3.1 The PDS-U and the SFS-U Structures

The molecular simulation was performed using DFT calculations[19] at the B3LYP[20]/6-311G** level for U(BSAPDA)₂. The results showed that uranium (IV) could be coordinated by two BSAPDA molecules. These formed a complex of U(BSAPDA)₂ with two different structures and a ratio of 1:2. The optimized configurations were shown in Figure 1 (PDS-U) and Figure 2 (SFS-U), respectively. In Figure 1, a and b show the two different perspectives, i.e., the front view and the top view, respectively. The two BSAPDA molecules lay upwards and downwards at the uranium in similar "∨" and "∧" shapes, respectively. The two BSAPDAs’ middle benzene rings (the benzene rings of o-phenylene-diamine) adopted a parallel dislocation structure (PDS) with an angle of 56.64^o, and uranium(IV) occupied the central position of the molecular structure. The numbers of atoms are listed in Figure 1 c and d, c and d as the top view and bottom view of the PDS-U, respectively. The other U(BSAPDA)₂ structures were shown in Figure 2. Uranium(IV) coordinated with four nitrogen atoms and four oxygen atoms of the two BSAPDA molecules give a staggered finger "+" structure (SFS). In the SFS, the benzene rings of o-phenylene-diamine of BSAPDAs were perpendicular to each other, and the transverse BSAPDA molecule was similar to "∨" shape. The uranium ion is located at the center of the "+" shape. The numbers of atoms are listed in Figure 2 g and h. This figure shows the top view and right side view (e), g shows the number of transverse BSAPDA ligands, and h is the number of the vertical BSAPDA for the SFS-U.

Blue, red, green and yellow balls (except the center U atom) stand for N, O, C and H atoms, respectively. The main structural parameters from the molecular simulation are shown in Table 1 and Table 2.

The calculated bond lengths and bond angles for the PDS-U and the SFS-U are listed in Tables 1 and 2. It was clear that the PDS-U was totally different from the SFS-U, in which the two BASPDA ligands combined with a U atom such that the middle benzene ring had a parallel dislocation angle of 56.64^o. Although the main bond lengths of the PDS-U were close to the SFS-U, the main bond angles were totally different from each other. This implied that there were two different structures for U(BSAPDA)₂.

There were no experimental data available for U(BSAPDA)₂. Thus, we compared the calculated results with that of Th(BSAPDA)₂ as recently synthesized by He et al. [17]. Note that only one structure of PDS-Th was obtained in their experiment (Tables 1 and 2). The comparison showed that the calculated parameters for PDS-U are very similar to that of PDS-Th, confirming the accuracy of our calculations.

3.2 Atomic charge distribution

The atomic charges obtained from the simulation for the PDS-U and the SFS-U are listed in Table 3. The numbers of atoms are shown in Figure 1 c and d and Figure 2 g and h. The positive electric charge values of U in the PDS-U and the SFS-U are +2.4009 and +2.3934, respectively. This indicates that the donor atoms in the PDS-U and the SFS-U could coordinate to the uranium ion with different binding modes.

3.3 Infrared Spectroscopy

The infrared spectra of the PDS-U and the SFS-U obtained by molecular simulations at the same level are shown in Figure 3. There was no imaginary frequency for the calculated IR vibrations of all geometry structures. This indicated that these two structures are quite stable. The C=N stretching vibration absorption peak of the PDS-U appeared at 1638.75 cm^-1, and the benzene ring stretching vibration absorption peak was at 1574.06 cm^-1. These observations are similar to that for the PDS-Th complex in the experiment, i.e., 1612 cm^-1 and 1574 cm^-1, respectively[17]. For the SFS-U complex, the C=N stretching vibration absorption peak appeared at 1640.98 cm^-1, and the benzene ring stretching vibration absorption peak was at 1574.30 cm^-1. These were very close to the corresponding peak for the PDS-U complex. However, the absorption intensities were significantly different, and the benzene ring stretching vibration absorption of the PDS-U was stronger than that of the SFS-U.

Fig. 3Full-screen View

Infrared spectra of the PDS-U and the SFS-U. Curve (1) was PDS-U and curve (2) is SFS-U

3.5 The thermal properties of complexes

To further investigate the stability of the two complexes, we calculated the changes of Gibbs free energy (ΔG) and enthalpy (ΔH) for the complexation reactions (U⁴⁺+ 2BSAPDA→U(BSAPDA)₂+2H⁺) under the same model. Table 4 shows the calculated thermal properties for the PDS-U and the SFS-U of U(BSAPDA)₂ at 298.15 K and 1 atm. The results showed that SFS-U has a ΔG value that is ~64 KJ/mol lower than that of PDS-U; the ΔH value is ~63 KJ/mol lower than that of PDS-U. These results indicated that SFS-U is more stable than PDS-U and the BSAPDA ligand coordinates more tightly to U(IV) in the SFS-U complex versus PDS-U complex.

3.6 Molecular orbitals and energies

Figure 4 presents the Kohn-Sham representations of HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) for the PDS-U and the SFS-U of U(BSAPDA)₂. The red and green portions represent the positive and negative phase of the molecular orbitals, respectively.

Fig. 4Full-screen View

Molecular orbitals and energies of the PDS-U and the SFS-U.

The highest occupied molecular orbital energy (E_HOMO) of the PDS-U is shown on the left in Figure 4 and was -3.7889 eV. The lowest unoccupied molecular orbital energy (E_LUMO) of the PDS-U is shown on the right and was -2.2903 eV. The energy gap between LUMO and HOMO was 1.4985 eV, namely, ΔE = 1.4985 eV. This indicated that to get the excited PDS-U, an energy of 1.4985 eV would be absorbed by the ground state molecule. For SFS-U, E_HOMO was -4.1296eV, the E_LUMO was -2.1075 eV; the energy gap (ΔE) was 2.0221 eV—this indicated that 2.0221 eV would be absorbed when the ground state SFS-U was excited. Thus, the ground state of SFS-U is harder to excite than the ground state of PDS-U. The SFS-U complex is thus more stable than the PDS-U complex (Table 5).

The HOMO energies were negative in the two U(BSAPDA)₂ complexes. This implied that the extent of electron delocalization increased after the complex formation. The energies of the occupied orbitals decreased, and stability was enhanced in the system.

3.7 Wiberg bond order indices

Generally, larger bond orders have larger bond energy and more stability[24]. Table 6 shows that the U-O WBIs ranged from 0.7179 to 0.7581 for the complexes on the same basis of NBO (natural bond orbital). The U-N WBIs changed from 0.3508 to 0.3948. The U-O Wiberg bond indices (WBIs) of the complexes were higher than the U-N Wiberg bond indices. This showed that the U-O bonds have more covalent characters than the U-N bonds. For the PDS-U complex, the WBIs of both the U-O bonds and U-N bonds were slightly less than those of the SFS-U complex. This indicated that the strengths of the U-O and U-N bonds in SFS-U complex were slightly stronger than that in the PDS-U complex.