Characterization of the binding of six actinyls AnO₂^2+/+ (An = U/Np/Pu) with three expanded porphyrins by density functional theory

Abstract

We reported a density functional theory study on the complexation of six hydrated actinyl cations, AnO₂(H₂O)₅^2+/+ (aq) (An = U/Np/Pu), with three expanded porphyrins, amethyrin (H₄L1), oxasapphyrin (H₂L2), and grandephyrin (H₃L3). The geometries have been fully optimized and analyzed, and the electronic structures, the binding free energies, and the NMR properties were calculated. Natural population analysis and Quantum Theory of Atoms in Molecules (QTAIM) topology analysis techniques were applied to understand the interaction modes between two entities of each complex. The calculations show that for the same ligand, PuO₂²⁺ and NpO₂²⁺ display stronger binding affinity than UO₂²⁺, UO₂⁺, NpO₂⁺, and PuO₂⁺, and among the three ligands tested, L2²⁻ fits better with the actinyl cations than L3³⁻ and H₂L1²⁻. The redox process was observed in the complexation of PuO₂²⁺ and NpO₂²⁺ with specific ligands, which agrees with the experimental results. In the characterization of the nature of the coordination bonding interactions, QTAIM gives a consistent description with the natural population analysis method and shows that the interaction between An and the electron donor atoms in the first coordination shell has a strong ionic feature, while the interaction between An and O^yl atoms of the actinyls in the complexes remains covalent. This work complements the earlier experimental studies by providing a molecular level of understanding on the interaction between actinyls and expanded porphyrins, and is expected to contribute to the communities of the chemistry of actinides and expanded porphyrins.

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1. Introduction

As an important ligand, porphyrins (Pors) are known to display strong affinity toward d-transition metal atoms, such as iron and zinc, and play important roles in biomolecular systems and have potential application in dye-sensitized solar cells. While their binding with actinides is rare, and limited examples^1–6include ThPor(acac)₂, MPor(Cl)₂L₂ (M = Th, U; L = Py, THF, or Bzn), UPor(Cl)₂(THF), and MPor₂ (M = Th, U), where the actinides form out-of-plane or sandwich complexes. Similar phenomena were also observed in an earlier work of Cao et al.,⁷ in which the coordination of three trivalent actinide cations (An(III)), Ac, Cm, and Lr, with texaphyrins and motexafins was studied and An(III) was observed to sit above the polypyrrolic macrocyclic ligand rather than being co-planar with the ligand. The texaphyrins and motexafins have five coordinating N atoms, thus with a cavity larger than that of Pors by about one fifth. This certainly indicates that the host and the guest do not fit each other well, which may be significantly improved if expanded porphyrins (ePors) with appropriate cavity size were developed.

Expanded porphyrins are a family of derivatives of normal tetradentate porphyrins, but with much rich chemistry since multiple types of heterocyclic rings may be fused into the macrocycle, leading to diverse conformers with modulated electronic and mechanical properties.⁸ The enlarged cavity size of ePor also gives them the ability to accommodate bigger cations that Por cannot hold, such as actinides.

The coordination of ePors with actinyls has been observed in early 1990s when Sessler et al.^9,10 developed a new class of pyrrole-based ePor which were capable of complexing uranyl (UO₂²⁺) effectively. This opened a new avenue of the host–guest chemistry of actinides in addition to calixarenes^11–14 and crown ethers,^15–18 and a variety of ePors were reported to form stable complexes with actinyls (AnO₂^2+/+), including pentaphyrin,^9,19 sapphyrin,^10,20 alaskaphyrin,²¹ grandephyrin,²² isoamethyrin,²³ oxasapphyrin,²⁴ and amethyrin.^25–27

The ePors provide a better “fit” for larger metal cations, and their affinity to bind with actinyls has been observed, while the nature of the interaction between actinyls and the ePors remains obscure. The specific binding behavior of ePors with actinyls may open a new avenue to the chemistry of actinyls and other cations of large size when coordinating with macrocyclic ligands which Pors cannot offer, and thus are potentially relevant to the communities of chemistry and biology. This motivated us to carry out a computational study to understand the interaction between actinides and ePors, which may facilitate the rational design of new ePor molecular systems for manipulatable binding of actinides.

2. Computational details

Three ePor compounds (Scheme 1) were used as ligands to study their interaction with six actinyls, i.e. uranyl(VI/V), neptunyl(VI/V), and plutonyl(VI/V). The ligands include amethyrin ([24] hexaphyrin(1.0.0.1.0.0) in the nomenclature rule of Franck and Nonn for porphyrinic systems^28–30) (H₄L1), oxasapphyrin (H₂L2), and grandephyrin (H₃L3).

Scheme 1 The neutral forms of the three ePor ligands with the numbering of heteroatoms (in green). The protonation scheme of each ligand showed here represents the energetically most stable conformer. The red signs label the rings.

Density functional theory (DFT)³¹ has been widely used to study the electronic properties and chemical processes of molecular actinide systems.^32–40 In this work, range-separated hybrid functional CAM-B3LYP⁴¹ implemented in Gaussian 09⁴² was employed to optimize and analyze the stationary points. This method has been used to study the nonlinear optical properties of an ePor and was shown to give correct qualitative results.⁴³ It was also tested in a recent comparative study of low valent uranium complexes, and was shown to give comparable geometrical parameters and energies with the recommended methods of B3LYP and B3PW91.⁴⁴

For the basis set, the locally dense basis set method was adopted, which has been qualified in earlier studies of geometry and electronic properties.^45–47 Quasi-relativistic 5f-in-valence small-core ECPs^48–50 (SC-RECPs), together with their optimized segmented basis set for the valence shells contracted as (14s13p10d8f6g)/[10s9p5d4f3g], were used to treat the actinide atoms. Such a treatment has been shown to yield reasonable results in many studies of molecular systems containing actinides.^51–56 For the N and O atoms of the first coordination sphere, the triple-ζ quality 6-311+G(d)^44,57–60 basis set was used, and for the H atoms bonding with N atoms in the native form of the ePors, only polarization functions were included based on the 6-311G basis set. For all the other atoms, including C, O, and H atoms, the double-ζ basis set 6-31G(d)^61,62 was used. Spin–orbit coupling is neglected with such treatments of the model systems.

