Zuschriften Cluster Compounds Very Important Paper
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Deutsche Ausgabe: DOI: 10.1002/ange.201804569 Internationale Ausgabe: DOI: 10.1002/anie.201804569
Atomically Precise Multimetallic Semiconductive Nanoclusters with Optical Limiting Effects Shuai Chen, Wei-Hui Fang, Lei Zhang,* and Jian Zhang* Abstract: For the first time, multinuclear noble-metal clusters have been successfully stabilized by Ti-oxo clusters. Two unprecedented Ag6@Ti16-oxo nanoclusters with precise atomic structures were prepared and characterized. The octahedral Ag6 core has strong Ag@Ag bonds (ca. 2.7 c), and is further stabilized by direct Ag@O@Ti coordination interactions. Moreover, as a result of different acidic/redox conditions in synthesis, the Ag6 core can adopt diverse geometric configurations inside the Ti16-O shell. Correspondingly, structural differences greatly influence their optical limiting effects. The transmittance reduction activity of the clusters towards 532 nm laser shows a nearly linear concentration dependence, and can be optimized up to about 43 %. This work not only opens a new direction for multimetallic semiconductive nanoclusters with interesting optical properties, but also provides molecular models for important noble-metal/TiO2 heterogeneous materials.
insight into the interface chemistry between metal particles and oxide supports, including metal–support bonding and catalytic mechanism.[35] Unfortunately, although some heterometallic titanium/noble-metal complexes have been prepared, all of them are small organometallic molecules with single metal sites.[36] The introduction of noble-metals to titanium-oxo clusters with direct Mx-TimOn interactions, which can act the real molecular models of heterogeneous metal–support systems, has never been achieved. Thus direct chemical understanding of the strong metal–support interactions remains quite challenging. Herein, we obtain the first two hybrid nanoclusters of noble-metal and titanium oxide, whose structures are fully characterized by X-ray diffraction analysis. Through encapsulating an Ag6 octahedron with two macrocyclic Ti8O40 clusters by direct Ag@O@Ti bonding (Scheme 1), two atomi-
In the past decades, there has been a rapid expansion of
research into nanostructural materials, with the most representative families as metallic and semiconductive nanoparticles.[1–5] They have found numerous applications both in academia and industry, in fields such as photonics, electronics, biomedicine, catalysis, energy and environment.[6–9] However, chemists have been greatly frustrated by the imprecise characteristics of these nanoparticles, such as polydispersity, blurry interfaces, and elusive surface compositions.[10–12] Therefore, intensive research has been devoted to their structural and reactive model molecular clusters, which can provide atomically precise structures to study the structure– property relationships.[13–18] To date, a great number of noblemetal and semiconductive nanoclusters with atomic precision were reported.[19–31] Despite such progress, these two cluster families are still quite distinct from each other. On the other hand, oxides of early transition metals (for example, TiO2) have been widely used as supports of latetransition-metal nanoparticles (for example, Au, Ag, Pt) with important photo or catalytic applications.[32–34] Therefore, early–late heterometallic complexes can be used as molecular models of such heterogeneous materials. They will provide [*] S. Chen, Dr. W.-H. Fang, Prof. Dr. L. Zhang, Prof. Dr. J. Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences 350002 Fuzhou (P. R. China) E-mail:
[email protected] [email protected] Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201804569.
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Scheme 1. Illustration of the assembly of the multimetallic semiconductive core–shell nanostructure.
cally precise nanoclusters with multimetallic cores and semiconductive shells were constructed and respectively formulated as [Ag(CH3CN)]2[Ag6Ti16(m3-O)18(m2-O)4(benzoate)26(CH3CN)2] (PTC-47) and Ag6Ti16(m3-O)16(m2-O)4(benzoate)24(CH3COO)4(CH3CN)2 (PTC-48). Relatively strong Ag@Ag bonds have been detected in these two complexes as indicated by short Ag@Ag distances of approximately 2.7 c. To our knowledge, PTC-47 and PTC-48 are the first two noble-metal-doped Ti-oxo clusters. More interestingly, although presenting similar cluster cores, the spatial configurations of Ag6 in PTC-47 and PTC-48 are different, which further results in distinct optical limiting effects. Through the solvothermal reaction of Ag(CH3COO)2 and Ti(OiPr)4 with benzoic acid in the presence of acetic acid in acetonitrile, colorless crystals of PTC-47 were obtained after cooling to room temperature. As revealed by single-crystal X-ray diffraction analysis, PTC-47 is mainly characterized by an Ag6Ti16 cluster core stabilized by 26 benzoate ligands (Figure 1 a). All Ti4+ ions in PTC-47 are coordinated by six
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Zuschriften
Figure 1. Crystal structures of a) PTC-47 and b) PTC-48.[43] c) Illustration of the triple Ag6@Ti16@(benzoate)26 core–shell structures of PTC47 (left) and PTC-48 (right), highlighting the capture of acetonitrile molecule through the window of Ag6@Ti16-oxo cluster. C, H, O, and Ti atoms are presented with a space-filing model. Green Ti; violet Ag; red O; blue N; gray C.
