Asymmetrically anchoring silver alkynyl cluster to the cobalt-containing polyoxometalate
Qing Li , Fangyu Fu , Mengyun Zhao , Yeqin Feng , Manzhou Chi , Zichen Zhao , Hongjin Lv , Guo-Yu Yang
The anion-template approach has been considered as one of the most effective strategies for the synthesis of atomically-precise metal clusters with aesthetic structures and fascinating properties [1-4]. Among various anionic templates, polyoxometalates (POMs), an anionic inorganic clusters of early transition metals in their highest oxidation states [5-16], exhibited rich bridging and terminal oxygen atoms, making them optimal large-sized anionic template in the syntheses of many intriguing silver (Ag) clusters [17-23]. These inorganic anionic POM templates can not only induce the aggregation of Ag cations through electrostatic interaction, but also adjust the physicochemical properties of Ag clusters [24-35]. Nevertheless, most of these reported POM-templated Ag (POM@Ag) clusters exhibited closely-arranged core-shell structures, where the anionic POM templates were symmetrically covered with organic ligands (e.g., alkynyl or thiolate) protected Ag shells.
In contrast, the atomically-precise Ag clusters with asymmetrical coverage of Ag shell on the surface of POM templates have been rarely reported, it is expected that such asymmetrical architecture would exhibit some intriguing properties due to the partially exposed POM surface and asymmetrically fused Ag shell [36]. To date, only four examples of asymmetrically covered POM@Ag clusters have been isolated. For instance, Jansen et al. reported the first asymmetrically covered POM@Ag cluster, [(CO3)@Ag42@(CoW12O40)2], containing a pair of Keggin-type {CoW12O40} anions located above and below the silver toroid [37]. The Ozeki and co-workers have constructed two asymmetrically covered POM@Ag clusters using asymmetrical {SiW9Nb3O40} and {P2W15Nb3O62} templates, where the Ag atoms were preferentially covered on the Nb-containing region with higher negative charge density [38,39]. Afterwards, Sun and co-workers have reported the construction of an asymmetrically covered POM@Ag cluster, [(P5W30O110)@Ag43], using a high-symmetry Preyssler {P5W30O110} as anion template. The resulting [(P5W30O110)@Ag43] revealed interesting dual functionalization of the POM template and silver cluster [40]. Based on above reports, it is easily concluded that the construction of asymmetrically covered POM@Ag cluster can greatly enrich the structural family of Ag cluster chemistry. However, these reported works have mainly focused on the design of new structures, it would be highly attracting to explore the physicochemical and catalytic properties of such specific type of asymmetrically covered POM@Ag clusters. In the other scenario, we expected that the introduction of different metal ions into the asymmetrically covered POM@Ag clusters could further generate novel materials with desirable properties.
In this context, we reported herein the successful solvothermal construction of a structurally new Ag cluster, H[Co(SiW11O39)Co4(OH)3(NO3)2(SiW9O34)@Ag37(tBuC≡C)23(NO3)2(DMF)3] (Ag37Co5), by asymmetrically anchoring Ag alkynyl motif to the cobalt-containing POM template. The resulting Ag37Co5 cluster has been fully characterized using various spectroscopic techniques. Detailed analyses revealed that the asymmetrical coverage of Ag cluster is attributed to the unevenly distributed negative charge density over the in situ generated [Co(SiW11O39)]6− and [Co4(OH)3(NO3)2(SiW9O34)]7− moieties. In addition, the resulting solid-state Ag37Co5 crystals exhibited interesting temperature-dependent photoluminescence property, efficient photothermal conversion behaviour with good recyclability, and good catalytic activity towards the detoxication of 4-nitrophenol.
