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DOI: 10.1039/D2QI01073A (Research Article) Inorg. Chem. Front., 2022, 9, 4350-4358

Synergic coordination of multicomponents for the formation of a {Ni30} cluster substituted polyoxometalate and its in situ assembly†

Sa-Sa Wang a, Xiang-Yu Kong ab, Weiming Wu a, Xiao-Yuan Wu a, Sheng Cai a and Can-Zhong Lu *ac
aCAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. E-mail: czlu@fjirsm.ac.cn
bSchool of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China
cUniversity of Chinese Academy of Science, Beijing 100049, China

Received 18th May 2022 , Accepted 3rd July 2022

First published on 7th July 2022

Abstract

Developing a new strategy for the synthesis of high-nuclear transition metal (TM) cluster substituted polyoxometalates (POMs) provides an avenue for new materials with unique functions. Herein, the synergetic coordination strategy of 1,2,4-1H-triazole (Htrz), ethylenediamine (en), and a lacunary POM was applied to construct a high-nuclear Ni cluster substituted POM. As a result, a {Ni30} cluster substituted POM, [Ni(trz)3]2@[Ni30(H2O)16]POM, representing the highest-nuclear Ni cluster containing POM at present, was discovered for the first time as the secondary building unit (SBU) of four in situ assembled frameworks, [Ni11(trz)12(en)4(H2O)22]H4{[Ni(trz)3(H2O)3]2[Ni30(μ3-OH)18 (H2O)16(en)6(PW9O34)6]}·4Htrz·222H2O (1), [Ni6(trz)6(μ3-OH)(en)(H2O)10]2H8{[Ni(trz)3 (H2O)3]2[Ni30+2(μ3-OH)18(O)4(H2O)16(en)6(PW9O34)6]}·4Htrz·94H2O (2), [Ni14(trz)12(en)6(H2O)36Cl2]{[Ni(trz)3(H2O)3]2[Ni30(μ3-OH)18(H2O)16(en)6(PW9O34)6]}·solvent (3), and [Ni3(trz)3(en)(H2O)8]2{[Ni(en)(H2O)2]H4[Ni6(μ3-OH)3(H2O)5(en)3(PW9O34)]}2{[Ni(trz)3 (H2O)3]2[Ni30(μ3-OH)18(H2O)16(en)6(PW9O34)6]}·4Htrz·174H2O (4), which served as stable heterogeneous catalysts in light driven hydrogen evolution. The {Ni30} cluster in the SBU held abundant water ligands. On one hand, these water ligands promoted the coordinating assembly of the [Ni(trz)3]2@[Ni30(H2O)16]POM by ligand replacement. On the other hand, they further coordinated with Ni2+ to expand the Ni cluster to a larger one. It is believed that the work will trigger tremendous interest in the fields of both synthesis chemistry of POM and hydrogen evolution catalysts.

Introduction

Developing new materials with unique properties is one of the hottest research topics in the world's science and technology frontier. The structural combination of two clusters leads to not only the combination of their properties, but also adjusting of their properties by each other, and consequently endows the mixed cluster with unique properties. Therefore, the structural combination of two clusters provides an avenue for developing new materials with unique properties. POMs are a class of discrete polyanion clusters with oxo-metal polyhedra of MOx (x = 5, 6) as the basic construction units. M generally represents early TMs in their high oxidation state including W, Mo, V, Nb, Ta, etc.1 Partial substitution of the polyanion by other oxo-metal clusters results in the production of mixed POM clusters, the properties of which are closely related to the nuclearity and structure of the substituting clusters. Driven by the aesthetically intriguing architectures and promising potential applications of high-nuclear TM cluster substituted POMs in the fields of high-density information storage, single-molecule magnets, nanotechnology, and catalysis, researchers strive for increasing the nuclearity of substituting TM clusters in mixed POM clusters,2 and substantial efforts have been devoted to the fundamental synthetic work of high-nuclear TM cluster substituted POMs in the past decades. To date, a series of representative TM cluster substituted POMs have been reported, such as {Mn14P4(PW9)4},3 {Mn19(SiW10)6},4 {Fe12(P2W15)4},5 {Fe15P4(SiW9)4},6 {Fe16P8W48},7 {Co16P4(PW9)4},8 {Co16P4(XW9)4} (X = Si/Ge/P/As),9 {Cu15(SiW9)4},10 {Ni6XW9} (X = P/Si),11 {Ni9P2(PW9)3},12 {Ni12W8(PW9)3},13 {Ni12C3P4(SiW9)3},14 {Ni13P4(SiW9)3},14etc. However, there are only a few examples with the nuclearity of substituting TM clusters higher than 20 so far, including {Fe28(P2W12)4},15 {Fe48(P2W12)8},16 {Cu20P8W48},17 {Ni25C2P6(SiW9)6},14 and {Zr24W4(GeW9)4(GeW8)2}.18 Actually, the synthesis of high-nuclear TM cluster substituted POMs is still a formidable challenge.

