Novel P-alkene and pincer-type POCOP ligands:
synthesis, coordination chemistry, and reactivity

Abstract

This thesis describes the development of a range of polydentate phosphorus-containing ligands relevant to catalysis. The focus is on developing a fundamental understanding of how changes to the substituents on a ligand impact on the environment at a coordinated metal centre.
Chapter 2 reports the synthesis and coordination chemistry of the phosphine-alkene ligands N-R2P-7-aza-benzobicyclo[2.2.1]hept-2-ene, R = Ph (2-1) and iPr (2-2). The electronic properties of 2-1 and 2-2 are probed by a variety of methods, which reveals them to be electron deficient. The coordination chemistry of 2-1 to various transition metal fragments is then explored, exhibiting a range of coordination geometries including tetrahedral ([Ni(κ2-P,C-2-1)2] (2-6)), square based pyramidal ([RhCl(κ2-P,C-2-1)2] (2-7)) and trigonal bipyramidal ([IrCl(κ2-P,C-2-1)2] (2-8)).
Chapter 3 introduces the problem of slow reductive elimination in some palladium-catalysed catalytic transformations along with methods of promoting this process, before describing the application of the electron deficient 2-1 in enhancing reductive elimination reactions. A detailed study of the formation of ethane by reductive elimination from a palladium dimethyl complex of phosphine-alkene ligand 2-1 has been undertaken. The mechanism of this process has been probed by a combination of experimental and computational studies and revealed that the mechanism proceeded via an associative mechanism through a 5-coordinate intermediate.
Chapter 4 describes the synthesis and systematic study of the steric and electronic impact of a range of POCOP pincer ligands 1,3- (4-1) and 1,3-, R = tBu (4-7), OiPr (4-8), NEt2 (4-9), morpholine (4-10) and pyrrole (4-12). The coordination chemistry of these ligands is then appraised to probe the steric impact of the ligand crystallographically. Subsequently, the electronic impact of the ligands are assessed by 31P NMR and infrared spectroscopy of the corresponding phosphine selenide compounds and palladium carbonyl complexes.
Chapter 5 reports exploratory reactions of a novel palladium hydride complex [PdH(κ3-P,C,P-4-1)] (5-1). The insertion of C=C (ethylene) and C=O (acetone, CO2) bonds into the Pd-H bond of 5-1 is attempted, notably showing facile insertion of CO2 to form the metal formate complex [Pd(OC(H)O)(κ3-P,C,P-4-1)] (5-4). Complex 5-1 is shown to be a catalyst for palladium-catalysed alkene isomerisation and aldehyde hydrosilylation, but the activity in both reactions is low.