Article pubs.acs.org/JPCA
Can Functionalized Cucurbituril Bind Actinyl Cations Efficiently? A Density Functional Theory Based Investigation Mahesh Sundararajan,*,† Vivek Sinha,‡ Tusar Bandyopadhyay,† and Swapan K. Ghosh*,†,§ †
Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400 085, India. Department of Physical Sciences, Indian Institute of Science Education and Research, Kolkata 700 064, India § Homi Bhabha National Institute, Mumbai 400 094, India ‡
S Supporting Information *
ABSTRACT: The feasibility of using cucurbituril host molecule as a probable actinyl cation binders candidate is investigated through density functional theory based calculations. Various possible binding sites of the cucurbit[5]uril host molecule to uranyl are analyzed and based on the binding energy evaluations, μ5-binding is predicted to be favored. For this coordination, the structure, vibrational spectra, and binding energies are evaluated for the binding of three actinyls in hexa-valent and penta-valent oxidation states with functionalized cucurbiturils. Functionalizing cucurbituril with methyl and cyclohexyl groups increases the binding affinities of actinyls, whereas fluorination decreases the binding affinities as compared to the native host molecule. Surprisingly hydroxylation of the host molecule does not distinguish the oxidation state of the three actinyls.
1. INTRODUCTION Designing novel ligands for the complexation of various actinide species has gained significant attention because of their importance in nuclear waste management.1 Although some nitrogen,2 phosphorus3 and oxygen containing ligands4−6 are known, chemists are making efforts to design new ligands such that the binding affinity and the selectivity of a particular actinide as compared to others can be increased. Recently, it has been recognized that macrocyclic ligands can be used for selective binding of actinide elements because of their less geometric relaxation of the macrocyle upon metal ion binding.7 Of the many known macrocyclic host molecules, crown ethers and porphyrins are known to bind metal ions efficiently.1 However, their binding affinity toward actinyl ions such as uranyl is poor.8−10 For instance, crown-[6] macrocyle binding toward uranyl species is unfavorable. Further, Clark et al. reported the selective binding of Np(V) to crown ether.7 Later, through density functional theory (DFT) calculations, Schreckenbach et al. suggested the preferential binding of Np(V) to be partly due to solvation.8 Further, Sessler and co-workers reported the binding of uranyl to porphyrin macrocycles can be improved upon by expanding the cavities of regular porphyrins which are known as alaskaphyrins.1,2 Nearly three decades ago, Freeman et al. reported a new host molecule whose geometric structure is similar to those of pumpkin, hence referred as Cucurbituril (CB) (Figure 1).11 Like crown ether and cyclodextrin, depending upon the number of monomer units, CB can be classified as CB-[5], CB-[6], CB-[7], CB-[8], and CB-[10] respectively. Further, CB possesses hydrophilic portals which can bind cations, whereas the presence of hydrophobic core facilitates the binding of drug © 2012 American Chemical Society
Figure 1. Structure of CB-[5].
molecules. Unlike the lower CB members (CB-[5] and CB[6]), the higher analogues are somewhat flexible and can adopt an oval shape. The lower analogues of CB host molecules are less soluble in aqueous environement, and hence complexation reactions in pure aqueous phase for the lower analogues are limited. Fortunately due to the pioneering work of Kim et al., the solubility problems of CB-[5] and CB-[6] can be overcome by functionalizing CB with hydroxo and cyclo-hexyl groups.12−14 Further, both methylation and partial functionalization of CB are also known.12−14 As far as actinide complexation with CB is concerned, Theury reported a series of framework type materials using CB[6] and CB-[7] as ligands partially coordinated to uranyl.15,16 Received: February 15, 2012 Revised: April 3, 2012 Published: April 3, 2012 4388
