Metal ions drive thermodynamics and photochemistry of the Bis(styryl) macrocyclic tweezer
Résumé
UV/Vis and NMR spectroscopy were used for the structural elucidation and thermodynamic and photochemical studies of the metal-coordinated crown-containing macrocyclic tweezer (E,E)-1. The bis(styryl) tweezer (E,E)-1 formed two types of complexes with magnesium(II): a 1:1 intramolecular asymmetric sandwich complex [(E,E)-1]·Mg2+ and a 1:2 complex [(E,E)-1]·(Mg2+)2. In the former case, there is direct cation intramolecular exchange (0.299 s-1, ΔG≠=69.4 kJ mol-1) between two parts of the bis(styryl) tweezer (E,E)-1. Addition of barium(II) to the bis(styryl) tweezer (E,E)-1 led to an intramolecular centrosymmetric sandwich 1:1 complex [(E,E)-1]·Ba2+. Irradiation of [(E,E)-1]·Ba2+ afforded reversible intramolecular [2π+2π] photocyclization with excellent stereoselectivity and quantitative yield. In contrast, irradiation of [(E,E)-1](Mg2+)2 resulted in reversible stepwise E,Z-isomerization.