Carbon Rich Organometallics: From Mixed Valency To Redox Active Ligands

Abstract

This thesis sets out to explore the electronic structures and redox properties of organometallic complexes. The mixed valence states of these complexes have been investigated and classified. Chapter 1 introduces the general area and provides a brief summary of the theory to electron transfer. The concepts discussed in Chapter 1 are developed further in subsequent Chapters.


A trimetallic molecular assembly featuring two redox-active organoiron fragments connected via a central ruthenium tetramesitylporphyrin socket through pendant pyridyl moieties was investigated using electrochemical and UV-vis-NIR/IR spectroelectrochemical studies. Electrochemical studies reveal three reversible oxidation waves, the first two of which can be attributed to the stepwise oxidation of the iron centres. The redox separation between these iron redox waves was found to be constant regardless of electrolyte composition, suggesting a “through bond” mechanism for electronic interactions between the iron centres. An analysis of the near-IR band in the mixed valence state suggests the assignment of [2(µ-Ru(TMP))]+ as a strongly coupled, Class II mixed valence complex in which the NIR band arises from genuine metal to metal charge transfer (IVCT) processes.



Iron alkynyl complexes Fe(C≡CR)(PP)(Cp’) are well known to exhibit metal localised redox properties, whilst the alkynyl ligand in analogous alkynyl ruthenium complexes is redox-active. In order to tune the metal coordination sphere, to explore further the redox activity of carbon rich ligands bound to ruthenium, a series of complexes of the general form Ru(CH=CHC6H4R-4)(CO)(PPh3)Tp where R = N(C6H4Me)2, OMe, CH3, CO2Me, NO2 were synthesised, crystallographically characterised and investigated using electrochemical, UV-vis-NIR/IR spectroelectrochemical studies and DFT computational methods. IR spectroelectrochemical studies and DFT computations are consistent, with descriptions of the vinyl ligand as being redox-active, as developed by Winter and co-workers for some related mono- and bimetallic ruthenium vinyl complexes. Studies of Ru(CH=CHC6H4R-4)(CO)(PPh3)Tp, Ru(CH=CHC6H4Me-4)Cl(CO)Cl(PMe3)3 and [2(µ-CH=CHC6H4CH=CH)] reveal that the bridging ligand is also heavily involved in the oxidation process. A three state model in which the bridge is appreciably involved in the stabilisation of charge is required to rationalise experimental data including electronic spectra, consistent with theoretical findings. In seeking to fine tune the electronic character of ruthenium based [2(µ-bridge)]n+, vinyl and alkynyl complexes featuring a dithia[3.3]cyclophane moiety were investigated. In both cases, the bridging moiety was found to be heavily involved in the oxidation processes.


The triarylamine ligand moiety has attracted attention due to its potential to promote electronic interactions between three metal sites, through a central nitrogen atom. The synthesis, crystallographic characterisation, electrochemical and spectroelectrochemical response of one, two and three cluster centres on a triarylamine core have been examined. Experimental results point towards a localised structure, with no electronic interaction between the cluster moieties, which are examples of Class I Robin and Day mixed valence complexes.

Pre-formed gold alkynyl and diyndiyl complexes can be cross-coupled with aryl halides in the presence of both Pd(II) and Cu(I) catalysts under mild conditions in ether based solvents at room temperature, without the need for an additional base. These gold-mediated Sonogashira reactions likely proceed via the transmetallation of the alkynyl fragment from Au(I) to Cu(I), prior to entry to a conventional Sonogashira reaction. These reactions present alternative methods to prepare differentially substituted diynes, without the need to expose the C-H functionality of longer chain polyynes, which can be inherently unstable.