Substituent effects on BTP's basicity and complexation properties with Ln^III lanthanide ions

Abstract

Bis(1,2-4 triazin-3-yl)pyridines “BTPs” represent a new class of N-donor extracting agents that separate trivalent actinides and lanthanides from nuclear waste solutions with high An/Ln separation factors. We report here QM calculations on the effect of R_para–BTP substituents on the protonation and complexation energies of these ligands (1 ∶ 1 and 1 ∶ 3 complexes with Ln^III lanthanides) in the gas phase. Both processes follow similar trends and are highly sensitive to the electron donor/acceptor character and polarizability of R. When compared to R–pyr analogues with pyridine, R–BTPs are found to be intrinsically much more basic, by ca. 20 kcal mol⁻¹. In aqueous solution, however (modelled by the continuum PCM model), BTPs and pyridines have a similar basicity, pointing to the importance of solvent environment on their protonation states. In the optimized Ln(R–BTP)₃³⁺ complexes with Ln = La, Eu, Yb, complexation energies E_c3 increase with the intrinsic basicity of the ligands, in the order R = NMe₂ > NH₂ > OMe > C₆H₅ > ^tButyl > Me > H > F > Cl. Furthermore, comparison of complexes with different Ln^III cations indicates that their stability increases in the order La^III < Eu^III < Yb^III, by the same amount with the different R-substituents. The relative contributions of central pyridinyl and lateral triazinyl nitrogens of BTPs are shown to depend on the stoichiometry of the complex and on the Ln^III size, possibly contributing to the subtle An^III/Ln^III discrimination by substituted BTPs.

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1. Introduction

Heterocyclic N-donor ligands L based on the bistriazinylpyridine (BTPs) skeleton have been recently developed to separate M^III trivalent actinides from lanthanide ions by liquid–liquid extraction from nuclear solutions.^1–3 For example, ⁿPrBTP extracts Am^III from water to an alkane/2-ethylhexanol mixture with a distribution coefficient D_Am of 300 and a separation factor SF_Am/Eu of 130. Extracted complexes are of ML₃³⁺ type where the cation, well shielded from solvent and counterions by three BTP ligands, is coordinated to nine soft N-atoms,^4–8 possibly explaining the preference for actinide over the harder lanthanide cations.^9–11BTP derivatives with lateral n-alkyl chains are degraded under acidic and radiolytic conditions, but their CyMe₄ derivatives (2,6-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo[1,2,4]triazine-3-yl) pyridines; see Fig. 1) are more stable while retaining satisfactory performance.¹² So far, little is known about the basicity and complexation ability of BTPs as a function of their substituents. This led us to undertake quantum mechanical “QM” studies on substituted R–BTPs to investigate the intrinsic effect of their R_para–pyridine substituent (R = NMe₂, NH₂, OMe, C₆H₅, ^tButyl, Me, H, F, Cl) in the gas phase. As a reference, we consider the protonation of R–pyr analogues in the pyridine series, well documented by experiment in the gas phase and in aqueous solution ^13,14 and by simulations.^15–17 Solvation effects play a major role in proton transfer, reactivity and complexation equilibria.^13,18,19 When used as extractant molecules, BTPs are solubilized in the organic phase, in equilibrium with acidic aqueous solutions. This medium is more polar than an organic phase based on e.g.alkanes where basicity and complexation processes are more “gas phase like”. It is however less polar than pure water (where BTPs are quasi insoluble). Thus, in order to gain insights into solvation effects on the BTPs basicity, QM calculations were also performed “in aqueous solution” to mimic a polar phase. In the following, unless otherwise indicated, “BTP” corresponds to the Me₄–BTP derivatives with four lateral Me groups. The unsubstituted H₄–BTP and the CyMe₄–BTP derivatives (see Fig. 1) will also be tested for comparison.

Fig. 1 Studied R–BTP(H⁺) (cis–cis form) and R–pyridine(H⁺) analogues (R = H, Cl, F, ^tBut, Me, C₆H₆, OMe, NH₂, NMe₂) and CyMe₄BTP.

