations in the cationic quinary cluster ion (HPPy ) (BF ) . Calculations show that this cooperative H-bond attraction compensates for the repulsive Coulomb forces and results in stable complexes that successfully compete with those in which the OH groups are predominantly attached to the counter anions. Our combined experimental and theoretical approach provides insight into the cooperative effects that lead to the formation of hydrogen bonded domains involving the cationic constituents of ILs.

Like-likes-Like: Cooperative Hydrogen Bonding Overcomes Coulomb Repulsion in Cationic Clusters with Net Charges up to Q=+6e

Chemphyschem : a European journal of chemical physics and physical chemistry, Jan 17, 2018

Quantum chemical calculations have been employed to study kinetically stable cationic clusters, w... more Quantum chemical calculations have been employed to study kinetically stable cationic clusters, wherein the monovalent cations are trapped by hydrogen bonding despite strongly repulsive electrostatic forces. We calculated linear and cyclic clusters of the hydroxy-functionalized cation N-(3-hydroxypropyl) pyridinium, commonly used as cation in ionic liquids. The largest kinetically stable cluster was a cyclic hexamer that very much resembles the structural motifs of molecular clusters, as known for water and alcohols. Surprisingly, strong cooperative hydrogen bonds overcome electrostatic repulsion and result in cationic clusters with a high net charge up to Q=+6e. The structural, spectroscopic, and electronic signatures of the cationic and related molecular clusters of 3-phenyl-1-propanol could be correlated to NBO parameters, supporting the existence of "anti-electrostatic" hydrogen bonds (AEHB), as recently suggested by Weinhold. We also showed that dispersion forces enha...

Dynamical heterogeneities in ionic liquids as revealed from deuteron NMR

Chemical communications (Cambridge, England), Jan 28, 2018

The heterogeneity in dynamics has important consequences for understanding the viscosity, diffusi... more The heterogeneity in dynamics has important consequences for understanding the viscosity, diffusion, ionic mobility, and the rates of chemical reactions in technology relevant systems such as polymers, metallic glasses, aqueous solutions, and inorganic materials. Herein, we study the spatial and dynamic heterogeneities in ionic liquids by means of solid state NMR spectroscopy. In theH spectra of the protic ionic liquid [TEA][OTf] we observe anisotropic and isotropic signals at the same time. The spectra measured below the melting temperature at 306 K could be simulated by a superposition of the solid and liquid line shapes, which provided the transition enthalpies between the rigid and mobile fractions. Consequently, we measured the spin-lattice relaxation times Tfor the anisotropic and the isotropic signals for the temperature range between 203 and 436 K. Both dispersion curves could be fitted to models including rotational correlation times, activation barriers and rate constants....

The influence of like-charge attraction on the structure and dynamics of ionic liquids: NMR chemical shifts, quadrupole coupling constants, rotational correlation times and failure of Stokes-Einstein-Debye

Physical chemistry chemical physics : PCCP, Jan 7, 2018

Ion pairing is one of the most fundamental atomic interactions in chemistry and biology. In contr... more Ion pairing is one of the most fundamental atomic interactions in chemistry and biology. In contrast, pairing between like-charged ions remains an elusive concept. So far, this phenomenon was observed only for large-scaled structures, assemblies, stabilizing frameworks, or in aqueous solution wherein like-charge attraction is supported by mediating water molecules. Recently, we reported the formation of cationic clusters in pure ionic liquids (ILs) which all include hydroxyl groups (OH) for possible hydrogen bonding. In such structures like-charge repulsion is overcome by cooperative hydrogen bonds. The vibrational bands in the OH-stretch region of the infrared spectra can be clearly assigned to H-bonded ion pairs (c-a) or to H-bonded cationic clusters (c-c). The equilibrium between both types of ionic clusters can be controlled by using the same cation but differently strong interacting anions. In the present work, we study the influence of the cationic cluster formation on structu...

Controlling the kinetic and thermodynamic stability of cationic clusters by the addition of molecules or counterions

Phys. Chem. Chem. Phys.

We discuss the stability of cationic clusters when adding molecules or counterions, and predict t... more