Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions

7Li NMR Spectroscopy: A Tool for Determining Dimerization Constants and Averaged Dimerization Constants of the Monomer/Dimer Equilibrium of Hierarchical Helicates

7Li nuclear magnetic resonance (NMR) spectroscopy is an ideal tool to study hierarchically assembled helicates of the form Li[Li3L6Ti2]. Internally bound and external lithium ions can be well distinguished by solution- or solid-state NMR spectroscopy and dimerization constants of the monomer/dimer equilibrium can be easily determined in solution. Averaged dimerization constants can be estimated in case of statistical mixtures of helicates formed from mixtures of ligands.

Advances and Prospects in Understanding London Dispersion Interactions in Molecular Chemistry

London dispersion (LD) interactions are the main contribution of the attractive part of the van der Waals potential. Even though LD effects are the driving force for molecular aggregation and recognition, the role of these omnipresent interactions in structure and reactivity had been largely underappreciated over decades. However, in the recent years considerable efforts have been made to thoroughly study LD interactions and their potential as a chemical design element for structures and catalysis. This was made possible through a fruitful interplay of theory and experiment. This review highlights recent results and advances in utilizing LD interactions as a structural motif to understand and utilize intra- and intermolecularly LD-stabilized systems. Additionally, we focus on the quantification of LD interactions and their fundamental role in chemical reactions.

Molecular Wind-Up Meter for the Quantification of London Dispersion Interactions

ConspectusThe experimental quantification of interactions on the molecular level provides the necessary basis for the design of functional materials and chemical processes. The interplay of multiple parameters and the small quantity of individual interactions pose a special challenge for such endeavors. The common method is the use of molecular balances, which can exist in two different states. Thereby, a stabilizing interaction can occur in one of the states, favoring its formation and thus affecting the thermodynamic equilibrium of the system. One challenge is determining the change in this equilibrium since various analytical methods could not be applied to fast-changing equilibria. A new and promising method for quantifying molecular interactions is the use of Molecular Wind-up Meters (MWM) in which the change in kinetics, rather than the effect on thermodynamics, is investigated. An MWM is transformed with an energy input (e.g. irradiation) into a metastable state. Then, the rate of thermal transformation back to the ground state is measured. The strength of interactions present in the metastable state controls the kinetics of the back reactions, allowing direct correlation. The advantage of this approach lies in the high sensitivity (energy differences can be larger by 1 order of magnitude) and, in general, allows the use of a broader range of solvents and analytical methods. An Azobenzene-based MWM has been established as a powerful tool to quantify London dispersion interactions. London dispersion (LD) represents the attractive part of the van der Waals potential. Although neglected in the past due to its weak character, it has been shown that the influence of LD on the structure, stability, and reactivity of matter can be decisive. Especially in larger molecules, its energy contribution increases overproportionately with the number of atoms, which has sparked increasing interest in the use of so-called dispersion energy donors (DED) as a new structural element. Application of the azobenzene-based MWM not only allowed the differentiation of bulkiness, but also systematically addressed the influence of the length of n-alkyl chains. Additionally, the solvent influence on LD was studied. Based on the azobenzene MWM, an increment system has been proposed, allowing a rough estimate of the effect of a specific DED.

Autocatalysis in Eschenmoser Coupling Reactions

The Eschenmoser coupling reaction (ECR) of thioamides with electrophiles is believed to proceed via thiirane intermediates. However, little is known about converting the intermediates into ECR products. Previous mechanistic studies involved external thiophiles to remove the sulfur atom from the intermediates. In this work, an ECR proceeding without any thiophilic agent or base is studied by electrospray ionization-mass spectrometry. ESI-MS enables the detection of the so-far elusive polysulfide species Sn , with n ranging from 2 to 16 sulfur atoms, proposed to be the key species leading to product formation. Integrating observations from ion mobility spectrometry, ion spectroscopy, and reaction monitoring via flow chemistry coupled with mass spectrometry provides a comprehensive understanding of the reaction mechanism and uncovers the autocatalytic nature of the ECR reaction.

