New cage and linked macrocyclic systems for metal ion and small molecule binding
Stepwise synthetic procedures have been developed for producing families of large cyclic ligand systems that include new oxygen-nitrogen donor cages as well as tri- linked macrocyclic species containing mixed nitrogen-sulfur donors. The crystal structure of a cage of the above type incorporating six phenoxy oxygens and two tribenzylamine (bridgehead) nitrogens reveals that the nitrogens adopt exo-ex0 configurations. Semiempirical MO calculations have been employed to investigate the topological rigidity of this hexabenzo cage relative to related aliphatic systems. The constraining of the bridgehead nitrogens (through the presence of tribenzylamine groups) has a significant effect on the overall flexibility of the system and appears to be largely responsible for inhibiting the inclusive coordination of metal ions by this cage. In contrast to the aliphatic 2,2,2-cryptand, which readily adopts an endo-endo arrangement, the present cage shows negligible affinity for sodium ions - as judged from comparative sodium picrate extraction experiments (water/ chloroform). New 'super' cages related to the above system but incorporating three additional pyridyl nitrogens in their heteroatom set have also been synthesised. Once again, the rigidity associated with the tribenzylamine bridgeheads introduces a measure of preorganisation into these cages. The X-ray structure of one system recrystallised from benzene shows that a benzene guest is encapsulated centrally in the cavity, seemingly held in position by 'T' orientation n-stacking interactions. In deutero- chloroform, this cage selectively binds phloroglucinol although it also shows significant affinity for resorcinol. Two new tri-linked S,N,-donor macrocycles incorporating three 16- membered rings connected by spacer groups to a phloroglucinol 'core' have been synthesised. Using an NMR titration procedure, these systems have been demonstrated to undergo stepwise interaction with silver(l), with the final product showing a 3: 1 (meta1:ligand) stoichiometry.