Solution methods ligand substitution reactions

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

The reaction dynamics of few excited complexes are known however, the opportunities provided by pulsed lasers promise to make this research area one of major emphasis of mechanistic studies. Such methods are necessary because few transition-metal complexes exist as electronically excited states in RT solutions with lifetimes exceeding 1 fjis, and many are shorter lived. Several competing processes lead to ES decay nonra-diative deactivation to the ground state (GS), radiative deactivation (i.e., emission) to the GS, unimolecular reaction to products (such as ligand substitutions or redox decomposition) or bimolecular electron transfer or energy transfer with another species, Q, in solution. These processes are indicated in Eqs. (a)-(e) for a hypothetical complex [MLJ" + ... 

Several natural products, for example siderophores, contain the N-hydroxy amide Y[CON(OH)] motif [138], Within a peptide backbone, this group increases the stability to enzyme degradation and induces characteristic conformational behavior [139]. In addition to the synthesis in solution of N-hydroxy amide-containing peptides (which is not trivial), a new solid-phase approach has recently been developed [140]. To explore the features of the N-hydroxy amide moiety using automated and combinatorial techniques, a method for the preparation of v /[CON(OH)] peptide ligands for MHC-I molecules has been elaborated [140], The strategy for the parallel preparation of these peptidomimetics on a solid support is illustrated in Scheme 7.9. The key step is the nucleophilic substitution reaction of resin-bound bromocarboxylic acids by O-benzylhydroxylamine, which requires several days. 

Let us return to our story line the measurement of rates of fast reactions in solution. Provided the rate of forming the excited state and establishing its new equilibrium is fast compared to the rate of deactivation of the excited state by fluorescence or radiationless transitions, a perturbation of the chemical equilibrium can occur and the relaxation of products back to reactants can be followed as a function of time. One advantage of the method is that there is no restriction to equilibrium processes not only can one measure rates in acid-base, redox, and ligand substitution equilibrium situations, but it is also possible to measure rates of irreversible processes,... 

Partenheimer showed (ref. 15) that when toluene was subjected to dioxygen in acetic acid no reaction occurred, even at 205 °C and 27 bar. He also showed that when a solution of cobalt(II) acetate in acetic acid at 113 °C was treated with dioxygen ca. 1 % of the cobalt was converted to the trivalent state. In the presence of a substituted toluene two reactions are possible formation of a benzyl radical via one-electron oxidation of the substrate or decarboxylation of the acetate ligand (Fig. 9). Unfortunately, at the temperatures required for a reasonable rate of ArCH3 oxidation (> 130 °C) competing decarboxylation predominates. As noted earlier, two methods have been devised to circumvent this undesirable... 

Reaction of metal complexes with S is a convenient method for directly introducing the ligand by substitution of other ligands. For example, NajS2 or polysulfide solutions can be used for this purpose (77, 107, 108, 120,146). 

---