A Quantum Chemistry Study of Actinide(III) and Lanthanide(III) Complexes with Tridentate Nitrogen Ligands D. Guillaumont CEA-Va

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A Quantum Chemistry Study of Actinide(III) and Lanthanide(III) complexes with tridentate nitrogen ligands

D. Guillaumont

CEA-Valrhô, DEN/DRCP/SCPS/LCAM, Bât. 166, BP 17171, 30207 Bagnols-sur-Cèze, France [email protected]

Abstract - The structure and bonding in large complexes of actinide(III) and lanthanide(III) with tridentate N-donor ligands and water molecules have been investigated through quantum chemistry calculations in order to characterize the nature of the lanthanide-ligand and actinide-ligand bonds. Calculations have 3+ 2+ been performed using relativistic density functional theory on [M(L)(H2O)6] , [M(L)(H2O)5Cl] and 3+ [M(H2O)9] clusters where M = La, Ce, Nd, U, Pu, Am or Cm and L = 2,2′:6′2″terpyridine (Terpy) or 2,6-bis(5,6-di-methyl-1,2,4-triazin-3-yl)pyridine (MeBtp). The calculated evolution of the M-L bond as a function of the cation shows that lanthanide-ligand distances decrease with the diminution of the ionic radius, whereas the actinide-ligand distances increase from uranium to americium and are shorter than Ln-N distances. These trends are explained by the presence of covalent effects in the metal-ligand decreasing in the order U > Pu > Am ≈ Cm ≈ Ln.

INTRODUCTION results on actinide(III) with the experimental work recently done on the systems of interest. Finding ligands able to separate trivalent minor Calculations have been carried out on n+ 3+ actinides (americium(III), curium(III)) from [M(L)(H2O)5X] and [M(H2O)9] clusters trivalent lanthanides through liquid/liquid where M is a trivalent cation belonging to the solvent extraction processes is a particularly first half of the lanthanides (lanthanum, cerium, difficult task. Hard Lewis bases form purely neodymium) or actinides (uranium, plutonium, ionic complexes and cannot achieve the americium, curium) series (Scheme 1). The separation. On the other hand, softer Lewis bases lanthanide cations were selected because of their are expected to form bonds with a slightly comparable ionic radii with the actinides of greater covalent character with actinides than interest. Two tridentate ligands L were chosen with lanthanides. This subtle electronic effect is for the study; Terpy and MeBtp. MeBtp was attributed to the ability of valence orbitals of taken as a model for the alkyl substituted RBtp actinides, especially 5f, to participate in bonding, ligands. The choice of the ligands was motivated whereas 4f orbitals of lanthanides are lower in by the available experimental data, in particular energy, less spatially expanded and are often X-ray structures resolved with lanthanide(III) considered as core orbitals. This electronic effect and uranium(III) [3-6] would provide a test of is expected to be small but has been exploited to the calculated structures. design some families of soft donor ligands and successful separations have been obtained H3C N N CH3 through liquid/liquid extraction processes. Nc Nd N d CH Among soft donor ligands, tridentate N-donor H3C N N 3 aromatic bases have shown a good ability to M OH2 3+ H2O [M(MeBtp)(H2O)6] separate americium(III) from lanthanide(III) and OH H2O 2 are some of the most extensively investigated H2O OH2 ligands. We report here the results of a systematic quantum chemistry study of actinide(III) and lanthanide(IIII) complexes with Nc N tridentate N-donor aromatic ligands in order to d Nd n+ characterize and compare the evolution of the [M(Terpy)(H2O)5X] H O M X metal-ligand bond within a series of trivalent 2 OH H2O 2 cations from a structural and electronic H2O OH2 standpoint. Actinide compounds remain a challenge for quantum chemistry and the study X =H2O, Cl M = La, Ce, Nd, U, Pu, Am, Cm was motivated in large part by the rare opportunity to confront quantum chemistry Scheme1