2. Methods

All structures were optimized at the DFT/B3LYP level of theory using the Gaussian03 software.²⁰ The H, C, N, O, F and Cl atoms of the ligands were described by the 6-31G(d,p) basis set.²⁰ As f-orbitals do not play an important role in metal–ligand bonding²¹ the 46 core and 4f electrons of the La^III, Eu^III and Yb^III lanthanides were described by quasi-relativistic effective core potentials (ECP) of the Stuttgart group.²² For the corresponding valence orbitals the affiliated (7s6p5d)/[5s4p3d] basis set was used²³ enhanced by an additional single f-function with an exponent of 0.591.²⁴

QM calculations “in aqueous solution” were conducted with the PCM model where the aqueous solvent is represented by a polarizable continuum medium with a dielectric constant of 80.²⁵ Spheres delineated by solute atoms are defined from standard atomic radii implemented in Gaussian03, and the solvent probe sphere has a radius of 1.385 Å.²⁰ This model does not account for specific (e.g. H-bonding) interactions with the solvent and cannot thus pretend to quantitatively predict basicities in solution, but allows for consistent comparisons within a series as a function of the R substituents. Total energies are given in Tables S1 and S2 (ESI†). For related theoretical studies on N-containing aromatic heterocyclic molecules, see e.g.ref. 15, 17 and 26.

3. Results

3.1 Protonation of R–BTPs and R–pyridines in the gas phase

Protonation energies E_Prot were calculated from eqn (1):

E_Prot: R–BTP + H⁺ → R–BTPH⁺ (1)

BTP and BTPH⁺ can adopt three planar conformations, depending on the cis/trans orientation of the lateral rings with respect to the N-pyridine lone pair or proton: cis–cis (as sketched in Fig. 1), ct or tt. While for the neutral H₄–BTP, trans–trans is more stable than cis–cis (by 3.0 kcal mol⁻¹), the protonated BTPH⁺ clearly prefers to be cis–cis.¹² Regarding the N₁versus N₂ sites of protonation, the former is calculated to be preferred (by 7.3 kcal mol⁻¹) and will be considered throughout the study.

The E_Prot energies reported in Table 1, calculated from the cc forms show that in the gas phase, basicities of R–BTP and R–pyr are modulated by the para-pyridine substituent R and follow the same order in both series. The more attractor R is (Cl and F), the less basic R–BTP is. Conversely electron donor groups (OMe, NH₂ or NMe₂) enhance the basicity of R–BTP. Analysis of substituent effects in terms of several components (e.g. effects of field, electronegativity, polarizability, π-resonance)²⁷ is beyond the scope of this paper. However, the importance of polarization²⁸ can be seen in the higher basicity of Cl–BTP compared to F–BTP, and of NMe₂–BTP compared to NH₂–BTP in the gas phase. Likewise, replacing R = H by alkyl or aryl groups enhances E_Prot, by up to 7.9 kcal mol⁻¹. Regarding the largest ΔE_Prot difference (from Cl to NMe₂ substituent), it is somewhat lower in the BTP series than in the Pyr series (16.5 and 22.7 kcal mol⁻¹, respectively), presumably due to dilution of the positive charge in the lateral rings of BTPH⁺s. For R–pyridines, the corresponding difference of experimental proton affinities at 300 K in the gas phase is somewhat smaller (17.9 kcal mol⁻¹)¹⁴ than ΔE_Prot. More quantitative agreement with experiment would require using larger basis sets with diffuse functions.^17,26 Note that the E_Prot energies correspond to molecules “at rest” in their energy minimum, whereas experimental proton affinities include thermodynamic contributions of rotational, vibrational and translational motions. Calculating the latter with Gaussian03 yields protonation enthalpies ΔH_Prot in the gas phase (see Table 1 for typical cases).²⁹ Comparing Cl–Pyr to NH₂–Pyr, one obtains a ΔΔH_Prot difference of 22.4 kcal mol⁻¹, differing by only 0.3 kcal mol⁻¹ from the ΔE_Prot energy. Likewise, in the R–BTP series, the ΔE_Prot difference between Cl–BTP and NMe₂–BTP (17.0 kcal mol⁻¹) is close to the calculated ΔΔH_Prot difference (16.1 kcal mol⁻¹), indicating that the calculated changes of E_Prot energies in the gas phase as a function of R are meaningful.

