A Versatile Electrochemical Protocol for the Synthesis of Transition Metal Complexes

This thesis concerns the application of a versatile electrochemical procedure to prepare organometallic compounds. Primarily, the method is employed to produce various transition metal complexes of N-heterocyclic carbenes (NHCs) from the electrochemical reduction of azolium salts. Importantly, the method generates hydrogen gas as the only side-product and takes place under mild and ambient conditions. The research focuses on the translation of this methodology into a continuous-flow protocol, improving the efficiency and widening the applications associated with the procedure. Using novel reaction engineering, the design and construction of a versatile and modular copper-plate reactor is presented which is capable of producing pure copper(I)-NHCs in flow.
It was found that the method could be extended towards several iron(II)-NHC complexes, through dissolution of a metallic iron electrode. Under these conditions, a family of N-pyridyl substituted imidazolium salts were electrochemically reduced and coordinated to iron, producing a variety of octahedral, low-spin iron(II)-NHCs in high yield. By adjusting the electronic properties of the ligand, an ammonia-bound iron(II)-NHC adduct was observed for the first time. Through tuning ligand denticity, the first example of a dinuclear iron(II)-NHC helicate complex was also prepared.
A wide array of Schiff base metal complexes have also been prepared using the same synthetic methodology. Performing an overall two-electron reduction of salen ligand precursors in the presence of metallic copper, nickel or zinc leads directly to their metal(II)-salen products in good yield. Synthetic practicality was demonstrated by the use of hydrous and aerobic reaction conditions, coupled with a straightforward isolation procedure. By modification of reaction conditions, further versatility was showcased in the selective synthesis of either iron(II)- or iron(III)-salen products from an iron(0) metal source. Using a bimetallic anodic alloy comprised of elemental manganese and nickel, selective liberation of Mn2+ ions was achieved to exclusively afford a number of manganese(II)- and manganese(IV)-salen complexes.
A family of novel palladium(II)-NHC complexes bearing electronically diverse N-pyridyl wingtip substituents are described. A combination of spectroscopic and crystallographic analyses have been used to affiliate dynamic solution-state behaviour with the solid-state structure of these compounds, with a view to design robust catalysts for challenging C-C bond forming reactions.
Amongst the scope of palladium(II)-NHC complexes explored, an unusual example of non-innocent ligand behaviour was observed for one electronically unique example. Modifying reaction conditions, this behaviour could be accentuated to selectively yield a σ-alkenyl palladacyclic product, which was investigated using a combined experimental and theoretical approach.