[Mo7O24]6 - ACS Publications - American Chemical Society

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Low-Percentage Ln3+ Doping in a Tetranuclear Lanthanum Polyoxometalate Assembled from [Mo7O24]6− Polyanions Yielding Visible and Near-Infrared Luminescence Anna M. Kaczmarek,*,† Kristof Van Hecke,‡ and Rik Van Deun*,† † 3

L -Luminescent Lanthanide Lab, Department of Inorganic and Physical Chemistry and ‡XStruct, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3, B-9000 Gent, Belgium S Supporting Information *

ABSTRACT: A rare case of low-percentage trivalent lanthanide doping in multinuclear lanthanide polyoxometalates (LnPOMs) was investigated. The [La4(MoO4)(H2O)16(Mo7O24)4]14− polyanion was chosen as the host material for this study. In this polyanion the central [La4(MoO4)]10+ core is coordinated by four heptamolybdate groups as well as 16 water molecules. The tetranuclear lanthanum POM was doped with 5% of Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, and Yb3+ (according to synthesis), and the structures and luminescence properties of the x%Ln:LaPOMs were investigated. Additionally a series of tetranuclear lanthanide POMs built from [Mo7O24]6− heptamolybdate polyanions with Eu3+, Tb3+, Sm3+, Dy3+, and Nd3+ instead of La3+ were synthesized, and a detailed analysis revealed that the tetranuclear clusters formed monomers or dimers linked through oxygen bridges. The smaller lanthanide ions, namely, Er3+ and Yb3+, did not form tetranuclear clusters, but instead mononuclear sandwich-type POMs were obtained. The obtained structures were shown to be lanthanide-specific, and not a result of different synthetic/crystallization conditions. The luminescence properties of the x%Ln:LaPOMs were compared with the luminescence properties of the LnPOMs.

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INTRODUCTION Polyoxometalates (POMs) are discrete anionic metal−oxygen clusters of early transition metals usually W, Mo, or V in a high oxidation state. They exhibit a huge variety in their size, shape, and nuclearities.1 Over the years many lanthanide POMs, including multinuclear lanthanide POMs, have been reported in literature. This has been reviewed in a very recent paper by X. Ma et al.2 In this publication a vast range of not only mono-, but di-, tri-, tetra-, up to icositetra-nuclear (24-nuclear) lanthanide POMs are reported. These multinuclear POMs can be built either from iso-polyanions or hetero-polyanions. Among the multinuclear structures composed from heteropolyanions those built from Keggin- and Dawson-type POMs are most often investigated. Only a few cases where Lindqvist or Anderson−Evans-type POMs are the basic building units of multinuclear POMs have so far been reported. The [Mo7O24]6− polyanion was originally proposed by Anderson to have a planar structure, and it was only later corrected by Lindqvist that [Mo7O24]6− has a bent structure; therefore, it is not an authentic Anderson-type POM.3,4 In general inorganic and hybrid organic−inorganic compounds doped with trivalent lanthanide ions (Ln3+) are widely used as luminescent materials in lighting and displays, scintillators, and lasers.5,6 The luminescence of these ions is a result of transitions within the partially filled 4f shells. As these transitions are parity-forbidden this leads to low molar absorption coefficients and long luminescence lifetimes.7 To © XXXX American Chemical Society

overcome the low molar absorption coefficients various inorganic materials and hybrid materials, which allow exciting the Ln3+ ions through charge-transfer bands or organic ligands, have been designed.8,9 To synthesize materials, which are cost-efficient and to avoid concentration quenching of luminescence properties many of the inorganic and hybrid organic−inorganic materials are designed to consist of a spectroscopically silent lanthanide, doped with an emissive lanthanide only at a low percentage, usually ranging from 1 to 10%. From an economical point of view lanthanum-based host materials are a good choice, as lanthanum oxide has a much lower price than other rare-earth oxides. Many such materials have been investigated in detail, for example, Eu3+:LaVO4,10 Eu3+:La2(MoO4)3,11 Eu3+,Tb3+:La(1,3,5-BTC)(H2O)6 (1,3,5-BTC = 1,3,5-benzenetricarboxylate),12 Eu3+, Tb3+:LaOHCO3,13 and others. Yet, among the various studied compounds lanthanide POMs doped with only a low percentage of an emissive lanthanide are not common. Recently a few reports on the possibility of employing lanthanides at a low doping percentage, or mixtures of two lanthanides in an appropriate ratio in POMs, have emerged, for example, Na9[HoxY(1−x)(W5O18)2]·nH2O, TBA8H4[{Ln(μ2− OH)2Ln′}(γ-SiW10O36)2] (where LnLn′; Ln = Gd3+, Dy3+; Ln′ = Eu3+, Yb3+, Lu3+; TBA = tetra-n-butylammonium), and Received: September 6, 2016

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DOI: 10.1021/acs.inorgchem.6b02130 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

powder X-ray diffraction that only the LaPOM1 form was obtained. The compound [La4(MoO4)(H2O)16(Mo7O24)4]14− (LaPOM1) crystallized in a cubic non centrosymmetric space group I4̅3d. The asymmetric unit consists of one-fourth of a total [La4(MoO4)(Mo7O24)4]14− polyanionic cluster, i.e. it is constructed from one [Mo7O24]6− polyanion and one [La(MoO)]2.5+ moiety (with Mo on a crystallographic fourfold axis) as well as four coordinated water molecules O26−O29 (Figure 1a). The asymmetric unit must also contain 3.5 counterions (H+ or NH4+) for charge balance, but it was impossible to unequivocally distinguish them from the lattice H2O molecules in this structure as well as the other structures described below. The MoO4 lies in the center of the polyanion

