Watson and Crick discovered the DNA double helix in 1953 and realized immediately that its replication involved a templated synthesis.1 Their work inspired Todd to suggest in 1956 that organic templates might one day be used to control laboratory syntheses in a

The binding of oligopyridyl ligands to butadiyne-linked zinc porphyrin dimers, trimers and tetramers has been investigated in detail using NMR and electronic spectroscopy. Pyridine binds to zinc porphyrin monomers in CH2C12 solution at 300 K with binding constants of ca. lo3 mo1-' dm3, while 4,4'-bipyridyl binds to the cyclic zinc porphyrin dimer with a binding constant of 1 x lo9 mol-' dm3, giving an effective molarity, or chelation factor, of 76 mol dmP3. The analogous linear dimer binds to this ligand 100 times less strongly, but adopts a similar conformation when bound. s-Tri(4-pyridy1)triazine has an affinity of ca. 10" mol-' dm3 for the cyclic zinc trimer, reflecting reasonably good host-guest complementarity. The affinity of 4,4'bipyridyl for the trimer is ca. lo5 mol-' dm3, implying two-point binding accompanied by host distortion and strain which reduce the binding constant; the trimer is therefore an elastic host, able to respond to the geometrical demands of rigid guests. The cyclic tetramer is a relatively flexible molecule, but its complexes with both bidentate and tetradentate ligands have more highly defined geometries. The cyclic dimer and t rimer have open pre-organised cavities, with no conformational barrier to ligand binding inside the cavity, whereas the cyclic tetramer can adopt many conformations in free solution.

Keywords: ab initio calculations . electrochemistry . lanthanide compounds [l] For a recent review on metallofullerenes. see:

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