Ligand substitution reactions transition metal complexes

Ruiz et al. (1989) studied ligand substitution in transition metal complexes. They found that the presence of a simple sodium salt completely changes the course of these reactions. One of the cases consists in the reaction of Fe(I) sandwiches with PMej in THE This reaction proceeds only in the presence of a sodium salt. 

Intramolecular aromatic substitution in transition metal complexes is an area of research where there is considerable current interest, and tri(phenyl-ds)-phosphine is a ligand often used in such investigations. This phosphine can be prepared from phenyl-ds-magnesium bromide by its reaction with PCI3 in diethylether, as described below. 

Ligand exchange in transition metal complexes, thermal and radiation-induced decomposition, pressure-induced reactions, electron exchange reactions, substitutional effects in sophisticated oxides... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

One of the commonest reactions in the chemistry of transition-metal complexes is the replacement of one ligand by another ligand (Fig. 9-3) - a so-called substitution reaction. These reactions proceed at a variety of rates, the half-lives of which may vary from several days for complexes of rhodium(iii) or cobalt(m) to about a microsecond with complexes of titanium(iii). 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

In this spirit, an attempt will be made to account for the magnitude of pressure effects on ligand substitution reaction rates. Attention will necessarily be confined to a few simple model systems two recent reviews (1, 2) of pressure effects on reactions of transition metal complexes in solution may be consulted for more comprehensive surveys of the field. 

The best enantioselectivity (35% ee) was observed in the reaction of l-(l-naphthyl)-2-propyn-l-ol with acetone in the presence of a complex bearing a 1-naphthylethylthio-lato moiety as a chiral ligand. Although the enantioselectivity is not yet satisfactory, this was the first example of an enantioselective propargylic substitution reaction catalyzed by transition metal complexes [27]. It is noteworthy that the chiral thiolate-bridged ligands work to control the chiral environment around the diruthenium site. 

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

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