AND EUROPIUM(III) with 6,6’-BIS-(5,6-DIETYHYL-1,2,4-TRIAZIN-3-YL)-2,2’-BIPYRIDINE and on THEIR FORMATION ENERGIES Wojciech P

44 CENTRE OF RADIOCHEMISTRY AND NUCLEAR CHEMISTRY THEORETICAL INVESTIGATIONS ON THE STRUCTURE AND BONDING IN CATIONIC COMPLEXES OF AMERICIUM(III) AND EUROPIUM(III) WITH 6,6’-BIS-(5,6-DIETYHYL-1,2,4-TRIAZIN-3-YL)-2,2’-BIPYRIDINE AND ON THEIR FORMATION ENERGIES Wojciech P. Ozimiński, Jerzy Narbutt

Bis-triazinyl-bipyridine (BTBP) derivatives, selec- calculations ensured that the obtained stationary tively extract tervalent actinides (An = Am, Cm, points were the true minima on the potential en- …) over lanthanides (Ln) from nitric acid solu- ergy surface. The structures of nonaaquaions, 3+ 3+ tions to organic solvents. The separation is essen- [Am(H 2O) 9] and [Eu(H 2O) 9] , optimized at the tial for further transmutation of long-lived nuclei same level of theory, have been reported in the of An, which will make it possible the safe storage previous report [8]. of the remaining radioactive waste from repro- For the analysis of bonding two approaches cessed nuclear fuel [1,2]. The origin of the selec- were employed: (i) analysis of orbital picture in tivity of the tetra-N-dentate BTBP ligands toward the form of localized molecular orbitals (LMO) actinides is not clearly explained yet though the applying the natural bond orbital (NBO) approach role of increased covalency of An-N bonds, due to [10], especially the natural semi-localized molecu- the higher spatial expansion of 5 f orbitals of An lar orbitals (NLMO); and (ii) topological analysis ions with respect to the 4 f orbitals of Ln ions, was of electron density within the Bader’s quantum concluded based on quantum chemical calcula- theory of atoms in molecules (QTAIM) [11], by tions of An/Ln complexes with similar ligands using the AIMAll Pro program [12]. The NBO ap- [3,4]. Two kinds of M-BTBP complexes – neutral proach made it also possible to analyze the parti ci- 3+ [M(BTBP)(NO 3)3] and charged [M(BTBP) 2] pation of metal s, d and f subshells in bonding in species – have been detected in the organic phase the complexes. The analysis was carried out by us- after solvent extraction, and molecular structure of ing NBO 5.0 program interfaced to Gaussian. The the neutral europium complex has been determin- wavefuctions used for QTAIM analysis were pre- ed [1,5]. Some doubts appear, however, regarding pared using the B01 release of Gaussian 09 where, the composition of these charged 1:2 complexes for the first time, the core densities associated which predominate at higher BTBP concentra- with effective core potentials have been properly tions [5]. TRLFS study made it possible to identify accounted for, thus enabling valid topological 3+ 3+ [Eu(BTBP) 2(H 2O)] and [Cm(BTBP) 2(H 2O)] analysis for the systems studied. species in water/propanol solutions [6]. In solvent For the calculations of the energy balance, extraction systems containing > 1 M HNO 3, also vari ous models of the complex formation were 2+ the [M(BTBP) 2(NO 3)] species can appear in the tested, related to the substrates and products in organic phase [5]. Recent ESI-MS examination of either gas phase or water [8]. The 6-31G(d) basis such organic phase identified two cationic 1:2 set appeared sufficient for geometry optimization 3+ complexes of Eu and Am: [M(BTBP) 2] and and produced reasonable bond lengths and angles 2+ [M(BTBP) 2(NO 3)] ; the latter significantly pre- at acceptable computational time. It was also good vailing [7]. enough for NBO and QTAIM analyses. However, In the present work theoretical investigations quantitative estimation of the energy was more were carried out on the structure and bonding in prone to basis set quality and thus required more cationic complexes of americium(III) and europ- sophisticated basis set. Therefore, for atoms other ium(III) with 6,6’-bis-(5,6-dietyhyl-1,2,4-triazin-3- than Am and Eu we applied the 6-311G(d,p) basis -yl)-2,2’-bipyridine (et-BTBP), as well as on the set of triple-zeta valence quality, with polarization energies of formation of these complexes in water, functions. As the calculations with this basis set complementing our previous study on the neutral are very time-consuming, we performed only single- [M(et-BTBP)(NO 3)3] complexes [8]. The calcula- -point calculations. The solvent (water) effect was tions were restricted to the species of symmetric modelled using PCM methodology (single-point 3+ structure – the [M(et-BTBP) 2] complexes (CN 8). calculation). A priori optimization of less symmetric structure Geometry optimization of the complexes. 