Ferrocene-Based Molecular Actuators

Abstract

Nature is bursting with molecular machines designed to achieve everything from the production of ATP, the energy currency of life, to muscle contraction and expansion. In an effort to create synthetic mimics of these machines, both interlocked and non-interlocked molecular actuators have been developed. The work presented herein was undertaken with the aims of developing a simple ferrocene-based actuator capable of reversible extension and contraction, building from that simple actuator to create larger oligomeric actuators, and eventually attaching these actuators onto a solid surface as a way of transferring the motion generated on actuation to the surrounding environment.

Chapter 1 provides a brief introduction to a selection of biological molecular machines and some of the more successful synthetic mimics. The synthetic actuators are broadly classed into interlocked and non-interlocked systems, with relevant examples from each class discussed, including the ferrocene-based two-state switch on which this current work is based. Chapter 2 details the attempted synthesis of a di-armed, or double-tiered, ferrocene-based actuator through Suzuki-Miyaura cross-coupling. Although this synthetic strategy worked well for the formation of the model 5-ferrocenyl-2,2'-bipyridine ligands (2a-b), the double-tiered actuators (15a-b)were not adequately accessible, with only trace amounts isolated. The isolated samples were enough to confirm that the double-tiered actuators were contracted/π-stacked in their native state, but complexation/extension studies could not be carried out. The model compounds, 2a-b, were used to screen a variety of extension stimuli. It was concluded from these studies that copper(I), in combination with the sterically bulky 6,6'-dimesityl-2,2'-bipyridine ligand, was a suitable candidate. Chapter 3 describes the Sonogashira cross-coupling approach to synthesising di-armed actuators. Using this strategy, two double-tiered actuators, 1,1’-di(5-yl-ethynyl-2,2’-bipyridine)ferrocene (22) and[1-(5-yl-ethynyl-2,2’-bipyridine)-1’-(N-methyl-3-yl-ethynylpyridine)ferrocene]PF6 (27), were successfully synthesised. These actuators, like their Suzuki-coupled counterparts, were found to adopt a contracted conformation. When complexed to copper(I) and the 6,6'-dimesityl-2,2'-bipyridine ligand, a combination of electronic and steric repulsion results in the actuators adopting an extended conformation. The actuators can be switched cleanly and reversibly between the extended and contracted using either chemical or electrochemical means. Chapter 4 presents the successful synthesis of a triple-tiered actuator (36b), generated through the established Sonogashira methodology. Like its double-tiered predecessors, the triple-tiered actuator is found to adopt a contracted conformation. Addition of copper(I) again affects extension, with an approximately three-fold change in length compared to the two-fold change in length achieved by the double-tiered system. This actuation is again reversible, with both chemical and electrochemical switching demonstrated. Chapter 5 offers an overarching conclusion to the thesis, as well as proposing a plethora of possible improvements and adjustments to the current system. This includes the attachment of the actuator to a surface, a key goal of this project which due to time constraints could not be realised.