View Article Online / Journal Homepage / Table of Contents for this issue PCCP Dynamic Article Links Cite this: Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 www.rsc.org/pccp PAPER 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 They concluded that light Ln(III) ions coordinate four carbonate ligands while heavier ones coordinate only three ligands. In The hydration properties of lanthanoids (Ln) in aqueous contrast, considering available crystallographic and spectroscopic solution have been widely studied both experimentally and data (including UV-vis, near infrared, and infrared), Janicki et al. theoretically.1–5 Such studies have primarily focused on concluded that in aqueous solution all Ln(III) ions form tetra- lanthanoids in their 3+ oxidation state, which are important carbonates when carbonate ions are not limited.16 These authors in nuclear waste remediation and medical imaging.6–8 In the also performed a set of theoretical calculations that suggest that context of nuclear waste, these ions are relevant because of the there is partial charge transfer between the Ln(III) ion and the challenge associated with separating them from actinide ions Published on 01 August 2012. Downloaded by Princeton University 07/07/2014 22:25:29. 9 carbonate ligand that introduces a degree of covalency to the (An). Ln(III) ions in deposited nuclear waste are expected to metal–ligand bonding. Another recent theoretical contribution in interact with carbonate as a counterion in so far as the presence of this area was a report by Sinha et al. on [Nd(CO ) ]5À using carbonates in geological media is ubiquitous. Interestingly, reliance 3 4 the Parameterized Model 3 (PM3) semi-empirical method.17 on differential lanthanide-carbonate interactions has been Notwithstanding these two studies, no systematic, quantitative proposed as a possible separation procedure for Ln(III)and 10 theoretical study has been undertaken in order to characterize An(III) ions in solution. Consequently, the characterization of the structures and bonding of lanthanoid(III) tri- and tetra- lanthanoid carbonate structures is central to understanding how carbonates, while, e.g., such kinds of studies were performed lanthanoid ions will behave in aqueous solutions with available on actinyl carbonate complexes.18,19 Among the questions that carbonate counterions that may act as supporting ligands. remain open: (i) what is the coordination geometry of the Crystallographic data for Ln3+ carbonate hydrates are 11 carbonate ligands for Ln(III) complexes in water?; (ii) which available for tri-carbonate ligands, and for Nd(III) Runde 12 5À stoichiometry dominates? and (iii) what is the degree of ionic et al. have suggested the formation of a [Nd(CO3)4H2O] vs. covalent bonding for the Ln(III)-carbonate interaction? structure at high carbonate concentrations. Recently Philippini Electronic structure methods, and in particular density- et al. have studied several Ln(III)-carbonate complexes in functional theory (DFT), have proven to be valuable tools solution using electrophoretic mobility measurements and time- for the study of heavy elements. Increasingly accurate lantha- resolved laser-induced fluorescence spectroscopy (TRLFS).13–15 noid and actinoid pseudo-potentials20 have been particularly a useful in this regard. In the present study, we focus on tri- and Universite´ d’Evry Val d’Essonne, CNRS UMR 8587 LAMBE, 3À 5À Bd F. Mitterrand, 91025 Evry Cedex, France. tetracarbonates ([Ln(CO3)3] and [Ln(CO3)4] , respectively) E-mail: [email protected] considering the Ln(III) ions lanthanum (La) and lutetium (Lu). b Department of Chemistry, Supercomputing Institute, and Chemical As these two elements begin and end the lanthanoid series, Theory Center, University of Minnesota, 207 Pleasant St. SE, respectively, they should establish limiting behavior with Minneapolis, MN 55455-0431, USA. E-mail: [email protected] w Electronic supplementary information (ESI) available. See DOI: respect to forming complexes with carbonates. In aqueous 10.1039/c2cp41996c solution with non-coordinating counterions, the difference in 14822 Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 This journal is c the Owner Societies 2012 View Article Online ionic radius for these two elements gives rise to a difference B3LYP optimized geometries, single-point energies were in hydration number (9-fold vs. 8-fold for La and Lu, calculated in a vacuum and implicit solvent with several other 21,22 respectively). Ln(III)-aquo interactions have been deter- functionals to evaluate sensitivity