At present, there are two main synthesis strategies for high-nuclear TM cluster substituted POMs: (1) employing as-prepared large TM clusters stabilized by organic molecules as starting materials; (2) using lacunary POMs as structure directing agents. Sometimes, two strategies are used synchronously.5,16 Lacunary POMs are regarded as excellent pluridentate inorganic ligands. They can efficaciously gather TM cations into clusters at the lacunary sites. However, the limitations of using pure lacunary POM ligands to construct high-nuclear TM cluster substituted POMs are destined by their confined lacunary space, big size and unusual rigidity. Therefore, the syntheses of most previously reported high-nuclear TM cluster substituted POMs are assisted by small inorganic ligands such as PO43−,6,8,9,14,19 CO32−,14,20 VO43−,19a AsO43−,21 or small organic molecules including AcO−,20a,b ethylenediamine (en)/1,2-diaminopropane (dap),11etc. However, similar geometries of the above inorganic ligands and sparse coordination sites of the organic ligands make the synthesis of high-nuclear TM cluster substituted POMs progress sluggishly. Keeping this knowledge in mind, we deduce that it is crucial to develop new auxiliary ligands to synergistically coordinate with lacunary POM for the production of high-nuclear TM cluster substituted POMs. Those with dense multiple coordination sites and flexible coordination modes probably act as prospective candidates to break the record of the nuclearity of substituting TM clusters in POMs. Azoles meet these criteria well. Furthermore, azoles take the advantage of similar size to the aforementioned inorganic ligands. For instance, the maximum distance between two non-hydrogen atoms of azole is about 2.2 Å. The value matches well with the size of CO32− (the distance between two O atoms is ca. 2.2 Å).22 The synergic coordination of lacunary POMs and azoles is expected to provide more opportunities for the formation of high-nuclear TM cluster substituted POMs. However, relevant studies have not yet been reported.

In this work, the synergic coordination strategy of Htrz, en, and trilacunary phosphotungstate Na9[A-α-PW9O34] (Na9PW9) was employed to construct high-nuclear Ni cluster substituted POMs. As a result, an unprecedented windmill-shaped {Ni30} cluster substituted POM, [Ni(trz)3]2@[Ni30(H2O)16]POM, was triumphantly achieved as the secondary building unit (SBU) of four frameworks ranging from one-dimension to three-dimensions, 1–4 (Fig. 1). To the best of our knowledge, the {Ni30} cluster represents the highest nuclearity of the Ni cluster in the POM at present. The coordinating assembly of the [Ni(trz)3]2@[Ni30(H2O)16]POM into 1–4 was realized via the in situ formation and tailoring of organometallic fragments or decorated POMs. The catalytic performances of these compounds in the light driven hydrogen evolution reaction (HER) were carried out, except compound 3 which was restricted by the extremely low yield. The results indicated that they efficaciously promoted the reaction as heterogeneous catalysts under the irradiation of visible light (≥420 nm).


image file: d2qi01073a-f1.tif
Fig. 1 Crystallographic structure of four frameworks and illustration of their assembly from the SBU [Ni(trz)3]2@[Ni30(H2O)16]POM (color code: orange, Ni; green, W; purple, P; red, O; blue, N; black, C. H atoms have been omitted for clarity).

