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

[摘要] English: Oxidative addition, and in particular the addition of iodomethane to rhodium(I)phosphine complexes is of great importance in catalytic processes. The Monsantoprocess for the production of acetic acid serves as one of the better-knownexamples.1,2 In the process of clearing up uncertainties in the mechanism ofoxidative addition, our group has been interested in the manipulation of the reactivityof the Rh(I) centre in the [Rh(LL�?(CO)(PX3)] type of complexes, where LL�?representsmonocharged bidentate ligands such as acetylacetonate,3,4 and substituted acacligands,5 thioacetylacetonate,6 8-hydroxyquinolinate,7 cupferrate,8 etc. containingdifferent donor atoms such as oxygen, nitrogen and sulphur. PX3 represents differentmonodentate 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 toprovide a significant variation in their electronic and steric properties. These andother ligand variations usually have a marked effect on the Lewis basicity of the metalcentre 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 ofsuch mechanisms, taking into account as much factors as possible. Moresophisticated apparatus enabled the unravelling of the oxidative addition of the acacsystem in the present study, coming to the conclusion that the mechanism involves an initial dissociative equilibrium of the Rh(I) complex. The equilibrium step isfollowed by oxidative addition leading to a postulated ionic intermediate, which reactsfurther 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 theSacac 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 twopossible reaction routes, via a three-centered transition state, or via a linear transitionstate (SN2 mechanism) were under consideration. The high-pressure studyconcluded that, on the basis of the solvent-independent ΔV* data, the reaction via thethree-centered transition state is more likely, since a significant solvent dependenceis expected for the linear transition state involving an ion-pair intermediate.The high-pressure study was extended to incorporate the cupferrate system. It wasconcluded that the formation of an ion-pair intermediate would be more favoured inmore polar solvents, indicating that this oxidative addition reaction most probablyproceeds via a linear transition state in more polar solvents. In less polar solventsthe observed ΔV* value can either be due to single-bond formation and partial chargecreation in the linear state, or due to simultaneous formation of two bonds in a threecentremechanism.Oxidative addition of the neocupferrate system resembles that of the cupferratesystem. Similar to what was proposed for cupferrate, oxidative addition proceeds viatwo 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 besolvent dependent. The solvent assisted k2 path can be viewed as a rare oxidativeaddition catalysis phenomenon, similar to the solvent effects in the migratoryinsertion of CO into transition metal alkyl bonds.The migratory CO insertion as observed in the cupferrate and neocupferrate systemswas studied in an effort to clarify the nature of solvent involvement. Using substitutedTHF and other steric manipulated solvents, solvents with different electronicproperties as well as a variation of bound ligands, proof was given that solventsparticipate in a coordinative way, rather than just stabilising the migratory insertiontransition state by solvation.Studying the effect of the varying steric and electronic properties of the phosphineligands on the oxidative addition of the neocupferrate system, a steric-electronicmodel, 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 abetter understanding and selection of the composition of catalysts.A number of novel rhodium complexes were synthesised during the study, of whichsome were characterised by X-ray crystallography. This established the ground statestereochemistry of these complexes, and although not specific for determining themechanistic pathway, aided the interpretation of kinetic data.

[发布机构] University of the Free State