Transuranic Hybrid Materials: Crystallographic and Computational Metrics of Supramolecular Assembly

Assembly of a family
of 12 supramolecular compounds containing
[AnO2Cl4]2– (An = U, Np, Pu),
via hydrogen and halogen bonds donated by substituted 4-X-pyridinium
cations (X = H, Cl, Br, I), is reported. These materials were prepared
from a room-temperature synthesis wherein crystallization of unhydrolyzed
and valence-pure [An­(VI)­O2Cl4]2– (An = U, Np, Pu) tectons is the norm. We present a hierarchy of
assembly criteria based on crystallographic observations and subsequently
quantify the strengths of the non-covalent interactions using Kohn–Sham
density functional calculations. We provide, for the first time, a
detailed description of the electrostatic potentials of the actinyl
tetrahalide dianions and reconcile crystallographically observed structural
motifs and non-covalent interaction acceptor–donor pairings.
Our findings indicate that the average electrostatic potential across
the halogen ligands (the acceptors) changes by only ∼2 kJ mol–1 across the AnO22+ series, indicating
that the magnitude of the potential is independent of the metal center.
The role of the cation is therefore critical in directing
structural motifs and dictating the resulting hydrogen and halogen
bond strengths, the former being stronger due to the positive charge
centralized on the pyridyl nitrogen, N–H+. Subsequent
analyses using the quantum theory of atoms in molecules and natural
bond orbital approaches support this conclusion and highlight the
structure-directing role of the cations. Whereas one can infer that
Columbic attraction is the driver for assembly, the contribution of
the non-covalent interaction is to direct the molecular-level arrangement
(or disposition) of the tectons.