Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Macrocycle-Encircled Polynuclear Metal Clusters: Controllable Synthesis, Reactivity Studies, and Applications Siqi Zhang and Liang Zhao*
Acc. Chem. Res. Downloaded from pubs.acs.org by WESTERN UNIV on 09/10/18. For personal use only.
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China
CONSPECTUS: Macrocyclic ligands have been extensively applied to recognize single metal ions with high selectivity and good affinity based on the size-match principle. The resulting metal−macrocycle complexes play a significant role in mimicking the function of natural metal ion carriers and understanding and reproducing the catalytic activity of metalloenzymes. Because of the known macrocyclic effect, those single metal−macrocycle adducts often show an enhanced kinetic and thermodynamic stability in comparison with their open-chain analogues. By virtue of such extraordinary coordination properties of macrocyclic ligands, it is expected that larger macrocycles with multiple coordination sites could properly act as an outer scaffold to direct the formation of multiatom species inside, such as polynuclear metal cluster aggregates, whose assembly may largely depend on the template positioning of coordinative atoms in the macrocyclic ring. Thus, the employment of polydentate macrocyclic ligands may provide a convenient tool to access polynuclear metal clusters in a controllable way. In this Account, we review our studies of the metal ion binding process of a class of polydentate macrocyclic ligands, azacalixpyridines (Py[n]s), and the application of Py[n]s as an outer template to direct the controllable synthesis of polynuclear metal clusters. Our investigations revealed that Py[n]s show a significant cooperative coordination effect in the metal ion binding process that facilitated the easy formation of a polymetallic assembled structure. Taking advantage of the cooperative coordination effect and the tunable and highly fluxional conformation of Py[n]s, we laid our focus on control of the nuclearity number by tuning the size of Py[n]s and the adoption of Py[n]s with different anionic centers in metal cluster synthesis. As an important example for application, this new established macrocycle-directed method has been employed to achieve a variety of metal-cluster-centered capsule, rotaxane, catenane, polygon, and other supramolecular assemblies. Furthermore, a cluster-tocluster transformation inside the cavity of Py[n]s is presented to showcase the use of the acquired metal cluster−macrocycle complexes to achieve unconventional metal cluster entities. With regard to the application of the newly synthesized macrocycle-encircled metal clusters, examples of the fabrication of functional materials and catalysts are presented. With the assistance of Py[n]s, a bulk-to-cluster-to-nanoparticle transformation of silver sulfide (Ag2S) and silver halides (AgX) has been conducted to produce a series of nonstoichiometric silver sulfide and halide nanoparticles. The resulting Ag−S nanoparticle material with a high Ag/S ratio, which is inherited from the Py[n]protected polysilver sulfide clusters, has a large energy gap relative to conventional Ag2S nanoparticles. Moreover, the nonstoichiometric silver halide nanoparticles can act as a new kind of electrocatalyst for the chlorine evolution reaction, showing excellent selectivity and high catalytic efficiency. Overall, in this Account we try to highlight the application of polydentate macrocycles as an outer template to guide the synthesis of polynuclear metal clusters in a controllable manner. This unique synthesis will provide a new avenue to access unconventional metal clusters of different metal kinds and diverse anionic centers, which are expected to have promising and significant applications in many interdisciplinary areas of chemistry.
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INTRODUCTION
of metal clusters, such as the nuclearity number and geometry of cluster aggregates and metal−metal interactions, play a significant role in determining their intrinsic properties and
Polynuclear metal clusters have attracted considerable attention in many interdisciplinary areas of chemistry on account of their unique structures for mimicking the active sites of metalloenzymes and their intriguing electronic, magnetic, and photophysical properties.1,2 Several crucial structural aspects © XXXX American Chemical Society
Received: June 14, 2018
A
DOI: 10.1021/acs.accounts.8b00283 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research Scheme 1. Synthesis of Polynuclear Metal Clusters inside Macrocyclic Py[n] Ligands
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COOPERATIVE COORDINATION EFFECT In contrast to the methylene-bridged conventional calixarene macrocycles, Py[n]s feature alterable hybrid configurations of the bridging nitrogen atoms and variable degrees of conjugation between the nitrogen atoms and adjacent pyridine rings.8 This unique electronic configuration suggests that every specific conformation of Py[n] corresponds to a distinguishable electron density distribution on the macrocycle interior surface. As an example, the surface electrostatic potential in neat azacalix[8]pyridine (Py[8]) is almost evenly distributed because of the presence of four similar conjugation systems between the bridging nitrogen atoms and their neighboring pyridine rings (Figure 1a).7 In contrast, Py[8] in the Ag4-cluster-encapsulated
extending the scope of their applications. Therefore, the development of straightforward synthetic approaches toward the controllable synthesis of metal clusters becomes a prior and important task before the in-depth studies of the properties and applications of metal cluster complexes. In this regard, polydentate organic macrocyclic compounds have many advantages. First of all, their multiple coordination sites suitably act as an outer template to position the metal atoms and thus direct the formation of multimetallic species inside. Second, the preorganized cyclic structure and the chelation bonding fashion of macrocyclic ligands can largely enhance their affinity for metal ions and form metal complexes with high kinetic inertness and thermodynamic stability due to the macrocyclic effect.3 Furthermore, the great advances in organic synthetic chemistry in the past decades make it feasible to design and synthesize desired macrocyclic compounds with a great deal of structural diversity.4 Subtle modulation of the size and conformation of macrocyclic ligands provides a promising method to adjust the nuclearity number and geometry of the resulting metal cluster complexes. A few macrocyclic compounds containing pendant donor groups (e.g., hydroxyl groups) have been applied in the synthesis of polynuclear metal cluster compounds.5 However, these macrocyclic ligands internally decorated with anionic pendant moieties (e.g., OH and SH) are often limited to the synthesis of O- or S-centered metal clusters. In order to achieve discrete metal clusters containing various externally introduced anions, neutral polydentate macrocycles will be a judicious choice. In 2010 we started a collaboration with Professor Mei-Xiang Wang to embark on studies of the controllable synthesis of polynuclear metal clusters by using a class of new emerging macrocycles, azacalix[n]pyridines (Py[n]s),6 as an outer template (Scheme 1). We envisioned that the characteristic flexible conformation and size tunability of Py[n]s7 would make them easily adapt to metal cluster aggregates with structural diversity in nuclearity number and geometry. Furthermore, the easy protonation of the pyridine rings of Py[n]s would help the release of the encapsulated metal cluster aggregates and enable hierarchical self-assembly of metal clusters to produce functional materials. In this Account, we mainly focus on the exploration and understanding of the cooperative coordination effect of Py[n]s and the newly established Py[n]-based method for the synthesis of polynuclear metal clusters. Special attention is paid to control of the nuclearity number by tuning the size of Py[n]s and the structurally complementary relationship between differently charged or multitopic anionic centers and Py[n]s. Reactivity studies and applications of the Py[n]-encircled cluster compounds are also exemplified.
