Synthesis of a 6-nm-Long Transition-Metal–Rare-Earth

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis of a 6‑nm-Long Transition-Metal−Rare-Earth-Containing Polyoxometalate Qing Han,† Zhong Li,‡ Xiangming Liang,† Yong Ding,*,†,§ and Shou-Tian Zheng*,‡

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†

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡ State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry, Fuzhou University, Fujian 350108, China § State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China S Supporting Information *

competing reactions caused by the oxophilic 4f cations and less reactive 3d cations toward POM ligands. To overcome the obstacle, three main synthetic strategies, including an auxiliary ligand protection strategy, a one-pot assembly strategy, and a presynthesized POM precursor strategy, have been developed for the syntheses of 3d−4fcontaining POMs. First, an auxiliary ligand protection strategy is widely used because the coordination of organic components with 3d or 4f cations can effectively improve the solubility and reduce the competitive reaction of 3d and 4f cations with POM ligands.7 Second, a one-pot assembly strategy of simple materials facilitates the formation of various building blocks to obtain unexpected 3d−4f POMs.8 Third, a presynthesized POM precursor strategy, such as the use of 3d-substituted POM precursors with 4f ions, 4f-substituted POM precursors with 3d ions, or POM precursors with 3d−4f complexes, is liable to synthesize certain targeted products in a controlled manner.9 Although various synthetic strategies have been developed, it still remains quite challenging to obtain intriguing high-nuclearity 3d−4f-containing POMs. Herein, we report the synthesis and structure of a novel giant high-nuclearity 3d−4f-containing POM, [(CH3)2NH2]4H52[Eu 16 Co 7 Se 16 W 128 O 448 (CIT) 10 (HCIT) 2 (NO 3 ) 4 (OH) 4 (H2O)52]·nH2O ([(CH3)2NH2]4H52 [1]·nH2O; H4CIT = citric acid and n ≈ 192), obtained by the combination of a one-pot assembly strategy and an auxiliary ligand protection strategy, that has many remarkable features: (1) Unlike most known giant nanoscale POMs with clusterlike structures, giant polyoxoanion 1 has a ribbonlike structure with an ultralong length of ca. 6.1 nm, which is by far the longest molecular POM. (2) Polyoxoanion 1 exhibits a rare giant inorganic− organic hybrid molecular POM and is the first 3d−4fcontaining POM based on Dawson-type selenotungstate(IV). (3) With 167 heavy atoms and a total of more than 800 non-H atoms, the longest polyoxoanion 1 is also the largest and the heaviest 3d−4f-containing POM. Polyoxoanion 1 was synthesized by a one-pot solution reaction of Na2WO4·2H2O, Na2SeO3, Eu(NO3)3·6H2O, Co(NO3)3·6H2O, (CH3)2NH2·HCl, and H4CIT in water at 80 °C

ABSTRACT: An extremely long ribbon-shaped polyoxometalate (POM) octamer, {Eu 16Co7Se16W128O 448 (CIT)10(HCIT)2(NO3)4(OH)4(H2O)52} (H4CIT = citric acid), with a maximum length of ca. 6 nm has been obtained, which is the longest molecular POM reported to date and the first Dawson-based 3d−4f-containing selenotungstate(IV). The nanoscale octamer is built from 8 new Dawson-type Eu2W2-substituted selenotungstates {(Eu2W2O4)Se2W14O52} bridged by 12 H4CIT ligands and 3 Co2+ ions, giving rise to a rare giant inorganic−organic hybrid molecular POM with more than 150 metal centers (7 CoII, 16 EuIII, 16 SeIV, and 128 WVI). Especially, with 128 W centers and more than 800 non-H atoms, the POM octamer also represents the largest known 3d−4f-containing polyoxotungstate.

T

he design and construction of large metal oxide clusters has long intrigued synthetic chemists as a fascinating research endeavor because of their applications in the multidisciplinary area associated with this class of materials.1 Polyoxometalates (POMs), a class of anionic metal oxo clusters of MoVI, WVI, VV, NbV, and TaVI, have captured extensive attention because of their intriguing structures, unique physicochemical properties, and wide applications in catalysis, magnetism, medicine, and material sciences.2−9 One hot topic recently on POM chemistry is the exploration of high-nuclearity POM clusters for their unique characteristics in nanodimensions. During the past 2 decades, a number of highnuclearity POMs have been fabricated,4−6 in which the lacunary POM anions are commonly employed as inorganic “superligands” to combine cationic 3d or 4f metal linkers. Among the high-nuclearity POM family, the construction of 3d−4f heterometallic species is of great interest considering the inherent contributions of both 3d and 4f electrons as well as the exchange interactions between these metal centers. However, reports on giant 3d−4f-containing POMs are relatively rare, especially for the intriguing cases with more than 100 metal centers. To our knowledge, such a known case is restricted to a nanocluster of {Dy30Co8Ge12W108O408(OH)42 (H2O)30}.6a The main challenge within this field is the © XXXX American Chemical Society