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Table 1. Structural Parameters (An-O) (Å) of Actinyl Aqua Complexes [AnO2(H2O)5]2+/1+ AnOyl
An-OH2O
this work Shamov et al.43 Austin et al.25 experiment this work Shamov et al.43 Austin et al.25 experiment
An = U(VI)
An = Np(VI)
An = Pu(VI)
An = U(V)
An = Np(V)
An = Pu(V)
1.769 1.751 (1.776) 1.79 1.76, 1.78 2.465 2.486 (2.472) 2.42 2.41
1.751 1.730 (1.758) 1.77 1.75 2.448 2.470 (2.457) 2.40−2.41 2.42
1.740 1.720 (1.749) 1.76 1.74 2.446 2.466 (2.453) 2.40−2.41 2.41
1.821 1.806 (1.824) 1.88−1.90
1.802 1.791 (1.807, 1.810) 1.85, 1.88 1.83 2.557 2.588 (2.567) 2.47−2.55 2.50−2.52
1.783 1.776 (1.796, 1.797) 1.81 1.81 2.557 2.577 (2.567) 2.50−2.52 2.47
2.557 2.585 (2.568) 2.44−2.53
In this study, we have used BP86 functional29,30 with def2SV(P) basis sets31 for geometry optimizations. For energetics, we used B3LYP functional32,33 with TZVP34,35 (triple-ζ quality) for all atoms. For actinide ions, a def-SV(P) (for geometry optimizations) and def-TZVP basis set (for energetics) was used to describe the valence orbitals and the core orbitals were modeled via def-ECP pseudo potential. It has been known in the past that such an approach saves computational time as their geometries are less sensitive to the size of basis sets, but the corresponding energies will be significantly influenced by a large basis set. To speed up the calculations, a resolution of identity (RI) approximation is invoked as implemented in TURBOMOLE 6.236 by using def2SV(P) auxiliary basis for Coulomb exchange terms. No such approximations have been used for the energy evaluation with TZVP basis set. Binding energies of the three actinyl ions with the CB-[5] host molecule are computed as shown below.
However, Fedin et al. reported the crystal structure of uranyl tetrachloride species weakly interacting with the walls of a CB host through hydrogen bonding.17 Very recently, the X-ray crystal structure and electronic and vibrational spectroscopic study of two uranyl species bound to one CB-[5] has been reported.18 Unlike the earlier structures, the portals of CB-[5] are completely sealed by the two uranyl cations. Unlike the rival host molecules of CB, only a handful of computational studies were carried out on CB. Gejji and coworkers carried out calculations on the structure, binding, and NMR chemical shifts of various host−guest complexes involving CB.19,20 We have investigated the possibilities of halide21 and chromate anions22 binding by calculating their affinities to CB and its congener host molecules. Recently, electron paramagnetic resonance (EPR) spin Hamiltonian parameters of nitronyl radical binding to CB-[8] were calculated at the hybrid quantum mechanics/molecular mechanics level (QM/MM).23 Because of the differing coordination complex possibilities of uranyl to CB, a theoretical investigation is necessary to understand the preferential coordination mode of CB to actinyl which will be the general focus of the present study. In this Article, we have investigated the various coordination modes of CB-[5] and CB-[6] host molecule to uranyl. Further, we have studied the structure, vibrational spectra, and binding of uranyl, neptunyl, and plutonyl complexes in hexa-valent and pentavalent oxidation states with CB-[5]. Further, we have explored the ways in which the binding energies of CB-[5] can be improved by functionalizing the host molecule. We have considered functionalization with fluorination (F-CB-[5]), hydroxylation (OH-CB-[5]), methylation (Met-CB-[5]), and cyclohexanation (CH-CB-[5]) at the methine hydrogen positions of CB-[5].
CB‐[5] + [AnO2 (H 2O)5 ]n + → [CB‐[5]‐AnO2 ]n + + (H 2O)5
In the above equation, five water molecules formed as part of the product are not the individual five molecules, but they form a cyclic planar hydrogen bonded complex. Our calculation shows that the formation of cyclic hydrogen bonded network of five waters (B3LYP) is associated with the binding energy of 38.9 kcal mol−1 and is close to the value reported by Bryantsev et al. (∼40 kcal mol−1).37 Further, the binding energies are evaluated using the COSMO continuum solvation model using the dielectric constant of water.