2+ of [M(BTBP) 2(NO 3)] complexes (CN 10) seems Figure shows the optimized gas-phase structure 3+ unlikely to succeed due to severe SCF conver- of the [Am(et-BTBP) 2] complex. Coordination gence problems. number (CN 8) of the metal ion results from its Optimization of the molecular structures of tetradentate coordination by the two et-BTBP li- 3+ the [M(et-BTBP) 2] complexes was performed at gands. A very similar structure has been obtained the B3LYP level of theory with the use of Gaussian for the analogous europium complex (Table 1). 09 suite of programs [9], with Stuttgart-Dresden Table 1 presents some calculated lengths of (SDD) energy-consistent pseudorelativistic basis metal-ligand bonds and angles in the complexes set with most of inner electrons replaced by small- studied. Due to symmetry of the BTBP ligands -core effective core potentials (ECPs) for Am and and of the complex molecules, only two types of 3+ 3+ Eu atoms. In both Am and Eu ions the 35 va- metal-ligand bonds were analysed: M-N py (two lence electrons were treated explicitly. Frequency central pyridine rings), and M-N tr (two lateral tri- CENTRE OF RADIOCHEMISTRY AND NUCLEAR CHEMISTRY 45 azine rings). The M-N tr bonds are shorter than the quantum mechanics without any arbitrary assump- respective M-N py ones in both complexes, which tions, and provides a unique way to partitioning (in agreement with the results of bonding analysis) the electron density in a molecule into atomic indicates stronger bonding of the metal ions to the basins. It also enables us to partition various prop- erties of the molecule into atomic contributions. In the recent years a great interest arose in the application of QTAIM [11] to transition metal complexes [15], because the analysis of bonding in these molecules presents ongoing challenge to theoretical methods and to the definition of chemical bonding itself. In Table 2 we present the atomic charges on selected atoms, obtained by both NBO-based natural population analysis (NPA) which can be considered an “improved Mulliken analysis”, and by QTAIM method which integrates the electron density within atomic basins separated by zero-flux surface where the electron density gradient vanishes. Table 2. NPA charges and QTAIM charges on selected 3+ atoms in the [M(et-BTBP) 2] complexes, obtained at the B3LYP/Stuttgart ECP small-core/6-31G(d) level of theory.

Atom (ligand) NBO 5.0 QTAIM Am +1.64 +2.10 N21 -0.36 -0.78 (triazine, bonding) N24 -0.20 -0.68 (triazine, nonbonding) Fig. The optimized gas-phase structure of the [Am(et-BT- 3+ N22 (triazine, distant) -0.44 -1.20 -BP) 2] complex. N7 (pyridine) -0.52 -1.28 triazine than pyridine rings. The greater length of Am-N than Eu-N distances is due to the differ- Eu +2.01 +2.12 ences in the metal ions radii, as in the case of neu- N21 tral [M(et-BTBP)(NO 3)3] complexes [8]. -0.39 -0.79 (triazine, bonding) 3+ Table 1. Selected geometric parameters of [M(et-BTBP) 2] N24 complexes, calculated at the B3LYP/Stuttgart ECP small- -0.21 -0.67 -core/6-31G(d) level of theory. For the numbers of atoms (triazine, nonbonding) see Fig. N22 (triazine, distant) -0.45 -1.20 Parameter Am Eu N7 (pyridine) -0.54 -1.29 M-N21 (N – triazine) [Å] 2.572 2.549 tr In spite of significant differences in the numer- M-N7 (N py – pyridine) [Å] 2.599 2.565 ical values obtained from the NBO and QTAIM N21-M-N30 [deg] 169.5 167.6 analyses (Table 2), similar conclusions can be drawn from both approaches. The positive charges on the N21N7N13N30 dihedral [deg] 3.5 2.5 central metal ions, much smaller than the nominal value +3, show a significant donation of the elec- Bonding analysis . Two different approaches tron density from the ligands to the metal ions. to the analysis of bonding between central ion and The charge on the central americium ion, lower ligands were used: (i) the analysis of the topology than that on