Conformational Re-engineering of Porphyrins as Receptors with Switchable N-H⋅⋅⋅X-Type Binding Modes

The selectivity and functional variability of porphyrin cofactors are typically based on substrate binding of metalloporphyrins wherein the pyrrole nitrogen units only serve to chelate the metal ions. Yet, using the porphyrin inner core system for other functions is possible through conformational engineering. As a first step towards porphyrin "enzyme-like" active centers, a structural and spectroscopic study of substrate binding to the inner core porphyrin system shows that a highly saddle-distorted porphyrin with peripheral amino receptor groups (1, 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(2-aminophenyl)porphyrin) coordinates analytes in a switchable manner dependent on the acidity of the solution. The supramolecular ensemble exhibits exceptionally high affinity to and selectivity for the pyrophosphate anion (2.26±0.021)×10⁹  m^-1 . ¹ H NMR spectroscopic studies provided insight into the likely mode of binding and the characterization of atropisomers, all four of which were also studied by X-ray crystallography.