Two-dimensional hydrogen-bonded networks in crystals of diboronic acids

Abstract

Crystallization of suitable diboronic acids favors the formation of hydrogen-bonded sheets, thereby underscoring the usefulness of the –B(OH)₂ group in crystal engineering.

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A productive strategy in crystal engineering is to build structures from molecules that are programmed to engage in multiple interactions with neighbors.¹ Hydrogen bonds are widely used as the principal interactions in this strategy because they are strong and directional.² Many different self-complementary hydrogen-bonding groups can be used to control association in the solid state and to produce networks with predictable structural features and properties. Recent work has revealed the usefulness of the –B(OH)₂ group in crystal engineering.^3–7 Boronic acids typically give cyclic hydrogen-bonded dimers analogous to those formed by carboxylic acids and amides (Fig. 1a).⁸

Fig. 1 (a) Cyclic hydrogen-bonded dimers of boronic acids, carboxylic acids, and amides. (b) Lateral hydrogen bonding of cyclic dimers of boronic acids and amides, with one dimer highlighted in red.

Unlike dimers of carboxylic acids, dimers of boronic acids and amides can associate further by forming lateral hydrogen bonds with nearby dimers (Fig. 1b).^3,6 These lateral interactions reinforce structures by adding hydrogen bonds and extending them in a new direction. As a result, boronic acids can be expected to play a special role in crystal engineering. For example, 1,4-benzenediboronic acid (1) crystallizes as expected to form chains of hydrogen-bonded dimers, and the chains then associate by lateral hydrogen bonding to create sheets.⁶ In this way, a 1-D network (chain) is transformed logically into a fully hydrogen-bonded 2-D motif (sheet). An analogous structure is formed by the corresponding amide, terephthalamide (2).⁹ In contrast, terephthalic acid (3) can only form hydrogen-bonded chains.¹⁰

Diboronic acid 1 can also crystallize as a tetrahydrate, in which the characteristic chains are present but use their capacity for lateral interactions to form hydrogen bonds with included water.⁷ This observation shows that crystallization in the presence of suitable guests can divert boronic acids from their normal tendency to form laterally paired dimers and thereby lead to the formation of inclusion complexes.

In an effort to engineer expanded versions of the structures formed by diboronic acid 1, we turned our attention to lengthened analogue 4, which incorporates a core derived from diphenylacetylene. Compound 4 was synthesized from bis(4-bromophenyl)acetylene¹¹ in 84% overall yield by lithium–halogen exchange (BuLi, −78 °C), followed by addition of triisopropylborate and hydrolysis. Crystals of diboronic acid 4 were grown by (i) diffusing vapors of hexane into a solution in THF; (ii) cooling a solution in dioxane; and (iii) cooling a solution in wet ethyl acetate.¹² Three different forms I–III were formed under conditions (i)–(iii), respectively, and their structures are discussed below.

Crystals of Form I were found to belong to the triclinic space group P1̄ and to have the composition 4·3THF. The included THF is partially ordered and occupies 56% of the volume of the crystal. As expected, each molecule of diboronic acid 4 interacts with two neighbors to form hydrogen-bonded chains (Fig. 2). The O⋯O distance within the chains is 2.759(4) Å. The repeating distance between the centers of adjacent molecules within chains constructed from diboronic acid 1 is 10 Å, whereas the value in chains built from lengthened analogue 4 is extended rationally to 17 Å. In both structures, the chains lie in layers, but the chains of diboronic acid 4 do not associate directly by forming lateral hydrogen bonds. Instead, they are separated by intervening molecules of THF (Fig. 2), which accept lateral hydrogen bonds donated by the chains (O⋯O distance 2.767(4) Å). THF is presumably included because it is a better acceptor of lateral hydrogen bonds than the chains themselves.¹³

Fig. 2 View of the hydrogen-bonded chains in crystals of diboronic acid 4 grown from THF/hexane (Form I). The chains are arranged in layers with intervening molecules of THF, which accept lateral hydrogen bonds donated by the chains. Carbon is shown in gray, hydrogen in white, oxygen in red, and boron in pink. Hydrogen atoms in THF are omitted for clarity, as are included molecules of THF not involved in hydrogen bonding.

As expected, crystals of diboronic acid 4 grown from dioxane (Form II) have a closely similar structure, with dioxane bridging the characteristic chains by serving as a bidentate acceptor of lateral hydrogen bonds (Fig. 3). Crystals of Form II were found to belong to the triclinic space group P1̄ and to have the composition 4·3dioxane. One of the included molecules of dioxane is ordered, and the other two are statistically disordered. Together, included dioxane occupies 58% of the volume of the crystal. The O⋯O distance within the chains is 2.746(2) Å, and the O⋯O distance between the chains and the bridging molecules of dioxane is 2.721(2) Å. Like crystals of diboronic acid 1, crystals of Form II consist of continuously hydrogen-bonded sheets, now co-assembled from dioxane and chains of lengthened diboronic acid 4. The new sheets have been extended in two dimensions, both in the direction of the chains (by lengthening the diboronic acid) and in the orthogonal direction (by inserting dioxane between the chains).

Fig. 3 View of the structure of crystals of diboronic acid 4 grown from dioxane (Form II), showing how the characteristic chains are bridged by dioxane. Hydrogen atoms in dioxane are omitted, as are included molecules of dioxane that are not involved in hydrogen bonding.

Because ethyl acetate is a poorer acceptor of hydrogen bonds than THF and dioxane, which are included in Forms I–II, we were not surprised to find that Form III has a different structure. The resulting crystals belong to the monoclinic space group P2₁/n and are closely packed, with no included solvent. As expected, based on the behavior of diboronic acid 1, the close-packed form of extended analogue 4 also incorporates hydrogen-bonded sheets. Contrary to expectation, however, the sheets are no longer built from chains held together by the standard dimeric motif shown in Fig. 1a. Instead, alternating molecules in chains of diboronic acid 4 are formally displaced, creating a new sheet in which each molecule now interacts with six neighbors via single hydrogen bonds (Fig. 4). The four hydrogen bonds parallel to the long axis of the molecule have an O⋯O distance of 2.764(1) Å, whereas the four lateral hydrogen bonds have an O⋯O distance of 2.843(1) Å. Similar motifs are rare among boronic acids and have only been reported in the structure of (4-methoxyphenyl)boronic acid.⁵ The hydrogen-bonded sheets then stack in close-packed layers, with no distinctive interactions between the layers.

Fig. 4 View of a single hydrogen-bonded sheet in close-packed crystals of diboronic acid 4 grown from ethyl acetate (Form III).

Together, our observations underscore the utility of the –B(OH)₂ group in crystal engineering. Unlike the structurally related –COOH group, which can donate only one hydrogen bond, the B(OH)₂ group can form characteristic motifs incorporating multiple hydrogen bonds. Placing two –B(OH)₂ groups in a single molecule thereby favors the formation of 2-D hydrogen-bonded networks. Modifications of molecular geometry and inclusion of guests can then be used to alter these networks with an important degree of predictability.

CCDC reference numbers 283685–283687. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b516402h.

Acknowledgements

We are grateful to the Natural Sciences and Engineering Council of Canada (NSERC), the Ministère de l'Éducation du Québec, the Canada Foundation for Innovation, and the Canada Research Chairs Program for financial support. We thank Dr S. G. Telfer for useful discussions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of 4,4′-diphenylacetylenediboronic acid; X-ray crystallographic studies. See DOI: 10.1039/b516402h

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