Figure 1. Crystal structures and surface electrostatic potential diagrams of Py[8] in (a) the neat azacalix[8]pyridine and (b) [(CF3SO3)Ag4(tBuCC)(Py[8])](CF3SO3)2 (1).
complex [(CF3SO3)Ag4(tBuCC)(Py[8])](CF3SO3)2 (1)9 exhibits a severely curved conformation with the bridging nitrogen atoms highly conjugated with four coordinated pyridine rings, finally enriching the electron density of the internal loop of Py[8] (Figure 1b). However, accompanying the enhanced structural tunability, Py[n]s often show very complicated host−guest binding behaviors. To gain insight into the metal ion binding process of Py[n]s, we mixed Py[8] with silver triflate at various stoichiometric ratios from 1:1 to 1:10. It was surprising to us that the same crystalline complex 2 ([Ag3(Py[8])(CF3SO3)](CF3SO3)2) was obtained in every trial (Figure 2a).10 This implies that the 3:1 metal/host adduct complex is more stable than the 1:1 and 2:1 complexes and that the 1:1 and 2:1 adducts are easily transformed into the 3:1 complex even under metalB
DOI: 10.1021/acs.accounts.8b00283 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. (a) Crystal structure and schematic diagram of the core unit in 2. (b) UV−vis titration curves of Py[8] in response to AgCF3SO3 from 0 to 4 equiv. (c) Partial 1H NMR spectra at −60 °C in the titration experiment of Py[8]. (d) Binding isotherm curve based on the UV−vis titration. (Inset) Hill plot for the binding of silver(I) ions to Py[8]. Reprinted with permission from refs 10 and 12. Copyright 2013 and 2018, respectively, Royal Society of Chemistry.
Figure 3. (a) Molecular structure of an (NHC)2Au-bridged polypyridine macrocycle and crystal structure of its protonated form. (b) Possible mechanism for the cooperative coordination effect of Py[8].
ion-deficient conditions. The NMR titrations clearly showed the formation of complex 2 upon the addition of just over 1 equiv of silver ions (Figure 2c). In addition, the sigmoidal binding isotherm curve acquired in the UV−vis titration (Figure 2b) and the Hill coefficient11 value of 3.27 (Figure 2d) both demonstrate
that Py[8] experiences a strongly cooperative silver ion binding process. Moreover, fitting of the UV−vis titration results for Py[8] using the Hyperquad 2003 program gave a large Ka1 ((1.66 ± 0.17) × 105 M−1) and Ka3 ((1.58 ± 0.16) × 105 M−1) relative to a small Ka2 ((1.90 ± 0.19) × 104 M−1),12 which C
DOI: 10.1021/acs.accounts.8b00283 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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aggregate that is held together by σ and π bonding of the tertbutylacetylide anion and argentophilic interactions (Figure 4a).17 A triflate group binds an edge of the Ag4 plane at the
further confirms the intermediate role of the 1:1 species and rationalizes the ready formation of the 1:3 product under silver(I) ion-deficient condition. The cooperative coordination effect is also present in the silver ion binding processes of Py[6]13 and Py[9].10 This ubiquitous cooperative coordination effect of Py[n]s suggests their superiority in the formation of multimetallic assembled structures. Besides Py[n]s, other macrocyclic analogues have also been investigated to clarify what structural factors significantly influence the cooperative coordination effect. For example, two linear and rigid N-heterocyclic carbene (NHC) silver or gold units were inserted into a Py[n] skeleton to generate new metalla-macrocycles (Figure 3a).14 These partially rigid metallamacrocycles showed poor affinity for metal ions but instead trapped protons to form a helical structure. Furthermore, when some restricting moieties were introduced to lower the flexibility of Py[n]15a or other aromatic rings without coordination sites (e.g., benzene)10 or with outward coordination sites (e.g., pyrimidine and pyrazine)15b,c were used to replace the pyridine rings of Py[n], the resulting macrocycles were often deprived of the cooperative coordination effect. We thus conjectured that the flexibility of the macrocyclic skeleton and the continuity and arrangement of coordination sites may both contribute to the cooperative coordination effect of Py[n]s. A mechanism for the cooperative coordination effect of Py[8] was then proposed (Figure 3b). During the step-by-step metal ion binding process, the coordination of the first metal ion by a pyridine of Py[8] may immobilize the conjugation between this pyridine ring and adjacent N(Me) units. This immobilization leads to a domino effect on the conjugation between the remaining pyridine rings and the corresponding bridging amino groups, consequently rigidifying the conformation of Py[8]. Meanwhile, the conformational change is also accompanied by mutual approach of coordination sites, which allows for the easy coordination of subsequent metal ions within a better-preorganized coordination environment. The remarkable cooperative binding effect of Py[n]s and the multiple coordination sites in Py[n]s are conducive to the formation of multimetallic assemblies as in complex 2, which is a prerequisite for the construction of polynuclear metal clusters.
