Structural control of co-receptor binding in porphyrin–bipyridinium supramolecular assemblies

Abstract

A new series of porphyrin-based receptors for the bipyridinium ions paraquat and diquat, consisting in each case of a porphyrin with an over-arching dibenzo-crown ether, and related structures with hydroquinol-containing polyether straps is described. In all cases, the appended superstructure is connected to the porphyrin via ether linkages from the o-positions of the meso-aryls of the octaalkyl-5,15-diaryl substituted porphyrins. Solution studies of the complexes are compared to those of a previously reported set of related structures with amide linkages in place of the ethers and, at variance with the amide series, reveal conformations in which the complexed bipyridinium is parallel to the porphyrin sub-unit rather than perpendicular. Binding constants for all of the free-base and zinc porphyrin derivatives with both paraquat and diquat in various solvents allow the following principles to be established: (i) the binding strength decreases with increasing solvent polarity, (ii) there is little difference in the binding strength between the free base and zinc derivatives for a given receptor, (iii) binding is stronger for the more constrained 4(i) compared to the looser 4(ii), (iv) in general, paraquat binds more strongly than diquat, (v) the single-strapped analogues 6 are relatively ineffective as receptors for these bipyridinium guests, (vi) for a given sized dibenzo-crown ether cap or polyether strap, the substitution of ether linkages for the amide linkages in the related family of receptors 1 results in stronger binding. A similar binding motif is described for a naphthoquinol-strapped porphyrin 5. The solution studies are supported by X-ray crystal structures of two of the paraquat complexes which indicate that the guest is held by face-to-face π–π interactions with the porphyrin, by C–H· · ·O hydrogen bonds, by electrostatic forces, and by either face-to-face π-stacking with the hydroquinol unit or by edge-to-face interactions in the case of the dibenzo-crown strapped molecule.