Tritopic (Cascade) and Ditopic Complexes of Halides with an Azacryptand

Structural aspects of the binding of halides in the octaaza cryptand (1,4,11,14,17,24,29,36-octa-azapentacyclo[12.12.12.26,9.219,22.231,34]-tetratetraconta-6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonaene, N(CH2CH2NHCH2-p-xylyl-CH2NHCH2CH2)3N), L1, were examined for fluoride, chloride, and bromide. Crystallographic results for two
different fluoride complexes indicated cascade-like coordination, with two fluoride ions inside the tren-based cavity,
bridged by a water molecule. In the two different chloride structures, a single chloride and a water molecule occupied
the cavity. The bromide structure contained two crystallographically independent cationic cryptands. Unit A consisted
of a bromide on one side of the cavity and three disordered water molecules situated between the cryptand arms
on the other side. Unit B also had a bromide inside the cavity at one side, but a single molecule of water was
centered at the other side of the cavity. Association constants for the three ions, determined by NMR titrations in
aqueous solution at pH 5, revealed log Ka = 3.15(5), 3.37(3), and 3.34(4) for fluoride, chloride, and bromide,
respectively.