All stationary points reported here were fully optimized and confirmed to be energy minima by frequency calculations. In order to better mimic the experimental condition, taking experimentally used methanol (MeOH) as solvent, the polarizable continuum model (PCM)^63,64 was used to re-optimize all of the stationary points followed by vibrational frequency analysis to identify the nature of each stationary point and obtain the zero-point energy (ZPE) and thermal correction. To take the entropy effect into account, the Gibbs free energy is used in the following discussions unless otherwise specified. The free energies in vacuum were calculated from standard determination emerged from the Gaussian 09⁴² output with ideal-gas translational entropy, and the ones in MeOH solvent were estimated using solution-phase translational entropy.⁶⁵

The Quantum Theory of Atoms in Molecules (QTAIM)^66–70 analysis was performed based on Kohn–Sham orbitals optimized by the Gaussian 09 program. The AIM analysis was done using the Multiwfn 3.2 package.⁷¹ The calculations of NMR shielding tensors were carried out by using the Continuous Set of Gauge Transformation (CSGT) method^72–75 and the Gauge-Independent Atomic Orbital (GIAO)^76,77 method to obtain the anisotropy of the current-induced density (ACID), which was analyzed by the ACID program,^78,79 and the Nucleus-independent chemical shifts (NICS) indices,^80–82 respectively.

3. Results and discussion

3.1. The ligands and the binding modes offered to actinyls

The three ePor ligands studied here differ from each other in their backbone and their coordinating atoms, as shown in Scheme 1. In their neutral forms, all of the three ligands may adopt multiple protonation schemes. In H₄L1, there are six pyrrole rings, with rings A and A*, B and B*, and C and C* being equivalent, respectively, leading to six potential neutral isomers according to different protonation sites, i.e. H₄L1_131*3*, H₄L1_131*2*, H₄L1_1*233*, H₄L1_1*2*23, H₄L1_232*3*, and H₄L1_1231*, where the four digits of the subscript stand for the protonated N atoms of the pyrrole rings defined in Scheme 1, respectively. Isomer H₄L1_131*3* was found to be the most stable, and the Gibbs free energies (Table S1, ESI†) of the other five isomers with respect to H₄L1_131*3*, ΔG_vacuum/ΔG_MeOH, were calculated to be 0.2/0.2, 13.6/10.5, 17.8/13.1, 22.9/18.8, and 33.2/19.7 kcal mol⁻¹ for H₄L1_131*2*, H₄L1_1*233*, H₄L1_1*2*23, H₄L1_232*3*, and H₄L1_1231*, respectively. This suggests that the two isomers, H₄L1_131*3* and H₄L1_131*2*, are essentially degenerated in view of free energy.

H₂L2 differs from H₄L1 in the number of pyrrole rings, and the presence of one furan ring, leading to two types of N atoms of H₂L2, numbered as 1 and 1*, and 2 and 2*, respectively, as seen in Scheme 1, and one O atom in the first coordination sphere when interacting with actinyls. There are four possible protonation isomers, with H₂L2_12* more stable by 1.7, 5.8, and 17.8 kcal mol⁻¹ in vacuum than H₂L2_11*, H₂L2_22*, and H₂L2₁₂, respectively.

H₃L3 is a Schiff-base type macrocyclic compound. It has four pyrrole rings and two imines which are bridged by a phenyl ring. The extension of the ring backbone makes H₃L3 more flexible. Similarly, there are ten different isomers (Table S1, ESI†) with the most energetically favorable structure of H₃L3_122* staying 3.4–44.6 kcal mol⁻¹ lower in vacuum than other isomers. The relative stability changes to 2.8–34.2 kcal mol⁻¹ when taking into account the solvent effect of MeOH. Such a protonation scheme is consistent with the structure of the grandephyrin methanolic adduct reported in a crystallographic study²² (see Fig. 7 and Table 2 in ref. 22).

These compounds in principle can appear as hexadentate (L1⁴⁻ and L3³⁻) or pentadentate ligands (L2²⁻). In view of the structures, it is reasonable to consider the pentadentate and hexadentate binding modes offered by H₂L2 and H₃L3 in their fully deprotonated states, respectively, while for H₄L1, it is more complex due to the enlarged cavity size and the potential competence of the six pyrrole N sites to binding with the electron-deficient actinide cations. To figure out the most favorable binding mode, we have investigated the thermodynamics of the binding processes featured by a sequential deprotonation of H₄L1 assisted by Et₃N. These reaction pathways are discussed in the ESI.†

According to our calculations, starting from the thermodynamically more stable conformers H₄L1_131*3* and H₄L1_131*2*, the first step, which involves the complexation of the deprotonated ligand and the dehydrated actinyl and the protonation of Et₃N, is strongly exothermic in the cases of hexavalent actinyls while endothermic for pentavalent actinyls. Taking into account the solvent effect of MeOH, the exothermicity decreases in both cases, while the preference of the ligand to bind to An(VI) over An(V) does not change. The further deprotonation in MeOH solvent brings more exothermicity for the An(VI) complexes due to the removal of repulsive interaction between the proton and the actinyl, while makes the An(V) complexes even less stable.

