Neutral C–H bond vs. electron pair of N(sp~2): A binding site effect study of macrocycle anion receptor

To evaluate the effect of neutral C–H bond or electron pair of nitrogen atom with sp2hybridization(N(sp2)) involving into the same chemical environment for anion binding, two analogous tetracationic imidazolium macrocycles, namely cyclo2](2,6-bis-(1H-imidazol-1-yl)pyridine) 2](1,3-dimethylenebenzene)(14+), and cyclo2](2,6-bis-(1H-imidazol-1-yl)pyridine)2](2,6-di methylenepyridine)(24+)were studied in detail as small inorganic anion receptors. The guest anions with different shapes are Cl,N3, NO3, and H2PO4. The host–guest interactions were characterized via1 H NMR spectroscopy,electrospray ionization mass spectrometry(ESI-MS) and single crystal X-ray crystallography. The results implied that macrocyclic hosts with similar backbone but two distinct binding sites(14+with neutral C–H vs. 24+with N(sp2)) vary markedly in their response to anions, including the binding modes and association constants. The finding will serve to the construction of new anion receptors, even improve insights into the anion binding process in biology.