Binding Properties of Octaaminocryptands

Complexation and protonation equilibria were studied in aqueous solution for a new range of aminocryptand ligands, N(CH2CH2NHCH2RCH2NHCH2CH2)3N, (R = m-xylyl, p-xylyl, 2,5-furan, 2,6-pyridine) and demonstrate that stability constants for first transition series ions Co2+ to Zn2+ are relatively high. X-ray crystallography shows that the cryptands are reasonably well preorganized for complexation. The furan-spaced cryptand L6·H2O crystallizes in the rhomobohedral space group R3̄ (no. 148) with a = 14.645(1), b = 14.645(1), and c = 25.530(4) Å, whereas the m-xylyl-spaced cryptand L4 crystallizes in the triclinic space group P1̄ (no. 2) with a = 9.517(1), b = 15.584(2), and c = 23.617(4) Å. The highest formation constant (log β21 = 33.07) is observed for the dicopper cryptate of a pyridine-spaced cryptand, suggesting involvement in complexation of donors from the spacer link. This pyridine-spaced host also shows good selectivity for copper(II) over zinc(II), making it a possible candidate for treatment of copper-excess pathology.