europium, indicates that more elec- of total electron density (QTAIM) and (ii) orbital- tron density was donated from the ligands in the -based NBO analysis. The two approaches led to former complex. Together with the more negative- similar conclusions, and the results from both will ly charged nitrogen donor atoms in the europium be presented altogether. The NBO theory is wide- complex, this makes the Am-N bonds more cova- ly used to interpret electronic structures, mainly in lent than Eu-N ones. organic species but also in transition metal com- The NBO methodology estimates the metal-li- pounds. However, as every orbital-based theory, gand bond orders which may be related to the NBO is based on some arbitrary assumptions, e.g. covalency [10]. The calculated values of selected 3+ exclusion of p-orbitals from highly occupied va- NLMO/NPA bond orders in the [M(et-BTBP) 2] lence shell. The latter has been, however, criti- complexes are shown in Table 3. Also included are cized [13], and the recent answer of the NBO au- the values of two other indicators of covalency, thors [14] seems not fully convincing. In contrast, based on QTAIM theory: (i) the electron density ! the QTAIM theory is based solely on the laws of at bond critical points (BCP), b, used in the re- 46 CENTRE OF RADIOCHEMISTRY AND NUCLEAR CHEMISTRY cent report [8]; and (ii) the delocalization index Perturbative estimation of stabilization energies DI(A,B), based on the topology of total electron arising from electron donation from lone pairs on density distribution, which indicates how many the ligand nitrogen atoms to empty orbitals of the electron pairs are delocalized between atoms A central metal ions shows that the total stabiliza- and B [16]. tion is greater for americium than for europium. The total bond orders, related to all metal-li- In fact, the sum of the calculated stabilization gand bonds in the whole molecules of the Am and energies for the two Am-N bonds in the complex, Eu complexes, are equal to 2.49 and 1.60, respec- 55.6 kcal/mol, is greater than that for the Eu-N tively, in the NLMO approach, while 2.31 and 2.01, bonds, 47.7 kcal/mol (Table 4). The values related respectively, from DI(M,N) data. The agreement to the whole molecules, including corrections for between the NBO and QTAIM trends in bonding beta spins and the eight M-N bonds, are much is remarkable. The calculated electron densities at Table 4. Main (> 2 kcal/mol) stabilizing donor-acceptor the BCPs of the metal-ligand bonds follow the in teractions between lone pair orbital on the nitrogen same trend. The very small values of particular (N21 and N7) atom in the et-BTBP ligand (donor) and M-N bond orders in the complexes indicate that empty orbitals on the Am or Eu cation (acceptor) in the 3+ the metal-ligand bonds are mainly ionic as expect- [M(et-BTBP) 2] complexes. The values shown report to ed for intermediate and heavy actinides and for alpha spins only, and beta spin values are of similar mag- lanthanides. The conclusions on larger covalency nitude. of (i) metal-ligand bonds in the Am than Eu com- Donor Acceptor orbital Interaction energy plexes and (ii) metal-triazine than metal-pyridine M bonds in both complexes remain in agreement atom character [kcal/mol] with the results of the charge distribution analyses Am N21 s 12.2 (Table 2). (triazine) One of the standard tools in NBO analysis is d 11.7 the donor-acceptor interaction analysis. In this d 2.8 scheme, fully localized natural bond orbitals are allowed to interact profitably with virtual NBOs, Total 26.7 provided their spatial relations are proper for the N7 s 9.5 maximum overlapping and their energy differ- (pyridine) ences are not too high. All possible interactions 37.4% d; 62.6% f 5.9 are being analysed and only those are selected d 7.5 which result in stabilization energies exceeding an arbitrarily chosen threshold. The formula for esti- d 6.1 mating these second-order perturbative correc- Total 29.0 tions is as follows: Eu N21 d 17.4 2 F(i, j) (triazine) E(2)" # E " q d 6.3 ij i $ % $ j i Total 23.7 $ $ where: q i – the occupancy of i-th donor NBO; i, j – the energies of donor and acceptor orbitals; F(i,j) N7 d 7.1 (pyridine) – an off-diagonal element of NBO Fock matrix, 