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.