Branched and cyclic multinuclear metallocene systems are of great interest due to their remarkable chemical and physical properties, which often differ greatly to their mononuclear, or linear, analogues. In particular, the presence of multiple redox centres, either directly covalently bound or linked by conjugated organic motifs, offers the prospect of studying a plethora of electronic features, such as electron transfer, electronic communication between redox centres and multi-valence states. Due to its easily accessible analogues, ease of handling, and redox properties, ferrocene-containing systems are by far the most studied. Investigation of such multinuclear species has been hindered by synthetic issues arising from the flexibility of the metallocene, resulting in complex synthetic procedures and low yields.
Described herein is the optimisation of the preparation of valuable bi- and tri-ferrocenyl synthons, Fc2I2 and Fc3I2, respectively, via the traditional route of halogenation of the lithiated intermediate. Ultimately, an improved methodology in which dilithioferrocene was reacted directly with diiodoferrocene (FcI2) gave notably improved yields of the multinuclear systems. To achieve covalently bonded metallocenes, Suzuki-Miyaura and Ullmann-like couplings were explored. Although mono-ferrocenyl boronic acids could be prepared, the bi-ferrocenyl derivatives could not and attempts to couple these to iodoferrocenes were unsuccessful. Such reactions resulted mainly in hydrodehalogenation products and degradation, however, in some cases, intramolecular cyclisation of Fc2I2 was observed under Suzuki-Miyaura conditions.
Replacing the NMP medium used in the CuTc-mediated Ullmann-like coupling of iodoferrocenes with acetonitrile revealed a change in reactivity. This allowed a competing oxygen arylation reaction, in which the thiophene carboxylate moiety is transferred from the copper centre to the Cp rings of ferrocene, to take place in tandem. The combination of both the C-C and C-O bond forming reactions allowed for the isolation of a series of ferrocene-based thiophene carboxylate derivatives. The same reactivity pathway gave the ruthenocene analogues in both NMP (N-methyl pyrrolidone) and acetonitrile solvents. The mono- and bis-metallocenyl thiophene carboxylate adducts were transformed into the corresponding metallocenol by means of a base-mediated ester hydrolysis reaction. Overall, this revealed a modified reaction pathway towards oxygen substituted metallocenes, the higher order analogues of which are novel.
A number of related synthetic pathways were explored for the preparation of branched and cyclic ethynylarene-containing materials, which heavily featured subsequent Sonogashira couplings and alkyne protection/deprotection strategies. Essentially, the nature of the organic functionalities introduced to both the bridging and terminal positions was found to dictate the efficacy of a given methodology. A series of novel, branched ferrocene systems, containing diethynylpyridine, diethynylbipyridine and diethynylthiophene bridging moieties, were prepared with a variety of terminal ligands by utilising symmetric and asymmetric protecting group strategies. It was found that materials containing the bipyridyl motif, in either the bridging or terminal positions, presented the most complications with regards to preparation and isolation. Attempts to synthesise analogous cyclic systems via both Sonogashira couplings and Ullmann-like couplings were also made with limited success, owing to the formation of side products and degradation of starting materials under the chosen conditions. This resulted in the isolation of a novel diethynylpyridine-bridged tris-ferrocenyl macrocycle as well as evidence of the analogous thiophenyl species. Evidence was also found for the formation of hydrodehalogenation products from the starting material utilised in the attempted Ullmann-like cyclisation reactions.