Figure 4. (a) Encapsulation of the [(tBuCC)Ag4(CF3SO3)] moiety into Py[8] to form a pseudorotaxane structure in 1. (b) Partial 1H NMR spectra (600 MHz, methanol-d4) of the Py[8] in 1 at different temperatures. Reprinted from ref 9. Copyright 2011 American Chemical Society.
opposite side of t BuCC. The linear [( t BuCC)Ag4(CF3SO3)] moiety is threaded through a Py[8] by the coordination of four alternating pyridyl nitrogen atoms, thus giving rise to a pseudorotaxane structure. In contrast to the parallelogram 1,3,4,6-alternate conformation of neat Py[8],7 the Py[8] ligand in 1 is folded into a cylinder-belt-like 1,5-planar2,4,7-alternate conformation. In the NMR spectrum of 1, only one set of broad signals corresponding to the pyridyl protons of the Py[8] ligand were observed (Figure 4b). As the temperature decreases, the broad signals gradually split into several sharp doublets and triplets. The NMR studies indicated that the Py[8] in 1 is fluxional in solution and that the eight pyridyl nitrogen atoms undergo a rapid dissociation−recombination equilibrium to bind to the central Ag4 aggregate. When the smaller macrocycle Py[6] is employed in place of Py[8], an acetylide-bonded trinuclear silver aggregate is formed in the concave cavity of Py[6] in 3.18 The Py[6] ligand exhibits a quasi-C3v bowl-shaped conformation with three coplanar pyridine rings bonding to the [tBuCCAg3] aggregate while the remaining three lateral ones encompass the Ag3 aggregate by silver−aromatic π interactions (Figure 5a). Because of the excellent size match between Py[6] and the Ag3 aggregate, the 1 H NMR spectrum of 3 exhibited two sets of well-resolved peaks for the coordinated and free pyridine rings of Py[6] rather than broad signals as shown in 1. Notably, such size matching leads to
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MONOANION-CENTERED SILVER CLUSTERS The cooperative coordination effect of macrocyclic ligands is a clear advantage for the construction of polynuclear metal clusters.16 Inspired by this understanding, we initially carried out the synthesis of polynuclear silver clusters with many common monoanions (e.g., acetylides, thiolates, and halides) and the single-atom dianion S2−. Because of the poor solubility of stoichiometric silver complexes of these anions (e.g., Ksp(Ag2S) = 8 × 10−51 at 25 °C), previously reported synthetic methods often need the use of precursor compounds (e.g., silylated sulfide sources for the sulfide anion) to avoid the prompt formation of precipitates. However, because of the remarkable cooperative coordination effect of Py[n]s that can largely enhance the effective concentration of silver ions, our synthetic approach features a direct transformation of highly insoluble silver complexes (e.g., silver acetylide [RCCAg]n) into polynuclear silver clusters. With the synthesis of 1 as a typical example,9 addition of Py[8] to a suspension of insoluble [AgCCtBu]n and silver triflate led to a clear pale-yellow solution promptly. Vapor diffusion of a poor solvent produced crystals of 1. In the crystal structure of 1, four silver atoms constitute a square-planar D
DOI: 10.1021/acs.accounts.8b00283 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 5. Crystal structures of polynuclear silver cluster complexes containing a single anion: (a) [(CF3SO3)1.5Ag3.5(tBuCC)(Py[6])(CH3OH)0.5](CF3SO3)·(H2O)0.5 (3); (b) [Ag3(p-MeOC6H4CC)(Py[6])](CF3SO3)2 (4); (c) [Ag4(tBuS)(CF3SO3)2(Py[6])](CF3SO3) (5); (d) [Ag3(p-tBuC6H4S)(CF3SO3)(Py[6])]2(CF3SO3)2·2CH3OH·CH2Cl2·2.5H2O (6); (e) [Ag5S(Py[6])](CF3SO3)3·CH3OH (7), in which both Ag5 and Ag6 have an occupancy ratio of 0.5; (f) [Ag4Cl(CF3SO3)3(Py[7])(CH3OH)] (8); (g) [Ag5Br(CF3SO3)2(H2O)4(Py[7])](CF3SO3)2·H2O (9); (h) [Ag4I(H2O)2(Py[7])](CF3SO3)3 (10).
Figure 6. Crystal structures of polynuclear silver clusters containing a dianion center: (a) ([Ag6(CC−(p-C6H4)−CC)(Py[6])2](CF3SO3)4· 2CH2Cl2 (11); (b) ([(CF3SO3)4Ag8(CC−CC)(Py[6])2](CF3SO3)2) (12); (c) {[Ag5(CC)(Py[6])2](CF3SO3)3}0.7{[Ag6(CC)(Py[6])2](CF3SO3)4}0.3 (13). Reprinted from ref 18. Copyright 2011 American Chemical Society.
included metal cluster is proportional to the size of Py[n], but the dynamic conformation of Py[n] seems to be influenced by the degree of metal cluster−macrocycle size matching. In order to further make clear the complementary relationship between the outer macrocycle template and the central anion, we selected a medium-sized macrocycle, Py[7], but included halide anions with a periodic variation in radius.21 The resulting
the invariable acquisition of a Ag3 aggregate in Py[6] no matter what kind of anionic centers are employed (e.g., phenylacetylide in 419 and thiolates in 5 and 620) (Figure 5b−d). When the single-atom dianion S2− serves as the cluster center, five silver atoms are found surrounding the S2− in 7, and three of them are coordinated by a bowl-shaped Py[6] macrocycle as in 3−6 (Figure 5e).13 It is clear that the nuclearity number of the E
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Figure 7. Crystal structures of dual-template-based polynuclear silver clusters: (a) [(CF3SO3)4Ag6(CC−CC)(Py[8])(H2O)5] (14); (b) [Ag10{1,4-(CC)2C6H4}2(Py[8])2(CH3OH)2](BF4)6·2H2O (15).
