Substituent-controlled assembly of helical complexes: synthesis, crystal and molecular structures of double helical silver(I) complexes with substituted quinquepyridines

Both 6,6″″-dimethyl-(dmqpy) and 6,6″″-dimethyl-4′,4‴-diphenyl-2,2′:6′,2″:6″,2‴:6‴,2″″-quinquepyridine (dmpqpy) have been found to form dinuclear double helical silver(I) complexes [Ag2(dmqpy)2][ClO4]21 and [Ag2(dmpqpy)2][ClO4]2·2H2O 2. The introduced terminal methyl groups play a crucial role in the assembly processes. According to their crystal structures, the ligand adopts the usual [2 + 3] co-ordination mode in 1 and the silver(I) is five-co-ordinated with a flattened and distorted trigonal-bipyramidal geometry. In 2 the ligand dmpqpy adopts a [2 + 1 + 2] mode and is unexpectedly tetradentate with the central pyridyl nitrogen atom having no direct co-ordination and acting as a rigid spacer. The silver(I) atoms are four-co-ordinated. It is supposed that the increased conjugation and rigidity caused by the phenyl groups at the 4′,4‴ positions makes it difficult for the central pyridine nitrogen to co-ordinate to AgI. Strong intramolecular π–π stacking interactions exist in complex 1, but the stacking effects in 2 are much weaker. The NMR results indicate that both complexes adopt more symmetric configurations in solution than in the solid state.