A Series of Cube-Shaped Polyoxoniobates Encapsulating Octahedral Cu12XmOn Clusters With Hydrolytic Decomposition for Chemical Warfare Agents

This study reported a series of cube-shaped polyoxoniobates, {MCu12O8)(Cu12XmOn)(Nb7(OH)O21)8} [M = Nb(1, 2), Ln3+(3), X = I(1, m = 3, n = 3; 2, m = 5, n = 1), Br(3, m = 5, n = 1)]. As the first octahedral Cu12XmOn cluster incorporated polyoxoniobate, the cube-shaped three-shell structure of {MCu12O8)(Cu12XmOn)(Nb7(OH)O21)8} polyanion contains a {MCu12O8} body-centered cuboctahedron, a {Cu12XmOn} octahedron and a {Cu12(Nb7(OH)O21)8} cube. Compounds 1, 2, 3 show effective catalytic activities for the hydrolytic decomposition of chemical warefare agent simulants.


RESULTS AND DISCUSSION
Single crystal X-ray diffraction reveals the brown compound 1 crystallizes in triclinic P1 space group. Compound 1 possesses a 3D extended inorganic polyoxometalates framework constructed from cupreous-halide incorporated { ( polyanion in 1 possesses a centrosymmetric structure, which can be described as a three-shell structure. The innermost shell is a bodycentered cuboctahedral {NbCu 12 O 8 } cluster unit ( Figure 1A). The Nb atom located in the center of the cuboctahedron adopts special eight-coordinated cube geometry. In the {NbO 8 } core, the central Nb atom is disordered at seven positions, and the site occupancies of Nb 3A , Nb 3A '(1-x, 1-y, −4-z), Nb 4A , Nb 4A '(1x, 1-y, −4-z), Nb 5A , Nb 5A '(1-x, 1-y, −4-z), Nb 37 atoms are 0.120, 0.120, 0.126, 0.126, 0.128, 0.128, and 0.252, respectively. There is, therefore, one Nb atom in the central position. The Nb-O bond lengths are located in the range of 1.965 (6)−2.421 (4) Å (Supplementary Figure 1). This phenomenon is probably attributed to the steric effect because the Nb atom is located in the central of the regular cuboctahedron. Bondvalences sum calculations (BVS) show that the valance states of the Cu atoms are +2, which is verified by XPS analysis (Supplementary Figure 2). As shown in Figures 1A,C, 12 Cu atoms are linked by eight µ 4 -O atoms to form a 13-nuclearity body-centered cuboctahedron {NbCu 12 O 8 } cage-cluster unit containing six Cu 4 square and eight triangular Cu 3 windows. It is noteworthy that such a body-centered cuboctahedron {NbCu 12 O 8 } cage-cluster is rarely found in polyoxoniobates.
The third shell is a regular cubic cage-cluster  connected to one six-coordinated Cu atom by two bridged O atoms (Supplementary Figure 5).
It is possible to effectively vary the central core metal atoms and the halogen atoms to obtain compound 3.  Supplementary Figures 11, 12; EDS-Mapping, Supplementary Figures 26, 27). Two {LnO 8 Cu 24 Br 5 O)(Nb 7 (OH)O 21 ) 8 } clusters are bridged by two Na + to form a polyanion dimer. The polyanion dimers are arranged in parallel along the a, b, and c axes (Supplementary Figure 13).

