Anion binding, aryl-extended cyclotriguaiacylenes and an aryl-bridged cryptophane that provides snapshots of a molecular gating mechanism

Abstract

Cyclotriguaiacylene, ( ± )-CTG, reacted with a series of [(η⁵-C₅H₅)Fe^II(η⁶-chloroarenes)]⁺ to yield a family of aryl-extended, [(η⁵-C₅H₅)Fe^II]⁺-functionalised cavitands ( ± )-2 ³⁺. Crystal structures of [BF₄]⁻  or mixed [BF₄]⁻ /[PF₆]⁻  salts of these cavitands shed light on their conformational dynamics and anion binding properties. These hosts adopt conformations that project the [(η⁵-C₅H₅)Fe^II(η⁶-arene)]⁺ substituents ‘up’ from the rim of the cavitand, creating deep cavities that are occupied by a [BF₄]⁻  anion. Hosts ( ± )-2a–f ³⁺ appear to show some selectivity for the formation of penetrated ion pairs with [BF₄]⁻  in preference to [PF₆]⁻ . The compounds were photochemically demetalated, giving aryl-extended CTGs ( ± )-3a–d,f. The crystal structure of ( ± )-3a reveals that the molecule forms a centrosymmetric, self-included dimer in the solid state. Cryptophanes syn-5 and ( ± )-anti-4 were synthesised by the photochemically induced demetalation of the putative metalated cryptophanes syn-4[PF₆]₃ and ( ± )-anti-4[PF₆]₃, obtained by the ‘capping’ of the cavitand ( ± )-2d ³⁺ by ( ± )-CTG. Single crystal structures of five different solvates of ( ± )-anti-5 reveal host–guest encapsulated complexes that are interpreted as revealing ‘snapshots’ of the molecular gating process. The first crystallographically characterised example of an empty cryptophane, namely ( ± )-anti-5, reveals that this host can also readily adopt a conformation that fills the bulk of its own cavity.

Keywords:

• cyclotriveratrylene
• cryptophane
• encapsulation
• gating
• penetrated ion pair

Acknowledgements

This work was supported in part by the US National Science Foundation (DMR-0349316), the University of Missouri and Georgetown University. We thank Prof. Christian Wolf (Georgetown University) for useful advice regarding the notation of stereochemistry for compounds 2b,c ³⁺.

Notes

^†This paper is contributed in memory of Prof. Dmitry M. Rudkevich, who positively influenced our thinking in so many ways.