The Monomer-Dimer Equilibrium of Triscatechol Titanium(IV)-Based Hierarchical Helicates as a Tool for the Development of Molecular Balances and Molecular Switches

ConspectusHierarchical helicates are formed by noncovalent connection of two or more monomeric metal complex units, e.g., by bridging metal cations. A unique kind of hierarchical helicate is obtained from 3-carbonyl substituted catechol ligands with titanium(IV) ions in the presence of lithium cations. This kind of supramolecular complex shows in solution a "monomer-dimer" equilibrium. There are different possibilities (solvent, countercation, substituents at carbonyl unit, etc.) to shift this equilibrium to either the monomer or the dimer side. Thus, the lithium-bridged catecholate-based hierarchical helicates resemble a molecular switch. In this Account, different aspects are discussed of how this unique behavior of the dimeric titanium catecholates can be used for application.Thorough investigation of the energetics of the monomer-dimer equilibrium leads to a deeper understanding of the thermodynamic and kinetic effects of the dimerization (or dissociation) process. In this context, even weak interaction of substituents in the periphery of the complexes can be observed. Hereby on the one hand, solvent effects have an important influence and can be easily evaluated. The thorough understanding of the behavior of the monomer-dimer equilibrium allows one to develop some novel applications. In this respect, the use of the hierarchical helicate-based switch as a platform for reaction control and catalysis is described. Decent enantioselectivities up to ee = 58% can be found in Diels-Alder reactions in the periphery of the dimers, while switching to the monomer as a reaction platform still allows the cycloaddition reaction but turns the selectivity off. Additionally, it is described that catalytically important units can be introduced and hydrogenation reactions as well as Michael-type reactions are catalyzed at the helicates.Covalent connection of two catechol ester units leads to classical helicates. Depending on the alkaline metal cation, those can be switched from a compressed to an expanded form or vice versa. Hereby the monomer-dimer equilibrium is transformed into a structural switch. The switching process can be initiated by removal or addition of lithium cations (e.g., by addition of [2.1.1]cryptand). Alternative switching possibilities are based in the case of glycol bridged helicates on cation translocation isomerism and with thioester derivatives it occurs spontaneously in DMSO. Introduction of chiral tethers results in a three state switch allowing expansion/compression as well as switching of the helicity.

Kinetics of ligand exchange in solution: a quantitative mass spectrometry approach

Complex speciation and exchange kinetics of labile ligands are critical parameters for understanding the reactivity of metal complexes in solution. We present a novel approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). The introduction of isotopically labelled ligands to a solution of metal host and unlabelled ligands allows the quantitative investigation of the solution-phase equilibria. Furthermore, ion mobility separation can target individual isomers, such as ligands bound at specific sites. As a proof of concept, we investigate the solution equilibria of labile pyridine ligands coordinated in the cavity of macrocyclic porphyrin cage complexes bearing diamagnetic or paramagnetic metal centres. The effects of solvent, porphyrin coordination sphere, transition metal, and counterion on ligand dissociation are discussed. Rate constants and activation parameters for ligand dissociation in the solution can be derived from our ESI-MS approach, thereby providing mechanistic insights that are not easily obtained from traditional solution-phase techniques.

Probing the Size Limit of Dispersion Energy Donors with a Bifluorenylidene Balance: Magic Cyclohexyl