TBAnHm[{FeM4{Ln(L)2}2O2(A-α-SiW9O34)2] (where M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+; L = acetylacetonate, hexafluoroacetylacetone).14−16 These materials were mostly studied for their interesting magnetic properties. Among the lanthanide POMs many show interesting luminescence properties, yet almost all of the reported compounds consist of only the emissive lanthanide at a 100% concentration.17−20 Materials with a 100% concentration of the emissive lanthanide are expensive in production and often suffer from concentration quenching. Despite that it is not a common approach to investigate multinuclear rare-earth POMs doped with a low percentage of an emissive lanthanide ion. The group of Yamase studied Tb3+→Eu3+ energy transfer in several terbium POMs, which had been codoped with Eu3+ ions.21 Three types of multinuclear POMs were explored: K15H3[Tb1.4 Eu1.6(H 2O)3 (SbW9O33)(W5O18)3 ] × 25.5H 2O Na 7H19[Tb4.3Eu1.7O2 (OH)6 (H 2O)Al 2(Nb6O19)5 ] × 47H 2O (NH4)12 H 2[Tb3.1Eu 0.9(MoO4 )(H 2O)16 (Mo7O24 )4 ] × 13H 2O

Although the Eu 3+ concentrations cannot really be considered as low it is nonetheless one of the few studies where two lanthanide ions are present in one POM material. Despite the large amount of research performed on POMs, studies on RE POMs (where RE is a nonemissive rare-earth ion) doped with a low percentage of emissive lanthanide ions are still lacking. This could be of high potential interest for “real applications” of these RE/Ln POMs for, for example, bioimaging probes, light-emitting diodes, and sensory probes or in fields not related to luminescence such as magnetism.2 This study was undertaken with the motivation to explore the luminescence properties of heteronuclear lanthanide POMs, which were doped with a low percentage of an emissive lanthanide ion. For this investigation we chose the multinuclear [La4(MoO4)(H2O)16(Mo7O24)4]14− polyanion. The cluster consists of four La3+ ions, four [Mo7O24]6−, and one [MoO4]2− tetrahedron. We also prepared [Mo7O24]6− clusters containing 100% of each of the studied lanthanides (Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, and Yb3+) and investigated their crystal structures as well as luminescence properties. Throughout the studied lanthanides different structures are obtained. Our study shows the potential of using multinuclear lanthanide POM clusters as luminescent materials, which are doped or codoped with emissive lanthanides at only low doping percentages. This would lead to avoiding concentration quenching and lowering the cost of luminescent materials based on POMs.

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RESULTS AND DISCUSSION Synthesis and Characterization. All of the x%Ln:LaPOM samples were characterized through powder X-ray diffraction (XRD) to confirm no change in the crystal structure after Ln3+ doping (Figure S1). It could be clearly seen that substituting x % of La3+ for a different lanthanide ion (Ln3+ = Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+) did not influence the crystal structure of the LaPOM. For the synthesis of the [(La0.95Ln0.05)4(MoO4)(H2O)16(Mo7O24)4]14− polyanions it was crucial to add the La(NO3)3·xH2O and Ln(NO3)3·xH2O salts (Ln3+ = Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+; x = 5−6) simultaneously to avoid the formation of two types of POMs. Crystal Structures. The compound LaPOM crystallized in two forms - LaPOM1 and in rare cases also LaPOM2 (in these cases LaPOM1 was obtained as the major phase). For all of the x% Ln3+ codoped lanthanide POMs it was confirmed by

Figure 1. (A) Coordination environment of the MoO4 and LaO units in the LaPOM1 structure. Symmetry codes: (i) 1 − x, 0.5 − y, z; (ii) 0.25 + y, 0.75 − x, 1.25 − z; (iii) 0.75 − y, −0.25 + x, 1.25 + z. The asymmetric unit is highlighted. (B) Molecular structure of the LaPOM1 tetranuclear polyanionic cluster in the cubic unit cell. (C) Asymmetric unit in the crystal structure of LaPOM2, consisting of two complete polyanionic tetranuclear clusters. B


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Inorganic Chemistry

(H2O)13(Mo7O24)4]14− polyanionic cluster constructed from four [Mo7O24]6− units and one [Tb4(MoO4)]10+ moiety as well as 13 coordinated water molecules. Three of the four Tb3+ anions are nine-coordinated by oxygen atoms, showing a spherical tricapped trigonal prismatic environment, while one Tb3+ anion is eight-coordinated, closest to a square antiprism. Two polyanionic clusters form dimers by four Tb−O bridges, that is, two oxygen atoms, of the coordination sphere of the eight-coordinated Tb3+ anion, are shared with the octahedral coordination sphere of one Mo atom of a [Mo7O24]6− unit of a symmetry-equivalent polyanionic cluster (Figure 2).

with the O−Mo−O angles ranging from 109.1(3)−110.3(7)°, which are close to those of an ideal tetrahedron. The corresponding Mo(8)−O(25) bond lengths for the MoO4 are all 1.79(1) Å, which shows the partial double bond character. Around the MoO4 unit four La atoms are bonded through La− O−Mo bridges. The coordination environments of the La atoms are exactly the same (because of the cubic space group symmetry), i.e. each La atom is eight-coordinated to an oxygen atom (O25) from the MoO4 unit, four oxygen atoms (O20, O22, and O23, O24) from three different [Mo7O24]6− units and four H2O molecules (O26−O29) (Figure 1a). The coordination environments of the La atoms can be described as being closest to a spherical tricapped trigonal prism. All coordination environments were determined by use of the SHAPE v2.1 program.22,23 The compound [La 4 (MoO 4 )(H 2 O) 16 (Mo 7 O 24 ) 4 ] 2 14− (LaPOM2) crystallized in the triclinic centrosymmetric space group P1.̅ The asymmetric unit consists of two [La4(MoO4)(Mo7O24)4]14− polyanionic tetranuclear clusters, which are each constructed from four [Mo7 O 24 ] 6− polyanions and a [La4(MoO4)]10+ moiety as well as 16 coordinated water molecules, and which are highly similar to the tetranuclear clusters observed for the cubic LaPOM1 (Figure 1b,c). The coordination environments of the eight crystallographically independent La atoms range from a spherical capped square antiprism (La2, La3) to spherical tricapped trigonal prisms (La1, La4, La5, La6, La7, and La8). For LaPOM2, the O−Mo− O angles of the two central MoO4 units range from 108.9(2)− 110.1(2)° and 108.5(3)−110.5(2)°, respectively, again close to those of ideal tetrahedrons. The corresponding Mo−O bond lengths for the two MoO4 units are in the 1.753(5)−1.772(5) Å and 1.763(5)−1.770(5) Å range, respectively, representing partial double bond character, as in the former LaPOM1 structure. Although crystallized in a completely different unit cell and space group, the solvent accessible surfaces are quite comparable, i.e. 15.3% and 12.6% of the unit cell volumes, for LaPOM1 and LaPOM2, respectively (Figure S2). Additionally, crystal structures of LnPOMs (Ln3+ = Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+) were obtained. Depending on the lanthanide ion, tetranuclear POMs, which form monomers or dimers attached through Ln−O bonds, or even combinations of both monomers and dimers, were obtained. Only for the two smallest studied lanthanide ionsEr3+ and Yb3+a completely different type of structure was obtained. In this case two [Mo7O24]6− polyanions are connected by one Er/ Yb atom yielding a mononuclear sandwich-type POM. The compound [Eu 4 (MoO 4 )(H 2 O) 16 (Mo 7 O 24 ) 4 ] 14− (EuPOM) crystallized in the monoclinic centrosymmetric space group C2/c. This compound is isostructural to the previously reported [{Eu(H2O)4}4(MoO4)(Mo7O24)4]14− cluster by Naruke et al.,24 despite employing a slightly different synthesis and crystallization approach. The central [Eu4(MoO4)(H2O)16]10+ core of the cluster is coordinated by four [Mo7O24]6− polyanions, yielding a ninefold coordination of Eu3+ by oxygen atoms from Mo7O24, MoO4, and H2O, all showing a spherical tricapped trigonal prismatic coordination environment. The compounds [Tb 8 (MoO 4 ) 2 (H 2 O) 26 (Mo 7 O 24 ) 8 ] 28− (TbPOM) and [Dy 8 (MoO 4 ) 2 (H 2 O) 2 6 (Mo 7 O 2 4 ) 8 ] 2 8 − (DyPOM) are isomorphous and crystallized in the triclinic centrosymmetric space group P1̅. Hence only the structure of [Tb8(MoO4)2(H2O)26(Mo7O24)8]28− will be discussed in more detail. The asymmetric unit consists of one [Tb4(MoO4)-

