Boron-Nitrogen Lewis Pairs in the Assembly of Supramolecular Macrocycles, Molecular Cages, Polymers, and 3D Materials

Covering an exceptionally wide range of bond strengths, the dynamic nature and facile tunability of dative B-N bonds is highly attractive when it comes to the assembly of supramolecular polymers and materials. This Minireview offers an overview of advances in the development of functional materials where Lewis pairs (LPs) play a key role in their assembly and critically influence their properties. Specifically, we describe the reversible assembly of linear polymers with interesting optical, electronic and catalytic properties, discrete macrocycles and molecular cages that take up diverse guest molecules and undergo structural changes triggered by external stimuli, covalent organic frameworks (COFs) with intriguing interlocked structures that can embed and separate gases such as CO2 and acetylene, and soft polymer networks that serve as recyclable, self-healing, and responsive thermosets, gels and elastomeric materials.

Flow-Chemistry-Enabled Synthesis of 5-Diethylboryl-2,3'-bipyridine and Its Self-Assembly Dynamics

5-Diethylboryl-2,3'-bipyridine (1), which is inaccessible by conventional batch methods, was synthesized by using a flow microreactor. Compound 1 was obtained as an equilibrium mixture of a cyclic trimer and a cyclic tetramer in solution, the latter of which was crystallized in benzene by vapor diffusion of hexane at 7 °C. The dynamic nature of this system was confirmed by solvent- and concentration-dependent experiments. Notably, the dynamics was verified by using flow NMR spectroscopy, which revealed that the time required to reach equilibrium was influenced by the solvent ratio (<18 s, 24-28 s, and 34-42 s in 2 : 1, 1 : 1, and 1 : 2 mixtures of [D₆ ]acetone and C₆ D₆ , respectively). Compound 1 and 3-[4'-(diethylboryl)phenyl]pyridine (2) exhibited different self-assembly behavior in solution and crystals. Density functional theory calculations suggested that this difference was largely due to enhanced planarity between two consecutive aromatic rings.

3-[4'-(Diethylboryl)phenyl]pyridine: Exclusive Crystallization of the Cyclic Tetramer

The crystallization of 3-[4'-(diethylboryl)phenyl]pyridine (1), which formed a mixture of oligomers in solution with the cyclic trimer as a major component, in acetone at 0 °C afforded a cyclic tetramer that co-crystallized with solvent molecules. Similarly, solutions of compound 1 in toluene at 10 °C and in benzene at 8 °C furnished the cyclic tetramer with the incorporation of toluene and benzene molecules, respectively, thus suggesting that the cyclic tetramer was the minor component. ¹³ C CP/MAS NMR spectroscopy of precipitates of compound 1 suggested that precipitation from acetone and toluene each afforded mixtures of the cyclic trimer and the cyclic tetramer, whereas precipitation from benzene exclusively furnished the cyclic tetramer. Therefore, it appeared that crystallization readily shifted the equilibrium towards the cyclic tetramer in benzene. The thermodynamic parameters for the equilibrium between these two oligomers in [D₆ ]benzene, as determined from a van't Hoff plot, were ΔH°=-8.8 kcal mol^-1 and ΔS°=-23.7 cal mol^-1  K^-1 , which were coincident with previously reported calculations and observations.

Self-Assembly of 4-(Diethylboryl)pyridine: Crystal Structures of the Cyclic Pentamer and Hexamer and Their Solvent-Dependent Selective Crystallization

Two distinct oligomeric structures were obtained by the self-assembly of 4-(diethylboryl)pyridine (1). In the (1)H NMR spectrum of 1 in CDCl3, at least two sets of signals were observed for the pyridyl α- and β-hydrogen atoms. ESI-MS, VPO, and TLC analysis revealed that 1 assembles mainly into a mixture of cyclic pentamers and hexamers in solution via intermolecular boron-nitrogen coordination bonds. Crystallization of 1 in THF by vapor diffusion of EtOH or in CHCl3 afforded the cyclic hexamer incorporating one THF molecule (16·THF) or 1.5 mol equiv of chloroform molecule (16·CHCl3), respectively. Similarly, a solution of 1 in a mixture of benzene and hexane furnished the cyclic pentamer bearing two benzene molecules (15·C6H6). It seems that the solvent differences affected the crystallization of the two distinct cyclic oligomers of 1, either of which was cocrystallized predominantly with the solvent molecule. Thermogravimetric analysis of the crystals and NMR studies of the solution revealed that the noncovalent interactions between the host and guest are not strong enough to hold the guest molecule in the cavity.

Supramolecular gels based on boronate esters and imidazolyl donors

Dative boron-nitrogen bonds can be used for the controlled assembly of boronate esters and N-donor ligands. So far, most investigations have focused on crystalline assemblies. In contrast, there are very few reports about soft materials based on dative B-N bonds. Here, we describe the formation of gels by aggregation of diboronate esters and ditopic bisimidazolyl donor ligands. Analyses of low molecular weight model compounds by NMR spectroscopy, isothermal titration calorimetry, and X-ray crystallography reveal that imidazolyl ligands form particularly strong dative bonds with boronate esters. The strong interaction allows gel formation in competitive polar solvents such as THF and acetone. In less polar solvents, very low critical gel concentrations of down to 0.02 wt% are observed. By using a diboronate ester linked by an azobenzene group, it is possible to form a photoresponsive gel.