What anions do inside a receptor's cavity: A trifurcate anion receptor providing both electrostatic and hydrogen-bonding interactions

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 (lgKass>7) in the presence of fluoride and bromide. The high stability reflects the receptor7s 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+.