Synthesis, structure, and luminescent properties of a family of lanthanide-functionalized peroxoniobiophosphates

Eight new lanthanide derivatives containing 6-peroxoniobio-4-phosphate building block, [LnIII(H2O)6]2[H4(NbO2)6P4O24]·nH2O [Ln = Eu (1), Gd (2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb (8), 1–5, 7, 8 n = 12; 6 n = 9], have been successfully obtained using an in-situ strategy and fully characterized in the solid state by single-crystal X-ray diffraction, IR spectra, TG-MS, PXRD. Structural analyses indicate that these isostructural polyanions 1–8 consist of one [P4(NbO2)6O24]10− (P 4 (NbO 2 ) 6) clusters and two pendant Ln3+ cations. In these compounds, P 4(NbO 2 ) 6 clusters are connected by lanthanide cations to form extended two-dimensional architectures. The approach takes advantage of the ability of in-situ formed P 4(NbO 2 ) 6 cluster to build frameworks by using it as ligands to lanthanide ions. The photoluminescence (PL) and lifetime decay behaviors of 1, 3 and 4 in solid state have been performed at room temperature. The PL emission of 1, 3 and 4 is mainly derived from the characteristic 5D0→7FJ (J = 1, 2, 3, 4), 5D4→7FJ (J = 6, 5, 4, 3) and 4F9/2→6H J (J = 15/2, 13/2, 11/2) transitions of the EuIII, TbIII and DyIII cations, respectively.


Results and Discussion
Lindqvist type [Nb 6 O 19 ] 8− (Nb 6 ) anions are known to be stable above pH 10.5 7 , a state in which lanthanide ions are easy to hydrolysis and thus make it difficult to investigate polyoxoniobate-lanthanide (PONb-Ln) materials. In this paper, a new strategy for the synthesis of PONb-Ln derivatives has been developed. Its major experimental strong point is the facile preparation of target compounds from in-situ formed P 4 (NbO 2 ) 6 (3.0 M), being as solution A. While solution B comprises LnCl 3 ·nH 2 O (4.0 mmol) in 2.0 mL of water, which is added into solution A and the pH of the resultant mixture is adjusted to 1.2-2.0, depending on the lanthanide metal used. The mixtures are then heated at 90 °C for 4 hours and filtered and then left at room temperature to crystallize. Single crystals are collected after about two weeks with the average yield 10-20% based on niobium.
Interestingly, the colour of the hexaniobate solution turns from colorless to bright-yellow with the addition of phosphoric acid, suggesting the formation of peroxo {NbO 2 } group. This is common in the previous {NbO 2 }-substituted polyoxotungstate chemistry 20,31 , and also reinforced by the fact that the solution comprising P 4 (NbO 2 ) 6 is intensely yellow, whereas the solution turns to colorless quite rapidly with the addition of NaHSO 3 . Importantly, the color of the resultant compounds 1, 2, 3, 4 and 8 is almost same to that of P 4 (NbO 2 ) 6 owing to the corresponding colorless lanthanide chloride, while the color of compounds 5, 6 and 7 is somewhat different, depending on the color of lanthanide chloride used (Fig. 1).
There are several aspects of the synthetic conditions that can influence the formation of these clusters, taking compound 1 as example, a series of reaction sets under different experimental conditions of pH, temperature, heating time and the mole ratio of Eu III /Nb 6 were investigated. If the solution of the reactants were left to heat at 90 °C for less than 2 h, we have been unable to isolate single crystals of compound 1, however, it was found that there was no obvious increase in the yields when the heating time was more than 4 h. In addition, the reaction is not particularly sensitive to the amount of Eu III because 1 could be obtained with the mole ratios of Eu III /Nb 6 ranging from 6:1 to 16:1, with 10:1 giving the highest yield. It is worth mentioning here that compounds 1−8 can be isolated without the need for extra cations, different from an essential templating role of Cs + in the formation of Keggin and Wells-Dawson-type niobium-substituted polyoxotungstates 28-31, 41-43 . Single-crystal X-ray diffraction analysis reveals that 1−8 are isostructural and comprise a neutral [Ln III (H 2 O) 6 ] 2 [H 4 (NbO 2 ) 6 P 4 O 24 ] (Ln 2 P 4 (NbO 2 ) 6 , Ln = Eu (1), Gd (2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb (8)) subset and some lattice water molecules. The self-assembly of all presented crystal structures can be traced back to coordinative bonding forces and the lanthanide ions are coordinated to POMs, connecting them to two-dimensional frameworks (Fig. 2). All of the compounds crystallize in the space group orthorhombic Pbca and with almost identical unit cell dimensions. This is unsurprising given that the only difference between the clusters is the lanthanide metal present, and thus the structural description is only exemplified by 1.
