Intermolecular Donor-Acceptor Interactions of B(OH)2 Group in Phenylboronic Acids – Important or Not?
|Paulina K. Rogowska 1, Michal K. Cyranski 1, Andrzej Sporzyński 2|
1. Uniwersytet Warszawski, Wydział Chemii, Warszawa, Poland
The arylboronic acids are widely used in organic chemistry for Suzuki coupling reactions but have also many applications in biochemistry and in medicinal chemistry. Similarly as in benzoic acids, the most basic structural motive found in the crystals of phenyl boronic acids are dimers. Since the boronic group (B(OH)2) may be a donor of two hydrogens to hydrogen bonds, usually more complex structures are formed, which offers expanded possibilities in crystal engineering. The formation of complex structures is facilitated by appreciable high rotational flexibility of the group. So far, successful attempts have been made to form heterodimers with aminoacids , with other systems containing carboxylate groups  and the complexes with pyridine derivatives . Since boron is an electron deficient atom, apart from hydrogen bonds formed by hydroxyl groups, the donor-acceptor interactions should also influence the chemical behavior of the group and are expected to be a potentially important building factor determining the architecture of a crystal. In fact, this kind of interactions has been found in the ortho substituted systems having a nitrogen atom in the beta position N→B . A problem arises whether the donor-acceptor interactions are also decisive in intermolecular interactions. To answer this question, we have traced the crystal structures of phenylboronic acid and its para substitued derivatives, where the substituent significantly differs by van der waals radius and electronegativity: -F, -Cl, -Br and -I. Our attempts to determine the crystal structure of p-flurophenylboronic acid failed since the system transformed to tris(4-fluorophenyl)boroxin. The molecular structures of chloro- and bromo- derivatives are similar to the structure of the parent molecule. The dimers are linked through a hydrogen-bonded network to adjacent pairs which lie almost perpendicular to each other, forming infinitely extended chains, as shown in the figure below. An important stabilising factor are the Cl…Cl and Br…Br interactions which are 3.528 Å and 3.562 Å, respectively. In contrast, the iodo- derivative (which cocrystalises with water molecule) reveals short contact between the iodo substituent and boron atoms and the molecules are perpendicular each other (as shown in the figure). The B…I distance is 3.670 Å and 3.744 Å.
A similar arrangement of molecules can be observed in the structure of p-bromophenylboronic acid cocrystalising with ethanol . Taking into account the difference in van der waals radius between I and Br the distance of 3.645 Å and 3.682 Å B…Br appeals, however, for much less efficient donor-acceptor interaction. The analysis based on crystal structures from Cambridge Structural Database leads to the conclusion that in contrast to donor-acceptor intramolecular interactions, the intermolecular ones generally lose the competition with hydrogen bonds formed by hydroxyl groups.
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Presentation: poster at 18th Conference on Physical Organic Chemistry, Posters, by Paulina K. Rogowska
See On-line Journal of 18th Conference on Physical Organic Chemistry
Submitted: 2006-07-07 21:06 Revised: 2006-08-04 20:20
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