46.1% d; 53.9% f 5.9 which reflects the strength of the interaction between the given orbitals. As the result of these in- d 7.2 teractions some charge density flows from the oc- 54.7% d; 45.0% f 3.8 cupied to virtual NBOs, and these new localized orbitals (which now extend from the lone pair to Total 24.1 the central metal ion) are called natural semi-localized molecular orbitals. Table 3 reports these greater, but nevertheless they must be considered 3+ only a rough, qualitative indicator of the relative interaction energies for the M(et-BTBP) 2] complexes (M = Am or Eu). stability of the complexes. The calculations of the energies of complex formation will be dealt with Table 3. Selected NLMO/NPA bond orders, electron den- in the next section. However, an important con- ! sity at the M-N bond critical point ( b) and delocaliza- clusion can be done that the most stabilizing do- tion indexes DI(M,N) between M and N atoms in the 3+ nor-acceptor interactions in the Am complex [M(et-BTBP) 2] complexes. All values presented are aver- aged over the four equivalent nitrogen atoms in both li- stem from delocalization of the ligand lone pairs gands. to the s orbitals of Am, which has not been found in its Eu counterpart. NLMO/NPA Visualization of the occupation of particular Bond ! DI(M,N) bond order b subshells of the central metal ions in the com- Am-N21 (triazine) 0.163 0.0476 0.298 plexes studied has been done by means of natural electronic configuration analysis (Table 5). Am-N7 (pyridine) 0.150 0.0449 0.281 Natural electron configurations of the central 3+ Eu-N21 (triazine) 0.116 0.0439 0.257 metal ions in the cationic [M(et-BTBP) 2] complexes are very similar to those obtained for the Eu-N7 (pyridine) 0.107 0.0423 0.244 neutral [M(et-BTBP)(NO 3)3] species [8], in par- CENTRE OF RADIOCHEMISTRY AND NUCLEAR CHEMISTRY 47

3+ Table 5. Natural charges and natural electronic configurations of Am and Eu ions in the [M(et-BTBP) 2] complexes, calculated using NBO 5.0 methodology.

Natural Number of electrons in core/valence/Rydberg spaces M Natural electron configuration charge core valence Rydberg total Am 1.64 85.98 7.02 0.36 93.36 [core] 7 s(0.29) 5 f(6.09) 6d (0.64) 6 f(0.28) Eu 2.00 53.98 6.97 0.05 61.00 [core] 6 s(0.06) 4 f(6.10) 5 d(0.81) ticular: (i) no significant difference in the popula- Energy calculations . The energies of the Eu tion of the valence 5 f orbitals of Am and 4 f ones of and Am species involved: the bare M 3+ ions, hy- 3+ 3+ Eu; (ii) significant population of virtual NBO-Ryd- drated ions [M(OH 2)9] and [M(et-BTBP) 2] 3+ berg type orbitals (mostly 6 f) in [Am(et-BTBP) 2] complexes have been computed using two differ- and (iii) higher populations of 7 s orbitals of Am ent models of the system – the gas phase and the than 6 s of Eu, and 5 d orbitals of Eu than 6 d of liquid (water) modelled by polarizable continuum Am. The latter observation, in line with the con- model (PCM), and the formation energies of the # clusion on significant delocalization of the ligand complexes studied, Ef,M , were calculated as those lone pairs to the s orbitals of Am (Table 4), can be in ref. [8]. The structures optimized in the gas phase attributed to relativistic effects, in particular to were used. Because the use of the B3LYP/6-31G(d) relativistic stabilization of the Am 7 s vs . Eu 6 s or- basis set led to unreliable formation energy values, bital and destabilization of the Am 6 d vs . Eu 5 d more negative for the Eu than for the Am complex, orbital [17]. Lack of electron populations of the the more extended basis set, B3LYP/6-311G(d,p), valence p orbitals of both metal ions is the result was optionally used (Table 6). of the assumption of the NBO 5.0 methodology For both basis sets, the gas phase model gave # # used for the calculations, that p-type orbitals do unreliable, positive ( Ef)Am/Eu values for the 3+ not belong to the so-called natural minima basis, [M(et-BTBP) 2] complexes, as in the case of the # # –1 3+ Table 6. Differences in the energies of complex formation, ( Ef)Am/Eu [kJ mol ], for [M(et-BTBP) 2] and [M(et-BTBP) # # (NO 3)3] [8] complexes (M = Am or Eu) for various models and various basis sets. (The reliable negative ( Ef)Am/Eu val ues in Table 6 are given in bold.)