oxygen atoms with Ti@O bonds of 1.792–2.137 c. Actually, the Ti16-oxo sub-cluster can be seemed as the connection of two cricoid Ti8O40 units. There are two kinds of links between the up and down Ti8O40 units, with six Ti@O@Ti connectivities and two carboxylate groups of two benzoate ligands. The hexanuclear silver cluster in PTC-47 contains two crystallographically independent Ag atoms including four Ag1 and
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two Ag2. As shown in Figure 2 a, Ag1@Ag1 distances are 2.6901(18) and 2.7346(18) c, while the Ag1@Ag2 distances are 2.7669(12) and 2.7755(19) c, respectively. All of these values are clearly shorter than the Ag@Ag distance (ca. 2.889 c) of bulk Ag metal and the sum of the van der Waals radii of silver atoms (3.44 c), indicating the existence of strong Ag@Ag bonding and closed-shell interactions. Meanwhile, the Ag@Ag distances in PTC-47 are comparable to those of reported [Ag6]4+ units (2.70–2.86 c), but shorter than those of [Ag6]6+ (2.81–3.28 c).[37, 38] Therefore, the silver cluster in PTC-47 should also be [Ag6]4+ with delocalized electrons. The presence of subvalent silver atoms can further be confirmed by its similar X-ray photoelectron spectrum as the former [Ag8]6+ and [Ag22]12+ containing silver clusters reported by Zheng et al. (Figure S18 in the Supporting Information).[39] Then the Ag6Ti16 unit in PTC-47 displays a charge of @2 which is precisely balanced by another two Ag+ ions attached through Ag@O bonds of 2.291 c. Based on the above structural analysis, the construction of PTC-47 can be simplified as encapsulating an Ag6 core in the cavity of a Ti16-oxo cluster (Figures 2 b,c). Therein, each Ag1 atom is bonded to two O atoms with Ag1@O distances of 2.394 and 2.440 c, while each Ag2 is bonded to four O atoms with Ag2@O distances of 2.457 and 2.577 c. To our knowledge, this is for the first time that a noble-metal cluster has been entirely surrounded by a metal-oxo cluster. It is worth noting that even the Ti@O moiety is further stabilized by benzoate ligands. Therefore, the structure of PTC-47 is actually a triple core–shell construction of Ag6@Ti16@(benzoate)26. As shown in Figure 1 c, two pairs of Ag1 atoms are exposed to the windows from both sides, allowing the capture of two acetonitrile molecules. And the N atom of each acetonitrile is disorderly coordinated to two Ag1 atoms with Ag1@N1 distance of 2.480 c (Figure S5).
Figure 2. a) Structural details and coordination environments of the Ag6 core in PTC-47, highlighting the Ag@Ag and Ag@O distances [b]. b,c) Side and top-view of the encapsulation of the Ag6 core by the Ti16-oxo cluster in PTC-47. d) Structural details and coordination environments of the Ag6 core in PTC-48. e,f) Side and top-view of the encapsulation of Ag6 core by the Ti16-oxo cluster in PTC-48. C, H, and N atoms have been omitted for clarity. Green polyhedra TiO6 ; violet polyhedra Ag6. Angew. Chem. 2018, 130, 11422 –11426
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Zuschriften From a geometric point of view, the Ag6 cluster should be able to adopt different configurations inside the Ti16 cavity. Indeed, by introducing the more acidic and reductive formic acid into the reaction system instead of acetic acid, a new heterometallic Ag6@Ti16-oxo cluster of PTC-48 was obtained (Figure 1 b). Structural analysis indicates that the [Ag6]4+ core of PTC-48 is more symmetric than that of PTC-47, represented by two kinds of Ag@Ag bonds as four Ag1@Ag1 of 2.7408(19) c and eight Ag1@Ag2 of 2.7307(27) c (Figure 2 d). Actually, the configuration of [Ag6]4+ in PTC-48 can be seemed as rotating the {Ag6} octahedron in Figure 3. PTC-47 around an Ag@Ag axis for about 4588 (Figure S4). Thus, now only two Ag2 atoms are exposed to the windows of the cluster core, which are further coordinated by two acetonitrile molecules (Ag2@N1 2.101 c; Figures 1 c and Figure S5). It is interesting that only Ag1 is connected to O atoms with four Ag@O bonds of 2.439 and 2.562 c (Figure 2 d). The Ag2 atoms are only stabilized by Ag@Ag bonding and the terminal acetonitrile ligands. Accordingly, the linkages between the two cricoid Ti8O40 units in PTC-48 are now provided by four Ti@O@Ti and four acetates. Considering that benzoate stabilized Ti8-oxo rings have already been isolated,[40] it should be rational to assume the following synergistic assembly of PTC-47 and PTC-48. Firstly, eight Ti ions were bridged by benzoate ligands to form cricoid Ti8-O moieties. Then the unstable [Ag6]4+ species in solution were bonded to Ti8-O by Ag@O interactions, which also induced the aggregation of two Ti8-O rings to form the Ti16-O sub-cluster. And such encapsulation further stabilized the [Ag6]4+ species. Slight changes in the reaction environment