Figure 8. (a) Synthetic strategy for a metal-cluster-pillared supramolecular prism. (b−d) Crystal structures of (b) [Ag9{1,3,5-(C C)3C6H3}(Py[6])3](CF3SO3)6 (16), (c) [Ag20.5{1,3,5-(CC)3C6H3}2(Py[7])6(CF3SO3)4](CF3SO3)10.5 (17), and (d) [Ag15{1,3,5-(C C)3C6H3}2(Py[8])3(CF3SO3)3](CF3SO3)6 (18). Reprinted with permission from ref 25. Copyright 2012 Royal Society of Chemistry.
also provides an effective means to regulate the nuclearity number by adopting differently sized macrocycles. We then envisioned that employing a di- or multitopic anion may integrate several macrocycle-protected metal cluster entities together to achieve cluster aggregates with higher nuclearity numbers and structural diversity. For example, employing 1,4phenylenediacetylide dianion engenders the formation of a dumbbell-like structure in complex 11 that contains two C CAg3⊃Py[6] units. Therein, the two Py[6] ligands are arranged in a parallel face-to-face fashion (Figure 6a). When the carbon chain is shortened to 1,3-butadiynediide, two similar Ag3−Py[6] fragments are bridged by the C42− to produce a clamlike structure in 12 (Figure 6b).18 The two C2-related Py[6] ligands are arranged in an eclipsed face-to-face fashion with a dihedral angle of 23°. Further shrinkage of the carbon chain makes two separated CCAg3 aggregates approach each other and fuse together, finally generating a C22−-encapsulated silver cage in 13 (Figure 6c).18 The C2@Ag5−6 silver carbide cluster is stabilized by two parallel bowl-shaped Py[6] ligands that are associated together by multiple C−H···π and C−H···N interactions. In a short summary, the relatively small Py[6] macrocycle facilitates the persistent formation of a Ag3 aggregate at each acetylide terminal. In this way, reactive and unstable metal clusters can be stabilized by this steady macrocycle encapsulation.23
complexes 8−10 all comprise a halide-centered three- or fourmembered silver cluster aggregate inside a Py[7] (Figure 5f−h). Because of the coordination restriction of Py[7], the Ag−Cl bond lengths in 8 are approximately 0.2 Å shorter than in reported pyramidal [Ag3Cl] clusters.22 Thus, the chlorine atom is 0.34 Å above the Ag1−Ag2−Ag3 plane. Similarly, the long Ag−Br lengths cause the bromine atom in 9 to be 1.05 Å above the Ag3 triangle. Because of the size mismatch between Py[7] and the small [Ag3Cl] and [Ag3Br] clusters, one or two additional silver atoms are included in the cavity of Py[7] in 8 and 9. In the iodide-centered complex 10, the long Ag−I bonds result in the inclusion of one silver atom capping on the silver triangle. The iodide anion (0.92 Å above the Ag1−Ag2−Ag2A plane) finally bonds to four silver atoms to produce a trigonalbipyramidal [IAg4] cluster. This study indicates that Py[n]s can significantly alter the metal−anion bonding distances by coordination restriction. In this way, metal clusters with novel intrinsic properties can be achieved by this method.
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DI- AND MULTITOPIC-ANION-CENTERED SILVER CLUSTERS The remarkable protecting function of Py[n] not only facilitates the formation of stable monoanion-centered silver clusters but F
DOI: 10.1021/acs.accounts.8b00283 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Ag4(CF3SO3)] chain is threaded through a Py[8] macrocycle to construct a pseudorotaxane structure.9 Two such clustercentered pseudorotaxane units are bridged by the central phenylene−acetylene moiety to afford a semicircular [3]pseudorotaxane structure (Figure 9a). Two such semicircles
To gain more insight into the adaptive relationship between multitopic anions and Py[n]s, the larger macrocycle Py[8] was then employed to incorporate cluster aggregates of the ditopic anions [CC−CC]2− and [1,4-(CC)2C6H4]2− as in 11 and 12. Structural studies revealed that in the resulting complexes 14 and 15, the peripheral macrocycle Py[8] acts as a “soft” template to adaptively encircle different cluster aggregates by coordination while the central anion can be considered as a “hard core” template to dictate the nuclearity number and geometry of the encapsulated clusters.24 In contrast to the curved quartet columnlike conformation of Py[8] in 1, the Py[8] in 14 adopts a parallelogram-like conformation to include the dumbbell-like [Ag3CC−CCAg3] inside, finally producing a discrete cocoonlike structure (Figure 7a). In contrast, since the 1.2 nm long [Ag3CC−C6H4−CCAg3] aggregate is beyond the limit of Py[8], in 15 two [Ag3CC−C6H4−C CAg3] aggregates are fused together to generate a bigger cluster aggregate, [Ag5(CC−C6H4−CC)2Ag5], and the Py[8] macrocycle tunes its conformation to adaptively encircle this zigzag-chain-like Ag5 aggregate (Figure 7b). Clearly, the extraordinary conformational tunability of large and flexible macrocycles enhances their adaptability for different cluster aggregates, spotlighting the advantage in the synthesis of a wide range of metal cluster complexes. In addition to the linear dianions, some multitopic anionic species have also been used to achieve high-nuclearity metal clusters and metal-cluster-centered metallosupramolecular architectures. For example, the use of the panel-like polyacetylide anion [1,3,5-(CC)3C6H3]3− together with size expansion of the coordinative macrocycles from Py[6] to Py[8] finally give rise to a metal-cluster-centered metallocage (Figure 8a).25 In the Py[6]-based complex 16, each acetylide moiety is bonded to a typical Ag3−Py[6] unit, and the resulting [1,3,5-(Ag3CC)3C6H3]6+ species is finally encircled by three Py[6] macrocycles to form a trefoil structure (Figure 8b). The same [1,3,5-(Ag3CC)3C6H3]6+ species is also observed in complex 17, while three Py[7] ligands exhibit different conformations (Figure 8c). In contrast to the sharp proton NMR signals of Py[6] in 16, the signals of Py[7] in 17 become very broad because of mismatched coordination between the large Py[7] and the small Ag3 aggregate. Along with the size expansion of the peripheral macrocycles from Py[7] to Py[8], at each wing of [1,3,5-(CC) 3 C 6 H 3 ] 3− two [CCAg 3 ] aggregates fuse together to generate a Py[8]-encircled Ag5 chain in 18. In this way, two [1,3,5-(CC)3C6H3] groups and three Py[8]-protected Ag5 aggregates act as the panels and pillars, respectively, to construct a triangular prism structure (Figure 8d). This assembled 3D structure is composed of a number of components (three Py[8] macrocycles, 15 silver atoms, two [1,3,5-(CC)3C6H3] units, and triflate anions) but shows very good stability and structural integrity in solution, as evidenced by DOSY NMR spectra and high-resolution ESI-MS. Therefore, we believe this macrocycle-based synthesis can provide an efficient pathway to achieve metal-cluster-centered 2D polygonal and 3D polyhedral architectures that should be complementary to the extensively reported single-metal-based coordination self-assembled structures.26 With reference to the directional bonding strategy of coordination-driven self-assembly,26 we also employed angled ditopic ligands to construct desired metal-cluster-centered supramolecular architectures.9,27 For example, when the angled ligand 1,3-bis((3-ethynylphenyl)ethynyl)benzene was utilized to obtain complex 19, at each terminal a [(CC)-
Figure 9. (a) Semicircular-cluster-centered [3]pseudorotaxane structure in the complex [(CF3SO3)Ag4{CC−(m-C6H4)−CC−(mC6H4)−CC−(m-C6H4)−CC}Ag4(CF3SO3)(Py8)2](CF3SO3)4 (19). (b) Hexagonal catenane-like structure in 19. Reprinted from ref 9. Copyright 2011 American Chemical Society.
