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Discovery of Heteropolytantalate: Synthesis and Structure of Two 6‑Peroxotantalo-4-phosphate Clusters Dongdi Zhang,† Zhijie Liang,† Suyi Liu,‡ Longsheng Li,‡,§ Pengtao Ma,‡ Shufang Zhao,† Haiying Wang,† Jingping Wang,*,‡ and Jingyang Niu*,† †

Henan Key Laboratory of Polyoxometalate Chemistry and ‡College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, P. R. China S Supporting Information *

ABSTRACT: Polyoxometalates (POMs) of Nb and Ta are greatly different from those of Mo, W, and V that have been studied extensively and developed well. The latter can be formed simply by acidification of their aqueous monomeric oxoanions and has found application areas from catalysis to magnetism, materials science, medicine, and nanotechnology. Even now the polyoxoniobate (PONb) chemistry has accelerated dramatically over the last 15 years, and a vast expansion of available PONbs has been reported. However, after nearly 200 years of POM research, Ta-based POM chemistry is still at its infant stage and only dominated by the isopolyoxotantalate ions (Ta6 and Ta10) and transition-metal-capped Ta6 species, along with two Tisubstituted polyoxotantalates [Ti2Ta8O28]8− and [Ti12Ta6O44]10− reported very recently. In this study, we discover two novel peroxotantalophosphate clusters [P4(TaO2)6O25]12− (1) and [P4(TaO2)6O24]10− (2) by incorporating phosphorus heteroatom into Ta-oxo framework, which represent the first two examples of heteropolytantalate. Interestingly, two P2Ta3 half-units are cisand trans-condensed in 1 and 2, leading to “open” and “closed” configurations, respectively. These two chemically and structurally related clusters can be isolated in a controlled manner, and the yields are relatively high. Both compounds were characterized in the solid state by single-crystal X-ray diffraction, 31P MAS NMR, FT-IR, TGA, and elemental analysis as well as by 31P NMR in solution. The results presented here provide a strategy to be applicable to other heteroatom-incorporated polyoxotantalates and further expand the phase space for polyoxotantalate chemistry.

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INTRODUCTION Polyoxometalates (POMs)1,2 are a class of polynuclear anionic metal oxo clusters with properties suitable for many applications including catalysis, magnetism, biomedicine, materials science, and nanotechnology.3−7 Generally, POMs can be divided into two broad families, namely, isopoly- and heteropolyoxoanions, based on whether it includes the heteroatom (commonly Si, Ge, P, As, Sb, Bi) or not.8 Up to now, a large number of isopolyoxometalates with nuclearities ranging from 2 to 368 in a single cluster have been reported.9,10 The number of publications on heteropolyoxometalates over the past 20 years has increased greatly as a result of the use of lacunary heteropolyoxoanions, which function as multidentate ligands to bind other metal ions, giving a plethora of new species.11−13 It is noteworthy that POMs of Nb and Ta are different from those of group VI. The monomeric molybdate and tungstate oxoanions can easily self-assemble to polynuclear clusters by acidification. Although the research on polyoxoniobates is not as extensive as that on polyoxotungstates or polyoxomolybdates, it is nonetheless necessary and rapidly growing.14−22 Because niobium and tantalum are in adjacent groups with tungsten and molybdenum on the periodic table, they are © XXXX American Chemical Society