We report the synthesis of 14 2,2'-disubstituted 9,9'-bifluorenylidenes as molecular balances for the quantification of London dispersion interactions between various dispersion energy donors. For all balances, we measured ΔG_Z/E at 333 K using ¹H NMR in seven organic solvents. For various alkyl and aryl substituents, we generally observe a preference for the "folded" Z-isomer due to attractive London dispersion interactions. The cyclohexyl-substituted system shows the largest Z-preference in this study with ΔG_Z/E = -0.6 ± 0.05 kcal mol^-1 in all solvents, owing to the rotational freedom of cyclohexyl groups paired with their large polarizability that maximizes London dispersion interactions. On the other hand, rigid and sterically more demanding substituents like tert-butyl unexpectedly favor the unfolded E-isomer. This is a result of the close relative position in which the functional groups are positioned in this molecular balance. This close proximity is the reason for the increase of Pauli repulsion in the Z-isomers with large rigid substituents (tert-butyl, adamantyl, and diamantyl) which leads to an equilibrium shift toward the unfolded E-form. While we were able to reproduce most of our experimental trends qualitatively using contemporary computational chemistry methods, quantitative accuracy of the employed methods still needs further improvement.

Silyl Groups Are Strong Dispersion Energy Donors

We present an experimental and computational study to investigate noncovalent interactions between silyl groups that are often employed as "innocent" protecting groups. We chose an extended cyclooctatetraene (COT)-based molecular balance comprising unfolded (1,4-disubstituted) and folded (1,6-disubstituted) valance bond isomers that typically display remote and close silyl group contacts, respectively. The thermodynamic equilibria were determined using nuclear magnetic resonance measurements. Additionally, we utilized Boltzmann weighted symmetry-adapted perturbation theory (SAPT) at the sSAPT0/aug-cc-pVDZ level of theory to dissect and quantify noncovalent interactions. Apart from the extremely bulky tris(trimethylsilyl)silyl "supersilyl" group, there is a preference for the folded 1,6-COT valence isomer, with London dispersion interactions being the main stabilizing factor. This makes silyl groups excellent dispersion energy donors, a finding that needs to be taken into account in synthesis planning.

London Dispersion Favors Sterically Hindered Diarylthiourea Conformers in Solution

We present an experimental and computational study on the conformers of N,N'-diphenylthiourea substituted with different dispersion energy donor (DED) groups. While the unfolded anti-anti conformer is the most relevant for thiourea catalysis, intramolecular noncovalent interactions counterintuitively favor the folded syn-syn conformer, as evident from a combination of low-temperature nuclear magnetic resonance measurements and computations. In order to quantify the noncovalent interactions, we utilized local energy decomposition analysis and symmetry-adapted perturbation theory at the DLPNO-CCSD(T)/def2-TZVPP and sSAPT0/6-311G(d,p) levels of theory. Additionally, we applied a double-mutant cycle to experimentally study the effects of bulky substituents on the equilibria. We determined London dispersion as the key interaction that shifts the equilibria towards the syn-syn conformers. This preference is likely a factor why such thiourea derivatives can be poor catalysts.

Playing with the weakest supramolecular interactions in a 3D crystalline hexakis[60]fullerene induces control over hydrogenation selectivity

Weak forces can play an essential role in chemical reactions. Controlling such subtle forces in reorganization processes by applying thermal or chemical stimuli represents a novel synthetic strategy and one of the main targets in supramolecular chemistry. Actually, to separate the different supramolecular contributions to the stability of the 3D assemblies is still a major challenge. Therefore, a clear differentiation of these contributions would help in understanding the intrinsic nature as well as the chemical reactivity of supramolecular ensembles. In the present work, a controlled reorganization of an hexakis[60]fullerene-based molecular compound purely governed by the weakest van der Waals interactions known, i.e. the dihydrogen interaction - usually called sticky fingers - is illustrated. This pre-reorganization of the hexakis[60]fullerene under mild conditions allows a further selective hydrogenation of the crystalline material via hydrazine vapors exposure. This unique two-step transformation process is monitored by single-crystal to single-crystal diffraction (SCSC) which allows the direct observation of the molecular movements in the lattice and the subsequent solid-gas hydrogenation reaction.

