and highlights the unique geometries of the metal cluster cores – geometries that are not present in bulk gold or larger gold particles.
Figure 5.4 Structures of Au8 (left), Au9 (middle), and Au11 (right) in space‐filled (top) and ball‐and‐stick visualizations with isolated cluster cores shown in the bottom row.
Source: Anderson et al. 2013 [39]. PCCP Owner Societies. CC BY 3.0
. (See online version for color figure).
It is worth highlighting how well the ligands can protect cluster metal cores, which are barely visible in the “space‐filled” (top row, Figure 5.4) structures – remember this for the discussion of catalyst activation later in the chapter.
If a mixture of clusters is obtained during the synthesis, the individual components of the mixture can sometimes be separated using chromatography [40]. If sufficient quantities of each individual cluster can be separated and collected, attempts to crystallize each cluster can be made (in order to resolve their structures using X‐ray crystallography). However, if crystallization fails, high‐resolution mass spectrometry can also be used to confirm the identity of the atomically precise clusters in conjunction with modeling of the expected exact mass of each cluster, as well as the corresponding isotopic and fragmentation patterns [40].
There is a family of materials related to metal clusters called metal colloids [41]. Both clusters and colloids have cores containing metal atoms bonded to each other that are surrounded by a protective layer. In the case of clusters, the layer is formed by ligands, while in the case of colloids the terms “capping agents” and “surfactants” are used more often to describe this layer. Figure 5.5 illustrates key features of both clusters and colloids with respect to metal core size and size distribution.
Figure 5.5 Illustration of the size regimes and particle size distributions of clusters and colloids. (See online version for color figure).
Clusters are atomically precise species (definitely in the light of the original definition by Cotton), while colloids have particle size distributions (i.e. particles with different sizes are present in a mixture). Clusters have metal core sizes ranging from two to hundreds of atoms, with sizes of larger clusters typically sub‐3 nm. A famous example of the large cluster with solved crystallographic structure is Au102 [42]. While colloids can have sizes just below 3 nm, most of the time their particle sizes are greater and can be as large as hundreds of nanometers. Ultrasmall colloids with narrow particle size distribution are often called clusters in the literature [43]. The reason to bring up this similarity is that two prominent examples of colloids were assigned cluster‐like formulas – Au55 and Au101 [44, 45] – whereas direct imaging by high‐resolution transmission electron microscopy (TEM) confirmed the existence of a particle size distribution [46]. Such species proved impossible to crystallize or to confirm their identity by high‐resolution mass spectrometry, but, although they are not truly atomically precise, they constitute an important complementary example of cluster‐like colloids.
The stability of classical organometallic complexes with one central atom is often explained based on the 18‐electron rule (think octet rule based on 2 + 6 electrons to fill s‐ and p‐orbitals, plus 10 electrons to fill d‐orbitals of the transition metal central atom). Similar electron‐counting rules such as effective atomic number rule (e.g. average of 18 electrons per metal atom in clusters) or Wade's rules (a better fit for delocalized bonding model) were successfully applied to relatively small metal carbonyl clusters [24]. The “magic‐number” concept was popular for rationalization of the high stability of some larger clusters. The idea behind such “magic numbers” is that closed‐shell, highly symmetric structures have a low free energy per atom. For example, Mackay icosahedra are uniquely defined by the number of closed shells around the initial central atom, with the total number of atoms per cluster with increase in number of shells being N = 13, 55, 147, 309, 561, 923, 1415, 2057, 2869, 3871, etc. The abovementioned assignment of the formula Au55 to a synthetically made gold cluster was inspired by this concept [44]. A very recent study demonstrated that, for larger (100+ atoms) cluster sizes, oscillation occurs with respect to the most stable structure between icosahedra, decahedra, and face‐centered cubic (typical for bulk gold) structures [47]. An important recent development was recently proposed – a unified view of ligand‐protected gold clusters as superatom complexes [48]. In this approach, exceptional stability is assigned to systems with electron counts corresponding to filling of shells (n = 2, 8, 18, 34, 58, 92, 138, etc.) and takes into account interactions of the cluster core and ligands. It was successfully applied to explain the stability of a wide range of clusters, including that of Au102 mentioned above [42, 48].
5.4 Catalysis Using the Chemically Synthesized Metal Clusters
Having made, purified, and fully characterized chemically synthesized, atomically precise clusters, one can start using these species in catalysis. There are a limited number of fundamental studies focused on chemically made clusters as model homogeneous catalytic systems in the gas phase by employing mass spectrometry approach [49, 50]. There are studies that used ligated clusters as homogeneous catalysts in the liquid phase [41, 51–53]. Lewis proposed an interesting criterion with respect to catalysis by clusters: “unusual or enhanced reactivity of a catalyst composed of two or more metal atoms which differs from the individual metal components indicates cluster catalysis” [41]. However, studies comparing mono‐atomic catalysts and cluster catalysts are rare. Recently, selective hydrogenation of α,β‐unsaturated ketones and aldehydes was demonstrated using thiol‐protected cluster Au25(SR)18 as a homogeneous catalyst, whereby 100% selectivity toward unsaturated alcohols was observed [54]. Classical use of chiral ligands to induce stereoselectivity employed in homogeneous catalysis with monometallic organometallic complexes was recently applied to metal cluster catalysts with some success [55]. In the case of larger metal clusters, the chirality of the system may arise from the specific geometry of both the metal core and non‐chiral ligands bonded to the core – an unexplored opportunity with respect to achieving stereoselective control using clusters [56].
However, care must be taken not to assign observed activity to intact clusters as the results of a recent study show that “very small gold clusters (3 to 10 atoms) formed from conventional gold salts and complexes can catalyze various organic reactions at room temperature, even when present at concentrations of parts per billion” and that such clusters formed, in situ, “give reaction turnover numbers of 107 at room temperature” [57]. This is an important issue since ligands bound to the cluster core stabilize clusters, help to solubilize them in an appropriate solvent, and, at the same time, should be blocking reagents from accessing metal core (as space‐filled images in Figure 5.4). Removal of ligands is likely to compromise the stability of the original clusters and could lead to formation of ultrasmall clusters, which could be responsible for the observed activity. A potential solution could be the use of stable, atomically precise nanoclusters with uncoordinated sites at the metal core, of which synthesis was only very recently developed
