Catalytic studies of low valent palladium \\(N\\)-heterocyclic carbene complexes

This thesis describes the synthesis of low valent \\(N\\)-heterocyclic carbene palladium complexes. The aim of this project was to investigate the identity of \\(in\\) \\(situ\\) formed \\(N\\)-heterocyclic carbene palladium species under catalytically relevant reaction conditions from palladium(II) precatalysts. This aim was underpinned by the importance of neutral and cationic palladium(II) complexes widely used as precatalysts for carbon monoxide/ethene copolymerisation as well as various related C-C coupling reactions. The use of bulky chelating ligands is an important factor for catalytic performance in these reactions. Consequently, we focussed on a variety of arylsubstituted methylene bridged bis(N-heterocyclic carbene) palladium complexes. In order to achieve our aims, various attempts to synthesise complexes under reductive/basic conditions were undertaken, which led to the synthesis of several known precursors and novel complexes. Reaction of the dicationic Pd(II) complexes [{1,1'-di(aryl)-3,3'-methylenediimidazolin-2,2'-diylidene} palladium(II)bis(acetonitrile)] [PF\\(_6\\)]\\(_2\\) (aryl = 2,4,6-mesityl and 2,6-diisopropylphenyl) with sodium carbonate in methanol afforded novel dipalladium(I) hydride complexes [{bis(˜í¬¿-carbene)}\\(_2\\)Pd\\(_2\\)H][PF\\(_6\\)]. Steric influences led to major structural differences; Pd-Pd-H core (for 2,4,6-mesityl) and linear Pd-H-Pd core (for 2,6-diisopropylphenyl). The complexes were characterised by \\(^1\\)H, \\(^{13}\\)C, 2D and VT NMR spectroscopy (revealing differences in fluxional behaviour), mass spectroscopy, microanalysis and X-ray and neutron (2,4,6-mesityl only) crystallography. The metal oxidation states of both dipalladium hydride complexes have been shown to be Pd(I) by DFT calculations, but the precise nature of the Pd-Pd-H and Pd-H-Pd interactions remains to be confirmed. The synthesis of analogous complexes with less bulky phenyl and \\(tert\\)-butyl substituents were unsuccessful. The reactivity of these cationic dinuclear Pd(I) hydride complexes as catalysts were examined in carbon monoxide/ethene copolymerisation as well as various C-C coupling reactions (2,4,6-mesityl substituent only for the latter). Their poor performance indicated that the dipalladium(D hydride complexes were inefficient catalysts for carbon monoxide/ethep.e copolymerisation, which may suggest that a palladium methoxide species is the actual catalyst instead of palladium hydride species