Oxidative activation of iron- and ruthenium-alkynyl complexes : toward square-shaped molecules with four redox-active metal centres.

Abstract

The synthesis of square molecules containing four redox-active metal centres and two positive charges which would be interesting as potential candidates for molecular Quantum-dot Cellular Automata (QCA) models constitutes the aim of this thesis. In this new paradigm, the binary information is encoded in the charge configuration of the QCA cell, and in the case of a molecular QCA, in the charge configuration of a single molecule. In order to synthesise such molecules with metal centres of general formula M(PP)Cp’ [M = Fe, Ru; PP = dppe, (PPh₃)₂; Cp’ = Cp, Cp*], new synthetic methods have been developed. By chemically oxidising mono- or bi-metallic alkynyl complexes, radical coupling can occur, mainly depending on the nature of the metal (Fe or Ru) and the length of the carbon chain (C₂, C₄ or C₆), to give dimers with original geometry. Therefore, this thesis describes the oxidation studies of iron- and ruthenium-alkynyl complexes containing short (C₂) to long carbon chains (C₆), and the characterisations of the oxidised products. The reactivity of the mononuclear 17-electron species [Ru(C≡CR)(PPh₃)₂Cp]•+ (16, R = Ph; 19, R = Tol) and [Fe(C≡CC≡CR)(dppe)Cp*]•+ (2a, R = Ph; 2b R = Tol) was investigated, dimerisation occurring in both cases. Intermolecular radical coupling of 16•+ afforded a linear dimer by coupling at the C[subscript]β and C[subscript]para positions, whereas dimerisation of 2•+ gives a single dicationic complex [27][PF₆]₂ containing a squared C₄ ring centre and two Fe(dppe)Cp* units. The reactivity of the bimetallic 35-electron species [{Cp’(dppe)M}(C≡CC≡CC≡C) {M(dppe)Cp’}]•+ (M = Fe, Ru; Cp’ = Cp, Cp*) was also investigated. The mixed-valence systems containing the M(dppe)Cp* (M = Fe, Ru) fragments were revealed to be stable and isolable: the first crystal structures of mixed-valence complexes with a carbon chain longer than C4 were resolved for [{Cp*(dppe)Fe}₂(μ-C≡CC≡CC≡C)]PF₆ [30]PF₆ and [{Cp*(dppe)Fe}(C≡CC≡CC≡C){Ru(dppe)Cp*}]PF₆ [34]PF₆. Electronic delocalisation in these stable mixed-valence complexes, between the two metal centres and through the C₆ bridge, was revealed to be strong which was unexpected for the unsymmetrical system [34]PF₆. In contrast, the mixed-valence systems containing the Ru(dppe)Cp fragment were not stable at room temperature and dimerised to afford dicationic square-shaped tetrametallic complexes with a C₄ ring centre. Two unsymmetrical dimers were characterised: one containing four Ru(dppe)Cp centres [43][PF₆]₂ and the other containing two Ru(dppe)Cp and two Fe(dppe)Cp* fragments [44][PF₆]₂. Compound [43][PF₆]₂ has been fully characterised and the positive charge revealed to be fully delocalised over the whole molecule. Even if unsymmetrical, these molecules are interesting for being potential molecular QCA models. It has been shown that TCNQ acts as an oxidising agent for iron- and ruthenium-alkynyl complexes. The organometallic 17-e species generated further react by coupling between the cationic and anionic radical [TCNQ]•- to give specifically TCNQ adducts. These new complexes which contain two electrophores possess unique properties. The σ-linked electron donor organometallic centre and the organic electron withdrawing group via an alkyndiyl bridge allow intramolecular charge transfer. The X-ray crystal structure analyses, electrochemistry and UV-Vis spectroscopy have been investigated and reveal the interesting properties of these molecules.