An unprecedented organic–inorganic hybrid based on the first {Nb₁₀V₄O₄₀(OH)₂}¹²⁻clusters and copper cations

Abstract

An unprecedented organic–inorganic hybrid {[Cu₆L₆(H₂O)₃][Nb₁₀V₄O₄₀(OH)₂]}₂·13H₂O (1) (L = 1,10-phenanthroline) containing the unreported {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ building blocks has been successfully synthesized and its photoluminescent properties, IR spectra, thermogravimetric analyses and single-crystal X-ray diffraction were investigated.

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Polyoxometalates (POMs) are an outstanding class of nanosized metal oxoanions with wide structural diversity and interesting properties that have potential applications in catalysis, electrical conductivity, photochemistry, medicine and magnetism.¹ However, up to now, the development of polyoxometalate chemistry is still dominated by the polyoxotungstates, polyoxomolybdates and polyoxovanadates, because their polyanions can be easily formed over a wide pH range and sometimes can be obtained simply by acidification and fusion of their oxoanion solutions at standard pressure and ambient temperature.² As a unique branch of POMs, polyoxoniobates (PONs) also display multiple applications in virology, nuclear waste treatment and the base-catalyzed decomposition of biocontaminants.³ However, in comparison with other polyoxometalates, the development of polyoxoniobates is still in its infancy and primarily controlled by the hexaniobate Lindqvist anion [Nb₆O₁₉]⁸⁻. The feature that the [Nb₆O₁₉]⁸⁻ anion exists only under alkaline conditions and forms Nb₂O₅ precipitates under acidic conditions has severely restricted the development of polyoxoniobates.⁴ Fortunately, in recent years, polyoxoniobate chemistry has gained great progress due to the pioneering work of Nyman and co-workers on the heteropolyniobates,⁵ and further developed by the series of research by Casey and co-workers.⁶ The other representative examples are as follows: [Nb₂₀O₅₄]⁸⁻,⁷[{Cu(H₂O)L}₂(CuNb₁₁O₃₅H₄)]⁵⁻ (L = 1,10-phenanthroline, 2,2′-bipyridine),^1k[H₉Nb₂₄O₇₂]¹⁵⁻,⁸[HNb₂₇O₇₆]¹⁶⁻ and [H₁₀Nb₃₁O₉₂(CO₃)]²³⁻.⁹ In addition, niobium-substituted POMs synthesized by the reaction of lacunary polyoxotungstates and K₇HNb₆O₁₉·13H₂O in acidic solution have also made great progress, such as the trisubstituted heteropolytungstates {MW₉Nb₃O₄₀} (M = Si^IV, Ge^IV), dimer {H₁₅Ge₂W₁₈Nb₈O₈₈}, and tetramer {Ge₄W₃₆Nb₁₆O₁₆₆} cluster.¹⁰ Lately, the first example of the vanadoniobate cluster with ‘trans-vanadium’ bicapped Keggin-type {VNb₁₂O₄₀(VO)₂} was reported by Hu's group.¹¹ These reported polyoxoniobates may assist toward the fundamental understanding of chemistry and are expected to find potential application in catalysis.

Inspired by the prominent work of Finke, Hill, Liu and their co-workers,^4a,10 we considered that vanadium might be a good candidate for construction of niobium-substituted POMs due to the similar ion radii and coordination numbers of V and Nb. Herein, we report the synthesis and structural characterization of an unprecedented compound {[Cu₆L₆(H₂O)₃][Nb₁₀V₄O₄₀(OH)₂]}₂·13H₂O (1) (L = 1,10-phenanthroline), which contains the unique polyoxoanion unit {Nb₁₀V₄O₄₀(OH)₂}¹²⁻.‡

