Two-Dimensional Inorganic Cationic Network of Thorium Iodate Chloride with Unique Halogen–Halogen Bonds

A unique two-dimensional
inorganic cationic network with the formula [Th3O2(IO3)5(OH)2]Cl was synthesized hydrothermally.
Its crystal structure can best be described as positively charged
slabs built with hexanuclear thorium clusters connected by iodate
trigonal pyramids. Additional chloride anions are present in the interlayer
spaces but surprisingly are not exchangeable, as demonstrated by a
series of CrO42– uptake experiments.
This is because all chloride anions are trapped by multiple strong
halogen–halogen interactions with short Cl–I bond lengths
ranging from 3.134 to 3.333 Å, forming a special Cl-centered
trigonal-pyramidal polyhedron as a newly observed coordination mode
for halogen bonds. Density functional theory calculations clarified
that electrons transformed from central Cl atoms to I atoms, generating
a halogen–halogen interaction energy with a value of about
−8.3 kcal mol–1 per Cl···I
pair as well as providing a total value of −57.9 kcal mol–1 among delocalized halogen–halogen bonds, which
is a new record value reported for a single halogen atom. Additional
hydrogen-bonding interaction is also present between Cl and OH, and
the interaction energy is predicted to be −8.1 kcal mol–1, confirming the strong total interaction to lock
the interlayer Cl anions.