Computational studies on metallophosphoranes as intermediates in palladium-phosphine chemistry
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The work detailed in this thesis describes density functional (DF) calculations that were performed to investigate the possible role of metallophosphoranes in the reactions of palladium-phosphine complexes. A study was conducted on X/R exchange in [M(Cl)X(PH3)(PR3)] (X = F, OH, NH2, Me, Ph, Cl; R = H, Me, F; M = Ni, Pd, Pt), to see how metallophosphorane formation, followed by R-transfer, was affected by varying the X ligand, nature of the accepting phosphine (PR3) or the metal centre. Metallophosphorane formation was easiest with the small, electronegative X ligands such as F and OH and was promoted by electron-withdrawing R substituents on the accepting phosphine. When the metal centre was varied, the trend for the activation barriers was Ni < Pd < Pt. Subsequent R-transfer was always more facile than metallophosphorane formation. The mechanism of disproportionation of [Pd(Cl)OH(PPh3)2] was studied using DF calculations on the model system [Pd(Cl)OH(PH3)2] and hybrid DF:HF methods on the full experimental complex. In the small model a reaction profile was found via a metallophosphorane intermediate formed via a transition state at 28.3 kcal/mol (from the trans reactant) or a transition state at 21.8 kcal/mol (from the cis reactant). A similar mechanism was found with the full experimental complex (highpoint 27.5 kcal/mol) but a more accessible route was located via a zwitterion intermediate, [PdPPh3Cl]-[P(OH)Ph3]+ (highpoint 26.7 kcal/mol). Possible mechanisms for experimentally-observed Ph/Ph exchange in trans- [PdX(Ph)(PPh3)2] (X = Cl, Br, I) and R/Ph exchange in trans-[PdI(R)(PPh3)2] (R = Me, CH2CF3) were studied using DF:HF calculations. For Ph/Ph exchange, the most accessible pathway involved pre-dissociation of a phosphine, followed by intermolecular attack of the palladium-bound phenyl on the remaining phosphorus. The lowest activation barriers were seen when X = I. The equivalent mechanism was also the most accessible for Me/Ph exchange, although the analogous mechanism without pre-dissociation of phosphine was very competitive. R/Ph exchange was computed to be considerably less accessible when R = CH2CF3 than when R = Me.