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[9 H -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[9 H -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[9 H -fluorene] ( 8 ) and 2,2‘,7,7‘-tetrakis(3-cyanophenyl)-9,9‘-spirobi[9 H -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.