Two Dawson-type U(VI)-containing selenotungstates with sandwich structure and its high-efficiency catalysis for pyrazoles

Mengyun Cheng, Yufeng Liu, Weixin Du, Jingwen Shi, Junhu Li, Hiying Wng,Ke Li, Guoping Yng,?, Dongdi Zhng,?

a Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China

b Jiangxi Province Key Laboratory of Synthetic Chemistry, Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China University of Technology, Nanchang 330013, China

ABSTRACT Two novel uranium-containing selenotungstates Na3[H19(UO2)2(μ2–O)(Se2W14O52)2]·41H2O (U2) and(NH4)10[H4(SeO)2(UO2)2(H2O)2(H2Se2W14O52)(Se2W14O52)]·66H2O (Se2U2) based on the {Se2W14O52} unit were successfully prepared and fully characterized.To our knowledge, the uranium is firstly introduced into the selenotungstates.Moreover, it is notable that U2 exhibits excellent Lewis acid-base catalytic activities in the condensation cyclization of sulfonyl hydrazides with diketones to synthesize polysubstituted pyrazoles.All the desired products were obtained in moderate to good yields (up to 99%).

Keywords:Selenotungstates Uranium Dawson-type Condensation reaction Pyrazoles

Pyrazoles form an important class of heterocycles and have recently attracted much attention due to a wide range of pharmacological and biological activities [1–3], such as antimicrobial,antibacterial, anti-inflammatory, antifungal, antiproliferative [4,5].Typical drugs examples containing pyrazole skeletons are celecoxib and rimonabant which have been sale in market [6,7].Because of the prominent roles, various methods have been reported for the synthesis of pyrazoles [8–10].Among the reported strategies, the most efficient method is the direct condensation of 1,3-diketones with hydrazines in the presence of acid, such as polystyrenesupported sulfonic acid (PSSA) [11], sulfuric acid [4], H2SO4·SiO2[12],Fe/MWCNTs [13], and so on [14,15].However, most of the methods suffer from certain drawbacks including long reaction times, unsatisfactory yields, and toxic nonreusable catalyst.Many encouraging advances have been reported, but the development of efficient catalytic systems for pyrazoles is still highly desirable.

Polyoxometalates (POMs) [16,17], large anionic molecular metaloxygen clusters of the group V and VI elements, have attracted increasing interest because of their strong acid and etching ability in catalysis, magnetism, biomedicine, materials science, and nanotechnology [18–25].POM ions have ideal multidentate oxygendonor ligands toward various electrophiles, making them promising candidates for the encapsulation of polynuclear d- and fblock metal-oxo fragments, leading to products with interesting properties [26–30].In this context, 3d transition metal-containing POM ions represent a large and particularly versatile subclass of POM chemistry [31–36].Given the same group in the periodic table and the similar behavior in coordination chemistry,we think many POMs long known for 3d or 4f ions should be realized for the actinides.However, only a handful of uraniumcontaining POMs (UCPOMs) examples are known.Indeed, we are aware of only seven Dawson-type-based UCPOMs since the pioneering work of Popeet al.in 1999, in which two [PW9O34]9–groups sandwich two UO22+and two Na+cations [37].In particular, Pope’s group has been active in this area, reporting three dimeric and one tetrameric tungstophosphate complexes in 2003,[U(α1-P2W17O61)2]16–(Fig.1a), [U(α1-P2W17O61)(α2-P2W17O61)]16–(Fig.1b), [U(α2-P2W17O61)2]16–(Fig.1c), and {U12(P2W15)4}(Fig.1d), in which two isomeric [P2W17O61]10–units and four[P2W15O56]12–anions are linked together by one uranium and four trigonal {U3} groups, leading to the first examples of 2:1 type and tetrahedral clusters, respectively [38,39].In 2008, Kortzet al.have reported the first uranium–peroxo derivative of‘P6W36’ion {U8(P2W12)3} (Fig.1e), in which three ‘P2W12’units combine to a gapped ring encircling a central U8–peroxo moiety [40].Recently, reaction of the macrocycle {P8W48} precursor with UO2(NO3)2·6H2O in formic acid buffer aqueous solution and lithium acetate solution leads to the formation of monodimensional chain constructed by {U7.2P8W48} units (Fig.1f) and a peroxouranium-containing W48wheel {[(UO2)4(O2)4]2P8W48}(Fig.1g), which is isolated by Duvalet al.and Kortzet al.in 2019 and 2020, respectively [41,42].

Fig.1.Summary of the reported Dawson-type of UCPOMs up to now.

It should be noted that there are few studies of the potential applications on UCPOMs, and only limited reports on its photochemical properties.To the best of our knowledge, the important application of Lewis acid-base catalytic activity of POMs in organic synthesis has drawn much attention [3,8], but rare catalytic performances of UCPOMs have been reported before.More recently,we have continued our efforts by developing an alternative synthetic strategy by using the template effect of heteroatoms to form new ‘open’structures with unsaturated nucleophilic oxygen atoms,which show strong affinity towards electrophilic linkers to construct bigger assemblies.Also, wehave been dedicated to investigate the catalytic activity of POMs for a long time [43–49].

