BALANCE OF FORCES BETWEEN BROMO CONTACTS AND HYDROGEN BONDS IN A SUPRAMOLECULAR SYSTEM

Thomas H. Brehmer¹, Felíx H. Cano², E. Weber¹

¹ Institute of Organic Chemistry, TU Bergakademie Freiberg, Leipziger Strasse 29, D-09599 Freiberg/Sachs.
² Departmento de Cristalografia, Instituto Química-Física Rocasolano, CSIC Serrano 119, E-28006 Madrid, Spain

Keywords: supramolecular chemistry, crystalline inclusion compounds, hydrogen bonding, bromo interactions, polymorphism

Various forces determine a crystal structure and sometimes an unstable balance of forces between different interactions is the reason for polymorphism. We have synthesized the new model compound cis 9,10-bis(4-bromophenyl)-9,10-dihydroxy-9,10-dihydroanthracene 1 which contains two hydroxyl groups and two 4-bromophenyl substituents in a cis-conformation attached at different sides to the basic 9,10-dihydroanthracene subunit. Thus, two potential contact regions of this molecule are distinguishable: one (region 1) which is around the two bromine atoms being rather highly polarizable (soft) and the other (region 2) including the two hydroxyl groups having a high affinity to form hydrogen bonds.

A variety of three possible interactions (Br...Br , Br...H-O, O-H...O) involving this two regions for different molecules is realized in the two crystal modifications 1A and 1B. Linear Br...Br contacts are the only type of halogen interactions shown in the modification 1A besides strong and dominant OH-interactions. In the crystalline modification 1B all three contacts including the perpendicular Br...Br interaction and also the Br...H-O and O-H...O contacts are present to form dimers. These dimers are arranged by two 4-center closed loops to give aggregated blocks.

In addition, the solvents which were used in the crystallization process give rise to a strong influence on the packing motif. Compound 1 with enclathrated chloroform as guest solvent crystallizes in a packing structure similar to 1B. On the other hand, the molecular geometry of host compound 1 is drastically changed in the crystalline inclusion structure involving the dipolar solvent DMSO. Here, the oxygen atom of DMSO acts as a double electron donor for both OH-groups in region 2 which causes a contraction of the distance between hydroxyl groups and a conformational rotation of the phenyl rings.