Two Lanthanide-Substituted Polyoxometalates Featuring Novel

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Two Lanthanide Substituted Polyoxometalates Featuring Novel CrescentShaped Ln5 Clusters: Structures, Ion Conductivities and Magnetic Properties Yu Hao, Li Zhong, Hao-Hong Li, and Shou-Tian Zheng Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01717 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Crystal Growth & Design

Two Lanthanide Substituted Polyoxometalates Featuring Novel Crescent-Shaped Ln5 Clusters: Structures, Ion Conductivities and Magnetic Properties Hao Yu†, Zhong Li†, Hao-Hong Li,* Shou-Tian Zheng* State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China ABSTRACT: By adopting trilacunary [A-α-GeW9O34]4- as precursor under hydrothermal conditions, two novel polyoxometalates (POMs) substituted by crescent-shaped Ln5 clusters, i.e. Na4K4(H2pip)6(H2O)10{Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4,11)-GeW10O38]2}·29H2O (1-La for Ln=La, 1-Ce for Ln= Ce, pip=piperazine) have been successfully synthesized. The 1-D chains of two lanthanide substituted POMs are constructed from edge-sharing of nanoclusters with size of 2.5×2.9×1.0 nm3, which contain two crescent-shaped five-nuclearity Ln5 clusters and two [β(4,11)-GeW10O38]12- polyanions. Furthermore, 1-La exhibits good ion-conducting performancec with proton conduction activation energy of 0.49 eV, which follows Vehicular mechanism. 1-Ce illustrates antiferromagnetic behavior, which can be attributed to spin-orbital coupling interactions within the crescent-shape Ln5 clusters.

INTRODUCTION Polyoxometalates (POMs) generated from oxygen-bridged early transition-metal ions (WIV, MoIV, NbV, TaV, VV) with high-oxidation states have captured sustainable interests owing to their charming molecular aesthetics1-5 together with promising utilizations as functional materials such as ion conductivity,6-8 magnetism,9-12 catalysis13-17 and medicine.18 Currently, the discovery of novel POMs-based materials remains a great challenge even the first POM (NH4)3PMo12O40 was prepared nearly two centuries ago.19,20 One effective strategy for exploration new POM materials is to introduce a second metal (including transition-metals, lanthanide metals) into POM skeleton, resulting a fast developing subclass of POMs called metal-substituted POMs (MSPs). Since the first MSP was reported in 1962, series of MSPs have been documented.21-24 However, most of attention in this area was paid to the transition-metal substituted POMs (TMSPs).25-29 In contrast, lanthanide substituted POMs (LMSPs) attracted much less attention. Compared with TMSPs, LMSPs could be expected to exhibit more fascinating performances due to the existence of unpaired 4f electrons, for example,longer life-time emissions,30-32 more complicated magnetic behaviors.33-36 But up to now, the study about LMSPs is still in its infancy, which is generally blocked by the synthesis difficulties. For example, lanthanide ions can often hydrolyze into unreactable lanthanide oxides. In all, this system is subject to higher oxophilicity, larger size, higher coordination number and more flexible coordination geometries of lanthanide centers compared with transition-metals. Therefore, the exploration of novel LMSPs is still a big challenge. So far, the nuclearities of incorporated lanthanide clusters in documented LMSPs range from two to ten, including the representative examples of [Ln2(PW11O39H)2(H2O)6]6-,37 [Y3(H2O)3(CO3)(PW9O34)2]11-,38 [Ln4(GeW10O38)2(H2O)6]12-,39 [{Yb6(μ6-O)(μ3-OH)6(H2O)6}(P2W15O56)2]14-,40 [Eu8(PW10O38)4(W3O14)]30-,41 [Ce10O6(OH)6(CO3)(H2O)11(P2W16O59)3]19-.42 Recently, two giant LMSPs [La27Ge10W106O406(OH)4(H2O)24]59and [La29Ge10W106O408(OH)2(H2O)28]53- have been reported by our group, whose lanthanide clusters possess the highest-nuclearity to date..43 These lanthanide-substituted polyoxometalates can be expected to

