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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

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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 chem- ical bonding itself. In Table 2 we present the atomic charges on selected atoms, obtained by both NBO-based na- tural population analysis (NPA) which can be con- sidered an “improved Mulliken analysis”, and by QTAIM method which integrates the electron den- sity within atomic basins separated by zero-flux sur- face 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.
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