PORPHYRIN-CROWN ETHER MACROTRICYCLIC CO-RECEPTORS FOR BIPYRIDINIUM CATIONS

In order to employ non-covalent interactions to position methyl violog en close to a photoactive centre (in this case a porphyrin) a dibenzo- crown ether, containing meta-substituted aromatic rings and tetraethyl ene glycol ether chains, was strapped across a porphyrin to produce a macrotricyclic host. This host was observed to complex methyl viologen (paraquat) (K-a 50 and 250 dm(3) mol(-1) and Delta G degrees -2.3 and -2.9 kcal mol(-1) at 298 K and 263 K. respectively) and [Pt(bpy)(NH3) (2)](PF6)(2) (K-a 1350 dm(3) mol(-1), Delta G degrees -4.3 kcal mol(-1 ) at 298 K) by an induced fit mechanism, as a result of rotational bar riers associated with the diaryl amide linkage. Efforts to improve the complexing ability of these types of hosts involved a shortening of t he linking ether chains and reduction of the amide group linking the c rown ether and porphyrin moieties. Of these two directions the reduced host was most successful, complexing all the bipyridinium dications e xamined, although ligand exchange reactions complicated the binding of the platinum complex (for paraquat K-a 480 dm(3) mol(-1), Delta G deg rees -3.6 kcal mol(-1) at 298 K; for diquat K-a 80 dm(3) mol(-1), Delt a G degrees -2.6 kcal mol(-1) at 298 K). In contrast, the host contain ing shortened ether chains was observed to have similar complexing abi lities to its predecessor, as a result of comparable solution conforma tions (for paraquat K-a 50 dm(3) mol(-1), Delta G degrees -2.3 kcal mo l(-1) at 298 K; for [Pt(bpy)(NH3)(2)](PF6)(2)K-a 365 dm(3) mol(-1), De lta G degrees -2.9 kcal mol(-1) at 298 K). However, in this case bindi ng by the platinum complex within the constricted cavity of the host r esulted in deformation of the porphyrin and an increased activation ba rrier to complexation.