Metal atom-solvent clusters

A similar method is to allow metal atoms to cluster in cold organic solvents. This method can be practiced on large scale with relatively low expense, and so has been widely practiced for almost 20 years, serving as a forerunner of all of those clustering methods. (25-30) Kinetic control of cluster growth may be realized, and unique structure/reactivity of such materials has been demonstrated many times.(4) Magnetic and electrical properties of such clusters are often unusual as well.(29,57)... 

However, a strong weakness of the QSAR approach is the lack of molecule-surface interaction description. Recently, attempts to model the molecule-surface interaction have been made. The surface was modeled by considering one single metal atom, a cluster (typically 10-20 atoms), or a periodic infinite surface in vacuum or in interaction with a solvent. Also the molecular coverage was considered, from one isolated molecule to one adsorbed layer. 

Colloidal solutions of gold in different solvents have been one of the most intensively studied and well-understood systems. Polar solvents such as acetone, dimethylformamide, tetrahydrofuran... and nonpolar solvents such as toluene, hexane, cyclohexane, decane... were broadly used as reaction media or solvent-madiated media. Acetone, as a polar solvent, solvates the metal atoms and clusters during the warmup stage [23]. In this way steric stabilization is achieved and some metal colloids can be stable for months. This behavior is the main motivation for choosing polar solvent as an initial solvent or co-stabilizer. Generally, the additional stabilizing agent such as alkylamine, alkylthiol, or alkylalcohol is mostly needed for the stabilization of final metal fluid. As reported [24, 25], two types of stabilization are characteristic for these systems ... 

Trivino GC, Klabunde KJ, Dale EB (1987) Living colloidal palladium in nonaqueous solvents. Formation, stability, and film-forming properties. Clustering of metal atoms in organic media. 14. Langmuir 3 986-992... 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

The complex forms purple air-stable crystals and has good solubility in both polar and nonpolar solvents (e.g., acetone, dichloromethane, tetrahydro-furan, benzene, and hexane). It has been characterized crystallographically.7 The structure is an open, but folded, ladder-like array of six metal atoms with the two platinum atoms in the center. The IR spectrum (hexane) exhibits V(co> bands at 2085 (m), 2062 (vs), 2035 (vs), 2016 (w). Pt2Ru4(CO)18 has been found to be very useful for the synthesis of new platinum-ruthenium cluster complexes.10,12,13... 

What is usually observed when metal atoms are codeposited with excess organic solvents at -196°C is the formation of a frozen matrix where the atoms are isolated and weakly solvated. Upon warming atoms begin to migrate in the cold liquid phase, and thousands of atoms cluster into particles of 4-9 nm. Continued warming and/or solvent evaporation leads to flocculation (without amalgamation) of these "monomer" clusters into super clusters or chains, and eventally yielding powders(29) or films(32-34). 

This method of preparation of supported metal catalyst requires a closed reactor to perform the preparation in the absence of water, so both the organic solvent and the oxide support must be carefully dehydrated. The method is based on the following principle the metal is evaporated and co-condensed with the organic to 77 K on the walls of the reactor. Under dynamic vacuum, the co-condensate is then warmed up to 195 K, and melted. The oxide support is impregnated with the solvated metal atom (cluster) at the same temperature, After a given time of contact, the slurry is warmed up to ambient temperature, and the solvent is eliminated, after which the sample can be dried. 

The section above described how alkali metal species such as solvated M+ ions and M ions can be formed and isolated within the lattices ofcertainhquid solvents. It is also possible to obtain single alkah metal atoms (M) and clusters (usually cationic ones, (M) +) in various solid hosts such as zeolites and graphite. There is considerable interest in clustered species since they are especially relevant to the question of what has been called the "metal/nonmetal transition, that is, how many atoms need to be gathered together before metallic (rather than molecular, nonmetalhc) properties become apparent The few electron alkah elements lend themselves to state-of-the-art... 

Figure 1. Schematic diagram of the energies of the anion ground state (M S), the lowest covalent state (M-S ), ion-pair state (M S ) and cation ground state (M+S ) versus /j M = alkali metal atom S = solvent molecules). The ion-pair state is expected to correlate with the ground state of the solvated electron plus the solvated M+ ion in bulk fluids. The ground-state electronic character of the neutral cluster changes from covalent to ion-pair type at a certain critical size.

---