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, 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+.[2]
Segment of the Co(OAc)2(H2O) chain,[3] illustrating two modes for bridging carboxylate ligands.
Homoleptic complexes
Homoleptic carboxylate complexes are usually coordination polymers. But exceptions exist.
A molecular monocarboxylate is silver acetate, Ag2(OAc)2.
Mononuclear triacetates include derivatives of 1-adamantanecarboxylic acid, which have the formula [M(O2CC10H11)4]− (M = Co, Ni, Zn).[4]
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 ethylhexanoates and related metallic soaps.[5] These lipophilic complexes are used as catalysts in oxidation reactions, e.g. oil drying agents.
Synthesis
Many methods allow the synthesis of metal carboxylates. From preformed carboxylic acid, the following routes have been demonstrated:[6]
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:
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.
Applications
Metal naphthenates and ethylhexanoates
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[8][9] 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.<[9] Illustrative is the use of cobalt naphthenate for the oxidation of tetrahydronaphthalene to the hydroperoxide.[10]
65% Solution of cobalt(II) bis(2-ethylhexanoate) in mineral spirits, tilted vial to illustrate color and viscosity.
Active site of hemerythrin, an O2-carrying iron-carboxylate
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.[12]
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.[13]
Palladium(II) acetate
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).[14]
^Brignole, Alicia B.; Cotton, F. A. (1972). "Rhenium and Molybdenum Compounds Containing Quadruple Bonds". Inorganic Syntheses. 13: 81–89. doi:10.1002/9780470132449.ch15.
^Catterick, Janet; Thornton, Peter (1977). Structures and Physical Properties of Polynuclear Carboxylates. Advances in Inorganic Chemistry and Radiochemistry. Vol. 20. pp. 291–362. doi:10.1016/S0065-2792(08)60041-2. ISBN9780120236206.
^Zhang, Gao; Lin, Jian; Guo, Dong-Wei; Yao, Shi-Yan; Tian, Yun-Qi (2010). "Infinite Coordination Polymers of One- and Two-dimensional Cobalt Acetates". Zeitschrift für Anorganische und Allgemeine Chemie. 636 (7): 1401–1404. doi:10.1002/zaac.200900457.
^Fursova, E. Yu.; Romanenko, G. V.; Tolstikov, S. E.; Ovcharenko, V. I. (2019). "Mononuclear Transition Metal Adamantane-1-Carboxylates". Russian Chemical Bulletin: 1669–1674. doi:10.1007/s11172-019-2610-4. S2CID203592748.
^García-Rodríguez, Raúl; Hendricks, Mark P.; Cossairt, Brandi M.; Liu, Haitao; Owen, Jonathan S. (2013). "Conversion Reactions of Cadmium Chalcogenide Nanocrystal Precursors". Chemistry of Materials. 25 (8): 1233–1249. doi:10.1021/cm3035642.
^M. Landau. 1993. "Driers and metallic soaps", in J. Kroschwitz, ed., Kirk-Othmer Encyclopedia of Chemical Technology. New York: John Wiley & Sons. Vol. 8, pp. 432-445. doi:10.1002/0471238961.0418090512011404.a01
^Grennberg, Helena; Foot, Jonathan S.; Banwell, Martin G.; Roman, Daniela Sustac (2001). "Palladium(II) Acetate". Encyclopedia of Reagents for Organic Synthesis. pp. 1–35. doi:10.1002/047084289X.rp001.pub3. ISBN978-0-470-84289-8.
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![Molybdenum(II) acetate, illustrating the Mo-Mo quadruple bond.[1]](/en/File:Mo2(OAc)4.svg){kind=small}
![[CoO(acetate)]4, the "Das cubane"](/en/File:DasCubane.svg)
2(H2O).svg)
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metal complex with the EDTA anion
Ferric bis(iminodiacetate)
Active site of hemerythrin, an O2-carrying iron-carboxylate-acetate-tetrahydrate-3D-balls.png)
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