Table 1 Protonation energies E_Prot (in kcal mol⁻¹) of R–BTP's (cis–cis forms) and of R–pyridines

R	R–BTP			R–pyridines
	E Prot	ΔEProt^a	ΔHProt^c	E Prot	ΔEProt^a	ΔHProt^c	PA exp^d
a Difference with respect to the unsubstituted H–BTP or H–pyridine. b Protonation of the NH2 group instead of the N-ring atom is calculated to be less stable by 37.5 kcal mol^−1. c Calculated protonation enthalpies, assuming an enthalpy of 1.5 kcal mol^−1 for H^+.^29 d Experimental proton affinities, based on PA(NH3) = 205.5 kcal mol^−1.^14
Cl	−258.9	3.5	−251.9	−231.5	4.1	−224.3	217.8
F	−259.8	2.6		−232.2	3.4		217.2
H	−262.4	0	−255.4	−235.6	0	−228.3	220.4
Me	−265.4	−2.9		−240.1	−4.5		223.7
^tBut	−266.8	−4.4		−242.5	−6.9
C6H6	−267.7	−5.3		−243.5	−7.9
OMe	−268.1	−5.6		−243.8	−8.2		226.8
NH2	−272.7^b	−10.3		−250.2	−14.6		231.0
NMe2	−275.9	−13.5	−268.0	−254.2	−18.6	−246.7	235.7

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Lateral substituents borne by the two triazinyl rings also markedly influence the BTP basicity: from H₄–BTP to Me₄–BTP, E_Prot increases in magnitude by 11.5 kcal mol⁻¹. Likewise, from Me₄–BTP to CyMe₄–BTP, E_Prot increases further by 2.3 kcal mol⁻¹.

3.2 Relative basicities of R–BTPs and R–pyridines in PCMwater solvent

Protonation energies E_Prot–Wat of R–BTP and R–pyr calculated “in water” with the PCM model are given in Table 2.

Table 2 Protonation energies (in kcal mol⁻¹) of pyridine and Me₄BTP derivatives in aqueous solution. Calculated E_Prot–Wat energies in PCM water and experimental values for R–pyridines

R	R–pyridine					R–BTP		ΔEProt–WatBTP/Pyr
	E Prot–Wat ^a	ΔEProt–Wat	pKa^b	ΔGexp^c	ΔΔGexp	E Prot–Wat ^a	ΔEProt–Wat
a Calculated PCM values have been corrected by subtracting the ΔHgas→water(H^+) enthalpy of −269.8 kcal mol^−1 (from ref. 14). b Experimental values. From V. A. Palm, Tables of Rate and Equilibrium Constants of Heterolytic Organic Reactions, Academy of Science of the USSR, Moscow, 1976, vol. 2. c From ref. 14.
Cl	−19.1	1.8	3.8	−5.2	1.9	−16.5	3.0	2.6
F	−20.4	0.5				−17.7	1.8	2.7
H	−20.9	0.0	5.2	−7.1	0.0	−19.5	0.0	1.4
Me	−23.5	−2.6	6.0	−8.1	−1.0
^tBut	−23.4	−2.5				−20.3	−0.8	3.1
C6H6	−23.3	−2.4				−21.4	−1.9	1.9
OMe	−24.8	−3.9		−9.0	−1.9	−21.9	−2.4	2.9
NH2	−26.5	−5.6	9.1	−12.4	−5.3	−28.0	−8.5	−1.5
NMe2	−29.7	−8.8		−12.9	−5.8	−27.0	−7.5	2.7

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In the pyridine series, basicities follow the same order of R-substituents in water as in the gas phase, i.e.Cl–Pyr is least basic and NMe₂–Pyr is most basic. The energy gap ΔE_Prot–Wat between these compounds (10.6 kcal mol⁻¹) is about half of that in the gas phase (22.7 kcal mol⁻¹). Likewise, in the BTP series, the basicity order is similar to the one in the gas phase, with one exception in the case of NH₂/NMe₂ order that is reversed in solution. The calculated energy range ΔE_Prot–Wat (11.5 kcal mol⁻¹) between Cl– and NMe₂– derivatives is similar to the one in the pyridine series, and also about half the range calculated in the gas phase. These features are fully consistent with experimental observations in the pyridine series where, according to Aue et al., “the gas- and aqueous-phase free energies of protonation correlate roughly with one another, with an approximate twofold attenuation of the gas-phase substituent effects in the aqueous basicities”.¹⁴ A similar correlation between basicities in the gas-phase and in acetonitrile solution has been observed.³⁰ Regarding the NH₂– and NMe₂– substituted pyridines, they are known to deviate from the linear relationship between water solution and gas phase complexation enthalpies.³¹