Received: July 25, 2019

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DOI: 10.1021/acs.inorgchem.9b02236 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

atom of one CIT from dimer-1 (Figure S7; Co1−O, 2.041− 2.126 Å; Co1−Ow, 2.062−2.126 Å). It should be noted that both dimer-1 and dimer-2 contain two rare in situ generated tetravacant Dawson-type selenotungstate(IV) fragments {Se2W14O52} that can be derived from the removal of four W units in the belts of the known prototypical POM {W18O56(SeO3)2(H2O)2} (Figure 2a).10 Interestingly, two tetravacant {Se2W14O52} fragments

and pH 4.2 for 2 h. The multidentate ligand H4CIT is chosen for its acidity to control the reaction pH and its rich O-atom donors, which may allow coordination with several d- and fblock metals, perhaps leading to the formation of giant 3d−4fcontaining POMs. The structure, elemental contents, and phase purity of 1 have been characterized and confirmed by single-crystal X-ray diffraction, elemental analysis, powder Xray diffraction, thermogravimetry, and Fourier transform infrared spectroscopy (Figures S1−S3). The solid luminescent properties of 1 were systematically studied and manifest the remarkable fluorescence feature of Eu3+ cations (Figure S4). The diffuse-reflectance UV−vis spectrum of the powder sample was recorded to gain the band-gap value (2.25 eV) of 1 (Figure S5). Besides, magnetic measurement for 1 was carried out in an applied 1000 Oe direct-current field with the temperature ranging from 300 to 1.8 K and did not exhibit single-molecule magnet behavior (Figure S6). As shown in Figure 1, the intriguing polyoxoanion 1 crystallizes in the triclinic space group P1̅ and represents a

Figure 2. (a) Structures of tetravacant selenotungstate {Se2W14O52}, Eu2W2-substituted {Eu2W2Se2W14O56}, and CIT. (b) Structures of dimer-1 and dimer-2. (c−f) Different coordination behaviors of CIT ligands in 1.

are further linked by the novel inorganic−organic hybrid HCIT-bridged octanuclearity Eu−W-based heterometal−organic clusters {Eu4W4O8(CIT)3(OH)(H2O)6} (EuW-1) or {Eu4W4O8(CIT)2(HCIT)(NO3)(OH)(H2O)5} (EuW-2), furnishing the assembly of dimer-1 and dimer-2 (Figure 2b,c). One fascinating structural feature of 1 is the existence of 12 HCIT ligands and their diverse coordination manners. As can be seen from Figure 2b−f, there are six crystallographically independent HCIT molecules in dimer-1 (CIT1, CIT2, and CIT3) and dimer-2 (CIT4, CIT5, and CIT6), engaging in bonding with the Eu, Co, and W atoms via abundant O sites to produce heterometallic clusters (EuW-1 and EuW-2), which also play a significant role in the formation and stabilization of the giant framework. Overall, all of these three CIT4− ligands (CIT-1/2/3) in the EuW-1 cluster can be viewed as pentadentate ligands, of which two (CIT-1/2) act as pillar linkers between the two {Eu2W2(μ3-O)2O2} cores and respectively bond to three Eu3+ ions and one W6+ ion (Figure 2c). The remainder one (CIT-3) coordinates to one Eu3+ ion and one W6+ ion from the same {Eu2W2(μ3-O)2O2} core and one Co2+ ion from the [Co(H2O)4]2+ bridge between dimer-1 and dimer-2, leaving one carboxyl group free to serve as a “hook” contributing to the capture of a decorating [Co(NO3)-

Figure 1. View of the ultralong giant molecular polyoxoanion 1.

hierarchical, multicomponent, centrosymmetric inorganic− organic hybrid POM octamer made up of two identical POM tetramers of {Eu8Co3Se8W64O224(CIT)6(NO3)2(OH)2(H2O)26} bridged by the coordination of an additional [Co3(H2O)4]2+ complex to two carboxyl O atoms of two CIT ligands from the POM tetramers (Figure S7; Co3−O, 1.935 Å; Co3−Ow, 2.059−2.138 Å). Further, each POM tetramer is composed of two sandwich-type 3d−4f-containing POM dimers {[Eu4W4O8(CIT)3(OH)(H2O)6][Co(NO3)(H2O)4](Se2W14O52)2} (dimer-1) and {[Eu4W4O8(CIT)3(NO3)(OH)(H2O)5][Co(H2O)5](Se2W14O52)2} (dimer-2) oriented at ca. 38° with respect to each other and also linked via an additional [Co(H2O)4]2+ complex but bonding to one terminal WO O atom from dimer-2 and one carboxyl O B