Species Model Basis set complex metal ion metal ion in complex in 6-31G(d) 6-311G(d,p)

3+ M(et-BTBP)(NO 3)3 M gas phase gas phase +15.1 - 3+ [M(OH 2)9] gas phase gas phase -0.13 -10.2 3+ $ $ [M(OH 2)9] H2O ( = 78) H 2O ( = 78) -13.1 -20.7

3+ 3+ [M(et-BTBP) 2] M gas phase gas phase +51 - 3+ [M(OH 2)9] gas phase gas phase +36 +11.2 3+ $ $ [M(OH 2)9] H2O ( = 78) H 2O ( = 78) +31 -10.1 therefore they are not significantly occupied. The neutral [M(et-BTBP)(NO 3)3] species [8], which is use of an older NBO version free of such restric- in contrast with the results of bonding analysis and tions, the NBO 3.1 program, led to nearly the with the experiment [1,5]. Only the calculations same populations on the 7 s, 5 f and 6 d orbitals of involving hydrated metal ions as the substrates, Am, except for the population on Rydberg’s 6 f or- and the aqueous medium (polarizable continuum bital which was not occupied; instead the equi- model with $ = 78.4) replacing the gas phase, re- # # valent population of 7 p orbital has been found. sulted in reliable, negative ( Ef)Am/Eu values for However, the old NBO 3.1 procedure does not both pairs of complexes studied (in 6-311G**). properly deal with transition metals; in our case it This result confirms the conclusion that interac- completely failed for europium atom in the cat- tions with the aqueous medium significantly affect ionic complex. Because the reliability of the cur- the energies of the substrates and products of the rent NBO implementation for f-block elements reaction of complex formation [8]. seems to be in doubt, the controversy whether the The analysis has shown that with the differences # # (n) p functions should be included into the va- in the energies of complex formation, ( Ef)Am/Eu lence space of transition metal ions or not [18,19] in the range of -10 kJ mol –1 to -20 kJ mol –1 (as seems to be still unresolved. Therefore, using only those given in Table 6), the calculated separation the natural electronic configuration analysis we factor, SF Am/Eu [8], is well comparable with the ex- can hardly conclude on the role of the p and f or- perimental value, SF Am/Eu = 160 ± 16 [1]. bitals in the metal-ligand bonding. On the other The work has been carried out within the col- hand, the role of the relativistically stabilized 7 s laborative project “Actinide Recycling by Separa- orbital of americium in strengthening the Am-N tion and Transmutation” (ACSEPT), FP7 – Eur- bonds must be emphasized. atom Fission, No. FP7-CP-2007-211 267. 48 CENTRE OF RADIOCHEMISTRY AND NUCLEAR CHEMISTRY References port 2009. Institute of Nuclear Chemistry and Tech- nology, Warszawa 2010, pp. 35-39. [1]. Drew M.G.B., Foreman M.R.S., Hill C., Hudson M.J., [9]. Gaussian 09. Revision B.01. Gaussian Inc., Walling- Madic C.: Inorg. Chem. Commun., 8, 239-241 (2005). ford CT 2009. [2]. Madic C. et al. : EUROPART. European research [10]. Reed A.E., Curtiss L.A., Weinhold, F.: Chem. Rev., programme for the partitioning of minor actinides 88, 899-926 (1988). within high active wastes issuing from the reprocess- [11]. Bader R.W.F.: Chem. Rev., 91, 893-928 (1991). ing of spent nuclear fuels. In: Proceedings of the Con- [12]. AIMAll (Version 10.12.13). Keith T.A. 2010 (aim.tk- ference FISA-06, Luxembourg, 13-16 March 2006. gristmill.com). [3]. Guillaumont D.: J. Phys. Chem. A, 108, 6893-6900 [13]. Maseras F., Morokuma K.: Chem. Phys. Lett., 195, (2004). 500-504 (1992). [4]. Petit L., Adamo C., Maldivi P.: Inorg. Chem., 45, [14]. Landis C.R., Weinhold F., J., Comput. Chem., 28, 8517-8522 (2006). 