Porphyrin–crown ether macrotricyclic co-receptors for bipyridinium cations

Abstract

In order to employ non-covalent interactions to position methyl viologen close to a photoactive centre (in this case a porphyrin) a dibenzo-crown ether, containing meta-substituted aromatic rings and tetraethylene 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 dm³ mol^–1 and ΔG°–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 dm³ mol^–1, ΔG°–4.3 kcal mol^–1 at 298 K) by an induced fit mechanism, as a result of rotational barriers associated with the diaryl amide linkage. Efforts to improve the complexing ability of these types of hosts involved a shortening of the linking ether chains and reduction of the amide group linking the crown ether and porphyrin moieties. Of these two directions the reduced host was most successful, complexing all the bipyridinium dications examined, although ligand exchange reactions complicated the binding of the platinum complex (for paraquat K_a 480 dm³ mol^–1, ΔG°–3.6 kcal mol^–1 at 298 K; for diquat K_a 80 dm³ mol^–1, ΔG°–2.6 kcal mol^–1 at 298 K). In contrast, the host containing shortened ether chains was observed to have similar complexing abilities to its predecessor, as a result of comparable solution conformations (for paraquat K_a 50 dm³ mol^–1, ΔG°–2.3 kcal mol^–1 at 298 K; for [Pt(bpy)(NH₃)₂](PF₆)₂K_a 365 dm³ mol^–1, ΔG°–2.9 kcal mol^–1 at 298 K). However, in this case binding by the platinum complex within the constricted cavity of the host resulted in deformation of the porphyrin and an increased activation barrier to complexation.