Article pubs.acs.org/accounts
Role of Anions Associated with the Formation and Properties of Silver Clusters Quan-Ming Wang,* Yu-Mei Lin, and Kuan-Guan Liu State Key Lab of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, P. R. China CONSPECTUS: Metal clusters have been very attractive due to their aesthetic structures and fascinating properties. Different from nanoparticles, each cluster of a macroscopic sample has a well-defined structure with identical composition, size, and shape. As the disadvantages of polydispersity are ruled out, informative structure−property relationships of metal clusters can be established. The formation of a high-nuclearity metal cluster involves the organization of metal ions into a complex entity in an ordered way. To achieve controllable preparation of metal clusters, it is helpful to introduce a directing agent in the formation process of a cluster. To this end, anion templates have been used to direct the formation of high nuclearity clusters. In this Account, the role of anions played in the formation of a variety of silver clusters has been reviewed. Silver ions are positively charged, so anionic species could be utilized to control the formation of silver clusters on the basis of electrostatic interactions, and the size and shape of the resulted clusters can be dictated by the templating anions. In addition, since the anion is an integral component in the silver clusters described, the physical properties of the clusters can be modulated by functional anions. The templating effects of simple inorganic anions and polyoxometales are shown in silver alkynyl clusters and silver thiolate clusters. Intercluster compounds are also described regarding the importance of anions in determining the packing of the ion pairs and making contribution to electron communications between the positive and negative counterparts. The role of the anions is threefold: (a) an anion is advantageous in stabilizing a cluster via balancing local positive charges of the metal cations; (b) an anion template could help control the size and shape of a cluster product; (c) an anion can be a key factor in influencing the function of a cluster through bringing in its intrinsic properties. Properties including electron communication, luminescent thermochromism, single-molecule magnet, and intercluster charge transfer associated with anion-directed silver clusters have been discussed. We intend to attract chemists’ attention to the role that anions could play in determining the structures and properties of metal complexes, especially clusters. We hope that this Account will stimulate more efforts in exploiting new role of anions in various metal cluster systems. Anions can do much more than counterions for charge balance, and they should be considered in the design and synthesis of cluster-based functional materials.
1. INTRODUCTION The term cluster coined by F. A. Cotton in the early 1960s refers to compounds containing metal−metal bonds. As the advance of cluster chemistry, metal clusters evolve to have a broader definition including multi-metal-center aggregates linked by bridging ligands (even without metal−metal bonding).1 Metal clusters are big molecules with dimension up to nanometer scale. Different from nanoparticles, each cluster of a macroscopic sample has a well-defined structure with identical composition, size and shape. Thus, the disadvantages of polydispersity are ruled out, which is very important for the study of structure−property relationships. The formation of a high-nuclearity metal cluster is a complex process involving multicomponents, and it is challenging to organize many metal centers together in an ordered way; that is, a controllable preparation is demanding. If a directing agent can be introduced in the formation process of a cluster, one will © XXXX American Chemical Society
be able to prepare predictable cluster species. To this end, anion templates2 have been used to induce the formation of high nuclearity clusters. In contrast to the extensively studied cationic or neutral templates, anions functioning as directing agents in synthetic chemistry have not been attracted much attention. In the past decade, the role of anions in determining the structures and properties of silver clusters appears very important. The role of the anions can be threefold: (a) an anion template could help control the size and shape of a cluster product; (b) an anion is advantageous in stabilizing a cluster via balancing local positive charges of the metal cations; (c) physical properties can be brought in the cluster system when functional anions are employed. In this Account, we will Received: January 8, 2015
A
DOI: 10.1021/acs.accounts.5b00007 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
clusters have been isolated with the templation of various anions including Cl−, CO32−, SO42−, CrO42−, and even polyoxometalates. X-ray single crystal structure analysis has revealed that these silver alkynyl clusters have a general structural type as illustrated in Chart 2, which consists of a spherical silver cage, an inner anion (X) as the directing agent, peripheral alkynyl ligands, and counterions (Y).
survey the development of silver cluster chemistry in terms of the anion directing effects.
