NASC 2023: Showcasing Diversity in North American Supramolecular Chemistry.

Diversity in supramolecular chemistry can showcase itself in many ways. This includes the diversity of thought and topics covered in research (from fundamental science to applications in biology and materials), as well as the diversity of people (e.g.

Transition State Stabilizing Effects of Oxygen and Sulfur Chalcogen Bond Interactions.

Non-covalent chalcogen bond (ChB) interactions have found utility in many fields, including catalysis, organic semiconductors, and crystal engineering. In this study, the transition stabilizing effects of ChB interactions of oxygen and sulfur were experimentally measured using a series of molecular rotors. The rotors were designed to form ChB interactions in their bond rotation transition states.

Empirical Model of Solvophobic Interactions in Organic Solvents.

An empirical model was developed to predict organic solvophobic effects using N-phenylimide molecular balances functionalized with non-polar alkyl groups. Solution studies and X-ray crystallography confirmed intramolecular alkyl-alkyl interactions in their folded conformers. The structural modularity of the balances allowed systematic variation of alkyl group lengths.

Contributions of London Dispersion Forces to Solution-Phase Association Processes.

ConspectusDespite their ubiquity and early discovery, London dispersion forces are often overlooked. This is due, in part, to the difficulty in assessing their contributions to molecular and polymeric structure, stability, properties, and reactivities. However, recent advances in modeling have revealed that dispersion interactions play an important role in many important chemical and biological processes.

Pnictogen Interactions with Nitrogen Acceptors.

Stabilizing nitrogen pnictogen bond interactions were measured using molecular rotors. Intramolecular C=O⋅⋅⋅N interactions were formed in the bond rotation transition states which lowered the rotational barriers and increased the rates of rotation, as measured by EXSY NMR. The pnictogen interaction energies show a very strong correlation with the positive electrostatic potential on nitrogen, which was consistent with a strong electrostatic component.

Electrostatically-gated molecular rotors.

The ability to control molecular-scale motion using electrostatic interactions was demonstrated using an -phenylsuccinimide molecular rotor with an electrostatic pyridyl-gate. Protonation of the pyridal-gate forms stabilizing electrostatic interactions in the transition state of the bond rotation process that lowers the rotational barrier and increases the rate of rotation by two orders of magnitude. Molecular modeling and energy decomposition analysis confirm the dominant role of attractive electrostatic interactions in lowering the bond rotation transition state.

Analysis of the Orbital and Electrostatic Contributions to the Lone Pair-Aromatic Interaction Using Molecular Rotors.

The attractive interaction between carbonyl oxygens and the π-face of aromatic surfaces was studied using -phenylimide molecular rotors. The C═O···Ar interactions could stabilize the transition states but were half the strength of comparable C═O···C═O interactions. The C═O···Ar interaction had a significant electrostatic component but only a small orbital delocalization component.

Large transition state stabilization from a weak hydrogen bond.

A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O-H⋯O[double bond, length as m-dash]C hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.

-Arylimide Molecular Balances: A Comprehensive Platform for Studying Aromatic Interactions in Solution.

Noncovalent interactions of aromatic surfaces play a key role in many biological processes and in determining the properties and utility of synthetic materials, sensors, and catalysts. However, the study of aromatic interactions has been challenging because these interactions are usually very weak and their trends are modulated by many factors such as structural, electronic, steric, and solvent effects. Recently, -arylimide molecular balances have emerged as highly versatile and effective platforms for studying aromatic interactions in solution.

Transition-State Stabilization by n→π* Interactions Measured Using Molecular Rotors.

A series of 16 molecular rotors were synthesized to investigate the ability of n→π* interactions to stabilize transition states (TSs) of bond rotation. Steric contributions to the rotational barrier were isolated using control rotors, which could not form n→π* interactions. Rotors with strong acceptor π* orbitals, such as ketones and aldehydes, had greatly increased rates of rotation.

Electrostatically Driven CO-π Aromatic Interactions.

A series of -arylimide molecular balances were developed to study and measure carbonyl-aromatic (CO-π) interactions. Carbonyl oxygens were observed to form repulsive interactions with unsubstituted arenes and attractive interactions with electron-deficient arenes with multiple electron-withdrawing groups. The repulsive and attractive CO-π aromatic interactions were well-correlated to electrostatic parameters, which allowed accurate predictions of the interaction energies based on the electrostatic potentials of the carbonyl and arene surfaces.

