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DOI: 10.1039/D5CC04198H (Communication) Chem. Commun., 2025, Advance Article

A tetrameric polyoxometalate with two orderly and dense double-layered {Ni12W8/3} cluster centers

Chen Lianab and Guo-Yu Yang*a
aMOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, China. E-mail: ygy@bit.edu.cn
bCollege of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang, 464000, China

Received 24th July 2025 , Accepted 14th August 2025

First published on 15th August 2025

Abstract

A germanotungstate aggregate, [H2N(CH3)2]8Na2H18[{Ni12W8/3O14–(OH)3(H2O)3}{BO(OH)2}{B2O3(OH)2}(B-α-GeW9O34)2]2·14H2O (1), with the highest nuclearity Ni cluster core among known poly(polyoxometalate) tetramers has been developed, which contains two unprecedented orderly and dense double-layered central Ni–W–O cluster cores that are further decorated by different oxo-B groups. Moreover, 1 is an efficient heterogeneous catalyst for accelerating the Knoevenagel reactions of various aldehydes with malononitrile.

Polyoxometalates (POMs), a class of nanosized anionic metal–oxygen clusters of V, Nb, Ta, Mo and W in their high oxidation states, have remarkable structural features and promising applications in the domains of medicine, catalysis, materials science, photochemistry and molecular magnetism.1–6 As an essential branch, transition-metal added POMs (TMAPs) are being researched with increasing intensity owing to their unique physical and chemical properties and potential for putative and realized applications in many fields.7–9 So far, utilizing lacunary POM fragments to induce TM ion aggregation has been proven to be a feasible synthesis strategy for constructing new TMAPs.10–13 And after long-term unremitting efforts, a library of TMAPs has been established and constitutes a huge subclass of POMs,14–21 including many exciting high-nuclearity TM–O cluster added poly(POM)s, such as {Mn20(L6)6(PW9)8},15 {Fe14P6(PW9)2},16 {Co14P2(P2W15)4},17 {Co16P4(PW9)4},18 {Ni14P4(P2W15)4},19 {Ni25(CO3)2P6(SiW9)6},20 {Cu9As6(AsW9)2},21 etc.

In most of the above-mentioned POMs, the high-nuclearity aggregation of TM–O clusters occurred under aqueous solution conditions and required the support or assistance of organic/inorganic ligands. On the basis of the lacunary-directed synthetic strategy,10 our group has developed some isolated and extended organic–inorganic hybrid high-nuclearity TMAPs by a hydrothermal method.10,22–24 With the continuous advancement of the research subject, now, the synthesis of novel TMAPs with fascinating structural diversity and interesting physicochemical properties in the presence of inorganic boron sources has gradually become the main direction of our future study. As for inorganic oxo-B clusters, diverse and versatile assemblies are originating from the fact that triangular BO3 and tetrahedral BO4 can be interlinked by corner/edge sharing.25–27 Various oxo-B cluster units with different shapes and sizes can not only function similar to CO3 and PO4 groups, satisfying the coordination requirements of metal centers to enhance the structural stability, but also possess the potential to exert intra/intermolecular connectivity as “elastic” ligands by self-polymerization into changeable configurations during the synthetic process, thereby capturing the in situ generated TM–O clusters via hydroxyl condensation or coordinated water substitution reactions to construct multifunctional POM oligomers or aggregates.28–30

However, POMs containing inorganic oxo-B units have rarely been reported and the research is still in its infancy. Only three series of heterometallic Ni–Ln based structures with 22 and 30 boron atoms and several cases of small oxo-B group decorated Ni-added poly(POM)s have been reported to date, in which oxo-B groups with different sizes condense with the hydroxyl groups or substitute the coordination water molecules of metal ions to form unique metal–nonmetal central cluster cores.31–37 These results demonstrated the feasibility of developing novel POMs in the presence of inorganic boron sources and encouraged us to further explore and develop this field.