Comparing different types of bases (BTPs versuspyridines) is a more difficult task.³¹ Before doing so, we decided to test the performance of the PCM model on a challenging case, NH₃versusaniline, where relative basicities in water are opposite to those in the gas phase.¹⁸ According to our simulations, aniline is indeed intrinsically more basic than NH₃ (E_Prot = −223.5 and −218.6 kcal mol⁻¹, respectively, in the gas phase). The difference of 5.9 kcal mol⁻¹ is comparable to the difference in experimental proton affinities in the gas phase (6.9 kcal mol⁻¹).³² In PCM aqueous solution, the order is inversed and NH₃ is calculated to be more basic than aniline (by 13.4 kcal mol⁻¹). Agreement with experiment is only qualitative however, since the pK_a difference in water (9.25–4.62) translates to a free energy difference of 6.3 kcal mol⁻¹, about half of the PCM value. Thus, care should be taken when comparing different types of bases with this model.³³

Comparing however the PCM-calculated basicity of R–BTPs versus R–pyrs with the same R-substituent indicates that “in water”, both have similar basicities. They differ by only 1.4 to 3.1 kcal mol⁻¹, depending on R (Table 2), indicating that BTPs and pyridines should have similar basicities. This feature can be understood from the lesser proton solvation in R–BTPH⁺, compared to R–pyrH⁺ where the proton is more accessible to the solvent and therefore better solvated. Thus, the relative basicities of R–BTP and R–pyr markedly depend on the solvent environment. In a dry apolar organic phase of low dielectric constant ε (e.g.alkanes with ε ≈ 2), relative basicities should be similar to those in the gas phase, i.e. BTPs should be much more basic than pyridines. In the extreme case of aqueous solutions, BTPs are calculated to be slightly less basic. In reality, in liquid–liquid extraction systems, the organic phase is heterogeneous and contains water, extractants, synergistic agents (e.g. diamides, alcohols), acid and extracted complexes.⁵ In such media or at water–oil interfaces, basicities are expected to be intermediate between those in the gas phase and in aqueous solution, and are hardly predictable from simulations only. Furthermore, as seen above, the precise nature of BTPs (e.g. lateral substituents) also markedly influences their basicity, calling for care with general conclusions. Anyway, assuming that BTP's basicities are comparable to those of pyridines, they should be protonated in the presence of nitric acid solutions.

Coming back to the R-substituent effect in the liquid–liquid biphasic extraction systems, it is expected to be somewhat larger in the organic phase than in water, but smaller than in the gas phase.

3.3 Lanthanide 1 ∶ 1 and 1 ∶ 3 complexes of R–BTPs

We considered 1 ∶ 1 and 1 ∶ 3 complexes of R–BTPs with Ln^III ions (Fig. 2) to gain insights into the effect of the R-substituent on the corresponding complexation reaction energies E_c1 and E_c3 in the gas phase defined in eqn (2) and (3) and reported in Tables 3 and 4. Snapshots of 1 ∶ 3 complexes are shown in Fig. S1 (ESI†).

Ln(NO₃)₃ + R–BTP → Ln(NO₃)₃R–BTP (2)

Ln³⁺ + 3 R–BTP → Ln(R–BTP)₃³⁺ (3)

Complexation energies. Complexation of different R–BTPs by Ln(NO₃)₃ to form 1 ∶ 1 complexes has been studied for Eu^III and La^III ions. The corresponding E_c1 energies range from −40.4 (Cl–BTP) to −47.0 kcal mol⁻¹ (NMe₂–BTP), indicating that the R–BTP ligands are attracted by the neutral complex, by different amounts depending on the R-substituent. The range ΔE_c1, 9.5 kcal mol⁻¹, is about half the corresponding ΔE_Prot range in the gas phase. Complexation energies E_c1 roughly follow the same order as the basicities, with exceptions, though, in the case of alkyl- and aryl-substituted BTPs that form less stable complexes than expected from a linear correlation (Fig. 3). As mentioned above, these R-groups are polarizable, and polarization effects are indeed weaker upon complexation of R–BTP to the neutral species Ln(NO₃)₃ than upon protonation.