exhibit superior properties in fluorescent, catalytic, magnetic, and electrochemical properties due to their rich f electrons. Compared with transition metal ions, rare earth ions have larger radius and higher charges. So during their reactions with highly negative polyanions, precipitation will happen more frequently than crystallization. In this work, by overcome these difficulties, another two novel LMSPs, namely, Na4K4(H2pip)6(H2O)10{Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4, 11)-GeW10O38]2}·29H2O (1-La for Ln=La, 1-Ce for Ln= Ce, pip= piperazine) have been successfully prepared. In 1-La and 1-Ce, novel crescent-shaped five-nuclearity lanthanide clusters were incorporated. It’s worth mentioning that 1-La and 1-Ce are the first examples of LMPSs containing aggregated five-nuclearity lanthanide crescent-shape clusters. Ion conductivity measurement shows that 1-La is a good proton conducting material. Antiferromagnetic behavior of 1-Ce can be attributed to spin-orbital coupling interactions within the crescent-shape Ln5 clusters.

EXPERIMENTAL SECTION Materials and Measurements All chemicals used for syntheses were purchased from commercial sources, and no further purifications were conducted before their usages. Na2K8[A-α-GeW9O34]·25H2O was prepared according the literature procedure.44 Elemental analyses on C, H and N were performed on a Vario EI III elemental analyzer. IR spectra collected in the range of 4000-400 cm-1 were obtained on a Nicoleti S50 spectrometer in form of KBr tablet. The powder X-ray diffraction (PXRD) patterns were executed on a Rigaku DMAX 2500 diffractometer equipped with CuKα radiation (λ = 1.54056 Å). Thermal stabilities from room temperature to 800°C were measured on Mettler Toledo Star TGA/DSC1 instrument (heating rate: 10°C/min, Ar atmosphere). Magnetic performance was investigated by variable-temperature susceptibility measurement on a Quantum Design PPMS-9T magnetometer (temperature range:2-300 K, magnetic field: 0.1 T). Ion Conductivity Measurement Zennium/IM6 electrochemical workstation was used to determine ion conductivity property. The accessories include a SI 1260 IMPEDANCE/GAINPHASE analyzer (AC impedance measurements, frequency range: 0.1-5 MHz, applied

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

voltage: of 50 mV) and a STIK Corp. CIHI-150B incubator (controlling the temperatures and relative humidity). During the measurement, a three-probe method was adopted. In detail, a cylindrical pellet using crystalline sample (1 mm in thickness and 0.196 mm2 in area) coated with C-pressed film was used as working electrode, .two silver electrodes attached on both sides of pellet act as end terminal electrode.. Crystal Structure Determination: The structures of 1-La and 1-Ce were determined at 175 K under nitrogen atmosphere by single-crystal X-ray diffraction method. The apparatus is a Bruker Apex Duo CCD diffractometer and the X-ray resource is a graphite-monochromatized Mo Kα radiation with wavelength of 0.71073 Å. The structures of 1-La and 1-Ce were solved through direct methods and full-matrix least-squares refinements based on F2 were taken during their structural resolution based on SHELX-2014 program.45,46 Non-hydrogen atoms were refined anisotropically, and hydrogen atoms of pip were generated geometrically. SQUEEZE program was applied to remove the disordered solvents and counter anions in 1-Ln. The crystallographic data crystal structure refinement details are summarized in Table 1. Their crystallographic data can be queried with CCDC numbers of 1819553 and 1819556 at the Cambridge Crystallographic Data Centre. Synthesis of 1-La and 1-Ce K8Na2[A-a-GeW9O34]·25H2O (0.320 g, 0.103 mmol), La(NO3)3·6H2O (0.100 g, 0.233

