Phenylboronic acids can exist, in principle, in three different conformers (syn,syn; syn,anti and anti,anti) with distinct energy profiles. In their native state, these compounds prefer the energetically favored syn,anti-conformation. In molecular complexes, however, the functionality exhibits conformational diversity. In this paper we report a series of co-crystals, with N-donor compounds, prepared by a design strategy involving the synthons based on the syn,syn-conformation of the boronic acid functionality. For this purpose, we employed compounds with the 1,2-diazo fragment (alprazolam, 1H-tetrazole, acetazolamide and benzotriazole), 1,10-phenanthroline and 2,2′-bipyridine for the co-crystallization experiments. However, our study shows that the mere presence of the 1,2-diazo fragment in the coformer does not guarantee the successful formation of co-crystals with a syn,syn-conformation of the boronic acid. 
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Typical cyclic O-H…N hydrogen bonds formed by boronic acids with alprazolam (a) and 1,10-phenanthroline (b).

Phenylboronic acids in crystal engineering:Utility of the energetically unfavorable syn,syn-conformation in co-crystal design

Because of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest in using their aryl derivatives in materials science and supramolecular chemistry has risen. Nevertheless, their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of the first double-H-bonded DD·AA-type complexes of a series of aromatic boronic acids that adopt a syn–syn conformation with suitable complementary H-bonding acceptor partners. The first determination of the association constant (Ka) of ortho-substituted boronic acids in solution showed that Ka for 1:1 association is in the range between 300 and 6900 M–1. Crystallization of dimeric 1:1 and trimeric 1:2 and 2:1 complexes enabled an in-depth examination of these complexes in the solid state, proving the selection of the −B(OH)2 syn–syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho-substituted boronic acids resul...

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Versatile self-adapting boronic acids for H-bond recognition: from discrete to polymeric supramolecules

Structural equivalence is a general tool applied in crystal engineering for the predictable construction of cocrystals. We studied 17 molecular complexes of 4-halophenylboronic acids (Cl, Br, and I) with different N-donor compounds in the context of the structural equivalence of halogen bonded Br and I, as proposed by Jones and co-workers (Chem. Mater. 2008, 20, 6623−6626; Chem.—Eur. J. 2008, 14, 747–753). The 4-Cl- and 4-Br-phenylboronic acids make isostructural complexes with 4,7-phenanthroline (phen) and 1,2-bis(4-pyridyl)ethene (bpye); in contrast, with phenazine (phenz) and acridine (acr) I and Br derivatives are analogous. Thus, the structural equivalence of Cl/Br or Br/I derivatives is equally probable and is largely determined by the interplay between hydrogen and halogen bonds, besides the size, electron charge shifting ability, and steric crowding adjacent to the binding sites of coformers. Thanks to its bigger size and higher polarizability of the electron cloud, I is capable of making I···I, I...

Molecular Complexes of 4-Halophenylboronic Acids: A Systematic Exploration of Isostructurality and Structural Landscape

An analysis of crystal structures of a series of fluoro-substituted phenylboronic acids is presented. Interplay between the structure of a basic H-bonded dimeric R22(8) synthon and a higher-order supramolecular organization is highlighted. The elucidation of hydrogen bonds formed by the boronic B(OH)2 moiety is supported by energy calculations based on a one-dimensional H-bond model as well as by Hirshfeld surface analysis. The results revealed that intramolecular O–H···F hydrogen bonds are insignificant compared to O–H···O ones in dimers in controlling the syn–anti conformation of the boronic group. Depending on the strength of H-bonds in the basic motif, forces constituting so-called large synthons change from O–H···O hydrogen bonds to stacking interactions. This differentiation entails the twist of the boronic group with respect to the phenyl ring. The large synthons serve as main building blocks for three-dimensional structures either by their close packing or by the aid of weak secondary interaction ...

Hydrogen-Bonded Dimeric Synthon of Fluoro-Substituted Phenylboronic Acids versus Supramolecular Organization in Crystals

A series of new crystal structures of salts containing 3- and 4-pyridineboronic acid (3- and 4-pba) with chlorometallate have been prepared: [4-pbaH][MnCl2] (1), [4-pbaH][CdCl2], (2), [4-pbaH]2[CuCl4] (3), [4-pbaH]2[PdCl4] (4), [3-pbaH]2[CdCl4] (5), [3-pbaH][CuCl3(OH2)] (6), and [3-pbaH][PdCl2] (7). In these salts five structural forms for the chlorometallate species are observed: mononuclear square planar [M = Pd (4 and 7)], dimeric square-pyramidal [M = Cu (3)], polymeric square-pyramidal [M = Cu (6)], polymeric trans-edge-sharing octahedral [M = Mn (1), Cd (2)], and polymeric cis-edge-sharing octahedral [M = Cd (5)]. The cyclic R2,2(8) boronic acid dimer is formed in the salt of 1–5. NH···Cl2M synthons A have been exploited in 1–4, 6, and 7. Both NH···Cl2M (sythons A) and B(OH)2···Cl2M (sythons B) interactions give rise to a synthon of form I in 6. The uncommon folded conformation of the NH···Cl2M (synthons A) are formed in 7. The NH or OH donors and consequent branching of the NH···Cl hydrogen-bond ne...

Mayura Talwelkar

Papers

-B(OH)2 versus -OH in supramolecular synthesis: molecular complexes of 4-hydroxyphenylboronic acid with aza-donor compounds