Probing the Influence of Phosphonate Bonding Modes to Uranium(VI) on Structural Topology and Stability: A Complementary Experimental and Computational Investigation

Systematic control of the reactions
between U­(VI) and 1,4-phenylenebis­(methylene))­bis­(phosphonic
acid) (pmbH4) allows for alterations
in the bonding between these constituents and affords three uranyl
phosphonate compounds with chiral one-dimensional (1D), two-dimensional
(2D), and three-dimensional (3D) structures, namely, [TPA]­[UO2(pmbH3)­(pmbH2)­H2O]·2H2O (1), [NH4]2[UO2(pmb)] (2), UO2(pmbH2) (3), and the first uranyl mixed phosphite/phosphonate compound
[TMA]2[(UO2)2­(pmb)­(HPO3)] (4) (TPA = NPr4+, TMA = NMe4+). These compounds crystallize in the
space groups P212121, P1̅, P21/c, and Cmcm, respectively. Further investigation
of the local uranyl coordination environment reveals that in 1 only oxygen atoms from PO moieties ligate the uranium
centers; whereas in 2 only P–O– oxygen atoms are involved in bonding and yield a layered topology.
Compound 3 differs sharply from the first two in that
conjugated PO and P–O– oxygen atoms
chelate the uranium centers resulting in a 3D framework. In compound 4, a phosphonate group bridges three uranyl centers further
coordinated with a phosphite ligand HPO32–, which is a product of pmbH4 decomposing, forming a 2D layered structure. Compounds 3 and 4 also contain a different coordination
environment for U­(VI) than that found in 1 or 2. In this case, tetragonal bipyramidal UO6 units occur
instead of the far more common UO7 pentagonal bipyramids
found in 1 and 2. Interestingly, 1 converts to 3 at elevated reaction temperatures, indicating
that the formation of 1 is likely under kinetic control.
This is supported by thermal analysis, which reveals that 3 has higher thermal stability than 1 or 2. UV–vis–near-IR absorption and fluorescence spectroscopy
show that the absorption and photoluminescence intensity increases
from 1 to 4. Density functional theory electronic
structure calculations provide insight into the nature of the interactions
between U­(VI) and the phosphonate ligands.