Oxidation of Water and Organic Substrates by Seven-Coordinate Ruthenium and Osmium Oxo Species

釕和鋨含氧七配位化合物用於水和有機物的氧化

Student thesis: Doctoral Thesis

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Author(s)

  • Yingying LIU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date29 Aug 2016

Abstract

Nowadays, the global energy demand is mostly provided by burning of non-renewable fossil fuels, such as nature gas, oil and coal. The energy crisis and environmental problems caused by dependence on fossil fuels have urged the scientific community to search for new, renewable and clean fuel sources. Photosynthesis in green plants provides excellent inspiration to design artificial photosynthetic system for energy production. The four-electron oxidation of H2O to O2 is a key process in both natural and artificial photosynthesis due to its molecular complexity and high reaction overpotential, and in recent years there have been considerable efforts in designing robust catalysts for this very important process. Notably, for a number of highly active ruthenium catalysts, seven-coordinate ruthenium oxo species have been proposed as active intermediates.

In chapter two, a series of ruthenium(II) WOCs bearing 2,2':6',2'':6'',2'''-quaterpyridine (qpy) and various axial ligands have been synthesized and characterized by 1H NMR spectroscopy, ESI mass spectrometry and single crystal X-ray crystallography. A maximum turnover number (TON) of 2000 was achieved with [Ru(qpy)(MeOpy)2](ClO4)2 (2.6, MeOpy = 4-methoxypyridine) in CeIV-driven {CeIV = (NH4)2[Ce(NO3)6], CAN} water oxidation. A polypyridyl-N,N’’’-dioxide complex [Ru(ONNO)(pic)2](PF6)3 (2.14, ONNO = 1,1'''-dioxide-2,2':6',2'':6'',2'''-quaterpyridine; pic = 4-picoline) was isolated from the reaction of [Ru(qpy)(pic)2](ClO4)2 (2.5) with CAN. Isotope-labeling experiments and mass spectrometric studies suggest that 2.14 is the real catalyst for water oxidation. A seven-coordinate RuV=O species is proposed to be the active intermediate, but it is too active to isolate.

In chapter three, the qpy ligand was used in an attempt to synthesize a seven-coordinate osmium oxo species. The first example of a seven-coordinate group 8 metal-oxo species, [OsV(O)(qpy)(pic)Cl]2+ (3.3), was successfully synthesized and characterized. The X-ray crystal structure of this complex shows that it has a distorted pentagonal bipyramidal geometry with an Os=O distance of 1.7375 Å. This oxo species undergoes facile O-atom and H-atom transfer reactions with various organic substrates. Notably it can abstract H-atoms from alkylaromatics with C-H bond dissociation energy as higher 90 Kcal mol-1. It suggests that highly active oxidants may be designed based on group 8 seven-coordinate metal-oxo species.

In chapter four, a Schiff-base type anionic ligand was introduced to design ruthenium water oxidation catalysts with lower oxidation potentials. A series of ruthenium(III) complexes bearing 6,6'-bis(2-hydroxyphenyl)-bipyridine (dObpy) ligand and its derivatives were synthesized and characterized. A maximum TON of about 22000 with 88% efficiency based on oxidant was achieved by [Ru(dObpy)(ptz)2]PF6 (4.1, ptz = phthalazine) using CAN as oxidant. Mass spectrometric study reveals that the dObpy ligand undergoes ligand oxidation during water oxidation process.