Ligand transfer reactions mechanism

A more detailed discussion on the mechanisms and steric courses of imide-oxygen ligand transfer reactions may be found in a paper by Cram and co-workers (8). 

Figure 3. Proposed Mechanism for the Oxidative Ligand Transfer Reaction Catalyzed by [Fe(TPA)X2] Complexes...

A chain mechanism is proposed. The first step is oxidation of a carboxylate ion coordinated to Pb(IV) with formation of alkyl radical, carbon dioxide, and Pb(III). The alkyl radical then abstracts halogen from a Pb(IV) complex, generating a Pb(III) species that decomposes to Pb(II) with release of an alkyl radical, which can continue the chain process. The step involving abstraction of halide from a complex with a change in metal ion oxidation state is quite similar to the ligand-transfer reaction described earlier. 

The further reactions of these compounds and the mechanisms of the ligand-transfer reactions will also be discussed. 

Figure 16.1 Schematic of assisted ion-transfer reaction mechanism, where the empty squares symbolize the ligand and filled squares the complex.

The symmetrization produces solely the cw-isomer, at least in dichloromethane solution. The most attractive mechanism for such stereospecific symmetrization involves a cyclic dinuclear transition state (6), in which both platinum atoms are five-co-ordinate. A similar mechanism [5 B2(cyclic)] with an analogous transition state has been suggested for closely related ligand-transfer reactions of the type ... 

Above it was noted that the replacement of Cl with Br increases the initial reaction rate. This inaease in rate is consistent with the involvement of a ligand transfer reaction in the rate limiting step, eq 9 in Scheme 1, in the proposed mechanism (for detailed discussion see section 8 and 9). However, there is another factor, which considerably affects the overall reaction rate. In acetonitrile at [CEES] >0.1 M a considerable part of total Au(lll) is in the form of the inactive complex 2, which results in a samration of the initial rate with increasing concentration (Figure 4). Eq 20 perfectly... 

N2 recognized as a bridging ligand in ((NH3)5RuN2Ru(NH3)5] by D. F. Harrison, E. Weissterger, and H. Taute. (H. Taute, 1983 Nobel Prize for chemistry for his work on the mechanisms of electron transfer reactions especially in metal complexes ). 

Demonstration of ligand transfer is crucial to the proof that this purticitlar reaction proceed.s via an inner-sphere mechanism, and ligand transfer i.s indeed a usual feature of inner-sphere redox reaction.s, but it is not an essential feature of oil such reactions. 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

The methanolic cupric bromide oxidation of propargyl alcohol to trans-BrCH-CBrCH20H (30%) and Br2C=CBrCH20H (18%) and, under other reaction conditions, Br2C-CBr-CH20H (93 %) follows simple second-order kinetics with a rate coefficient of 1.5 x 10 l.mole . sec at 64 °C. A mechanism of ligand-transfer in a 7t-complex is proposed. ... 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

Charge transfer reactions at ITIES include both ET reactions and ion transfer (IT) reactions. One question that may be addressed by nonlinear optics is the problem of the surface excess concentration during the IT reaction. Preliminary experiments have been reported for the IT reaction of sodium assisted by the crown ether ligand 4-nitro-benzo-15-crown-5 [104]. In the absence of sodium, the adsorption from the organic phase and the reorientation of the neutral crown ether at the interface has been observed. In the presence of the sodium ion, the problem is complicated by the complex formation between the crown ether and sodium. The SH response observed as a function of the applied potential clearly exhibited features related to the different steps in the mechanisms of the assisted ion transfer reaction although a clear relationship is difficult to establish as the ion transfer itself may be convoluted with monolayer rearrangements like reorientation. 

During the course of the work, we also conducted further studies of the reactions of proton hydrates with CH3COCH3 and CH3COOCH3. The reaction mechanisms were found to change from proton transfer to ligand switching and ultimately to an association process, which would be equivalent to adsorption in the case of bulk systems. 

---