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.

Structure and bonding
Carboxylates bind to single metals by one or both oxygen atoms, the respective notation being κ1- and κ2-. In terms of electron counting, κ1-carboxylates are "X"-type ligands, i.e., a pseudohalide-like. κ2-carboxylates are "L-X ligands", i.e. resembling the combination of a Lewis base (L) and a pseudohalide (X). Carboxylates are classified as hard ligands, in HSAB theory.

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 [M3O(OAc)6(H2O)3]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) including rhodium(II) acetate, copper(II) acetate, molybdenum(II) acetate, and chromium(II) acetate. 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 [M(O2CC10H11)4]− (M = Co, Ni, Zn).

Synthesis
Many methods allow the synthesis of metal carboxylates. From preformed carboxylic acid, the following routes have been demonstrated:
 * acid-base reactions: LnMOR' + RCO2H -> LnMO2CR + R'OH
 * protonolysis: LnMalkyl + RCO2H -> LnMO2CR + alkane
 * oxidative addition: LnM + RCO2H -> Ln(H)MO2CR

From preformed carboxylate, salt metathesis reactions are common:
 * LnMCl + RCO2Na -> LnMO2CR + NaCl

Metal carboxylates can be prepared by carbonation of highly basis metal alkyls:
 * LnMR + CO2 -> LnMO2CR

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:
 * Mo2(O2CCH3)4 + 4 HCl + 4 KCl -> K4[Mo2Cl8] + 4 CH3CO2H

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

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). 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.

Other carboxylates
Many carboxylates form complexes with transition metals. Alkyl and simple aryl carboxylates behave similarly to the acetates. Trifluoroacetates differ in mononuclear complexes because it is usually monodentate, e.g. [Zn(κ2-O2CCH3)2(OH2)2] vs [Zn(κ1-O2CCF3)2(OH4)2].

Metal naphthenates and ethylhexanoates

 * [[Image:Naphthensäure.svg|200px|thumb|Structure of a representative naphthenic acid, which forms a variety of complexes with transition metals.]]

Naphthenic acids, mixtures of long chain and cyclic carboxylic acids extracted from petroleum, form lipophilic complexes (often called salts) with transition metals. These metal naphthenates, have the formula M(naphthenate)2, or M3O(naphthenate)6, have diverse applications including synthetic detergents, lubricants, corrosion inhibitors, fuel and lubricating oil additives, wood preservatives, insecticides, fungicides, acaricides, wetting agents, thickening agent, and oil drying agents. Industrially useful naphthenates include those of aluminium, magnesium, calcium, barium, cobalt, copper, lead, manganese, nickel, vanadium, and zinc.< Illustrative is the use of cobalt naphthenate for the oxidation of tetrahydronaphthalene to the hydroperoxide.



Like naphthenic acid, 2-ethylhexanoic acid forms lipophilic complexes that are used in organic and industrial chemical synthesis. They function as catalysts in polymerizations as well as for oxidation reactions as oil drying agents. Metal ethylhexanoates are referred to as metallic soaps.

Aminopolycarboxylates
A commercially important family of metal carboxylates are derived from aminopolycarboxylates, e.g., EDTA4-. 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.

Metal organic frameworks (MOFs)
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.

Reagents for organic synthesis
Palladium(II) acetate has been described as being "among the most extensively used transition metal complexes in metal-mediated organic synthesis". Many coupling reactions utilize this reagent, which is soluble in organic solvents and which contains a built-in Bronsted base (acetate).

Dirhodium tetrakis(trifluoroacetate) is widely used catalyst for reactions involving diazo compounds.

Related topics

 * transition metal oxalate complex