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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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.