are arranged in a face-to-face fashion and are connected by the F···F interactions between the attached triflates, thus generating a hexagonal nanometer-sized catenane-like structure (Figure 9b).
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ORGANOMETALLIC TRANSFORMATIONS WITHIN A METAL−MACROCYCLE CAPSULE Besides the synthesis of polynuclear metal clusters, Py[n]s can also act as molecular flasks to conduct organometallic transformations of the included metal cluster species. Previously, many reported metal−ligand container architectures have been extensively applied in encaging guest molecules and accelerating otherwise sluggish chemical reactions.28 A variety of reactions (e.g., Diels−Alder reaction, photoaddition, and reductive elimination) have been performed within metallacage hosts,29 G
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Figure 10. Molecular structures of substrates and polysilver-bonded heteroaromatics formed within a metal−macrocycle capsule and crystal structures of the complexes [Ag5(C8NH5)(Py[8])](CF3SO3)3 (20), [Ag5(C9NH5)H(Py[8])](CF3SO3)4 (21), [Ag5(C16N2H10)(Py[8])](CF3SO3)3 (22), and [Ag5(C16NH9)(Py[8])](CF3SO3)3 (23). Reprinted with permission from ref 12. Copyright 2018 Royal Society of Chemistry.
theoretical studies deepen our understanding of basic organometallic concepts such as metal-perturbed aromaticity and multicentered bonding, which is conducive to the advancement of more efficient and versatile synthetic methods based on polymetallic catalysts.
yielding products with unusual selectivity and/or enhanced activity. However, structurally well-defined reaction intermediates are rarely isolated within the supramolecular coordination capsules. In view of the cooperative coordination effect of Py[n]s that facilitates the formation of capsulelike polymetallic structures, we purposefully utilized the Py[8]−Ag3 capsule in complex 2 as a dynamically flexible molecular flask to conduct organic transformations of diverse substrates.12 For example, treatment of o-ethynylaniline with a solution of 2 and additional silver triflate resulted in the occurrence of an intramolecular 5-endodig cyclization, finally yielding polymetalated indole complex 20. The indole ring is negatively charged at two vicinal carbon atoms, which are coordinated by a Py[8]-stabilized coplanar Ag4 rectangle. Such coordination-capsule-triggered cyclization is also applicable to other substrates containing both amine and ethynyl groups (Figure 10), which undergo an unusual 6-endodig cyclization or cascade or multistep cyclization within the Py[8]−Ag3 capsule to generate the quinolinium, 2,2′-biindole, and benzo[a]carbazole skeletons in 21−23, respectively. The metal−macrocycle capsule holds great potential to adjust its conformation to adapt to different unprecedented polysilver heteroaromatic species. The acquisition of these unprecedented structurally well-defined polysilver heteroaromatics and related
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APPLICATIONS OF MACROCYCLE-PROTECTED METAL CLUSTERS The above synthesis of macrocycle-protected silver clusters has shown that because of the coordination restriction of Py[n]s, the included cluster aggregates exhibit uncommon metal−anion bond lengths and unusual elemental ratios between the central anions and silver atoms. We thus expected that these exceptional metal cluster aggregates may be applied as starting materials to afford new functional compounds. Two recent examples concerning the hierarchical assembly of Py[n]-protected silver sulfide and halide clusters are discussed below. Silver sulfide as a type of narrow-band-gap semiconductor has attracted considerable attention because of its good stability, low toxicity, and extensive potential applications in photovoltaic cells, infrared detectors, and near-infrared imaging.30 Bulk αAg2S has a band gap of 0.9−1.1 eV, and the synthesis of nanosized Ag2S provides an efficient way to finely enlarge the H
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Figure 11. (a) Crystal structures of [Ag5S(Py[6])2](CF3SO3)3 (24) and [Ag12S2(Py[6])2](CF3SO3)8·H2O·CH3OH (25). (b) Macrocycle-assisted bulk-to-cluster-to-nanoparticle transformation of Ag2S. (c) Gap energy determination of the nonstoichiometric silver sulfide nanoparticles using the Bardeen or Tauc equation. (d) Optimized structures of Ag8S4 and Ag11S3(OH)5 and their HOMO and LUMO orbitals. The calculated energy gaps are listed in the inset table. Reprinted with permission from ref 13. Copyright 2015 Royal Society of Chemistry.
