Thermodynamics and Electronic Properties of Heterometallic Multinuclear Actinide-Containing Metal–Organic Frameworks with “Structural Memory”

Thermodynamic studies of actinide-containing
metal–organic
frameworks (An-MOFs), reported herein for the first time, are a step
toward addressing challenges related to effective nuclear waste administration.
In addition to An-MOF thermochemistry, enthalpies of formation were
determined for the organic linkers, 2,2′-dimethylbiphenyl-4,4′-dicarboxylic
acid (H2Me2BPDC) and biphenyl-4,4′-dicarboxylic
acid (H2BPDC), which are commonly used building blocks
for MOF preparation. The electronic structure of the first example
of An-MOF with mixed-metal AnAn′-nodes was influenced through
coordination of transition metals as shown by the density of states
near the Fermi edge, changes in the Tauc plot, conductivity measurements,
and theoretical calculations. The “structural memory”
effect (i.e., solvent-directed crystalline–amorphous–crystalline
structural dynamism) was demonstrated as a function of node coordination
degree, which is the number of organic linkers per metal node. Remarkable
three-month water stability was reported for Th-containing frameworks
herein, and the mechanism is also considered for improvement of the
behavior of a U-based framework in water. Mechanistic aspects of capping
linker installation were highlighted through crystallographic characterization
of the intermediate, and theoretical calculations of free energies
of formation (ΔGf) for U- and Th-MOFs with 10- and 12-coordinated secondary building
units (SBUs) were performed to elucidate experimentally observed transformations
during the installation processes. Overall, these results are the
first thermochemical, electronic, and mechanistic insights for a relatively
young class of actinide-containing frameworks.