3. RESULTS AND DISCUSSION Before discussing the actinyl bound structures to CB host molecule, we first present results of optimized structures of the bare CB host and solvated species of [AnO2(H2O)6]2+/1+. (i). Structures of Bare Host and Hydrated Actinyl Complexes. Important geometric parameters of CB-[5] and CB-[6] host molecules such as depth, widths at the core and at the portals were calculated. The overall geometric features of CB-[5] and CB-[6] such as symmetry (D5h and D6h), cavity diameter and height (∼6.00 Å) are reproduced quite well. Further, our calculated geometric parameters are consistent with previously reported values using B3LYP functionals.38−41 Our optimized structural parameters of the three actinyl complexes in both hexa- and penta-valent oxidation states [AnO2 (H2 O) 5]2+/1+ are shown in Table 1. Numerous calculations were carried out on aqua complexes of actinyls to understand the number of coordination and the actinide
2. COMPUTATIONAL DETAILS For modeling the actinide complexes, apart from the choice of theoretical method and basis sets, one needs to take care of how to incorporate the relativistic and solvation effects at an appropriate level.24 However, it is now being recognized that DFT level of theory in conjunction with small core-effective core potential (SC-ECP) which includes relativistic effects for the description of actinide elements seems accurate. We can employ different models which vary in the number of shells of water molecules that can be treated explicitly.25 The most usual approach is to include the first shell explicitly, and to use a continuum model to describe the bulk solvent. Such a costeffective strategy may be sufficient if we compare the relative trends for the formation of host guest complexes.26−28 4389
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contraction trends.25,42−44 Particularly Schreckenbach and coworkers proved that the use of small core pseudo potential for actinyl is crucial for predicting the actinide contraction.43 Our computed geometric parameters for all the six species are comparable to those reported by Shamov and Schreckenbach43 and those of Austin et al.25 We do predict actinide contraction trends for all six species similar to those predicted in earlier calculations. From Table 1, it is very clear that the choice of our computational method for modeling actinide complexes is validated. (ii). Structures and Vibrational Spectra of Uranyl-CB Complexes. A careful inspection of the D5h lowest unoccupied molecular orbital (LUMO) of bare CB-[5] can reveal the possible binding site of uranyl to the host molecule. The portals of CBs are anionic, and it is expected that the uranyl binding preferentially takes place at the portals (Supporting Information, Figure S1). We have made attempts to optimize the penta aqua uranyl complex encapsulated to CB-[5] and CB-[6]. However, because of the small cavity size of CB-[5] and CB[6], we find that encapsulation of penta-aqua uranyl complex is unfavorable and the uranyl species are expelled out of the host molecules. Our computation predicts that encapsulation of uranyl is unfavorable which is consistent with the experimental findings where the tetra-chloro uranyl complex interacts with CB only via hydrogen bonding. Further, we have investigated three more possible types of binding of uranyl to CB-[5] (Figure 2). In the first binding site, the coordinated five water molecules to uranyl interact with the portals of CB-[5] through hydrogen bonding (denoted as μ0). Here the two UO bond lengths are dissimilar (1.781 and 1.767 Å) and the water molecules are strongly hydrogen
bonded (1.590 Å). Such noncovalent binding of uranyl to CB[5] has a strongly favorable binding energy (−16.7 kcal mol−1). In the second binding site, uranyl binds to two carbonyl oxygens units of CB-[5], and the three bound water molecules to uranyl are now interacting with the carbonyl groups of CB[5] via hydrogen bonding (1.554−1.559 Å) (denoted as μ2). In the optimized structure, the UO bond lengths are nearly symmetric (1.782, 1.786 Å); their binding affinity is indeed favorable (−17.0 kcal mol−1) and is of magnitude similar to those of the completely hydrogen bonded structure (μ0). In the final binding site, all five water molecules are replaced by the CB-[5] molecule to form a C5v symmetric structure (denoted as μ5). Here the UO bond lengths are again asymmetric, where the uranyl oxygen is buried inside the core, and this bond is 0.02 Å longer as compared to the freely exposed UO bond. The calculated binding energy is −28.5 kcal mol−1, which is the strongest among the three possible binding sites. Further, we have characterized all three CB bound uranyl species as mimima by performing vibrational frequency calculations on the optimized structures. We note that the UO bond of the μ0-uranyl complex is short (and strong) as compared with that of the other two uranyl complexes (μ2 and μ5) which is reflected in the calculated symmetric (υsym) and asymmetric (υasym) stretching frequencies of the UO bond. As the binding energies of the μ0- and μ2-complexes have similar complexation energies with CB-[5], perhaps these υsym (871 cm−1 vs 850 cm−1) and υasym (965 cm−1 vs 952 cm−1) stretching frequencies of UO can be used as a “f ingerprint” to distinguish the two species. The υsym and υasym UO stretching of μ0 is stronger (20 cm−1 and 15 