The exothermicity for the deprotonation of An(VI) complexes is reversed starting from the third step in MeOH solvent. Concerning the thermodynamics of the various channels, the product of the optimal doubly deprotonation processes, i.e. H₂L1²⁻ with only N1 and N3 remaining protonated, was considered as the most favorable precursor for the binding of actinyls to H₄L1, and the following discussion is based on the complexes of AnO₂(H₂L1)^0/−.

The optimized geometries of the three ligands in their deprotonated forms, i.e. H₂L1²⁻, L2²⁻, and L3³⁻, and their complexes with actinyls bound are shown in Fig. 1 and Fig. S1 (ESI†), and the key geometrical parameters, including bond distances d_An–N and d_An–O, and bond angle O–An–O, are given in Table 1. In their unbound deprotonated forms, the hetero atoms of the ligands are nearly coplanar as a result of the backbone strain and the conjugation.

Fig. 1 The optimized structures of H₂L1²⁻, L2²⁻, and L3³⁻ ligands and their complexes with UO₂²⁺ in vacuum (hydrogens binding with carbon atoms are omitted for clarity): (a) top view; (b) side view. The schematic structures of complexes with other antinyls are similar and given in Fig S1 (ESI†).

Table 1 Key geometrical parameters (bond length in Å, bond angle in degree) for AnO₂^2+/+ and their complexes with H₂L1²⁻, L2²⁻, and L3³⁻ ligands in vacuum

	U^VI			Np^VI			Pu^VI
	H2L1^2−	L2^2−	L3^3−	H2L1^2−	L2^2−	L3^3−	H2L1^2−	L2^2−	L3^3−
a The distances between An and N1* and N2* are the same as their counterparts for AnO2L2^0/− and AnO2L3^−/2−, thus are not shown; for AnO2H2L1^0/−, they are given as An–N1*/An–N1 and An–N2*/An–N2. b An–N3*/An–N3 for AnO2H2L1^0/− and AnO2L3^−/2−, and An–O3 for AnO2L2^0/−. c Arithmetic mean of the bond lengths in the equatorial plane of actinyls. d The distances in the penta-aquo complexes of actinyls AnO2(H2O)5^2+/+. The data in parenthesis are obtained with solvent effect of MeOH taken into account.
An–N1^a	2.49/2.81	2.58	2.67	2.58/2.94	2.65	2.75	2.57/2.94	2.65	2.68
An–N2^a	2.41/3.64	2.48	2.54	2.52/3.51	2.56	2.59	2.51/3.50	2.56	2.51
An–N3(O3)^b	2.48/3.68	2.70	2.83	2.60/3.50	2.73	2.88	2.60/3.49	2.73	2.83
d̅	2.92	2.56	2.68	2.94	2.63	2.74	2.93	2.63	2.67
An–O^yl	1.74/1.75	1.75	1.75	1.77/1.79	1.79	1.79	1.75/1.77	1.76	1.72
∠O–An–O	171.8	175.5	178.5	174.6	177.1	179.1	176.3	178.3	179.2
ν s(An–O^yl)	923	926	913	833	837	820	833	836	895
ν as(An–O^yl)	999	999	986	908	898	877	916	912	1000
An–O^yl (hyd.)^d	1.73(1.74)			1.71(1.71)			1.69(1.69)
An–O^w (hyd.)^d	2.47(2.44)			2.46(2.43)			2.44(2.41)
^bareAn–O^yl	1.68			1.66			1.64

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	U^V			Np^V			Pu^V
	H2L1^2−	L2^2−	L3^3−	H2L1^2−	L2^2−	L3^3−	H2L1^2−	L2^2−	L3^3−
An–N1^a	2.56/3.02	2.57	2.59	2.57/3.02	2.69	2.74	2.56/3.01	2.69	2.75
An–N2^a	2.50/3.61	2.48	2.56	2.49/3.64	2.57	2.61	2.48/3.62	2.57	2.60
An–N3(O3)^b	2.59/3.55	2.73	2.98	2.57/3.58	2.77	2.95	2.57/3.57	2.77	2.94
d̅	2.97	2.57	2.71	2.98	2.66	2.77	2.97	2.65	2.76
An–O^yl	1.80/1.82	1.75	1.76	1.78/1.80	1.79	1.80	1.76/1.78	1.77	1.78
∠O–An–O	171.2	175.3	176.8	173.4	176.7	178.7	175.5	178.0	179.0
ν s(An–O^yl)	812	912	900	818	827	807	820	826	806
ν as(An–O^yl)	874	983	973	891	884	866	902	899	881
An–O^yl (hyd.)^d	1.79(1.81)			1.76(1.78)			1.75(1.77)
An–O^w (hyd.)^d	2.55(2.53)			2.54(2.52)			2.55(2.53)
^bareAn–O^yl	1.73			1.71			1.70

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Upon the binding of actinyl cations, significant deformation was observed in the conformations of the ligands. The deformation energy (ΔE^deform) was calculated as a measure of the energy required to deform the ligand in order to adapt to its coordination with actinyl cations, and was calculated as the energy difference between the conformations before (E^ePor₀) and after (E^ePor₁) the coordination of the ligand with actinyl, i.e. ΔE^deform = E^ePor₁ − E^ePor₀. The results are plotted in Fig. 2.

Fig. 2 The deformation energy of the three ligands when coordinated with the antinyls in vacuum (left) and in MeOH (right).

