Manganese Coordination Complexes: From Magnetism to Water Oxidation
Citation:
TANDON, SWETANSHU, Manganese Coordination Complexes: From Magnetism to Water Oxidation, Trinity College Dublin.School of Chemistry, 2020Download Item:
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Abstract:
Ever increasing energy demands and the closely related issue of global warming derived from fossil fuel combustion have encouraged the development of renewable energy sources and energy storage concepts as one of the most important scientific challenges today. H2 serves as a clean alternative to fossil fuels, and the electrolysis of water is promising for H2 generation and hence the development of the H2 economy. Catalysing the oxidation-half reaction of water splitting represents the key obstacle for the utilisation of H2 as an alternative clean fuel. Since H2O is diamagnetic and O2 is paramagnetic, the electronic structure and hence the magnetic nature of the catalyst affects this half-reaction. In plants, this highly endergonic reaction is facilitated in photosystem-II by the oxygen evolving complex (OEC) which comprises of a Mn based coordination complex. Since Mn contains a large number of unpaired electrons, it provides the possibility of tuning the electronic structure to facilitate the oxidation of water. In addition, the presence of unpaired electrons also gives rise to interesting magnetic properties in Mn coordination complexes.
This work focusses on investigating Mn coordination complexes for these applications using a combined computational and experimental approach. In this thesis, we report the development of a computational methodology for investigating the magnetic properties of coordination complexes, and describe a novel methodology for determining the mechanism of coupling via each ligand bridge in instances where multiple ligand groups bridge a given pair of metal centres. The computational investigation of a {Mn6} cage complex reveals that the oxidation of the cage results in switching of the nature of coupling between some Mn centres and stabilisation of intermediate spin states. The versatility and reliability of the novel methodology developed for understanding the mechanism of coupling between Mn centres is demonstrated using this complex along with a {Mn13} complex.
We also report a synthetic approach for the synthesis of Mn based high nuclearity coordination complexes and demonstrate the application of this approach for modifying previously known structures, for accessing rare geometrical arrangements and for synthesising structural mimics of the OEC. In addition, we report the magnetic properties and the electrochemical water oxidation activity of selected complexes synthesised in this work.
Using carboxylate based ligands, three high nuclearity complexes have been synthesised and among these, one of the complex has been found to bear structural similarities with the OEC. The electrochemical investigation of this complex reveals high catalytic activity and rapid decomposition similar to the OEC.
With a mixture of carboxylate and phosphonate based ligands, a series of novel high nuclearity complexes structurally related to the truncated tetrahedron geometry have been synthesised. These complexes show interesting magnetic behaviour that varies with the extent of distortion from the truncated tetrahedral geometry. The electrochemical investigation of a {Mn12} and a {Mn15} complex shows that these complexes can catalyse the water oxidation reaction.
Using phosphonate based ligands, a novel {Mn12} complex, and a series of {Mn13} complexes that are structurally related to the cuboctahedral geometry have been synthesised. Some of these {Mn13} complexes have also been observed to be active in catalysing the water oxidation reaction under electrochemical conditions.
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Irish Research Council (IRC)
Description:
APPROVED
Author: TANDON, SWETANSHU
Advisor:
Watson, GraemePublisher:
Trinity College Dublin. School of Chemistry. Discipline of ChemistryType of material:
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