cm−1) as compared to that of the μ2 complex. For the 1:1 μ5-desolvated complex the υsym and υasym stretching frequencies of UO bond are 846 cm−1 and 953 cm−1, which are close to those of the partially solvated complex. A crystal structure of two uranyl bound to CB-[5] in 2:1 guest host ratio in μ5 coordination was recently reported.18 Here, both the portals are sealed by two uranyl in symmetric μ5-bindng. To further validate the computational protocol used here, we have optimized this D5h symmetric structure using the BP86 functional with the same basis as discussed above. The optimized structure is shown in Figure 2, and we note that the calculated UO bond lengths are very close to the experimental data.18 Further, the calculated υsym and υasym stretching frequencies of UO are 853 and 969 cm−1 which are again close to the experimental estimates (859 and 948 cm−1).18 The calculated binding energy of 19.2 kcal mol−1 per uranyl is lower as compared with that of the 1:1 complexes (by ∼10 kcal mol−1). However, the nitrate counterions interacting with the walls of CB-[5] through hydrogen bonding will increase the binding affinities of uranyl further. We have also made attempts to optimize the C6v symmetric structure with CB-[6] (denoted as μ6), where the coordinated five water molecules of uranyl are now coordinated by six portal oxygens. We find that the calculated binding energy remains unfavorable (+34.9 kcal mol−1), which suggest that such a species may not be feasible experimentally. Hence, in the forthcoming sections, we will focus on the structures, vibrational frequencies, and binding affinities of actinyls (An = U, Np, Pu) for both hexa- and penta-valent oxidation states to bare and functionalized CB-[5] in 1:1 ratio. In all axial complex formation, the five water molecules of the
Figure 2. Optimized structures of various uranyl binding to CB-[5]. Values in parentheses in 2:1 complexes are derived from experimental data.18 4390
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actinyl aqua complexes are now replaced by the five portal oxygens of CB-[5] (μ5). Before proceeding to the actinyl binding to functionalized CB-[5], we glance through the essential molecular orbitals (MOs) which contribute to the bonding of uranyl to CB-[5] (Figure 3). The MOs are derived from B3LYP/def2-SV(P)
basis sets. For uranium a more accurate Segmented All Electron Relativistically Contracted (SARC)45,46 basis sets was used in conjunction with Zeroth Order Regular Approximation (ZORA)47 as implemented in the ORCA 2.8 program.48 For comparison, we have also plotted the MOs of bare uranyl. The valence ground state electronic configuration of bare uranyl is πg4, πu4, σg2 and σu2.49 Both σu and πu have mostly contributions from the uranium 5f and oxygen 2p orbitals, whereas the πg and σg have mainly uranium 6d and oxygen 2p contributions. Upon binding with CB-[5], the order of these valence orbitals are slightly changed because of bonding. Further, the oxygen 2p contribution of “yl” oxygen in bare uranyl is nearly 70% for πg4 for MOs, whereas in CB bound species, it is only ∼41%. For the πu4 MO, the f-orbital contribution to the MO is larger in the complex as compared to the bare uranyl. In the UO2-CB-[5], although the πg4 MOs are lowest, the πu4 is slightly above σg2, whereas in the bare uranyl, it is the πu4 that lies below the σg MO. We note that the CB-[5]-uranyl bonding MO lies above the σu MO. The highest occupied molecular orbital (HOMO) of bare uranyl is the σu MO which is somewhat more stabilized in the complex. However, the HOMO of the uranyl bound CB[5] complex is the oxygen 2p of portal CB-[5] which is not involved in bonding with uranyl. Hence, these orbitals are likely to interact with the second uranyl to form the 2:1 uranyl:CB[5] ratio complex as observed experimentally.18 (iii). Structure of Actinyl-CB-[5] Complexes. As compared to the higher analogues of CB, CB-[5] is a relatively rigid molecule. During complexation, the geometric structure of the host molecule undergoes less relaxation, a vital feature for the formation of efficient host−guest complexes. Hence, we explored the possibility of using CB-[5] molecule, a selective binder of a specific actinyl. Further, we explored the ways in which the binding affinities of CB-[5] to actinyls can be improved through functionalization of CB-[5]. As mentioned earlier, the 10 methine hydrogens of CB-[5] molecule are now replaced by fluorine (F-CB-[5]), hydroxy((OH-CB-[5]), methyl (Met-CB-[5]), and cyclohexyl (CH-CB-[5]) groups. We use these functionalized CB-[5] to understand whether the binding of actinyls can be improved. Prior to actinyl binding, we have optimized all the four functionalized CB-[5] and find that the geometric parameters such as well depth, width at the core and at the portals are essentially similar to those of the unfunctionalized counterpart. The calculated HOMO−LUMO gap of the functionalized CB-[5] increases by up to 0.2 eV for OH-CB-[5] as compared to the native CB-[5]. For the other three complexes, this gap is within 0.1 eV. The optimized geometric parameters of AnO22+/1+ binding to functionalized CB-[5] are listed in Table 2, and their optimized structures are given in Figure 4. In all optimized structures of An-CB-[5] complexes and irrespective of the functional groups
Figure 3. Important molecular orbitals of bare and CB-[5] bound uranyl.