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mmol)/Ce(NO3)3·6H2O (0.100 g, 0.230 mmol) and pip (0.050 g, 0.580 mmol) were mixed in 5 mL distilled water. After stirring for 1 hour, the suspensions were moved into a 25 mL Teflon-lined autoclaves. The temperature was heated to 140°C and kept for 72 hours. Afterwards, colorless/yellow block crystals were produced after cooling to room temperature. Yields: 40 mg (12.86%) for 1-La/35 mg (11.2%) for 1-Ce (based on Ln3+). It’s worth mentioning that the addition of appropriate amount of sustained-releasing agents (such as dimethylamine hydrochloride, piperazine) will be beneficial to the formation of aggregated lanthanide clusters substituted POMs, which can not only reduce the fast reaction of rare earth with polyoxometalate clusters, but also can act as a countercations to promote crystallization. Generally, the rare earth cluster-substituted polyoxometalates can not be obtained in one step, and a post-treatment after hydrothermal reaction seems necessary (Table S2). Elemental analysis (%): For 1-La, calcd for C8H22N4O188Na4K4W40Ge4La10 (Fw = 12464.12): C, 2.82; H, 1.43; N, 1.57. Found: C, 2.86; H: 1.45; N, 1.53. For 1-Ce, calcd for C8H22N4O184Na4K4W40Ge4Ce10 (Fw = 12412.21): C, 3.00; H, 1.49; N, 1.58. Found: C, 2.92; H: 1.49; N, 1.58. IR (solid ATR, v/cm-1): for 1-La, 3420(m), 3020(w), 1615(w), 1450(w), 1389(w), 943(m), 875(m), 791(vs). For 1-Ce, 3427(m), 3013(w), 1618(w), 1457(w), 1384(w), 950(m), 876(m), 777(vs).

Table 1 Crystal Data and Structure Refinement for 1-La and 1-Ce

Empirical formula Formula weight

1-La C8H22N4O188Na4K4W40Ge4La10 12464.12

1-Ce C8H22N4O184Na4K4W40Ge4Ce10 12412.21

Crystal system

Triclinic

Triclinic

Space group a (Å)

P-1 13.9972(14)

P-1 13.9981(23)

b (Å)

18.9495(19)

18.9644(31)

c (Å)

21.300(2)

21.2349(35)

V α (º)

5091.9(9)

5077.2(14)

107.5830(10)

107.579(2)

β (º)

105.8510(10)

106.032(2)

γ (º) Z F(000)

94.9790 (10) 2 5358

94.860 (2) 2

4.057

(Å3)

ρcalcd

(g∙cm-3)

5336

Temperature (K)

175

4.052 175

μ(mm-1)

25.305

25.513

Refl. Collected

36488

31177

Independent relf.

17924

17640

Parameters

1210

1162

GOF on F2 1.074 1.097 Final R indices (I= 2σ(I)) R1 = 0.0406;wR2 = 0.1108 R1 = 0.0768;wR2 = 0.2348 R indices (all data) R1 = 0.0570;wR2 = 0.1116 R1 = 0.1085;wR2 = 0.2294 R1 = ∑||Fo| – |Fc||/∑|Fo|. wR2 = [∑w(Fo2– Fc2)2/∑w(Fo2)2]1/2; w = 1/[σ2(Fo2) + (xP)2 + yP], P = (Fo2 + 2Fc2)/3, where x =0.0516, y =121.2962 for 1-La; where x =0.1472, y = 0 for 1-Ce


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Figure 1 a) Polyhedral and ball-stick illustration of {Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4,11)-GeW10O38]2}20polyanion. b-c) Ball-stick and polyhedral structures of the five-nuclearity [La5O26(H2O)5(μ3-O)]39- cluster. d) Structure of [α(1,8)GeW10O38]12- dilacunary fragment. e) Structure of the [β(4,11)-GeW10O38]12- dilacunary fragment.

