Preorganized anion traps for exploiting anion-π interactions: An experimental and computational study

Abstract

1,3-Bis(pentafluorophenyl-imino)isoindoline (A(F)) and 3,6-di-tert-butyl-1,8-bis(pentafluorophenyl)-9H-carbazole (B-F) have been designed as preorganized anion receptors that exploit anion- interactions, and their ability to bind chloride and bromide in various solvents has been evaluated. Both receptors A(F) and B-F are neutral but provide a central NH hydrogen bond that directs the halide anion into a preorganized clamp of the two electron-deficient appended arenes. Crystal structures of host-guest complexes of A(F) with DMSO, Cl-, or Br- (A(F):DMSO, A(F):Cl-, and A(2)(F):Br-) reveal that in all cases the guest is located in the cleft between the perfluorinated flaps, but NMR spectroscopy shows a more complex situation in solution because of E,Z/Z,Z isomerism of the host. In the case of the more rigid receptor B-F, Job plots evidence 1:1 complex formation with Cl- and Br-, and association constants up to 960M(-1) have been determined depending on the solvent. Crystal structures of B-F and B-F:DMSO visualize the distinct preorganization of the host for anion- interactions. The reference compounds 1,3-bis(2-pyrimidylimino)isoindoline (A(N)) and 3,6-di-tert-butyl-1,8-diphenyl-9H-carbazole (B-H), which lack the perfluorinated flaps, do not show any indication of anion binding under the same conditions. A detailed computational analysis of the receptors A(F) and B-F and their host-guest complexes with Cl- or Br- was carried out to quantify the interactions in play. Local correlation methods were applied, allowing for a decomposition of the ring-anion interactions. The latter were found to contribute significantly to the stabilization of these complexes (about half of the total energy). Compounds A(F) and B-F represent rare examples of neutral receptors that are well preorganized for exploiting anion- interactions, and rare examples of receptors for which the individual contributions to the binding energy have been quantified.