Results and discussion

Formation of the [Ni(trz)3]2@[Ni30(H2O)16]POM

Frameworks 1–4 were obtained under hydrothermal conditions from a one-pot reaction of NiCl2·6H2O with Htrz, en, and Na9PW9. Their crystallographic data are presented in Table S1.† The TG curves of 1, 2, and 4 suggested that their decomposition temperatures were around 400 °C, indicating their good thermal stabilities (Fig. S1†). The sharp weight losses before 200 °C were ascribed to the removal of water molecules in the structures. Frameworks 1–4 share a SBU, [Ni(trz)3]2@[Ni30(H2O)16]POM, of ∼3 nm diameter, which displays an aesthetic rotiform structure (Fig. 2a). The [Ni(trz)3]2@[Ni30(H2O)16]POM can be viewed as a hexamer of {Ni5}-capped PW9 fragments (Ni5POM) alternatingly pointing up and down (Fig. 2b). A floriform moiety consisting of two triangular {Ni(trz)3} complexes arranging in a face-to-face manner with 60° rotation is located in the centre of the [Ni(trz)3]2@[Ni30(H2O)16]POM (Fig. 2c). It is reasonable that the gathering of Ni5POM into the [Ni(trz)3]2@[Ni30(H2O)16]POM has been driven by the in situ forming [Ni(trz)3]2 moiety. The large {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM assembling from six planar {Ni5} clusters represents the largest Ni cluster in the POM at present (Fig. 2d). It can be speculated that the Ni5POM is a metastable species in the present reaction and its stability is significantly inferior to previously reported Ni6POMs containing a triangular {Ni6} cluster (Fig. 3).23 As far as we know, discrete Ni5POM species have not been isolated. It is apparent that the shape of the subcluster {Ni5} is identical to that of the {Ni6} cluster in Ni6POMs except for the loss of one vertex, which makes it possible to assemble the subunit into a larger Ni cluster.24 In fact, the production of 1–4 in our system was fiercely competed by Ni6POM-1 and Ni6POM-2, suggesting that the [Ni(trz)3]2@[Ni30(H2O)16]POM and Ni6POMs might share the same intermediate Ni5POM. Namely, the triangular {Ni6} cluster may also evolve from the metastable {Ni5} cluster. In the absence of in situ forming [Ni(trz)3]2, {Ni5} prefers to assemble into {Ni6}, rather than {Ni30}. The appearance of [Ni(trz)3]2 breaks the original assembling balance. The speculation was also confirmed by the formation of a hexamer of Ni6POM (denoted as [Ni6POM]6 for convenient description) reported by Kortz, which showed a similar profile to the [Ni(trz)3]2@[Ni30(H2O)16]POM but left the centre of the framework hollow (Fig. S2a†).25 It is worth emphasizing that the six {Ni6} clusters in [Ni6POM]6 are discrete despite the use of a dinuclear coordination complex [Ni2(μ-OH2)(O2CCMe3)4(HO2CCMe3)4] as the nickel source. In [Ni6POM]6, neighbouring Ni6POMs connect with each other via four-fold Ni–O[double bond, length as m-dash]W and two-fold Ni–O–W linkers, attesting to the high activity of the O atoms on the surface of the POM (Fig. S2b†). However, the neighbouring Ni5POM fragments in the [Ni(trz)3]2@[Ni30(H2O)16]POM are connected together through Ni–O[double bond, length as m-dash]W and Ni–O–Ni linkers (Fig. S3†). Additionally, two Ni2+ ions respectively belonging to two neighbouring Ni5POMs are further bound by a trz of the [Ni(trz)3] complex, improving the interaction between the neighbouring fragments. The fact further suggests the pivotal role of the [Ni(trz)3]2 moiety in the formation of the [Ni(trz)3]2@[Ni30(H2O)16]POM.
image file: d2qi01073a-f2.tif
Fig. 2 (a) Structure of the SBU [Ni(trz)3]2@[Ni30(H2O)16]POM; (b) Ni5POM fragment; (c) [Ni(trz)3]2 template in the center of the {Ni30} cluster; (d) consecutive rotiform {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM.

image file: d2qi01073a-f3.tif
Fig. 3 Structural illustration of (a) Ni6POM-1; (b) Ni6POM-2; (c) {Ni6} cluster in Ni6POM-1; (d) {Ni6} cluster in Ni6POM-2 (color code: black C; bule N; red O; green O of coordinating water; orange Ni).