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COMPUTATIONAL DETAILS Table 1 Calculated and Experimental M-N bond distances (Å) in Terpy and MeBtp complexes * The calculations were performed using density M-Nc M-Nd 2+ functional theory (DFT) methods with the [La(Terpy)(H2O)5Cl] calc. 2.67 2.65 2+ a Amsterdam Density Functional (ADF) program [La(Terpy)(H2O)5Cl] exp. 2.688(4) 2.658(3) 2+ [Ce(Terpy)(H2O)5Cl] calc. 2.63 2.62 package [1]. Relativistic effects were considered 2+ a [Ce(Terpy)(H2O)5Cl] exp. 2.658(5) 2.643(4) by the zeroth-order regular approximation 2+ [Nd(Terpy)(H2O)5Cl] calc. 2.59 2.57 (ZORA) [2]. Uncontracted triple-ζ Slater type 2+ a [Nd(Terpy)(H2O)5Cl] exp. 2.622(4) 2.613(3) orbitals valence orbitals with one set of 3+ [U(Terpy)(H2O)6] calc. 2.52 2.53 polarization functions were used for all atoms. 3+ b [U(Terpy)3] exp. 2.623(2) 2.63(4) The frozen-core approximation was used where 3+ [La(MeBtp)(H2O)6] calc. 2.70 2.67 3+ b the core density was obtained from four- [La (MeBtp)3] exp. 2.67(2) 2.63(2) 3+ component Dirac-Slater calculations on all the [Ce(MeBtp)(H2O)6] calc. 2.65 2.62 3+ c atoms and kept frozen during molecular [Ce(MeBtp)3] exp. 2.64(2) 2.61(2) 2 3+ calculations. 1s core electrons were frozen for [U(MeBtp)(H2O)6] calc. 2.56 2.54 10 n 3+ c carbon, nitrogen and oxygen and (1s2s2p) for [U(Pr Btp)3] exp. 2.55(2) 2.54(2) chlorine. The valence space of the heavy a ref [3] b ref [4] c ref [5] * elements includes 5s, 5p, 5d, 4f, 6s shells of Average of the two M-Nd distances lanthanides and 6s, 6p, 6d, 5f, 7s shells of actinides. The density functional consists of a local density part using the parametrization of TRENDS IN THE METAL-LIGAND BOND Vosko, Wilk, and Nusair and exchange- correlation gradient corrected parts of Becke and The trends in calculated metal-nitrogen bond Perdew. distances versus the reciprocal of the ionic radius of the metals are shown in Fig. 1 for 3+ [M(L)(H2O)6] complexes. COMPARISON OF CALCULATED WITH OBSERVED STRUCTURES 2.7 2.68 2.66 Metal-ligand distances corresponding to the most 2.64 Ln-N(MeBtp) relevant calculated and crystal structures are 2.62 given in Table 1. 2.6 An-N(MeBtp)

Calculated and experimental distances in (Å) 2.58 Ln-N(Terpy) 2+ 2.56 [Ln(Terpy)(H2O)5Cl] (Ln=La, Ce, Nd) agree 2.54 An-N(Terpy) within 0.01-0.04 Å whereas calculated distances 2.52 3+ in [U(Terpy)(H2O)6] are 0.10 Å shorter than U- 2.5 0.83 0.84 0.85 0.86 0.87 0.88 0.89 N distances in the crystal structure of La U Ce Pu Nd Am Cm 3+ 1/r (Å-1) [U(Terpy)3] . The M-N bond contraction ionic observed experimentally from cerium to uranium Figure 1. Evolution of M-N bond lengths versus is slightly overestimated by the calculations; the the ionic radius of the trivalent metal in 3+ shortening of the mean M-N bond distance is [M(L)(H2O)6] . 3+ 0.06 Å in [M(Terpy)(H2O)6] structures whereas 3+ it is equal to 0.02 Å in [M(Terpy)3] and to 0.05 A purely ionic bonding model would give a + Å in [M(Terpy)2I2] crystal structures [4,6]. regular decrease of M-N distances with 1/rionic. The experimental M-N distances observed in As can be seen from Fig. 1, opposite trends are 3+ [M(Btp)3] complexes are well reproduced by obtained for An-N and Ln-N bond distances. the calculations for all the cations. The computed While metal-nitrogen distances computed for the U-N/Ce-N bond contraction agree with X-ray lanthanides follow the order La-N > Ce-N > Nd- measurements: the bond length contraction has N as expected from the diminishing ionic radius n 3+ been measured in [M(Pr Btp)3] as equal to 0.09 in the series, the calculated actinide-nitrogen distances increase at the beginning of the series Å for M-Nc and to 0.08 Å for the mean M-Nd [5] whereas the calculated shortening in in the order U-N < Pu-N < Am-N whereas the 3+ [M(MeBtp)(H2O)6] is 0.09 Å (M-Nc) and 0.06 size of the ions increases in the series. The Å (M-Nd). shortest M-N distances are obtained for uranium, which is the largest of the cations investigated here; for instance U-N(MeBtp) distances are ~

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0.07 Å shorter than Cm-N(MeBtp) whereas the calculations on the free ligands, the π* of MeBtp ionic radius of uranium(III) is 0.07 Å larger than is at lower energy than the π* of Terpy, and for curium(III). From Am to Cm, the An-N interact better with U(5f). distances decrease with the diminishing ionic radii of the ions. It is remarkable that An-N distances are all shorter than Ln-N distances for a given ligand and the considered metal ion. The smallest differences between An-N and Ln-N distances correspond to Am / Nd, which have a similar ionic radius. Terpy and MeBtp ligands give rise to very similar trends in M-N bond distances. The main difference is the larger M-N diminution calculated from Pu to U for MeBtp than for σ1 σ2 Terpy.

