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 [Ag-2(dmqpy)(2)][ClO4](2) 1 and [Ag-2(dmpqpy)(2)][ClO4](2) . 2H(2)O 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 Ag-I. Strong intramolecular pi-pi 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.