BASED-CATALYSIS PROPERTIES
Previous investigations indicate that PONbs can catalyze the hydrolytic decomposition of chemical warfare agent simulants, such as dimethyl methylphosphonate (DMMP) and diethyl cyanophosphonate (DECP) (Guo et al., 2016). Taking this into account, we tested the catalytic performance of 1, 2, and 3 in the hydrolytic decontamination of the nerve agent simulants DMMP and DECP.
Because purity and stability are important in the viability of a catalyst, we conducted experiments on the purity and stability of 1, 2 and 3 (Supplementary Figures 14-27). Compounds 1, 2, and 3 were immersed in water and recollected for IR spectra. The IR spectra of 1, 2, and 3 after immersion were consistent with those before. This indicates that compounds 1, 2, and 3 maintain physical integrity and that no other new phases were generated (Supplementary Figures 28-30). To evaluate the catalytic properties of compounds 1, 2 and 3 for CWA destruction, we first analyzed the hydrolysis of dimethyl 4-nitrophenyl phosphate (DMMP). Fifty milligram samples 1, 2, or 3 were used as catalysts, and 15.5 mM of DMMP was dispersed in 1 mL of H 2 O and 0.6 mL D 2 O at room temperature and 1 atm. The results showed that 26.5, 20.63, and 19.35% DMMP was converted to methyl phosphoric acid in 264 h when compounds 1, 2, and 3 were used, respectively (Figure 3 and Supplementary Figures 31-33). In contrast, no non-toxic degradation production methyl phosphonic acid (MP) of DMMP can be detected in the absence of 1, 2, and 3, suggesting that all of them are efficient DMMP catalysts. Their hydrolytic reactivity is weaker than that of KGeNb (54% conversion under the same reaction conditions) (Guo et al., 2016). Compound 1 is more active than 2 and 3 relatively. The IR spectra of compounds 1, 2, and 3 after catalytic hydrolysis were consistent with those before catalytic reaction, respectively, indicating that the structures of compounds 1, 2, and 3 remain unchanged (Supplementary Figures 34-36).
We further investigated the catalytic performance of compounds 1, 2, and 3 in the hydrolytic degradation of another chemical warfare agent, DECP to diethyl hydrogen phosphate (DEHP). As shown in Figure 3, 100% of DECP was converted by 1 in 35 min, by 2 in 20 min, and by 3 in 30 min, respectively. The hydrolysis of the P-CN bond was monitored by 31 P NMR spectroscopy. The results reveal that compounds 1, 2, and 3 can greatly accelerate the hydrolytic reaction. These catalytic activities are comparable to that of KGeNb (Guo et al., 2016) reported by Hill and co-workers.
The reusability of compounds 1, 2, and 3 was also evaluated in the degradation of DECP (Supplementary Figures 37-39). The catalytic activities of compounds 1, 2, and 3 were maintained after 3 cycles. The IR spectra and XRD patterns of compounds 1, 2, and 3 after the three-cycle catalytic degradation of DECP reveal their crystalline integrity in catalytic reaction (Supplementary Figures 40-45). Compared with the decomposition of DMMP, the complete hydrolytic degradation of DECP under mild ambient conditions is of interest and has practical applications in providing human protection in realworld environments. Compounds 1, 2, and 3 contain basic {(MCu 12 O 8 )(Cu 12 X m O n )(Nb 7 O 22 ) 8 } clusters with high negative charges. The protonation of the {(MO 8 Cu 24 X m O n )(Nb 7 (OH)-O 21 ) 8 } could be the key step of the overall mechanism . The incorporation of X into the cluster is favorable for the hydrolytic degradation of chemical warfare agents.

CONCLUSIONS
The first series of cube-shaped CuX-incorporated polyoxoniobates 1, 2, and 3 have been constructed based on the {MCu 12 O 8 } cluster, the {Cu 12 X m O n } cluster, and the {Nb 7 (OH)O 21 } 8− subunits under hydrothermal conditions. The cuboctahedron-in-octahedron-in-cube structure of {(MO 8 Cu 24 X m O n )(Nb 7 (OH)O 21 ) 8 } polyanion is a new structure of CuX-incorporated heteropolyniobates. Compounds 1, 2, and 3 can effectively catalyze the hydrolytic degradation of the nerve agent simulants DECP (conv. 100% in 20-35 min) and DMMP. The incorporation of a cupreous-halid cluster into PONbs not only enriches the limited structural type of PONbs but also improves the hydrolytic activities.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
All experimental work was performed by Y-LW under the guidance of S-TZ, Y-QS, and X-XL. The manuscript was written by Y-LW with contributions and corrections from Y-QS and S-TZ. All authors contributed to the article and approved the submitted version.