The detailed synthetic procedure of Ag37Co5 cluster was described in Supporting information. Briefly, the mixture of [tBuC≡CAg]n [28] and TBA4H6[SiW9O34] [41] was stirred in 5 mL CH3CN for 3 h, to which Co(NO3)2 and 1 mL DMF were then added. After solvothermal reaction at 70 ℃ for 48 h, pink block crystals of Ag37Co5 were crystallized from the supernatant in the dark (Scheme S1 in Supporting information). It is noted that the Ag37Co5 crystals can be isolated in a two-component solvent system (CH3CN:DMF = 5:1), but not so in pure CH3CN solvent, indicating the essential role of DMF in the formation of Ag37Co5. Indeed, three DMF molecules as protecting ligands were found in the structure of Ag37Co5 (Scheme S1 and Fig. 1a).
Single-crystal X-ray diffraction (SCXRD) analysis revealed that the Ag37Co5 cluster crystallized in the triclinic P1 space group (Table S1 in Supporting information). The thermal ellipsoid diagram of Ag37Co5 cluster was supplemented in Fig. S1 (Supporting information). Compound Ag37Co5 consists of a penta-Co-containing anionic POM template ({Co(SiW11O39)Co4(OH)3(NO3)2(SiW9O34)}, Figs. 1a and b), a bowl-shaped Ag37 shell (Fig. 1f) co-protected by 23 tBuC≡C−, 2 NO3−, and 3 DMF (Fig. 1c) with a size of 22.3 Å × 20.9 Å × 18.5 Å (Fig. S2 in Supporting information). The Fourier transform infrared (FT-IR) spectrum of compound Ag37Co5 revealed the characteristic vibrational modes of confirmed the presence of alkynyl [24], NO3− [42], and the POM template in the range of 4000−400 cm−1 (Fig. S3 in Supporting information). Also, the X-ray photoelectron spectroscopy (XPS) survey spectrum confirmed the presence of Ag, Co, W, and O in the Ag37Co5 cluster (Fig. S4a in Supporting information). The corresponding oxidation states were further revealed by the high-resolution binding energy spectra of each element (Figs. S4b–d in Supporting information), confirming that the oxidation states of Ag, Co, and W elements are +1, +6, and +2, respectively, which are also consistent with the bond valence sum (BVS) calculations of W and Co atoms (Table S2 in Supporting information). Structural interpretation also implied that the anionic {Co(SiW11O39)Co4(OH)3(NO3)2(SiW9O34)} template is composed of a mono-Co-containing [Co(SiW11O39)]6− (Fig. 1d) and a tetra-Co-containing [Co4(OH)3(NO3)2(SiW9O34)]7− (Fig. 1e) moieties connected via three μ–O atoms of W–O–Co linkages with Co–O bond distances ranging from 1.985 Å to 2.316 Å (Table S3 in Supporting information). The monoprotonation state of three O atoms in the {Co4(OH)3} motif is confirmed by their BVS values of 1.104, 1.073, and 1.225 (Table S4 in Supporting information). It is noted that the mono-lacunary [SiW11O39]8− species was in situ generated from the transformation of tri-lacunary [SiW9O34]10− ingredient during solvothermal reaction. Considering the more negative charge density of [Co4(OH)3(NO3)2(SiW9O34)]7− moiety compared to that of [Co(SiW11O39)]6− moiety, or in other words, more negative charge density of [SiW9O34]10− versus [SiW11O39]8−, the Ag ions would be preferentially covered onto the surface of [Co4(OH)3(NO3)2(SiW9O34)]7− moiety, resulting in the formation of asymmetrically anchored {Ag37(tBuC≡C)23(NO3)2(DMF)3} motif (Fig. 1c). Similar to these reported asymmetrically-covered Ag clusters [36,40], the end-capping effect of tBuC≡C− ligands could also prohibit the overall coverage of Ag ions on the [Co(SiW11O39)]6− moiety, which might contribute to the formation of discrete Ag37Co5 cluster in the crystal packing structure (Fig. S5 in Supporting information).
