Open Access Dissertation
Date of Award
Claudio N. Verani
The work presented in the dissertation is focused on developing catalysts for water oxidation. In this regard, a series of unsysmmetrical ruthenium complexes of type [Ru(terpy-R)(phen-X)Cl]PF6 where terpy-R= 4'-(4-methylmercaptophenyl)-2,2':6'2"-terpyridine and phen-X=H (1), X= 5-nitro (2), X=5,6-dimethyl (3), and X= 3,4,7,8-tretramethyl (4) was synthesized as precursors for self-assembled monolayers. Water oxidation properties of these complexes were evaluated in the presence of (NH4)2[Ce(NO3)6] as the sacrificial oxidant by measuring catalytic turnover number (TON) after 24 h of reaction time and rate of dioxygen evolution in solution during early stages of catalysis. The results revealed that all complexes 1 - 4 are catalytic towards water oxidation. However, the presence of electron-withdrawing nitro substituent on phenanthroline dramatically decreased the water oxidation activity (TON 60, rate of dioxygen evolution 2.40 × 10-4 µmol O2/s) compared to the complex 1 (TON 410, rate 8.19 × 10-4 µmol O2/s). In contrast, complex 4 with electron-donating methyl substituents on phenanthroline indicated enhanced rate of dioxygen evolution (27.3 × 10-4 µmol O2/s) in solution than complex 1. Furthermore, reactivity of complexes 1 - 4 towards (NH4)2[Ce(NO3)6] followed a trend similar to rate of dioxygen evolution in solution and suggests chloride ligand was likely to retain in the coordination sphere during formation of high-valent ruthenium species. This is an important study as it challenges the accepted mechanism of water oxidation catalysts with halogens as axial ligands. In addition, 1H-NMR and ESI mass analysis of complex 1 isolated after catalysis indicated catalyst core remained intact during catalysis. However, the -PhSCH3 substituent on terpyridine was oxidized to -PhSO2CH3 in the presence of strong oxidizing environment.
The need of a sacrificial oxidant such as CeIV to initiate the catalytic cycle can be avoided if a catalyst was deposited on to an electrode surface and activated electrochemically. In this regard, use of Langmuir-Blodgett technique to deposit catalyst onto a surface was studied. Amphiphilic nature essential for LB film formation was introduced to the procatalytic core of [Ru(terpyridine-X)(phenanthroline)Cl]PF6 by long alkane chain attached to the terpyridine(X = C6H6-O(CH2)8CH3). The LB film formation indicated that the film collapsed at 50 mN/m. UV-visible spectra and infrared reflection absorption spectra (IRRAS) of the complex deposited onto an ITO surface were in good agreement with the complex in solution confirming the catalytic core was intact after LB deposition process. The complex indicated 86 catalytic turnover cycles during a period of 24 h in solution in the presence of CeIV as the sacrificial oxidant. The catalytic wave for water oxidation appeared at 1.5 V vs. Ag/AgCl indicating 0.36 V overpotential compared standard potential of water oxidation. In addition, catalytic wave increased linearly with number of LB monolayers, suggesting catalytic properties were not confined to the outer most LB monolayer.
In the next series of ruthenium/terpyridine complexes, capping ligand was changed to redox non-innocent 3,5-di-tert-butyl-2-(phenylamino)catechol (L1) and 3,5-di-tert-butyl-catechol (L2). Following complexes [Ru(TpyOC9)(L1)Cl]PF6, [Ru(TpyOC9)(L2)Cl], and [Ru(TpyOC9)(L2)DMSO]Cl were synthesized and characterized. The change of ligand architecture to from L1 to L2 and change in axial ligand from chloride to dimethyl sulfoxide resulted in dramatic changes in electronic and redox properties. LB isotherm and Brewster angle micrographs of complexes with L1 and L2 ligands indicated a uniform monolayer formation. However, UV-visible, redox, and IRRAS studies indicated only the complex with L1 ligand can be successfully transferred onto a substrate. Unfortunately, none of the above mentioned complexes with L1 and L2 ligands were catalytic towards water oxidation.
In summary, studies on ruthenium/terpyridine complexes found that, (i) water oxidation properties depends on the nature and the position of electron-withdrawing/donating substituents on phenanthroline, (ii) LB technique can be used to deposit water oxidation catalysts onto a surface, (iii) the complexes with redox non innocent ligands indicated that the ligand architecture and the axial ligand coordinated to the ruthenium center played a significant role in regulating redox processes and electronic spectral properties.
Wanniarachchi, Dakshika Chandrashani, "Development Of Ruthenium/terpyridine Complexes For Water Oxidation" (2013). Wayne State University Dissertations. 935.