Methanesulfonyl-polarized halogen bonding enables strong halide recognition in an arylethynyl anion receptor

Solution Studies of a Water-Stable, Trivalent Antimony Pnictogen Bonding Anion Receptor with High Binding Affinities for CN-, OCN-, and OAc. Inorg Chem 2023. [PMID: 37499143 DOI: 10.1021/acs.inorgchem.3c01887] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis.

Competition between Binding to Various Sites of Substituted Imidazoliums

The imidazolium cation has a number of different sites that can interact with a nucleophile. Adding a halogen atom (X) or a chalcogen (YH) group introduces the possibility of an NX···nuc halogen or NY···nuc chalcogen bond, which competes against the various H-bonds (NH and CH donors) as well as the lone pair···π interaction wherein the nucleophile lies above the plane of the cation. Substituted imidazoliums are paired with the NH₃ base, and the various different complexes are evaluated by density functional theory (DFT) calculations. The strength of XB and YB increases quickly along with the size and polarizability of the X/Y atom, and this sort of bond is the strongest for the heavier Br, I, Se, and Te atoms, followed by the NH···N H-bond, but this order reverses for Cl and S. The various CH···N H-bonds are comparable to one another and to the lone pair···π bond, all with interaction energies of 10-13 kcal/mol, values which show very little dependence upon the substituent placed on the imidazolium.

A family of powerful halogen-bond donors: a structural and theoretical analysis of triply activated 3-iodo-1-phenylprop-2-yn-1-ones

A new group of powerful halogen-bond donors have been synthesized and evaluated using structural and computational tools.

Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

We have performed a database survey and a structural and computational study of the potential and the limitations of halogenopyridinium cations as halogen bond donors. The database survey demonstrated that adding a positive charge on a halogenopyridine ring increases the probability that the halogen atom will participate in a halogen bond, although for chloropyridines it remains below 60%. Crystal structures of both protonated and N-methylated monohalogenated pyridinium cations revealed that the iodo- and bromopyridinium cations always form halogen-bonding contacts with the iodide anions shorter than the sum of the vdW radii, while chloropyridinium cations mostly participate in longer contacts or fail to form halogen bonds. Although a DFT study of the electrostatic potential has shown that both protonation and N-methylation of halogenopyridines leads to a considerable increase in the ESP of the halogen σ-hole, it is generally not the most positive site on the cation, allowing for alternate binding sites.

Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

We performed a structural study of N-alkylated halogenopyridinium cations to examine whether choice of the N-substituent has any considerable effect on the halogen bonding capability of the cations. For that purpose, we prepared a series of N-ethyl-3-halopyridinium iodides and compared them with their N-methyl-3-halopyridinium analogues. Structural analysis revealed that N-ethylated halogenopyridinium cations form slightly shorter C−X⋯I− halogen bonds with iodide anion. We have also attempted synthesis of ditopic symmetric bis-(3-iodopyridinium) dications. Although successful in only one case, the syntheses have afforded two novel ditopic asymmetric monocations with an iodine atom bonded to the pyridine ring and another on the aliphatic N-substituent. Here, the C−I⋯I− halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor. This trend in halogen bond lengths is in line with the charge distribution on the Hirshfeld surfaces of the cations—the positive charge is predominantly located in the pyridine ring making the pyridine iodine atom σ-hole more positive than the one on the alkyl chan.

Multidentate, V-Shaped Pyridine Building Blocks as Tectons for Crystal Engineering

The formation of supramolecular structural units through self-assembly is a powerful method to design new architectures and materials endowed with specific properties. With the aim of adding a group of versatile tectons to the toolkit of crystal engineers, we have devised and synthesised four new V-shaped building blocks characterised by an aryl acetylene scaffold comprising three substituted pyridine rings connected by two triple bonds. The judicious choice of different substituents on the pyridine rings provides these tectons with distinctive steric, electrostatic and self-assembly properties, which influence their crystal structures and their ability to form co-crystals. Co-crystals of the tectons with tetraiododifluorobenzene were obtained both via traditional and mechanochemical crystallisation strategies, proving their potential use in crystal engineering. The energetic contributions of the supramolecular interactions at play in the crystal lattice have also been evaluated to better understand their nature and strength and to rationalise their role in designing molecular crystals.

