The alkali cation (Li+, Na+,
K+, Rb+, and Cs+) binding
properties of cryptands [2.1.1], [2.2.1], and
[2.2.2]
were investigated under solvent-free, gas-phase conditions using
Fourier transform ion cyclotron resonance mass
spectrometry. The alkali cations serve as size probes for the
cryptand cavities. All three cryptands readily form
1:1
alkali cation complexes. Ligand−metal (2:1) complexes of
[2.1.1] with K+, Rb+, and
Cs+, and of [2.2.1] with Rb+
and Cs+ were observed, but no 2:1 complexes of [2.2.2]
were seen, consistent with formation of “inclusive”
rather
than “exclusive” complexes when the binding cavity of the ligand is
large enough to accommodate the metal cation.
Kinetics for 2:1 ligand−metal complexation, as well as molecular
mechanics calculations and cation transfer equilibrium
constant measurements, lead to estimates of the radii of the cation
binding cavities of the cryptands under gas-phase
conditions: [2.1.1], 1.25 Å; [2.2.1], 1.50 Å; [2.2.2], 1.65
Å. Cation transfer equilibrium studies comparing
cryptands
with crown ethers having the same number of donor atoms reveal that the
cryptands have higher affinities than
crowns for cations small enough to enter the cavity of the cryptand,
while the crowns have the higher affinity for
cations too large to enter the cryptand cavity. The results are
interpreted in terms of the macrobicyclic cryptate
effect: for cations small enough to fit inside the cryptand, the
three-dimensional preorganization of the ligand leads
to stronger binding than is possible for a floppier,
pseudo-two-dimensional crown ether. The loss of binding by
one
ether oxygen which occurs as metal size increases for a given cryptand
is worth approximately 25 kJ mol-1, and
accounts for the higher cation affinities of the crowns for the larger
metals. The Li+ affinity of
1,10-diaza-18-crown-6 is ∼1 kJ mol-1 higher than that of 18-crown-6,
while the latter has lower affinity than the former for all
of
the larger alkali cations (about 7 kJ mol-1 lower for
Na+, and about 15 kJ mol-1 lower for
K+, Rb+, and Cs+).
The
equilibrium measurements also allow the determination of relative free
energies of cation binding for a number of
crown ethers and cryptands. Molecular mechanics modeling with the
AMBER force field is generally consistent
with the experiments.
