Substituent effects in the crystal packing of derivatives of 4'-phenyl-2,2':6',2"-terpyridine

We report the preparation of a series of new 4'-substituted 2,2':6',2"-terpyridines: 4'-(3,5-dimethylphenyl)-2,2':6',2"-terpyridine ( 2 ), 4'-(3-fluoro-5-methylphenyl)-2,2':6',2"-terpyridine ( 3 ), 4'-(3,5-difluorophenyl)-2,2':6',2"-terpyridine ( 4 ), and 4'-(3,5- bis(trifluoromethyl)phenyl)-2,2':6',2"-terpyridine ( 5 ). The compounds have been characterized by mass spectrometry, solid-state IR spectroscopy and solution NMR and absorption spectroscopies. The single-crystal X-ray diffraction structures of 3 , 5 and 6 . EtOH ( 6 = 4'-(3,5-bis( tert -butyl)phenyl)-2,2':6',2"-terpyridine) have been elucidated. The molecular structures of the compounds are unexceptional. Since 3 and 5 crystallize without lattice solvent, we are able to understand the influence of introducing substituents in the 4'-phenyl ring and compare the structures with that of the previously reported 4'-phenyl-2,2':6',2"-terpyridine ( 1 ). On going from 1 to 3 , face-to-face p-stacking of pairs of 3-fluoro-5-methylphenyl rings contributes to a change in packing from a herring-bone assembly in 1 with no ring p-stacking to a layer-like packing. The latter arises through a combination of p-stacking of aromatic rings and N¼H-C hydrogen bonding. On going from 3 to 5 , N¼H-C and F¼H-C hydrogen-bonding is dominant, supplemented by p-stacking interactions between pairs of pyridine rings. A comparison of the packing of molecules of 6 with that in 1 , 3 and 5 is difficult because of the incorporation of solvent in 6 . EtOH