Figure 2. Polyanionic dimer in the crystal structure of [Tb8(MoO4)2(H2O)26(Mo7O24)8]28−. The four Tb−O bridges are highlighted in yellow.

The compound [Sm 4 (MoO 4 )(H 2 O) 16 (Mo 7 O 24 ) 4 ] 14− (SmPOM) crystallized in the triclinic centrosymmetric space group P1.̅ The asymmetric unit consists of one [Sm4(MoO4)(Mo7O24)4]14− polyanionic cluster. The structure only contains discrete polyanionic cluster; no connections to other polyanionic clusters are observed in the packing (Figure S3). The coordination environments of the Sm atoms can be best described as spherical tricapped triganol prisms. The compound [Nd8(MoO4)2(H2O)28(Mo7O24)8][Nd4(MoO4)(H2O)16(Mo7O24)4]42− (NdPOM) crystallized in the triclinic centrosymmetric space group P1.̅ Here, the asymmetric unit consists of two tetranuclear cluster units. One of these polyanions is connected with its symmetry-equivalent (inversion center) through one eight-coordinated Nd−O link, while the other cluster is found as monomeric (Figure 3). The Nd coordination environments are all closest to spherical tricapped trigonal prisms, for the nine-coordinated Nd atoms (except for Nd6, which is closest to a spherical capped square antiprism), while again a square antiprism is found for the eightcoordinated Nd atom. The NdPOM structure is isostructural with the previously reported Ce structure (ICSD 380536).25 The compounds [Yb(H2O)4(Mo7O24)2]218− (YbPOM) and [Er(H2O)4(Mo7O24)2]9− (ErPOM) are isostructural and crystallized in the triclinic centrosymmetric space group P1̅. Only the [Yb(H2O)4(Mo7O24)2]218− structure will be discussed in more detail. The asymmetric unit consists of two [Mo7O24]6− units and one Yb3+ anion (Figure 4). The Yb atom is coordinated to four oxygen atoms from the two [Mo7O24]6− units, creating a mononuclear sandwich-type POM. The coordination of the ytterbium is completed by four water C


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Inorganic Chemistry

Figure 3. Polyanionic dimer in the crystal structure of [Nd8(MoO4)2(H2O)28(Mo7O24)8][Nd4(MoO4)(H2O)16(Mo7O24)4]42−. The two Nd−O bridges are highlighted in yellow.

luminescence properties of these materials were discussed in the subchapters below. Photoluminescence of EuPOM and x%Eu:LaPOM. The photoluminescence properties of the EuPOM and x% Eu:LaPOM were investigated in detail. The combined excitation and emission spectra of the EuPOM and x% Eu:LaPOM are presented in Figure 5a,b. All of the labeled peaks were assigned in Table 1 and Table S2. In the excitation spectrum of the EuPOM a weak broad band in the 250−350 nm region is present (Mo−O charge-transfer band) as well as a series of strong sharp peaks, which can be assigned to the characteristic f−f transitions of Eu3+. In the excitation spectrum recorded for the EuPOM the Mo−O band is stronger, yet the f−f transition peaks are still more intensive. Also in the case of the x%Eu:LaPOM the charge-transfer band is slightly shifted toward higher wavelengths. The emission of the two samples was observed after exciting into the Mo−O transition (at 332.0 or 342.0 nm) as well as into the 5L6←7F0 (392.0 or 392.6 nm). It could be clearly seen that exciting into the Mo−O band yielded weaker emission than exciting into the f−f transition. In europium compounds the relative intensities of the 5D0→7F2 and 5D0→7F1 transition peaks and the splitting of the 5D0→7F0−2 peaks give information on the Eu3+ ions environment. The fact that for both the EuPOM and x%Eu:LaPOM there is only one 5 D0→7F0 peak, and the 5D0→7F1 peak is split 2 times indicating the presence of one Eu crystallographic site with axial symmetry. The 5D0→7F2/5D0→7F1 ratio suggests that the Eu3+ ions are located in a site without inversion center. Clearly, the two Eu POMs can be efficiently excited both into the Mo− O charge transfer band as well as directly into the 5L6←7F0 yielding strong red emission. The decay times of the compound were recorded. After exciting into the Mo−O band the x% Eu:LaPOM (0.308 ms) yielded a longer luminescence lifetime than the EuPOM (0.255 ms); see Figure 5c. Photoluminescence of TbPOM and x%Tb:LaPOM. As can be seen from Figure 6a for both the TbPOM and x% Tb:LaPOM in the excitation spectra only the Mo−O chargetransfer band is present. No f−f transition peaks could be detected in either of the excitation spectra. This indicates that the energy transfer from Mo−O to Tb3+ ions is very efficient in this POM. For both Tb3+ POMs the characteristic transitions peaks 5D4→7F6−3 are present. The peaks presented in the figure were assigned to the appropriate transitions in Table 2 and Table S3. The luminescence lifetime of the x%Tb:LaPOM is

