"Click" Functionalised Ferrocene-based Molecular Actuators

Abstract

This thesis describes the synthesis of 2-pyridyl-1,2,3-triazole functionalised ferrocene ligands, their coordination chemistry, and their ability to act as molecular actuators. It consists of five chapters.

Chapter 1 briefly introduces molecular machinery discovered in the realm of biological cells, and goes on to describe pioneering examples of synthetic molecular machinery. Synthetic molecular switches and motors are broadly classed as either mechanically interlocked (comprising rotaxane and catenane architectures), or non-interlocked structures. The introductory chapter is rounded off by describing existing examples of molecular rotary switches, including the work on ferrocene-based rotors exhibiting stable extended and contracted states, accessed through reversible electrochemistry, which led directly to the research described in chapters 2-5.

Chapter 2 outlines the synthesis of 1,1’-disubstituted ferrocene ligands having 2-pyridyl-1,2,3-triazole chelates, and the corresponding mono-substituted model analogues. The new ligands are characterised and evidence for the free di-substituted ferrocene ligands assuming a contracted π-π stacked conformation in their native state is given. The coordination chemistry of the ligands described in Chapter 2 is explored in the remaining chapters.

Chapter 3 explores the metallosupramolecular complexes formed through selfassembly of the new ferrocene-based ligands with the metal ions Cu(I), Ag(I) and Pd(II). It is demonstrated that the rotationally flexible linking ferrocene units lead to a range of architecture types including coordination polymers, and metallomacrocycles of varying size. The thermodynamic and intramolecular forces which govern the formation of the resulting metallo-structures are then discussed.

Chapter 4 initially describes the synthesis and characterisation of Ru(II) bis-bipyridyl complexes of the 2-pyridyl-1,2,3-triazole ferrocene ligands as an extension mechanism. Consequently, the potential of utilising the UV-light promoted ejection of the pyridyl-triazole chelates from the metal centre is investigated as the corresponding contraction stimulus for the ferrocene rotors. It was conceived that heating of the resulting mixture of photo-products would cause recoordination, resulting in a reversible extension/contraction motion using heat/irradiation as the stimuli. Unfortunately, it was found that while the Ru(II) centres are indeed photo-ejected, the UV irradiation decomposes the ferrocene ligands, rendering the switching mechanism inappropriate.

Chapter 5 discusses the use of the previously employed copper-based switching mechanism, in the context of pyridyl-triazole ligands. The extension mechanism makes use of the sterically demanding 2,6-dimesitylbipyridine ligand directing the formation of heteroleptic Cu(I) complexes of the ferrocene rotor ligands to generate the extended conformation. In the previously reported bipyridine-based systems, oxidation of the Cu(I) centres to Cu(II) alters the coordination sphere preference such that the copper ions leave the ferrocene rotors for a higher denticity ligand allowing the contracted rotor state to be reassumed. Due to pyridyl-triazole ligands being weaker chelators than bipyridine, the presence of the higher denticity ligands causes exchange of Cu(I) ions between the ligands in the mixture, meaning the extended state cannot be quantitatively accessed under switching conditions. It is then proposed that the barrier to exchange in the Cu(I) state would be higher in an equivalent mechanically-interlocked system, and accessing each state quantitatively may therefore be possible. The design and synthesis of a ferrocene-based [3]rotaxane rotor is thus described, where the shuttling of bipyridine-containing macrocycles along the “arms” of the ferrocene-rotor thread is proposed to induce rotary motion about the ferrocene “joint”.