Table 2. Optimized Structural Parameters (Å) of Actinyl Binding to Functionalized CB-[5] UOylb CB-[5] U(VI) U(V) Np(VI) Np(V) Pu(VI) Pu(V) b
1.771 1.804 1.753 1.792 1.774 1.775
(1.794) (1.841) (1.778) (1.822) (1.763) (1.804)
F-CB-[5] 1.766 1.797 1.748 1.785 1.740 1.769
(1.795) (1.841) (1.776) (1.823) (1.762) (1.804)
OH-CB-[5] 1.771 1.803 1.756 1.795 1.747 1.778
(1.794) (1.841) (1.775) (1.822) (1.762) (1.804)
U−OCB Met-CB-[5] 1.775 1.808 1.756 1.795 1.747 1.778
(1.794) (1.841) (1.775) (1.822) (1.762) (1.804)
CH-CB-[5] 1.775 1.808 1.756 1.795 1.747 1.778
(1.794) (1.840) (1.775) (1.821) (1.762) (1.803)
CB-[5]
F-CB-[5]
OH-CB-[5]
Met-CB-[5]
CH-CB-[5]
2.429 2.555 2.421 2.559 2.425 2.556
2.436 2.568 2.431 2.573 2.433 2.570
2.429 2.557 2.419 2.561 2.424 2.557
2.421 2.542 2.413 2.550 2.418 2.546
2.423 2.544 2.414 2.554 2.419 2.546
Values in parentheses are for buried UO bond length. 4391
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The strength of actinide binding to various functionalized CB-[5] will be reflected in the AnO bond lengths. Indeed, we note that the length of the AnO bond decreases in the following order: AnOMet‑CB‑[5] (or CH‑CB‑[5] < AnOCB‑[5] < AnOOH‑CB‑[5] < AnOF‑CB‑[5]. The variance in the bond lengths is solely introduced by the functional groups which possess different electronic effects (inductive effect). Compared to hydrogen, functional groups such as cyclo-hexyl and methyl groups are electron donating groups, whereas fluorine is a wellknown electron withdrawing group which alters the charge transfer ability of the carbonyl oxygen to the actinyl. For both oxidation states of actinyl complexes bound to Met-CB-[5], the axial AnO bond length is ∼0.01 Å longer as compared to FCB-[5]. It is interesting to note that this electronic effect is affecting only the AnO bond length which is freely exposed and not the buried AnO bond which remains unaltered irrespective of the substitutions. Corresponding to the axial AnO bond elongation, a contraction of An-OCB is noted which can be attributed to the electronic effects played by the functional groups attached to CB-[5]. For the one electron reduction of hexavalent actinyl to pentavalent forms of aqua complexes, the change in AnO bond length is more than 0.05 Å as predicted by us and others. Except Pu binding to native CB-[5], we note that the AnO bond length is longer for the penta-valent state by ∼0.03 Å for both U and Pu, but for Np, this value is ∼0.04 Å. Surprisingly, we note that for Pu binding to native CB-[5], the change in PuO bond is only 0.001 Å. For the buried oxygen, we generally find that the bond length elongation upon one electron reduction is larger (by 0.01 Å) as compared to the free AnO bond length. However during the reduction process, the An-OCB bond length elongation is rather systematic (by 0.13 Å) in all the three actinyls and for all four functonalized CB-[5]. Finally, the extent of actinyl displacement to the plane of CB[5] portal oxygens is calculated. Unsurprisingly, the pentavalent actinyls are more displaced as compared to the hexavalent state (∼0.05 Å). Within the three actinyls, uranyl is the most displaced molecule in both hexa-(0.19 Å) and pentavalent (0.24 Å) oxidation states. (iv). Vibrational Spectra. Because of the size and complexity of the systems studied, we have carried out vibrational frequency calculations only for selected systems. We first compare the vibrational spectra for the three actinyls in both oxidation states for native CB-[5]. These calculations reveal the trends in the vibrational spectrum of the three actinyls for a given host. We have also computed the vibrational spectrum of UO22+/1+ bound to native CB-[5], F-CB-[5], OHCB-[5], and Met-CB-[5]. Because of the similarities in structural parameters and electronic effects of Met-CB-[5] and CH-CB-[5], and the larger size of the CH-CB-[5] (more than 140 atoms) as compared to Met-CB-[5], we have not
Figure 4. DFT optimized uranyl complexes with functionalized CB[5].