We note that the deformation energies of the ligands decrease going from An(VI) to An(V), with an exception of U(V) for L2²⁻ and L3³⁻ and Pu(VI) for L3³⁻. Population analysis shows that there is electron transfer from U(V) to the macrocyclic ligands L2²⁻ and L3³⁻ while Pu(VI) remains in its hexavalent state when binding with L3³⁻. This indicates that more energy is needed for the ligands in these cases to adopt the conformation with their spin density increased or unchanged to interact with the dicationic actinyls, and is in contrast to the case of NpO₂L3⁻ where L3³⁻ donates one electron to Np(VI) and less energy is needed to deform its conformation. These show the delicate balance between the electron affinity of L3³⁻ and its ability to resist the deformation of its geometry when interacting with oxidizing dications (NpO₂²⁺ and PuO₂²⁺) and reducing monocation (UO₂⁺). The calculations also suggest that upon complexation, less energy is needed for L2²⁻ to adapt to the coordination to the actinyl moiety than that for H₂L1²⁻ and L3³⁻, as shown in Fig. 2, except for the case of U(V) where U(V) is oxidized when coordinating with the L2²⁻ and L3³⁻ ligands.

The larger deformation of H₂L1²⁻ and L3³⁻ than L2²⁻ may be explained in terms of aromaticity. In their neutral states, there are 24 and 22 π electrons in the shortest conjugation paths of H₄L1 and H₂L2, respectively, suggesting that H₂L2 is aromatic globally, while the aromaticity of H₄L1 displays a localized feature, according to Hückel's definition of aromaticity. In H₃L3, though there are 22 π electrons in its shortest conjugation path, the phenyl ring does not favor the delocalization of its electrons through the Schiff-base N atoms, leading to a broken ring current, which is to be shown later in the discussion of ACID.

In AnO₂(H₂L1)^0/− (An = U/Np/Pu) (Fig. 1 and Fig. S1, ESI,†Table 1), for all of the six actinyl cations, the actinide atoms bind stronger with N1*, N2*, and N3* than with the other three N atoms, as seen from the shorter An–N1*/N2*/N3* bond lengths of 2.4–2.6 Å compared to the values of 2.8–3.7 Å for the other three An–N bonds. The protons on N1 and N3 deviate from the planar significantly due to the repulsive interaction rising from the binding of actinyls. As a result of interaction between AnO₂^2+/+ and the ligand H₂L1²⁻, the An–O^yl bonds were perturbed with a bond length increment of 0.01–0.08 Å, and larger changes were observed for Np(VI) and Pu(VI), which is in agreement with the larger charge transfer character in these two complexes.

The fully deprotonated L3³⁻ is a hexadentate ligand with higher backbone flexibility. In our calculations, this ligand, both in its intact form and binding state, was found to be strongly deformed, which is different from the conformation solved from the crystallographic experiment²² where a planar conformation was reported. The difference may be due to the lack of the crystal packing in our calculations. In view of key geometric parameters of UO₂L3⁻, including the U–O^yl and U–N distances, there is good agreement between the calculated values and the experimental data with 1.75 vs. 1.76 Å for U–O^yl, (2.54, 2.67, 2.83 Å) vs. (2.54, 2.72, 2.86 Å) for the three pairs of equivalent U–N bonds. The ligand deformation was also observed by Shamov and Schreckenbach in their study⁸³ of the interaction between uranyl and alaskaphyrin, which is a hexadentate macrocyclic Schiff-base ligand.

In the actinyl oxasapphyrin complexes AnO₂L2^0/−, for all of the six actinyl moieties, the An–O bond length is longer than that of An–N bonds by 0.08–0.25 Å. Such a difference is relevant to the distinct nucleophilicity of the two types of heteroatoms in view of their atomic charges and the unsaturated valence of N atoms that bring more orbital overlap between actinyl and N atoms.

3.2. Free energy of the binding process

In experimental studies, triethyl amine (Et₃N) appeared as an additive when actinyl compounds were mixed with ePors in MeOH. By assuming that the ePor ligands exist in their neutral forms in MeOH, the binding of actinyls to the ePor ligands studied here accompanies with the deprotonation of the ligands assisted by Et₃N, which may be described as below:

H_kL + AnO₂(H₂O)₅ⁿ⁺ + (k − m)Et₃N → AnO₂LH^{(k − n − m)−}_m + (k − m)Et₃NH⁺ + (H₂O)₅

where H_kL represents the neutral forms of the three ligands, i.e. H₄L1, H₂L2, and H₃L3; An represents the hexavalent or pentavalent U/Np/Pu; m is the number of protons remaining on the ligand in the coordination complex, and n is the positive charge the actinyl carries. The changes in Gibbs free energy during the binding process may be obtained via the following relations:

ΔG = G(AnO₂LH^{(k − n − m)−}_m) + (k − m)G(Et₃NH⁺) + G((H₂O)₅) − G(H_kL) − G(AnO₂(H₂O)₅ⁿ⁺) − (k − m)G(Et₃N)

As shown in Table 2, in vacuum, the complexation of plutonyl(VI) with H₂L1²⁻ or L2²⁻ is in general much more exothermic than that of uranyl(VI) and neptunyl(VI), while for L3³⁻, the reaction energies of the three hexavalent actinyls are comparable within a difference of 5.3 kcal mol⁻¹.

Table 2 The calculated free energy changes (in kcal mol⁻¹) for the complexation of AnO₂(H₂O)₅^2+/+ with H₄L1, H₂L2, and H₃L3 with the assistance of Et₃N in vacuum and in MeOH

	U^VI	Np^VI	Pu^VI	U^V	Np^V	Pu^V
Vacuum
H2L1^2−	−82.0	−104.9	−123.0	99.7	92.5	99.9
L2^2−	−113.7	−113.3	−134.4	88.5	79.2	83.4
L3^3−	−21.3	−17.9	−23.2	255.5	240.2	247.5
MeOH
H2L1^2−	−19.7	−45.3	−60.6	4.4	2.3	8.1
L2^2−	−42.4	−52.5	−72.4	5.9	−17.7	−12.5
L3^3−	−35.1	−46.7	−35.3	32.5	1.9	10.6

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The actinyls with U, Np, and Pu in their pentavalent states display strong propensity to resist their binding with the deprotonated L3³⁻ ligand, and their interaction with H₂L1²⁻ and L2²⁻ ligands is also much weaker than that between the ligands and the three actinyls(VI). This is consistent with their low ability to form corrdination complexes. The much stronger binding strength between actinyls(VI) and the ligands than that between actinyls(V) and the ligands is relevant to the more positive charge the actinyls(VI) carry, and in the cases of Np(VI) and Pu(VI), the redox reaction occurred upon their binding to the ligands, which is to be discussed later.