expected to present similar behaviors because of their related electronic configurations, as well as have virtually identical ionic sizes caused by lanthanide contraction. However, we still know little about the polyoxotantalates (POTas) even after the first POTa cluster reported in 1954.23 Rare examples were communicated as iso-POTas on the basis of the [Ta6O19]8− (Ta6) ion24−29 along with the [Ta10O28]6− (Ta10)30 and transition-metal-capped Ta6,31−34 in addition to tantalum-substituted polyoxotungstates35 and Ti-substituted POTas36 reported in 2012 and 2016, respectively (Figure S1). As far as we know, no hetero-POTa has been observed or isolated until now. Herein, we present for the first time two unprecedented peroxotantalophosphate clusters [P 4 Ta 6 (O 2 ) 6 O 25 ] 1 2− (P4Ta6O25, 1) and [P4Ta6(O2)6O24]10− (P4Ta6O24, 2), which was isolated as cesium salt Cs3[H9P4Ta6(O2)6O25]·9H2O (1a) and guanidinium salt (CN3H6)6[H4P4Ta6(O2)6O24]·4H2O (2a), respectively. Therefore, 1a and 2a represent the first two phosphorus-incorporated hexatantalate derivatives and thus the first two hetero-POTas. Furthermore, compounds 1a and Received: October 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 2a are thoroughly characterized by single-crystal X-ray diffraction, 31P MAS NMR, FT-IR, TGA, and elemental analysis in solid state.

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Table 1. Crystal Data and Structure Refinement of Compounds 1a and 2a

EXPERIMENTAL SECTION

empirical formula formula weight, g mol−1 T/K crystal system space group λ/Å a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g cm−3 crystal size/mm3 limiting indices

Materials and Methods. All reagents and solvents were obtained from commercial sources and used without treatment. K8[Ta6O19]· 16H2O was prepared using literature method.24 Synthesis of Compound 1a. K8[Ta6O19]·16H2O (1.50 g, 0.747 mmol) was dissolved in a solution consisting of 13.5 mL of 30% aqueous H2O2 and 165 mL of water. Diluted phosphoric acid (3 mol/ L, 6.5 mL) was added dropwise under rapidly stirring for 15 min, resulting in a clear solution. The pH of the solution was adjusted to ca. 3.8 by 2 mol/L NaOH aq and then heated to 80 °C for 3 h. After this period, the beaker was removed from the hot bath and left to cool to room temperature, followed by the addition of CsCl (1.29 g, 7.66 mmol). The solution was then stirred for 30 min and filtered. The clear colorless filtrate was kept in an open beaker at room temperature to allow slow evaporation. Subsequent crystallization about 1 week yielded a colorless block with the yield of 62% (based on Ta). IR (KBr-pellet): 1112, 1056, 1005, 854, 837, 818, 784, 677, 590, 526 cm−1. Analysis (calcd., found for Cs3[H9P4Ta6(O2)6O25]·9H2O): Cs (16.81, 16.67), P (5.22, 5.18), Ta (45.78, 45.19). Synthesis of Compound 2a. Same procedure as for 1a but with pH ca. 2.5. After the hot bath, the solution was added guanidine hydrochloride (CN3H5·HCl, 0.72 g, 7.54 mmol). Yield: 65% (based on Ta). IR (KBr-pellet): 1168, 1076, 1021, 956, 849, 832, 798, 673, 587, 530 cm−1. Analysis (calcd., found for (CN3H6)6[H4P4Ta6(O2)6O24]· 4H2O): C (3.24, 3.32), H (2.18, 2.21), N (11.35, 11.33), P (5.58, 5.52), Ta (48.86, 48.25). Physical Measurements. Infrared spectra (4000−400 cm−1) of all samples were recorded on a Bruker VERTEX 70 IR spectrometer using KBr pellets. C, H, and N elemental analyses were measured on a PerkinElmer 2400 II CHNS/O analyzer. Cs, P, and Ta elemental analyses were obtained with a PerkinEimer Optima 2100 DV Inductively Coupled Plasma Optical Emission Spectrometry. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance instrument with Cu Kα radiation (λ = 1.5418 Å) in the angular range 2θ = 5−50° at 293 K. Thermogravimetric analyses (TGA) were measured on a NETZSCH STA 449 F5 Jupiter thermal analyzer in flowing N2 with a heating rate of 10 °C·min−1. 31P HPDEC/MAS NMR were measured on a Bruker AVANCE III 500 MHz NMR spectrometer (11.7 T) with a 4 mm commercial probe. The spin rate is 12 kHz. 31P NMR in water were detected on a Bruker Avance 400 MHz spectrometer operating at 161.98 MHz. X-ray Crystallography. Suitable single crystals were selected from their respective mother liquors and placed in a thin glass tube due to efflorescence. X-ray diffraction intensity were recorded on a Bruker Apex-II CCD diffractometer at 296(2) K with Mo Kα monochromated radiation (λ = 0.710 73 Å). Structure solution and refinement were carried out with SHELXS-2014 and SHELXL-2014 program packages.37,38 No hydrogen atoms were located from the difference Fourier map. CCDC 1546487 for 1a and 1472584 for 2a contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre. Crystallographic data and structure refinement parameters are summarized in Table 1.

