Molecular tectonics. Porous hydrogen-bonded networks built from derivatives of 9,9'-spirobifluorene.

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.