attached to CB-[5], we note an asymmetric AnO bond length for both hexa-valent and penta-valent oxidation states (Table 2). For a typical hexa valent actinyl complexes such as those of halides, aqua, hydroxides, carbonates, and acetate, the asymmetric AnO bond lengths are not generally seen. However, for the penta-valent actinyl species, such asymmetric AnO are often encountered because of the variable hydrogen bonding interaction between the solvent molecules and the axial oxygen as discussed by Austin et al.25 for both aqua and carbonate complexes. In the present case, the two oxygens of actinyl experiences two different electronic environments. We find that one oxygen exposed to the core of the host CB-[5] molecule is elongated as compared to its counterpart. Although the extent of elongation may vary within the actinyl series, the increased AnO bond length at the core can be attributed to the electronic repulsion between the core CB-[5] and those of the oxygens attached to the AnO molecule. Table 3. Calculated Vibrational Frequencies (cm−1) U(VI) U(V) Np(VI) Np(V) Pu(VI) Pu(V)
CB-[5]
F-CB-[5]
OH-CB-[5]
Met-CB-[5]
846.0, (953.1) 786.2 (876.1) 839.9 (957.3) 783.6 (882.5) 816.7 (949.7) 777.1 (883.7)
848.3 (959.9) 790.4 (885.3)
846.2 (953.2) 791.2 (877.2)
844.9 (948.8) 786.1 (870.9)
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Table 4. Calculated Complexation Energies (kcal mol−1) complexation energies CB-[5] CB-[5] CB-[5] CB-[5] CB-[5] CB-[5]
+ + + + + +
UO2(H2O)5]2+→ (UO2-CB-[5])2+ + (H2O)5 UO2(H2O)5]1+→ (UO2-CB-[5])1+ + (H2O)5 NpO2(H2O)5]2+→ (NpO2-CB-[5])2+ + (H2O)5 NpO2(H2O)5]1+→ (NpO2-CB-[5])1+ + (H2O)5 PuO2(H2O)2]2+→ (PuO2-CB-[5])2+ + (H2O)5 PuO2(H2O)5]1+→ (PuO2-CB-[5])1+ + (H2O)5
CB-[5]
F-CB-[5]
OH-CB-[5]
Met-CB-[5]
CH-CB-[5]
−28.5 −20.6 −27.9 −21.0 −26.0 −20.8
+13.6 +3.9 +14.2 +2.5 +14.9 +2.4
−14.4 −12.3 −13.6 −13.1 −12.1 −13.2
−33.2 −23.2 −32.6 −23.8 −30.6 −23.4
−33.2 −23.5 −32.6 −24.1 −30.8 −23.8
[5], whereas for F-CB-[5], the binding is largely unfavorable (Table 5). Between the two oxidation states, the higher An(VI) binding is stronger as compared to An(V) for all the species except for Pu complexed with OH-CB-[5]. In this case, we find that Pu(V) binding is favorable over Pu(VI) by 1 kcal mol−1. Further, the extent of favorable binding of An in the higher oxidation state is larger for Met-CB-[5] and CH-CB[5] (∼10 kcal mol−1 for U) as compared to the native CB-[5] (8 kcal mol−1). For the OH-CB-[5], both oxidation states bind with equal strengths (Table 4). Further, within the actinyls, we note that the binding decreases in the following order, U < Np < Pu which nicely correlates with the actinide contraction and with the blue-shift in the frequency. We have analyzed the origin of favorable binding at the functionalized CB-[5] with actinyls. We note that the extent of charge transfer from CB-[5] to actinyl is crucial for favorable binding. In Table 5, we have listed the net charge transfer that
computed the vibrational frequencies for CH-CB-[5] species (Table 3). Our calculated vibrational frequencies of actinyl-CB-[5] complexes along the series follow an interesting trend as previously observed for the actinyl aqua complexes. For hexavalent oxidation state, the AnO υsym stretching for uranium is stronger (846 cm−1) as compared with neptunium (839 cm−1) and plutonium (816 cm−1). However, the corresponding An O υasym stretching Np(VI) is stronger (957 cm−1) than uranium (953 cm−1) and plutonium (950 cm−1). Although, the AnO bond lengths are shortened as we move from U to Pu, a blue shifting of vibrational frequencies are rather unexpected. Similar observations were previously noted by Hay et al.42 and also in the studies of Shamov and Schreckenbach43 for actinyl aqua complexes. The natural bond order analysis of actinide CB-[5] complexes indicates that in actinyls, only the f- (2.61) and dorbitals (1.39) participate in bonding as compared to s- (2.15) and p- orbitals (5.75). From uranyl to plutonyl, the f-electrons are populated in the fδ orbitals and no significant contributions arise from s- and p-orbitals, although the net overlap population decreases steadily because of actinide contraction. Hence, the actinyl bond becomes stronger and weaker along the series mainly