14822 Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 Electronic structure and bonding of lanthanoid(III) carbonatesw Yannick Jeanvoine, a Pere Miro´, b Fausto Martelli, a Christopher J. Cramer* b and Riccardo Spezia* a Received 14th June 2012, Accepted 31st July 2012 DOI: 10.1039/c2cp41996c Quantum chemical calculations were employed to elucidate the structural and bonding properties of La(III) and Lu(III) carbonates. These elements are found at the beginning and end of the lanthanoid series, respectively, and we investigate two possible metal-carbonate stoichiometries (tri- and tetracarbonates) considering all possible carbonate binding motifs, i.e., combinations of mono- and bidentate coordination. In the gas phase, the most stable tricarbonate complexes coordinate all carbonates in a bidentate fashion, while the most stable tetracarbonate complexes incorporate entirely monodentate carbonate ligands. When continuum aqueous solvation eﬀects are included, structures having fully bidentate coordination are the most favorable in each instance. Investigation of the electronic structures of these species reveals the metal–ligand interactions to be essentially devoid of covalent character. 1. Introduction The hydration properties of lanthanoids (Ln) in aqueous solution have been widely studied both experimentally and theoretically. 1–5 Such studies have primarily focused on lanthanoids in their 3+ oxidation state, which are important in nuclear waste remediation and medical imaging. 6–8 In the context of nuclear waste, these ions are relevant because of the challenge associated with separating them from actinide ions (An). 9 Ln(III) ions in deposited nuclear waste are expected to interact with carbonate as a counterion in so far as the presence of carbonates in geological media is ubiquitous. Interestingly, reliance on diﬀerential lanthanide-carbonate interactions has been proposed as a possible separation procedure for Ln(III) and An(III) ions in solution. 10 Consequently, the characterization of lanthanoid carbonate structures is central to understanding how lanthanoid ions will behave in aqueous solutions with available carbonate counterions that may act as supporting ligands. Crystallographic data for Ln 3+ carbonate hydrates are available for tri-carbonate ligands, 11 and for Nd(III) Runde et al. 12 have suggested the formation of a [Nd(CO 3 ) 4 H 2 O] 5structure at high carbonate concentrations. Recently Philippini et al. have studied several Ln(III)-carbonate complexes in solution using electrophoretic mobility measurements and time- resolved laser-induced ﬂuorescence spectroscopy (TRLFS). 13–15 They concluded that light Ln(III) ions coordinate four carbonate ligands while heavier ones coordinate only three ligands. In contrast, considering available crystallographic and spectroscopic data (including UV-vis, near infrared, and infrared), Janicki et al. concluded that in aqueous solution all Ln(III) ions form tetra- carbonates when carbonate ions are not limited. 16 These authors also performed a set of theoretical calculations that suggest that there is partial charge transfer between the Ln(III) ion and the carbonate ligand that introduces a degree of covalency to the metal–ligand bonding. Another recent theoretical contribution in this area was a report by Sinha et al. on [Nd(CO 3 ) 4 ] 5using the Parameterized Model 3 (PM3) semi-empirical method. 17 Notwithstanding these two studies, no systematic, quantitative theoretical study has been undertaken in order to characterize the structures and bonding of lanthanoid(III) tri- and tetra- carbonates, while, e.g., such kinds of studies were performed on actinyl carbonate complexes. 18,19 Among the questions that remain open: (i) what is the coordination geometry of the carbonate ligands for Ln(III) complexes in water?; (ii) which stoichiometry dominates? and (iii) what is the degree of ionic vs. covalent bonding for the Ln(III)-carbonate interaction? Electronic structure methods, and in particular density- functional theory (DFT), have proven to be valuable tools for the study of heavy elements. Increasingly accurate lantha- noid and actinoid pseudo-potentials 20 have been particularly useful in this regard. In the present study, we focus on tri- and tetracarbonates ([Ln(CO 3 ) 3 ] 3and [Ln(CO 3 ) 4 ] 5, respectively) considering the Ln(III) ions lanthanum (La) and lutetium (Lu). As these two elements begin and end the lanthanoid series, respectively, they should establish limiting behavior with respect to forming complexes with carbonates. In aqueous solution with non-coordinating counterions, the diﬀerence in a Universite ´ d’Evry Val d’Essonne, CNRS UMR 8587 LAMBE, Bd F. Mitterrand, 91025 Evry Cedex, France. E-mail: firstname.lastname@example.org b Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455-0431, USA. E-mail: email@example.com w Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cp41996c PCCP Dynamic Article Links www.rsc.org/pccp PAPER Published on 01 August 2012. Downloaded by Princeton University on 07/07/2014 22:25:29. 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Citethis:Phys. Chem. Chem. Phys.,2012,14 ,1482214831 … · 14822 Phys. Chem. Chem. Phys., 2012,14 ,1482214831 This ournal is c the Owner Societies 2012 Citethis:Phys. Chem. Chem.
Table 5 Reaction free energies (DG, kcal mol�1) at the B3LYP/ECP/6-31+G(d) level of theory in both vacuum and water (described withthe PCM continuum solvation model). In bold we highlight the DGcorresponding to the most favorable product in vacuum or water
14830 Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 This journal is c the Owner Societies 2012
Fully bidentate binding of carbonate ligands is preferred both
in the gas phase and water for tricarbonates of lanthanum(III)
and lutetium(III). By contrast, for the corresponding tetra-
carbonates fully monodentate binding is preferred in the gas
phase and fully bidentate binding in aqueous solution. The
stronger repulsion energy associated with four carbonate
ligands drives the different behavior for the tetracarbonate in
the gas phase compared to the tricarbonate, but aqueous
solvation effectively compensates for this effect. The energy
of the tri-carbonate structure relative to the tetra-carbonate
alternative is thus lower for Lu than La in the gas phase, in line
with some experimental suggestions,13–15 while in solution
La and Lu behave similarly. This deserves further studies
and developments, in particular to have access to free energy
differences in liquid systems explicitly considering the solvent
and the experimental conditions (pH, ionic strength, etc.). This
is the direction of our current research.
Topological analysis of the electron density, energy decom-
position analysis, and natural orbitals for the chemical valence
analysis all agree that the Ln-carbonate interaction is predo-
minantly closed shell/ionic in nature. Thus, the known differ-
ence in ionic radii across the lanthanoid series should be
the key physical quantity determining the properties of
Ln/carbonate complexes. A contrasting, and certainly inter-
esting situation could arise for the case of An(III)/carbonate
complexes, where the 5f orbitals, which have more valence
character than do 4f analogs, could determine differences in
binding through covalent interactions, as recently shown by
Gagliardi, Albrecht-Schmitt and co-workers.67,68
Finally, the highly closed-shell/ionic nature of lanthanoid(III)-
carbonate interactions highlighted by the present analysis
paves the way for developing classical force fields for these
systems. Simulations of lanthanoid solutions by means of
finite temperature molecular dynamics with explicit solvent will
be crucial to address questions related to the formation and
equilibrium of these complexes as a function of salt concen-
tration, as has recently been shown for lanthanoid-chloride,
thorium-chloride and thorium boride salts.69,70 The present
study suggests that the extension of such techniques to
Ln/carbonate salts in explicit water should be feasible to study
statistically the equilibrium between different complexes.
We would like to acknowledge Thomas Vercouter and Pierre
Vitorge for interesting discussions. This work was partially
supported by the French National Research Agency (ANR)
on project ACLASOLV (ANR-10-JCJC-0807-01) (Y.J., F.M.
and R.S.). PM and CJC acknowledge the National Science
Foundation (grant CHE-0952054).
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