It is worth noting that the polyanion P 4 (NbO 2 ) 6 resembles structurally the previously reported [H 7 Nb 6 P 4 O 24 (O 2 ) 6 ] 3− (1′) cluster of Casey and co-authors 15 . This centrosymmetric cluster can be viewed as two P 2 Nb 3 units fused by two Nb-µ 2 -O-Nb and two P-µ 2 -O-Nb bridges (Fig. 3b). The P 2 Nb 3 can be regarded as a peroxohexaniobate 33 with a contiguous longitudinal strip of three Nb(O 2 ) groups (one on equatorial position and two on axial position) replaced by two PO 4 groups (Figs 3c and S1). In 1, each of the six Nb atoms is ligated by  Ball-and-stick/polyhedral representations of Eu 2 P 4 (NbO 2 ) 6 (a), P 4 (NbO 2 ) 6 (b), P 2 Nb 3 (c) and coordination environment of NbO 7 (d) and EuO 8 (e). All solvent water molecules have been omitted for clarity. Color code: NbO 7 blue polyhedral, PO 4 pink polyhedral, EuO 8 green dodecahedral, Nb blue spheres, P pink spheres, Eu green spheres, O red spheres, peroxo bond red.
The metal-oxygen bond lengths in 1−8 are sorted and plotted in the order of their lengths (Fig. 4), it can be clearly seen that the bond lengths of Nb-peroxo and Nb-µ 3 -O in 1−8 are almost the same. The Ln-O bonds length are gradually reduced, which are generally in agreemnt with the ion radius trend in the lanthanide elements.
Bond valence sum (BVS) calculations 45 are carried out on all the Ln, Nb, P and O centers (Table S3) and the results show that all the Ln, Nb and P atoms are in the +3, +5 and +5 oxidation states, respectively. The BVS values of the µ 2 -O oxygen atoms bridging Nb1-Nb2 (O3) are in the range of 1.29-1.31, suggesting that these oxygen atoms are monoprotonated. In addtition, charge-balance considerations with counter cations suggested that compounds 1−8 should contain the two additional protons, and we think these two protons are delocalized in the polyoxoanions on the basis of the previous studies by Nyman and Niu 46 (Figures S2-3). As expected, the overall IR spectra of 1-8 are almost the same because of the isostructural nature (Table 1). All compounds 1-8 exhibit strong and medium bands in the range of 1200-1000 cm −1 , associated with antisymmetric stretching of the P-O bond 15  The significant changes in IR spectra (Fig. 5) of 1−8 compared to that of Nb 6 are the appearance of strong intensity bands at 850 cm −1 and in the region 1200-1000 cm −1 , which is characteristic of the antisymmetric stretching vibrations of peroxo group 29, 31 and P-O bond, respectively. This is in satisfactory agreement with the solid-state structure. Additionally, the above-mentioned results confirm that the 6-peroxoniobio-4-phosphate framework formed in-situ remains intact under the condition of the synthesis, and further indicate that the developed strategy may be further applicable to molecules of the class of PONb-based lanthanide derivatives.
The photoluminescence behaviours of compounds 1, 3 and 4 in solid state at room temperature are depicted in Fig. 6, which displays intense photoluminescence upon excitation at 394, 378 and 388 nm for compounds 1, 3 and 4, respectively. The emission spectrum of compound 1 exhibits four characteristic emission bands at 590, 612, 653 and 698 nm, corresponding to 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 transitions of the Eu 3+ ions (Fig. 6a). These are in good agreement with previous results 39, 48 . The 5 D 0 → 7 F 1,3 transitions are magnetic dipole transitions and insensitive to their coordination environments, while 5 D 0 → 7 F 2,4 transitions are electric dipole transitions and sensitive to their local environments 49 . The transition at 590 nm belongs to the magnetic dipole 5 D 0 → 7 F 1 transition and its emission intensity scarcely varies with the strength of the ligand field exerted on the Eu 3+ ions, whereas the highest relative intensity of the 5 D 0 → 7 F 2 transition at 612 nm is the electric dipole transition and implies red emission light of 1. Further, the intensity of 5 D 0 → 7 F 2 transition is extremely sensitive to chemical bonds in the vicinity of the Eu 3+ ions. The 5 D 0 → 7 F 2 / 5 D 0 → 7 F 1 ratio is widely regarded as a measured of the coordination state and site symmetry of the lanthanide 50 . However, it should be noted that this ratio is easily influenced by other factors such as the polarizability of the ligands. For 1, the value is ca. 13.5 implying the low site symmetry of the Eu 3+ ions, which agrees well with the distorted dodecahedral geometry of Eu 3+ ions in 1. Furthermore, the excitation spectrum of 1 monitored at the Eu 3+ 5 D 0 → 7 F 2 transition (612 nm) contains a narrow band and several weak bands (Fig. 6b). The narrow band at 394 nm is attributed to the 7 F 0 → 5 L 6 transition of the intra-4f 6 , the three weak peaks in the range of 300-445 nm are assigned to 7 F 0 → 5 D 4 (362 nm), 7 F 0 → 5 G 2 (385 nm), 7 F 0 → 5 D 3 (416 nm) transition, respectively 51 . In order to obtain the lifetime, the luminescence decay curve of 1 was measured at room temperature by monitoring the strongest emission of 5 D 0 → 7 F 2 (Figs 6c and S4), which can be fitted successfully to a single exponential function as the equation I = A exp(−t/τ). The affording lifetime (τ) is 148.38 μs with a pre-exponential factor (A) of 2045. 82.