The {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM presents a six-segment windmill shape, the cusps of which are coordinated by six en molecules (Fig. 2d). From the overall point of view, the {Ni30} cluster is sandwiched by six PW9 fragments from two sides. We hypothesize that the formation of the {Ni5} cluster is cooperatively facilitated by PW9 and en, and the assembly of them into a larger {Ni30} cluster should be attributed to the template of the triangular [Ni(trz)3] complexes. In brief, the synergic coordination of trz, en, and PW9 results in the formation of the {Ni30} cluster. It is believed that the formation of the triangular [Ni(trz)3] in the centre of the [Ni(trz)3]2@[Ni30(H2O)16]POM is not accidental. Its size well matches that of the pore entrapped by six Ni5POM fragments. In order to further confirm the role of en and trz in the formation of the {Ni30} cluster, the reactions for 1–4 were carried out in the absence of en or Htrz. As was expected, no {Ni30} or {Ni5} cluster containing compounds were found in these reactions. Therefore, both en and Htrz are indispensable for the formation of the {Ni30} cluster. Furthermore, a close inspection of the {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM reveals that each of them comprises 18 ternary quasi-cubane {Ni3O4} units counting the shared atoms. Such {Ni3O4} or {Ni4O4} subunits are well-known in POM chemistry and closely related to their properties.19a,26

In situ coordinating assembly of the [Ni(trz)3]2@[Ni30(H2O)16]POM

The structural combination of POMs and metal–organic frameworks (MOFs) not only integrates their advantages into the resulting framework, but probably endows the framework with extra unique properties such as high stability. The further assembly of the giant [Ni(trz)3]2@[Ni30(H2O)16]POM into frameworks 1–4 is achieved by in situ tailoring of metal–organic bridges or other decorated POMs, rather than expensive large organic ligands, via the coordination of {Ni30} clusters with trz molecules in bridges and/or the coordination of terminal oxygen atoms on the surface of the [Ni(trz)3]2@[Ni30(H2O)16]POM with Ni2+ ions in bridges. As mentioned before, the formation of the [Ni(trz)3]2@[Ni30(H2O)16]POM was fiercely competed by discrete Ni6POM-1 and Ni6POM-2, which hold 6 and 8 water ligands by the {Ni6} cluster, respectively (Fig. 3c and d).23 But the yields of 1–4 can be improved by meticulously optimizing the reaction conditions including the dosage of en, the amount of water and the reaction temperature.27 The structural information of 1–4 reveals that the {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM holds 16 residual coordinated water molecules (Fig. 4). According to the fact that several frameworks with Ni6POM units as the SBU have been achieved through the replacement of the water ligand by organics,28 the formation of 1–4 is rationalized by undergoing the same procedure. Therefore, the {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM may initially hold 18 ternary water ligands before assembling into frameworks 1–4, which equally distribute on six {Ni5} subclusters and can also be replaced by other ligands (Fig. S4†). But only two of them are replaced by bridges to form 1–4. It is noteworthy that the distribution and orientation of 16 residual water ligands on the {Ni30} cluster are distinguishing for each framework, implying that they may derive from different primitive patterns of the {Ni30} cluster (Fig. S4†) and multifold replacement modes possibly occur.
image file: d2qi01073a-f4.tif
Fig. 4 The substitution mode of water ligands in (a) 1, (b) 2, (c) 3, and (d) 4. (color code: black, C; blue, N; red, O; green, O of coordinating water; orange, Ni; yellow, Ni coordinating with a bridge).

It is interesting that the [Ni(trz)3]2@[Ni30(H2O)16]POM was found as the SBU of 1–4, while Ni6POM-1 and Ni6POM-2 were detected as discrete structures in our reaction system. The Ni6POM-based framework was absent throughout the work, suggesting that the [Ni(trz)3]2@[Ni30(H2O)18]POM is more active than Ni6POMs. The assembling behaviour by ligand replacement commenced immediately once the [Ni(trz)3]2@[Ni30(H2O)18]POM formed. Actually, we have tried to synthesize the discrete [Ni(trz)3]2@[Ni30(H2O)18]POM by optimizing the reaction temperature and input ratio of the starting materials. But the results were disappointing. Besides, decreasing the coordination capacity of Htrz by introducing a sterically hindered substituent group to prevent the [Ni(trz)3]2@[Ni30(H2O)18]POM from assembling into frameworks was also carried out. Unfortunately, we failed again when 3-methyl-1H-1,2,4-triazole and 3-amino-1H-1,2,4-triazole were used instead of Htrz. These results can be rationalized by the high activity of the [Ni(trz)3]2@[Ni30(H2O)18]POM originating from numerous active terminal O atoms on its surface and multiple commutable water ligands on the {Ni30} cluster. It is believed that more frameworks with the [Ni(trz)3]2@[Ni30(H2O)18]POM as the SBU can be expected in the future.