Bumpy Roads Lead to Beautiful Places: The Twists and Turns in Developing a New Class of PN-Heterocycles

The Haley and Johnson labs at the University of Oregon have been collaborating since 2006, combining skillsets in synthetic organic, physical organic, and supramolecular chemistries. This joint project has produced many examples of host molecules that bind anionic guests and give chemical, photophysical, and/or electrical responses. Many of these receptors utilize two-armed arylethynyl backbones that have a variety of hydrogen- or halogen-bonding functional groups appended. However, in attempts to produce a bisamide-containing host using a peptide-coupling protocol with P(OPh)3 present, we isolated something unexpected – a heterocycle containing neighboring P and N atoms. This ‘failed’ reaction turned into a surprisingly robust synthesis of phosphaquinolinones, an unusual class of PN-heterocycles. This Account article tells the rollercoaster story of these heterocycles in our lab. It will highlight our key works to this field, including a suite of fundamental studies of both the original PN-naphthalene moiety, as well as a variety of structural modifications to the arene backbone. It will also discuss the major step forward the project took when we developed a phosphaquinolinone-containing receptor molecule capable of binding HSO4 – selectively, reversibly, and with recyclability. With these findings, the project has gone from hospice care to making a full, robust recovery.

1 Introduction

2 Initial Discovery

3 Setbacks Breathe New Life

4 A New Dynamic Duo Develops Dozens of Derivatives

5 Physicochemical Characterization

5.1 Fluorescence

5.2 Molecular Structures

5.3 Solution Dimerization Studies

6 Applying What We Have Learned

6.1 Development of Supramolecular Host

6.2 Use of PN Moiety as an Impressive Fluorophore

7 Conclusions and Outlook

Dramatic Enhancement of Binding Affinities Between Foldamer-Based Receptors and Anions by Intra-Receptor π-Stacking

As a synthetic model for intra-protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer-based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen-bonding sites for anion binding but different aryl appendages that simply provide additional π-stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron-deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.

Triple-Pnictogen Bonding as a Tool for Supramolecular Assembly

Supramolecular assembly utilizing simultaneous formation of three pnictogen bonds around a single antimony vertex was explored via X-ray crystallography, solution NMR, and computational chemistry. An arylethynyl (AE) ligand was designed to complement the three electrophilic regions around the Sb compound. Though solution studies reveal large binding constants for individual pyridyl units with the Sb donor, the rigidity and prearrangement of the AE acceptor proved necessary to achieve simultaneous binding of three acceptors to the Sb-centered pnictogen-bond donor. Calculations and X-ray structures suggest that negative cooperativity upon sequential binding of three acceptors to a Sb center limits the utility of triple-pnictogen bonding pyridyl acceptors. These limitations can be negated, however, when positive cooperativity is designed into a complementary acceptor ligand.

Anion Influence on the Packing of 1,3-Bis(4-Ethynyl-3-Iodopyridinium)-Benzene Halogen Bond Receptors

Rigid and directional arylethynyl scaffolds have been widely successful across diverse areas of chemistry. Utilizing this platform, we present three new structures of a dicationic 1,3-bis(4-ethynyl-3-iodopyridinium)-benzene halogen bonding receptor with tetrafluoroborate, nitrate, and hydrogen sulfate. Structural analysis focuses on receptor conformation, anion shape, solvation, and long range packing of these systems. Coupled with our previously reported structures, we conclude that anions can be classified as building units within this family of halogen bonding receptors. Two kinds of antiparallel dimers are observed for these dicationic receptors. An off-centered species is most frequent, present among geometrically diverse anions, and assorted receptor conformations. In contrast, the centered antiparallel dimers are observed with receptors adopting a bidentate conformation in the solid-state. While anions support the solid-state formation of dimers, the molecular geometry and characteristics (planarity, rigidity, and directionality) of arylethynyl systems increases the likelihood of dimer formation by limiting efficient packing arrangements. The significantly larger cation may have considerable influence on the solid-state packing, as similar cationic arylethynyl systems also display these dimers, suggesting.