Cationic clusters synthesis

A specific feature of reactions occurring in the autoclave is that the least soluble compounds are always precipitated from the homogeneous phase of the reaction. As a result, the equilibrium of the reaction is always shifted to the formation of these very insoluble compounds. Thus, it becomes clear that by varying the composition of the reaction mixture (mainly due to the introduction of new cations and anions) practically all types of the cluster forms being generated in the given system can be obtained in the solution. This is a clear advantage of the hydrothermal technique for cluster synthesis in the autoclave. 

Figure6.32 Structure of the cationic cluster complex [Eu4(p.3-0H)4(nic)6(H20)8]2 + (nic= nicoti-nate). Hydrogen atoms are removed for clarity [57]. (Redrawn from X. Kong et al., Hydrolytic synthesis and structural characterization of lanthanide hydroxide clusters supported by nicotinic acid, Inorganic Chemistry, 48, 3268-3273, 2009.)...

Proton formation in cluster synthesis and Hj reduction of metal cations-oxide cluster (M O ) were reported, and it was suggested that the protons are attached to zeolite interior lattice oxygen, giving acidic OH groups (3800-36(X) cm ) which may accommodate the prepared cluster anions inside zeolite cages ... 

Table 2. Gaseous heats of formation, Ai/ab[AX ], for a number of Lewis acid-base AX complexes of relevance for the synthesis of cationic clusters.

The oxidizing superacids are versatile and effective for the generation of clusters in solution but severe difficulties are associated with crystallization and recovery of solid cluster phases. These difficulties were to a great extent overcome by the observation that the pentaffuorides of antimony and, even better, arsenic in super-acidic or inert solvents quantitatively oxidize post-transition elements to cationic clusters. The method was presented in 1969 and was first used in the synthesis of the (AsFfi)" salt of (Ss)" 4 in HF solution, according to reaction (7). ° ... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

There have been many recent studies in support of this mechanistic approach. Stepwise reductive formation of Ag3+ and Ag4+ clusters has been followed using spectroscopic methods by Henglein [33], Reduction of copper (II) to colloidal Cu protected by cationic surfactants (NR4+) through the intermediate Cu+ prior to nucleation of the particles [36] as monitored by in situ x-ray absorption spectroscopy is another example. The seed-mediated synthesis also serves as evidence in support of this mechanism [38-41],... 

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