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

Abstract

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(1) complexes [Ag,(dmqpy),][ClO,], 1 and [Ag2(dmpqpy)2][C10,]2~2H20 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 + 31 co-ordination mode in 1 and the silver(1) is five-co-ordinated with a flattened and distorted trigonal-bipyramidal geometry. In 2 the ligand dmpqpy adopts a [2 + 1 + 21 mode and is unexpectedly tetradentate with the central pyridyl nitrogen atom having no direct co-ordination and acting as a rigid spacer. The silver(1) 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'. Strong intramolecular n-n 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.

Synthesis and Theoretical Studies of a Double Helical Complex with the Ligand 4′,4′′′‐Bis(ferrocenyl)‐2,2′:6′,2′′:6′′,2′′′:6′′′,2′′′′‐quinquepyridine

“…Despite the covalence of the bonding to each silver atom in the compound, the AgϪN distances are within the expected range. The Ag(1)···Ag (2) [31] These values are in accordance with a shorter silver···silver contact. It has been suggested that it is the steric effect of the terminal groups in flexible ligands with various donor atoms that forces the ligand to adopt a double helical disposition, since the planar configuration is not possible.…”

“…In Figure 4, we show a comparison of the calculated AgϪAg and AgϪN distances (Å ) with those found in the corresponding section of the experimental structure of the [Ag 2 (Fc 2 qpy) 2 ] 2ϩ helix (notice that for clarity, silver atoms are represented twice, as each chain binds two silvers), and also of the [Ag 2 (dmqpy) 2 ] 2ϩ complex (owing to symmetry, only one set of distances is given). [31] Although the AgϪAg distance is either slightly too long or too short, depending on the structure considered, some AgϪN distances compare relatively well. Each helix binds two different Ag atoms, using three N atoms for the first Ag, and two N atoms for the second.…”

“…The shorter distance is similar to the stacking distances in [Ag 2 (dmqpy) 2 ](ClO 4 ) 2 (3.38Ϫ3.42 Å ), and the larger distance is similar to those between di-silver units in the same compound (3.70 Å ). [31] No such interactions were observed between di-silver units, since the rings of those which are adjacent are not coplanar. These interactions between di-units are not present in [Ag(dmpqpy) 2 ](ClO 4 ) 2 ·2H 2 O, probably due to the presence of the bulky substituents.…”

“…[19] Pentacoordinated silver atoms are present in [Ag 2 (dmqpy) 2 ](ClO 4 ) 2 (dmqpy ϭ 6,6ЈЈЈЈ-dimethyl-2,2Ј:6Ј,2ЈЈ:6ЈЈ,2ЈЈЈ:6ЈЈЈ,2ЈЈЈЈ-quinquepyridine). [31] Here we report on the synthesis of the Ag I complex of the 4Ј,4ЈЈЈ-bis(ferrocenyl)-2,2Ј:6Ј,2ЈЈ:6ЈЈ,2ЈЈЈ:6ЈЈЈ,2ЈЈЈЈ-quin-quepyridine ligand. The crystal structure of the silver() derivative shows a double helical arrangement with a short Ag···Ag interaction.…”

“…There are few examples of doubly helical silver complexes with quinquepyridine ligands. In the complex [Ag 2 (dmpqpy) 2 ](ClO 4 ) 2 · 2H 2 O (dmpqpy ϭ 6,6ЈЈЈЈ-dimethyl-4Ј,4ЈЈЈ-diphenyl-2,2Ј:6Ј,2ЈЈ:6ЈЈ,2ЈЈЈ:6ЈЈЈ,2ЈЈЈЈ-quinquepyridine) [31] the ligand acts as tetradentate donor, and the central pyridine nitrogen atoms play the role of rigid spacers, rather than of donor atoms. The silver center is also tetracoordinate in the heterodinuclear derivative [AgCo(qpy) 2 ] 3ϩ .…”