RESULTS AND DISCUSSION Crystal Structure 1-La and 1-Ce are isostructures with triclinic system and space group P-1. Herein, only 1-La is selected as an example for structural description. The independent unit of 1-La is composed by a centrosymmetric {Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4,11)-GeW10O38]2}20polyanion (Figure 1a), six protonated piperazine (H2pip)2+ cations, four Na+ ions, four K+ ions, ten coordinated water ligands and twenty-nine lattice waters. The structure of {Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4,11)-GeW10O38]2}20polyanion can be viewed as a tetrameric structure, in which two isolated five-nuclearity [La5O26(H2O)5(μ3-OH)]38- clusters (La5, Figure 1d) are stabilized by two [α(1,8)-GeW10O38]12- (Figure 1b-1c) and two [β(4,11)-GeW10O38]12- (Figure 1e) dilacunary fragments. In La5 cluster, the La- La distances fall in the range of 3.818 ~ 4.183 Å, and the La-O-La angles range from 56.365 to 112.779°, which are approximate to the reported LMSPs.40,42 In detail, five unique La3+ ions are in three kinds of coordination environments (Figure S1). Among them, La1 employs a ten-coordinated distorted biscapped tetragonal prismatic geometry, whose polyhedron is saturated by four O atoms from a [α(1,8)-GeW10O38]12- fragment, four O atoms from a [β(4,11)-GeW10O38]12- fragment, an two coordinated waters. La2, La3 and La4 are all in distorted monocapped tetragonal prismatic geometry (nine-coordinated) with different coordination environments. In detail, La2 is surrounded by two O donors from a [β(4,11)-GeW10O38]12- fragment, six O atoms from two [α(1,8)-GeW10O38]12- polyanions and one μ3-OH group. La3 is binded by two O atoms from a [α(1,8)-GeW10O38]12- polyanion, five O atoms from a [β(4,11)-GeW10O38]12- fragment, one μ3-OH group and one terminal water. La4 is coordinated by five O atoms from a [α(1,8)-GeW10O38]12- polyanion, three O atoms from a [β(4,11)-GeW10O38]12- polyanion, and one μ3-OH group. The coordinated number of La5 is eight, whose polyhedron employs bicapped trigonal prismatic geometry, among which four O atoms is

provided by a [β(4,11)-GeW10O38]12- polyanion, two O atoms stem from two [α(1,8)-GeW10O38]12- polyanions, and two ones are coordinated waters. The La-O bond distances fall in the range of 2.373 ~ 2.870 Å, which is comparable with that in reported LMSPs.47,48 In La5 cluster, three nine-coordinated La3+ ions (La2, La3, La4) are integrated by a μ3-OH group (O53) to generate a {La3O20(μ3-OH)} trimer. A LaO8 and a LaO10 polyhedron connect to {La3O20(μ3-OH)} trimer to generate a crescent-shaped five-nuclearity La5 cluster vis face-sharing mode (Figure 2a). To the best of our knowledge, such a crescent-shaped five-nuclearity La5 cluster has never been reported so far in POMs. Another structure feature is the further assembly of La5 clusters with two kinds of different dilacunary fragments. As shown in Figure 1a, a [β(4,11)-GeW10O38]12- polyanion inserts into the equatorial vacancy of La5 cluster, resulting in a {[La5O13(μ3-OH)(H2O)5][β(4,11)-GeW10O38]} building block. Such a building block and its centrosymmetric one are placed in a back-to-back manner, and further joined by two [α(1,8)-GeW10O38]12polyanions to produce an uncommon nanocluster with size of 2.5×2.9×1.0 nm3 (Figure S1). Bond valence sum (BVS)49 calculations indicate that the oxidation states of all La atoms is +3.(Table S1) Currently, although LMSPs containing ten discrete lanthanide centers in one cluster have been reported,20,50,51 in which the rare earth atoms were linked indirectly by La-O-X-O-La (X=W, Ge, As, P, etc). However, the linkage way of lanthanide centers in this work is significantly different, in detail, lanthanide atoms are directly bonded by La-O bonds to generate a penta-nuclearity crescent-shaped Ln5 cluster, which was reported for the first time. Furthermore, each nanosized polyanion shares four La-O-W bonds with neighboring ones to form a unique 1-D infinite chain (Figure 2b).

