Synthesis, and structural and catalytic investigation of N-heterocyclic carbene metal complexes derived from electron-rich alkoxybenzimidazolin-2-ylidenes

Peter Simpson

Research output: ThesisDoctoral Thesis

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Abstract

This thesis presents an investigation into the synthesis, properties and application of a series of transition metal complexes bearing N-heterocyclic carbene ligands. There were four main areas of focus: the synthesis of alkoxy functionalised benzimidazolium salts and the assessment of their electron-donating abilities; the synthesis of transition metal complexes bearing electron-rich N-heterocyclic carbene ligands, and an investigation of their properties; an investigation into the catalytic activity of the palladium and rhodium complexes; and a preliminary study into the possibility of immobilising a palladium- NHC complex onto silica gel for use as a solid-supported heterogeneous catalyst. Various butoxy-functionalised benzimidazolium salts, including mono- (benzimidazolium) salts, bis(benzimidazolium) salts and benzimidazolium cyclophanes were prepared as precursors to electron rich NHC-metal complexes. Preparation of the (NHC)RhI(CO)2 complexes I - III and subsequent analysis by IR spectroscopy confirmed the increased electron richness of the butoxy-functionalised NHCs compared to the non-butoxy-functionalised analogues. Some of the benzimidazolium salts were used to prepare Ag(I)-NHC complexes for the purpose of transmetallation agents. Treatment of the Ag(I)-NHC complex IV with [RuCl2(p-cymene)]2 yielded the dinuclear cationic Ru(II)-NHC complex V, which exhibits !2-binding of the quaternary carbons of each xylyl ring to the Ru centre. This unique complex was also converted to the hexafluorophosphate complex in excellent yields. A range of Pd(II)-NHC complexes were synthesized. The complexes possessed mono(NHC), chelating bis(NHC) (bridged on one or both sides), and mixed NHC/ancillary ligand systems, in order to assess the most favourable structural characteristics of a potential catalyst. Reaction of the bis(benzimidazolium) salt VI with Pd(OAc)2 gave two complexes, VII and VIII, which were subsequently isolated. The unusual complex VII, in which the xylyl ring is folded under the metal centre, was found to isomerise to IX over approximately twenty-six days in CDCl3. The conformations of the xylyl ring in VII and IX was confirmed using selective NOESY experiments. Two trans-o-xylyl-linked Pd(II)-NHC complexes were prepared and characterised, and represent the only fully characterised complexes of this type. During efforts to synthesize the dinitrophenyl-functionalised bis(NHC)-Pd(II) complex X under strict anhydrous and anaerobic conditions, instead, the trinuclear Abstract v palladium complex XI was isolated, where one dinitrophenyl group of each ligand has been displaced, presumably by attack from adventitious water or acetate present in the reaction mixture. Pd N N N N Br. Many of the Pd(II)-NHC complexes were tested in an extensive set of experiments designed to provide an insight into favourable structural motifs for a potential catalyst and confirm the beneficial nature of the electron donating butoxy groups. It was found that the Pd(II)-NHC complexes possessing chelating bis(NHC) ligands were the most active in the Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions, possibly due to the robustness of these complexes at the temperatures necessary to promote the reactions. Gratifyingly, the complexes possessing butoxy groups were found to be more active than the non-butoxy-functionalised analogues.
Original languageEnglish
QualificationDoctor of Philosophy
Publication statusUnpublished - 2010

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