The structure of bowl-shaped Ag37 shell was further analyzed in detail, which consisted of seven Ag triangles, one quadrilateral, and several irregular Ag rings (Fig. S6a in Supporting information). Among them, two pairs of triangles shared the Ag20 and Ag9 atoms, respectively; and the other three triangles were connected by sharing edges. In addition, two hexatomic Ag rings (blue), two discrete irregular heptatomic Ag rings (red), and a heart-shaped octoatomic Ag ring (orange) were found in the bowl-shaped Ag37 shell (Figs. S6b and d in Supporting information). The Ag···Ag interaction distance ranges from 2.740 Å to 3.440 Å (Table S3). In such asymmetrically-covered Ag37Co5 cluster, 15 Ag atoms of the Ag37 shell coordinated to the [Co4(OH)3(NO3)2(SiW9O34)]7− moiety with the Ag–OW bond distances of 2.270–2.720 Å (Fig. S7 and Table S3 in Supporting information). It is worth mentioning that such bowl-shaped Ag37 structure is structurally unique, which is distinct from the Cl−-templated Ag37 thiolate cluster recently reported by Sun's group [43]. In Ag cluster chemistry, surface protecting ligands could significantly contribute to the structural stability and diversity as well as the physicochemical property of the resulting metal clusters [44]. The coordination mode analyses of surface ligands in Ag37Co5 cluster revealed that the tBuC≡C− ligands were regularly arranged in adjacent positions on the surface of the bowl-shaped Ag37 shell (Figs. S8a and b in Supporting information). These 23 tBuC≡C− ligands exhibited two different kinds of coordination modes, 18 of them adopted μ3-ησ1:ƞσ2:ƞπ1 bridging modes (Fig. S8c in Supporting information), and other 5 ligands adopted μ3-ησ2:ƞσ1:ƞσ2 ligation modes (Fig. S8d in Supporting information) with Ag–C bond distances of 1.94−2.84 Å (Table S3). This bowl-shaped Ag37 shell was further anchored by four inorganic NO3− ligands (Fig. S8e in Supporting information), adopting four different coordination types: one μ1 and one μ2-κ1:κ1 NO3− ligands linked to the Ag atoms through Ag–ON bond (2.4761–2.6993 Å, Table S3), and unprecedentedly, one μ3-κ1:κ1:κ1 and one μ4-κ1:κ1:κ1:κ1 NO3− ligands co-coordinated to both Ag atoms of the Ag37 coat (Ag–ON distances: 2.3946–2.6993 Å, Table S3) and three Co atoms of the Co4(OH)3 motif (Co–ON distances: 2.0334–2.1011 Å, Table S3). The Ag37 shell was additionally stabilized by three neutral μ1 DMF molecules with Ag–OC bond distances of 2.4021–2.5511 Å (Fig. S8f and Table S3 in Supporting information).
The UV–vis spectroscopy was used to study the optical properties of Ag37Co5 cluster. The UV–vis absorption spectrum of Ag37Co5 in DMSO showed a shoulder peak at around 420 nm and a prominent low-energy band at 565 nm (Fig. S9 in Supporting information), which could be attributed to the tBuC≡C− ligand-based π → π* transition and the ligand-to-metal charge transfer (LMCT) [45].The photoluminescence property of Ag37Co5 cluster has also been investigated. It is found that neither solid-stated nor solution-stated Ag37Co5 cluster was emissive at room temperature upon irradiation of a hand-held UV lamp (λex = 365 nm), but the crystals of Ag37Co5 exhibits a bright orange luminescence at cryogenic temperature (Fig. 2a). Therefore, the emission spectra of solid-state Ag37Co5 crystals were measured in a temperature range from 293 K to113 K with an interval of 30 K (Fig. 2a). Upon excitation at 355 nm, the photoluminescence spectrum of Ag37Co5 displayed a maximum emission peak position of 630 nm at 293 K, which might originate from excited states involving argentophilic Ag centers with mixed character of ligand-to-metal charge transfer [45]. Gradually decreasing the temperature to 113 K can result in a slight red-shift of the maximum emission peak to 636 nm and the significant enhancement of photoluminescence intensity by 91 times. The red-shift of the emission peak could be probably attributed to the reduction of the distances between Ag···Ag, leading to a reduced energy gap between the ground and excited states at low temperatures as reported in the literature [46]. Moreover, plotting emission intensity versus temperature exhibits a perfect single exponential behaviour in the range of 113−293 K with the formula Imax = 460,806 × exp(−T/32.60) + 86.32 (Fig. 2b), revealing its potential functionality as a molecular luminescent thermometer.
