PORPHYRIN-CROWN ETHER BASED MACROCYCLIC RECEPTORS FOR BIPYRIDINIUM CATIONS

Design modifications to potential receptors for the bipyridinium catio ns-paraquat, diquat and [Pt(bpy)(NH3)(2)](2+), in which a porphyrin is appended by benzo crown ethers in two different configurations are re ported. A porphyrin strapped by a single dibenzo crown ether 4, and a porphyrin-based molecular tweezer 7, in which a porphyrin is surmounte d by two benzo-15-crown-5 units, are described. The macrobicyclic rece ptor 4 failed to complex either the paraquat or diquat dications to an y significant extent, yet formed an inclusion complex with [Pt(bpy)(NH 3)(2)](2+) (K-a 155 dm(-3) mol(-1), Delta G(0) -2.9 kcal mol(-1)) with an orthogonal orientation of the guest relative to the porphyrin plan e. This lack of paraquat and diquat complexation may be due to either an incorrect number or geometry of ether oxygen atoms for electrostati c interactions, and/or unfavourable orientations of the. crown aromati c rings due to their increased conformational mobility within 4 when c ompared to the macrotricyclic analogue 1 reported previously by us. Th e complexation of [Pt(bpy)(NH3)(2)](2)(PF6)(2) by 4 was accompanied by a conformational change within the crown ether subunit, and a deforma tion of the porphyrin nucleus indicating that the ether strap is stret ched to accommodate [Pt(bpy) (NH3)(2)](2+). This suggests that (i) dec reasing the number of ether chains from two to one only partially remo ves the steric effects from the ether chain on the crown aromatic ring s, or (ii) the amide bond is still dominant in influencing the solutio n 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 c rown ether subunit of 9 was found to be conformationally solvent depen dent, Two conformational extremes were identifiable; in acetone the cr own 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, d iquat, and [Pt(bpy)(NH3](2)](2+) in acetone solution with association constants and free energies of complexation of 32, 20, 140 dm(3) mel(- 1), and -2.1, -1.8, -2.9 kcal mol(-1), respectively. The stronger comp lexation of [Pt(bpy)(NH3)(2)](2+) presumably reflects the additional s tabilisation of the inclusion complex by hydrogen bonding between the ammine ligands of [Pt(bpy)(NH3)(2)](2+) and the oxygen atoms of the cr own tweezer. The inclusion geometry of these guests within the pincers of 7 is consistent with the aromatic rings of the guest sandwiched be tween the aromatic benzo-crown rings as predicted. An analogous tweeze r molecule 8 lacking the crown ether chains did not bind any of the gu ests. 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 remova l of the bridging ether chains allows an increase in the conformationa l mobility of the crown aromatic rings the nett result is a decrease i n the level of pre-organisation. and a reduction in the receptor bindi ng strength.