Density functional theory calculations and electrochemistry of octahedral M(L,L’-BID)3complexes, L and L’ = N and/or O and M = selected transition metals
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Date
2019-01
Authors
Ferreira, Hendrik
Journal Title
Journal ISSN
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Publisher
University of the Free State
Abstract
In this thesis “Density functional theory calculations and electrochemistry of octahedral M(L,L’-BID)3
complexes, L and L’ = N and/or O and M = selected transition metals” the focus is on density
functional theory (DFT) calculations and electrochemistry of octahedral M(N,N,N)22+ and M(N,N)3
2+
complexes with M = Co or Fe and N,N,N = the tridentate terpyridine(tpy) ligand (with three N donor
atoms) and N,N = bipyridine (bpy), phenanthroline (phen) or substituted bpy and phen ligands (with
two N donor atoms). Many linear correlations were obtained between the experimentally measured
redox potentials and DFT calculated energies of the different series of complexes. DFT may thus
assist to decrease research time and cost in the lab through the use of these correlations to design
related complexes with the desired redox potential as needed for mediators and dyes in dye
sensitized solar cells (DSSC). The results obtained on the ligands and complexes investigated, are
presented in four main publications, namely on (i) phenanthroline and substituted phen ligands, (ii)
Co(phen)3
2+ where phen = phenanthroline and substituted phen ligands, (iii) polypyridine ligands
(tpy, bpy or substituted bpy ligands) and Co(II)-polypyridine complexes and (iv) a series of Fe(II)
complexes containing tpy, bpy, phen and substituted bpy and phen ligands. The correlations made between the experimentally determined reduction potentials of the
uncoordinated, substituted phenanthrolines as well as the density functional theory calculated
energies and properties of the ligands (both the neutral and reduced phenanthrolines) are
presented first. The electrochemical study shows irreversible reduction of the uncoordinated free
phenanthrolines. Chloride substituents, which are electron withdrawing, on the 4 and 7 positions of
the phenanthrolines, increase the measured reduction potential by 0.3 V and the methyl
substituents, which are electron donating, lead to the decrease, or lowering, of the reducing
potential when compared with the unsubstituted 1,10-phenanthroline. Linear correlations are
obtained between the DFT calculated properties and energies when compared with the
experimental reduction potentials of phenanthrolines containing non-aromatic substituents. Nonaromatic
substituents affect the electron density across the phenanthrolinic ring system solely via σ-
induction effects. Phenylic substituents on the phenanthroline ring system donate electron density
through both σ-induction and π-resonance effects, which leads to a deviation from the trends
observed. These dual donation effects, allow the phenanthroline system to more easily accept
electrons at less negative, or higher, potentials than expected. Comparison between the reduction
potential of metal coordinated phenanthrolinic complexes (M = Fe, Ru and Cu) and the reduction
potential of unbound ligands, provided linear correlations. Electrochemical studies of a series of phenanthrolines coordinated to a Co(II) metal center are
presented and illustrate 3 redox events in each of the investigated series (containing both
substituted and unsubstituted phenanthrolines). An electrochemically and chemically reversible
Co(III/II) couple is observed as well as a Co(II/I) couple, also reversible in both respects. A ligand
based reduction is also observed at potentials lower than the potentials observed for both of the
metal centered redox events. The electron withdrawing or donating capability of the substituents on
the coordinated phenanthroline ligands influences the density of electrons on the Co metallic center
similarly to those results obtained in the electrochemical and DFT study of the uncoordinated, free
ligands, leading to linear correlations between the different redox couples and calculated theoretical
energies. The next material presented is an investigation into the properties of a series of bipyridines that are
coordinated to the Co(II) metal center. The density functional theory (DFT) calculations focussed on
the structure of the Co(II) complexes, as well as the oxidized Co(III) and reduced Co(I) complexes,
also identifying the locus of the experimentally observed redox processes. Low spin DFT calculations
of the Co(II)-bpy complexes showed shorter equatorial and longer axial Co-N bonds which is
classified as elongation Jahn-Teller distortion. The Co(II)-tpy complex is shown to possess four longer
distal Co-N bonds and two shorter axial Co-N bonds which is classified as compression Jahn-Teller
distortion. Similar trends were observed in the calculations performed of the high spin Co(II)
complexes. The electrochemical investigation showed three redox couples, that are both
electrochemically and chemically reversible, which are the Co(III/II) couple, Co(II/I) couple as well as
the ligand based reduction (at lower potentials than the potentials of the metal centered redox
processes), similar to the results obtained for the series of Co-phen complexes. Comparison of the
free, uncoordinated ligand’s reduction potential with the results from this study, shows a reduction
potential 0.5 V more negative than the reduction potential observed for the reduction of the
coordinated ligand in the associated Co(I) complex. Lastly a comparative investigation of the oxidation of an Fe(II) metal center coordinated series of
phenanthrolines, bipyridines and terpyridine are presented. The electrochemical results showed the
Fe(II/III)oxidation range varies from 0.363 V up to 0.894 V with tris(3,6-dimethoxybipyridyl)Fe2+
exhibiting the lowest and tris(5-nitrophenanthroline)Fe2+ the highest oxidation potential. Also noted
from this study is the role of the substituent’s position on the coordinating ligand on the
electrochemical properties of the Fe(II) complexes, i.e. the oxidation potential is 0.669 V for the
complex containing a methyl substituent on the 5-phen position (on the inner phenanthroline ring)
and 0.613 V for the complex containing a methyl substituent on the 4-phen position (on the outer phenanthroline ring). Density functional theory calculations, performed on the oxidized, reduced
and neutral complexes provided optimized electronic energies for each state allowing for
correlations between the calculated energies and the experimentally determined results.
Considerations between closely related complexes, which allows for linear correlations, showed
good correlations for the two considered series (bipyridine and phenanthroline), with R2 > 0.9.
Renderings of the molecular orbitals (HOMO and LUMO) illustrate that the top three HOMOs are
metal-centric, with the directional transfer of the charge during UV/vis excitation to the six lowest
LUMOs, which are ligand-centric.
Description
Keywords
Dye-sensitized solar cell (DSSC), Polypyridyl, Density functional theory (DFT), Octahedral, Electrochemistry, Molecular orbitals, Linear correlations, Redox, Ligand, Substituent, Thesis (Ph.D. (Chemistry))--University of the Free State, 2019