Metal-mediated molecular machines
Howgego, David Christopher
Nature abounds with ingenious nanoscopic machines employed to carry out all of the requisite tasks that collectively contribute to the molecular basis of life. This thesis focuses primarily on a sub-set known as "molecular walkers" which can perambulate along intracellular molecular motorways carrying out such essential tasks as vesicle transport and muscle contraction. A summary of these incredible natural motors is presented in Chapter I along with a review of the artificial small-molecule mimics reported to date. When elucidating a set of design principles for synthetic analogues, inspiration is taken from the mechanism of the biological bipedal motor protein kinesin with a focus on potential strategies to enable directional walking. Transition metal-ligand chemistry is utilised as one such strategy in Chapter II through the governance of walker-track interactions in the design, synthesis and operation of a bimetallic molecular biped. A palladium(II) moiety is selectively and intramolecularly stepped between pyridine-derivative binding sites in the track using a thermal stimulus in the presence of a coordinating solvent. Acid-base manipulations facilitate directional stepping by means of an energy ratchet mechanism allowing the track to do work on the biped unit and ultimately drive it away from equilibrium. The potential of malleable transition metal binding-event energetics is explored further in Chapter III with the design and synthesis of a platinum(II)-complexed rotaxane. Thermodynamic and kinetic stimuli are investigated as means to mediate selective shuttling of a Pt-complexed macrocycle between two ligand binding sites in the thread. The substitution pattern of the ligands and the kinetic stability of the metal-ligand bonds afford exceptional metastability to the co-conformers of the molecule in the absence of an external stimulus providing the possibility for long-term information storage. In Chapter IV, a novel macrocycle is used to demonstrate the chemical orthogonality of acid-mediated hydrazone exchange with respect to the palladium(II) stepping mechanism described in Chapter II and show that two such motifs can be independently addressed within a single molecule. These linkages are then utilised as mutually exclusive chemo-selective switches to individually operate opposing feet in an unprecedented first-generation small-molecule walker-track system.