Photoresponsive crown ethers. Part 7. Proton and metal ion catalyses in the cis—trans isomerisation of azopyridines and an azopyridine-bridged cryptand

Abstract

A 2,2′-azopyridine-bridged crown ether (5) has been synthesised for the purpose of controlling the ionbinding functions by an on-off light switch mechanism. Since the trans-azopyridine moiety of compound (5)[i.e. trans-(5)] is vertically over the crown ether ring and the photoisomerised cis-azopyridine moiety of (5)[i.e. cis-(5)] is almost parallel to the crown ether plane, it would be expected that only the pyridine nitrogens of trans-(5) are capable of co-ordinating to metal ions bound into the crown ether ring. The thermal Isomerisation of cis-2,2′-azopyridine [cis-(3)] to trans-2,2′-azopyridine [trans-(3)] was speeded up either by protonation of the pyridine nitrogen or by complexation with heavy-metal ions (e.g. Cu²⁺, Ni²⁺, and Co²⁺). Similarly, the thermal Isomerisation of cis-(5) to trans-(5) was speeded up by protonation of the azopyridine, but the metal ion catalysis was observed only for the metal ions which were bound into the crown ether ring (e.g. Cu²⁺ and Pb²⁺). The result of solvent extraction of alkali-metal ions with (5) was very similar to that with an azobenzene-bridged crown ether (1), indicating that the 2,2′-azopyridine-bridge of (5) has almost no effect on the extraction of alkali-metal ions. On the other hand, trans-(5) was capable of extracting considerable amounts of heavy-metal ions (Cu²⁺, Ni²⁺, Co²⁺, and Hg²⁺), whereas photoisomerised cis-(5) scarcely extracted these metal ions. Such a difference in the extractability was not observed between trans-(1) and photoisomerised cis-(1). Neither the trans- nor the cis-form of 6,6′-bis(morpholinocarbonyl)-2,2′-azopyridine[non-crown analogue of (5)] could extract these metal ions under comparable extraction conditions. These results suggest that pyridine nitrogens of trans-(5) are directed towards the crown ether plane so as to co-ordinate to metal ions in the crown ether ring, whereas those of cis-(5) have no such co-ordination ability due to the distorted configuration. Therefore, compound (5) would act as a ‘photoresponse cryptand’ for heavy metal ions.