In MeOH, the solvation effect significantly mitigates the binding strength of An(VI) with H₂L1²⁻ and L2²⁻, while brings enhancement to the coordination of An(VI) with L3³⁻ moderately and that of An(V) with the three ligands strongly. The difference rises from their distinct charge features. The coordination of actinyls(VI) with H₂L1²⁻ and L2²⁻ gives neutral species, while their binding with L3³⁻ and the coordination of actinyls(V) with the three ligands produce charged complexes. This is similar to that observed in the study⁸⁴ of [AnO₂(18-crown-6)]^2+/+ (An = U/Np/Pu), in which Schreckenbach et al. reported that polar solvent brought more negative solvation free energy change (ΔΔG_solv) for the ligand exchange reaction of hydrated monocationic actinyls(V) with the crown ether ligand than that of hydrated dicationic actinyls(VI), and proposed that this resulted from the dependence of the stabilization of a charged species due to solvation on its charge and its distance with the polarizable solvent medium. This suggests that the smaller a charged solute and the more charge it carries, the stronger stabilization it feels from the polarizable medium. This rationale still holds in the current case.

3.3. Electronic structure

3.3.1 Atomic charge and electronic configuration. In Table 3, the natural atomic charges of the actinyls in their hydrated states and their coordination complexes with the three ligands are collected. In view of the less populated natural atomic charges on O^yl in hydrated NpO₂²⁺ (−0.35e) and PuO₂²⁺ (−0.26e) than in the other hydrated actinyls, there is less charge transfer from the O^yl atoms to U(VI), U(V), Np(V), and Pu(V) than to Pu(VI) and Np(VI), which suggests stronger electron affinity of the latter ones than the former ones. This is consistent with the larger binding free energies of NpO₂²⁺ and PuO₂²⁺ than UO₂²⁺ and the pentavalent actinide species when coordinating to the deprotonated ligands H₂L1²⁻, L2²⁻, and L3³⁻, respectively, as seen in Table 2.

Table 3 The natural charges of An, O^yl, and the group of AnO₂^2+/+ in its hydrated form (AnO₂(H₂O)₅^2+/+) and in its complexes with the ligands H₂L1²⁻, L2²⁻, and L3³⁻ in vacuum

	An				O^yl				AnO2^2+/+
	Aquo	H2L1^2−	L2^2−	L3^3−	Aquo	H2L1^2−	L2^2−	L3^3−	Aquo	H2L1^2−	L2^2−	L3^3−
U^VI	1.88	1.86	1.74	1.57	−0.47	−0.54	−0.53	−0.54	0.93	0.78	0.69	0.49
Np^VI	1.63	1.53	1.39	1.28	−0.35	−0.64	−0.62	−0.64	0.92	0.25	0.16	0.00
Pu^VI	1.45	1.44	1.30	1.27	−0.26	−0.58	−0.56	−0.36	0.93	0.28	0.18	0.56
U^V	1.61	1.71	1.73	1.62	−0.70	−0.75	−0.54	−0.56	0.21	0.21	0.65	0.50
Np^V	1.42	1.56	1.44	1.32	−0.60	−0.66	−0.64	−0.66	0.22	0.25	0.15	0.01
Pu^V	1.40	1.49	1.34	1.23	−0.54	−0.60	−0.58	−0.60	0.32	0.30	0.18	0.04

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Upon binding actinyls with the ligands, there is significant ligand-to-metal charge transfer in the actinyl(VI) complexes, as seen from the decrease of group charges of the actinyl moiety with respect to those of hydrated actinyls shown in Table 3. Compared to the charge distribution of the hydrated actinyl ions, the charge transfer does not always neutralize the positive charge on the actinide atoms, but increases the negative charge accumulation of the O^yl atoms. This may be the consequence, in other words, an indication of the strong interaction between the actinide atoms and the ePor ligands, which repels the electron clouds of O^yl atoms that were in the An–O^yl bonding region, leading to the weakening of the An–O^yl covalent bond feature, as observed from the increase of the bond lengths and the decrease of the Wiberg bond index (WBI) of the An–O^yl bonds.

In the neptunyl(VI) (Np^VIO₂H₂L1⁰/L2⁰/L3⁻) and plutonyl(VI) (Pu^VIO₂H₂L1⁰/L2⁰) complexes, except for the charge redistribution, we also observed that the spin density of Np(VI) in all three complexes and Pu(VI) in PuO₂H₂L1 and PuO₂L2 becomes larger than in their formal oxidation states, which should be 1e and 2e, respectively. As seen in Fig. 3, the spin densities of Np(VI) and Pu(VI) were calculated to be about 2.2e and 3.4e, respectively, which is comparable to that of Np(V) and Pu(V) upon their coordination with the corresponding ligands. This suggests that redox reaction may occur upon the binding of neptunyl(VI) and plutonyl(VI) with these ligands, i.e. reduction of the actinide(VI) and oxidation of the macrocyclic ligands. This is consistent with the experimental studies²² where the reduction of Np(VI) and Pu(VI) has been observed in view of the presence of the characteristic Np(V) UV-Vis signals or the disappearance of the Pu(VI) UV-Vis signals except for Pu(VI) in its grandephyrin (PuO₂L3⁻) complex.