198-203 (2007). [5]. Foreman M.R.S., Hudson M.J., Drew M.G.B., Hill [15]. Macchi P., Sironi A.: Coord. Chem. Rev., 238/239, C., Madic C.: Dalton Trans., 1645-1653 (2006). 383-412 (2003). [6]. Trumm S., Lieser G., Foreman M.R.S., Panak P.J., [16]. Macchi P., Sironi A.: Chapter 13. In: The quantum Geist A., Fanghänel T.: Dalton Trans., 39, 923-929 theory of atoms in molecules. Eds. C.F. Matta and (2010). R.J. Boyd. Wiley 2007. [7]. Retegan T., Berthon L., Ekberg C., Fermvik A., Skar- [17]. Siekierski S., Burgess J.: Concise chemistry of the nemark G., Zorz N.: Solv. Extr. Ion Exch., 27, ele ments. Horwood Publishing, Chichester 2002, pp. 663-682 (2009). 40-42. [8]. Oziminski W.P., Narbutt J.: Theoretical investigations [18]. Frenking G., Frölich N.: Chem. Rev., 100, 717-774 on the structure and bonding in neutral trinitrate (2000). complexes of americium(III) and europium(III) with [19]. Pyykkö P.: J. Organomet. Chem., 691, 4336-4340 6,6’-bis-(5,6-diethyl-1,2,4-triazin-3-yl)-2,2’-bipyridine (2006). in solvent extraction systems. In: INCT Annual Re-

VITRIFICATION OF NUCLEAR WASTES BY COMPLEX SOL-GEL PROCESS Andrzej Deptuła, Magdalena Miłkowska, Wiesława Łada, Tadeusz Olczak, Fabio Zaza 1/ , Andrzej G. Chmielewski 1/ Italian National Agency for New Technologies, Energy and Environment (ENEA), CR Casaccia, Rome, Italy

Vitrification of hazardous nuclear wastes is an at- ed at approximately 800 oC. Formation of any crys- tractive technological alternative suitable to be talline phase at this step is reflected in DTA curves. adapted for more effective management of spent This conclusion is confirmed by XRD patterns fuel and radioactive waste. The advantage of the which indicate that the elements studied are inte- method is that a large number of chemical ele- grally bonded with the silicate glass structures ments that can be incorporated into the glass, within the investigated temperature range. forming a highly durable and small volume waste The infrared spectra show conclusively that or- [1,2]. Recently, sol-gel techniques was successfully ganic impurities do not exist in detectable amounts o used for the preparation of porous glasses – host at 70 C. H 2O and OH groups are identified, but matrices for nuclear wastes components [3,4] . In they are not observed at higher temperatures (ex- the present study, our complex sol-gel process (CSGP) patented in 1997 [5], has been developed to synthesize silica glasses capable to host high- -level nuclear wastes. As a model, elements/ions Cs, Sr, Co, and Nd (model for actinides), denoted Me, were tested. The Me:SiO 2 molar ratio was 1:10. Gels in the form of powders and/or monoliths were prepared by hydrolysis and subsequent poly- condensation of silicon tetraethoxide (TEOS)/Me nitrate solutions containing ascorbic acid as cata- lyst, instead of HCl or NH 4OH routinely used in glass synthesis (see flow chart in Fig.1). Thermal decomposition of gels (dried under vacuum at 70 oC in Rotavapor) is illustrated in Fig.2. Infrared spectra and XRD patterns of gels and doped silicate glasses led to the conclusions that in dried gels no organic compounds were present (IR data), the continuous mass losses being presuma- bly connected with the detachment of water and OH groups. The weight loss becomes slower over Fig.1. Flow-chart of CSGP for preparation Cs-, Sr-, Co-, 350 oC, and the thermal decomposition is complet- and Nd-doped silica gels.