Substitution Reactions in Nonaqueous Solvents

Water may not be a suitable solvent in all cases due to reactant insolubility (of the precursor complex and/or the ligand, but usually the latter), extreme inertness that demands use of a higher boiling point solvent for reaction, or high stability of undesired hydroxo or oxo species that prevent or interfere with formation of the desired products. This is usually solved by using another solvent, either a conventional molecular organic solvent or a low melting point ionic liquid. The aqueous chemistry of particularly Al(III) Fe(III) and Cr(III) involves formation of strong M O bonds and in basic aqueous solution hydroxide species (or unreactive oligomers) usually precipitate preferentially and rapidly because many added ligands are strong bases that cause a rise in solution pH. The behaviour of Fe (III) in a weakly basic aqueous solution is a good example and can be represented simplistically by (6.12).

Hydroxide is an effective ligand and where the hydroxo complexes are more stable than those of the added basic ligand, it is the role of the added ligand as a base, to promote formation of hydroxo complexes, that dominates its potential role as a ligand. This can be avoided simply by working in the absence of water, using instead an anhydrous aprotic solvent. For example using a hydrated salt of chromium(III)in water(6.13) or an anhydrous salt in anhydrous diethyl ether (6.14) with 1.2-ethanediamine as introduced ligand has distinctly different outcomes due to the absence of water in the latter reaction.

In the first case, the basic diamine effectively extracts protons from coordinated water molecules leaving an insoluble metal hydroxide rather than initiating any ligand substitution. In the aprotic solvent where no water is present, thus preventing formation of any hydroxo species, the diamine achieves coordination. It is also possible to prepare solvated salts other than hydrated ones for use. For example, [Ni(OH₂)₆] (CIO4)₂ may be replaced by [Ni(DMF)₆] (CIO₄)₂ in dimethylformamide (DMF) as solvent providing a way of en- suring that the initial complex coordination sphere is nonaqueous in form as well as the solvent.

Where solubility alone is the issue, simply changing solvent to permit all species to be dissolved allows the chemistry to proceed essentially as it would in aqueous solution were species soluble. Typical molecular organic solvents used in place of water include other protic solvents such as alcohols (e.g. ethanol) and aprotic solvents such as ketones (e.g. acetone) amides (e.g. dimethylformamide) nitriles (e.g. acetonitrile) and sulfoxides (e.g. dimethyl sulfoxide). Recently, solvents termed ionic liquids, which are purely ionic material that are liquid at or near room temperature, have been employed for synthesis; typically, they consist of a large organic cation and an inorganic anion (e.g. N.N'-butyl(methyl)- imidazolium nitrate) and their ionic nature supports dissolution of particularly ionic complexes.

In some case, where hydrolysis reactions are not of concern, mixtures or organic solvents and water can be employed. For example, it is possible to mix an aqueous solution of the

metal salt with a miscible organic solvent containing the organic ligand, with the two reactants sufficiently soluble in the mixed solvent to remain in solution and react. Further, it is possible sometimes to react an insoluble compound with a dissolved one with sufficient stirring and heating, since 'insoluble' usually does not mean absolute insolubility with sufficient dissolving to permit reaction, and its 'removal' from solution by complexation allowing further compound to dissolve and react.

Some complexes may not form or else are not stable in water because they are not thermodynamically stable in that solvent. Their formation in the total absence of water can be achieved however as exemplified by the sequence of reactions below where the neutral tetrahedral complex can be isolated readily and is stable in the absence of water, although dissolution in water leads to very rapid formation of the octahedral Co2+aq cation and dissociation of the initial ligands (6.15).

Neutral complexes of the type [M(acac)2] and [M(acac)3] where acac− is the 2.4 dioxopentan-3-ido anion, usually show good solubility in organic solvents and serve as useful synthons, since they undergo ligand substitution reactions by other chelates fairly readily. Moreover, they tend to stabilize otherwise relatively unstable oxidations states; for example the Mn(III) complex [Mn(acac)3] (6.16) is very stable whereas the hydrated ion Mn3+aq readily undergoes a disproportionation reaction (6.17) in water (to Mn(II) and Mn(IV)). The former complex is itself readily prepared from reaction of MnCl2·4H2O and 2.4-dioxopentane (acacH) in basic solution with a strong oxidant (6.16).

Subsequent reaction of [Mn(acac)3] in an organic solvent with potential polydentate ligands offers a route to a wide range of complexes.

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مواضيع ذات صلة

Reactions Involving the Metal Oxidation State

Catalysed Reactions

Chiral Complexes

Substitution Reactions without using a Solvent

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)

Block d: Transition Metals

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

A New Body - Stereochemical Change

Lability and Inertness in Octahedral Complexes