Furthermore, the emission spectrum of 3 upon excitation at 378 nm exhibits a maximum at 546 nm corresponding to the 5 D 4 → 7 F 5 transition of Tb 3+ ion, while the peaks located at 488, 588 and 620 nm are attributed to the 5 D 4 → 7 F 6 , 5 D 4 → 7 F 4 and 5 D 4 → 7 F 3 transitions of Tb 3+ ion (Fig. 6d), respectively [52][53][54] . Interestingly, the excitation spectrum of 3 upon the excitation at 546 nm consists of three dominant emission bands at 343, 353 and 378 nm, which can be ascribed to the 7 F 6 → 5 L 6 , 7 F 6 → 5 L 9 and 7 F 6 → 5 G 6 transitions (Fig. 6e), respectively 55 . The luminescence lifetime of 3 was monitored and can also conform to a single exponential function with a lifetime 18.67 μs (Figs 6f and S5). Meanwhile, the emission spectrum of 4 under excitation at 388 nm displays one high-intensity emission peak at 573 nm and two low-intensity emission peaks at 481 and 663 nm, which is assigned to the 4 F 9/2 → 6 H 13/2 , 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 11/2 transitions of Dy 3+ ions (Fig. 6g), respectively 56 . It is noteworthy that the intensity of the 4 F 9/2 → 9 H 13/2 electric dipole transition is much higher than that of the 4 F 9/2 → 6 H 15/2 magnetic dipole transition, illustrating that the Dy 3+ ions reside in low symmetrical environments without inversion. The excitation spectrum of 4 collected by monitoring the emission at 573 nm is presented in Fig. 6h, and the most intense peak is observed at 388 nm ( 6 H 15/2 → 4 I 13/2 ), whereas the other three relatively weak peaks are located at 324 nm ( 6 H 15/2 → 6 P 3/2 ), 350 nm ( 6 H 15/2 → 6 P 7/2 ) and 364 nm ( 6 H 15/2 → 6 P 5/2 ), respectively. Interestingly, the lifetime curve for 4 can be well-fitted using a second-order exponential function I = A 1 exp(t/τ 1 ) + A 2 exp(t/τ 2 ), affording the luminescence lifetimes τ 1 and τ 2 as 2.77 μs (57.65%) and 10.09 μs (42.35%) (Figs 6i and S6), respectively. The average lifetime τ* is calculated as 5.87 μs based on the formula τ* = (A 1 τ 1 2 + A 2 τ 2 2 )/(A 1 τ 1 + A 2 τ 2 ) 57 . The flat, tongue-shaped CIE chromatic diagram represents an internationally agreed method of color identification by combining three primary colors (red, green and blue), which will be seen in light with a wavelength. And with a certain conversion, it is important that only two new values (x and y) can be shown on a two-dimensional chart, where x and y represents the horizontal and vertical axis, respectively. In order to name colors, the emission spectra of 1, 3 and 4 were converted into the x and y coordinates in the CIE chromatic diagram (Fig. 7). The (x, y) values for 1, 3 and 4 are found to be (0.65, 0.35), (0.36, 0.48) and (0.38, 0.43), respectively, corresponding the reddish orange for 1, green for 3 and yellowish green for 4.

Conclusion
In summary, we have developed a new synthetic method for the synthesis of a new family of PONb-based lanthanoid complexes, 1-8. In order to accomplish the synthesis, a new strategy was developed and successfully applied. The experimental aspects of the strategy include in-situ formation of P 4 (NbO 2 ) 6 building block in acidic media, addition of lanthanide chloride salt and pH adjustment to the desired value. All compounds have been fully characterized in the solid state by single-crystal X-ray diffraction, IR spectra, TG-MS, PXRD. Moreover, their luminescence and lifetime decay behaviors were also investigated systematically. This study not only enriches the structural diversity of lanthanide derivatives containing PONb aggregates, but also provides a convenient synthetic route to PONb-based lanthanoid clusters. In future work, we will extend this approach to isolate various symmetries with the aim of providing in-situ formed building block that can be used to design and systematically tailor new luminescent materials with control.

Experimental Section
Materials and methods. All     Physical measurements. IR spectra (ν = 4000-400 cm −1 ) of the samples were recorded on a PerkinElmer FT-IR spectrometer using KBr pellets. 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 N 2 with a heating rate of 10 °C·min −1 . Photoluminescence properties were performed on EDINBURGH FLS 980 fluorescence spectrophotometer.
X-ray crystallography. Suitable single crystals of 1-8 were selected from their respective mother liquors and placed in a thin glass tube. X-ray diffraction intensity was recorded on a Bruker Apex-II CCD diffractometer at 296(2) K with MoKa monochromated radiation (λ = 0.71073 Å). Structure solution and refinement were carried out by using the SHELXS-97 and SHELXL-2014 program package 59, 60 for 1-8. Selected details of the data collection and structural refinement of compounds 1-8 can be found in Table S5. Further details of the crystal structure investigation can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the depository CSD numbers 432370-432377 for 1-8.