Framework 1 was initially obtained at 160 °C. It crystallized in the triclinic P[1 with combining macron] space group and was found as a one-dimensional chain structure. In 1, the [Ni(trz)3]2@[Ni30(H2O)16]POM is bridged by in situ formation of a metal–organic fragment [Ni11(trz)12], which contributes two Ni2+ ions (yellow in Fig. S5,† left) and two N atoms (dark purple, Fig. S5,† left) from two trz anions to coordinate with terminal O atoms of the giant SBU and Ni2+ ions from the {Ni30} cluster, respectively. As mentioned before, 16 commutable water ligands remain on the {Ni30} cluster of 1 (Fig. 4a). For four {Ni5} subclusters, the orientations of three water ligands on each of them are identical, while for the other two {Ni5} subclusters on which one Ni2+ coordinating with bridge, two residual water ligands on each of them are nearly perpendicular to each other. Therefore, framework 1 was regarded to derive from the primitive [Ni(trz)3]2@[Ni30(H2O)18]POM with the water distribution of pattern 1 (Fig. S4a†). It was achieved through the replacement of two coordinated water at opposite sides of the {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM (Fig. S5,† right). The assembly behaviour profited from both unoccupied coordination sites rooting from the dissociation of water ligands on the {Ni30} cluster and the highly active terminal O atoms on the surface of the [Ni(trz)3]2@[Ni30(H2O)16]POM.

The dosages of the starting materials for 1 and 2 are quite resemblant. When the reaction temperature was elevated to 180 °C, framework 2 became the principal product, accompanied by a small amount of 1. Framework 2 crystallized in the monoclinic C2/c space group. The in situ forming [Ni6(trz)6] fragments join [Ni(trz)3]2@[Ni30(H2O)16]POMs into an undulating two-dimensional layer through the coordination of Ni2+ and trz with terminal O atoms and the {Ni30} cluster of the [Ni(trz)3]2@[Ni30(H2O)16]POM, respectively (Fig. 1). Each [Ni6(trz)6] connects with three [Ni(trz)3]2@[Ni30(H2O)16]POMs and each [Ni(trz)3]2@[Ni30(H2O)16]POM is bound by six [Ni6(trz)6] bridges that can be divided into three groups (labelled as A, B and C) according to different coordination modes (Fig. S6†). Bridge A accepts three terminal O atoms on the SBU surface by four Ni2+ ions, one of which develops into a μ3-O, to form Ni–O–W linkers. Bridge B accepts one terminal O atom on the SBU surface by one Ni2+ and donates one N atom from trz to coordinate with the {Ni30} cluster. Bridge C connects the SBU by the coordination of one Ni2+ ion with one terminal O atom of the SBU. In a word, each [Ni6(trz)6] contributes five coordinating sites including one N sites and four Ni sites (Fig. S7†). In framework 2, two Ni2+ ions of the {Ni30} cluster and ten terminal O atoms on the surface of the [Ni(trz)3]2@[Ni30(H2O)16]POM participate in the coordination with bridges. The formation of a two-dimensional layer also profited from both the commutable water ligands of the {Ni30} cluster and the highly active terminal O atoms of the SBU. It is proposed that the orientations of three water ligands on each {Ni5} subcluster are identical in the primitive {Ni30} cluster of 2, corresponding to pattern 2 of the {Ni30} cluster in the [Ni(trz)3]2@[Ni30(H2O)16]POM (Fig. S4b†).

Excitingly, two extra Ni2+ ions adhere to the {Ni30} cluster in 2via coordinating with two water ligands with an identical orientation to form an expanded {Ni30+2} cluster (Fig. 4b), suggesting that a larger Ni cluster can be expected in the present system in the light of numerous water ligands of the {Ni30} cluster. In brief, the water ligands on the {Ni30} cluster can contribute to the synthesis chemistry of POMs via two avenues: forming frameworks by ligand replacement and expanding the Ni cluster in the POM to a larger one, the two most principal branches of POM synthesis chemistry.