μ/mm−1 no. of reflns collected/unique Rint GOF on F2 R1,a wR2b [I>2σ(I)] R1,a wR2b [all data]

1a

2a

Cs3H27O46P4Ta6 2371.48 296.15 orthorhombic Pnma (No. 62) 0.71073 19.0333(11) 20.7940(12) 11.0811(6) 90 90 90 4385.7(4) 4 3.569 0.37 × 0.29 × 0.26 −22 ≤ h ≤ 22 −24 ≤ k ≤ 24 −11 ≤ l ≤ 13 17.629 4003/3181 0.0378 1.034 0.0378, 0.0940 0.0521, 0.1038

C6H48N18O40P4Ta6 2222.12 296.15 monoclinic P21/c (No. 14) 0.71073 10.9958(6) 20.1314(12) 12.3269(7) 90 115.2430(10) 90 2468.1(2) 2 2.985 0.41 × 0.35 × 0.26 −13 ≤ h ≤ 13 −24 ≤ k ≤ 22 −14 ≤ l ≤ 14 13.499 4339/4001 0.0250 1.041 0.0250, 0.0567 0.0284, 0.0582

R 1 = ∑∥F 0 | − |F c ∥/∑|F 0 |. ∑[w(F02)2]}1/2. a

b

wR 2 = {∑[w(F 0 2 − F c 2 ) 2 ]/

Scheme 1. Formation of Polyanions 1 and 2, by Changing the pH and the Cation Present

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RESULTS AND DISCUSSION Synthesis. Both 1a and 2a were synthesized by a similar one-pot reaction of potassium hexatantalate with phosphoric acid in aqueous hydrogen peroxide solution (Scheme 1). We observed that the pH value and the nature of cation have decisive effet on the formation of 1a or 2a. For 1a, the pH was adjusted to ca. 3.8 by using NaOH upon stirring. The mixture was heated, filtered, and followed by the addition of CsCl. Notably, the same solution with lower pH value (around 2.5) and the addition of CN3H5·HCl yields a remarkable trans

structure. This is in agreement with reported conclusions that the nature of the inorganic cation has a significant impact on the assembly of POMs.39−41 Interestingly, the only countercation that formed suitable single crystals for X-ray diffraction analysis was the cesium and guanidinium ion for 1a and 2a, respectively. Our attempts to crystallize 1a or 2a without addition of countercation salt were failure and the same goes for lithium, sodium, and potassium counterions. B

DOI: 10.1021/acs.inorgchem.6b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Structural Analysis. The molecular structure of 1, represented in Figure 1, consists of two identical P2Ta3

Figure 3. Representation of TaO7 coordination enviorment (a), 1 and 2, highlighting the μ3-O bridging atom O7 in 1 (b) and O5 in 2 (c). Color code: Ta, blue spheres; P, pink spheres; peroxo bond, red; μ3-O atom, yellow spheres.

ment (Figures 4 and S3). It is interesting to note that the 180° rotation of P2Ta3 subunit in the condensation of 2 leads the

Figure 1. Ball-and-stick/polyhedral representations of the structure of polyanion 1: left, the basket-shaped architecture; right, polyhedral representation of the building block. Color code: TaO7, blue polyhedral; PO4, pink polyhedral; Ta, blue spheres; P, pink spheres; peroxo bond, red.