Herein, we present two novel, previously unknown uraniumcontaining selenotungstates (UCSTs), Na3[H19(UO2)2(μ2–O)(Se2W14O52)2]·41H2O (abbreviated as U2) and (NH4)10[H4(SeO)2-(UO2)2(H2O)2(H2Se2W14O52)(Se2W14O52)]·66H2O (abbreviated as Se2U2).In contrast with the previously reported complexes mentioned above, two {Se2W14} units are linked together with U2and Se2U2to give sandwich-dimeric species (Fig.2).As we know,such a dimeric structure has not yet been observed.The structures have been fully analysed by single-crystal X-ray structure analysis,infrared spectroscopy (IR), thermogravimetric (TG) analysis, and powder X-ray diffraction (PXRD).Moreover, U2as an efficient catalyst to catalyze the condensation cyclization reaction of sulfonyl hydrazides with diketones to synthesize polysubstituted pyrazoles has also been studied.

Compounds U2and Se2U2were obtained by utilizing a one-pot synthetic strategy employing SeO32?as a heterotemplate to generate multivacant selenotungstate (ST) clusters, which can further bind the uranyl UO22+cationic group.It is well-known that the self-assembly of POMs is quite sensitive to subtle synthetic parameters, such as pH and counter cations.For U2, {α-Se2W14} framework was always the preferable product when the pH value was 5.4.Notably, even if U2does not contain any dimethylamine hydrochloride (DMAHC) ligand, its presence during the synthetic process seems crucial since it has not been possible to obtain U2in absence of this reactant.DMAHC was introduced into the reaction system so that it can facilitate enhancement of the solubility of U cations in the Se-O-W system [50].However, under similar reaction conditions, but with lower pH 4.0 and in the presence of NH4Cl,the complex Se2U2is obtained.The two complexes show how it is possible to control the transformation of the clusters (from {α-Se2W14} to {α-Se2W14} and {γ-Se2W14}) by slightly changing the synthetic conditions.

Fig.2.(a, c) Combined polyhedral/ball-and-stick representations of U2 and Se2U2.(b) Simplified view of the four UVI centers in U2.(d) Simplified view of the central tetra-Se/UVI cluster in Se2U2.Color code: Se, lavander; W, green; U, yellow; O, red.

Single-crystal X-ray diffraction analysis of the obtained yellow block crystals (Table S1 in Supporting information) reveals that U2and Se2U2crystallizes in the monoclinic and triclinic system with the space groupC2/mandP-1, respectively.U2consists of two identical {α-Se2W14} units connected by four UVIcenters through sixteen terminal oxygen atoms from WO6octahedra of each {α-Se2W14} group (Fig.2a).Thus, the structure of cluster U2can also be simplistically viewed as two tetravacant {α-Se2W14} units linked together by four uranium atoms (Fig.S1a in Supporting information).Each of four U atoms is coordinated by four terminal oxygen atoms from two {α-Se2W14} units with the average bond length of 2.344, one doubly-bridging oxygen atom with the average bond length of 2.509, and two terminal oxo groups, forming a pentagonal bipyramidal coordination geometry (Fig.S1b in Supporting information).The axial U–O bond distances in U2are similar to those in structures previously reported [51], in which the external and the internal bond distances for U1–O28 and U1–O30 are in the range of 1.816–1.836and 1.752–1.818, respectively.The uranyl groups have a small deviation from linearity with angles of 179.6° for O28–U1–O30 and 176.1° for O29–U2–O31, which are within the expected range for the UO22–entity.Interestingly,the four UVIcenters form a regular rectangle with the edges in the range of 4.662?4.714and the U1?U2A?U1A angle 90.0°(Fig.2b).It should be noted that crystallographic refinement indicates that occupancy factors of theμ2–O atoms and the four uranium positions are all 0.5, leading to the formula [(UO2)2(μ2–O)(α-Se2W14O52)2]22?.

Table 1 Screening the reaction conditions.a

The {α-Se2W14} unit is a tetravacant Dawson fragment which is formed by removing four corner-sharing WO octahedra from the plenary Dawson cluster {α-Se2W18}, and thus it possesses eight unsaturated oxygen atoms, having the potential for further coordination (Fig.3).Further, the 60° rotation of the two edgesharing triads of cap-position in {α-Se2W14} leads to the isomer{γ-Se2W14}.Noteworthy, {Se2W14}-based structures are rarely observed in POM chemistry.Su and co-workers presented the first examples of {Se2W14}-based POM clusters in 2014–2016 [52–54].Very recently, Zhaoet al.reported several rare-earth-embedded Dawson ST derivatives [55,56].

Fig.3.Ball-and-stick representations of plenary {α-Se2W18} cluster (left), tetravacant {α-Se2W14} (middle) and {γ-Se2W14} (right) segments, highlighting the transformation of {α-Se2W18} to {α-Se2W14} and {α-Se2W14} to {γ-Se2W14}.Color code:Se, lavander; W, green; O, red.