Fig. 2 Simulated 1 ∶ 1 Ln(NO₃)₃(R–BTP) complexes (Ln = La, Eu) and 1 ∶ 3 Ln(R–BTP)₃³⁺ complexes (Ln = La, Eu, Yb).

Fig. 3 E _c1 (top) and E_c3 (bottom) complexation energies (Eu^III complexes; in kcal mol⁻¹) as a function of the protonation energies E_Prot.

Table 3 E _c1 complexation energies (in kcal mol⁻¹) of Ln(NO₃)₃ with R–BTPs to form 1 ∶ 1 complexes

R	Eu		La
	E c1	ΔEc1	E c1	ΔEc1
Cl	−40.4	3.9	−40.2	3.9
F	−42.8	1.6	−42.5	1.6
H	−44.3	0.0	−44.1	0
Me	−44.8	−0.5	−44.5	−0.5
^tBut	−43.9	0.5	−43.6	0.5
C6H6	−43.5	0.9	−43.2	0.9
OMe	−44.8	−0.5	−44.5	−0.5
NH2	−46.7	−2.4	−46.7	−2.3
NMe2	−47.0	−2.6	−46.7	−2.6

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Table 4 E _c3 complexation energies (in kcal mol⁻¹). Difference ΔE_c3 with respect to the unsubstituted H–BTP. ΔE_Eu/La and ΔE_Eu/Yb are the differences with respect to the Eu^III complexes

R	La		Eu			Yb		ΔEEu/La	ΔEEu/Yb
	E c3	ΔEc3	E c3	ΔEc3	E c3 PCM ^a	E c3	ΔEc3
a Calculated using the PCM energy of −34.9299 u.a. for “naked” Eu^III.
Cl	−608.1	17.4	−656.3	17.5	−173.1	−696.8	17.6	48.2	−40.5
F	−614.4	11.1	−662.5	11.3		−703.0	11.4	48.1	−40.5
H	−625.5	0.0	−673.8	0.0		−714.4	0.0	48.3	−40.6
Me	−632.8	−7.3	−681.1	−7.3		−721.7	−7.3	48.3	−40.6
^tBut	−634.2	−8.7	−682.6	−8.8		−723.2	−8.8	48.3	−40.6
OMe	−637.3	−11.8	−685.7	−11.9		−726.3	−11.9	48.3	−40.6
NH2	−650.7	−25.2	−701.0	−27.2		−739.7	−25.3	50.3	−38.8
NMe2	−657.9	−32.4	−706.4	−32.5	−189.8	−747.0	−32.6	48.5	−40.7

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Complexation energies E_c3 corresponding to the 1 ∶ 3 complexes range from −608 to −747 kcal mol⁻¹ (see Table 3). They are much larger in magnitude than the E_c1 energies, because each ligand formally interacts with a +3 charged instead of a neutral species, and polarization effects in the ligand should be larger in 1 ∶ 3 complexes.³⁴ As a result, for the La^III, Eu^III and Yb^III complexes, the order of E_c3 energies exactly fits the order of protonation energies E_Prot, as sketched in Fig. 3. As for the 1 ∶ 1 complexes, the least basic Cl–BTP ligand forms the least stable 1 ∶ 3 complex, whereas the most basic NMe₂–BTP ligand forms the most stable complex. The energy difference between these complexes, ca. 50 kcal mol⁻¹, is about twice the difference ΔE_Prot in protonation energies, indicating that substituent effects are magnified in complexation compared to protonation processes in the gas phase.