subunits connected through three μ2-O bridging ligands to form a basket-shaped architecture with idealized C2v symmetry. To the best our knowledge, such geometry of this polyanion has not been reported previously in POM chemistry. The P2Ta3 can be regarded as a supposed peroxohexatantalate fragment with a contiguous longitudinal strip of three Ta(O2) groups (one on equatorial position and two on axial position) replaced by two PO4 groups (Figure 2). Each of the three Ta Figure 4. Ball-and-stick representations of 1 (left) and 2 (right) highlighting the cis- and trans-dimeric mode of P2Ta3 subunits. Color code: Ta, blue spheres; P, pink spheres; O, red spheres.

trans configuration with differs distinctly from the cluster 1, in which the equatorial {PO4} ligands are on opposite sides. The structure of 2 is similar to that of the P4Nb6 cluster,44 with four phosphate ligands stabilizing the peroxo-{M6} cluster (M = Nb or Ta, Figure S4). This novel centrosymmetric cluster can be viewed as two P2Ta3 units fused by two Ta−μ2-O−Ta and two P−μ2-O−Ta bridges. The metal−oxygen bond lengths in 1 and 2 are classified and plotted in the order of their lengths, as shown in Figure 5, the bond lengths of Ta-peroxo and Ta−μ3-O are almost the same. As expected, the bond Figure 2. Ball-and-stick representations of [Ta6O19]8− ({Ta6}), supposed peroxo-{Ta6}, P2Ta3, and 1. Color code: Ta, blue spheres; P, pink spheres; O, red spheres.

atoms is ligated by one μ3-O bridging atom, four μ2-O bridging atoms, and one terminal peroxo group (Figure 3). All the Ta− O distances are similar, within the range 1.91(2)−2.097(8) Å, and the average value of the Op−Op bond in 1 (1.51 Å) is slightly longer than that for noncoordinated O22− (1.49 Å)42 but nearly identical to those of the peroxotantalum-substituted polyoxotungstates35 and tetraperoxotantalates (1.50 Å).43 The average Ta−Op distance of 1.96 Å is 0.1 Å shorter than that in the anion [Ta(O2)4]3−, whereas the average Op−Ta−Op angle of 45.3° is bigger to that in [Ta(O2)4]3− (43.7°) (Figure S2 and Table S1). In polyanion 2, P4Ta6O24, four μ2-O bridging ligands link the P2Ta3 units to each other, giving a centrosymmetric arrange-

Figure 5. Comparison of the metal−oxygen bond lengths in 1, 2, and P4Nb6. The inset figure is Ta−O bond lengths of 1 and 2. C

DOI: 10.1021/acs.inorgchem.6b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry lengths of [P 4 Ta 6 (O 2 ) 6 O 25 ] 12− and [P 4 Ta 6 (O 2 ) 6 O 24 ] 10− compare well to that in the previously isolated peroxoniobophosphate cluster [P4Nb6(O2)6O24]10−.44 Bond valence sum (BVS) calculations45 are carried out on these tantalum, phosphrous, and oxygen atoms (Table S2). The formulas of 1a and 2a from the X-ray crystal structure determination and the element analysis required additional nine and four protons for charge balance, respectively. The P− O(terminal) bond lengths in 1a are in the range 1.52−1.55 Å, and BVS calculations suggest that these six terminal oxygen atoms (shown in turquoise in Figure S5) have one protons associated with them (P−OH). According to element analysis, there are three cesium counter cations and polyanion 1 can then be described as {Cs3[H6P4Ta6(O2)6O25]}3−, which should be balanced by additional three protons. In addition, the BVS of the μ2-O oxygen atoms bridging Ta1−Ta2 (O3) and Ta2−Ta3 (O10) are 1.34 and 1.39, respectively. The most reasonable assumption is that the two O3 atoms are monoprotonated (shown in turquoise in Figure S5) while O10 atoms are O/OH ligands (shown in green in Figure S5). Meanwhile, the intermediate BVS values of 1.37−1.40 in 2a lead us to believe that we are looking at an oxygen and a hydroxo ligand disordered over these eight sites (shown in green in Figure S5). This is common in POM chemistry and has been reported in previous POM clusters.46,47 The total charges of polyanion 1 and 2 are therefore 3− and 6−, which are balanced by three cesium and six guanidinium counter cations in the solid state, respectively. Infrared Spectroscopy and XRPD. The Fourier transform infrared spectra (FTIR) of 1a, 2a, and K8[Ta6O19]·16H2O (Ta6) are shown in Table 2 and Figure S6. The overall IR

