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 center dot 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 packing in 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 pi-stacking of pairs of 3-fluoro-5-methylphenyl rings contributes to a change in packing from a herringbone assembly in 1 with no ring pi-stacking to a layer-like packing. The latter arises through a combination of pi-stacking of aromatic rings and N center dot center dot center dot H-C hydrogen bonding. On going from 3 to 5, N center dot center dot center dot H-C and F center dot center dot center dot H-C hydrogen-bonding is dominant, supplemented by pi-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 center dot EtOH.