Chelate ring formation

Figure 3.8 Chelate ring formation in metal complex azo dyes...

Reactions of selected metal complexes of multidentate amines with formaldehyde and a range of carbon acids (such as nitroethane) have led to ring-closure reactions to yield a series of three-dimensional cage molecules (see Chapter 3). Condensations of this type may also be used to produce two-dimensional macrocycles (Comba et al., 1986) - see [2.20], In such cases, it appears that imine intermediates are initially produced by condensation of the amines with formaldehyde as in the Curtis reaction. This is followed by attack of the conjugate base of the carbon acid on an imine carbon. The resulting bound (new) carbon acid then reacts with a second imine in a cis site to yield chelate ring formation. 

Chelate ring formation may be rate-limiting for polydentate (and especially macrocyclic) ligand complexes. Further, the rates of formation of macrocyclic complexes are sometimes somewhat slower than occur for related open-chain polydentate ligand systems. The additional steric constraints in the cyclic ligand case may restrict the mechanistic pathways available relative to the open-chain case and may even alter the location of the rate-determining step. Indeed, the rate-determining step is not necessarily restricted to the formation of the first or second metal-macrocycle bond but may occur later in the coordination sequence. 

KMLJKM(Lh coincides with the EM for the chelate ring formation from M L L. In the general case, the ratio whose common logarithm defines the chelate effect has units of Mopen chain w-dentate ligand. 

Rate constants for reaction of cis-[Pt(NH3)2(H20)Cl]+ with phosphate and with S - and 5/ -nucleotide bases are 4.6xl0-3, 0.48, and 0.16 M-1s-1, respectively, with ring closure rate constants of 0.17 x 10 5 and 2.55x10-5s-1 for subsequent reaction in the latter two cases 220). Kinetic aspects of interactions between DNA and platinum(II) complexes such as [Pt(NH3)3(H20)]2+, ds-[Pt(NH3)2(H20)2]2+, and cis-[Pt(NH3)2(H20)Cl]+, of loss of chloride from Pt-DNA-Cl adducts, and of chelate ring formation of cis-[Pt(NH3)2(H20)(oligonucleotide)]"+ intermediates implicate cis-[Pt(NH3)2(H20)2]2+ rather than cis-[Pt(NH3)2 (H20)C1]+, as usually proposed, as the most important Pt-binder 222). The role of aquation in the overall scheme of platinum(II)/DNA interactions has been reviewed 223), and platinum(II)-nucleotide-DNA interactions have been the subject of molecular modeling investigations 178). 

Examples of carbanion additions to cobalt-coordinated imines will be considered in the next section (7.4.3.2) as chelate ring formation is a consequence in some cases. 

A,TV-Dimethyl-A -phenylthiourea has been shown to coordinate to Rh111 as an N—S bidentate involving four-membered chelate ring formation.154 N-Substituted thioamides also may bond in this manner.155 156 l-Amidino-2-thioureas (44) may behave either as N—S or as N—N bidentates, with this donor choice being dependent mainly on pH and the nature of the metal ion.157 As N—S donors they are known to stabilize lower oxidation states.158 As part of a study on Mo—S-containing complexes as models for redox-active molybdoenzymes, Dilworth et al. have shown that some p-(substituted)phenylhydrazines may coordinate as N—S bidentates in three different ways to one metal atom.159 The three diazenido, diazene and hydrazonido forms vary in their degree of deprotonation and therefore their anionic nature. 

Source Data from Spike and Parry, Thermodynamics of chelation I. The statistical factor in chelate ring formation, J. Amer. Chem. Soc. 75 2726, 1953. 

Poly(ethyleneimine) cross-linked (CPEI) with ethylenedichloride forms stable complexes with copper (II) as well as with cobalt (II). The RC1 type of cross-linked poly(ethyleneimine) having an anion-exchange capacity of 6.8 meq g 1 retains copper from CuS04 and cobalt from 1 M aq. CoCl2 solutions [55], PEI is by itself a weak basic anion-exchange resin and forms stable complexes with anions and cations. The process is probably accompanied with chelate ring formation ... 

Later, Werner and Vilmos (20) took advantage of metal chelate ring formation in determining the stereochemistry of the dichloroethylene-diaminecobalt(III) chlorides. The isomer which was found to react readily with oxalate to give oxalatobisethylenediaminecobalt(III) chloride (IV) was considered to be the cis form because the steric requirements of the oxalato chelate ring are such that the oxalate oxygens must occupy cis positions in the coordination sphere of the cobalt(III) ion. 

Thus, it is seen that the effect described by Schwarzenbach has precise thermodynamic meaning—the change in the entropy of translation that accompanies metal chelate ring formation. The entropy effects estimated by Schwarzenbach, up to 2.0 log K units, agree quite well with the value obtained with the thermodynamic approximation. Experimentally, one would expect wide deviations from this value (7.9 entropy units per chelate ring) because of the variations in solvation and internal entropies of complexes and ligands that occur in the displacement reaction. 

Enthalpy Changes Associated with Chelate Ring Formation... 

This discussion of the data in Table II demonstrates the importance of obtaining enthalpies and entropies of reaction to gain an understanding of the nature of chelate ring formation in aqueous solution. In the absence of absolute structural proofs, the number of chelate rings formed in solution can only be inferred from thermodynamic data. The examples in Table II show that the ideas resulting from stability data alone may be misleading and can lead to different conclusions than one would draw on the basis of more complete thermodynamic data. 

Other coulombic forces involved in chelate ring formation. 

Effects are either unique for metal chelate ring formation, as contr ted to coordination by unidentate ligands, or are factors which are considerably different when chelation is involved. 

The above table contains a relatively complete list of the factors involved in determining enthalpies and entropies of metal chelate formation in aqueous solution. Of these, the effects designated by footnote a are either unique for metal chelate ring formation, as contrasted to coordination by unidentate ligands, or are factors which are considerably different when chelation is involved. 

Decisive properties of the phosphinomethanides are their stability towards reduction, the separation of the negative charge of the anionic ligand (centered at the formal carbanion) from the electron rich E(II) (E = Si, Ge, Sn) center by the phosphane donor atoms, the thus generated additional electrostatic stabilization and the four-membered chelate ring formation [1]. 

Metals such as iron and aluminum, which markedly catalyze nonenzymatic transaminations in vitro, probably do so by promoting formation of the Schiff base and maintaining planarity of the conjugated system thnrugh chelate ring formation. which requires the pre.scnce of the phenolic group. This chelated metal ion also provides an additional electron-... 

Macrocycles obtained from functionalized dicarboxylates capable of chelate ring formation are generally smaller than the analogous macrocycles obtained from the unsubstituted dicarboxylate (compare... 

Chelate ring formation may not be ideal in terms of the fit of the ligand to the metal. Potential mismatch resolution involves in large part (but not exclusively) adjustment in the metal-donor distances and the angles around the metal. 

Denticity defines the number of donor groups of a ligand that are coordinated. Apart from one (monodentate), those providing two (didentate) or more (polydentate) donors may involve chelate ring formation. For each chelate ring, two linked donor groups bind the central atom, forming a cyclic unit that includes the central atom. 

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