Ligand binding by butadiyne-linked porphyrin dimers, trimers and tetramers

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 CH2Cl2 solution at 300 K with binding constants of ca. 103 mol–1 dm3, while 4,4′-bipyridyl binds to the cyclic zinc porphyrin dimer with a binding constant of 1 × 109 mol–1 dm3, giving an effective molarity, or chelation factor, of 76 mol dm–3. The analogous linear dimer binds to this ligand 100 times less strongly, but adopts a similar conformation when bound. s-Tri(4-pyridyl)triazine has an affinity of ca. 1010 mol–1 dm3 for the cyclic zinc trimer, reflecting reasonably good host–guest complementarity. The affinity of 4,4′-bipyridyl for the trimer is ca. 105 mol–1 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 trimer 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.