Compound 1 was obtained from heating a mixture of K₇HNb₆O₁₉·13H₂O, Cu(Ac)₂·3H₂O, NaVO₃·2H₂O, 1,10-phenanthroline, and NaOH at 58 °C for 8 h.† Compared with other transition metal cations, copper-amine cations have even more potential in isolating new niobate cluster geometries. The possible reasons are as follows: their strong affinity for organic amine or aromatic N-ligands originating from the distinct Jahn–Teller effect of the Cu²⁺ with d⁹ configuration, their flexible coordination modes, and their stability in alkaline solutions.¹¹ Compound 1 is stable in air and insoluble in acidic and alkaline solutions. In 1, the oxidation states of all Nb, V and Cu centers are +5, +5 and +2, respectively, based on the charge balance consideration and bond valence sum calculations.¹² The phase purity of 1 is confirmed by the agreement between the experimental X-ray powder diffraction (XRPD) pattern and the simulated pattern based on structural analysis (Fig. S5, ESI†). X-Ray structural analysis indicates that 1 contains two identical {Nb₁₀V₄O₄₀(OH)₂}¹²⁻clusters decorated by twelve {CuL} fragments. The surrounding twelve {CuL} fragments are linked to the clusters through twenty-four terminal oxygens and eight bridging oxygen atoms (Fig. 1 and Fig. S1, ESI†). The {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ unit is derived from two fundamental building blocks, namely, the {Nb₁₀O₃₂} cluster and the {VO₄} tetrahedron (Fig. 2e). Although the decaniobate ion, first reported in 1977 by Edward J. Graeber and B. Morosin (Fig. 2b),¹³ is a well-known polyoxoniobate (PON), the structure of the decaniobate ion described here has never been observed to date (Fig. 2d). The new decaniobate cluster is made up of four additional {NbO₆} octahedra and an interesting {Nb₆O₂₀} unit (Fig. 2c). Each {NbO₆} octahedron is considerably distorted around the niobium center, with a spread of Nb–O bond distances which range from 1.767(13) to 2.220(9) Å. Similar distortions of {NbO₆} octahedra were also observed in classical Lindqvist hexaniobate (Fig. 2a). Four such {NbO₆} octahedra bond to the {Nb₆O₂₀} fragment by the edge-sharing mode into a unique {Nb₁₀O₃₂} unit. Compared with classical Lindqvist hexaniobate, the Nb atoms of the new {Nb₆O₂₀} unit display six-coordinate and seven-coordinate environments (Fig. S2, ESI†). The formation of the unprecedented {Nb₁₀O₃₂} structure is attributed to the seven-coordinate Nb. To our knowledge, there is no similar species reported in polyoxotungstates, polyoxomolybdates and polyoxovanadates chemistry. The {Nb₁₀O₃₂} unit has four types of O atoms: nine terminal oxygen atoms (O_t), ten bridging μ₂-oxygen atoms, six bridging μ₃-oxygen atoms, and one central μ₆-oxygen atom. The Nb–O_t, Nb–μ₂-O, Nb–μ₃-O, and Nb–μ₆-O bond lengths in {Nb₁₀O₃₂} anions range from 1.716(12)–1.810(11), 1.908(10)–2.068(10), 1.982(10)–2.219(10), and 2.321(9)–2.589(9) Å, respectively, and the Nb⋯Nb distances are in the range 3.261(2) to 3.359(2) Å. Each {Nb₁₀O₃₂} cluster is further connected to four {VO₄} tetrahedra by a corner-sharing mode into a novel {Nb₁₀V₄O₄₀(OH)₂}¹²⁻cluster (Fig. 2e and Fig. S2, ESI†). All V ions of the {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ unit are tetrahedrally coordinated. The V–O bond lengths range from 1.644(16)–1.88(4) Å. There are two different copper coordination environments in compound 1 (Fig. S2, ESI†). The Cu1, Cu3, Cu4, Cu6, Cu7, Cu9, Cu10 and Cu12 cations exhibit five-coordinate square pyramidal geometry, in which the square plane is defined by two N atoms from a 1,10-phenanthroline ligand and two terminal O atoms from the {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ unit (Cu–N: 1.982(15)–2.018(14) Å, Cu–O: 1.875(11)–1.945(13) Å), and the apical site is occupied by one coordinate water (Cu–O_W: 2.221(16)–2.295(14) Å). While Cu2, Cu8, Cu11 and Cu5 cations show a four-coordinate square-planar geometry defined by two N atoms from a 1,10-phenanthroline ligand and two bridging O atoms from the {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ unit (Cu–N: 1.993(14)–2.003(13) Å, Cu–O: 1.878(10)–1.930(10) Å).¹ⁱ The adjacent {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ units form a dipolymer through a Cu–O_b–Cu bond. Furthermore, the {[Cu₆L₆(H₂O)₃][Nb₁₀V₄O₄₀(OH)₂]}₂·13H₂O molecules close-pack in the solid-state to form a 3D supramolecular network via π–π stacking interaction between the neighboring 1,10-phenanthroline ligands with the distance of 3.472 Å (face to face) (Fig. 3). Interestingly, there exist one-dimensional quadrangular channels of approximately 6.9 × 10.9 Å along the [−1 1 −2] direction that are occupied by the free water molecules. PLATON analysis shows that the effective free volume of 1 is about 31.5% of the crystal volume (2182.3 ų out of the 6939 ų).