Regarding the solid-state structure of Se2U2, the cluster geometry resembles that of U2but with subtle difference.Firstly, the building units in U2are two identical {α-Se2W14} fragments, while Se2U2comprises two different Dawson fragments.Secondly, two of the four central UVIcenters in anion U2have been replaced by two SeIVions (Fig.S5 in Supporting information).Thirdly, the UVIoccupancy factors in Se2U2are 1 instead of 0.5 in U2.In addition, the central tetra-Se/UVIcluster in Se2U2can be simplified as a parallelogram with the angles of 80.6° for U1?Se6?U2 and 99.4°for Se6?U1?Se5.Bond-valences sum (BVS) calculations show that the W6+and Se4+centers in U2and Se2U2are hexacoordinated and tetracoordinated with an octahedral geometry and triangular cone geometry, respectively (Table S2 in Supporting information).Although reports on TM- or RE-substituted STs are burgeoning, the sandwich-type STs are still very limited.Clusters U2and Se2U2revealed a novel, previously unknown sandwich-type UCST structure.Moreover, U2and Se2U2are fully characterized by IR, TG and PXRD(Figs.S7-S10 in Supporting information).

Subsequently, in order to explore the catalytic performance of the U2and Se2U2, they were applied to catalyze the condensation of benzenesulfonohydrazide 1a and acetylacetone 2a to evaluate the catalytic activity.The desired product 3a was obtained with 34% yield at room temperature for 15 min without catalyst(Table 1, entry 1).When the reaction carried out in the presence of U2or Se2U2, the yields of 3a increased to 77% and 69%, respectively (entries 2 and 3).These above results indicated that the U2and Se2U2have catalytic activity for this reaction and the U2perform better.Then, the U2was chosen as catalyst for reaction time screening, the yield of 3a was improved to 85% with the reaction time increased to 60 min (entries 4 and 5).The results of temperature screening show that increasing the temperature benefited this condensation reaction and the 3a was obtained in 97% yield at 80°C (entries 6–8).On further increasing the catalyst loading of U2to 0.2 mol%, the yield of 3a was obtained in 99% (entry 9).

Table 2 Substrates scope of the cyclization of sulfonyl hydrazides and 1,3-diketones.a

Having the optimized reaction conditions in hand, we next explored the scope of this reaction.All results were summarized in Table 2, acetylacetone 2a, 3-methylpentane-2,4–dione 2b,3-chloropentane-2,4–dione 2c and 2,6-dimethylheptane-3,5–dione 2d could react with benzenesulfonohydrazide 1a smoothly, delivering the desired products 3a-3d in 99%, 99%, 97% and 99%yields, respectively (entries 1–4).Furthermore, the different sulfonyl hydrazines were also tested with the above of four diketone.By usingp-toluenesulfonhydrazide 1b as reaction partner,the corresponding products 3e-3h were obtained with 94%, 93%,95% and 95% yields, respectively (entries 5–8).Further, the 4-chlorobenzenesulfonohydrazide 1c reacted with acetylacetone 2a,2b, 2c and 2d, giving the desired products 3i-3l in 95%, 95%, 95%and 98% yields, respectively (entries 9–12).

More practically, this reaction could be performed on a gramscale (5 mmol scale), clearly showing its potential application in organic synthesis (Scheme 1).The model reaction of benzenesulfonohydrazide 1a and acetylacetone 2a were investigated under the standard conditions, and the desired product 3a were obtained in 97% yields, respectively.

Combining the nucleophilic Lewis base surface of POMs with Lewis acid of U cations, the compound U2could function as Lewis acid–base catalysts [3,57] and effectively promote the condensation reaction.As shown in Scheme 2, the U cations in U2act as effective Lewis acid for activation of 1,3-diketones, and the peripheral oxygen atoms of W-O-W species act as Lewis base for activation of sulfonyl hydrazides.

Scheme 1.Gram-scale reactions.

Scheme 2.Proposed mechanism.

In conclusion, two Dawson-type-{Se2W14}-based dimerirc uranium-containing selenotungstates Na3[H19(UO2)2(μ2–O)(Se2W14O52)2]·41H2O (U2) and (NH4)10[H4(SeO)2(UO2)2(H2O)2(H2Se2W14O52)(Se2W14O52)]·66H2O (Se2U2) were successfully prepared and thoroughly characterized.The clusters represent the first examples of Dawson-type UCSTs in POM chemistry.Furthermore,U2as an efficient catalyst, the polysubstituted pyrazoles can be synthesized in the condensation cyclization of sulfonyl hydrazides with diketones under moderate conditions to excellent yields.This work may benefit to the development of POMs chemistry,catalytic chemistry and actinides chemistry.Further researches on the synthesis of novel U-containing POMs and their catalytic performance in our group are underway.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos.22071045, 22001034), Excellent Youth Science Fund Project of Henan Province (No.202300410042), and the Open Fund of the Jiangxi Province Key Laboratory of Synthetic Chemistry (No.JXSC202008).Dr.D.Zhang thanks for the start-up fund of Henan University.

Supplementary materials

Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.cclet.2021.11.059.