London Dispersion Interactions Rather than Steric Hindrance Determine the Enantioselectivity of the Corey-Bakshi-Shibata Reduction

The well-known Corey-Bakshi-Shibata (CBS) reduction is a powerful method for the asymmetric synthesis of alcohols from prochiral ketones, often featuring high yields and excellent selectivities. While steric repulsion has been regarded as the key director of the observed high enantioselectivity for many years, we show that London dispersion (LD) interactions are at least as important for enantiodiscrimination. We exemplify this through a combination of detailed computational and experimental studies for a series of modified CBS catalysts equipped with dispersion energy donors (DEDs) in the catalysts and the substrates. Our results demonstrate that attractive LD interactions between the catalyst and the substrate, rather than steric repulsion, determine the selectivity. As a key outcome of our study, we were able to improve the catalyst design for some challenging CBS reductions.

London Dispersion in Alkane Solvents

The importance of London dispersion interactions in solution is an ongoing debate. Although the significance of dispersion for structure and stability is widely accepted, the degree of its attenuation in solution is still not properly understood. Quantitative evaluations are derived mostly from computations. Experimental data provide guidelines to include London dispersion in solution phase design. Herein, dispersive interactions were examined with an azobenzene probe. Alkyl substituents in meta positions of the azobenzene core were systematically varied and the effect on the half-lives for the thermally induced Z to E isomerization in several alkane solvents was determined. The results show that intramolecular dispersion is only marginally influenced. In solvents with low surface tension, reduced destabilizing solvent-solvent interactions increase the half-life up to 20 %. Specific individual interactions between alkyl chains on the azobenzene and those of the solvent lead to additional fluctuations of the half-lives. These presumably result from structural changes of the conformer ensemble.

Hierarchically assembled helicates as reaction platform - from stoichiometric Diels-Alder reactions to enamine catalysis

The stereoselectivity of a Diels-Alder reaction within the periphery of hierarchically assembled titanium(IV) helicates formed from mixtures of achiral, reactive and chiral, unreactive ligands was investigated in detail. Following the pathway of the chiral induction, the chiral ligands, solvents as well as substituents at the dienophile were carefully varied. Based on the results of the stoichiometric reaction, a secondary amine-catalyzed nitro-Michael reaction is performed as well which afforded reasonable diastereoselectivities.

Solvent Dependence of the Monomer-Dimer Equilibrium of Ketone-Substituted Triscatecholate Titanium(IV) Complexes

Hierarchical helicates based on ketone-substituted titanium(IV)triscatecholates show different monomer-dimer behavior depending on different solvents. The dimerization constants of a whole series of differently alkyl-substituted complexes is analyzed to show that the solvent has a very strong influence on the dimerization. Hereby, effects like solvophobicity/philicity, sterics, electronics of the substituents and weak side-chain-side-chain interactions seem to act in concert.

Alkyl-Alkyl Interactions in the Periphery of Supramolecular Entities: From the Evaluation of Weak Forces to Applications

Supramolecular chemistry is based on weak intermolecular forces which nevertheless are of importance for chemical processes. In this report the relevance of alkyl-alkyl interactions in supramolecular assemblies is discussed. We show how hierarchically formed helicates can be used to evaluate weak interactions of alkyl groups based on solvent-supported London dispersion. In addition, the role of nano-segregation of alkyl groups in the periphery of supramolecular assemblies is described, as well as how this can be used to improve the properties of liquid-crystalline materials by controlling the alkyl-chain-mediated aggregation process.

Catechol Thioesters: Ligands for Hierarchically Formed Lithium-Bridged Titanium(IV) Helicates and Helicate-Based Switches

The thioester moiety is introduced as a lithium binding unit for the hierarchical formation of titanium(IV) catecholate-based lithium-bridged helicates. In solution, the coordination compounds show a monomer-dimer equilibrium which -in comparison to the oxo esters- is significantly shifted towards the monomers. In addition, the influence of the thioester side chain on the dimerization behavior is investigated and an expansible/compressible molecular switch is synthesized. In the latter case expansion and compression are performed reversibly in methanol, whereas in DMSO spontaneous expansion occurs.