because of actinyl contraction. For the penta-valent complexes, we find that AnO υsym stretching again follows the higher oxidation state trend, whereas in AnO υasym stretching, both Np and Pu are slightly stronger as compared to U(V). Upon one electron reduction, we note that the UO υasym and υsym stretching decrease by 77 cm−1 and 60 cm−1 respectively. For the corresponding higher analogues, the vibrational frequency shift is less, which is again consistent with the actinyl aqua complexes. The quantitative UO elongation due to functionalization of CB-[5] can be readily understood from the calculated vibrational spectra. For instance, because of the presence of electron withdrawing groups at the CB, the UO bond is shorter as compared with the native uranyl-CB complex. Indeed the calculated υasym and υsym stretching for U(VI) binding to FCB-[5] are stronger, whereas the uranyl binding to Met-CB-[5] species is weakest (Table 3). Similarly, the U(V) binding to FCB-[5] is red-shifted, and the Met-CB-[5] binding is blueshifted as compared to native CB-[5]. However, it must be noted that the changes in the UO υasym and υsym stretching due to functionalization is larger for the penta-valent oxidation states as compared to the higher oxidation state (Table 3). Hence, although the changes in the U−Oyl bond lengths are very minimal, the vibrational spectra can actually distinguish the two actinyl binding to functionalized CB-[5] in a quantitative manner. (v). Binding Energies. The formation of actinyl-CB-[5] complexes for all species are listed in Table 5. Compared to native CB-[5], both Met-CB-[5] and CH-CB-[5] bind actinyls strongly in both oxidation states. Although the binding of OHCB-[5] is favorable, it is weaker as compared to the native CB-
Table 5. Net Charge Transfer (a.u) from Actinyl to Functionalized CB-[5] U(VI) U(V) Np(VI) Np(V) Pu(VI) Pu(V)
CB-[5]
F-CB-[5]
OH-CB-[5]
Met-CB-[5]
CH-CB-[5]
1.254 0.808 1.262 0.719 1.144 0.641
1.201 0.762 1.204 0.671 1.078 0.598
1.241 0.796 1.241 0.705 1.128 0.629
1.271 0.823 1.271 0.732 1.161 1.654
1.268 0.820 1.276 0.730 1.157 0.651
occurred from actinyl to functionalized CB-[5]. As compared to bare actinyl ([AnO2]2+/1+), we note that for favorable binding to CB-[5], the ability of the CB-[5] to accept the positive charges of the highly charged actinyl is more for Met-CB-[5] and CH-CB-[5] as compared to F-CB-[5]. Hence, the binding at these sites are more favorable over the F-CB-[5]. Finally, within the three actinyls of the hexa-valent complexes, we note that charge transfer is more for Np as compared to U and Pu. For the penta-valent complexes, the charge transfer is more for U as compared to Np and Pu. (vi). CB versus Other Macrocyles. We now compare the actinyl binding affinities of CB to macrocyles such as 18-crown[6]-ether, alaskaphyrin (an expanded porphyrin), and Schiff base complexes (Table 6). The calculated binding energies of the crown system (oxygen coordinating system)8−10 and the other two Schiff base complexes (nitrogen donor) are taken from Shamov and Schreckenbach.50 From Table 6, it is quite clear that the binding affinities of actinyls to CB-[5] are stronger as compared to 18-crown-[6]-ether by large amount. Between the N-donor macrocycle of Schiff base complexes and the O-donor complexes of CB-[5], the binding with the latter is more preferred. It must be remembered that during complexation, the Schiff base complexes are dianionic whose binding is expected to be stronger as compared to neutral CB-[5]. 4393
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Table 6. Binding Affinities (kcal mol−1) of Various Host Molecules with Actinyls An
CB-[5]
Met-CB-[5]
18-crown-[6]8
Alaskaphyrin50
Bipyen 250
Bipytmd 350
U(VI) U(V) Np(VI) Np(V) Pu(VI) Pu(V)
−28.5 −20.6 −27.9 −21.0 −26.0 −20.8
−33.2 −23.2 −32.6 −23.8 −30.6 −23.4
−0.2 −3.2 −1.1 −2.0
−28.1 −13.3
−33.2 −4.9
−20.9 +5.4
−2.1
However, the extent of negative ion stabilization of the ligand prior to binding of actinyls in aqueous phase is more, which effectively leads to decreased binding affinities. Although the calculated vibrational frequencies of the Schiff base bound actinyls are red-shifted, the binding affinities of the CB-[5] are as effective as the dianionic N-donor complexes.
