Reports: DNI1050439-DNI10: Dynamic Assembly of Porous Boronate Ester Macromolecules

Brian H. Northrop, PhD , Wesleyan University

Materials containing nano- to mesoscale pores play vital roles in the production, purification, separation and storage of chemical fuels. The primary goal of this research is to develop the synthesis and self-assembly of discrete boronate ester macromolecules that possess well-defined nanoscale pores and are soluble in common organic solvents. We are tentatively referring to these boronate ester macromolecules as covalent organic polygons (COPs), as they share some structural and design characteristics with infinitely periodic, though insoluble, covalent organic frameworks (COFs). Herein we report our progress to date toward our goal of synthesizing boronate ester macromolecules.

As described in the original proposal, the successful synthesis of discrete boronate ester macromolecules first requires the synthesis of oligo-catechols and oligo-boronic acids that maintain a high degree of rigidity such that their catechol and boronic acid moieties are held at fixed distances and angels with respect to each other; they are “directional.” To enforce this directionality we initially designed and synthesized bis-catechols and bis-boronic acids built upon fused aromatic backbones. During the first six months of the current reporting period we were able synthesize five target bis-catechols (based upon phenyl, trypticene, anthracene, 9,10-anthraquinone, and triphenylene backbones) as well as three target bis-boronic acids (based upon anthracene, phenanthrene, and pyrene backbones). All eight target compounds, however, were poorly soluble to insoluble in aprotic solvents of moderate polarity (e.g. CHCl­3, THF, CH2Cl2, EtOAc) and were fully soluble only in highly polar or protic solvents (e.g. DMSO, DMF, MeOH, EtOH). This result was undesired given that highly polar and protic solvents shift the equilibrium of boronate ester self-assembly almost exclusively toward their catechol and boronic acid starting materials, thus rendering boronate ester macromolecules unstable in such media.

While the eight initially synthesized target compounds did not have the desired solubility in moderately polar solvents, we were able to obtain evidence supporting their assembly into boronate ester macromolecules under solvent free conditions. Mechanical grinding of complementary pairs of bis-catechols and bis-boronic acids was achieved both with a mortar and pestle and in a ball mill. These solid state reactions produced new solids whose IR spectra showed the disappearance of bands associated with catechol (C)O–H or boronic acid (B)O–H stretching and the appearance of bands associated with B–O(C) stretching, indicative of boronate ester formation. These solids, however, were insoluble in all available solvents and therefore their structures could not be confirmed by solution phase NMR spectroscopy. We are currently in the process of sending samples of these solids for mass spectrometry and powder X-ray diffraction analysis to better understand their molecularity and their structure. These preliminary results, while promising, are not in line with the goal of developing the synthesis of organic soluble boronate ester macromolecules.

Given the insolubility of initial bis-catechol and bis-boronic acid compounds built upon polycyclic aromatic hydrocarbon backbones we then set out to synthesize functionalized analogues of these aromatic compounds to improve their solubility. Toward this aim we have targeted four bis-catechols: two functionalized with hydrophobic hexyl chains and two functionalized with hydrophilic diethylene glycol monomethyl ether chains. To date, preparative quantities of both hydrophobic and hydrophilic phenanthrene-based bis-catechol targets have been synthesized and we are nearing completion of hydrophobic and hydrophilic triphenylene bis-catechol targets. In an important demonstration of the feasibility of our approach to the self-assembly of soluble boronate ester macromolecules we successfully condensed hydrophobic and hydrophilic phenthrene-based bis-catechols with 4-(tert-butyl)phenylboronic acid to give boronate ester products (see TOC graphic).  These dynamic self-assembly reactions were carried out in CDCl3 and showed complete and quantitative conversion of the catechol and boronic acid starting materials to their boronate ester products. Monitoring these reactions by 1H NMR spectroscopy showed complex mixtures of several interconverting products within 10 min of mixing. In both the hydrophobic and hydrophilic systems these kinetic intermediates were, in fewer than 30 min total, dynamically converted to the most favorable thermodynamic product: the desired symmetric boronate ester assemblies. It’s important to note that both boronate ester products are fully soluble in CDCl3.

We are currently in the process of scaling up the hydrophobic and hydrophilic phenanthrene-based bis-catechol starting materials and completing the synthesis of hydrophobic and hydrophilic triphenylene-based bis-catechols. With the preliminary demonstration that these soluble bis-catechol derivatives are able to dynamically self-assemble with boronic acid derivatives in common organic solvents we are now targeting the synthesis of discrete, soluble covalent organic polygons by using 1,4-phenyl bis-boronic acid and 2,7-pyrene bis-boronic acid in place of the 4-(tert-butyl)phenylboronic acid. The successful synthesis and self-assembly of these target covalent organic polygons will be disseminated both in the chemical literature and at local and national conferences and seminars.

To date the research outlined above has significantly impacted one graduate student and four undergraduate students working in my lab as well as my academic career. Two undergraduate students who performed many of the initial synthesis and computational modeling involved with this project have since graduated and both have gone on graduate school: one in chemistry and the other in materials science and engineering. Furthermore, we have published the senior thesis research of one of these undergraduate students whose research involved computational and spectroscopic investigations of dynamic self-assembly. This publication is the first of my independent academic career and will prove instrumental in helping me attract additional students to the project and also help in securing additional external funding for the future. The graduate student working on these boronate ester macromolecules has made significant progress on all fronts of the project. She has gained significant exposure to a variety of new synthetic and analytical techniques and has grown considerably in her ability to think critically and independently. There are also two new undergraduate students who joined this project this past summer. This is the first time either student has been involved in independent research and they are both developing their laboratory skills apace. Over the summer, one of these undergraduates developed a new synthetic route to a valuable intermediate we had been struggling to synthesize for the past year.

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