relationship between the gap energy and the Ag/S ratio (Figure 11d). Compared with the two model clusters Ag8S4 and Ag10S5, Ag11S3(OH)5 and Ag12S3(OH)6 with a higher Ag:S elemental ratio have more localized HOMO−LUMO orbitals and indeed have larger energy gaps. Therefore, this method represents a viable means to tune the band gap of binary nanomaterials independent of their size. In addition, the present macrocycleassisted bulk-to-cluster-to-nanoparticle transformation can also be applied to the fabrication of thiolate-protected Ag2S nanoclusters20 and alkynyl-protected silver nanoclusters.19 The above macrocycle-assisted bulk-to-cluster-to-nanoparticle transformation was further employed to fabricate nonstoichiometric silver halide nanoparticles [AgmXn](m−n)+ (m > n; X = Cl, Br, I).21 Addition of tetrafluoroboric acid to a methanol solution of complex 8 interrupts the coordination between the central [Ag3Cl] cluster and the surrounding Py[7]. This protonation process together with the addition of the stabilizing surfactant polyvinylpyrrolidone (PVP) led to the formation of nonstoichiometric silver chloride nanoparticles (Figure 12a). Similar synthetic procedures were applied for complexes 9 and 10 to produce PVP-stabilized silver bromide and silver iodide nanoparticles, respectively. The newly synthesized nonstoichiometric silver halide nanoparticles [AgmXn](m−n)+ (m > n; X = Cl, Br, I) can act as a highly efficient electrocatalyst for the chlorine evolution reaction (CER) with excellent selectivity. The CER is a significant electrochemical reaction in industry because of the extensive application of chlorine. The positively charged nature of the [AgmXn](m−n)+ nanoparticles expedites chloride transport by electrostatic attraction and facilitates the formation of the catalytically active silver polychloride species. In addition, the nonstoichiometric elemental ratio between silver and halogen atoms makes coordinatively unsaturated silver atoms easily exposed to catalyze the chloride oxidation (Figure 12b). As a result, the [AgmXn](m−n)+ nanoparticles can function as an
band gap as a result of the quantum confinement effect. However, the use of exotic ligand- or surfactant-stabilized silver and sulfide ions or their precursors and the requirement of elevated temperature and high pressure in most cases make the synthesis of uniformly sized Ag2S nanocrystals very arduous.31 Clearly, direct transformation of bulk Ag2S solid into its nanometer-sized prototype would be a concise and ideal synthetic strategy. With reference to the synthetic method for the Py[6]protected [Ag5S]-containing complex 7, we varied the amount of Py[6] to produce two new crystalline complexes 24 and 25.13 24 comprises a [Ag5S] aggregate protected by two face-to-face Py[6]s. In 25, a dumbbell-shaped [Ag12S2] silver sulfide cluster aggregate embraced by two Py[6] macrocycles at the upper and nether sides was observed (Figure 11a). As a result of the coordination restriction of Py[6], the Ag−S bond lengths in 24 and 25 are ∼0.2 Å shorter than the values in bulk Ag2S.32 Successful isolation of single [Ag5S] and joint [Ag12S2] silver sulfide clusters by varying the amount of Py[6] suggests the viability of achieving nonstoichiometric Ag−S binary nanoparticles through coalescence and fusion of Py[6]-encapsulated [Ag5S] clusters. As shown in Figure 11b, the protective Py[6] macrocycles were removed by protonation, and the released clusters coalescenced and fused together to form silver sulfide nanoparticles. X-ray photoelectron spectroscopy revealed the +1 oxidation state of the silver atoms in the nanoparticles and determined the Ag/S molar ratio to be 3.7. Energy-dispersive Xray spectroscopy further offered the Ag/S atomic ratio of 3.5. We hypothesized that such a high Ag/S ratio is actually inherited from the Py[6]-protected silver-rich Ag−S clusters. The bandgap energy of the nanoparticle sample was determined to be 4.0 eV (Figure 11c), greatly blue-shifted relative to that of bulk αAg2S. We then calculated the HOMO−LUMO gap of silver sulfide clusters with different Ag/S ratios to make clear the I
DOI: 10.1021/acs.accounts.8b00283 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
which are being applied in mimicking multicopper oxidases. Furthermore, other types of macrocyclic ligands with symmetrical or unsymmetrical structures can be elaborately designed and synthesized to direct the controllable synthesis of both homonuclear and heteronuclear metal clusters. In particular, the latter is still a challenging task for chemists at present. Lastly, the macrocycle-protected clusters with structural diversity provide an ideal molecular platform to study the structure−property relationship of metal clusters in view of their satisfying stability. In-depth studies of enzyme mimics and polymetallic complex-based catalysis within macrocyclic ligands may lead to the exploration of new synthetic methodologies and new functional materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID Figure 12. (a) Schematic diagram of the fabrication of nonstoichiometric silver halide nanoparticles as electrocatalysts for the CER. (b) Cyclic voltammograms (CVs) at a glassy carbon electrode in an aqueous solution of NaCl (1 M) without (black) and with (red) the nonstoichiometric [AgmCln](m−n)+ nanoparticles (cAg+ = 5.30 μM) and with (blue) stoichiometric AgCl nanoparticles (cAg+ = 4.89 μM). (c) CVs at a glassy carbon electrode in an aqueous solution of NaCl with the nonstoichiometric [AgmCln](m−n)+ nanoparticles (cAg+ = 0.53 μM) as a catalyst after degassing (red) or after bubbling of oxygen (black). Reprinted with permission from ref 21. Copyright 2017 Royal Society of Chemistry.
Liang Zhao: 0000-0003-4646-2887
efficient and selective electrocatalyst to catalyze the CER at a very low overpotential (10 mV) and within a wide concentration range of chloride (0.05−1 M). Meanwhile, the [AgmXn](m−n)+ nanoparticles have good catalytic selectivity for the CER over the oxygen evolution reaction because their positively charged surface attracts chloride anions more easily than neutral water molecules (Figure 12c). This study showcases a promising approach to achieve highly efficient catalysts from the macrocycle-protected metal clusters. We believe that many other unconventional nonstoichiometric metal catalysts with high catalytic efficiency and selectivity will be produced by similar synthetic protocols.
Liang Zhao (born in 1981 in Ning Xia, China) received his Bachelor’s degree at Peking University in 2002. In 2003 he enrolled in the Chemistry Department of the Chinese University of Hong Kong under the guidance of Prof. Thomas C. W. Mak and graduated with a Ph.D. degree in 2007. Subsequently, he joined Prof. Peter J. Stang’s group at the University of Utah and completed a two-year postdoctoral study until September 2009. In November 2009 he joined Chemistry Department at Tsinghua University. His current research focuses on the controllable synthesis and reactivity studies of polynuclear organometallic clusters and supramolecular self-assembly of metal clusters.
