Metal-directed Reactions

There exist both metal-catalysed and metal-directed reactions which require definition to distinguish the two classes. Metal-catalysed reactions are those in which the metal containing species in the reaction is regenerated in each reaction cycle so stoichiometric amounts are not required. It is the transition state of the catalysed reaction rather than the product, which is most strongly complexed by the metal. Thus, it is the rate of establishment of equilibrium (kf and kb) rather than the position of the equilibrium (K = kf/kb) that is altered (6.38).

Homogeneous transition metal catalysts usually employ their coordination sphere as the site of the chemistry they promote. One example is the rhodium catalyst used for promoting ethene hydrogenation. The keys to the process are an addition reaction of dihydrogen to the rhodium centre and a substitution reaction that also introduces ethene to the coordination sphere in place of a solvent molecule. It is in the intermediate produced that the former adds to the latter to produce ethane which then, as an exceedingly poor ligand, departs the coordination sphere and leaves vacancies for the process to occur again (Figure 6.4). Here, the catalyst is reused, many times.

Figure 6.4 A simplified catalytic cycle for the hydrogenation of ethene, which employs a rhodium(I) catalyst.

For a metal-directed reaction the product is formed as a metal complex and stoichio metric amounts of metal are consumed in the process (6.39).

Reactants + M→ [Products-M]

In virtually all cases, the driving force for metal-directed reactions is the stabilization associated with the formation of a chelating ligand from monodentate ligands, or the conversion of a weakly chelating ligand to a stronger chelator. Often the major product in the presence of the metal ion is not even detected from the same reaction in the absence of the metal ion. The metal has either caused an extreme displacement of an equilibrium, or promoted a new and rapid reaction pathway by complexation and stabilization of an otherwise inaccessible transition state. Normally metal-directed reactions refer to those reactions that are involved in synthesis of a larger organic molecule from smaller components with the product being an effective ligand. In fact these reactions result in the organic product being bound to the metal, which is therefore consumed in stoichiometric amounts. There are several general principles considered to be of importance in governing metal-directed reactions, the most important of which are:

Chelation– This is probably the most important factor. In nearly all cases, it is the formation of a (more) stable metal chelate as the primary reaction product that drives the equilibrium to favour that product. Lig and Polarization– Nucleophilic and electrophilic reactions of organic molecules (such as condensation hydrolysis alkylation and solvolysis amongst others) can be greatly enhanced by their coordination to metal ions as ligands. The metal ion can act variously as a Lewis acid-acid or-donor to alter electron density or distribution on the bound organic molecule (or ligand), thus altering the character of the ligand as a nucleophile or electrophile and hence its reactivity. Template Effects– The metal ion can act as an ‘organizer’ or ‘collector’ of ligands into arrangements around it that are most suitable for the desired reactions. There are, in addition, some other contributing effects. Enantiomer discrimination relates to the fact that ligand binding and reactivity can be affected by other ligands not actually participating in the reaction (the so-called ‘spectator’ ligands– like spectators at a football    game can ‘lift’ their teams’ performance without actually playing themselves, spectator ligands will influence what happens at reaction sites by their presence). If these spectator ligands are bulkyandoptically active, or if the metal centre is madechiralbyadissymmetric and rigid spectator ligand, differential binding of another chiral ligand, or stereospecificity of reactions, can be introduced. Metal ion lability is the ability of a metal ion to exchange its ligands rapidly, which is of importance in template reactions. Very slow exchange will effectively prevent substitution by reaction components and hence limiter actions occurring. However, this doesn’t mean that inert complexes are not relevant, as inert metal ions that retain chirality throughout are important for certain stereospecific syntheses. Redox effects may occasionally play a role. Metals in high oxidation states may act in some cases as stoichiometric oxidizing agents of a ligand functional group. Also, because varying the oxidation state may lead to more stable complexes, electron transfer reactions may be assisted.

Metal ions can direct spontaneous self-assembly of larger and often cyclic molecules from smaller components through the above effects. Nature does this exceptionally well but is not alone in being able to build cyclic molecules readily. Simple one-step comparable syntheses can be achieved in an open beaker, directed by a metal ion, as exemplified in Figure 6.5. This reaction occurs spontaneously in high yield when appropriate amounts of copper (II) nitrate 1.2-ethanediamine, aqueous formaldehyde and nitroethane are mixed in methanol in a beaker, warmed for a short period, and then stood overnight to allow crystallization. The metal ion acts as a ‘collector’ of ligands as well as a promoter of ligand reactivity by means of chelation and polarization effects. Areaction in the absence of a metal ion may differ completely from what occurs in the presence of a metal ion, as exemplified in Figure 6.6. The linear organic molecule formed in the presence of stoichiometric amounts of a metal ion is totally absent in the metal-free chemistry, where small heterocyclic ring formation occurs.

Figure 6.5

An example of a spontaneous self-assembling template reaction, in which small organic components are organized by the metal ion and undergo reaction to form a large cyclic organic product that includes the metal ion.

Figure 6.6

An example of the normal organic reaction route compared with the template reaction in the presence of a metal ion; a distinctly different path is followed. Metal-directed reactions have been used to prepare a wide range of cyclic and acyclic ligand systems. Often, they involve reaction of a coordinated amine with an aldehyde or ketone. Reaction of a carbon acid anion with an electron-deficient site is also commonly featured. Zinc(II)-directed condensation between an aldehyde (R CHO) and an aromatic nitrogen heterocycle (pyrrole) has been used to prepare substituted porphyrin rings (aro matic tetraaza macrocycles, analogous to hemes found as iron complexes in blood) since the 1960s. Once formed, the new ligands can be removed from the templating metal ion and different metal ions bound to it. This usually involves one of the following: treatment with acid to protonate the ligand and cause it to dissociate; reduction or oxidation of the metal to an oxidation state which will not bind the ligand effectively, allowing it to be re moved; treatment with a strongly-binding anion (such as CN−) that removes the metal ion competitively, leaving the free ligand; addition of another competing metal ion to which the ligand binds preferentially to a solution of the templated product, causing metal exchange (or ligand transfer to the added metal ion). Polynuclear complexes form through self-assembly also, where both the ligand and precursor metal complex geometry play a role in the outcome– the pieces tend to fit together in a particular way like children’s building blocks. An early example involves the self-assembly of 4.4-bipyridyl and [Pd(en)(ONO₂)₂]. The two cis-disposed O-bound nitrate ions are readily substituted by the preferred pyridine nitrogen donors, but they impose an L-shape when including the palladium, while the ligand imposes a rod-like shape; thus an assembly of four Pd ‘corner’ L-shapes and four ligand ‘rod’ shapes creates a square ‘picture frame’ shape which is now a large cyclic molecule (Figure 6.7). The product in the above reaction is of low solubility, and its precipitation from the reaction solution drives further formation, leading to a good yield of the product. The reaction is, in effect, a sequence of substitution steps with coordinated nitrate ions each replaced in turn by the pyridine nitrogen donors.

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مواضيع ذات صلة

Pretty in Red? - Colour and the Spectrochemical Series

Peak Performance– Illustrating Selected Physical Methods

Probing the Life of Complexes– Using Physical Methods

Organometallic Synthesis

Reactions of Coordinated Ligands

Reactions Involving the Metal Oxidation State

Catalysed Reactions

Chiral Complexes

Substitution Reactions without using a Solvent

Substitution Reactions in Nonaqueous Solvents

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)