molecules making the ytterbium eight-coordinated, exhibiting a triangular dodecahedron environment. The four oxygen atoms (O19, O24, and O43, O48) from the two different [Mo7O24]6− units form bond lengths in the 2.149(11)−2.485(17) Å range. An exhaustive study confirmed that in these specific synthesis and crystallization conditions the appropriate lanthanide ions always crystallized in the same form. The synthesis of each compound was repeated several times, and multiple crystals from each batch were screened to confirm the reproducibility of the structures. In summary, going from the largest to smallest Ln atoms, two polymorphic structures were obtained for the LaPOMs, containing only discrete tetranuclear polyanionic clusters; however, the cubic structure was found to be the dominant phase. For the NdPOM, the POM nuclearities change to a combination of dimeric (octanuclear) and monomeric (tetranuclear) clusters. For Sm3+ and Eu3+, only discrete tetranuclear clusters are observed, while for Tb3+ and Dy3+, isomorphous structures were found, showing only octanuclear dimers of polyanionic clusters. For the NdPOM dimer, as well as for TbPOM and DyPOM, the dimers are formed through Ln−O bridges, with eight-coordinated Ln-atoms, showing a biaugmented trigonal prism environment, instead of a ninecoordinated tricapped trigonal prism. For the smallest Er3+ and Yb3+ lanthanide ions, mononuclear sandwich-type structures were observed, exhibiting eight-coordinated Ln-atoms in a different triangular dodecahedron environment. Photoluminescence Properties of x%Ln:LaPOMs and LnPOMs (Ln3+ = Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Er3+, Yb3+). As the structures of the LnPOMs (Ln3+ = Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+) are not isostructural to that of LaPOM1 (which for simplicity will be referred to just as LaPOM in the luminescence section of the paper) the luminescence properties cannot be compared in a straightforward way. Although all of the compounds (with the exception of YbPOM and ErPOM) have essentially the same central [Ln4(MoO4)(H2O)x]10+ core coordinated by either four [Mo7O24]6− units yielding ninefold coordination the presence of oxygen bridges linking the clusters together can influence the luminescence properties of these materials. For all of the prepared x%Ln:LaPOMs and LnPOMs (Ln3+ = Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+) visible (Vis) or near-infrared (NIR) emission could be detected when exciting either into the Mo−O charge transfer band, directly into the strongest f−f transition peaks, or both. The detailed D


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Inorganic Chemistry

two different wavelengths: 350.0 nm (Mo−O), and 401.8 nm (4L13/2←6H5/2). As expected, in both emission spectra the shape and relative intensities of the peaks are very similar. Much stronger emission is observed when exciting the material at 401.8 nm directly into the f−f transition. Exciting into the Mo−O band yields significantly weaker emission. When exciting at these wavelengths no broad band in the 500−550 nm region is visible. This indicates that all energy is efficiently transferred from the Mo−O to the Sm3+ when exciting into the Mo−O band and that no energy is transferred to Mo−O from the Sm3+ ions. The decay time of the SmPOM was determined to be 10.35 μs (excited at 401.8 nm; Figure S9). In the case of the x%Sm:LaPOM in the excitation spectrum a very weak Mo−O band and strong sharp peaks of the f−f transitions are present. To investigate the energy transfer abilities of this material it was excited at two wavelengths: 335.0 nm (Mo−O) and 402.4 nm (4L13/2←6H5/2). As can be clearly seen in Figure 7b exciting at both wavelengths results in the four characteristic Sm 3+ emission peaks: 4 G 5/2 → 6 H 5/2 , 4 G5/2→6H7/2, 4G5/2→6H9/2, and 4G5/2→6H11/2. The emission spectrum of the x%Sm:LaPOM sample is very similar to that of the SmPOM sample. No broad band from Mo−O is visible in

Figure 4. Molecular structure of [Yb(H2O)4(Mo7O24)2]218−, showing a mononuclear sandwich-type POM.

slightly longer compared to the TbPOM (0.978 vs 0.928 ms; Figure 6b) Photoluminescence of SmPOM and x%Sm:LaPOM. The combined excitation and emission spectra for SmPOM and x%Sm:LaPOM were presented in Figure 7a,b. For the SmPOM compound a weak broad Mo−O band as well as a series of sharp f−f transitions is observed in the excitation spectrum (for peak assignment see Table 3 and Table S4). Emission spectra were recorded after exciting the material at

Figure 5. Combined excitation−emission spectra of (a) EuPOM (exciting into the 5L6←7F0 at 392.0 nm yields stronger emission than exciting onto the Mo−O band at 332.0 nm), (b) x%Eu:LaPOM (stronger emission is observed when the sample is excited at 392.6 nm); (c) luminescence lifetimes of EuPOM and x%Eu:LaPOM when excited into the Mo−O charge-transfer band. E