Figure 6. IR spectra of 1a, 2a, and Ta6 in the region 1250−400 cm−1.

water molecules and nine protons in the form of aqua ligands as well as the cleavage of six peroxide groups. The results of experimental (14.1%) value are in complete agreement with the calculated weightlessness (14.3%). This suggests that only one oxygen atom is released from each peroxide group, which is previously reported by Liu et al.50 For compound 2a, the onestep weight loss of 23.8% can be attributed to four lattice water molecules, six oxygen atoms from peroxide groups together with the oxidation of the six guanidinium organic matter (calc. 25.4%), with the decomposition products in the form of H2O, CO2, and NO components. Soild-State 31P MAS NMR. The 31P MAS NMR spectra for a crystalline sample of two compounds are shown in Figure 7. The 31P NMR spectrum of 1a displays two peaks at δ = +4.82 and −11.0 ppm; however, the latter peak exhibits splitting. The peak at δ = +4.82 ppm can be attributed to axial phosphorus atoms, whereas the splitting peak at δ = −10.1 and −11.0 ppm can be assigned to equatorial phosphorus atoms. It is should be noted that the P−O distances of axial phosphorus atoms (1.522(8)−1.548(9) Å) have a smaller variation in comparison with those of the bridging equatorial phosphorus atoms (1.517(9)−1.547(9) Å). BVS results indicate that there are nine protons localized in 1a, and the odd number of protons may form nonequivalent phosphorus atoms and thus result in the splitting at δ = −11.0 ppm. In particular, O3 and O10 atoms in the equatorial ring are OH and O/OH ligands, respectively, which may also aggravate this inequivalence. The spectrum of 2a exhibits two resonances at δ = −1.0 and −15.6 ppm, which is expected from the axial and equatorial phosphate group in the structure of polyanion 2, respectively. These results agree well with X-ray crystal structure. In addition, the difference between the two chemical shifts of 1a and 2a is probably due to the different outer environmentsthe degree of protonation of oxygen atoms linking with P. To complement our solid-state characterization results on these two compounds, we also investigated their solution studies by room temperature 31P NMR spectroscopy. Although we cannot obtain a decent 31P spectra owing to their poor solubility in water, the 31P spectra of 1a and 2a exhibit two expected peaks at +4.6 and −9.1 ppm with relative 1:1 intensities for 1a and +3.8 and −13.6 ppm with relative 1:1 intensities for 2a (Figure 8), which clearly suggests the two nonequivalent kinds of phosphorus atoms and is excellent

Table 2. Comparison of the IR Spectra for 1a, 2a, and Ta6 ν(P−O)

ν(O−O)