of results to choice of mined to be mainly electrostatic in nature, as one might expect functional, including: BLYP,31,32 M05,33 M05-2X,34 PBE0,35 36 37 38 given the ‘‘hard’’ characters of both Ln(III) ions and water. As BHandH, TPSS, and VSXC. These functionals are of such, the variation in ionic radius is the main physical quantity different constructions: generalized gradient approximation, that affects hydration properties.22,23 The fact that ionic radii GGA (BLYP), meta-GGA (TPSS and VSXC), hybrid GGA can dictate the complexation properties has also been pointed (B3LYP and PBE0), meta-hybrid GGA (M05) and two hybrids out for the case of ligands that are potentially less hard than with a higher percentage of Hartree–Fock exchange: the hybrid water, like hexacyanoferrate.24 Nevertheless, carbonates are GGA BHandH and the meta-hybrid GGA M05-2X. MP2 softer ligands than water, and it is also possible that the single point calculations were also performed in both gas phase metal–ligand interaction may change across the spectrum of and continuum aqueous solution to have results from a wave the lanthanoid series. The difference between La and Lu offers function theory model against which to compare. insight into the extrema for the whole series if the interaction is In general, molecular geometries are not especially sensitive mainly electrostatic and/or if the contribution of 4f orbitals is to choice of (modern) density functional.39 We have verified negligible to Ln/carbonate interaction. This last situation is to that geometry optimizations with various functionals lead to be expected since 4f orbitals are compact around lanthanoids changes in geometries and energy orderings that are minimal and rarely invoked as contributing to valence bonding; indeed (relative energy differences are below 1 kcal molÀ1, see this behavior rationalizes the key role that ionic radius plays in Table S17 in ESIw). In the interest of brevity, we thus report dictating interactions with water as a ligand.25 As we will show below only results obtained with B3LYP geometries. in the present study, this is indeed the case for carbonate as We also examined all-electron calculations including relati- well and thus the difference between La and Lu complexes vistic effects. In particular, using the geometries optimized at does likely span the lanthanoid spectrum. the B3LYP/ECP/6-31+G(d) level of theory, single-point calcu- We study differences in Ln-carbonate interactions as a lations on all species were performed using the Amsterdam function of the lanthanoid, focusing on the number and Density Functional program (ADF 2010.02) developed by coordination geometries of the carbonate ligands. The influ- Baerends, Ziegler, and co-workers.40 For these computations ence of aqueous solvation has been included through the use the B3LYP functional was employed with an all-electron of implicit solvation methods, which are useful for predicting triple-z plus two polarization functions basis set on all atoms. the electrostatic component that dominates the free energies of Relativistic corrections were introduced by the scalar-relativistic solvation for these highly charged species. Finally, topological zero-order regular approximation (ZORA).41,42 Gas-phase analysis of the electron density and examination of valence and implicit aqueous solution calculations were performed, natural orbitals are undertaken to address the nature of the with continuum solvent effects included via the COSMO43 various Ln-carbonate bonds. solvent model with standard radii except for La (R = 2.42 A˚ ) and Lu (R = 2.24 A˚ ) centres.44 2. Computational details Published on 01 August 2012. Downloaded by Princeton University 07/07/2014 22:25:29. All geometries were fully optimized at the density functional 3. Results and discussion theory level with the Gaussian 03 electronic structure program 3.1 Structure of lanthanum and lutetium carbonates suite26 using the hybrid three parameter functional incorpor- ating Becke exchange and Lee–Yang–Parr correlation, also Structures of lanthanum(III) and lutetium(III) tri- and tetra- known as B3LYP.27 For La and Lu atoms, we have used the carbonates have been fully optimized at the B3LYP/ECP/ energy-consistent pseudopotentials (ECP) of the Stuttgart/ 6-31+G(d) level of theory (Fig. 1 and 2). The carbonate Cologne group which are semi-local pseudopotentials adjusted ligands can coordinate the metal centre in either a mono- 28,29 1 2À 2 2À to reproduce atomic valence-energy spectra. Amongst the dentate (Z -CO3 ) or bidentate (Z -CO3 ) fashion.