Fig. 3 Natural electron configuration for the actinide atoms in AnO₂^2+/+ cations and their complexes in vacuum (filled symbols) and in methanol (open symbols). n_α: electrons with α spin; n_β: electrons with β spin.

The population analysis shows that the transferred electron mainly locates in the 5f orbitals of Np(VI) and Pu(VI), and the occupation in the 5f orbitals increases moderately compared to the values in the bare cations, as seen in Fig. 3. Analysis of the frontier orbitals of these complexes shows that in the frontier orbitals of these complexes, there is significant 5f contribution from Np(VI) and Pu(VI), which displays much little overlap with the p-component of the ligand atoms in the same orbital, leading to an assignment of α-spin density in the 5f-component and the β-spin density in the p-components of the ligand atoms. Such a phenomenon has been observed and discussed by Kaltsoyannis⁸⁵ and Prodan et al.⁸⁶ in their studies of AnCp₃ and AnCp₄ (An = Th-Cm; Cp = η⁵-C₅H₅) and AnO₂ (An = Th-Es), respectively.

We note that the reduction of Np(VI) and Pu(VI) was also observed in the fragments of Np^VIO₂(TMOGA)₂²⁺ and Pu^VIO₂(TMOGA)₂²⁺, where TMOGA represents tetramethyl-3-oxa-glutaramide, via collision induced dissociation (CID) by electrospray ionization,¹⁰³ which otherwise remains in its +6 oxidation state. In these complexes, actinyl is coordinated to the O atom of the carbonyl or the ether group of the ligand, which has stronger electronic affinity, or “harder”, than N atoms in the ligands studied in this work. Thus, in the native complexes, Np(VI) and Pu(VI) cannot abstract one electron from TMOGA, while they are reduced when complexing with the macrocyclic ligands L1 and L2 or the fragmented TMOGA, showing the delicate balance of the electron distribution in the complexes.

3.3.2 Nature of interactions between actinides and coordinating atoms. The QTAIM topology analysis according to Bader⁷⁰ and Mata and Boyd⁶⁸ was done by using the Kohn–Sham orbitals obtained from the optimizations using the Gaussian 09 program. AIM analysis focuses on molecular electron density, which is different from the population analysis techniques and provides an alternative way to investigate chemical bonding.

In the framework of AIM, a bond critical point (BCP) is defined as the point along the bond path of an atom pair where the gradient of the electron density is zero, i.e. depending on the property of a bond, the electron density may accumulate (a shared bond) or be depleted (a closed-shell bond) at a BCP. Four descriptors⁶⁸ are used to identify the nature of chemical bonding and non-bonding interactions, i.e. the electron density ρ_b and its Laplacian ∇²ρ_b, the total electron energy density H_b(r) at a BCP, and the delocalization index between atom pair A and B δ(A, B).

According to our calculations (Table S14, ESI†), the electron density ρ_b at the BCP of the U–O^yl bond is 0.32–0.33 e⁻ bohr⁻³ in the vacuum, which is similar to those of the U≡O triple bonds,^66,87,88 suggesting a covalent bond between U and O^yl in the complexes of UO₂²⁺ with H₂L1²⁻, L2²⁻ and L3³⁻, according to Bader⁷⁰ that a ρ_b value greater than 0.2 e⁻ bohr⁻³ may be used as the lower bound to judge a covalent bond and a value less than 0.1 e⁻ bohr⁻³ as an upper bound for closed-shell interactions including ionic interactions. The values of ρ_b of An–O^yl for the other actinyls are also over 0.2 e⁻ bohr⁻³, i.e. 0.29–0.30, 0.30–0.34, 0.26–0.32, 0.28–0.29, and 0.29–0.31 e⁻ bohr⁻³ for Np^VI, Pu^VI, U^V, Np^V, and Pu^V, showing similar bonding character. This is also supported by the value of H_b(r) (Table S14, ESI†), the positive and negative values of which is used, respectively, as an alternative way of defining ionic and covalent bonds. In the calculations, the values of H_b(r) fall between −0.22 and −0.34 for all the actinyls studied here. Since the An–O^yl bond is of multiple bond feature, the delocalization index between An and O was also calculated (Table S16, ESI†), and all are in the range of 2.7–3.0, suggesting a triple bond character of An–O^yl.

In contrast to the An–O^yl bond, there is essentially little build-up of electron density between An and the coordinating atoms of the ligands in the first coordination shell, suggesting a dominant contribution of ionic interactions. It is recommended that the positive value of ∇²ρ_b is for ionic bonds while negative for a single covalent bond. In these complexes, the Laplacian is positive with a ρ_b-value smaller than the value of 0.08 e⁻ bohr⁻³ for the typical ionic LiF molecule,^66,67 suggesting only minor electron accumulation between actinide and the ligands, and the interaction between An and the electron donor atoms in the first coordination shell is best described as ionic interaction.

Taking the solvent effect of MeOH into account (Tables S15 and S16, ESI†), the values of the four descriptors fluctuate marginally, but does not change the bonding character.

In the framework of QTAIM, δ(A, B) is defined as the integration of the electron density ρ_b in the bonding region between two atoms A and B, thus it is strongly correlated with ρ_b. Matta and Hernandez-Trujillo⁸⁹ proposed that δ(A, B) may be used to calibrate the correlation between ρ_b and bond order via the relation δ(A, B) = exp[A(ρ_b − B)], where A and B are constants depending on the nature of the two atoms evaluated. Here we will not fit the relation due to insufficient samples, while it is interesting to see how δ(A, B) is correlated with ρ_b in the model systems under study, which is plotted in Fig. 4.

