Reactions of Coordinated Ligands

It has been known for decades that a range of reactions occurs that involve chemistry of the ligands and in which metal–ligand bond cleavage is not involved. We can regard these as reactions of coordinated ligands. These early and deceptively simple studies provide fine examples of chemical detective work. One of the earliest studies probed the preparation of a H₂O-Co (III) species from a O₂CO-Co (III) precursor a reaction which was seen to

Figure 6.7

An example of a self-assembly reaction of a monomeric complex and ligand, forming a macrocyclic tetranuclear complex.

release CO₂ gas. Two quite different options for this reaction are: dissociation of carbonate ion from the complex followed by release of CO₂ from decomposition of the released anion, with a solvent water molecule entering the coordination sphere in its place; or else cleavage of a C-O bond on the coordinated ligandto release CO₂ while leaving the residual O atom bound to cobalt, with di protonation of the residual bound oxygen dianion to form coordinated water. In the former case a completely different ligand is inserted, making it a traditional substitution reaction; in the latter case part of the original ligand remains behind with the metal–donor atom bond staying intact making it a different class of reaction which we now define as a reaction of a coordinated ligand. The key to distinguishing these reactions was to use isotopically-labelled 18OH₂ as solvent, which showed that the product contained just the normal dominantly 16OH2 coordinated with no entry of 18OH₂ into the coordination sphere– the CO₂ must depart from the coordinated carbonate, as in (6.40).

Not only can this type of reaction occur for this and a range of other coordinated oxyanions, but also what is effectively the reverse reaction can occur, where the reactive species is a HO− Co(III) complex, which as the 18O-labelled form retains the label essentially ex clusively in the product, proving that it is a reaction between the nucleophilic coordinated hydroxide and an electrophile, as exemplified for the following well-known reaction that produces coordinated nitrite ion (6.41). In this case the formation of the thermodynamically unstable O-bound nitrite rather than the thermodynamically stable N-bound nitrite is sup porting evidence. The formed O-bound form does spontaneously isomerize to the N-bound form in a relatively slow isomerization reaction, but the rate is sufficiently slow that the O-bound form can be isolated readily.

Other reactions of simple anions that may occur in the absence of coordination can also be observed to occur for the complexed form, with the rate of reaction usually changed significantly as a result of complexation. This is anticipated since a coordinated ion is bonded directly to a highly-charged metal ion which must influence the electron distribution in the bound molecule and hence its reactivity. Two well-known examples where the product is ammonia occur through either reduction of nitrite with zinc/acid or oxidation of thiocyanate with peroxide. The former example is exemplified in (6.42) below.

Transition metal complexes can promote reactions by organizing and binding substrates. We have already seen this in terms of metal-directed reactions. Another important function is the supply of a coordinated nucleophile for the reaction, which is incorporated in the product. We have already seen a coordinated nucleophile at work in the reaction discussed above of Co −-OH with NO+; nucleophiles, which are electron-rich entities, are best represented in coordination chemistry by coordinated hydroxide ion, formed by proton loss from a water molecule; this is a common ligand in metal complexes. Normally, water dissociates only to a very limited extent. Via

for which we define Kw = [H+] [−OH]/ [H₂O] and for which pKw ≈ 14. However, when bound to a highly-charged metal ion, its acidity is very significantly enhanced, to the extent that, at neutral pH, a coordination complex will have a significant part of its M-OH2 present as M −OH .via

Although the coordinated hydroxide is a slightly worse nucleophile than free hydroxide, due to electronic effects of the bonded metal cation, its substantially higher concentration in the bound form at any pH more than compensates. A coordinated water molecule with a pKa of7willbe50%in the hydroxide form at neutral pH for example. Importantly because it can often be placed adjacent to a bound substrate (thus pre-organized for reaction) it is very effective, and marked catalysis is commonly observed. Although the most important, hydroxide is not the sole example of a coordinated nucleophile met in coordination chemistry. The next most important as a result of the prevalence of ammonia as a ligand, is the amide ion. Ammonia is usually thought of simply as a base  but it has the capacity to lose protons and thus act as an acid,

although this reaction has such a high pKa that it is not of significance for free ammonia in water. However, as for coordinated water, acidity of ammonia is significantly enhanced through coordination

so that sufficient concentrations of bound amide can form to permit reaction. Overall the coordinated amide anion is a far better nucleophile than free amide ion. Alkylamines can show the same activity as nucleophiles,

as long as they have at least one amine hydrogen atom to release as a proton. Reactions of coordinated ligands with organic substrates usually occur where the organic molecule enters the coordination sphere in a position adjacent to the nucleophile, and the subsequent reaction involves attack of the coordinated nucleophile at a relatively electron deficient site on the organic substrate. These reactions lead to a new organic molecule that is usually chelated to the metal ion. This product may depart from labile complex centres through substitution by other ligands (providing a mechanism for repeating the reaction or catalysis), or else may occur with inert metal centres as a single stoichiometric reaction. These reactions can also induce a particular stereochemistry, and may be defined as stere o specific (producing exclusively one isomer) or stereoselective (producing an excess of one isomer). Selectivity can be introduced simply by preference for a particular conformation in a chelate ring equilibrium, as illustrated in Figure 6.8. Here the conformation (left hand side, Figure 6.8) is preferred and not the con formation (right hand side, Figure 6.8) of the chelate ring, as steric clashing (of the ring methyl substituent with other axial ligands on the complex) is minimized in the former. Any subsequent reaction will ‘carry forward’ this selectivity into the reaction outcome, leading to selectivity in the product. Either a coordinated −OH or −NH₂ group is able to initiate chemistry with appropriate ligands present in an adjacent (cis) site. This reactivity was probed in detail for several

Figure 6.8

Two conformations of a substituted ethylenediamine chelate ring. The left-hand conformer has the methyl substituent on the chelate ring displaced away from the rest of the molecule (equatorial), whereas the right-hand conformer places it in an axial disposition where it may clash with another axial ligand, which is thus unfavourable.

