title: Electronic structural studies of transition metal complexes with chelating sulfur ligands
Creators: name: Macleod, Cailean Iain
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Sulfur, cobalt, single-molecule, magnet, electronic, structure, XAS, UVVis, electronic, spectroscopy, chemistry, inorganic, EPR, X-ray, paramagnetic, resonance, absorption, copper, nickel, cu(III), co(II), 1,1-dithiolate, 1,2-dithiolates, dithiolate, dithiocarbamate, ligand, complex.

description: The coordination chemistry of transition metals with sulfur-based ligands is of huge importance to our everyday life and with increased demand for data storage and processing, it could become more important still. This is because sulfur-ligated metal complexes, particularly pseudotetrahedral CoII complexes, offer great potential for the developing high-performance single-molecule magnets. However, before such systems can be developed the coordination chemistry of such complexes must be further understood and to this end, three different areas of cobalt-sulfur chemistry were studied. The first of these examined the effect of modifications to the second-coordination sphere on the electronic and chemical properties of CoII arylthiolate complexes. Using electronic and XAS spectroscopy and magnetic susceptibility measurements it was shown that, even as the chemical behaviour changes dramatically, the electronic structures of the {CoS4} moieties remain relatively stable across the series. In the course of study two [Co4(SAr)10]2− clusters were isolated which showed similar amounts of antiferromagnetic exchange coupling, with the yield proving contingent on the electron-withdrawing strength of the arylthiolate substituents. The second area of investigation was into 1,1-dithiolate coordination complexes. Research initially focussed on CoII complexes, to investigate the effect of the tight bite-angle on the electronic and magnetic properties of the complexes. Electronic absorption spectroscopy proved the link between the energy and intensity of the 4A2 → 4T1(P) transitions in each D2d complex and the electron-withdrawing strength of the ligand substituents, but magnetic susceptibility measurements proved inconclusive. In the solid-state results consistent with a S = 1/2 were obtained, whilst fluid solution results were consistent with S = 3/2. Co K-edge XAS confirmed the square-planar nature in the solid-state, with the difference attributed to the fluxional CoII species changing {CoS4} coordination in solution. S K-edge XAS confirmed the link between substituent electron-withdrawing strength and {CoS4} electronic structure, with transition energies correlating strongly with the substituent electron-withdrawing strength in both the CoII complexes and the free ligand salt. To probe this further NiII and CuII 1,1-dithiolate complexes were prepared. Electronic absorption spectroscopy confirmed the trend, with the energy and intensity of the ligand field transitions increasing as the electron-withdrawing strength decreased in both cases. EPR of the CuII species showed g- and A-values to be broadly similar across the series, however, whilst still revealing that the substituent-facilitated ligand charge dominates the in-plane π-bonding in the complexes. The out-of-plane π-bond covalency was shown to be more influenced by the metal-ligand bite-angle. Cyclic voltammetry confirmed the presence of reversible CuIII/II redox events in each 1,1-dithiolate species, with the analogous NiIII/II oxidation events proving irreversible. Once again the substituent effects were clear, with the redox potentials lowering as the substituent electron-withdrawing strength was reduced. The new CuIII species [Cu(i-ect)2]− was successfully isolated, with Cu K-edge of the CuII and CuIII species confirming oxidation of the parent species. S K-edge XAS revealed the same trend as the CoII complexes and the free ligands, with the C−S π* covalency also increasing as substituent electron-withdrawing strength does. Transition metal tetrathiotungstate chemistry was the final area investigated. Electronic absorption spectra confirmed the formation and purity of the Co, Ni, Cu and Zn bis(tetrathiotungstate) species, with [Co(WS4)2]z− (z = 2, 3) the focal point of the investigation. Magnetic susceptibility measurements showed a reduced magnetic moment in the reduced species, with the introduced electron coupling antiferromagnetically to the S = 3/2 system. Co K-edge XAS showed the CoII to be partially reduced, with the bulk of the reduction taking place on the tetrathiotungstate ligands. S K-edge showed the impact of the reduction, with the 1s → 4p transition energy of [Co(WS4)2]3− higher than both [Co(WS4)2]2− and [Zn(WS4)2]2−.