Metal steric stabilization

Of all the known sites for metal-ion binding to the heteroatoms of DNA bases, G-N3 is the most elusive. The adjacent 2-amino group is often considered to offer steric hindrance to binding at this site. However, while this undoubtedly influences the chemistry it does not preclude binding. The tri-metalated [ [Pt(N]3(9-Et G N1,N3,N7)]5 compound has for many years been the only structurally characterized example of an N3-coordinated guanine (66). A second example has now been reported, the tetranuclear octacation 16 (56). In this complex both the N7 and N3 atoms are bound to Pd2+ (Fig. 22). The molecule presents an interesting new architecture for a guanine-tetramer. Such structures are well known in DNA chemistry and are almost inevitably metal-ion stabilized (67,68). 

Kitchens, C.L., McLeod, M.C. and Roberts, C.B. (2003) Solvent effects on the growth and steric stabilization of copper metallic nanoparticles in AOT reverse micelle systems. Journal of Physical Chemistry B, 107 (41), 11331-11338. 

Protective Colloids. Another approach in preparing and stabilizing metal colloids is by adsorption of macromolecules on their surfaces. A wide variety of materials have been used including gummy gelatinous liquids,(J 0) albumin,(27) Icelandic moss,(28) latex,(22) polyvinylpyrrolidone, (29) antibodies, ( 30 ) carbowax 20M, ( 31 ) polyvinylpyridine, (31 ) and various polymer-water/oil-water mixtures.( 2) These studies clearly indicate that "steric stabilization of metal colloids is also important (along with electronic stabilization).(33)... 

Small metal particles are unstable with respect to agglomeration to the bulk. At short interparticle distances, two particles would be attracted to each other by van der Waals forces and, in the absence of repulsive forces to counteract this attraction, an unprotected sol would coagulate. To counteract this, stabilization can be achieved in two ways electrostatic stabilization and steric stabilization. 

The term steric stabilization may also be used to describe protective transition-metal colloids with traditional ligands or solvents. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [16-18], thiols [19-22], amines [21, 23-26], oxazolines [27] or carbon monoxide [18] or (ii) weak interactions with solvents such as tetrahydrofur-an (THF) or various alcohols [18, 28-31]. 

Organic polymers are very often used for the stabilization of metal nanoparticles by providing a steric stabilizing effect Due to this embedding effect, it is generally considered that the diffusion of substrates through the polymer matrix can be limited. Nevertheless, some interesting results have been obtained. 

A promising strategy towards stable and catalyticaUy active metal colloids is their preparation inside the core of micelles formed by amphiphilic block copolymers. This strategy offers a number of advantages (i) micelles represent a nano-structured environment which can be exactly tailored by block copolymer synthesis (ii) polymers act as effective steric stabilizer ]36] (iii) metal leaching might be avoided (iv) micelles allow control over particle size, size distribution and particle solubility [37] and (v) micelles are also supposed to effect catalytic activity and selectivity [38]. 

Two general methods are available for tbe assembly of tbe sterically stabilized species 16-24, both starting from metallic derivatives of diazo compounds (147) (16). Tbe latter have two nucleopbilic centers and can, in principle, react with electrophiles at C giving the functionalized diazo compounds (148), or at N, which... 

In order to reduce quantitatively Co or Ni species to the zero-valence state the reaction must be carried out in boiling polyols (EG, PEG, mixture EG + DEG), i.e., at a temperature lying in the range 180-220°C. Owing to the low tendency of the metal particles to coalesce during the growth step, the addition of a polymer is not needed to ensure their steric stabilization. The metal precursor may be either very soluble in the polyol (acetate was used rather than chloride) or only slightly soluble (hydroxide, hydroxycarbonate). 

In view of the extensive documentation outlined above, the usefulness of the polarity alternation concept as a primary guide for evaluation of substituent effects can hardly be denied. The influence of a substituent on the ipso site has not been discussed in this article but an even more direct and important effect is implicit. Among the innumerable examples one may cite the preferential formation of geminal dimetallic species [5] in hydrometalation and carbometalation of vinylmetals and acetylenes. On the other hand, chemical systems are usually very complex, inter- and intramolecular forces including steric and stereoelectronic factors may dominate over polarity alternation. Thus, chelation by a proximal donor often directs metalation and stabilizes certain organometallic entities. In these instances the stability gaining from polarity alternation is overwhelmed. 

Metal nanoparticles are unstable with respect to agglomeration to the bulk, van der Waals forces between the particles attract them to such an extent that metal metal bond formation occms. The fact that nanoparticles can be kept in solution is due to either electrostatic or steric stabilization. For instance, the ruby red gold sols, prepared from [AuCLi]" and sodium citrate as reducing agent, are stabilized by an electrical double layer formed by adsorbed citrate and chloride... 

Figure 3 Different types of steric stabilization of metal nanoparticles. (a) polymer molecules on the particles surface (b) micelle (c) inverse micelle (d) ligand stabilized particle...

Both steric and electronic effects contribute to the magnitude of the formation constant. The effect of sterics are often assessed with the use of molecular mechanics calculations (Chapter 6) and electronic effects, by studying linear free energy relationships— the relationship between free energies and rates of complex formation. HSAB and CFT (or LFT) are used regularly to predict and rationalize metal complex stability. 

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