Protective ligand shell

Independent of the dimensionahty, the organization of metal nanopartides seemed to be linked with the existence of a protecting ligand shell that enveloped the particles surfaces. The most important reason for this condition was seen as a necessity to grow the arrangements from solution. 

One of the most attractive features of monolayer-protected AuNPs is the possibility for facile introduction of a wide variety of different thiolated molecules into the ligand shell. This may be achieved using different strategies (1) direct synthetic methods,97 124 170 (2) the so-called place-exchange reaction, (3) postsynthetic modifications, or (4) displacement of weak ligands (or protective agents) by functional ligands. 

Small molecules such as phosphines and alkane thiols stabilize metal nanoparticles in a very effective maimer. Very stable covalent metal-phosphorus or metal sulfur bonds lead to such strong ligand shells that in some cases the protected particles can even be isolated in solid state, which can never be done with electrostatically stabilized particles. The chemical nature of the protecting ligand molecules is responsible for the solubility of the particles. Thus, the use of organic solvents has become very useful for several reasons. Figme 3 shows a sketch of the three types of steric stabihzations of metal nanoparticles. 

Metal clusters and colloids cannot be isolated in an unprotected form, as coalescence processes set in immediately by contact between particles to give amorphous or polycrystalline powders. Therefore, colloids have always been used in highly diluted dispersions, in polymers or in matrices [1]. Clusters are well known as stable compounds if they are protected by a shell of appropriate ligands. To make colloids available as isolable molecules (e. g. for homogenous catalysts), one developed a method to stabilize them by a ligand shell similar to that of clusters [2-4]. Colloids thus became applicable for numerous chemical and physical investigations [2, 5]. Even bimetallic particles have been stabilized and made useful in various practical applications [5]. 

A 1 wt.% Pd 8 catalyst with di[2,9-(2-methylbutyl)]-l,10-phenanthroline-pro-tected clusters on active carbon hydrogenates acetophenone to phenylethanol at room temperature and at a pressure of 0.1 MPa of hydrogen quantitatively with a TOF value of 45.5 h". Substitution of the 1,10-phenanthroline by (-)-cinchoni-dine ligands leads to an almost complete deactivation of the catalyst. Even temperatures of 60 °C and hydrogen pressures up to 10 MPa do not change the catalyst s activity. On the other hand, the same (-)-cinchonidine-protected eight-shell Pd clusters catalyze the hydrogenation of various unsaturated carbonic acids. 

Ligand shell protective layer, allows to tune properties such as solubility... 

---