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 x 5.9 A(2) and 5.2 x 8.6 A(2) in cross section at the narrowest points, whereas those in crystals of tetraphenylsilane 2 are approximately 6.4 x 6.4 A(2) and 6.4 x 9.0 A(2). 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) A), 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.
Molecules with multiple sites that induce strong directional association tend to form open networks with significant volumes available for the inclusion of guests. Such molecules can be conveniently synthesized by grafting diverse sticky sites onto geometrically suitable cores. The characteristic inability of 9,9'-spirobifluorene to form close-packed crystals suggests that it should serve as a particularly effective core for the elaboration of molecules designed to form highly porous networks. To test this hypothesis, various new tetrasubstituted 9,9'-spirobifluorenes with hydrogen-bonding sites at the 3,3',6,6'-positions or 2,2',7,7'-positions were synthesized by multistep routes. Four of these compounds were crystallized, and their structures were determined by X-ray crystallography. In all cases, the compounds form extensively hydrogen-bonded networks with high porosity. In particular, 43% of the volume of crystals of 3,3',6,6'-tetrahydroxy-9,9'-spirobifluorene (28) is available for the inclusion of guests, whereas the porosity is only 28% in crystals of tetrakis(4-hydroxyphenyl)methane, a close model that lacks the spirobifluorene core. Similarly, the porosities found in crystals of 2,2',7,7'-tetra(acetamido)-9,9'-spirobifluorene (33) and 2,2',7,7'-tetrasubstituted tetrakis(diaminotriazine) 39 are 33% and 60%, respectively. Moreover, the porosity of crystals of 2,2',7,7'-tetrasubstituted tetrakis(triaminotriazine) 40 is 75%, the highest value yet observed in crystals built from small molecules. These observations demonstrate that a particularly effective strategy for engineering molecules able to form highly porous networks is to graft multiple sticky sites onto spirobifluorenes or other cores intrinsically resistant to close packing.
Useful derivatives of tetraphenylmethane and tetraphenylsilane have been synthesized by efficient methods that give crystalline products without chromatographic purification. New compounds include tetrakis(4-hydroxyphenyl)methane (21), tetrakis(4-formylphenyl)methane (22), tetrakis[(4-hydroxymethyl)phenyl]methane (23), tetrakis(4-bromophenyl)silane (24), tetrakis(4-iodophenyl)silane (25), tetrakis(4-hydroxyphenyl)silane (26), tetrakis[(4-hydroxymethyl)phenyl]silane (27), and tetrakis[(4-chloromethyl)phenyl]silane (28). These compounds are valuable precursors for the construction of complex molecules with tetrahedral geometries.Key words: organic synthesis, molecular and supramolecular design and construction, tetraphenylmethane, tetraphenyl silane, tetrahedral building blocks.
Will the real dictyostatin please stand up? There were five finalists as stereostructures for the potent anticancer agent dictyostatin; ten, if one were to include enantiomers. A total synthesis of (−)‐dictyostatin (1) has ended the decade‐old masquerade and identified the winner as a structure recently proposed by Paterson and Wright.
Tetrakis(3-hydroxyphenyl)silane (1), tetrakis(4-hydroxyphenyl)methane (2), and tetrakis(4-hydroxyphenyl)silane (3), in which phenolic hydroxyl groups are attached to tetrahedral tetraphenylsilyl and tetraphenylmethyl cores, produce a series of hydrogen-bonded networks when crystallized from CH3COOC2H5. Each hydroxyl group in meta-substituted tetraphenol 1 participates in two intermolecular hydrogen bonds as both donor and acceptor, producing helical chains of hydrogen bonds running along the c axis. Each molecule of tetraphenol 1 is linked to four symmetrically oriented neighbors by a total of eight hydrogen bonds, thereby creating a diamondoid network. No interpenetration is observed, and no significant volume remains for the inclusion of guests. The hydrogen-bonded networks derived from para-substituted analogues 2 and 3 are markedly different. Each molecule of tetraphenol 2 is hydrogen-bonded to six neighboring tetraphenols, and the resulting network defines zigzag channels that run parallel to the c axis, measure about 3.3 × 4.4 Å at the narrowest point, and include CH3COOC2H5 as guest. Approximately 28% of the volume of crystals of tetraphenol 2 is available for inclusion. Tetraphenol 3 crystallizes as a monohydrate, and H2O is incorporated as a structural element in the resulting network. Each molecule of tetraphenol 3 forms hydrogen bonds with two molecules of H2O and four unsymmetrically oriented neighboring molecules of tetraphenol 3, producing a structure that is closely packed. The variety of structures obtained from compounds 1−3 under similar conditions shows that the hydroxyl group of phenols is not a highly predictable director of supramolecular assembly.
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