Access Type

Open Access Dissertation

Date of Award

January 2017

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Claudio N. Verani

Abstract

The development of affordable water splitting catalysts from Earth-abundant transition metal ions such as Co and Mn is of immense scientific interest. Aiming to develop an efficient water splitting catalyst, a Co(II) complex featuring an asymmetric, pentadentate quinolyl-bispyridine ligand with a phenylenediamine backbone was synthesized and characterized by spectroscopic, spectrometric, and X-ray analysis. The Co ion was selected because of its ability to undergo redox conversions from 3d5 CoIV through 3d8 CoI thereby making it a suitable catalyst that can withstand harsh structural and electronic changes during catalysis. The electrocatalytic water reduction activity of the catalyst at neutral pH gave a turnover frequency (TOF) of 970 moles of H2/h at an overpotential of 0.65 V. Sustained catalytic water reduction over 18 hours gave a TON of 12,100 and (%FE) of 97% suggesting a stable catalyst. Post-catalytic analysis of a grafoil electrode using SEM, EDS and U.V-visible spectroscopy shows no evidence of catalyst degradation or transformation into other species thus confirming the molecular nature of the catalyst. [CoII(LQpy)H2O]ClO4 is active towards water oxidation as well, operating with a %FE of 91% during catalysis in a 0.1 M borate buffer (pH 8.0), and giving a TON of 97, at an applied potential of 1.50 VAg/AgCl. By using a series of experimental methods as well as DFT techniques, we fully isolated and characterized the catalytic oxidized intermediates for [CoII(LQpy)H2O]ClO4, and proposed a ‘water nucleophilic-attack’ (WNA) mechanism of water oxidation where, the highly electrophilic 3d5 [HSCoIV=O] intermediate is attacked by a nucleophilic water molecule thus forming the essential O-O bond and releasing dioxygen. The photocatalytic activity in the presence of [Ru(bpy)3]2+ and ascorbic acid in acetate buffer (pH 4) shows a TON of 295 and TOF of 50 moles of O2/h.

Monometallic cobalt complexes have been shown to efficiently catalyze water reduction and therefore, enhanced activity is expected from binuclear analogs of these monometallic catalysts. The close proximity between two Co centers could trigger cooperativity either by facilitating homolytic pathways or by enabling electron transfer between the metallic centers, thus avoiding the formation of a CoIII–H– species. We hypothesize that cooperativity will be dependent on (i) the distance between the Co centers, (ii) the relative topology of the coordination environments, and (iii) the degree of orientation and overlap between redox-active orbitals. We analyzed the catalytic potential of the bimetallic complex [CoII2(L1’)(bpy)2]Cl4, by means of electrochemical, spectroscopic, and computational methods and observed that it effectively reduces H+ to H2 in acetonitrile in the presence of 100 equiv. of acetic acid with a TON of 18 and %F.E of 94 after 3h at –1.6 VAg/AgCl. This observation allows us to propose that this bimetallic cooperativity is associated with distance, angle, and orbital alignment of the two Co centers, as promoted by the unique Co-Namido-Co environment offered by L1’. Experimental results reveal that the parent [CoIICoII] complex undergoes two successive metal-based 1e– reductions to generate the catalytically active species [CoICoI], and DFT calculations suggest that addition of a proton to one CoI triggers a cooperative 1e– transfer by each of these CoI centers. This 2e– transfer is an alternative route to generate a more reactive [CoII(CoII–H–)] hydride avoiding the CoIII–H– required in monometallic species. This [CoII(CoII–H–)] species then accepts another H+ in order to release H2.

The manganese ion, with its broad range of oxidation states and considerable Earth-abundance, is an appropriate choice for the study of electron transfer processes involved in catalytic water oxidation as it has been used as an efficient electron donor in PS II. It has been proposed that incorporation of phenolate moieties into manganese species could lead to catalytic activity as well. We synthesized two manganese complexes, the hexacoordinate [MnIIIL1CH3OH] and the pentacoordinate [MnIIIL2], with pentadentate tris-phenolate ligands H3L1 and H3L2 respectively. Detailed results from the structural, spectroscopic, and electrochemical evaluation of the two Mn complexes suggest that whilst both complexes show ligand-based oxidations favoring formation of a [MnIII/phenoxyl] species, the hexacoordinate analog could form a [MnIV/phenolate] species. This is specifically due to the low energy difference between the frontier orbitals (<5 kcal/mol) of the Mn center, and the redox-active phenolate ligands. This low energy barrier allows electronic interaction between the Mn ion, and the phenolate ligand, causing valence tautomerism through electron transfer. We, therefore, tested the hexacoordinate [MnIIIL1CH3OH] for water oxidation catalysis and observed an overpotential of 0.77 V and TON of 53 in three hours with the catalyst operating at a %F.E. of 85. This study is particularly useful because it provides a basis for ligand design that favors either a radical or a high-valent metal pathway for catalytic water oxidation.

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