Stability of Hierarchically Formed Titanium(IV) Tris(catecholate ester) Helicates with Cyclohexyl Substituents in DMSO

A cyclohexyl substituent strongly prefers the chair conformation with large substituents in equatorial positions, while other cycloalkyls are structurally more flexible. In hierarchically formed dimeric titanium(IV) tris(catecholates) equatorial versus axial connection of the cyclohexane to the ester results in either a more compact (axial) or more expanded (equatorial) structure. In DMSO solution the axial position results in a compact structure which minimizes solvophobic effects, leading to higher stability. However, computational investigations indicate that additionally intramolecular London dispersion interactions significantly contribute to the stability of the dimer. Thus, weak side-chain-side-chain interactions are responsible for the high stability of cyclohexyl ester derivatives with axial compared to equatorial ester connection.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1 H NMR dimerization constants for the monomer-dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π-π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n-alkyls, n-Ω-alkenyls or n-Ω-alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

Exploring London Dispersion and Solvent Interactions at Alkyl-Alkyl Interfaces Using Azobenzene Switches

Interactions on the molecular level control structure as well as function. Especially interfaces between innocent alkyl groups are hardly studied although they are of great importance in larger systems. Herein, London dispersion in conjunction with solvent interactions between linear alkyl chains was examined with an azobenzene-based experimental setup. Alkyl chains in all meta positions of the azobenzene core were systematically elongated, and the change in rate for the thermally induced Z→E isomerization in n-decane was determined. The stability of the Z-isomer increased with longer chains and reached a maximum for n-butyl groups. Further elongation led to faster isomerization. The origin of the intramolecular interactions was elaborated by various techniques, including 1 H NOESY NMR spectroscopy. The results indicate that there are additional long-range interactions between n-alkyl chains with the opposite phenyl core in the Z-state. These interactions are most likely dominated by attractive London dispersion. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra- as well as an intermolecular setting.

From Hierarchical Helicates to Functional Supramolecular Devices

Catechol ligands with aldehyde, ketone or ester groups attached in 3-position form, in the presence of titanium(IV) triscatecholate, titanium(IV) complexes. If lithium cations are the counterions, they can bind in a successive step to the salicylate units of the complex and form a dimeric triple-lithium-bridged dinuclear helicate. In solution, the dimer is in equilibrium with the monomer and the thermodynamics of the dimerization can be easily evaluated. Thus, the hierarchically assembled titanium(IV) helicates represent a lithium-dependent molecular switch. The investigation of different derivatives of the complex allows for an estimation of the influence of side chain functionalities on the energetics of the dimerization. Thus, the hierarchically assembled helicates can be used as a kind of molecular balance to determine weak interaction energies (solvophobic effects and even dispersive effects). In addition, tethering of two ligands leads to "classical" helicates. Removal or addition of lithium cations allows for a reversible switching between a compressed and expanded state, which in the case of chiral ligands can be even performed stereospecifically.

Kinetic investigation of the dissociation of dinuclear hierarchically assembled titanium(iv) helicates

Hierarchically assembled helicates consisting of lithium-bridged triscatecholate titanium(iv) complexes represent a powerful self-assembled supramolecular system with applications as e.g. molecular balances for the evaluation of weak interactions, stereoselectivity switches in asymmetric synthesis or molecular switches.

Importance of Molecular Meshing for the Stabilization of Solvophobic Assemblies

The effect of the methyl groups in neutral gear-shaped amphiphiles (GSAs) on the stability of nanocubes was investigated using a novel C_{2 v}-symmetric GSA, which was synthesized using selective alternate trilithiation of a pentabrominated hexaphenylbenzene derivative. The lack of only one methyl group in the GSA decreased the association constant for the assembly of the nanocube by 3 orders of magnitude. A surface analysis recently developed by the authors (SAVPR: surface analysis with varying probe radii) was carried out for characteristic isomers of the nanocube consisting of C_{2 v}-symmetric GSAs. It was found that the methyl groups near the equator of the nanocube play a significant role in the stabilization of the nanocubes.