Cover Picture

Abstract

The cover picture shows the influence of pressure on the 1 H NMR signals (right) of the protons of the p-xylyl bridge in the Ag complex of the CHIRAGEN ligand depicted in light blue color. The doubling of the signal is due to the presence of two circular helicates, a hexamer (top left) and a tetramer (top right), both with predetermined absolute configurations. Increasing pressure shifts the equilibrium from the hexamer to the tetramer. The reaction volume has a remarkably large negative value. The study of this equilibrium system is described in more detail by A. von Zelewsky and A. Merbach et al. on p. 533 ff.

Self-assembly of Ag(I) helicates with new enantiopure 5,6-Chiragen type ligands

“…For Ag(I) also an isostructural hexanuclear circular helicate has been obtained while in solution an equilibrium between hexa-and tetranuclear species has been demonstrated [23]. We have shown as well that chirality plays a crucial role in the self-assembly of these discrete supramolecular species.…”

“…1) and their derivatives, have seen a rapid development [2]. Indeed, many research groups worldwide are still synthesising new ligands and their metal complexes based on this pinene: bipyridine [3], pyridine [4] or phenantroline [5] framework.…”

“…Another hypothesis would be that circular helicates are formed at 50°C (for L1 complex) but at RT they are unstable and open to form the polymeric helicate. This hypothesis, put forward previously for the L1 0 Ag(I) complex [2], appears to be more favoured here because of the formation of a single stranded helix (Fig. 6).…”

“…The spectrum does not change at lower temperature indicating the presence of a single, discrete molecular species. In similar concentration and temperature conditions, the ligand L2, whose paraxylene bridge has the same geometry but is one benzene cycle shorter, form two circular mono-stranded helicates, one hexanuclear (characterised also by X-ray analysis) and the other tetranuclear [23]. The equilibrium between these two species is dependent on concentration, temperature and pressure.…”

“…A 5,6-Chiragen ligand possessing a para-xylene bridge (L2 0 , Fig. 2) self-assembles with Cu(I) and Ag(I) cations circular helicates in solid state [22] and in solution [23]. The use of (À)-5,6-pinene bipyridine as coordinating unit in these Chiragen ligands ensures a complete stereocontrol at the metal centers (K) and the transfer of their chirality to the helical chains (P).…”