Kinetically inert metal centres

The use of kinetically inert cobalt(III) complexes has led to important developments in our understanding of the metal ion-promoted hydrolysis of esters, amides and peptides. These complexes have been particularly useful in helping to define the mechanistic pathways available in reactions of this type. Work in this area has been the subject of a number of reviews.21-24 Although most of the initial work was connected with cobalt(III), investigations are now being extended to other kinetically inert metal centres such as Rhin, lrni and Ru111. 

Both intramolecular and intermolecular attack by M—OHn+ species are well established for cobalt(III) and other kinetically inert metal centres (Section 61.4.2.2.3). However, reactions of this type are not as well defined with labile metal ions. In copper(II) complexes, the pKa values for coordinated water ligands usually fall within the range pKa 6-8. If coordinated hydroxide ion is an important nucleophile in copper(II)-promoted reactions, the reactions would be expected to become independent of [OH-] at pH 8 when the bulk of the complex was converted to the active hydroxo species. Studies of the pH dependence of a number of copper(II)-promoted reactions to such pH levels have been carried out and no evidence obtained for the production of catalytically active hydroxo complexes however, some reactions do proceed by this pathway. 

Figure 5-41. Ammine ligands co-ordinated to a kinetically inert metal centre may undergo rapid base-catalysed deuterium exchange reactions.

Although we approached this section in terms of kinetically inert metal centres, it is also possible to build such encapsulating ligands about relatively labile metal ions. For example, although the square-planar nickel(n) complex 7.2 may be formed, this can react with an excess of dimethylglyoximedihydrazone and formaldehyde to give the nickel(n) analogue of 7.3. 

Figure 8-2. The bromination of aniline in the complex [Cr(PhNH2)3Cl3] to give a complex containing 2,4,6-tribromoaniline. The use of a kinetically inert metal centre allows us to ensure that the reaction is one of the co-ordinated ligand, even though the organic product is the same as that obtained with the free ligand.

We have already touched on some aspects of inorganic reaction mechanisms kinetically inert metal centres such as Co(III) (Section 21.10) and organometallic reaction types (Section 23.7). Now, we discuss in more detail the mechanisms of ligand substitution and electron-transfer reactions in coordination complexes for the substitution reactions, we confine our attention to square planar and octahedral complexes, for which kinetic data are plentiful. 

Studies of Lewis acid catalysis have been carried out with kinetically labile metal ions (Cu(II), Ni(II), Zn(II) etc.) and with kinetically inert metal centres such as cobalt(III). The use of inert cobalt(III) complexes in kinetic studies presents many advantages as the metal complex is well defined and it is often possible to obtain more detailed mechanistic information from such systems. 

For the tris(amino acidato)metal(III) complexes four geometrical and chiral isomers are possible, as shown in Scheme 2, where NO is the chelated amino acid anion, mer and fac refer to the meridional and facial geometrical isomers and A and A refer to the configuration at the metal centre. For the glycine complexes A and A are an enantiomeric pair, while for the optically pure forms of the other amino acids they form a diastereomeric pair and hence are easier to separate. For most of the simple bidentate amino acids of the kinetically inert metal ions Cr" , Co " and Rh ", the four isomers have been obtained. The isomers are distinguishable by their UV/visible, CD and NMR spectra. Not all the isomers can be found in certain cases, for example, with L-proline the k-fac isomer could not be prepared, a fact which was predicted on steric grounds." ... 

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. 

Chromium(III) forms stable complexes with adenosine-S -triphosphate.840,841,842 These are kinetically inert analogues of magnesium ATP complexes and may be used to study enzyme systems. The complexes prepared are chiral and may be distinguished in terms of chirality at the metal centre (198,199).843 The related complex of chromium(lll) with adenosine-5 -(l-thiodiphosphate) has been prepared the diastereoisomers were separated.844 The stereospecific synthesis of chromium(III) complexes of thiophosphates has been reported845 by the method outlined in equation (47), enabling the configuration of the thiophosphoryl centre to be determined. The availability of optically pure substrates will enable the stereospecificity of various enzyme systems to be investigated.845... 

Figure 4-46. A metal-assisted Arbuzov reaction involving a kinetically inert cobalt(m) centre.

Template Condensations at Kinetically Inert Octahedral Metal Centres... 

What should we do to observe a three-dimensional template effect First, we should choose a reaction type that we know to be effective for the formation of macrocyclic ligands and extend the methodology to a kinetically inert cP or d6 metal centre. Let us reconsider the reaction, that we first encountered in Fig. 6-11. In this reaction, a dioximato complex reacted with BF3 to give the nickel(n) complex of a dianionic macrocycle (Fig. 7-1). 

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