Fig. 4 The delocalization index δ of key bonds as a function of the electron density ρ_b in the complexes of actinyls with H₂L1²⁻, L2²⁻, and L3³⁻. Left: An(VI), right: An(V).

It is clear that δ(An, O^yl) is not sensitive to the variation of ρ_b for U, Np, and Pu both in their hexa- and penta-oxidation states. In contrast, for the An⋯N/O dative bonds, δ(An, N/O) increases nearly linearly as ρ_b becomes higher. The inert response of the An–O^yl bond in its δ(An, O^yl) to the variation of ρ_b may reflect the saturation of electron density in the bond region. The variation of ρ_b is a consequence of the distinct polarization mode of the An–O^yl bonds in the models studied here.

As mentioned above, upon the binding of actinyls to the ligands, Np(VI) was reduced to Np(V) in all of the three complexes, and so does Pu(VI) in its complexes with H₂L1²⁻ and L2²⁻. While for actinyl(V), all remain in their +5 oxidation state except for U(V) when coordinated to L2²⁻ and L3³⁻, where U(V) was oxidized to U(VI). Keeping this in mind, we observe in Fig. 4 that the electron density at the BCP, ρ_b, of An^VI–O^yl bonds, including the oxidized ones in the U^VO₂L2⁻ and U^VO₂L3²⁻, is always larger than that in the An^V–O^yl bonds, including the reduced ones.

3.3.3 Aromaticity of the macrocyclic ligands. In order to study the influence of actinyl binding on the aromaticity of the ePor ligands, we have calculated the anisotropy of induced current density (ACID) and the nucleus-independent chemical shifts (NICS) indices.

In general, the induced current density mainly circulates around nuclei where the electron density is high. For ring-shaped molecules, the current density may flow around the molecular ring. In these molecules, aromaticity is associated with diatropic ring current, which results in induced magnetic field with opposite direction to the external field, while paratropic current enhances the external field and indicates antiaromaticity.

According to our calculations, macrocyclic paratropic current was observed in H₄L1, H₃L3 and their deprotonated states, while macrocyclic diatropic current was observed in H₂L2/L2²⁻, as shown in Fig. 5. Note that, in H₂L1²⁻ and L3³⁻, there is diatropic current in local pyrrolic and phenyl rings, which is similar to that observed by Wu, Fernandez and Schleyer in their study⁹⁰ of aromaticity in porphyrinoids. Such a phenomenon may be illustrated as below in Scheme 2.

Fig. 5 ACID plots of the three ligands in their neutral (top) and deprotonated states (bottom) at an isosurface value of 0.05. L1 and L3 have paratropic macrocyclic circulation, and L2 has diatropic macrocyclic circulation.

Scheme 2 Schematic illustration of the co-play of macrocyclic (dotted line) and local pyrrolic (solid red line) circulations. Diatropic/paratropic ring currents are labeled in red/green lines and are clockwise/counterclockwise.

NICSs have been proposed to be used as a measure of the aromaticity of a molecular system.^91–93 Negative NICS values in interior positions of rings or cages, i.e. magnetically shielded, indicate the presence of induced diatropic ring currents or “aromaticity”, whereas positive values, i.e., deshielded, at each point denote paratropic ring currents and “antiaromaticity”. Thus, NICS is typically computed at ring centers, which is referred to as NICS(0), or at points 1 Å above the ring center for planar or nearly planar molecules, where the π orbitals have their maximum density, and NICS(1) was recommended as a better measure of the π electron delocalization than NICS(0).^93,94

Note that NICS has been shown^95–98 to be able to identify the aromaticity of organic molecules, while suffers from its limitations to work always as reliable descriptor of aromaticity,^90,99–101 and its use for some model systems has been warned, including the organometallic and metallic cluster systems.¹⁰² Here, as a complement of the ACID calculations, we also obtained the NICSs of the macrocyclic and the pyrrolic and furanic subunits and tried to analyze the data with caution. The values of NICS(0) and NICS(1) are −13.62 and −10.09 for pyrrole, −11.88 and −9.38 for furan, and −8.03 and −10.20 for benzene at the B3LYP/6-311+G(d,p) level, according to Chen and Schleyer and coworkers.⁸²

In this work, the NICS(0) and NICS(1) values of H₂L1²⁻, L2²⁻, and L3³⁻ and their corresponding complexes AnO₂Lⁿ⁻ (An = U, Np, Pu) have been calculated at the same level as optimization based on the fully optimized structures to investigate how the presence of actinyls and the deformation of the ligands upon their complexation with actinyls may modulate the aromaticity of the polycyclic systems. In the calculations, the presence of actinyls was mimicked by putting the Mulliken atomic charges of An and the two O^yl atoms at the corresponding sites, and the NICS(1) results are listed in Table 4. Due to the limitation of the program we used in the calculation of NICS, we have only investigated the model systems that did not experience electron transfer between the actinyl and the macrocyclic ligands.