As a continuation of our recent study, herein, we successfully obtained an unprecedented germanotungstate tetramer, [H2N(CH3)2]8Na2H18[{Ni12W8/3O14(OH)3(H2O)3}{BO(OH)2}{B2O3(OH)2}(B-α-GeW9O34)2]2·14H2O (1). The isomerization and partial re-organization of lacunary A-α-GeW9O34 precursors led to the entrapment of an orderly and dense double-layered central Ni–W–O cluster core that is decorated by a BO(OH)2 ({B}) group and a binuclear B2O3(OH)2 ({B2}) cluster. 1 not only constitutes the poly(POM) with the densest central Ni–W–O cluster core, but also represents the tetramer with the highest number of Ni atoms by far.

Green crystals of 1 were obtained by the hydro(solvo)thermal reaction of K8Na2[A-α-GeW9O34]·25H2O, NiCl2·6H2O, H3BO3 and Li2CO3 at 170 °C for 3 days. Single-crystal X-ray diffraction shows that 1 crystallizes in the triclinic space group P[1 with combining macron]. Its molecular structure contains a unique 24-Ni-added polyoxoanion, [{Ni12W8/3O14(OH)3(H2O)3}{BO(OH)2}{B2O3(OH)2}(B-α-GeW9O34)2]228− (1a, Fig. 1a, b and Fig. S1, SI), assembled from two dimeric subunits [{Ni12W8/3O14(OH)3(H2O)3}{BO(OH)2}{B2O3(OH)2}(B-α-GeW9O34)2]14− (1b, Fig. 1c) linked by 12 Ni–O–W bonds. Each 1b features a unique three-component B–Ni–W–O cluster center of {Ni12W8/3O14–(H2O)}{BO(OH)2}{B2O3(OH)2} ({Ni12W8/3BB2}) ligated by two trilacunary B-α-GeW9O34 ({GeW9}) fragments via the Ni–O–W and Ni–O–Ge linkages, leading to a previously unobserved sandwich configuration (Fig. 1d and e). The {GeW9} fragments are generated from the isomerization of the A-α-GeW9O34 precursor (Fig. 1e and f), and the included angle between two {GeW9} units is 68.22°, creating a space suitable for accommodating the {Ni12W8/3BB2} cluster core (Fig. S2, SI). 1b differs from the reported {(Ni8BB2)@(GeW9)2} segment where two {GeW9} units form a slightly larger angle of 70.63° and enclose a {Ni8BB2} cluster core.35


image file: d5cc04198h-f1.tif
Fig. 1 (a) and (b) The side and top views of polyoxoanion 1a. (c) View of subunit 1b. (d) View of the central {Ni12W8/3BB2} cluster core. (e) View of the [B-α-GeW9O34]10− fragment. (f) View of the [A-α-GeW9O34]10− fragment. The color codes for polyhedra and atoms: WO6, red; GeO4, light purple; NiO6, green.