Comparing now the different lanthanide cations, one sees that, for a given R–BTP ligand, the E_c3 complexation energies decrease in magnitude in the order Yb^(III) > Eu^(III) > La^(III), i.e. as the lanthanide cation gets bigger and less “hard”. At a more quantitative level, the preference for Eu^III over La^III is quasi independent on the R-substituent (except for R = NH₂): ΔE_Eu/La = 48.1 to 48.5 kcal mol⁻¹. Likewise, the preference for the hardest Yb^III cation over Eu^III is quasi independent on R (ΔE_Eu/Yb = −40.5 to −40.7 kcal mol⁻¹). Thus, changing the R-substituent should have a minor effect on the selectivity of a given R–BTP towards different Ln^III ions.

Structural features of the complexes. Typical Ln–N distances reported in Tables S3 (1 ∶ 1 complexes) and S4 (1 ∶ 3 complexes) in ESI† reveal several interesting features. First, as expected, all Ln–N bonds shorten when the complex gets more stable, i.e. as the cation gets smaller, in the order La > Eu > Yb. This is consistent with previous computational and experimental results (see ref. 35 and references cited therein). Regarding the R-effect on Ln–N bonds, in both types of complexes, Ln–N bonds shorten when the interaction gets stronger. Ln–N₁ bond with the central pyridine ring shortens by ca. 0.04 Å along the R-series (from Cl to NMe₂ groups), whereas the Ln–N₂ bonds with the lateral rings decrease less (by ca. 0.01 Å), as expected. Generally, for a given Ln^III ion and R–BTP ligand, Ln–N₁ bonds are shorter in the 1 ∶ 3 than in the 1 ∶ 1 complexes, in keeping with the stronger cation–ligand interactions in the former. Comparing now the Ln–N₁ to the Ln–N₂ bonds reveals different trends, depending on the Ln^III cation size and on the stoichiometry of the complex. In 1 ∶ 1 complexes, Ln–N₁ bonds are longer than Ln–N₂, indicating a weaker contribution of the central pyridine ring, compared to the lateral rings to the ligand binding, and this feature is most pronounced with the biggest cation. In the 1 ∶ 3 complexes, the relative bond “ordering” depends on the cation size. With the Yb^III and Eu^III cations, Ln–N₁ distances are shorter than the Ln–N₂ distances whatever the R-substituent is. With the biggest ion La^III, the order depends on R. For the best ligands (R = NMe₂ or NH₂), La–N₁ is shorter than La–N₂, whereas for the other BTP derivatives, La–N₁ is longer. This analysis makes clear that the relative contributions of the pyridine and triazinyl rings (and their derivatives) depend on the cation size and hardness, a feature that may contribute to discriminate actinideversus lanthanide ions.^36,37

Electronic features of the complexes. The evolution of Mulliken atomic charges q_i as a function of the R-substituent, the stoichiometry of the complex and the nature of Ln^III are quite interesting (see Tables S3 and S5, ESI†). First, for a given Ln^III ion, the q_Ln charge is quasi constant in the R-series. For 1 ∶ 1 complexes, q_Eu = 1.37 to 1.38 e, and q_La = 1.48 e. For 1 ∶ 3 complexes, q_La = 1.66 to 1.67 e, q_Eu = 1.56 to 1.57 e and q_Yb = 1.54 to 1.55 e. Thus, when the R-group evolves from acceptor to donor, the electron transfer to the Ln^III ion remains little affected. The increased stability of the complex along the R-series seems to be rather due to polarization of the ligand, in the direction R^δ+⋯N^δ− for the pyridine moiety. Indeed, the q_N1 charge is more negative in the complexes (−0.55 to −0.65 e) than in the corresponding free ligands (−0.51 to −0.55 e). Furthermore, in 1 ∶ 3 and 1 ∶ 1 complexes of a given Ln^III ion, when R is changed from Cl to NMe₂, the q_N1 charge becomes more negative (by ca. 0.05 e), whereas q_N2 remains quasi constant. Regarding the q_Ln charges for different Ln^III ions, they decrease in the order La > Eu > Yb for each type of complex, indicating increased ligand to cation electron transfer in the lanthanide series as the complex becomes more stable. Comparing now q_Ln in the 1 ∶ 1 versus 1 ∶ 3 complexes, it is found to be lower in the former (by ca. 0.20 e with Eu^III) as expected from the coordination of negatively charged nitrate ligands in Ln(NO₃)₃R–BTP complexes, instead of neutral ligands in Ln(BTP)₃³⁺ complexes.