Table 4 The NICS(1) values of the three ligands in their deprotonated free states (H₂L1²⁻, L2²⁻, and L3³⁻) and their complexation states with AnO₂^2+/+ (An = U/Np/Pu) in vacuum. The definition of the rings is given in Scheme 1

	Actinide	A Ring	B Ring	C Ring	A* Ring	B* Ring	C* Ring	G Ring
H2L1^2−	H2L1^2−	−9.11	−6.01	−3.83	−11.66	−6.76	−7.70	4.26
	U^VI	−17.12	−5.15	−13.89	−5.89	−1.41	−6.15	−0.93
	U^V	−12.62	−4.64	−7.95	−9.90	−4.88	−6.80	4.01
	Np^V	−12.62	−4.66	−7.87	−9.96	−4.77	−6.84	4.12
	Pu^V	−12.88	−4.67	−7.79	−9.92	−4.81	−6.77	4.11
L2^2−	L2^2−	−5.43	−7.62	−8.05	−5.59	−7.23	—	−4.63
	U^VI	−14.74	−10.76	−14.64	−13.46	−10.50	—	−7.91
	Np^V	−7.34	−8.43	−12.25	−7.33	−8.25	—	−9.23
	Pu^V	−7.28	−8.28	−12.14	−7.30	−8.10	—	−8.98
L3^3−	L3^3−	−7.17	−18.37	−9.18	−6.88	−20.40	—	1.39
	U^VI	−8.56	−11.77	−11.51	−8.56	−13.09	—	−1.51
	Pu^VI	−8.54	−11.53	−11.34	−8.55	−12.89	—	−0.67
	Np^V	−7.29	−11.51	−10.64	−7.22	−12.83	—	0.15
	Pu^V	−7.37	−11.32	−10.61	−7.25	−12.70	—	0.30

---

According to our calculations, the aromaticity of the deprotonated H₂L1²⁻ and L3³⁻ ligands displays a highly localized feature in view of their positive NICS(0) (Table S17, ESI†) and NICS(1) values for the macrocycles, while negative for their ring subunits. For H₂L1²⁻, both the NICS(0) and NICS(1) indices of A and A* rings are about double of the values of the other rings, suggesting stronger π character of A and A* rings than the others. As seen in Scheme 1, the difference between the two equivalent A and A* rings and the others is that these two rings are directly neighboured to two aromatic pyrrole rings, while the others are bridged by a meso-C atom, leading to more significant delocalization of π electrons to be redistributed in a broader region. Similar phenomena exist in L3³⁻, where B and B* displays stronger π character than A and A* rings. For ligand L2²⁻, both the NICS(0) and NICS(1) indices of the whole ligand and its ring subunits are negative, among which ring C, i.e. the furan ring, has the larger indices. These results are consistent with the ACID results discussed above.

Upon the complexation with actinyl cations, both the NICS(1) indices of the sub-rings of H₂L1²⁻ and L3³⁻ become less negative except for A and C rings of H₂L1²⁻ and A, A* and C rings of L3³⁻. In contrast, the NICS(1) indices of L2²⁻ become much more negative in the complexes, indicating that the deformation due to the complexation with actinyls enhances the aromaticity of L2²⁻. These suggest that ligand L2²⁻ benefits more than the other two ligands upon complexation with actinyls, and explains the larger exothermicity of L2²⁻ than H₂L1²⁻ and L3³⁻ when coordinating with the same actinyl.

4. Conclusion

The complexation of six penta-aquo actinyl complexes, AnO₂(H₂O)₅^2+/+ (An = U/Np/Pu), with three expanded porphyrins (ePors), amethyrin (H₄L1), oxasapphyrin (H₂L2), and grandephyrin (H₃L3), have been studied by range-separated hybrid functional CAM-B3LYP. The geometries, the electronic structures, the binding free energies, and the NMR properties have been analyzed, and quantum theory of atom-in-molecules (QTAIM) topology analysis was also done to understand the interaction modes between to two entities of each complex.

In view of binding free energy, in MeOH, PuO₂²⁺ and NpO₂²⁺ display stronger propensity to coordination with the ePor ligands than UO₂²⁺, UO₂⁺, NpO₂⁺, and PuO₂⁺ for the same ligand, and the binding affinity of the L2²⁻ ligand to the actinyls is larger than the other two ligands, suggesting that the introduction of the furan ring in L2²⁻ favors the binding of actinyls. In the complexes of Np(VI) and Pu(VI), we observed higher spin density accumulation on the actinide atoms. The stronger electron affinity of Np(VI) and Pu(VI) than that of U(VI), U(V), Np(V), and Pu(V) enables the former to more significantly influence the electronic structures of the ligands than the latter. This leads to electronic configurations of f⁰, f², f³, f¹, f² and f³ for U(VI), Np(VI), Pu(VI), U(V), Np(V) and Pu(V) in their H₂L1²⁻ complexes, f⁰, f², f³, f⁰, f², and f³ in their L2²⁻ complexes, and f⁰, f², f², f⁰, f², and f³ in their L3³⁻ complexes, respectively. The redox reactions upon the binding of Np(VI) and Pu(VI) to the specific ligands are consistent with the experimental observations.

The QTAIM was used here to analyze the bonding mode of AnO₂^2+/+ (An = U/Np/Pu) with H₂L1²⁻/L2²⁻/L3³⁻ and agrees well with the messages delivered by the analysis of geometric parameters and free energies along the ligand exchange reactions. Our calculations suggest that the interaction between An and the electron donor atoms in the first coordination shell is best described as ionic interaction, and covalency remains between An and O^yl in the actinyl upon their complexation with the ligands studied here.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (21573021, 21073013, and 91226105) and the CAS Hundred Talents Program (Y2291810S3) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: The optimized structures of the complexes with H₂L1²⁻, L2²⁻, and L3³⁻ ligands and their complexes with AnO₂(H₂O)₅^2+/+ (An = U, Np, Pu) in vacuum and in methanol; the calculated energies of different isomers of the three ligands in vacuum and in methanol; the influence of protonation on the complexation; key geometrical parameters of the actinyls and the corresponding complexes; the atomic charges and the atomic spin densities of key atoms and groups, and the bond orders of the complexes in methanol; the QTAIM analysis of the complexes in methanol; the NICS values of the ligands and their complexation states with AnO₂^2+/+ in methanol; the integration accuracy and the convergence criteria in our calculations; and the coordinates and energies of stationary points. See DOI: 10.1039/c6nj01615d

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