Synthesis, electrochemical and electrocatalytic properties of transition metal complexes based on cyclohexane-supported bis-imino pyridines

Child, Simon (2017) Synthesis, electrochemical and electrocatalytic properties of transition metal complexes based on cyclohexane-supported bis-imino pyridines. Doctoral thesis, University of East Anglia.

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Abstract

This thesis concerns the design and synthesis of transition metal complexes based on cyclohexane-supported bis-imino pyridines for use as electrocatalysts for hydrogen evolution, towards the aim of renewable energy storage.

To investigate the effect of secondary coordination interactions on electrochemistry and electrocatalytic response to protons, a series of transition metal complexes with the same fundamental bis-imino pyridine chelating groups, but with different ligand backbones of cyclohexane and cyclohexanol were synthesised. Two isomers of the cyclohexanol ligands were synthesised giving the hydroxyl groups positioned either distal or proximal to the metal centre, due to the conformational lock that occurs on metal coordination.

Coordination of these ligands to metal salts of Zn, Mn, Co, Ni and Cu gives a range of geometries. Ligands with no hydroxyl group (10, L1) or a distal (trans) hydroxyl (5, L2) both give octahedral geometries, whereas a proximal (cis) alcohol (9, L3) gives trigonal prismatic geometry for Zn, Mn and Co, but square-pyramidal geometry for Cu.

All of the complexes synthesised were shown to give rich electrochemistry, largely due to the redox non-innocent bis-imino pyridine ligands. The presence or absence and position of the hydroxyl group are shown to have a large effect on the cyclic voltammetry. This includes evidence for molecular motion involving the interconversion between different geometries: specifically trigonal prismatic and octahedral or square-based pyramidal.

Electrocatalysis is focused on the series of cobalt complexes as these were shown to give the largest response to acetic acid. Catalytic parameters, η, and TOFmax, of the cobalt complexes follow a trend in increasing η and TOFmax from the propane backbone, to cyclohexane backbone, to the distal cyclohexanol, to the proximal cyclohexanol. However bulk electrolysis of CoL3N with acetic acid indicates that proton reduction to hydrogen is not the primary electrocatalytic process for this complex, and suggests reduction of solvent. Future studies must address the nature of this process, and unambiguously establish those of the other complexes.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Chemistry
Depositing User: Gillian Aldus
Date Deposited: 28 Mar 2018 13:07
Last Modified: 31 Oct 2019 01:38
URI: https://ueaeprints.uea.ac.uk/id/eprint/66634
DOI:

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