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Inorganic Chemistry

when exciting at 387.1 nm. For the DyPOM compound when exciting at all three wavelengths a broad band is additionally present in the emission spectrum. This band most likely originates from Mo−O emission. This suggests incomplete energy transfer from Mo−O to Dy3+ (when sample is excited at 335.0 nm), and that energy is transferred from the Dy3+ ions to the Mo−O energy levels. Exciting the compound at 351.5 nm (6P7/2←6H15/2) or 387.1 nm (4F7/2←6H15/2) resulted in the characteristic Dy3+ emission: 4F9/2→6H15/2, 4F9/2→6H13/2, and 4 F9/2→6H11/2. The 4F9/2→6H15/2 peak is superimposed on a strong broad band originating from the Mo−O emission. The decay of the compound was calculated to be 8.78 μs (when excited at 355 nm; Figure S9). The x%Dy:LaPOM was excited at 282.0 nm (weak Mo−O band) and 350.5 nm (6P7/2←6H15/2). Exciting into the weak Mo−O band yielded the three characteristic emission peaks of Dy3+. Although the emission is weak no broad band suggesting incomplete energy transfer from Mo−O to Dy3+ is present in the emission spectrum. When the sample was excited at 350.5 nm, as expected, the emission is much stronger. Much stronger Dy3+ emission is observed in this case; aside from that in the 400−500 nm region a weak broad band is also visible revealing that some energy is passed on to the Mo−O energy level. The decay of the compound was calculated to be 12.53 μs (when excited at 355 nm; Figure S9). Some similarities in the luminescence properties of the isostructural TbPOM and DyPOM can be observed. In the emission spectra of both compounds the presence of the Mo− O charge transfer band can be detected. This can most likely be explained by the fact that the accepting energy levels of the Tb3+ and Dy3+ ions are located higher compared to the other lanthanide ions investigated in this work. Most likely there is some energy back transfer to the Mo−O charge-transfer band, and therefore the band is visible in the emission spectrum. Therefore, although this is observed in both isostructural compounds it has no connection to the fact that they form dimeric clusters. Photoluminescence of NdPOM and x%Nd:LaPOM. Examples of purely inorganic lanthanide POMs emitting in the NIR region are rather scarce. We observed NIR emission for both LnPOMs and x%Ln:LaPOM (Ln = Nd3+, Er3+, and Yb3+).

the two emission spectra. Even at an excitation wavelength of 335.0 nm, when the emission peaks are very weak, no emission band from the Mo−O is present. This implies that in the x% Sm:LaPOM compound the energy transfer from the Mo−O to Sm3+ is complete. Also when exciting directly into the f−f transitions of Sm3+ no energy is transferred to the Mo−O energy levels similarly to what we have observed in the SmPOM. The decay time of the x%Sm:LaPOM compound was calculated to be 68.48 μs (excited at 402.4 nm; Figure S9). The lifetime of the x%Sm:LaPOM is significantly longer than that of the SmPOM. Photoluminescence of DyPOM and x%Dy:LaPOM. In the excitation spectra of both DyPOM and x%Dy:LaPOM the f−f transition peaks are present, and only a weak Mo−O band is visible (Figure 8a,b; for peak assignment see Table 4 and Table S5). The DyPOM compound was excited at three different wavelengths: 335.0 nm (Mo−O), 351.5 nm (6P7/2←6H15/2), or 387.1 nm (4F7/2←6H15/2) resulted in the characteristic Dy3+ emission: 4F9/2→6H15/2, 4F9/2→6H13/2, and 4F9/2→6H11/2. Even when the sample is excited into the weak Mo−O band some Dy3+ emission is visible. The strongest emission is observed Table 1. Assignment of Peaks Labeled in Figure 5a (EuPOM) label

wavelength (nm)

wavenumber (cm−1)

transition

excitation a b c d e

361.2 379.9 392.0 414.6 461.9

f g h i j

578.2 590.7 613.2 652.0 702.1

D4←7F0 G2←7F0 5 L6←7F0 5 D3←7F0 5 D2←7F0

27 685 26 323 25 510 24 119 21 650

5

17 295 16 929 16 308 15 337 14 243

5

5

emission D0→7F0 D0→7F1 5 D0→7F2 5 D0→7F3 5 D0→7F4 5

Figure 6. (a) Combined excitation−emission spectra of TbPOM and x%Tb:LaPOM, (b) luminescence lifetimes of TbPOM and x%Tb:LaPOM when excited into the Mo−O charge-transfer band. F


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Inorganic Chemistry Table 2. Assignment of Labeled Peaks in the Emission Spectrum of TbPOM (Figure 6a) label

wavelength (nm)

wavenumber (cm−1)

a b c d

485.4 540.2 586.6 618.2

20 602 18 512 17 047 16 176

Table 3. Assignment of Peaks Labeled in Figure 7a (SmPOM)

transitions label

D4→7F6 5 D4→7F5 5 D4→7F4 5 D4→7F3 5

The fact that the Ln3+ ions are well-shielded from the environment in these multinuclear cluster POMs allows us to observe the NIR emission, which is easily quenched by the environment (by O−H, C−H, N−H vibrations). The emission and excitation spectra of NdPOM were presented in Figure 9a. Exciting the sample both into the Mo− O charge-transfer band at 290.0 nm as well as into the f−f transitions (into the 2G7/2, 4G5/2←4I9/2 transitions at 584.8 nm, see Figure S2) yields the three characteristic emission peaks of Nd3+: 4F3/2→4F9/2 (895.4 nm), 4F3/2→4F11/2 (1065.1 nm), and 4 F3/2→4F12/2 (1344.2 nm). The 2G7/2, 4G5/2←4I9/2 transition peak is more intense than the Mo−O charge-transfer band, and exciting the NdPOM compound at 584.8 nm yields stronger emission. All of the peaks were assigned to appropriate transitions in Table 5. The combined excitation and emission spectrum of x% Nd:LaPOM were presented in Figure 9b. The excitation spectrum consists of a broad Mo−O band in the 250−400 nm region (maximum at 352.0 nm) and several peaks, which could be assigned to f−f transitions of Nd3+ (see Table S6). As in the case of NdPOM the 2G7/2, 4G5/2←4I9/2 transition peak is stronger than the Mo−O charge-transfer band. Exciting into the Mo−O band as well as into the f−f transitions yields the characteristic emission peaks of Nd3+. As can be seen in Figure 9b more intensive emission is obtained upon excitation into the strongest f−f transition at 579.0 nm. The decay times of the NdPOM and x%Nd:LaPOM were measured and were calculated to be 5.13 and 4.92 μs, respectively (Figure S10). These lifetime values can be considered identical within the error margin. Photoluminescence of ErPOM and x%Er:LaPOM. The emission and excitation spectra of ErPOM were presented in the Supporting Information (Figure S5 and Figure S6). The

wavelength (nm)

a b c

362.2 374.1 401.8

d e f g

415.7 439.6 462.7 479.1

h i j k

560.4 594.6 642.9 705.8

wavenumber (cm−1) excitation 27 609 26 731 24 888 24 056 22 747 21 612 20 872 emission 17 844 16 818 15 555 14 168

transition D5/2, 6P5/2, 4D3/2←6H5/2 K13/2, 6P7/2←6H5/2 4 L15/2, 4K11/2, 6P3/2, 4 L13/2←6H5/2 6 P5/2, 4P5/2←6H5/2 4 G9/2, 4M17/2←6H5/2 4 I13/2←6H5/2 4 I11/2, 4M15/2←6H5/2 4 4