ELECTRONIC STRUCTURE OF 3+ [M(L)(H2O)6] COMPLEXES

The metal-ligand bond in these systems is predominantly ionic. Covalency can be described σ π in terms of a ligand-to-metal donation, involving 3 1 Figure 2. Metal-Terpy bonding molecular the filled ligand σ and π molecular orbitals and orbitals. the empty metal ns, (n-1)d and (n-2)f orbitals

(with n=7 for An and n=6 for Ln) and metal-to- ligand back-donation from the filled metal 4f or

5f orbitals to the empty ligand π* molecular Table 2 Percentage contribution of metal and orbitals. This view of the bonding is supported ligand orbitals to the bonding M-L molecular by an analysis of the molecular orbitals presented orbitals in [M(Terpy)(H O) ]3+ (Boys-Foster in Table 2 and Fig. 2 where the molecular 2 6 localized α-spin orbitals). orbitals involved in the metal-ligand bonding are listed for the Terpy complexes. For all the metal Description M (%) Terpy (%) centre, ligand-to-metal donation is found in three d f s total bonding molecular orbitals involving mainly the La σ1(L→M) 6 91 ligand σ orbital and the empty metal d orbitals. σ2(L→M) 6 91 The contributions (%) of the metals and ligand σ3(L→M) 5 92 orbitals show that the molecular orbitals are Ce σ1(L→M) 6 91 mainly localized on the ligand. On the contrary, σ2(L→M) 6 91 metal-to-ligand back-donation takes place σ3(L→M) 5 92 mainly for uranium. For U, the donation involves Nd σ1(L→M) 7 91 the 5f and a π* with significant % contribution σ2(L→M) 7 90 on the ligand, 16% on Terpy and 27% on MeBtp. σ3(L→M) 5 91 The most striking result is the absence of retro- U σ (L→M) 7 1 1 91 donation from plutonium and americium. 1 σ (L→M) 7 2 1 88 The participation of empty s and f metal orbitals 2 σ (L→M) 7 2 1 90 is greater for actinides than for lanthanides, but 3 remains weak for all the cations. Overall, the π1(M→L) 2 76 16 total participation of the An centre per σ bond is Pu σ1(L→M) 6 2 1 89 a few percent greater than the participation of σ2(L→M) 6 3 1 90 Ln, for similarly size cations. The larger σ3(L→M) 6 1 1 9 contraction calculated for U-N(MeBtp) with Am σ1(L→M) 6 1 92 respect to U-N(Terpy) distances in the actinides σ2(L→M) 6 1 91 series is related to the larger 5f-π*(MeBtp) σ3(L→M) 6 1 90 mixing than 5f-π*(Terpy). According to the

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CONCLUSION Ephritikhine, J. Chem. Soc., Dalton Trans., 3265 (2002). Comparing the calculated and experimental 5. L. I. Semenova, A. H. White, Aust. J. structures shows that the ZORA calculations Chem., 52, 539 (1999). correctly reproduce the measured bond distances. 6. J. C. Berthet, C. Riviere, Y. Miquel, M. Calculations reproduce particularly well the Nierlich, C. Madic, M. Ephritikhine, Eur. J. shortening of the uranium-ligand bond / cerium- Inorg. Chem., 1439 (2002). ligand bond observed experimentally. This is an important result as it is a strong indication of the capability of the DFT/ZORA approach to describe correctly the structures of 4f/5f ions although the lack of crystal structures for americium(III) and curium(III) complexes prevents complete validation of this theoretical approach. According to the calculations, the lanthanide- ligand distances decrease with the diminishing ionic radius, whereas the actinide-ligand bond increases from uranium to americium and are shorter than Ln-N distances. These trends are explained by the presence of stronger covalent effects in the metal-ligand bond for actinides than lanthanides. However, the increased covalency is significant for uranium, but reaches the limit of the calculated approximation for americium and curium. According to this study there is no significant 5f contribution of americium and curium to the bonding. As previously shown for other uranium(III) complexes, the present study confirms the significant participation of the uranium 5f orbital in the metal-ligand bond, with greater mixing for MeBtp ligand than for Terpy, leading to a more significant shortening of the U-N(MeBtp) bond than for U-N(Terpy). These results are coherent with experimental findings that indicate significantly shorter U-N bonds but very small energy differences between the thermodynamic parameters of americium(III), curium(III) and lanthanide(III).

REFERENCES

1. ADF2002.03, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com. 2. E. van Lenthe, A. Ehlers, E. J. Baerends, J. Chem. Phys., 110, 8943 (1999). 3. C. J. Kepert, L. Weimin, B. W. Skelton, A. H. White, Aust. J. Chem., 47, 365 (1994). 4. J. C. Berthet, Y. Miquel, P. B. Iveson, M. Nierlich, P. Thuéry, C. Madic, M.

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