Structure and Bonding in Uranyl(VI) Peroxide and Crown Ether Complexes; Comparison of Quantum Chemical and Experimental Data

The
structure, chemical bonding, and thermodynamics of alkali ions in
M­[12-crown-4]+, M­[15-crown-5]+, and M­[18-crown-6]+, M­[UO2(O2)­(OH2)2]+4,5, and M­[UO2(O2)­(OH)­(OH2)]n1–n (n = 4, 5) complexes have been explored by
using quantum chemical (QC) calculations at the ab initio level. The
chemical bonding has been studied in the gas phase in order to eliminate
solvent effects. QTAIM analysis demonstrates features that are very
similar in all complexes and typical for electrostatic M–O
bonds, but with the M–O bonds in the uranyl peroxide systems
about 20 kJ mol–1 stronger than in the corresponding
crown ether complexes. The regular decrease in bond strength with
increasing M–O bond distance is consistent with predominantly
electrostatic contributions. Energy decomposition of the reaction
energies in the gas phase and solvent demonstrates that the predominant
component of the total attractive (ΔEelec + ΔEorb) energy contribution is
the electrostatic component. There are no steric constraints for coordination
of large cations to small rings, because the M+ ions are
located outside the ring plane, [On],
formed by the oxygen donors in the ligands; coordination of ions smaller
than the ligand cavity results in longer than normal M–O distances
or in a change in the number of bonds, both resulting in weaker complexes.
The Gibbs energies, enthalpies, and entropies of reaction calculated
using the conductor-like screening model, COSMO, to account for solvent
effects deviate significantly from experimental values in water, while
those in acetonitrile are in much better agreement. Factors that might
affect the selectivity are discussed, but our conclusion is that present
QC methods are not accurate enough to describe the rather small differences
in selectivity, which only amount to 5–10 kJ mol–1. We can, however, conclude on the basis of QC and experimental data
that M­[crown ether]+ complexes in the strongly coordinating
water solvent are of outer-sphere type, [M­(OH2)n+]­[crown ether], while those in weakly
coordinating acetonitrile are of inner-sphere type, [M-crown ether]+. The observation that the M­[UO2(O2)­(OH)­(OH2)]n1–n complexes are more stable in solution than those of M­[crown
ether]+ is an effect of the different charges of the rings.