Iminophosphine complexes of palladium and platinum: catalysis and metallacycloalkanes synthesis

Doctoral Thesis


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University of Cape Town

A series of N-functionalized 2-diphenylphosphinobenzaldimino ligands (3.1 â 3.6) bearing pendant groups on the imine moiety were prepared by the Schiff-base condensation reaction of 2-diphenylphosphinibenzaldehyde and appropriate primary amines. The ligands were subsequently used to synthesize a range of palladium complexes of the types [Pd(P^N)Cl2] (3.7 â 3.12) and [Pd(P^N)(Me)Cl)] (3.13 â 3.18) from precursor complexes [Pd(COD)Cl2] and [Pd(COD)(Me)Cl], respectively. Platinum complexes of the type [Pt(P^N)Cl2] (3.19 â 3.24) were synthesized by the ligand displacement reaction between [Pt(COD)Cl2] and ligands 3.1 â 3.6. All compounds were characterized by multinuclear NMR and infrared spectroscopies as well as elemental analysis. In addition, the structure of complex 3.14 was determined by x-ray crystallography. Palladium complexes 3.8 â 3.10 and 3.16 were evaluated as pre-catalysts in the Suzuki- Miyaura coupling reaction. These complexes were found to be highly active and tolerant of a wide range of reaction conditions and functional groups on substrates. Low catalyst loadings (0.1 mol% Pd) were required, while high conversions and short reaction times were maintained. Having a substituent bearing a donor atom on the imine moiety of the ligand (ligands 3.3 and 3.4) was found to enhance catalytic activity. Palladium methyl chloride complexes were found to show slightly more activity than their palladium dichloride counterparts. Reaction of [Pt(P^N)Cl2] complexes with BrMg(CH2)4MgBr in an attempt to synthesize platinacycloalkane complexes resulted in the formation of bromobutyl complexes [Pt(P^N)(C4H9)Br] (3.25 and 3.26) instead. Successful synthesis of platinacyclopentane complexes, 5.1 â 5.6, and platinacycloheptane complexes, 5.7 â 5.12, was achieved by the reaction of [Pt(COD)Cl2] with appropriate di-Grignard reagents, followed by ligand displacement with the iminophosphine ligands. All complexes were fully characterized using various NMR spectroscopies, mass spectrometry and elemental analysis. Crystal structures of the bromobutyl and platinacyclopentane complexes 3.25 and 5.1 were determined. Studies on the thermal decomposition of the platinacycloalkane complexes were carried out. Platinacyclopentane complexes 5.1 â 5.6 were found to be markedly stable, with the decomposition reaction requiring temperatures higher than 100 °C. Reaction temperature and duration were found to have a significant influence on the organic product distribution obtained. These reactions gave 1-butene (for the platinacyclopentane complexes) and 1- hexene (for the platinacycloheptane complexes) as major products. Kinetic data obtained for the decomposition of 5.1 and 5.7 shows that the decomposition reaction follows first order kinetics for the initial 30% of the decomposition reaction. Thereafter, reaction order deviates from first order behaviour, indicating increasing involvement of products in the reaction mechanism. The generally accepted β-hydride elimination/reductive elimination reaction mechanism for the decomposition of metallacycloalkanes was investigated using DFT methods. The simplified complex, 5.13B, was used as a model for platinacyclopentane complexes. Results from these calculations show that intramolecular β-hydride elimination from the carbocyclic ring of platinacyclopentane complexes is unlikely to occur as this process requires an extremely high energy barrier (>64 kcal.mol-1). Furthermore, these calculations reveal that ligand hemilability is energetically disfavoured in the β-elimination reaction while it is favoured in the reductive elimination reaction.