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ASSOCIATED CONTENT
S Supporting Information *
HOMO and LUMO of CB-[5] are shown. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
CONCLUSIONS In this paper, we have addressed the feasibility of using host molecule CB-[5] for the complexation of three actinyls in hexa and penta-valent oxidation states using DFT calculations. We initially addressed the various complexation types with CB-[5] complexes with uranyl. On the basis of binding energy evaluations, we concluded that the formation of μ5-coordination is energetically favorable by more than 10 kcal mol−1 as compared to hydrogen bonded and μ2-coordination. The calculated vibrational spectra can be used as fingerprint to differentiate the μ0 and μ2 binding modes. We also note that encapsulation or μ6-coordination is not energetically favorable for CB-[5] and CB-[6] host molecules. Further, we believe that the cavity created by the portals of CB-[5] fit with the equatorial binding of actinyls as compared to higher CB analogues. Further, for the higher analogues, we hope that CB will be interacting with actinyl only with μ1 or μ2 coordination modes. The calculated structures and vibrational spectra for 2:1 uranyl complex with CB-[5] are in excellent agreement with the recent experimental data.18 On the basis of the electronic structure investigation of uranyl binding to CB-[5], we have shown that the experimental formation of 2:1 uranyl-CB-[5] is feasible because of the donor ability of portal 2p oxygens of CB[5] to the uranyl. We next addressed the binding affinities of μ5-coordination of three actinyls (U, Np, and Pu) with a functionalized CB-[5] host molecule. The calculated structures and vibrational spectra do follow actinide contraction trends as observed for well studied actinyl aqua complexes. We note that the binding affinities of actinyls are significantly enhanced when both MetCB-[5] and CH-CB-[5] are used as host molecules. Additionally on the basis of calculated binding energies, we note that OH-CB-[5] does not distinguish the oxidation state of the actinyl as compared to the other three host molecules. Surprisingly, we note that Pu(V) is found to be marginally more stable with OH-CB-[5] as compared to the Pu(VI) oxidation state. The preferential origin of binding energies can be attributed to the extent of charge transfer from the host molecule to the actinyl. For both Met-CB-[5] and CH-CB-[5] host molecules, the charge transfer is maximum, whereas for FCB-[5], the charge transfer is minimum as compared to the native CB-[5]. Our calculations suggest that CBs can be efficiently used for binding actinyls as compared to other macrocylic host molecules which may have significance in nuclear waste management and separation processes.
*Phone: +91 22 25593829 (M.S.), +91 22 25595092 (S.K.G.). Fax: +91 22 25505151 (M.S.), +91 22 25505151 (S.K.G.). Email:
[email protected] (M.S.),
[email protected] (S.K.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.S. and S.K.G. thank Dr. T. Mukherjee for his kind support and the BARC computer center for providing the high performance paraller computing facility (Ameya and Ajeya Systems). Funding from the INDO-EU project MONAMI is also gratefully acknowledged. The work of S.K.G. has also been supported by a Sir J. C. Bose fellowship of DST, India. V.S. thanks HBCSE, TIFR as a part of National Initiative on Undergraduate Science (NIUS).
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REFERENCES
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