A charge-neutral tetrahedral [(Pd3X)4L6] cage assembly built from a trinuclear
polyhedral building unit (PBU), [Pd3X]3+, cis-blocked
with an imido P(V) ligand, [(NiPr)3PO]3– (X3–), and oxalate
dianions (L2–) is reported. Use of benzoate or ferrocene
dicarboxylate anions, which do not offer wide-angle chelation as that
of oxalate dianions, leads to smaller prismatic clusters instead of
polyhedral cage assemblies. The porosity of the tetrahedral cage assembly
was determined by gas adsorption studies, which show a higher uptake
capacity for CO2 over N2 and H2.
The tetrahedral cage was shown to encapsulate a wide range of neutral
guest solvents from polar to nonpolar such as dimethyl sulfoxide,
benzene, dichloromethane, chloroform, carbon tetrachloride, and cyclopentane
as observed by mass spectral and single-crystal X-ray diffraction
studies. The 1H two-dimensional diffusion ordered spectroscopy
NMR analysis shows that the host and guest molecules exhibit similar
diffusion coefficients in all the studied host–guest systems.
Further, the tetrahedral cage shows selective binding of benzene,
CCl4, and cyclopentane among other solvents from their
categories as evidenced from mass spectral analysis. A preliminary
density functional theory analysis gave a highest binding energy for
benzene among the other solvents that were structurally shown to be
encapsulated at the intrinsic cavity of the tetrahedral cage.