Unsymmetric PCN Pincer Palladium and Nickel Complexes : Synthesis, Characterization and Reactivity

Abstract

New palladium and nickel complexes based on unsymmetric PCN pincer ligands have been synthesized and fully characterized using different techniques. The substituents on the nitrogen side arm of the ligand have great influence on the cyclometallation and the reactivity of the new complexes particularly in case of nickel. The less sterically hindered PCNMe ligand enabled formation of the trivalent nickel halide complexes through a straightforward reaction with anhydrous copper halide salts. The trivalent nickel halide complexes were isolated and characterized using EPR, magnetic moment and elemental analysis. The reactivity of the PCN nickel halide complexes in Kharasch addition was tested.
The PCN palladium and nickel complexes relevant to CO2 insertion reactions were synthesized, and their reactivity towards CO2 were investigated giving facile insertion reactions at room temperature in case of the hydroxo and amido complexes. Insertion of CO2 into the nickel methyl bond was conducted under mild reaction conditions using the more electron donating and the sterically hindered PCNi-Pr ligand. Insertion of CO2 into the palladium phenyl acetylide complex offered the corresponding hydrogen carbonate complex. Investigation of the identity of the later complex led to the development of catalytic decarboxylative cross coupling reaction.
A short synthetic route was developed to prepare the PCNPy nickel complexes including pyridine as a nitrogen side arm. The new PCNPy allowed C-H activation at room temperature and enhanced the stability of the alkyl nickel complexes. The PCN nickel complexes mediate the catalytic Kumada coupling reaction.

Abstract (Swedish)

Using transition metals in organic synthesis has led to the discovery of new reactions. These novel reactions have enabled different valuable transformations to take place through short synthetic routes and most importantly using a small amount of the metal. This small amount is called a catalytic amount and the metal which is in the form of a complex (with an array of groups of atoms bonded to the metal, so called ligands) is then known as a catalyst. Transition metal complexes have been extensively employed as catalysts to mediate many transformations. These transformations provide our society with useful things, e.g. pharmaceuticals to treat diseases for example cancer, killing a lot of people around the world, clean fuels to protect our planet from pollution and solar cells to provide a clean source of energy. All these useful things start in a small scale at laboratories in universities and companies and are then applied on a large scale in industry.
In general, an organometallic catalyst can be divided into two parts, the metal center and the surrounding inorganic or organic molecule or atom, the ligand. The ligand is responsible for tuning the electronic and steric properties of the catalyst. Changing the donor atoms of the ligand scaffolds greatly influences the catalytic activity. Different types of ligands have been used in combination with a variety of transition metals in order to prepare new catalysts. Palladium and nickel have gained a lot of interest in this respect. Also, monodentate and bidentate ligands (with one and two binding atoms, respectively) have been extensively employed with these metals because most of them are commercially available and easily accessible. However, these ligands do not always give stable complexes, which result in decomposition affecting the outcome of the catalytic process. This problem induced chemists to develop new type of ligands having a more robust interaction with the metal in a tridentate fashion (with three binding atoms). Indeed, complexes supported by tridentate ligands displayed unique reactivities compared to those including other ligands.
In the current thesis, we have designed and synthesized new palladium and nickel complexes containing tridentate PCN ligands (with phosphorus, carbon and nitrogen binding to the metal). Our choice of palladium and nickel is in line with their successful and great applications in catalysis. We used the new complexes in the activation of CO2 and formation of new molecules, which are relevant to pharmaceuticals. Using different donor atoms were found to be very important to enhance the catalytic activities of these complexes.

• author: Mousa, Abdelrazek
• supervisor: Ola Wendt; Daniel Strand
• opponent: Professor Ott, Sascha, Uppsala universitet
• organization: Centre for Analysis and Synthesis
• publishing date: 2018-11
• type: Thesis
• publication status: published
• subject: Inorganic Chemistry
• keywords: Unsymmetric PCN, Palladium, Nickel, Cyclometallation, Carboxylation, Cross-coupling
• pages: 186 pages
• defense location: Hörsal Blå hallen, Ekologihuset, Sölvegatan 37, Lund
• defense date: 2018-12-17 09:15:00
• ISBN: 978-91-7422-608-9
• ISBN: 978-91-7422-609-6
• language: English
• LU publication?: yes
• id: d103d6eb-76b5-4796-8247-42e932288194
• date added to LUP: 2018-11-20 12:59:14
• date last changed: 2018-11-21 21:43:28