Chemistry that holds water: A honeycomblike water sheet having a hitherto unknown structure is described. The crystal lattice of bulk water in the presence of 1,4‐phenylenediboronic acid (see picture) contains hexameric rings with chair and boat conformations. The hydrogen‐bonding pattern is completely elucidated and gives two different motifs for each of the three independent hydrogen‐bonded rings.

4,4′-Bipyridine has been combined with boric acid and four different boronic acids (1,3- and 1,4-benzenediboronic acid, 3-aminophenylboronic acid, 4-acetylphenylboronic acid) to give 1:1 (with boric acid) and 1:2 adducts (with arylboronic acids), which have been characterized by X-ray diffraction analysis. The supramolecular solid-state structures are composed of hydrogen-bonded networks with (B)O−H···N, (B)O−H···O, C−H···O, C−H···N, C−H···π, π···π and C−H···B interactions. The comparative analysis of the boric/boronic acid−4,4′-bipyridine adducts has revealed that water molecules play an important role as spacer molecules in RB(OH)2···py synthons, since their incorporation in the hydrogen-bonding patterns allows optimization of π−π interactions. The structural relationship between the dihydroxyboryl and the carboxyl group has been analyzed, showing that the former can form at least three different hydrogen-bonding patterns with pyridines. This can be attributed to the presence of two acidic hydrogen atoms instead of one (B(OH)2 ↔ C(O)OH). The three motifs have been examined also by ab initio calculations, confirming that for the three cases the (B)O−H···N interaction energies are similar.

The hydrogen-bonding interaction energies have been computed for a series of five homo-and heterodimers formed between boronic acids, carboxylic acids, and carboxylate anions using ab initio methods at the MP2/6-31G(d,p) level. The results show that the stability of these systems decreases in the following direction: RB(OH) 2 ‚‚‚ -OOCR > RCOOH‚‚‚ -OOCR >> RCOOH‚‚‚HOOCR > RB(OH) 2 ‚‚‚HOOCR > RB(OH) 2 ‚‚‚(HO) 2 BR; (R ) Me, Ph). Five crystals containing boronic acids and carboxylate anions have been prepared and characterized both in solution and the solid-state using spectroscopic as well as X-ray crystallographic methods. The interaction energies of the RB(OH) 2 ‚‚‚ -OOCR heterodimeric motifs are more than double those of the homodimeric RCOOH‚‚‚HOOCR and RB(OH) 2 ‚‚‚(HO) 2 BR motifs. This was proved by the fact that these anionic systems were stable in solution, even in polar solvents such as DMSO. The solid-state analysis provided detailed geometric data for one of these systems. The predicted interaction energies for the RB(OH) 2 ‚‚‚HOOCR heterodimers are intermediate between the energies for the corresponding RCOOH‚‚‚HOOCR and RB(OH) 2 ‚‚‚(HO) 2 BR homodimers; nevertheless, crystals containing such a system could not be obtained. Despite of this, these experiments provided two new systems containing hydrogen-bonding interactions between neutral carboxylic and boronic acids.

The study of compounds containing boron continues to have an important impact on virtually every area of chemistry. One of the few areas in which compounds of boron have been neglected is crystal engineering, which seeks to develop and exploit an understanding of how the structure and properties of crystals are related to the individual atomic or molecular components. Although detailed predictions of crystal structures are not yet reliable, crystal engineers have developed a sound qualitative strategy for anticipating and controlling structural preferences. This strategy is based on the design of special molecules, called tectons, which feature carefully selected cores and multiple peripheral functional groups that can direct association and thereby place neighboring molecules in predetermined positions. Recent work has demonstrated that molecular crystals with unique properties can be constructed logically from tectons with boron in their cores or sticky sites of association. In particular, the -B(OH)2 group of boronic acids engages in reliable patterns of hydrogen bonding, and its use as a sticky site in tectons has emerged as an effective tool for controlling association predictably. In addition, replacement of tetraphenylsilyl or tetraphenylmethyl cores in tectons by tetraphenylborate leaves the overall molecular geometry little changed, but it has the profound effect of introducing charge. Tectons derived from tetraphenylborate can be used rationally to construct porous charged molecular networks that resemble zeolites and undergo selective ion exchange. In such ways, boron offers chemists exciting new ways to engineer molecular crystals with predetermined structures and properties.