2. ANION LIGATION WITH METAL CATIONS Traditionally, coordination chemistry deals with a metal complex containing a metal center surrounded by neutral or anionic ligands. In supramolecular chemistry, “anion coordination chemistry” refers to the recognizing of anions with cyclic organic ligands through hydrogen bonds.3 However, the investigation of the case regarding of an anion ligated to metal cations has not attracted much attention until recent years. The understanding of possible coordination modes of an anion, that is, how an anion is ligated by metal cations, can shed important light on the design and preparation of clusters and coordination networks. In 1999, Mak and Guo proposed the highest ligation number (HLN) concept of an anion.4 The silver(I) ion has been utilized to probe HLN, because of its various coordination preferences and the argentophilic interaction that makes extra contribution to the structural stability. A number of novel ligation modes of various anions including nitrate, carboxylate, cyanide, thiocyanate, azide, and all-carbon anions (C22−, C42−, C62−, C82−) have been revealed.4−11 Some of the ligation modes with high ligation number are illustrated in Chart 1.
Chart 2. General Structure of Anion Templated Silver Alkynyl Clusters
3.1.1. Simple Anions as Templates. The first aniontemplated synthesis of a silver alkynyl cluster could be traced back to 2001. In attempt to prepare a silver(I) complex analogous to the gold(I) catenate {[AuCCBut]6}2, Vilar and Mingos et al. isolated accidentally a novel rhombohedral silveralkynyl cage compound [Ag14(CCBut)12Cl]OH.14,15 This silver(I) cluster consists of 14 silver(I) ions and 12 tertbutylacetylide ligands and an enclosed chloride ion. The silver(I) centers are in an arrangement of a near-regular rhombic dodecahedron of Oh symmetry (Figure 1). In the cage,
Chart 1. High Ligated Modes of Various Anions
3. ANIONS AS SYNTHETIC TEMPLATES IN CONSTRUCTION OF SILVER CLUSTERS Cation templates have been extensively used in the synthesis of organic macrocycles, e.g. the potassium ion enhances the formation of crown ether 18-C-6. Similarly, anion templates can be used in the synthesis of silver clusters. Silver ions are positively charged, so anionic species could be utilized to control the formation of silver clusters, and the size and shape of the resulted clusters can be dictated by the templating anions. 3.1. Silver Alkynyl Clusters with Anion Templates
Figure 1. Halide-templated assembly of the rhombic silver alkynyl clusters [Ag14(CCBut)12Cl]BF4. Color code: Ag, purple; Cl, green; C, gray. Hydrogen atoms have been omitted.
Alkynyl ligands are versatile in terms of the coordinating abilities, as the triple bond of an alkynyl ligand may coordinate to metal centers in σ or/and π modes.12 Alkynyl ligands have been used for the preparation of silver coordination compounds, because of their strong affinity to silver ions. However, simple metal alkynyls are insoluble in common solvents, which hinders the studies of their structures and properties. It was found that the addition of excess soluble silver salt could disrupt the polymeric structures of silver alkynyls. With the soluble precursors,13 a series of silver alkynyl oligomers have been synthesized. Furthermore, a series of silver
the Cl···Ag distances between 3.116(2) and 3.297(1) Å are much longer than those of conventional Ag−Cl bonds (ca. 2.6 Å). Analogues with fluoride and bromide could also be prepared. Electrospray mass measurements demonstrated the retention of the cages (Cl− and Br− but not F−) in methanol solution. The anion templating effect is clearly important in the formation of the cages. In addition, weak argentophilic interactions are also believed to be significant for the assembly and the stability of the structures. B
DOI: 10.1021/acs.accounts.5b00007 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