DOI: 10.1021/acs.inorgchem.9b02236 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry (H2O)4]+ complex (Figure 2d). It is worth noting that the EuW-1 cluster features rare W atoms coordinated with the carboxylic ligand in POM structures. Although EuW-1 and EuW-2 have similar compositions and geometric arrangements of metals and ligands, the CIT ligands in EuW-2 display different coordination behaviors. Unlike the pentadentate CIT4− ligands in EuW-1, the HCIT3− ligand (CIT-6) and CIT4− ligands (CIT-4/5) in EuW-2 respectively serve as tri-, penta-, and hexadentate ligands (Figure 2c,e,f) via bonding to two metals (one Eu atom and one W atom), four metals (three Eu atoms and one W atom), and five metals (three Eu atoms, one W atom, and one Co atom). Additionally, except for CIT ligands, some additional small molecules, such as H2O and NO3−, are also involved in EuW-1 and EuW-2 as terminal ligands to meet the coordination requirements of metal ions. As a result, the Eu3+ ions (Figure S8) in 1 form bicapped (eight-coordinate) or tricapped (ninecoordinate) trigonal prisms with Eu−O/(H2O) bond lengths in the range of 2.275−2.925 Å, while all of the Co2+ and W6+ ions form octahedra with Co−O and W−O bond lengths of 1.931−2.238 and 1.672−2.467 Å, respectively. Bond-valence-sum calculations reveal that the bond valences for all Eu, Co, and W atoms are 3+, 2+, and 6+,11 respectively. Interestingly, as outlined above, polyoxoanion 1 is a dimer of the dimeric {Eu 8 Co 3 Se 8 W 64 O 224 (CIT) 6 (NO 3 ) 2 (OH) 2 (H2O)26} (dimer-1 plus dimer-2), and dimer-1 or dimer-2 is a dimer of Dawson-type POM motifs {Se2W14O52}. This hierarchical arrangement renders 1 an unprecedentedly long molecular POM with the maximum length of ca. 6.1 nm. Notably, the most common molecular POMs are spherical, followed by circular. In contrast, molecular POMs with elongated structures are rarely reported. A prominent example is the Gd-bridged bar-shaped POM {Gd8As12W124O432(H2O)22} with overall dimensions of about 4.8 nm,6e which was the longest molecular polyoxotungstate reported. Here, polyoxoanion 1 shows an even longer molecular polyoxotungstate. Besides, with 128 W, 16 Eu, 16 Se, and 7 Co atoms in the structure, polyoxoanion 1 is larger than other molecular 3d−4fcontaining polyoxotungstates. What is more, most known giant discrete POMs are pure inorganic species. Molecular polyoxoanion 1 contains 12 CIT ligands with different coordination and connection modes, contributing to the isolation of rare inorganic−organic hybrid giant POM with more than 100 W atoms. In summary, we have presented a combination “one-pot” and “the auxiliary ligand protection” synthetic strategy to synthesize a multi-CIT-mediated, longest discrete POM 1. So far, the POM octamer is also the largest inorganic−organic hybrid 3d-4f-incorporated POM comprising 128 W, 16 Eu, 16 Se, and 7 Co atoms, 12 organic ligands CIT, and hundreds of O atoms. The introduction of organic ligand CIT with diverse coordination modes plays a key role in the formation and stabilization of the exceptuallly long and large POM 1. These results indicate that the combination of aliphatic poly(carboxylic acid)s, mixed 3d−4f metal ions, and lacunary POM motifs is a feasible strategy for the construction of giant and rare inorganic−organic hybrid multicomponent POMs.

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Experimental details, crystallographic data for 1, additional structural figures, and additional characterizations (PDF) Accession Codes

CCDC 1895870 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Yong Ding: 0000-0002-5329-8088 Shou-Tian Zheng: 0000-0002-3365-9747 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21773096 and 21572084), Fundamental Research Funds for the Central Universities (Grant lzujbky2018-k08), and the Natural Science Foundation of Gansu (Grant 17JR5RA186).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b02236. C

DOI: 10.1021/acs.inorgchem.9b02236 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b02236 Inorg. Chem. XXXX, XXX, XXX−XXX