Structural and kinetic study of rhodium complexes of N-aryl-N-nitrosophydroxylamines and related complexes

English: Oxidative addition, and in particular the addition of iodomethane to rhodium(I) phosphine complexes is of great importance in catalytic processes. The Monsanto process for the production of acetic acid serves as one of the better-known examples.1,2 In the process of clearing up uncertainties in the mechanism of oxidative addition, our group has been interested in the manipulation of the reactivity of the Rh(I) centre in the [Rh(LL′)(CO)(PX3)] type of complexes, where LL′ represents monocharged bidentate ligands such as acetylacetonate,3,4 and substituted acac ligands,5 thioacetylacetonate,6 8-hydroxyquinolinate,7 cupferrate,8 etc. containing different donor atoms such as oxygen, nitrogen and sulphur. PX3 represents different monodentate phosphine ligands, such as PPh3, PCy3, P(o-Tol)3, PPh2C6F5, P(p- ClC6H4)3 and P(p-MeOC6H4)3. The phosphine ligands in such studies are selected to provide a significant variation in their electronic and steric properties. These and other ligand variations usually have a marked effect on the Lewis basicity of the metal centre and thus on its reactivity as well as on the type of product formed. The different outcome of the oxidative addition reactions of acetylacetonate, thioacetylacetonate and cupferrate showed the importance of in depth studies of such mechanisms, taking into account as much factors as possible. More sophisticated apparatus enabled the unravelling of the oxidative addition of the acac system in the present study, coming to the conclusion that the mechanism involves an initial dissociative equilibrium of the Rh(I) complex. The equilibrium step is followed by oxidative addition leading to a postulated ionic intermediate, which reacts further by different pathways to produce the final, presumably trans-addition product (D). Uncertainty regarding the nature of the transition state in the oxidative addition of the Sacac system urged the high-pressure investigation undertaken in the present study. The final product is the acyl complex (similar to C in the acac system) and two possible reaction routes, via a three-centered transition state, or via a linear transition state (SN2 mechanism) were under consideration. The high-pressure study concluded that, on the basis of the solvent-independent ΔV* data, the reaction via the three-centered transition state is more likely, since a significant solvent dependence is expected for the linear transition state involving an ion-pair intermediate. The high-pressure study was extended to incorporate the cupferrate system. It was concluded that the formation of an ion-pair intermediate would be more favoured in more polar solvents, indicating that this oxidative addition reaction most probably proceeds via a linear transition state in more polar solvents. In less polar solvents the observed ΔV* value can either be due to single-bond formation and partial charge creation in the linear state, or due to simultaneous formation of two bonds in a threecentre mechanism. Oxidative addition of the neocupferrate system resembles that of the cupferrate system. Similar to what was proposed for cupferrate, oxidative addition proceeds via two competitive pathways. The k1 path implies a nucleophilic attack on CH3I, giving a 16-electron 5-coordinate intermediate for which the degree of ion separation will be solvent dependent. The solvent assisted k2 path can be viewed as a rare oxidative addition catalysis phenomenon, similar to the solvent effects in the migratory insertion of CO into transition metal alkyl bonds. The migratory CO insertion as observed in the cupferrate and neocupferrate systems was studied in an effort to clarify the nature of solvent involvement. Using substituted THF and other steric manipulated solvents, solvents with different electronic properties as well as a variation of bound ligands, proof was given that solvents participate in a coordinative way, rather than just stabilising the migratory insertion transition state by solvation. Studying the effect of the varying steric and electronic properties of the phosphine ligands on the oxidative addition of the neocupferrate system, a steric-electronic model, developed by Tolman, was applied to oxidative addition reactions to evaluate the total effect of the phosphorous ligand in a particular system. This could lead to a better understanding and selection of the composition of catalysts. A number of novel rhodium complexes were synthesised during the study, of which some were characterised by X-ray crystallography. This established the ground state stereochemistry of these complexes, and although not specific for determining the mechanistic pathway, aided the interpretation of kinetic data.