Linear bridging ligands, supramolecular

When six coordination sites around octahedral metal ion are occupied by only bidentate ligands, stereoisomers around the metal ion are formed. However, the coordination of symmetric tridentate ligands to a six-coordinate metal ion leads to only one isomer. Furthermore, tridentate bridging ligands connect metals in a linear fashion, resulting in the formation of stereochemically well-defined supramolecular systems. The rigid structure of these systems is suitable for studies of electron or energy transfer events between the donor-acceptor dyads. 

Unfortunately, despite its poor luminescent properties, tpy is a desirable ligand from a structural point of view in the design of supramolecular photochemical devices. Metal bis(tpy) complexes are achiral, which means that they do not give rise to undesirable mixtures of diastereoisomers when more than one metal centre is present, and as a bridging ligand, substituted tpy compounds are easier to make into rigid linear connectors than bpy or phen derivatives. Furthermore, substituted tpy compounds do not exhibit facimer isomerism as do their bidentate counterparts (Figure 11.9). 

As unambiguously evidenced by NMR spectroscopy, titration of 9Zn in CDCI3 solution with one equivalent of a linear ditopic N-ligand leads rapidly to the quantitative assembling of sandwich-like 2 2 supramolecular adducts of formula [(9Zn)2(Ar-L)2j (L = 4,4 -bipy, 13 L = 4 -fransDPyP, 14 L = 4 -tra sDPyP-npm, 15), formed by two parallel metallacycles connected by two bridging ligands which are axially bound to the zinc-porphyrins (Scheme 3) [68,72,73]. 

The synthesis of organolead polymers has been reported by Fischer and coworkers by combining aqueous solutions of MejPbCl and K4[M(CN)6] (M=Fe, Ru), assemble to give supramolecular assemblies. Edelmann and coworkers have also reported the supramolecular assembly of triphenyllead(IV) dimethylphosphino-dithioate to produce a linear polymer with the structure 227 It was noted that the phosphinodithioate ligands were bridging with nearly linear S-Pb S segments. 

---