Promoting the conversion of ruthenium aqua to ruthenium oxo complexes with specially designed ancillary ligands

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Promoting the conversion of ruthenium aqua to ruthenium oxo complexes with specially designed ancillary ligands

 

Author: Cheung, Kwong-chak
Title: Promoting the conversion of ruthenium aqua to ruthenium oxo complexes with specially designed ancillary ligands
Degree: Ph.D.
Year: 2004
Subject: Hong Kong Polytechnic University -- Dissertations
Ruthenium
Electrocatalysis
Department: Dept. of Applied Biology and Chemical Technology
Pages: ix, 233 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1756573
URI: http://theses.lib.polyu.edu.hk/handle/200/516
Abstract: Ruthenium oxo complexes are important intermediates in ruthenium-catalyzed oxidation reactions such as epoxidation of alkenes, oxidation of hydrocarbons and the oxidation of water to dioxygen. The electrochemical interconversion of ruthenium aqua and ruthenium oxo species has been a subject of extensive interest in the past two decades. By generating the ruthenium oxo complexes electrochemically, oxidation reactions can be achieved under electrocatalytic conditions with catalysts recycled in-situ on the electrode surface. However, the electrochemical oxidation of ruthenium aqua to oxo species is kinetically slow on electrode surface. The objective of this project is to promote the electrochemical conversion of Ru-OH2 to Ru=O through specially designed ancillary ligands of the metal complexes. In the first part of this study, a new electropolymerizable pyrrole-containing ruthenium complex [Ru(tpy)(PPP)(H2O)](ClO4)2 (PPP = N-(3-N,N'-bis(2-pyridyl)propylamino) pyrrole) was prepared. The structure of the ruthenium complex [Ru(tpy)(PPP)(Cl)](ClO4) has been confirmed by X-ray crystallography. The ruthenium monomer can be electropolymerized onto glassy carbon electrode surface via anodic oxidation of the pyrrole function group in aqueous medium. This is the first example of a pyrrole-containing ruthenium aqua complex that can be electropolymerized in aqueous medium. It is believed that by placing the ruthenium centres close to each other in the polymer film on the electrode surface, interaction between the RuIII-OH moieties (which are eletrogenerated from RuII-OH2) is facilitated. This interaction should assist the deprotonation of the hydroxo ligand in the further oxidation of RuIII-OH to RuIV=O. The generation of RuIV=O species can be easily observed in the cyclic voltammograms. In the second part of the thesis, the X-ray structure of two ruthenium aqua complexes containing 6,6'-dichloro-2,2'-bipyridine (dcbpyt), cis-[Ru(dcbpy)2(H2O)2](CF3SO3)2 and [Ru(tpy)(bpy)(H2O)](CF3SO3)2 were reported. The cyclic voltammograms of these two complexes show that the oxidation of Ru-OH to Ru=O is much more facile than the analogues without the chloro-substituents. On comparing the structure of these complexes with analogues without the chloro-substituents, it is anticipated that intramolecular hydrogen bonding exists between the chloro-substituent at the ortho-position of the bipyridine ligand and the hydrogen of the aqua ligand on the ruthenium centre. It is suggested that the intramolecular hydrogen bonding facilitates the deprotoriation of the hydroxo ligand during the oxidation of Ru-OH to Ru=O. In the third part of the thesis, the synthesis of two novel binuciear ligands N,N,N'N'-tetra(2-pyridyl) ethylenediamine (ETHPY) and 1,8-bis(2,2'-dipyridylamino)anthracerie (BDPAA) and their ruthenium aqua complexes were reported. The structure of the ruthenium binuclear complex [Ru2(tpy)2(ETHPY)(CH3CN)2]4+ has been confirmed by X-ray crystallography. The O=RuIV-RuIV=OH/ HO-RuIII-RuIII-OH couple is only observed in the cyclic voltammogram of [Ru2(tpy)2(ETHPY)(H2O)2]4+ but not [Ru2(tpy)2(BDPAA)(H2O)2]4+. It is proposed that the flexibility of the ethylene bridge in [Ru2(tpy)2(ETHPY)(H2O)2]4+ facilitates the interaction of Ru-OH moieties and hence promotes the conversion of Ru-OH2 to Ru=O.

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