Porphyrin–crown ether based macrocyclic receptors for bipyridinium cations

Abstract

Design modifications to potential receptors for the bipyridinium cations paraquat, diquat and [Pt(bpy)(NH₃)₂]²⁺, in which a porphyrin is appended by benzo crown ethers in two different configurations, are reported. A porphyrin strapped by a single dibenzo crown ether 4, and a porphyrin-based molecular tweezer7, in which a porphyrin is surmounted by two benzo-15-crown-5 units, are described. The macrobicyclic receptor 4 failed to complex either the paraquat or diquat dications to any significant extent, yet formed an inclusion complex with [Pt(bpy)(NH₃)₂]²⁺(K_a 155 dm^–3 mol^–1, ΔG°–2.9 kcal mol^–1) with an orthogonal orientation of the guest relative to the porphyrin plane. This lack of paraquat and diquat complexation may be due to either an incorrect number or geometry of ether oxygen atoms for electrostatic interactions, and/or unfavourable orientations of the crown aromatic rings due to their increased conformational mobility within 4 when compared to the macrotricyclic analogue 1 reported previously by us. The complexation of [Pt(bpy)(NH₃)₂]₂(PF₆)₂ by 4 was accompanied by a conformational change within the crown ether subunit, and a deformation of the porphyrin nucleus indicating that the ether strap is stretched to accommodate [Pt(bpy)(NH₃)₂]²⁺. This suggests that (i) decreasing the number of ether chains from two to one only partially removes the steric effects from the ether chain on the crown aromatic rings, or (ii) the amide bond is still dominant in influencing the solution conformations of the crown aromatic rings. The crystal structure of an isomer 4 showed the ether chain twisted around the porphyrin, as a result of atropisomerisation during the crystallisation process.

The crown ether subunit of 9 was found to be conformationally solvent dependent. Two conformational extremes were identifiable; in acetone the crown aromatic rings are essentially coplanar with the meso-phenyl plane, whereas in chloroform the crown aromatic rings are nearly orthogonal to the meso-phenyl ring. The tweezer was observed to bind paraquat, diquat, and [Pt(bpy)(NH₃]₂]²⁺ in acetone solution with association constants and free energies of complexation of 32, 20, 140 dm³ mol^–1, and –2.1, –1.8, –2.9 kcal mol^–1, respectively. The stronger complexation of [Pt(bpy)(NH₃)₂]²⁺ presumably reflects the additional stabilisation of the inclusion complex by hydrogen bonding between the ammine ligands of [Pt(bpy)(NH₃)₂]²⁺ and the oxygen atoms of the crown tweezer. The inclusion geometry of these guests within the pincers of 7 is consistent with the aromatic rings of the guest sandwiched between the aromatic benzo-crown rings as predicted. An analogous tweezer molecule 8 lacking the crown ether chains did not bind any of the guests. Although 7 was observed to complex all three guests, the smaller association constants and free energies of complexation compared to 1 at 298 K reflect the lesser degree of preorganisation present in the crown-ethers of the complexing subunit of 7. Thus, although the removal of the bridging ether chains allows an increase in the conformational mobility of the crown aromatic rings the nett result is a decrease in the level of pre-organisation, and a reduction in the receptor binding strength.