Fig. 1 Polyhedral/ball-and-stick views of the anions of 1 (H atom and water molecules are omitted for clarity).

Fig. 2 (a) The classical Lindqvist [Nb₆O₁₉]⁸⁻ fragment. (b) The decaniobate ion reported by Edward J. Graeber and B. Morosin. (c and d) The new Lindqvist {Nb₆O₂₀} cluster and the new {Nb₁₀O₃₂} cluster in this communication. (e) Summary of the formation of {Nb₁₀V₄O₄₀(OH)₂}¹²⁻.

Fig. 3 Polyhedral representation of the three-dimensional supramolecular structure in compound 1 (H atom and water molecules are omitted for clarity).

The IR spectrum of 1 is recorded between 400 and 4000 cm⁻¹ with a KBr pellet, which is very useful for identification of characteristic vibration bands of PONs and organic components in products. In the IR spectrum of compound 1, the characteristic peaks at 903, 715, 622 and 442 cm⁻¹ are assigned to the ν(Nb–O_b), ν(Nb–O–M) and ν(V–O) (M = Nb^V or V^V) stretches. Bands in the 1423–1514 cm⁻¹ region are attributed to the 1,10-phenanthroline groups. The peak at 3423 cm⁻¹ is assigned to the water molecules.¹⁴

In order to characterize the compound more fully, a TGA experiment was carried out to investigate its thermal stability. A weight loss of ca. 4.99% occurred from 42 to 100 °C, corresponding to the loss of all lattice and coordinated water molecules (4.97%), followed by two-step continuous weight loss of 32.13% in the range 100 and 900 °C consistent with the removal of twelve 1,10-phenanthroline ligands (calcd 34.57%). The whole weight loss of 37.12% is consistent with the calculated value of 39.54%.

The solid-state luminescence property of 1 was investigated at room temperature. Compound 1 exhibits an intense emission maximum at ca. 424 nm upon excitation at ca. 365 nm (Fig. 4). As we know, the strongest emission peak for the free 1,10-phenanthroline ligand is at 362 nm and 380 nm (λ_ex = 227 nm).^15,16 The K₇HNb₆O₁₉·13H₂O exhibits a broad emission band at around 320 nm (λ_ex = 227 nm), which can be assigned to the O–Nb charge transfer.¹¹ Thus, the emission of 1 may be attributed to the mixture of the intraligand π_L*–π_L transitions from the 1,10-phenanthroline and the O → Nb and O → V charge transfer of the {Nb₁₀V₄O₄₀(OH)₂}¹²⁻ unit.¹⁵

Fig. 4 Solid-state emission spectrum of compound 1 at room temperature.

In summary, we have synthesized and structurally characterized an unprecedented organic–inorganic hybrid, {[Cu₆L₆(H₂O)₃][Nb₁₀V₄O₄₀(OH)₂]}₂·13H₂O, which contains the unique polyniobovanadate {Nb₁₀V₄O₄₀(OH)₂}¹²⁻. The successful synthesis of 1 adds a new number of Nb-based POMs and unveils the potential for further development of polyoxoniobate chemistry. In addition, it provides us an enlightening synthetic strategy for the introduction of other transition metal cations and large aromatic or chiral ligands into this system to construct organic–inorganic hybrid multidimensional PONs or chiral PONs. In the following exploration, we will continue to investigate the pertinent synthetic chemistry.

The authors gratefully acknowledge the financial support from the NNSF of China (No. 21001022, 21171033, 21131001), The National Grand Fundamental Research 973 Program of China (2010CB635114), Program for New Century Excellent Talents in Chinese University (NCET-10-0282), PhD Station Foundation of Ministry of Education (20100043110003), The Foundation for Author of National Excellent Doctoral Dissertation of P.R. China (FANEDD) (No. 201022), The Science and Technology Development Planning of Jilin Province (201001169,20100182), The Fundamental Research Funds for the Central Universities (09ZDQD003 and 10CXTD001).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Details of the synthesis, measurements, the coordination geometric frameworks, XRPD, TGA and IR data. CCDC 836458. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1cc15684e

‡ Crystallographic data for 1 (H₁₃₈Nb₂₀V₈O₁₀₃Cu₁₂C₁₄₄N₂₄), M_r = 6892.98, triclinic, space groupP1̄, a = 18.429(5) Å, b = 18.778(2) Å, c = 23.173(3) Å, V = 6939(2) ų, Z = 1, R1 = 0.0941 and wR2 = 0.2719 (R_int = 0.0399) for 21 366 independent reflections [I > 2σ(I)]. CCDC 836458.

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