Notes
The authors declare no competing financial interest. Biographies Siqi Zhang (born in 1994 in Jiang Su, China) graduated with a B.S. degree from Soochow University in China in 2016. She is now pursuing her Ph.D. degree at Tsinghua University under the guidance of Dr. Liang Zhao. Her research work is mainly focused on the designed synthesis of enzyme mimics of multinuclear copper clusters.
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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (21522206, 21772111, 21821001, and 21661132006) is gratefully acknowledged. We thank all of our co-workers, whose names are cited in references, for their great contributions to this research program. We are especially grateful to Profs. Mei-Xiang Wang (THU) and De-Xian Wang (ICCAS) for their kind help during the study of this program.
CONCLUSION AND OUTLOOK This Account describes our recent progress in the controllable synthesis of polynuclear silver clusters by using neutral polydentate macrocyclic ligands as an outer template. The cooperative coordination effect of Py[n]s and the adaptive binding between them and various anionic centers have been highlighted. Preliminary application studies of the macrocycleencircled metal clusters have presented some uncommon organometallic transformations and a Py[n]-assisted bulk-tocluster-to-nanoparticle transformation that can be used to fabricate novel nanomaterials with unique physical and catalytic properties. The research work presented herein is limited to silver clusters and Py[n]s. There is still much uncharted terrain in this interdisciplinary area that deserves to be explored. We believe that the macrocycle-directed synthetic approach can be generalized to achieve cluster compounds of other metal kinds. Actually, we have recently synthesized several Py[n]stabilized polynuclear Cu(I) or mixed-valence Cu(I/II) clusters,
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REFERENCES
(1) Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany, 1999. (2) For some recent reviews about the synthesis and applications of polynuclear metal clusters, see: (a) Buchwalter, P.; Rose, J.; Braunstein, P. Multimetallic catalysis based on heterometallic complexes and clusters. Chem. Rev. 2015, 115, 28−126. (b) Chakraborty, I.; Pradeep, T. Atomically precise clusters of noble metals: emerging link between atoms and nanoparticles. Chem. Rev. 2017, 117, 8208−8271. (c) Zheng, X.-Y.; Kong, X.-J.; Zheng, Z.; Long, L.-S.; Zheng, L.-S. High-nuclearity lanthanide-containing clusters as potential molecular magnetic coolers. Acc. Chem. Res. 2018, 51, 517−525. J
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Accounts of Chemical Research (3) Gloe, K. Macrocyclic Chemistry: Current Trends and Future Perspectives; Springer: Dordrecht, The Netherlands, 2005, and references therein. (4) For some recent reviews about macrocyclic compounds, see: (a) Ballester, P. Supramolecular capsules derived from calixpyrrole scaffolds. Isr. J. Chem. 2011, 51, 710−724. (b) Ogoshi, T.; Yamagishi, T.; Nakamoto, Y. Pillar-shaped macrocyclic hosts pillar[n]arenes: new key players for supramolecular chemistry. Chem. Rev. 2016, 116, 7937− 8002. (c) Kim, D. S.; Sessler, J. L. Calix[4]pyrroles: versatile molecular containers with ion transport, recognition, and molecular switching functions. Chem. Soc. Rev. 2015, 44, 532−546. (d) Xue, M.; Yang, Y.; Chi, X.; Zhang, Z.; Huang, F. Pillararenes, a new class of macrocycles for supramolecular chemistry. Acc. Chem. Res. 2012, 45, 1294−1308. (e) Zhang, M.; Yan, X.; Huang, F.; Niu, Z.; Gibson, H. W. Stimuliresponsive host-guest systems based on the recognition of cryptands by organic guests. Acc. Chem. Res. 2014, 47, 1995−2005. (5) For some reviews about the synthesis of multimetallic complexes within the cavities of macrocyclic compounds, see: (a) Frischmann, P. D.; MacLachlan, M. J. Schiff base macrocycles reliable templates for multinuclear metallocavitands. Comments Inorg. Chem. 2008, 29, 26− 45. (b) Park, S.-H.; Lee, S.-Y.; Park, K.-M.; Lee, S.-S. Supramolecular networking of macrocycles based on exo-coordination: from discrete to continuous frameworks. Acc. Chem. Res. 2012, 45, 391−403. (c) Frischmann, P. D.; MacLachlan, M. J. Metallocavitands: an emerging class of functional multimetallic host molecules. Chem. Soc. Rev. 2013, 42, 871−890. (6) Wang, M.-X. Nitrogen and oxygen bridged calixaromatics: synthesis, structure, functionalization, and molecular recognition. Acc. Chem. Res. 2012, 45, 182−195. (7) Gong, H.-Y.; Zhang, X.-H.; Wang, D.-X.; Ma, H.-W.; Zheng, Q.-Y.; Wang, M.-X. Methylazacalixpyridines: remarkable bridging nitrogentuned conformations and cavities with unique recognition properties. Chem. - Eur. J. 2006, 12, 9262−9275. (8) Wang, M.-X. Heterocalixaromatics, new generation macrocyclic host molecules in supramolecular chemistry. Chem. Commun. 2008, 4541−4551. (9) Gao, C.-Y.; Zhao, L.; Wang, M.-X. Designed synthesis of metal cluster-centered pseudo-rotaxane supramolecular architectures. J. Am. Chem. Soc. 2011, 133, 8448−8451. (10) He, X.; Xu, X.-B.; Wang, X.; Zhao, L. Positive homotropic allosteric binding of silver(I) ions in multidentate azacalixpyridine macrocycles: effect on the formation and stabilization of silver nanoparticles. Chem. Commun. 2013, 49, 7153−7155. (11) Conners, K. A. Binding Constants; Wiley: New York, 1987. (12) He, X.; Xue, Y.; Li, C.-C.; Wang, Y.; Jiang, H.; Zhao, L. Synthesis of stable polymetalated aromatic complexes through metal-macrocycle capsule-triggered cyclization. Chem. Sci. 2018, 9, 1481−1487. (13) He, X.; Wang, Y.; Gao, C.-Y.; Jiang, H.; Zhao, L. A macrocycleassisted nanoparticlization process for bulk Ag2S. Chem. Sci. 2015, 6, 654−658. (14) Liu, H.-X.; He, X.; Zhao, L. Metallamacrocycle-modified gold nanoparticles: a new pathway for surface functionalization. Chem. Commun. 2014, 50, 971−974. (15) (a) Fang, Y.-X.; Zhao, L.; Wang, D.-X.; Wang, M.-X. Synthesis, structure and metal binding property of internally 1,3-arylene-bridged azacalix[6]aromatics. J. Org. Chem. 2012, 77, 10073−10082. (b) Wu, J.C.; Zhao, L.; Wang, D.-X.; Wang, M.-X. Structural diversity in coordination self-assembled networks of a multimodal ligand azacalix[4]pyrazine. Inorg. Chem. 2012, 51, 3860−3867. (c) Wu, J.C.; Zhao, L.; Wang, D.-X.; Wang, M.-X. Synthesis, structure and coordination self-assembly of azacalix[4-n]pyridine[n]pyrazines (n = 1−3). Chin. J. Chem. 2013, 31, 589−597. (16) Sessler, J. L.; Tomat, E.; Lynch, V. M. Positive homotropic allosteric binding of silver(I) cations in a Schiff base oligopyrrolic macrocycle. J. Am. Chem. Soc. 2006, 128, 4184−4185. (17) Schmidbaur, H.; Schier, A. Argentophilic interactions. Angew. Chem., Int. Ed. 2015, 54, 746−784.
