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

• Main Author: Tuxworth, Luke William
• Published: Durham University 2014
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.600974

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-{(tBu2PO)2C6H4} (4-1) and 1,3-{(R2PO)2C14H20}, 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{1H} 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.