Oxalate chelation

Many of the usual physical techniques are of limited use for the identification of these various coordination modes, but two reactions that have aided characterization are those of protonation and reduction. The lack of protonation of the rf oxalate chelate [Co(NH3)4(C204)]+ contrasts with... 

Ca2 -chelators (B) prevent the enzymatic activity of Ca2+-dependent factors they contain COO groups that bind Ca2+ ions (C) citrate and EDTA (ethylenediaminetetraace-tic acid) form soluble complexes with Ca2+ oxalate precipitates Ca2+ as insoluble calcium oxalate. Chelation of Ca2+ cannot be used in vivo for therapeutic purposes because Ca2 concentrations would have to be lowered to a level incompatible with life (hypo-calcemic tetany). These compounds (sodium salts) are, therefore, used only for rendering blood incoagulable outside the body. This effect can be reversed at any time by addition of Ca2 ions. 

Oxalate chelation has long been used as a diagnostic tool in coordination chemistry, and consequently the complex oxalates have been extensively investigated. Since Werner s resolution of K3[Cr(0204)3] with strychnine in 1912, more resolution and racemization studies have been made of oxalato complexes than of any other class of coordination compounds. 

The first type of catalytic activity is better known and is illustrated by numerous examples, which include reactions in which the metal ion changes valence and reactions in which no change in the oxidation state of the metal takes place. Examples of metal-catalyzed redox reactions are the oxidation of oxalate through the formation of the 1 1 Mn(II) oxalate chelate compound, described by Taube (1), and the oxidation of ascorbic acid 2, 3) by chelation with the Cu(II) ion ... 

The examples in reactions (3.47), (3.49) and (3.51) can be viewed as intermolecular nucleophilic attack of coordinated OH on an electrophile. The same process can occur intramolecularly, as in the case of oxalate chelate ring opening studied by Andrade and Taube, whose isotope tracer results are sununarized by... 

Formation of [Cr(edta)] from the reaction of edta with the cis-[Cr(ox)2(OH2)2] ion has been studied in alkaline solutions. Both the uncatalyzed and HCOJ-catalyzed reactions were investigated. The initial step involves deprotonation of a coordinated water molecule, or its reaction with HCO J ion, followed by oxalate displacement by Hedta , either directly or via oxalate chelate-ring opening. ... 

Fig. 14. Ball and stick model of a silica-oxalate chelate on a quartz surface. The coordination of some organic acids on the surface would both alter the geometry at the chelated silicon center, and polarize framework bonds, thereby decreasing the energy barrier associated with hydrolysis and increasing dissolution rate...

Of nutrient chelates in the human diet, oxalates and phytates are the most common. OxaUc acid (8), found principally in spinach, rhubarb leaves, beet leaves, some fmits, and mushrooms, is a primary chelator of calcium. Oxalate present in pineapple, kiwifmit, and possibly in other foods, occurs as calcium... 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

Ethylenediamine tetraacetic acid (EDTA) [60-00-4] (Sequestrene), an anticoagulent at 1 mg of the disodium salt per mL blood, complexes with and removes calcium, Ca ", from the blood. Oxalate, citrate, and fluoride ions form insoluble salts with Ca " and chelate calcium from the blood. Salts containing these anticoagulants include lithium oxalate [553-91-3] 1 mg/mL blood sodium oxalate [62-76-0]2 mg/mL blood ... 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The pH effect in chelation is utilized to Hberate metals from thein chelates that have participated in another stage of a process, so that the metal or chelant or both can be separately recovered. Hydrogen ion at low pH displaces copper, eg, which is recovered from the acid bath by electrolysis while the hydrogen form of the chelant is recycled (43). Precipitation of the displaced metal by anions such as oxalate as the pH is lowered (Fig. 4) is utilized in separations of rare earths. Metals can also be displaced as insoluble salts or hydroxides in high pH domains where the pM that can be maintained by the chelate is less than that allowed by the insoluble species (Fig. 3). 

In addition to the oxide carboxylates, beryllium forms numerous chelating and bridged complexes with ligands such as the oxalate ion C204 , alkoxides, /9-diketonates and 1,3-diketonates. These almost invariably feature 4-coordinate Be... 

A few cases of optical isomerism are known for planar and tetrahedral complexes involving unsymmetrical bidentate ligands, but by far the most numerous examples are afforded by octahedral compounds of chelating ligands, e.g. [Cr(oxalate)3] and [Co(edta)] (Fig. 19.13). 

Complexes of cobalt(III) with O-donor ligands are generally less stable than those with Af-donors although the dark-green [Co(acac)3] and M iCo-( 204)3] complexes, formed from the chelating ligands acetylacetonate and oxalate, are stable. Other carboxylato complexes such as those of... 

Meyers has demonstrated that chiral oxazolines derived from valine or rert-leucine are also effective auxiliaries for asymmetric additions to naphthalene. These chiral oxazolines (39 and 40) are more readily available than the methoxymethyl substituted compounds (3) described above but provide comparable yields and stereoselectivities in the tandem alkylation reactions. For example, addition of -butyllithium to naphthyl oxazoline 39 followed by treatment of the resulting anion with iodomethane afforded 41 in 99% yield as a 99 1 mixture of diastereomers. The identical transformation of valine derived substrate 40 led to a 97% yield of 42 with 94% de. As described above, sequential treatment of the oxazoline products 41 and 42 with MeOTf, NaBKi and aqueous oxalic acid afforded aldehydes 43 in > 98% ee and 90% ee, respectively. These experiments demonstrate that a chelating (methoxymethyl) group is not necessary for reactions to proceed with high asymmetric induction. 

As seen in the preceding chapters, macrocyclic polyamines such as [18]aneN6 can be cation chelators, anion chelators, or both depending on the conditions. It was expected that with these dual properties, the macrocyclic polyamines might serve as litholytic agents by removing Ca2+ and phosphate or oxalate anions from insoluble calculi. 

Some ligands have more than one atom with an unshared pair of electrons and hence can form more than one bond with a central metal atom. Ligands of this type are referred to as chelating agents the complexes formed are referred to as chelates (from the Greek chela, crab s claw). Two of the most common chelating agents are the oxalate anion (abbreviated ox) and the ethylenediamine molecule (abbreviated en), whose Lewis structures are... 

Entry into the m-diammine system (Figure 2.50) uses the chelating ligand oxalate, as with the ammines use of NaBH4 as catalyst speeds this up. 

One study has been made with a chelate complex, tran5-l,2-diaminocyclohexane-tetraaceatomanganate(III), which is either pentadentate or hexadentate, a water molecule occupying the sixth or seventh coordination position respectively, and hence chelation of the oxalate is very unlikely. The reaction is first-order both in oxidant and oxalate and is retarded by increase of acidity. The HC204 ion is, therefore, attacked more slowly than the 204 " ion but both forms are oxidised as follows... 

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