As shown in Fig. 1d and Fig. S3a–d, SI, the central {Ni12W8/3BB2} is formed by one {B} and one binuclear {B2} unit bonded to a double layered Ni–W–O cluster, Ni12W8/3O14(OH)3(H2O)3 ({Ni12W8/3}), by 1 μ3-O and 2 μ4-O atoms, respectively. In the {Ni12W8/3} cluster, all of the Ni and W atoms adopt octahedral coordination modes. The number of W atoms is not an integer with an occupancy factor of 2/3 for W21, and the Ni1–Ni4 atoms occupy only 1/2 of the sites based on the crystallographic refinement rationality of their atomic thermal displacement parameters. Since there is no additional tungsten source, the WO6 octahedra in the {Ni12W8/3} cluster come from the degradation of the precursor. All hydroxyl groups of the B atoms (O92, 0.976; O93, 1.087; O94, 1.087; O95, 0.950) are confirmed by bond valence sum (BVS) calculations, and there are three terminal water molecules (O84, O89 and O96) on Ni with BVS values of 0.280, 0.355 and 0.462, respectively. It is worth noting that the {Ni12W8/3} cluster has a thickness of ca. 4.7 Å, in which the metal atoms show an orderly and dense double-layered distribution and can be divided into upper and lower moieties with different numbers of metal centers and shapes by the central horizontal plane of the bridging O atoms 4 μ6-O (50, 65, 69, 72), 2 μ4-O (26, 71) and 6 μ3-O (27, 73, 51, 61, 20, 33, 86, 79) (Fig. 2a–d). The upper Ni11/2W2O27 moiety is made of 6 NiO6 and 2 WO6 octahedra [Ni–O 1.985(16)–2.309(14); W–O 1.744(14)-2.333(13) Å]. The distribution of 5 fully-occupied Ni(6, 8, 9, 11, 14) centers and 2 fully-occupied W (19, 20) centers in the form of 2[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]2 leads to a hexagonal motif, {Ni5W2}, which is somewhat similar to the classical Anderson structure. In the hexagonal {Ni5W2}, a {B2} and a {B} group are, respectively, adhered to the three edge-sharing Ni atoms in the middle position and two edge-sharing Ni (6,14) on one side by hydroxyl condensation, and an additional Ni4 atom with a 1/2 occupancy factor connects to WO6 octahedra on the other side by sharing two edges (Fig. 2b). The other layered Ni13/2W2/3O30 cluster contains 5 fully-occupied Ni(5, 7, 10, 12, 13) atoms, 3 half-occupied Ni(1, 2, 3) atoms as well as a W21 atom with a 2/3 occupancy factor, showing a regular quadrangular arrangement [Ni–O/W–O 1.940(2)–2.360(16)/1.853(14)–2.210(13) Å] (Fig. 2c). Each side of the quadrangle is composed of 3 NiO6 groups and the side lengths range from 6.018 to 6.112 Å. The W21 sites are located at the midpoint of the quadrilateral by sharing the edges with surrounding NiO6 groups. The parallel interweaving of Ni11/2W2O27 and Ni13/2W2/3O30 layers by edge-sharing to form the integral {Ni12W8/3} cluster core, such as a Ni–W–O cluster, has not been reported so far. The presence of rich μ6-O, μ4-O and μ3-O bridges makes it more dense than the TM–O cluster cores in the previously reported POMs.


image file: d5cc04198h-f2.tif
Fig. 2 (a) Side view of the {Ni12W8/3} cluster. (b) and (c) Top view of the layered Ni11/2W2O27/Ni13/2W2/3O30 cluster. (d) View of the bridging O atoms located in the same plane and linking the Ni and W atoms from two adjacent Ni–W–O layers in the {Ni12W8/3} cluster.

From another point of view, the {Ni12W8/3} cluster core is assembled in an approximate way of putting together building units, where the edge-sharing {W8/3} cluster is coincidentally stuck in the gap of the {Ni12} cluster (Fig. S3e and f, SI). Additionally, it is worth mentioning that seven cubane M–O clusters (2 Ni4O4, 1 Ni3W2/3O4, 2 Ni3/2W5/3O4, 1 Ni3W8/3O4, and 1 Ni2O4) can be observed in the {Ni12W8/3} cluster, and they are gathered together in the edge-, corner-, and face-sharing fashions (Fig. S4, SI).

Knoevenagel condensation is a fundamental and useful synthetic method for the formation of C[double bond, length as m-dash]C bonds from active methylene-containing compounds and aldehydes. The products prepared via this reaction are widely used in therapeutic agents, fine chemicals, polymers with different groups, etc.38–43 The catalytic activity of 1 towards the Knoevenagel condensation was investigated in methanol solvent at ambient temperature. The initial experiment was carried out using benzaldehyde and malononitrile as model substrates at a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]2. The reaction proceeded efficiently and a complete conversion was achieved after 0.5 h with no trace amount of benzaldehyde being detected by GC, which was significantly superior to the blank experiment without catalyst 1 (Fig. S6 and S7, SI). Subsequently, various substrates including aliphatic aldehydes and benzaldehyde derivatives with different substituting groups were used to estimate the applicability of 1. It could be observed from the results in Table 1 that the catalytic reactions of most selected substrates proceeded smoothly with malononitrile to give the corresponding products in high yields. To be more specific, as for aliphatic aldehydes, a good catalytic effect could be achieved after extending the reaction time (entries 1 and 2). With regard to benzaldehyde derivatives, electron-withdrawing substituted benzaldehydes were obviously more reactive than those with electron-donating groups in the condensation reaction (entries 4–11).

