Transition Metal Carboxylate Complex

Transition metal carboxylate complexes are coordination complexes with carboxylate (RCO₂⁻) ligands. Reflecting the diversity of carboxylic acids, the inventory of metal carboxylates is large. Many are useful commercially, and many have attracted intense scholarly scrutiny. Carboxylates exhibit a variety of coordination modes, most common are κ¹- (O-monodentate), κ² (O,O-bidentate), and bridging.

Acetate and related monocarboxylates

Structure and bonding

Carboxylates bind to single metals by one or both oxygen atoms, respectively κ¹- and κ²-. In terms of electron counting, κ¹- and κ²- are "X", a pseudohalide, and "L-X ligands", i.e. a combination of a Lewis base (L) and a pseudohalide (X). Carboxylates are classified as hard ligands, in HSAB theory.

File:BasicFeacetate.png, Basic ferric acetate
File:Ag2(OAc)2.png, Silver acetate
File:BasicZnAcetate.png, Basic zinc acetate
File:Mo2(OAc)4.svg, Molybdenum(II) acetate, illustrating the Mo-Mo quadruple bond.
File:DasCubane.svg, oO(acetate)sub>4, the "Das cubane"

For simple carboxylates, the acetate complexes are illustrative. Most transition metal acetates are mixed ligand complexes. One common example is hydrated nickel acetate, Ni(O₂CCH₃)₂(H₂O)₄, which features intramolecular hydrogen-bonding between the uncoordinated oxygens and the protons of aquo ligands. Stoichiometrically simple complexes are often multimetallic. One family are the basic metal acetates, of the stoichiometry ₃O(OAc)₆(H₂O)₃sup>n+.

Homoleptic complexes

Homoleptic carboxylate complexes are usually coordination polymers. But exceptions exist.

*A molecular monocarboxylate is silver acetate, Ag₂(OAc)₂.
*Molecular diacetates are more common. Several diacetates adopt the Chinese lantern structure. Well studied examples include the dimetal tetraacetates (M₂(OAc)₄) where M = Cu(II), Rh(II), Cr(II), and Mo(II). Platinum diacetate and palladium diacetate feature Pt₄ and Pd₃ cores, further illustrating the tendency of acetate ligands to stabilize multimetallic structures.
*Mononuclear triacetates include derivatives of 1-adamantanecarboxylic acid, which have the formula (O₂CC₁₀H₁₁)₄sup>− (M = Co, Ni, Zn).

Reactions and applications

Attempts to prepare some carboxylate complexes, especially for electrophilic metals, often gives oxo derivatives. Examples include the oxo-acetates of Fe(III), Mn(III), and Cr(III).

Metal acetates are common catalysts or precatalysts. Particularly useful are the lipophilic complexes of ethylhexanoates, which are used as catalysts in oxidation reactions.

Synthesis

Many methods allow the synthesis of metal carboxylates. From preformed carboxylic acid, the following routes have been demonstrated:
:acid-base reactions:
:protonolysis:
:oxidative addition:

From preformed carboxylate, salt metathesis reactions are common:
:

Metal carboxylates can be prepared by carbonation of highly basis metal alkyls:
:

Reactions

A common reaction of metal carboxylates is their displacement by more basic ligands. Acetate is a common leaving group. They are especially prone to protonolysis, which is widely used to introduce ligands, displacing the carboxylic acid. In this way octachlorodimolybdate is produced from dimolybdenum tetraacetate:
:

Acetates of electrophilic metals are proposed to function as bases in concerted metalation deprotonation reactions.

Pyrolysis of metal carboxylates affords acid anhydrides and the metal oxide. This reaction explains the formation of basic zinc acetate from anhydrous zinc diacetate.

In some cases, monodentate carboxylates undergo O-alkylation to give esters. Strong alkylating agents are required.

Di- and polycarboxylates

Benzenedi- and tricarboxylates

Metal organic frameworks, porous, three-dimensional coordination polymers, are often derived from metal carboxylate clusters. These clusters, called secondary bonding units (SBU's), are often linked by the conjugate bases of benzenedi- and tricarboxylic acids.

Aminopolycarboxylates

A commercially important family of metal carboxylates are derived from aminopolycarboxylates, e.g., EDTA^4-. Related to these synthetic chelating agents are the amino acids, which form large families of amino acid complexes. Two amino acids, glutamate and aspartate, have carboxylate side chains, which function as ligands for iron in nonheme iron proteins, such as hemerythrin.

References

{{Coordination complexes

Ligands
Coordination chemistry
Salts of carboxylic acids