1a

1112, 1057, 1007

867, 853

2a Ta6

1168, 1075, 1019 none

862, 850 none

ν(TaO), ν(Ta−O−Ta) 985, 836, 818, 781, 675, 590, 523, 446 955, 832, 797, 674, 585, 529, 441 843, 684, 536

spectra of 1a and 2a are alomst the same, which indicates a structural similarity. Both 1a and 2a display strong and medium bands from 1200 to 1000 cm−1, corresponding to antisymmetric stretching of the PO bond. The medium to strong bands at approximately 985, 836, 818, 781, 675, 590, 523, and 446 cm−1 for 1a, 955, 832, 797, 674, 585, 529, and 441 cm−1 for 2a are ascribed to the antisymmetric stretching vibrations of the terminal TaO bonds and the TaOTa bridges, respectively. The two obvious differences in IR spectra of 1a, 2a and Ta6 are the appearance of bands at 850 cm−1 and strong intensity peaks in the regions 1200−1000 cm−1 (Figure 6), which originate from the peroxo group and PO bond,48,49 respectively. This agrees well with the solid-state structure. In addition, X-ray powder diffraction patterns of compounds 1a and 2a agree well with their simulated patterns based on the single crystal (Figures S7 and S8), indicating the phase purity of the materials. Thermogravimetric-Mass Spectrometry. Thermogravimetric-mass spectrometry (TG-MS) analyses of compounds 1a and 2a have been performed in the range 25−1000 °C (Figures S9−10). The TG curve of compound 1a indicates that the weight loss of 1a can be regarded as a continuous three-step weightlessness, corresponding to the release of nine lattice D

DOI: 10.1021/acs.inorgchem.6b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. Soild-state 31P MAS NMR of 1a and 2a.

Figure 8. Evolution of the 31P NMR spectra of compounds 1a (left) and 2a (right) in solution.

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consistent with the solid-state architecture of 1a and 2a, repectively. The unexpected peak at ca. 0 ppm is also poorly resolved, indicating the existence of a “free” phosphate group that may originate from the immediate dissociation of the phosphate group. Interestingly, addition of extra phosphate to the solution leads to an obvious increase of the signal at ca. 0 ppm. Therefore, we ascribe this signal to the two “bound” phosphate groups that actually are in equilibrium with “free” phosphate, as shown by the singlet at ca. 0 ppm. This is in agreement with the findings of Kortz et al.51 Moreover, these two compounds can be retained in solution over ca. 30 h. However, an unexpected resonance at +3.9 and −11.7 ppm for 1a and 2a appeared 30 h later, respectively, which may be attributed to the part dissociation of 1a and 2a.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02524. Bond distances and angles, figures of structures, BVS calculation results, solid-state FT-IR spectra, TGA spectra, XRD patterns, and NMR spectra (PDF) Accession Codes

CCDC 1472584 and 1546487 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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CONCLUSION

AUTHOR INFORMATION

Corresponding Authors

In summary, two unpresented heteropolyoxotantalates were successfully synthesized by accurately controlling pH and cation. Our results represent a step forward in polyoxometalate chemistry on different fronts. First, to the best of our knowledge, such a hetero group incorporated polyoxotantalate cluster has never been reported so far. Second, potassium and guanidinium cations combined with pH act as efficient factors to form such cis- and trans-condensed clusters. Moreover, the terminal oxygen ligands on the phosphorus centers can allow further derivatization reactions, which are expected to bring forward interesting complexes. All in all, this work opens a new route to the design of POM-based supramolecular functional materials.

*J. Wang. E-mail: [email protected]. *J. Niu. E-mail: [email protected]. ORCID

Jingyang Niu: 0000-0001-6526-7767 Present Address §

Longsheng Li, Nankai University, Tianjin, China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Numbers 21371048 and 21671056), the Postdoctoral Foundation of Henan Province E

DOI: 10.1021/acs.inorgchem.6b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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(2015031), 2015 Young Backbone Teachers Foundation from Henan Province (2015GGJS-017). J. Niu gratefully thanks Henan University and Key Lab of Polyoxometalate Science of Ministry of Education. The solid-state 31P MAS NMR study was conducted at Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, by Prof. Jiwen Feng. D. Zhang thanks Vikram Singh, worked as a Post-Doctor in our research group, to polish the English writing.

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DOI: 10.1021/acs.inorgchem.6b02524 Inorg. Chem. XXXX, XXX, XXX−XXX