Chelate complex, conformational

In addition to X-ray crystallographic studies, two-dimensional NMR solution experiments (i.e., COSY, 1D-NOE, and NOESY, discussed in Sections 3.5.9 and 3.5.10) have been carried out on many lanthanide(III), Ln(ffl), chelate complexes to confirm that the structure of the MRI imaging agent, used in aqueous solution, will correspond to the solid-state X-ray crystallographic structure. Two-dimensional exchange spectroscopy (2D-EXSY) has been applied to lanthanide chelates to study the dynamics of conformational equilibria (how acetate arms chelate and how... 

Brunsveld, L., Chiral Discotic Molecules Expression and Amplification of Chirality, 24, 373. Buckingham, D. A., Conformational Analysis and Configurational Effects for Chelate Complexes, 6, 219. 

Following the success with the titanium-mediated asymmetric epoxidation reactions of allylic alcohols, work was intensified to seek a similar general method that does not rely on allylic alcohols for substrate recognition. A particularly interesting challenge was the development of catalysts for enantioselective oxidation of unfunctionalized olefins. These alkenes cannot form conformationally restricted chelate complexes, and consequently the differentiation of the enan-tiotropic sides of the substrate is considerably more difficult. 

Crabtree and coworkers prepared a series of complexes with a methylene linker and different wingtips. Changing the bulk of the wingtip groups has a strong influence on whether a chelating conformation is achieved. For example, in the case of the terhbutyl group a monocarbene complex was formed (Scheme 3.5) instead of the chelate complex [21]. 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

Certain chiral phosphinite or aminophosphine ligands are also useful for amino acid synthesis. Hydrogenation with a CYCPHOS-Rh complex occurs rapidly to produce high optical yields. The efficacy of this reaction is ascribed to the flexibility of the ligand, which speeds the reaction and gives a fixed chelate ring conformation (26). 

The l3C NMR spectrum in CDC13 at ambient temperature displays two Mo—CO (<5 222.2 and 228.79 ppm) and two characteristic high-field coordinated C=C (<5 58.64 and 71.91 ppm) resonances, indicating that the solid state structure of the Mo-chelate complex is maintained in solution. Conformational rigidity is generally observed in many -complexes. Rotational barriers about the metal-olefin axes and conformational preferences in these complexes have been estimated using extended Huckel-type calculations283. 

FIGURE 5.6 Definition of the skew-line convention used to define the absolute configuration of tris- or bis(chelate) complexes and conformation of diamine chelates. 

There are practically no quantitative measurements concerning the equilibrium la lb. Dwyer and coworkers studied the correlation between chelate ring conformation and complex configuration in a series of ethylenediaminepropylenediamin-cobalt(III) complexes. From the results, they conclude that the methyl group of propylenediamine is always in an equatorial position25,26). 

For complexes with bidentate 1,2-diamines only one type of chelate ring conformation could be ascertained. The six-membered chelate rings of 1,3-diamines may exist in a chair 6a, skew boat 6b or boat 6c conformation. The least stable of the three types is 6c and was not taken into account so far. When the chelate ring carries... 

Buckingham, D. A., Conformational Analysis and Configurational Effects for Chelate Complexes, 6, 219. 

The situation becomes even more complicated when chelate complexes are considered. The chelate ligand forms a metallacycle with the central metal atom that can give rise to fluxional behaviour and thus to dynamic, interconverting conformations that add to the number of possible diastereomers [35-37]. 

Figure 5.6 Conformational flexibility of transition metal carbene chelate complexes.

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