could tune the Ag@O interactions to form different geometric configurations of Ag6, resulting in cluster cores of PTC-47 and PTC-48, respectively. Therefore, in addition to the previously intensively used thiolate, alkynyl and phosphine groups, the protecting ligands of noble-metal nanoclusters have now been extended to titanium-oxo moieties. It is believed that by further modulating the reaction conditions, especially redox aspects, larger silver cores will be encapsulated to give more heterometallic Ag/Ti-oxo clusters with fascinating structural attributes. The Ag6@Ti16-oxo nanoclusters in the single-crystals of PTC-47 and PTC-48 are packed together by supramolecular interactions like hydrogen bonding and Van der WaalsQ forces (Figure 3). And the packing fashions of these core–shell clusters are different from each other, with side-to-side in PTC-47 and face-to-face in PTC-48 along c-axis, respectively. PTC-47 and PTC-48 can be obtained in relative high purity (Figures S6, S7). Then the optical properties of these two samples were investigated. Solid-state absorption spectra of PTC-47 and PTC-48 both show strong peaks up to approximately 400 nm (Figure S12). Noble-metal-containing nanoparticles often show nonlinear absorption properties, attracting pronounced interest in these materials for potential nonlinear optical (NLO) applications such as optical limiting.[41] In contrast, such studies using atomically precise noble-metal nanoclusters still remain
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The packing view of PTC-47 (left) and PTC-48 (right) along c-axis.
rare.[42] To investigate the optical limiting characteristics of PTC-47 and PTC-48 and achieve some structure-property relationships, open aperture (OA) Z-scan measurements were carried out on their dichloromethane solutions (2 X 10@4 m). As indicated in Figure 4, both the recorded OA Z-scan curves of PTC-47 and PTC-48 exhibit obvious reverse saturated absorption behavior. At the focal point, the transmittance at 532 nm of PTC-47 reduces to a minimum
Figure 4. The open aperture Z-scan (points) and theoretical fit (solid lines) curves of PTC-47 and PTC-48 at 532 nm (upper), and different concentrations of PTC-47 (lower). Inset: transmittance reduction versus concentration plot.
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Zuschriften (Tmin) of approximately 72.5 %, which is much lower than the Tmin (ca. 90 %) of PTC-48. These results illustrate that PTC-47 presents significantly better optical limiting property under 532 nm than PTC-48. Considering their similar cluster moiety, such diversity in optical limiting might be attributed to the different geometric configurations of the Ag6 core in PTC-47 and PTC-48. Moreover, the opting effects of PTC-47 are almost linearly dependent on its concentration, with the optimized transmittance reduction up to about 43 % at 4 X 10@4 m dichloromethane solution. These results confirm that the observed optical limiting effects belong to the intrinsic characteristics of the prepared Ag6@Ti16-oxo clusters. In summary, we synthesized the first atomically precise noble-metal-doped titanium-oxo cluster. An acidic/redox control assembly strategy has been developed to construct two Ag6@Ti16-oxo nanoclusters with direct Ag@O@Ti bonds and core–shell structures. Although having the same Ti16-oxo shell, the relative configurations of the Ag6 core inside the cavity differ by about 4588 rotation. Attributed to its structure, PTC-47 displays better optical limiting activity towards 532 nm laser light than PTC-48. Moreover, the transmittance reduction effect of PTC-47 exhibits nearly linear dependence on its concentration, making it a potential candidate for the fabrication of future optical limiting devices. This work provides a breakthrough in the construction of molecular noble-metal/Ti@O materials that can be applied as structural models of technically important TiO2-supported metallic materials.
Acknowledgements This work is supported by NSFC (21425102, 21521061, 21673238, and 21771181), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000) and Natural Science Foundation of Fujian Province (2017J06009).
Conflict of interest The authors declare no conflict of interest. Keywords: cluster compounds · core–shell structure · optical limiting · silver · titanium-oxo cluster How to cite: Angew. Chem. Int. Ed. 2018, 57, 11252 – 11256 Angew. Chem. 2018, 130, 11422 – 11426
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[43] CCDC 1834932 (PTC-47) and 1834933 (PTC-48) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre. Manuscript received: April 19, 2018 Accepted manuscript online: May 14, 2018 Version of record online: May 30, 2018
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