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.