Figure 2 a) Structure evolution of [La5O28(H2O)3(μ3-O)]43- cluster. b) 1-D chain based on {Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4,11)-GeW10O38]2}20polyanions. In this work, trilacunary [A-α-GeW9O34]4- was used as precursor, and two different dilacunary fragments were present in the final products, indicating that the [A-α-GeW9O34]4- precursor has experienced the configuration transformations from trilacunary to



dilacunary geometries during the reaction processes. The detail structural evolution involving cluster dissembling and re-assembling processes are illustrated in Figure 3. Compared with trilacunary fragments, the dilanucary fragments have more isomers. Usually, the Keggin-type dilacunary fragments are beneficial for the generation of high-nuclearity LMSPs.38,39,41 Two interesting conclusions could be drawn from the above results. Firstly, this dilacunary GeW10 stemed from trilacunary GeW9 block is an important building unit for constructing high-nuclear Ln-substituted POMs, which has been proved by the previous work.52,53In our previous work, we have also used this GeW10 block to achieve Ln27 and Ln29 high-nuclear clusters.43Secondly, organic amine such as piperazine can not only relieve the precipitation of Ln ions, but also serve as countercations to stablize the structures, which can be verified by the good crystal quality and high stability.

Figure 3 Schematic of the structural transformations of the starting material [A-α-GeW9O34]10-, the final dilacunary fragments [α(1,8)-GeW10O38]12-, [β(4,11)-GeW10O38]12-, and the saturated [α-GeW12O40]4- and [β-GeW12O40]4-. Phase Purtiies and Thermal Stabilities Characterizations Phase purities of 1-La and 1-Ce are confirmed by the comparison of experimental PXRD patterns with the simulated ones calculated from single-crystal X-ray results (Figure S5). The good accordance suggests their purities are reliable, in which some diffraction peak intensity differences are caused by the anisotropy of the crystals. In addition, PXRD measurement on 1-La after proton conduction test reveals that its framework is maintained during the proton conductio process. The thermal stability and the number of lattice water molecules of 1-La and 1-Ce have been determined by TG analyses using freshly synthesized crystalline samples. Two weight loss steps can be observed for two compounds (Figure S6). The first weight loss stage from 30 to 200 °C with ratio of 3.21% corresponds to the liberation of 29 lattice waters, and the second one from 200 to 600 °C can be attributed to the dehydration of piperazine protons and the removal of coordinate waters. Above 600 °C, the skelstons begin to collapse. Ion conductivity Properties The good water thermal stability together with the existence of numerous lattice waters, protonated pip cations hint their possible ion conducting performances. Herein, alternating current (AC) impedance measurement on 1-La was conducted to disclose their ion conducting behavior, in which its conductivity was estimated from the semicircle fitting on Nyquist plots. Firstly, at given temperature of 30ºC, the the relative humidity (RH)-dependent ion conductivities were measured using various RH (55-98%) (Figure 4a).

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With the RH increasing from 55 to 98%, its conductivities augment from 8.66  10-7 to 4.68  10-5 S·cm-1 correspondingly. Afterwards, at given RH of 98%, the temperature-dependent conductivities were recorded in the temperature range of 20-85 ºC (Figure 4b). The proton conductivities also increase with the temperature elevating.54,55 When the temperature rises to 85 ºC, the conductivity were measured as 6.35  10-4 S·cm-1. The results show that temperature can affect the conductivity more efficiently than humidity. Finally, the ion-conductive activation energy at 98% RH was calculated as 0.49 eV using the Arrhenius equation (σT=σ0exp(-Ea/kBT))56 (Figure 4c). The activation energy of proton conduction Ea > 0.4 eV indicates that the ion conduction process in 1-La should be dominated by Vehicular mechanism.57 In other words, the presence of ion conduction channels and large number of proton carriers are necessary for electrically conductive transportation in this process. According to structural analysis, proton conduction channels, proton carriers such as isolated waters and pip can be presented in 1-La, which verify the reliability of Vehicular mechanism. Magnetic properties Due to the existence of numerous unpaired 4f electrons in Ln3+ ions, magnetic behaviors of Ln-containing is fascinated, in which spin-orbital coupling interactions is dominated. In their magnetic performance, stark levels stemming from 2S+1LJ states are important, because the energy level of 2S+1LJ ground state can be well separated, leading to its mere thermally population at low temperature.58 Here, variable-temperature magneic susceptibility χm of 1-Ce was investigated (temperature range: 2-300 K, external magnetic field: 1 KOe). As shown in Figure 5, the experimental χmT value of is 8.26 cm3·mol-1·K at room temperature. Comparably, for ten non-interacting Ce3+ ions under conditions of J=5/2 and g=6/7, the corresponding theoretical value is 8.04 cm3·mol-1·K. Wuth the decrease of temperature, the χmT value decreases gradually, and reaches to the minimium value of 4.52 cm3·mol-1·K at 2 K. Such magnetic performance hints the occurance of antiferromagnetic coupling among the Ce3+ ions in crescent-shape Ce5 cluster. Its antiferromagnetic behavior might probably be ascribed to the depopulation of excited states derived from splitting of 2F5/2 ground state.49,50 Furthermore, above 70 K, the temperature dependent reciprocal susceptibility (1/χm) of 1-Ce fits well with the Curie-Weiss law, the corresponding Curie constant C is 9.066 cm3∙K∙mol-l and Weiss constant θ is-42.31 K. The good consistence with Curie-Weiss law can be explained as the spin-orbital coupling interactions together with the generation of thermally populated excited states. The negtive Weiss constant further testifies the antiferromagnetic interactions within the crescent-shape Ln5 clusters in 1-Ce, which can be also observed in many Ln-substituted polyoxometalates.50,51