(18) Gao, C.-Y.; Zhao, L.; Wang, M.-X. Stabilization of a reactive polynuclear silver carbide cluster through the encapsulation within a supramolecular cage. J. Am. Chem. Soc. 2012, 134, 824−827. (19) Guo, H.; He, X.; Wan, C.-Q.; Zhao, L. A stepwise bulk-to-clusterto-particle transformation toward the efficient synthesis of alkynylprotected silver nanoclusters. Chem. Commun. 2016, 52, 7723−7726. (20) Chen, H.-Q.; He, X.; Guo, H.; Fu, N.-Y.; Zhao, L. Designed synthesis of size-tunable Ag2S nanoclusters via distinguishable C-S bond cleavage reaction of alkyl- and aryl-thiolates. Dalton Trans. 2015, 44, 3963−3966. (21) Zhang, Q.-Y.; He, X.; Zhao, L. Macrocycle-assisted synthesis of non-stoichiometric silver(I) halide electrocatalysts for efficient chlorine evolution reaction. Chem. Sci. 2017, 8, 5662−5668. (22) Liu, C. W.; Liaw, B.-J.; Liou, L.-S.; Wang, J.-C. A 2D honeycombshaped network based on a starburst cluster: [Ag 4 (μ 3 -Cl)(PPh2(CH2)2PPh2)1.5{S2P(OR)2}3] (R = Et, iPr). Chem. Commun. 2005, 1983−1985. (23) Galan, A.; Ballester, P. Stabilization of reactive species by supramolecular encapsulation. Chem. Soc. Rev. 2016, 45, 1720−1737. (24) Gao, C.-Y.; He, X.; Zhao, L.; Wang, M.-X. Dual templated synthesis of silver acetylide cluster-encapsulated supramolecular boxes. Chem. Commun. 2012, 48, 8368−8370. (25) He, X.; Gao, C.-Y.; Wang, M.-X.; Zhao, L. Designed synthesis of a metal cluster-pillared coordination cage. Chem. Commun. 2012, 48, 10877−10879. (26) For some recent reviews, see: (a) Cook, T. R.; Stang, P. J. Recent developments in the preparation and chemistry of metallacycles and metallacages via coordination. Chem. Rev. 2015, 115, 7001−7045. (b) McConnell, A. J.; Wood, C. S.; Neelakandan, P. P.; Nitschke, J. R. Stimuli-responsive metal-ligand assemblies. Chem. Rev. 2015, 115, 7729−7793. (c) Wang, W.; Wang, Y.-X.; Yang, H.-B. Supramolecular transformations within discrete coordination-driven supramolecular architectures. Chem. Soc. Rev. 2016, 45, 2656−2693. (27) Ji, G.; Zhang, S.; Hau, S. C. K.; Zhao, L. Macrocyclic-ligand induced synthesis of aryl ethynides with divergent silver(I) clusters. Chin. J. Chem. 2017, 35, 1824−1828. (28) Ballester, P.; Fujita, M.; Rebek, J. Molecular containers. Chem. Soc. Rev. 2015, 44, 392−393 and references in this theme issue.. (29) (a) Yoshizawa, M.; Takeyama, Y.; Kusukawa, T.; Fujita, M. Cavity-directed, highly stereoselective [2 + 2] photodimerization of olefins within self-assembled coordination cages. Angew. Chem., Int. Ed. 2002, 41, 1347−1349. (b) Hastings, C. J.; Pluth, M. D.; Bergman, R. G.; Raymond, K. N. Enzymelike catalysis of the Nazarov cyclization by supramolecular encapsulation. J. Am. Chem. Soc. 2010, 132, 6938− 6940. (c) Kaphan, D. M.; Levin, M. D.; Bergman, R. G.; Raymond, K. N.; Toste, F. D. A supramolecular microenvironment strategy for transition metal catalysis. Science 2015, 350, 1235−1238. (30) Sadovnikov, S. I.; Gusev, A. I. Recent progress in nanostructured silver sulfide: from synthesis and nonstoichiometry to properties. J. Mater. Chem. A 2017, 5, 17676−17704. (31) Kershaw, S. V.; Susha, A. S.; Rogach, A. L. Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties. Chem. Soc. Rev. 2013, 42, 3033−3087. (32) Santamaría-Pérez, D.; Marqués, M.; Chuliá-Jordán, R.; Menendez, J. M.; Gomis, O.; Ruiz-Fuertes, J.; Sans, J. A.; Errandonea, D.; Recio, J. M. Compression of silver sulfide: X-ray diffraction measurements and total-energy calculations. Inorg. Chem. 2012, 51, 5289−5298.
K
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