The templating effect of carbonates in silver alkynyl clusters was discovered in 2009.17 In the presence of tetramethylethylenediamine (TMEDA), the reaction of ButCCAg with AgOTf in methanol in air gave [Ag17(CCBut)14CO3]OTf in good yield. Structural determination revealed that the cation [Ag17(CCBut)14CO3]+ consists of 17 silver atoms linked by 14 alkynyl ligands, and there is a templating carbonate ion enclosed in the middle (Figure 3). The Ag−O bond distances are between 2.39 and 2.84 Å. There are totally 14 ButCC ligands around the cluster, and each bridges three silver atoms. In addition, it is found that the nuclearity of the product can be controlled by using a different counterion, probably as a result of packing effect. When AgBF4 was used in place of AgOTf, a new silver cluster [Ag19(CCBut)16CO3]BF4 was isolated. Inside this Ag19 cluster, the carbonate is coordinated with Ag− O distances being 2.45−2.86 Å. The cage is held by 16 μ3ButCC ligands. The carbonate is formed through the fixation of carbon dioxide from the air. The yield of [Ag19(CCBut)16CO3]BF4 can be significantly improved from 18% to 88% by deliberately adding potassium carbonate. The existence of carbonate in the clusters was unambiguously proved by solid-state 13C NMR spectra, where chemical shifts of 158.7 and 159.9 ppm corresponding to carbonate anions were found, respectively. The importance of the templates was proved by the fact that the isolation of the clusters were unsuccessful in the absence of CO2. Based on these serendipitous findings, a facile anion templating approach to synthesize high nuclearity silver alkynyl clusters was proposed as illustrated in Chart 2.18 Tetrahedral anions such as chromate and sulfate were chosen as the directing agents to demonstrate this synthetic method. Cluster [Ag22(CCBut)18CrO4](BF4)2 was prepared through the reaction of [ButCCAg]n with AgBF4 followed by adding K2Cr2O7 in the presence of TMEDA. Under the reaction condition (pH ∼ 8), Cr2O72− ions were transformed to CrO42− ions. The incorporation of chromates into the silver system could be easily verified as a yellow solution formed (characteristic of CrO 4 2− ) and yellow crystals of [Ag 22 (C CBut)18CrO4](BF4)2 were isolated. The existence of CrO42− was confirmed by IR bands at 898 and 835 cm−1. The cation [Ag22(CCBut)18CrO4]2+ consists of 22 silver atoms, 18 ButCC ligands and one chromate ion (Figure 4a). The
Although it was known that anionic species are important in promoting the assembly process of a silver alkynyl cluster, little advance has been made until 2008. Interestingly, another preparation of a chloride-templated silver alkynyl cluster by Wang and Bian was also a serendipitous result.16 With a chloride as the directing anion, 19 silver atoms form a doublecaged cluster with a snowman-like structure. The cluster is stabilized by peripheral ligands including 11 tert-butylacetylides and seven trifluoroacetates (TFA). One chloride is situated in the middle (Figure 2). There are four TFA ligands on the upper
Figure 2. Chloride-templated assembly of the silver double cage.
rim, each connecting a pair of silver ions. The middle part of the silver skeleton is bridged by three TFAs. The skeleton consists of two face-sharing silver cages. Cage A is a relatively regular square antiprism, and cage B has 15 silver atoms forming a pentacapped pentagonal prism. This cluster contains three types of polygons: triangle, tetragon, and pentagon. A templating chloride is located in the middle of cage B, which is believed to be derived from the CHCl3 solvent. In cage B (Cl@ Ag15), significant longer Ag···Cl distances 2.94−3.847 Å are observed in comparison with those found in [Ag14(C CBut)12Cl]BF4, because Cage B is a larger cluster than the rhombohedral cage Cl@Ag14. The crucial role of chloride is evident that running the reaction in nonchlorinated solvents resulted in the formation of one-dimensional silver alkynyl polymers.
Figure 3. Carbonate-templated assembly of the silver alkynyl clusters. C
DOI: 10.1021/acs.accounts.5b00007 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 5. (a) Structure of the Mo6O228− anion and its polyhedral presentation. (b) Core structure of [Ag60(Mo6O22)2(ButCC)38](CF3SO3)6. Green edge-sharing octahedra stand for Mo6O228−, and purple balls are silver atoms; Ag···Ag contacts (