Table 1 Knoevenagel condensation of various aldehydes with malononitrile
Entry Substrate Time (h) Product Yield (%)
Reaction conditions: substrate (1.0 mmol), catalyst 1 (1.0 μmol), malononitrile (2.0 mmol), 3 mL methanol, ambient temperature. Yields were detected by GC analysis using reference standards.
1 image file: d5cc04198h-u1.tif 0.5 image file: d5cc04198h-u2.tif 70
1.0 84
1.5 91
2 image file: d5cc04198h-u3.tif 0.5 image file: d5cc04198h-u4.tif 71
1.0 86
1.5 94
3 image file: d5cc04198h-u5.tif 0.5 image file: d5cc04198h-u6.tif 100
4 image file: d5cc04198h-u7.tif 0.5 image file: d5cc04198h-u8.tif 100
5 image file: d5cc04198h-u9.tif 0.5 image file: d5cc04198h-u10.tif 100
6 image file: d5cc04198h-u11.tif 0.5 image file: d5cc04198h-u12.tif 100
7 image file: d5cc04198h-u13.tif 0.5 image file: d5cc04198h-u14.tif 100
8 image file: d5cc04198h-u15.tif 0.5 image file: d5cc04198h-u16.tif 100
9 image file: d5cc04198h-u17.tif 0.5 image file: d5cc04198h-u18.tif 68
1.0 87
1.5 100
10 image file: d5cc04198h-u19.tif 0.5 image file: d5cc04198h-u20.tif 60
1.0 71
1.5 79
2.0 84
11 image file: d5cc04198h-u21.tif 0.5 image file: d5cc04198h-u22.tif 54
1.0 66
1.5 77
12 image file: d5cc04198h-u23.tif 0.5 image file: d5cc04198h-u24.tif 91
1.0 98

For the catalyst stability, the Knoevenagel condensation of benzaldehyde was chosen as the model reaction to perform the reusability experiments of 1 with five successive cycles. The results showed that the benzylidenemalononitrile yield was maintained at a high level, manifesting that 1 could be recovered and reused without an obvious reduction in activity (Fig. 3). Moreover, the IR spectra of 1 before and after five runs are shown in Fig. S8, SI, which showed good consistency, further indicating the structural stability of the compound.


image file: d5cc04198h-f3.tif
Fig. 3 Recycling experiments of 1 for the Knoevenagel condensation of benzaldehyde with malononitrile.

In conclusion, a novel poly(POM) tetramer has been prepared by the combination of a lacunary POM fragment, Ni2+ ions and inorganic oxo-B groups under hydro(solvo)thermal conditions, in which four {GeW9} fragments anchor two orderly and dense double-layered Ni–W–O cluster cores decorated by small oxo-B groups, and such a configuration is first discovered in the field of POM chemistry. Furthermore, 1 possesses good catalytic activity towards the Knoevenagel condensation reaction. This work provides an avenue for the construction of more novel high-nuclearity TMAPs with the participation of inorganic oxo-B clusters.

This work was supported by the NSFC (no. 21831001, 21571016 and 91122028) and the NSFC for Distinguished Young scholars (no. 20725101).

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors declare that the data supporting this work are available within the paper and its SI. Supplementary information available: Synthesis, characterization data, additional structures and catalytic pictures. See DOI: https://doi.org/10.1039/d5cc04198h

CCDC 2175069 contains the supplementary crystallographic data for this paper.44

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