A Loose Cage for Transition Metals

The octamine cage 5, L, incorporates NiII,
CuII, and ZnII in aqueous solution by a fast
and reversible process.
Equilibrium studies indicated the formation of metal complexes of
protonated forms of the ligands, i.e. MII(LH2)]4+ and
[MII(LH)]+, and of the neutral ligand,
[MII(L)]2+. The crystal structure
of a complex of the
monoprotonated ligand,
[NiII(LH)](ClO4)3, has
been determined by single-crystal X-ray crystallography.
The
complex salt
(C18H43N8Cl3NiO12·H2O)
crystallizes in the orthorombic Pbca space group, with cell
constants a =
14.173(2) Å, b = 14.383(1) Å, c =
30.622(3) Å, V = 6242(1) Å3, and
Z = 8. The NiII 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 nitrogen atom and an adjacent secondary one, it
is the latter that is protonated. The easy
access of protons to uncoordinated amine groups of the cage accounts
for the fast demetalation of
[NiII(L)]2+ and
[CuII(L)]2+ in acidic solution, which
was investigated by stopped-flow spectrophotometry. Dependence of
kobs
upon [H+] for
[NiII(L)]2+ and upon
[H+]2 for
[CuII(L)]2+ indicated that the
protonation of uncoordinated nitrogen
atoms of the cage is the key step of the demetalation
process.