It is well-documented that the emission quenching of potentially photothermal materials can result in the enhancement of the light-to-heat energy conversion efficiency [47-49], we were encouraged to further investigate the photothermal performance of Ag37Co5 considering its weak photoluminescent intensity at room temperature. Upon green laser irradiation (λ = 532 nm), the temperature of Ag37Co5 sample increased rapidly within 10 s at various laser power densities of 0.1–1.0 W/cm2, and the enhancement of temperature was positively correlated to that of power densities (Fig. 3a). For instance, at the power densities of 0.8 W/cm2, the temperature of the Ag37Co5 crystals can quickly increase ~125 ℃ with a heating rate of 8.73 ℃/s (Fig. S10 in Supporting information). Compared to those previously reported POM-templated silver clusters, Ag37Co5 exhibits decent photothermal conversion performance (Table S5 in Supporting information). In addition, Ag37Co5 exhibited excellent photothermal recycling stability for at least twenty heating-cooling recycles under 0.8 W/cm2 laser irradiation (Fig. 3b). The solid-state UV–vis diffuse reflectance and FT-IR spectra remained largely unchanged before and after twenty photothermal recycles (Figs. S11 and S12 in Supporting information), confirming the good stability and photothermal recyclability of Ag37Co5.
Then, considering the detrimental effects of nitrophenols on the environment and human health [50-52], we have further evaluated the catalytic performance of Ag37Co5 cluster by using the reduction of harmful 4-nitrophenol (4-NP) to less toxic 4-aminophenol (4-AP) as a model reaction (Fig. 4a). The catalytic reaction process was monitored by time-dependent UV–vis absorption spectra. After addition of Ag37Co5 catalyst to the mixture of 4-NP and NaBH4 (see details in Supporting information), the absorption of 4-NP in the UV–vis spectra at 400 nm quickly disappeared in 15 min along with the increase of the absorption band at 300 nm showing the formation of 4-AP (Fig. 4b). Under our specific catalytic conditions, an excess amount of NaBH4 was used for 4-NP reduction. Kinetic analysis revealed that such catalytic reaction proceeded a pseudo-first order with the kinetic rate constant following the equation of kt = −ln(C/C0). The apparent rate constant k was determined as 0.232 min−1 (Fig. S13 in Supporting information). To prove the catalytic role of Ag37Co5 catalyst, several control experiments by replacing the Ag37Co5 catalyst with molar equivalents of SiW9, SiW11, CoSiW11, Co4(PW9)2 and CoSiW11+ tBuC≡CAg have been conducted. It is found that all these comparative catalysts exhibited negligible or very low catalytic activities towards the reduction of 4-NP (Fig. S14 in Supporting information). In addition, it was found that the catalytic reaction process was dependent on the concentrations of 4-NP and Ag37Co5 catalyst. Typically, the higher concentration of Ag37Co5 catalyst, the better catalytic performance (Fig. S15 in Supporting information). These results confirmed the significant role of the unique Ag37Co5 structure in driving efficient catalysis. Previous studies showed that both reactants (BH4− and 4-NP) could be simultaneously adsorbed on the active site of the cluster catalyst prior to the catalytic reaction by following the Langmuir–Hinshelwood (LH) kinetic model [53,54]. In our case, we proposed that the high catalytic activity of Ag37Co5 catalyst could be also attributed to the exposed Ag sites on the cluster surface (Fig. S16 in Supporting information), providing accessible reactive sites for 4-NP and active hydrogen adsorption [55,56].
