Calixarene Metalloreceptors. Synthesis and Molecular Recognition Properties of Upper-Rim Functionalized Calix[4]arenes Containing an Organopalladium Binding Site

The compound
5,17-bis(2-chloroacetamido)-25,26,27,28-tetrapropoxycalix[4]arene,
4, was prepared in the cone
conformation by the published method. Compound 4 was
reacted with α,α‘-m-xylenedithiol and sodium
ethoxide
under high-dilution conditions in ethanol solution. The resulting
macrobicyclic calix[4]arene, HL1, 5,
contains a
symmetrically disposed macrocyclic loop across the upper rim of the
calix[4]arene. Palladation of 5
yielded
[Pd(L1)(CH3CN)][BF4],
6. Similarly
5,17-bis[2-(4-(chloromethyl)phenoxy)acetamido]-25,26,27,28-tetrapropoxycalix[4]arene, 8, was used to prepare HL2,
9. Compound 9 is similar to 5 but
contains an aromatic spacer group
separating the xylyl fragment from the calixarene unit.
Palladation of 9 yielded
[Pd(L2)(CH3CN)][BF4],
11. All
new compounds and complexes were characterized in solution by
1H NMR spectroscopy, and the molecular
structure of 5 was verified by a single-crystal X-ray
diffraction study. Compound 5 crystallized in the
space
group P21/c with a =
23.5095(1) Å, b = 9.9090(1) Å, c
= 23.3586(1) Å, β = 91.53(1)°, V =
5439.59(8) Å3, and
Z = 4. The structure was refined to
R(F) = 10.13% and
Rw(F2) = 21.91%
for 5799 reflections with Fo2
>
2σ(Fo2). Compounds
6 and 11 are calix[4]arene-based
metalloreceptors containing an organopalladium binding
site and a hydrophobic cavity provided by the
calix[4]arene. Binding of a substrate through
σ-bonding to the
palladium center and interaction within the hydrophobic site were
demonstrated in solution by 1H NMR
spectroscopy. These multiple receptor−substrate interactions are
used by 11 for the molecular recognition of
4-phenylpyridine in the presence of 2-phenylpyridine or
3-phenylpyridine.