CONCLUSION In summary, two novel polyoxometalates substituted by crescent-shaped Ln5 clusters have been prepared successfully using trilacunary [A-α-GeW9O34]4- as precursor under hydrothermal conditions. Keggin-type dilacunary fragments are crucial for the generation of high-nuclearity LMSPs. The 1-D chains of two LMSPs are constructed from a nanocluster, i. e. {Ln10(μ3-OH)2(H2O)10[α(1,8)-GeW10O38]2[β(4,11)-GeW10O38]2}20-, which contains two novel crescent-shaped five-nuclearity La5 clusters. 1-La exhibits good ion-conducting performancec with proton conduction ativation energy of 0.49 eV. The antiferromagnetic behavior of 1-Ce could be ascribed to the depopulation of excited states derived from splitting of 2F5/2 ground state in Ce5 centers. This work enriches the structural diversities of LMSPs with joined


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high-nuclearity Ln clusters and their magnetic/electrical performance. Further works about introducing LMSPs with higher-nuclearity joined Ln clusters are still ongoing.

Figure 5 Temperature dependence of χmT (black) and 1/χm (blue) for 1-Ce at 2-300 K

ASSOCIATED CONTENT Supporting Information Supporting information include other structural figures, TGA curves, IR spectra, PXRD patterns and bond valence calculations. These materials are available free of charge via the internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author

*Email: [email protected]. *Email: [email protected]. ORCID

Hao-Hong Li: 0000-0003-3543-7715 Shou-Tian Zheng: 0000-0002-3365-9747 Author Contributions

†These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Figure 4 (a) Nyquist plots under different RHs at given temperature; (b) Nyquist plots under different temperatures at given RH; (c) Arrhenius plots of the conductivity of 1-La.

This work was financially supported by National Natural Science Foundations of China (No. 21303018, 21371033, and 21401195) and National Natural Science Foundation of Fujian Province (2018J01684).

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Two Lanthanide Substituted Polyoxometalates Featuring Novel Crescent-Shaped Ln5 Clusters: Structures, Ion Conductivities and Magnetic Properties Hao Yu†, Zhong Li†, Hao-Hong Li,* Shou-Tian Zheng* Two novel ten-nuclearity lanthanide cluster substituted polyoxometalates have been obtained, in which a [β(1,8)-GeW10O38]12- polyanion insert into the equatorial vacancy of La5 cluster to generate a La5@GeW10O38 building block. Such a building block and its centrosymmetric one are placed in a back-to-back manner, and further joined by two [α(1,8)-GeW10O38]12- polyanions. Moreover, 1-La exhibits good ion-conducting performance and 1-Ce illustrates antiferromagnetic behavior.

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