Molecular tectonics. Use of the hydrogen bonding of boronic acids to direct supramolecular construction

Tetraboronic acids 1 and 2 have four -B(OH)₂ groups oriented tetrahedrally by cores derived from tetraphenylmethane and tetraphenylsilane. Crystallization produces isostructural diamondoid networks held together by hydrogen bonding of the -B(OH)₂ groups, in accord with the tendency of simple arylboronic acids to form cyclic hydrogen-bonded dimers in the solid state. Five-fold interpenetration of the networks is observed, but 60% and 64% of the volumes of crystals of tetraboronic acids 1 and 2, respectively, remain available for the inclusion of disordered guests. Guests occupy two types of interconnected channels aligned with the a and b axes; those in crystals of tetraphenylmethane 1 measure approximately 5.9 × 5.9 Ų and 5.2 × 8.6 Ų in cross section at the narrowest points, whereas those in crystals of tetraphenylsilane 2 are approximately 6.4 × 6.4 Ų and 6.4 × 9.0 Ų. These channels provide access to the interior and permit guests to be exchanged quantitatively without loss of crystallinity. Because the Si-C bonds at the core of tetraboronic acid 2 are longer (1.889(3) A) than the C-C bonds at the core of tetraboronic acid 1 (1.519(6) Å), the resulting network is expanded rationally. By associating to form robust isostructural networks with predictable architectures and properties of porosity, compounds 1 and 2 underscore the usefulness of molecular tectonics as a strategy for making ordered materials.