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 foun d 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 w ith a flattened and distorted trigonal;bipyramidal geometry. In 2 the ligand dmpqpy adopts a [2 + 1 + 2] mode and is unexpectedly tetradenta te with the central pyridyl nitrogen atom having no direct co-ordinati on and acting as a rigid spacer. The silver(I) atoms are four-co-ordin ated. It is supposed that the increased conjugation and rigidity cause d by the phenyl groups at the 4',4''' positions makes it difficult for the central pyridine nitrogen to co-ordinate to Ag-I. Strong intramol ecular pi-pi stacking interactions exist in complex 1, but the stackin g effects in 2 are much weaker. The NMR results indicate that both com plexes adopt more symmetric configurations in solution than in the sol id state.