Figure 6.9

Proposed competing mechanisms for reaction of coordinated and free hydroxide with an adjacent coordinated peptide to form a new chelated amino acid ligand; the coordinated hydroxide (Path A) is more efficient.

decades commencing in the 1960s by the groups of New Zealander D.A. Buckingham and Australian A.M. Sargeson. The coordinated hydroxide ion in particular has been the subject of extensive studies. In aqueous solution, of course, free hydroxide ion is present, that can in principle compete with coordinated hydroxide ion. Buckingham studied peptide cleavage by an inert cobalt(III) complex, employing isotopic labelling to probe the origin and fate of oxygen atoms in the product. This work showed clearly that, while pathways for both internal and external hydroxide attack exist, the coordinated hydroxide is the significant player, consistent with its enhanced acidity and pre-organized location. The alternate processes proposed are illustrated in Figure 6.9. For a coordinated −NH₂ group in aqueous solution, there is no capability for competition as above that would lead to the same product, and hence the intramolecular nature of the reaction is clearly defined. The reactivity of this group is illustrated in the simple reaction with [Co(NH₃)₅(OPO₃R)]+ in aqueous base (Figure 6.10). Here, the nucleophile can attack at the adjacent and relatively electron-deficient P atom leading to a new N P bond forming. The phosphorane intermediate formed has in effect one too many bonds around the P, relieved by a P O bond breaking to release an alkoxide ion. Amore complicated example of a reaction featuring a coordinated −NR2 nucleophile is the reaction of N-bound amino acetaldehyde with a coordinated polyamine (Figure 6.11). In this molecule, there are three amine groups sufficiently close (in positions adjacent or cis to the aldehyde) to participate in reaction, and yet the reaction occurs at only one of these three– it is therefore regiospecific. We discussed regiospecificity with respect to substitution reactions earlier, and it is the influence of a trans group that was used as the example. In this case, the regiospecificity arises in the same manner, since the trans chloride ion makes the secondary amine group opposite significantly more acidic than those in the other two sites, so that it is deprotonated much more readily to produce a reactive −NR₂

Figure 6.10

Proposed mechanism for reaction of a deprotonated ammine ligand with an adjacent coordinated phosphate monoester, leading to formation of a new chelated amino phosphate ligand.

group. Further, this reaction is stereospecific, since a particular chirality is introduced at the newly created tetrahedral carbon centre. This is ascribed to specific hydrogen bonding interactions in the transition state holding the carbonyl oxygen in a particular orientation that carries through into the product, once the amide ion attacks the carbon of the carbonyl to form a new N-C bond. Intramolecular hydrogen-bonding may often be important in directing stereoselectivity.

Stereospecific hydration of olefins is another reaction involving a coordinated hydroxide. This reaction with maleate monoester (Figure 6.12 top) involves essentially alkene hydration by the coordinated nucleophile, hydroxide. The reaction is

Figure 6.11

Proposed mechanism for reaction of a particular amine group (an example of regiospecificity) with a pendant aldehyde of a coordinated amino aldehyde leading to only one optical isomer of an aminol (an example of stereospecificity). The core reaction appears in the inside box.

Figure 6.12

Hydration of a pendant alkene by coordinated hydroxide (top), displaying specificity in site of attack, with the five-membered chelate ring only formed, and stereospecific attack of a deprotonated amine nucleophile (bottom) at the alkene of a chelate maleate ion. ring formation. With the transition state apparently important, it is difficult to be certain why there is exclusive formation of the five-membered ring, but which CH centre is more electron-deficient is likely to be involved. Whereas the maleate monoester met above can coordinate as a monodentate ligand through the one free carboxylic acid group, the diacid can employ both acid groups to bind as a didentate chelate, forming a seven-membered chelate ring. Chelated maleate dianion bound to Co(III) can also undergo intramolecular attack, but in this case the nucleophile is a deprotonated amine group of an adjacent chelated1.2-ethanediamine (Figure6.12 bottom) as no coordinated hydroxide is present. The reaction has a choice of two sites for attack (at either end of the C-C),but again there is stereospecificity observed. This specificity in ring formation may arise if we require in the transition state one carboxylate to be coplanar and thus conjugated with the diene. This leads to two discrete conformations, depending on which of the two carboxylate groups is coplanar. From examining models, it appears substantially more favourable for nucleophilic addition to occur at the ‘front’ CH group, as a result of the spatial orientation of the lone pair and the preferred conformation adopted by the chelate ring leading to a closer and appropriately directed approach at that site, depicted in Figure 6.12. It is notable that reaction does not occur between free maleate ion and free 1.2 ethane diamine although it does occur (but only very slowly) with maleate diester and 1,2-ethanediamine. Acceleration of the reaction of the maleate resulting from coordination to a metal ion is significant, and lies in the range 106–1010. Accelerations of this size are common for reactions of coordinated nucleophiles. The observation of stereospecificity and large accelerations suggests that these types of reactions may be relevant to the modes of action of certain metalloenzymes, where reactions are very rapid.

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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

Metal-directed Reactions

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)