Weakly Bonded Molecular Networks Built from Tetranitro- and Tetracyanospirobifluorenes

Three tetranitro and three tetracyano derivatives of 9,9‘-spirobifluorene were synthesized and
crystallized, and their structures were determined by X-ray crystallography. 2,2‘,7,7‘-Tetranitro-9,9‘-spirobi[9H-fluorene] (4) self-associates via C−H···O interactions to give an open supramolecular network, with 31% of the total
volume of the crystal available for including guests. Tetracyano analogue 5 also crystallizes to give a porous network,
but no significant C−H···N interactions are present; instead, molecular association is directed primarily by π-stacking.
An extended tetranitro analogue, 2,2‘,7,7‘-tetrakis(4-nitrophenyl)-9,9‘-spirobi[9H-fluorene] (6), crystallizes as a nearly
close-packed structure held together by a combination of C−H···O interactions and π-stacking. Tetracyano analogue
7 crystallizes to form an open structure maintained by π-stacking, without significant C−H···N contacts. Extended
meta-substituted analogues, 2,2‘,7,7‘-tetrakis(3-nitrophenyl)-9,9‘-spirobi[9H-fluorene] (8) and 2,2‘,7,7‘-tetrakis(3-cyanophenyl)-9,9‘-spirobi[9H-fluorene] (9), were also synthesized and crystallized. The two structures are closely
similar and show close-packed architectures involving mainly π-stacking. Together, these observations suggest that
cyano groups are not effective acceptors for the formation of C−H···N interactions strong enough to direct molecular
association in competition with aryl−aryl interactions, at least in the spirobifluorene system. In contrast, nitro
groups can be used to help direct association via C−H···O interactions, either in competition with or in cooperation
with other weak interactions.