G5/2→6H5/2 G5/2→6H7/2 5 G5/2→6H9/2 5 G5/2→6H11/2 5 5

excitation spectrum consists of a broad band in the 250−400 nm region (maximum at 320.0 nm) and several Er3+ f−f transition peaks (see Table S8). When exciting the sample into the broad Mo−O charge-transfer band a strong, broad peak located at 1528.0 nm, which can be assigned to the 4I13/2→4I15/2 “telecom” transition, was observed. The luminescence lifetime was recorded for this complex and was determined to be 3.46 μs (Figure S10). For the x%Er:LaPOM sample the excitation spectrum consisted of a broad Mo−O band in the 250−350 nm region with a maximum at 278.0 nm (Figure 9c). Additionally two weak peaks, which could be assigned to f−f transitions of Er3+ are visible in the spectrum (Table S7). Exciting into the Mo−O band yielded a broad peak located at 1522.0 nm, which could be assigned to the 4I13/2→4I15/2 transition. Exciting into one of the f−f transitions also yielded the characteristic Er3+ emission, yet much weaker than when exciting into the Mo−O transition band. For the ErPOM and x%Er:LaPOM the most efficient way of exciting the samples is through the Mo−O band. This is similar to the Tb3+ POM samples. The decay times of the x%Er:LaPOM were calculated to be 3.63 μs (Figure S10). Therefore, the decay times of the ErPOM

Figure 7. Combined excitation−emission spectra of (a) SmPOM (strongest emission is obtained when exciting at 402.8 nm) and (b) x% Sm:LaPOM (exciting into the f−f transition at 402.4 nm yields stronger emission). G


Article

Inorganic Chemistry

Figure 8. Combined excitation−emission spectra of (a) DyPOM (the emission spectra were obtained after exciting at three different wavelengths) and (b) x%Dy:LaPOM (exciting the sample at 350.5 nm, directly into the f−f transition yields stronger emission).

structure than in the 2:1 sandwich-type POM. In the isostructrural 2:1 sandwich-type ErPOM and YbPOM POMs, the lanthanide ions are at higher risk of luminescence quenching, as they are not as well-shielded from the environment as in the cluster POMs. As a result, both the ErPOM and YbPOM POM show rather weak excitation and emission intensity.

Table 4. Assignment of Labeled Peaks in Figure 8a (DyPOM) label

wavelength (nm)

a b c d e f

351.5 364.5 387.1 426.4 450.8 471.5

g h i

477.2 573.1 660.3

wavenumber (cm−1) excitation 28 450 27 435 25 833 23 452 22 183 21 209 emission 20 956 17 449 15 145

transition P7/2←6H15/2 P5/2←6H15/2 4 F7/2←6H15/2 4 G11/2←6H15/2 4 I15/2←6H15/2 4 F9/2←6H15/2 6 6

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CONCLUSIONS In this study we have investigated a rare case of low-percentage doping of an emissive Ln3+ ion in a novel multinuclear lanthanum POM. The tetranuclear lanthanum POM built from [Mo7O24]6− units was doped at a low percentage of Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, and Yb3+ ions and showed either visible or near-infrared emission in all of the cases. Additionally we have synthesized a series of [Mo7O24]6− tetranuclear lanthanide POMs with the Eu3+, Tb3+, Sm3+, Dy3+, and Nd3+ ions to compare them with the lanthanum POM with x% Ln3+ doping. Several different types of structures were obtained including monomers and dimers, linked through Ln−O bridges. For the Er3+ and Yb3+ ions mononuclear sandwich-type structures built from two [Mo7O24]6− units were obtained. Single crystals suitable for measurements were obtained for all of the compounds and allowed determination of their exact structure. As in the case of most reported POMs the structures are lanthanide-specific, and some trends can be observed. Especially in the case of the lanthanides with a small ionic radius a trend to form dimers linked through oxygen bridges was observed (Tb3+, Dy3+). The smallest of the investigated lanthanides (Er3+, Yb3+) did not form multinuclear clusters. Instead a sandwich-type POM, where one Yb3+/Er3+ is connected with two [Mo7O24]6− POM units, is yielded. A detailed luminescence study (excitation, emission, luminescence lifetime) was recorded for both the x% Ln POM (x−low percentage doping) samples as well as the 100% Ln POM samples. As the structures of the x% Ln POM samples are not identical to the 100% Ln POM samples a straightforward analysis was not possible. It can be observed though that in almost all of the cases the POMs can be excited both through the Mo−O charge-transfer band as well as by directly exciting the f−f Ln3+ transitions. For most of the lanthanides the energy transfer from Mo−O to Ln3+ seems to be efficient, although in

F9/2→6H15/2 F9/2→6H13/2 4 F9/2→6H11/2 4 4

and x%Er:LaPOM samples are practically identical (within error margin). Photoluminescence of YbPOM and x%Yb:LaPOM. The emission and excitation spectra of YbPOM were presented in the Supporting Information (Figure S7 and Figure S8). The excitation spectrum consists of a broad band in the 250−400 nm region (Mo−O band). This band consists of two shoulders and its maximum is at 358.0 nm. Exciting the YbPOM sample at 358.0 nm results in a strong broad band in the NIR region with a maximum at 989.0 nm. This peak could be assigned to the 2F5/2 →2F7/2 transition of Yb3+. In the excitation spectrum of the x%Yb:LaPOM a broad Mo−O band in the 250−400 nm is also present (Figure 9d). In this case the band is not split into two shoulders. The maximum of this band is at 350.0 nm. The significant difference in the excitation spectrum can be linked to the different crystal structure of the YbPOM and x%Yb:LaPOM. When exciting the x%Yb:LaPOM into the Mo−O band the characteristic emission of Yb3+ is observed; the emission spectrum was presented in Figure 9d. The decay times of the Yb3+ POMs were calculated to be 2.17 μs (for YbPOM) and 6.16 μs (for x %Yb:LaPOM). The decay profiles were presented in Figure S10. Here the x%Yb:LaPOM has significantly longer decay time, which can be explained by the fact that the Yb3+ ions are better shielded from the environment in the cluster-type H


Article

Inorganic Chemistry

Figure 9. Combined excitation−emission spectra of (a) NdPOM (emission spectrum was obtained when exciting into the Mo−O charge-transfer band), (b) x%Nd:LaPOM (emission spectra were obtained when exciting into the Mo−O charge transfer band or into a f−f transition), (c) x% Er:LaPOM (emission was obtained when exciting into Mo−O charge transfer band or directly into a f−f transition peak), and (d) x%Yb:LaPOM.