4. Discussion and conclusions

We report QM investigations on the effect of R-substituents on the basicity and complexation properties of R–BTPs, an important class of molecules that can discriminate between Ln^III and An^III ions, and thus of high potential for nuclear waste partitioning by liquid–liquid extraction. Intrinsically (in the gas phase), the basicity order of R–BTPs is the same as with R–pyridines, but R–BTPs are found to display a much higher protonation energy (by more than 20 kcal mol⁻¹) and should thus be more easily protonated in apolar or weakly polar media. In aqueous solution, mimicked by the simple PCM-continuum model, the basicity sequence of R–BTPs is calculated to be the same as for R–pyr and also the same as in the gas phase (with one exception when R = NH₂). R–BTPs are found to be somewhat less basic than R–pyr analogues, and those with alkyl, aryl or donor R-groups should thus be protonated in acidic aqueous solutions and, a fortiori, in organic solutions in contact with such phases.

Optimizations of 1 ∶ 3 complexes Ln(R–BTP)₃³⁺ in the gas phase reveal a quasi-linear relationship between intrinsic basicities of R–BTPs and complexation energies with trivalent lanthanides. In the 1 ∶ 1 complexes, BTPs with alkyl and aryl substituents deviate from the linear correlation, presumably because of different contributions of polarization effects in protonation versus complexation processes.

The studied alkyl lateral substituents R′_lat borne by triazinyl rings also markedly influence the basicity of BTPs and should therefore also affect their complexation properties, as shown by Guillaumont (when R′_lat = CH₃, OCH₃ and CN).⁹ What happens in solution cannot be predicted by QM simulations alone,³⁸ but the latter point to the versatile acido-basic and complexing character of BTPs as a function of their substituents and call for experimental studies along these lines. In fact, recent experimental studies fully support the predicted correlation between the basicity of BTPs and their complexation properties: in the R–BTP series with R = Cl, H and OMe, the basicity of the ligand increases (estimated pK_as of the protonated forms in ethanol are 2.1, 3.0 and 3.4, respectively) while the corresponding distribution ratio for Eu^III extraction from water to an organic phase increases (from 0.002 to 0.59), indicating an increased stability of the Eu(R–BTP)₃³⁺ complex.^39,40 Furthermore, it was found that increasing the BTP basicity enhances the An/Ln separation factor (by a factor of ca. 4 when R is changed from Cl to OMe),³⁹ a key feature in the context of nuclear waste partitioning. Beyond the BTP series, such a correlation can be important for other polyaromatic N-donor based extractants with pyridine moieties like the bis-triazine-bipyridine (BTBP) derivatives that display remarkable actinide/lanthanide discrimination.^41,42

Acknowledgements

The authors are grateful to IDRIS, CINES, Université de Strasbourg and GNR PARIS for computer resources, and to Etienne Engler for assistance. Support from the ACSEPT FP7 EEC collaborative project and Training Grant for GB is greatly acknowledged.

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34. In aqueous solution, the complexation energy would be much smaller, due to the desolvation energy of the “free” Ln^III ion (see e.g. L. Maron and D. Bourissou, Organometallics, 2009, 28, 3686 ). The experimental dehydration enthalpy of Eu^III is 839 kcal mol⁻¹ ( I. Marcus, Ion Solvation, Wiley, New York, 1985).
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37. Note that according to EXAFS data and QM calculations on M(BTP)₃³⁺ complexes with M = Eu and Cm, however, there is no significant difference between M–N₁ and M–N₂ bond distances with both cations ( M. A. Denecke, A. Rossberg, P. J. Panak, M. Weigl, B. Schimmelpfennig and A. Geist, Inorg. Chem., 2005, 44, 8418 ).
38. As for protonation energies, substituent effects are expected to be smaller in solution than in the gas phase. According to our QM calculations, the difference in complexation energies E_c3 for NMe₂–BTPversusCl–BTP to form Eu(R-BTP)₃³⁺ complexes drops from 50.0 kcal mol⁻¹ in the gas phase to 16.7 kcal mol⁻¹ in PCM–water (see Table 4).
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Footnote

† Electronic supplementary information (ESI) available: Total energies of neutral, protonated and complexed ligands and structural parameters of the complexes. See DOI: 10.1039/c0nj00527d

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