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Title: Secondary interactions which modify redox and other properties of organometallic systems
Author: Aljohani, Faizah
ISNI:       0000 0004 5919 6810
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2016
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Second coordination sphere interactions of metal centres involving hydrogen bonding, ion-pairing or dipolar bonding can play an important role in determining the physical and chemical properties of molecular and biomolecular systems. For example, the redox potentials of cubane iron-sulfur clusters in a redox protein or enzyme can range between -700mV and -400mV versus the standard hydrogen electrode as a consequence of the nature of the second-coordination sphere environment. Whilst this is largely explained in terms of hydrogen-bonding ionic, dipolar or other interactions may have a role in this. The first part of this thesis examines a model system in which a ligand based isomerism controls the redox potentials of two interconverting couples. Specifically this is a bis(cyclopentadienyl) MoV/IV system possessing a cysteinyl N,S coordinated metallocycle ring in which deployment of the amide O away or towards the metal centre by rotation is shown to lead to a ca 200 mV difference in the isomer redox potentials. The activation enthalpy for this rotation process is 18kcal mole-1 in MeCN. It is further shown that second coordination sphere interactions with an exogenous Lewis acid, Eu(fod)3 can control the isomer population, favouring the trans-amide conformation. The second part of this thesis examines second coordination sphere interactions of coordinated cyanide in a model for the diiron subsite of [FeFe]-hydrogenase. The natural subsite in its protein environment catalyses hydrogen evolution (or uptake) at very fast rates ca 104 s-1 near the reversible potential of the H+ /1/2 H2 couple but such rates have yet to be attained in model systems. Controlling the electronic and geometric iii properties of synthetic diiron subsites by hydrogen bonding or other interactions might provide a means of attaining the turnover rates of the natural system. This work takes some first steps towards introducing second coordination sphere interactions to modify subsite properties. Firstly, a metal Lewis acid Eu(fod)3 is shown to bridge to CN ligands in the model subsite to change its spectroscopic and electrochemical behavior. Secondly, it is shown that hydrogen bonding interactions with the ligated cyanide using certain activated amides also perturbs spectroscopic and redox properties and importantly, the rate of protonation at the metal-metal bond and the stability of the resulting μ-hydride.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available