LnPOMs. This can be explained by lanthanide concentration quenching in the 100% Ln POMs. These results are a rare study of multinuclear lanthanide POMs codoped with a low percentage of an emissive Ln3+ ion to obtain materials yielding good luminescent properties. They highlight the potential of employing multinuclear rare-earth POMs as visible and NIR luminescent materials for various applications.

Table 5. Assignment of Labeled Peaks in Figure 9a (NdPOM) label

wavelength (nm)

a

353.4

b c

429.4 473.7

d e f

528.6 584.8 686.4

g h i

895.4 1065.1 1344.2

wavenumber (cm−1)

transition

excitation 2 L15/2, 4D1/2, 2I11/2, 4D5/2, 28 297 4 D3/2←4I9/2 2 23 288 P1/2, 2D5/2←4I9/2 4 21 110 G11/2, 2D3/2, 2P3/2, 2G9/2, 2 K15/2←4I9/2 4 18 918 G9/2, 4G7/2, 2K13/2←4I9/2 2 17 100 G7/2, 4G5/2←4I9/2 2 14 569 H11/2←4I9/2 emission 4 11 168 F3/2→4F9/2 4 9389 F3/2→4F11/2 4 7439 F3/2→4F13/2

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EXPERIMENTAL SECTION

Synthesis. All chemicals (analytical-grade) were purchased from Sigma-Aldrich or VWR and used without further purification. 0.4 mmol (NH4)6Mo7O24 × 4H2O was dissolved in 6 mL of distilled H2O and 1 mL of methanol at 50 °C. Next, 0.2 mmol of Ln(NO3)3 × xH2O (Ln = La3+, Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+; x = 5−6) or La0.95Ln0.05(NO3)3 × xH2O (a mixture of two salts La(NO3)3 × xH2O and Ln(NO3)3 × xH2O where Ln = Eu3+, Tb3+, Sm3+, Dy3+, Nd3+, Er3+, Yb3+; x = 5−6) dissolved in 2 mL of distilled water was added and left to stir at room temperature for 30 min. The solutions were filtered, and the filtrate was set up for crystallization through vapor diffusion of methanol or diethyl ether into the water−methanol solution of the lanthanide POMs (at 7 °C). Crystals suitable for measurements were obtained after 1−5 d. They were filtered off and dried in air for luminescence measurements. A table with the actual calculated Ln3+ concentrations found for each sample (using ICP-MS) was presented in the Supporting Information (Table S9).

most cases (except for the Tb and Er POMs) exciting into f−f transitions is favorable. Only in the case of Dy POMs it can be observed that the energy transfer from Mo−O to Dy3+ ions is not complete. We have also observed that in most cases the x% Ln:LaPOMs show decay times higher than that for the I


Article

Inorganic Chemistry Characterization. For the reported structures X-ray intensity data were collected on an Agilent Supernova Dual Source diffractometer equipped with an Atlas CCD detector using ω scans and Mo Kα (λ = 0.710 73 Å) radiation for samples LaPOM2, EuPOM, DyPOM, and SmPOM or Cu Kα (λ = 1.541 84 Å) radiation, for samples LaPOM1, TbPOM, NdPOM, YbPOM, and ErPOM. The images were interpreted and integrated with the program CrysAlisPro (Agilent Technologies).26 Using Olex2 the structures were solved by direct methods using the ShelXS structure solution program and refined by full-matrix least-squares on F2 using the ShelXL program package.27,28 All atoms were anisotropically refined. Hydrogen atoms could not be unequivocally located on solvent water or ammonium molecules and were not included in the refinement. The crystals were measured either under the N2 cryostream at 100 K or at room temperature. ICSD reference numbers: ICSD 431587 (LaPOM2), ICSD 431590 (TbPOM), ICSD 431589 (SmPOM), ICSD 431591 (DyPOM), ICSD 431588 (NdPOM), ICSD 431592 (ErPOM), ICSD 431593 (YbPOM). Powder XRD patterns were recorded by a Thermo Scientific ARL X’TRA diffractometer equipped with a Cu Kα (λ = 1.5405 Å) source, a goniometer, and a Peltier-cooled Si (Li) solid-state detector. For the Ln3+ concentrations a quadrupole ICP-MS (ELAN DRC− from PerkinElmer) instrument was used. Approximate 0.02−0.04 g samples were weighed and taken up in 2 mL of 14 M HNO3 (purified by in-house sub-boiling distillation) + 1 mL of HF (analytical grade) and were left to stir on a hot plate at 130 °C for 24 h. Next to the samples a blank was prepared using the same digestion procedure but without sample. All samples were diluted 200−40.000 times before measuring. The concentrations were calculated using an internal standard Y (OES measurements) and Rh (ICP-MS measurements). In general: all the concentration measurements take into account a uncertainty of 5−10%. The luminescence of solid samples was studied. Solid powdered samples were put between quartz plates (Starna cuvettes for powdered samples, type 20/C/Q/0.2). Luminescence measurements were performed on an Edinburgh Instruments FLSP920 UV−vis−NIR spectrometer setup. A 450 W xenon lamp was used as the steady-state excitation source. Luminescence decay times were recorded using a 60 W pulsed Xe lamp, operating at a frequency of 100 Hz or a Continuum Surelite I laser (450 mJ @1064 nm), operating at a repetition rate of 10 Hz or using either the third harmonic (355 nm) or fourth harmonic (266 nm) as the excitation source, and the photomultiplier detectors mentioned below. A Hamamatsu R928P photomultiplier tube was used to detect the emission signals in the near-UV to visible range. A Hamamatsu R5509−72 photomultiplier was used to detect emission in the NIR region. All of the luminescence measurements were recorded at room temperature. To compare the measurements the same amounts of powders were used as well as the same settings for each measurement (same slit size, step, and dwell time). All of the excitation spectra are recorded observing at the strongest f−f emission peak. All emission spectra in the manuscript have been corrected for detector response. The luminescence decay curves of the samples were measured when excited into the maximum of the Mo−O charge transfer band (unless stated otherwise) and monitored at the appropriate wavelength. All of the decay curves could be well-fitted using a single exponential eq 1:

⎛ t⎞ I = I0exp⎜− ⎟ ⎝ τ⎠

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Anna M. Kaczmarek: 0000-0001-5254-8762 Rik Van Deun: 0000-0001-7091-6864 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.M.K. acknowledges Ghent University’s Special Research Fund (BOF) for a Postdoctoral Mandate (Project No. BOF15/ PDO/091). R.V.D. acknowledges the Hercules Foundation (Project No. AUGE/09/024 “Advanced Luminescence Setup”) for funding. K.V.H. and R.V.D. thank the Hercules Foundation (Project No. AUGE/11/029 “3D-SPACE: 3D Structural Platform Aiming for Chemical Excellence”) for funding. K.V.H. thanks the Research Fund Flanders (FWO) for funding.

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REFERENCES

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(1)

where I and I0 are the luminescence intensities at time t and 0, respectively, and τ is the luminescence lifetime.

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Powder XRD patterns, additional crystal structures, illustrated solvent-accessible surfaces, illustrated crystal packing, tabulated data collection and refinement statistics, tabulated peak assignments, emission spectra, excitation spectra, decay profiles, tabulated relative Ln3+ content for the x%Ln:LaPOM samples, tabulated Shape calculations (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02130. J


Article

Inorganic Chemistry (12) Krishna Bharat, L.; Vengala Rao, B.; Yu, J. S. Polyol mediated solvothermal synthesis and characterization of spindle shaped La2(MoO4)3: Eu3+ phosphors. Chem. Eng. J. 2014, 255, 205−213. (13) Liu, K.; You, H.; Zheng, Y.; Jia, G.; Huang, Y.; Yang, M.; Song, Y.; Zhang, L.; Zhang, H. Room-Temperature Synthesis of MultiMorphological Coordination Polymer and Tunable White-Light Emission. Cryst. Growth Des. 2010, 10, 16−19. (14) Kaczmarek, A. M.; Miermans, L.; Van Deun, R. Nano- and microsized Eu3+ and Tb3+-doped lanthanide hydroxycarbonates and oxycarbonates. The influence of glucose and fructose as stabilizing ligands. Dalton Trans. 2013, 42, 4639−4649. (15) Shiddiq, M.; Komijani, D.; Duan, Y.; Gaita-Arino, A.; Coronado, E.; Hill, S. Enhancing coherence in molecular spin qubits via atomic clock transitions. Nature 2016, 531, 348−351. (16) Sato, R.; Suzuki, K.; Sugawa, M.; Mizuno, N. Heterodinuclear Lanthanoid-Containing Polyoxometalates: Stepwise Synthesis and Single-Molecule Magnet Behavior. Chem. - Eur. J. 2013, 19, 12982− 12990. (17) Sato, R.; Suzuki, K.; Minato, T.; Yamaguchi, K.; Mizuno, N. Sequential Synthesis of 3d−3d′−4f Heterometallic Heptanuclear Clusters in between Lacunary Polyoxometalates. Inorg. Chem. 2016, 55, 2023−2029. (18) Artetxe, B.; Reinoso, S.; Felices, L. S.; Lezama, L.; GutiérrezZorrilla, J. M.; García, J. A.; Galán-Mascarós, J. R.; Haider, A.; Kortz, U.; Vicent, C. Cation-Directed Dimeric versus Tetrameric Assemblies of Lanthanide-Stabilized Dilacunary Keggin Tungstogermanates. Chem. - Eur. J. 2014, 20, 12144−12156. (19) Gupta, R.; Saini, M. K.; Doungmene, F.; de Oliveira, P.; Hussain, F. Lanthanoid containing phosphotungstates: the syntheses, crystal structure, electrochemistry, photoluminescence and magnetic properties. Dalton Trans. 2014, 43, 8290−8299. (20) Bosco, M.; Rat, S.; Dupré, N.; Hasenknopf, B.; Lacote, E.; Malacria, M.; Rémy, P.; Kovensky, J.; Thorimbert, S.; Wadouachi, A. Lewis-Acidic Polyoxometalates as Reusable Catalysts for the Synthesis of Glucuronic Acid Esters under Microwave Irradiation. ChemSusChem 2010, 3, 1249−1252. (21) Ritchie, C.; Moore, E. G.; Speldrich, M.; Kögerler, P.; Boskovic, C. Terbium Polyoxometalate Organic Complexes: Correlation of Structure with Luminescence Properties. Angew. Chem., Int. Ed. 2010, 49, 7702−7705. (22) In Polyoxometalate Chemistry;Pope, M. T.; Müller, A., Eds.; 2001; pp 187−203. Alvarez, S.; Alemany, P.; Casanova, D.; Cirera, D.; Llunell, M.; Avnir, D. Shape maps and polyhedral interconversion paths in transition metal chemistry. Coord. Chem. Rev. 2005, 249, 1693−1708. (23) Llunell, M.; Casanova, D.; Cirera, J.; Alemany, P.; Alvarez, S. SHAPE, v2.1; Universitat de Barcelona, 2013. (24) Naruke, H.; Yamase, T. Photoluminescence of (NH4)12H2[Eu4(MoO4)(H2O)16(Mo7O24)4]·13H2O. J. Lumin. 1991, 50, 55−60. (25) Wu, S.; Deng, B.; Jiang, X.; Li, R.; Guo, J.; Lai, F.; Huang, X.; Huang, C. Step-by-step assembly of 4d−4f−3d complex based on heptamolybdate anion. J. Solid State Chem. 2012, 196, 451−457. (26) Agilent. CrysAlis Pro; Agilent Technologies, U.K. Ltd: Yarnton, England, 2013. (27) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (28) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339−341.

K


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