π-Stacking between Pentafluorophenyl and Phenyl Groups as a Controlling Feature of Intra- and Intermolecular Crystal Structure Motifs in Substituted Ferrocenes. Observation of Unexpected Face-to-Face Stacking between Pentafluorophenyl Rings

Ferrocenes containing phenyl and pentafluorophenyl substituents on the cyclopentadienyl rings have been prepared, and their solid-state structures were determined by single-crystal X-ray studies. In the homoleptic ferrocene Fe(Me 2 PhCp) 2 ( 5 ), no intra- or intermolecular face-to-face π-stacking of the aryl rings is observed, as expected. The intramolecular conformation has staggered cyclopentadienyl rings with the phenyl groups in an anti conformation. However, the mixed ferrocene Fe(Me 2 PhCp)(C 6 F 5 Cp) ( 6 ) crystallizes with eclipsed cyclopentadienyl rings and intramolecular π-stacking between the phenyl and pentafluorophenyl substituents. In addition the π-stacking between phenyl and pentafluorophenyl rings occurs between neighboring molecules, and controls the intermolecular crystal motif. Yet a different molecular architecture is observed in the homoleptic ferrocene Fe(C 6 F 5 Cp) 2 ( 2 ) which crystallizes surprisingly with intramolecularly π-stacked pentafluorophenyl rings. The overall crystal architecture in this case in controlled by π-stacking between pentafluorophenyl rings on one molecule with the cyclopentadienyl rings of neighbors, to afford a third structural motif. These results illustrate that both the intra- and intermolecular solid-state structures of simple organometallic molecules can be controlled by stacking interactions involving pentafluorophenyl rings and suggest that these attractive interactions can be added to the repertoire of attractive forces used to effect crystal engineering in organometallic systems.