What Anions Do Inside a Receptor's Cavity: A Trifurcate Anion Receptor Providing Both Electrostatic and Hydrogen‐Bonding Interactions

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Abstract
The trifurcate receptor 13+ forms stable 1:1 complexes with halide and oxo anions in MeCN solution, as shown by spectrophotometric and 1H NMR experiments, and selectively recognizes chloride (lg Kass>7) in the presence of fluoride and bromide. The high stability reflects the receptor's ability to donate up to six hydrogen bonds (from three pyrrole NH and three CH fragments, polarized by the proximate positive charge) to the included anion. Addition of an excess of more basic anions (F− and CH3COO−) induces stepwise deprotonation of the NH groups, an event signalled by the appearance of a bright yellow color. Crystal and molecular structures are reported for the complex with NO3− and a capsule consisting of two interconnected trifurcate subunits, one of which includes an H‐bound Br− ion, while the other is doubly deprotonated and includes an H‐bound water molecule. Finally, evidence is given for the formation in solution of an authentic complex of OH−, in which H‐bound hydroxide is included within the cavity of 13+.