Novel Organometallic Mixed Valence Complexes

Abstract

Organometallic mixed valence complexes have been studied extensively over the past 30 years providing many synthetic and theoretical challenges. This thesis has sought to provide the field with a unique family of mixed valence complexes through which theories of electron transfer in weakly coupled systems can be tested. The metal fragment Mo(dppe)(η7-C7H7) is unique amongst its half-sandwich counterparts providing low formal oxidation states and a well resolved EPR signal and forms the basis of these studies.
Before undertaking a study of the electronic structure of [2]n+ systems, and associated issues regarding mixed valence characteristics and carbon-chain mediated metal-metal interactions, mono-metallic molybdenum acetylide complexes that serve as model systems were studied in detail and their electronic structure fully rationalised. Thus, in Chapter two, a range of para substituted molybdenum aryl acetylides, Mo(C≡CC6H4X-4)(dppe)(η7-C7H7), featuring a range of electron-donating and -withdrawing substituents, are described. These compounds have been studied using a range of spectroscopic, crystallographic, electrochemical, spectroelectrochemical, and computational techniques establishing metal centred oxidation character. This is a consequence of cycloheptatrienyl ring destabilising the filled dz2 metal d-orbtial which then forms the HOMO. The poor symmetry match of this dz2 orbital and the alkynyl π-system effectively decouples the molybdenum fragment from the alkynyl substituent. As a precursor to the synthesis and understanding of bi-metallic complexes containing all-carbon bridging moieties, a series of mono-metallic compounds containing diynyl and triynyl ligands have been studied in Chapter three. The subsequent elucidation of the influence of the length of the carbon chain on the electronic structure has been studied using a combination of spectroelectrochemical and computational techniques. These studies reveal that the length of the carbon chain, and the identity of the supporting ligand, (bipyridine or dppe) increases the chain character of the frontier orbitals.
Homo-bimetallic complexes containing a bis(ethynyl) substituted para-carborane bridging moiety were synthesised (Chapter four) together with the monometallic complex Mo(C≡CC2B10H11)(dppe)(η7-C7H7). The mono-metallic complex was first synthesised and studied to establish how the ethynyl carborane affects the electronic structure of the Mo(dppe)(η7-C7H7) centre and the nature of interaction between the molybdenum centre, the ethynyl fragment and the carborane cage. This preliminary work was followed by the synthesis of the bimetallic complex, [2]. Using a range of spectroscopic, spectroelectrochemical and computational techniques the electronic structure, and charge transfer process of [2]n+ (n = 0, 1 or 2) have been explored. The monocation [2]+ has shown to be a genuine example of a valence trapped, weakly coupled mixed valence complex allowing conventional descriptions of the intervalence transition to be compared with TD-DFT based interpretations. The literature surrounding the area of poly-carbon ligand chemistry indicates that the butadiyndiyl bridging moiety is an efficient conduit for electron transfer, due to its two orthogonal π-systems that span across the entirety of the ligand, leading to systems which are generally delocalised. An investigation of the mixed valence complex, [2(μ-C≡CC≡C)]+ reveals a weakly coupled, localised mixed valence electronic structure, which is unique amongst its poly-carbon counterparts (Chapter five). Through using a range of spectroscopic timescales (EPR /IR /UV /vis) the rate of electron transfer has been estimated. To fully account for the number of transitions in the NIR region and the shape of the resulting absorption bands, it is necessary to employ a three state approximation (which explicity indicates the bridge state) when describing the electron transfer process. The complex [2]n+ has been studied using a range of spectroscopic, electrochemical and computational methods to establish the nature and rate of electron transfer of the mixed valence complex (Chapter six). It has been demonstrated that the 1,4-diethynylbenzene bridge mixes more efficiently with the Mo(dppe)(η7-C7H7) than the 1,3-butadiyndiyl bridge. Spectroscopic analysis revealed a moderately coupled, localised mixed valence complex, where the rate of electron transfer is much faster than the diynyl complex but not faster than the infrared spectroscopy timescale. The application of the three state model in the description of the charge transfer process allows the increased electron transfer rate to be explained through the increased mixing of the bridge with the Mo(dppe)(η7-C7H7) moiety, characterised by the lowering of the LMCT transition in comparison to carboranyl and diynyl containing complexes. Metal complexes containing the cyanoacetylide moiety, C≡CC≡N, have been known for several decades, but despite the obvious synthetic advantages of cyanoacetylide as a bridging moiety compared to a butadiyndiyl bridge, C≡CC≡C, the C≡CC≡N ligand has been largely ignored. Chapter seven summarises attempts made to provide a convenient route to complexes containing the cyanoacetylide moiety so that a greater variety bimetallic complexes can be synthesised, thus allowing the investigation of the charge transfer characteristics of [(μ-C≡CC≡N)]n+ complexes. Reactions of cyanogen bromide with metal acetylide complexes yield novel mono- and di- bromovinylidenes rather than cyano containing complexes. The cyanation reagent of choice is 1-cyano-4-dimethylaminopyridinium tetrafluoroborate ([CAP]BF4) which allows the ready synthesis of mono- and di-cyanovinylidenes, as well as the synthesis of cyanoacetylide containing complexes. The cyanating agent [CAP]BF4 is able to cyanate a range of metal acetylides, thus expanding the number of potential bimetallic complexes. The hetero-bimetallic complex [(μ- C≡CC≡N)]PF6 has been synthesised and studied using a range of techniques and has demonstrated that the cyanoacetylide bridge promotes a more delocalised electronic structure for dicationic complexes than is found for the other ethynyl based ligands described in this thesis.