Ligand exchange, degenerate

NMR studies of degenerate ligand exchange in generation I and generation II ruthenium alkylidene pro-catalysts for alkene metathesis... 

Bis(trifluoromethyl) A3-iodane 6a undergoes degenerate ligand exchange with added alkoxide PhC(CF3)2OK more rapidly (second-order rate constant = 49 M 1s 1 at 56 °C) than that of dimethyl A3-iodane 6b (second-order rate constant =61 M 1s 1 at 93 °C), in which an associative mechanism involving the formation of [12-1-4] species was proposed [16]. The CF3 substituents, which lower the electron density on iodine(III) relative to the CH3 substituents, make the iodine of 6a more susceptible to attack by alkoxide ion. Dynamic 19F NMR of A3-iodane 7 showed an intramolecular ligand exchange via intermediacy of bicyclic tetracoordinated iodate with a AG of ca. 12 kcal/mol at - 80 °C [17]. 

V. I. Minkin and L. E. Nivorozhkin, Stereodynamics and Degenerated Ligand Exchange in the Solutions of Tetracoordinated Chelate Complexes of Non-Transition Metals in Complex Formation and Stereochemistry of Coordination Compounds , ed. Y. Beslaev, Nova Science Publishers, Commack, New York, 1996, p. 77. 

In mechanistic terms, the PMR states that the pathways for forward and reverse reactions at equilibrium are described by the same energy surface it does not state that the profile of such a surface must be symmetrical with respect to the reaction path 2 Application of the PMR to displacement reactions at phosphorus is aided by Fig. 9, which depicts all of the possible reaction pathways for degenerate ligand exchange at tetracoordinate phosphorus that proceed either via pentacoordinate transition states or via phosphorane intermediates capable of pseudorotation. The letters a and e in Fig. 9... 

Low-temperature NMR spectrum of triaryl-A3-iodane 73, prepared from iodo-A3-iodane by the ligand exchange with aryllithium, shows two methyl singlets at 6 2.24 and 2.41 [112]. Simultaneous broadening of the methyl and biphenyl aromatic resonances at higher temperatures demonstrates that the temperature dependence is caused by the degenerate isomerization of p-tri-fluoromethylphenyl substituent between two equivalent sites (73 73 ). The... 

SCHEME 7. Degenerate Et20 ligand exchange of (Li-8)2 Et20. The Et20 represents an ether molecule from the solvent... 

From temperature-dependent NMR studies it has been shown that uncomplexed diamine 6 exhibits ligand exchange with complexed 6 in (Li-6)2 6 by a dissociative mechanism with AG 228 = 7.8 kcalmol-1. The (Li-6)2 6 chelate undergoes a fast intra-aggregate diamine-amide interconversion via degenerate proton transfer between diamine and amide with AG 268 = 10.9 kcalmol-1 (Scheme 8)39. 

Several important spectroscopic structural studies of polyvalent iodine compounds in solution have been published [108-112, 189]. Reich and Cooperman reported low-temperature NMR study of triaryl-X -iodanes 27 (Scheme 1.1), which demonstrated that these compounds have a nonsymmetrical planar orientation of iodine-carbon bonds and that the barrier to unimolecular degenerate isomerization between 27 and 27 is greater than 15 kcal mol The exact mechanism of this degenerate isomerization is unknown both pseudorotation on iodine(III) and intermolecular ligand exchange may account for the isomerization of these compounds [189]. 

That initiation involves dissociative substitution of a phosphine ligand from catalysts 23 and 24 (rather than associative displacement) is indicated by several experimental observations. Metathesis activity for these catalysts is severely depressed in the presence of excess PR3. The values for degenerate phosphine exchange in these systems are 13eu, indicative of a dissociative mechanism (via the 14-electron alkylidene 25) for this process. Furthermore, the rate of this exchange is identical to the rate of initiation (when corrected for temperature), as measured by the irreversible reaction of 23 and 24 with ethyl vinyl ether (Scheme 13). Thus, the propensity for PR3dissociation is directly related to a given catalyst s ability to initiate and enter the catalytic cycle shown within the dotted lines of Scheme 12. 

The emissive Dq excited state for Eu is non-degenerate and Eu + species give rise to relatively simple emission spectra that may be readily analysed [17], For the highest energy Dq - Fq (AJ = 0) emission band at 580 nm one component arises for every chemically distinct species that is not undergoing chemical exchange on the millisecond scale. The transition is sensitive to the ligand field and symmetry [23], The intensity of... 

FIGURE 10.11 Degenerate exchange of hydride ligands in an osmium complex.29... 

Topomerization - A type of valence tautomerism involving the exchange of identical atoms or ligands to produce a molecule indistinguishable from the starting material (see also Degenerate rearrangement). For examples see Sections 24.3.3.l(v) and 2.5.5.2. 

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