A LOOSE CAGE FOR TRANSITION-METALS

The octamine cage 5, L, incorporates Ni-II, Cu-II, and Zn-II in aqueou s solution by st fast and reversible process. Equilibrium studies indi cated the formation of metal complexes of protonated forms of the liga nds, i.e. Mn-II-(LH2)](4+) and [M-II(LH)](+), and of the neutral ligan d, [M-II(L)](2+). The crystal structure of a complex of the monoproton ated ligand, [Ni-II(LH)](ClO4)(3), has been determined by single-cryst al X-ray crystallography. The complex salt (C18H43N8Cl3NiO12. H2O) cry stallizes in the orthorombic Pbca spice group, with cell constants a = 14.173(2) Angstrom, b = 14.383(1) Angstrom, c = 30.622(3) Angstrom, V = 6242(1) Angstrom(3), and Z = 8. The Ni-II ion is coordinated to six of the eight available nitrogen atoms, in a very distorted octahedral stereochemistry: of the two uncoordinated donor atoms, a tertiary nit rogen atom and an adjacent secondary one, it is the latter that is pro tonated, The easy access of protons to uncoordinated amine groups of t he cage accounts for the fast demetalation of [Ni-II(L)](2+) and [Cu-I I(L)](2+) in acidic solution, which was investigated by stopped-flow s pectrophotometry. Dependence of k(obs) upon [H+] for [Ni-II(L)](2+) an d upon [H+](2) for [Cu-II(L)](2+) indicated that the protonation of un coordinated nitrogen atoms of the cage is the key step of the demetala tion process.