Framework 3 was discovered accidentally as the by-product of 1 and only a few crystals were obtained despite the elaborate efforts made to improve the yield. Framework 3 crystallized in the monoclinic P21/n space group and showed a compact three-dimensional network that was achieved by a longer [Ni14(trz)12] bridge, which contributed to two N coordinating sites (dark purple, Fig. S8†) and six Ni2+ coordinating sites (yellow, Fig. S8†). One [Ni14(trz)12] bridge is bound by six [Ni(trz)3]2@[Ni30(H2O)16]POMs and one [Ni(trz)3]2@[Ni30(H2O)16]POM is also bound by six bridges (Fig. S9†). Framework 3 shares the same primitive form of the [Ni(trz)3]2@[Ni30(H2O)16]POM with 1 (Fig. S4a†), but the replacing sites are apparently different. Two residual coordinating water ligands on the {Ni5} cluster, on which ligand replacement occurs, point to the same orientation (Fig. 4c). Different primitive forms of the [Ni(trz)3]2@[Ni30(H2O)16]POM in 1 and 2 and different replacing sites on the {Ni30} clusters between 1 and 3 indicate that more replacing mode of the water ligands on the [Ni(trz)3]2@[Ni30(H2O)18]POM can be expected, suggesting that the [Ni(trz)3]2@[Ni30(H2O)18]POM will be a productive SBU. The structural information of frameworks 1–3 makes it clear that the dimensions of the frameworks are positively correlated with the numbers of coordinating sites provided by [Nix(trz)y] bridges (Table S2†). Thus, multiple coordination sites of in situ forming [Nix(trz)y] bridges also play a vital role in the successful realization of high-dimensional [Ni(trz)3]2@[Ni30(H2O)16]POM-based frameworks. Compared to pure large organic ligands, the in situ tailoring of [Nix(trz)y] bridges take advantage of apparently low-cost and synchronously holding multiple donor and acceptor coordination sites, providing much more opportunities for assembling such large SBUs into high-dimensional frameworks.

Distinguishing from the case in 1–3, [Ni(trz)3]2@[Ni30(H2O)16]POMs in chain-like 4 are bridged by {Ni6}-capped PW9 fragments (Ni6POM-3) in pair rather than the [Nix(trz)y] complex (Fig. 1). Ni6POM-3 is cognate with Ni6POM-1 and Ni6POM-2. Six Ni2+ ions of the {Ni6} cluster are also arranged in a triangular shape (Fig. S10a†). An accessional Ni2+ ion hangs on the surface of Ni6POM-3via W[double bond, length as m-dash]O–Ni bonds (yellow, Fig. S10a†). The Ni2+ ion at one vertex of the triangle and the one hanging on the surface of Ni6POM-3 are coordinated by terminal O atoms belonging to two [Ni(trz)3]2@[Ni30(H2O)16]POM units, respectively, to form an infinite chain. The coexistence of two substituted POMs in a structure makes framework 4 quite structurally unique. It also conveys a message that the forming conditions of the [Ni(trz)3]2@[Ni30(H2O)16]POM are very similar to that of Ni6POM-3. The result is in line with the isolation of Ni6POM-1 and Ni6POM-2 during the preparation of frameworks 1–4. In framework 4, although the {Ni30} cluster does not participate in the formation of the chain, the replacement of water ligands also occurs. Two [Ni3(trz)3] fragments decorate the SBU in 4via ligand replacement (Fig. S10b†). The replacing mode of the {Ni30} cluster in 4 is the same as that in 1 (Fig. 4d).

Catalytic performance

The present energy crisis and environmental pollution drive researchers to explore the performance of TM cluster substituted POMs in HERs.6,29 In particular, Ni cluster substituted POMs have exhibited promising catalytic performance in light driven HERs.19a,21,30 The UV-VIS-NIR spectra of 1, 2, and 4 were recorded before the evaluation of their photocatalytic properties (Fig. 5). The results revealed that they showed strong broad absorption at 200–518 nm, while their precursor Na9PW9 only absorbed the light in the ultraviolet region (λ < 350 nm), suggesting the occurrence of ligand-to-metal charge transfer between PW9 and the {Ni30} cluster and confirming the successful regulation of POM's band gap. The enlarged absorption region of 1, 2, and 4 will be beneficial for their photocatalytic properties.
image file: d2qi01073a-f5.tif
Fig. 5 Comparison of the UV-VIS-NIR spectra of 1, 2, and 4 with their precursor Na9PW9.

