Structure of lanthanide nitrates in solution and in the solid state: DFT modelling of hydration effects

Abstract

Five types of [Ln(NO3)3(H2O)4] neutral complexes with different mutual arrangement of nitrato and aqua ligands are established by the analysis of crystallographic data. Quantum chemical calculations performed at the level of density functional theory employing full-electron relativistic approach and effective core potential with different functionals and basis sets demonstrate that each of these structure types converges to a specific energy minimum, thus these types should be considered as isomers. Besides this, two isomers have been found for [Ln(NO3)3(H2O)3] complexes. In all structures of isomers optimized with the gas-phase approximation, Ln–ONO2 bonds are significantly shorter that Ln–OH2 bonds, in contrast to the results of X-ray diffraction and EXAFS studies of single crystals and aqueous solutions, respectively. This discrepancy arises from the fact that in crystals and solutions, complex molecules form numerous hydrogen bonds, which enhance charge transfer in the H2O → Ln3+ → NO3− chains and increase the relative stability of isomers where the nitrate anions are arranged separately from the aqua ligands. The effect of environment on electronic and spatial structures of [Ln(NO3)3(H2O)n] (n = 3, 4) complexes has been modelled at the levels of polarizable continuum model (PCM) and cluster approach. When using PCM, the similarity of quantum chemical and experimental bond lengths enhances, but complete agreement is not reached. Better agreement is achieved within the clusters {[La(NO3)3(H2O)4]∙nH2O} (n = 18, 21, 32, 50), where the effects of hydrogen bonding are modelled explicitly. These clusters are highly flexible with both hydration shells and the central core being easily rearranged under the effect of outer water molecules. With an increase in cluster size, new local energy minima appear related with the structures with monodentate nitrate anion and with nitrate anion positioned in the second coordination sphere of the cation. Joint analysis of simulation results together with literature data on the structure and dynamic behavior of lanthanide nitrates in aqueous solutions allows conclusion that the entire set of equilibria involving [La(NO3)3(H2O)n] complexes should be considered as a dynamic combinatorial library.