Anion-enhanced solvophobic effects in organic solvent.

The influence of salts on the solvophobic interactions of two non-polar surfaces in organic solvent was examined using a series of molecular balances. Specific anion effects were observed that followed the Hofmeister series and enhanced the solvophobic effect up to two-fold.

Guest control of a hydrogen bond-catalysed molecular rotor.

Herein, the control of a molecular rotor using hydrogen bonding guests is demonstrated. With a properly positioned phenol substituent, the N-arylimide rotors can form an intramolecular hydrogen bond that catalyses the rotational isomerization process. The addition of the guests disrupts the hydrogen bond and raises the rotational barrier, slowing the rotation by two orders of magnitude.

Measurement of Solvent OH-π Interactions Using a Molecular Balance.

A molecular torsion balance was designed to study and measure OH-π interactions between protic solvents and aromatic surfaces. These specific solvent-solute interactions were measured via their influence on the folded-unfolded equilibrium of an N-arylimide rotor. Protic solvents displayed systematically weaker solvophobic interactions than aprotic solvents with similar solvent cohesion parameters.

Synergy between experimental and computational studies of aromatic stacking interactions.

Aromatic stacking interactions are one of the most common types of non-covalent interactions. However, their fundamental origins and the ability to accurately predict their stability trends are still an active area of research. The study of aromatic stacking interactions has been particularly challenging.

Distance-Dependent Attractive and Repulsive Interactions of Bulky Alkyl Groups.

The stabilizing and destabilizing effects of alkyl groups on an aromatic stacking interaction were experimentally measured in solution. The size (Me, Et, iPr, and tBu) and position (meta and para) of the alkyl groups were varied in a molecular balance model system designed to measure the strength of an intramolecular aromatic interaction. Opposite stability trends were observed for alkyl substituents at different positions on the aromatic rings.

Solvent-induced reversible solid-state colour change of an intramolecular charge-transfer complex.

A dynamic intramolecular charge-transfer (CT) complex was designed that displayed reversible colour changes in the solid-state when treated with different organic solvents. The origins of the dichromatism were shown to be due to solvent-inclusion, which induced changes in the relative orientations of the donor pyrene and acceptor naphthalenediimide units.

How important are dispersion interactions to the strength of aromatic stacking interactions in solution?

In this study, the contributions of London dispersion forces to the strength of aromatic stacking interactions in solution were experimentally assessed using a small molecule model system. A series of molecular torsion balances were designed to measure an intramolecular stacking interaction a conformational equilibrium. To probe the importance of the dispersion term, the size and polarizability of one of the aromatic surfaces were systematically increased (benzene, naphthalene, phenanthrene, biphenyl, diphenylethene, and diphenylacetylene).

Measurement of Silver-π Interactions in Solution Using Molecular Torsion Balances.

A new series of molecular torsion balances were designed to measure the strength of individual Ag-π interactions in solution for an Ag(I) coordinated to a pyridine nitrogen. The formation of a well-defined intramolecular Ag-π interaction in these model systems was verified by X-ray crystallography and (1)H NMR. The strength of the intramolecular Ag-π interaction in solution was found to be stabilizing in nature and quantified to be -1.

Additivity of substituent effects in aromatic stacking interactions.

The goal of this study was to experimentally test the additivity of the electrostatic substituent effects (SEs) for the aromatic stacking interaction. The additivity of the SEs was assessed using a small molecule model system that could adopt an offset face-to-face aromatic stacking geometry. The intramolecular interactions of these molecular torsional balances were quantitatively measured via the changes in a folded/unfolded conformational equilibrium.

The CH-π interactions of methyl ethers as a model for carbohydrate-N-heteroarene interactions.

CH-π interactions have been cited as an important contributor to carbohydrate recognition. To determine whether N-heterocycles form stronger CH-π interactions, the interactions of methyl ether groups with heterocyclic and nonheterocyclic aromatic surfaces were studied. Both experimental and computational experiments found that N-heterocyclic aromatic surfaces formed stronger interactions.

Experimental study of the cooperativity of CH-π interactions.

A series of new torsional molecular balances was designed to study the cooperativity of CH-π interaction in the solid state and in solution. The measured interaction energies correlated better to the number of participating alkyl carbons than to the number of CH-π contacts. The methyl and ethyl groups displayed additive interaction energies.