Frameworks 1, 2, and 4 are insoluble in water and most conventional organic solvents, indicating that they may serve as heterogeneous catalysts in light-driven HERs. HERs with 1, 2, and 4 were investigated in acetonitrile with [Ir(ppy)2(dtbbpy)]PF6 ([Ir]) as the photosensitizer, triethanolamine (TEOA) as the proton donor and 1-benzyl-1,4-dihydronicotinamide (BNAH) as the sacrificial electron donor under the irradiation of visible light (λ ≥ 420 nm). As shown in Fig. 6, when the reaction with 1 was carried out in the dark, no H2 was detected after 5 h. However, when the reaction was exposed to visible light, H2 was rapidly generated and the amount reached up to 36.5 μmol in 2 h and the corresponding TON was 56.2, being indicative of the necessity of light. The values for 2 and 4 were 24.4 μmol (TON = 34.1) and 31.1 μmol (TON = 56.9), respectively. These values are comparable to that of previously reported results for H2 evolution (Table S3†). The better catalytic performances of 1 and 4 than that of 2 can be rationalized by the fully exposed active {Ni30} clusters in the 1D chain structures of 1 and 4. In the absence of a catalyst, the reaction produced a negligible H2 amount under the irradiation of visible light, further confirming the catalytic function of these frameworks. Equivalent Na9PW9, NiCl2·6H2O and their mixture exhibited extremely low activity under identical conditions. The H2 amounts for them were less than 3 μmol after 5 h. The results highlighted the benefit of accumulating Ni2+ ions at the lacunary sites for photocatalytic properties. The performance of Ni6POM-1 in the HER was also investigated. Only 4.9 μmol H2 was produced in 2 h under identical conditions (TON = 5). The results revealed that it was more inert than 1, 2, and 4.


image file: d2qi01073a-f6.tif
Fig. 6 (a) Time courses of H2 evolution over different catalysts; recyclability of (b) 1, (c) 2, and (d) 4.

The time courses of H2 evolution over 1, 2, and 4 showed that the H2 amount increased rapidly in the initial 1.5 h, and then plateaus were achieved, suggesting that no H2 was further produced. In order to confirm the retention of catalytic activities of these frameworks, they were recovered by filtration from the reaction mixture after 2 h. After being washed thoroughly with acetonitrile, water and ethanol in succession and then dried naturally, the recovered samples were used in the next reaction. After 5 reaction cycles, no activity loss occurred for these frameworks. Instead, an obvious increasing trend in their activities was presented. The phenomenon was rationalized by the gradual activation of the frameworks. The PXRD patterns of the recovered 1, 2, and 4 after 5 reaction cycles were highly consistent with that of the pristine samples (Fig. S11†), being indicative of the maintenance of their structural integrities, which should be responsible for the retention of catalytic activity.31 This point was further confirmed by the comparison of the FT-IR spectra of pristine and recovered 1, 2, and 4 (Fig. S12†). Therefore, it was deduced that the plateaus with 1, 2, and 4 resulted from the deactivation of the photosensitizer, rather than the catalysts. To confirm the supposition, an equivalent photosensitizer [Ir] to that for the initial reaction with 2 was replenished after 2 h. As was expected, the H2 evolution capacity of the system was totally recovered (Fig. S13†).

Conclusions

In summary, a {Ni30} cluster substituted POM, [Ni(trz)3]2@[Ni30(H2O)16]POM, with abundant water ligands on the {Ni30} cluster, representing the highest-nuclearity of the Ni cluster in the POM at present, was achieved by synergic coordination of trz, en, and PW9. The water ligands and highly active surface O atoms of the [Ni(trz)3]2@[Ni30(H2O)16]POM promoted its assembly into frameworks of 1–4. The water ligands on the {Ni30} cluster contribute to the synthesis chemistry of POMs via two avenues: forming frameworks by ligand replacement and further coordinating with Ni2+ to expand the Ni cluster to a larger one, the two most principal branches of synthesis chemistry of POMs. Frameworks 1, 2, and 4 exhibited remarkable activity, excellent stability, and prominent recyclability as heterogeneous catalysts for visible light-driven HERs. It is believed that the work will contribute to the further synthesis of high-nuclear TM cluster substituted POMs, the assembly of giant POMs into extended frameworks, and the development of POM-based hydrogen evolution catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21773247, 21521061, 21875252) and the Natural Science Foundation of Fujian Province (2006L2005).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Tables S1–S3, Fig. S1–S13, and crystallographic data. CCDC 2093863–2093865 and 2106591. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01073a
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