A Tetra-Porphyrin Host Exhibiting Interannular Cooperativity

Recently identified as another form of cooperativity, interannular cooperativity is rarely observed in supramolecular chemistry. A tetra-porphyrin molecular tweezer with two bis-porphyrin binding sites is reported that exhibits archetypal interannular cooperativity when complexing 1,4-diazabicyclo[2.2.2]octane (DABCO). The UV/Vis titration data best supported a 1:2 plus 2:2 plus 1:4 complexation model (host:guest), giving K₁₂=6.32×10¹³ m⁻², K₂₂=3.04×10²⁰ m⁻³, and K₁₄=1.92×10¹⁶ m⁻⁴ in CHCl₃. The NMR titration data supported the formation of two sandwich species, including tetra-porphyrin⋅(DABCO)₂ as the major species, although there are speciation differences between UV/Vis and NMR concentrations. Using statistical analysis, interannular cooperativity (γ) for tetra-porphyrin⋅(DABCO)₂ was determined to be negative (γ=2.41×10⁻³), which may be explained by DABCO being too small to be optimally bound simultaneously at both bis-porphyrin binding sites.