In summary, we have successfully constructed a new asymmetrically covered POM-templated Ag cluster, H[Co(SiW11O39)Co4(OH)3(NO3)2(SiW9O34)@Ag37(tBuC≡C)23(NO3)2(DMF)3] (Ag37Co5) by adopting a facile solvothermal approach. Various characterizations revealed that uneven negative charge distribution on the in situ generated [Co(SiW11O39)]6− and [Co4(OH)3(NO3)2(SiW9O34)]7− moieties accounted for the asymmetrical coverage of alkynyl-protected Ag shell the asymmetric package of Ag cluster. At cryogenic temperatures, the solid-state Ag37Co5 crystals exhibited orange light emission, and such photoluminescence behaviour was temperature-dependent. Upon green laser irradiation (λ = 532 nm, 0.8 W/cm2), the Ag37Co5 crystals displayed efficient photothermal conversion efficiency with the temperature reaching as high as ~125 ℃ at a heating rate of 8.73 ℃/s, which also exhibited robust photothermal stability for at least twenty heating-cooling recycles. Moreover, while using the reduction of harmful 4-NP to less toxic 4-AP as a model reaction, the Ag37Co5 cluster showed efficient catalytic activity with near-unity (>99.5%) conversion of 4-NP to 4-AP at decent apparent rate constant of 0.232 min−1. This work not only enriches the structural diversity of asymmetrically covered POM@Ag clusters, but also facilitates the exploration of their potential applications in emission, photothermal conversion, and catalysis, etc.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Qing Li: Writing – original draft, Investigation, Formal analysis, Data curation. Fangyu Fu: Writing – original draft, Supervision, Investigation. Mengyun Zhao: Visualization, Software. Yeqin Feng: Validation, Software. Manzhou Chi: Visualization, Software. Zichen Zhao: Visualization. Hongjin Lv: Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization. Guo-Yu Yang: Supervision, Project administration, Funding acquisition.
This work is financially supported by the National Natural Science Foundation of China (Nos. 21871025, 21831001), the Recruitment Program of Global Experts (Young Talents), and Beijing Institute of Technology (BIT) Excellent Young Scholars Research Fund. The instrumental support from the Analysis and Testing Center of Beijing Institute of Technology is also highly appreciated.
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a) The X-ray structure of the Ag37Co5 cluster. The fragment structures of (b) {Co(SiW11O39)Co4(OH)3(NO3)2(SiW9O34)}, (c) {Ag37(tBuC≡C)23(NO3)2(DMF)3}, (d) [Co(SiW11O39)]6−, (e) [Co4(OH)3(NO3)2(SiW9O34)]7−, and (f) bowl-shaped Ag37 shell. Color codes: green, Ag; red, O; light blue polyhedron, WO6; orange, Si; pink, Co; gray, C; dark blue ball, N; H atoms are omitted for clarity.
Figure 2 (a) Solid-state emission (λex = 355 nm) spectra of Ag37Co5 acquired at different temperatures. (b) Plot of emission intensity of Ag37Co5 versus temperature.
Figure 3 (a) The photothermal performance of Ag37Co5 sample under 532 nm laser irradiation at various laser power intensities. (b) The temperature variation curves of Ag37Co5 sample for 20 successive photothermal cycles upon 532 nm laser irradiation (0.8 W/cm2).
Figure 4 Catalytic performance of Ag37Co5 towards (a) a model reaction of 4-NP to 4-AP. (b) Time-dependent UV−vis absorption spectra of 4-NP reduction by NaBH4 in the presence of different Ag37Co5 catalyst. Conditions: 1.7 mg Ag37Co5 catalyst, 4-NP (6 mmol/L), 35 mg NaBH4, and 2.5 mL of H2O.
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