Bao-Lin Lee

Photosensitized water oxidation by use of a bioinspired manganese catalyst

In an artificial version of photosynthesis, sunlight and water are used to produce fuels. Our research focuses on the bottleneck in this process, the photooxidation of water. In the course of developing a water oxidation catalyst, a number of metal complexes have been synthesised, characterised, and studied for catalytic activity. Three of them are dinuclear complexes (Ru, Co and Cu) of 2,6-bis[(2-hydroxybenzyl)-(2-pyridylmethyl)aminomethyl]-4-methylphenol (H3bbpmp). The fourth is a dimeric Ru complex with a ligand containing imidazole and phenol motifs. Additionally, a dinuclear Mn complex with a ligand that contains benzimidazoles and carboxylates coordinating to the metal atoms was also developed. This Mn complex was then covalently linked to [Ru(bpy)3]2+-type photosensitisers, resulting in three different bimetallic dyads. Finally, a dinuclear Fe complex containing the same ligand as the dinuclear Mn complex was synthesised.The potential of the three H3bbpmp complexes as catalysts for oxidation of organic compounds was investigated and it was found that the Ru complex catalyses the oxidation of alcohols to the corresponding ketone or aldehyde using (diacetoxyiodo)benzene as oxidant. The Co complex functions as an electron transfer mediator in a coupled catalytic system for allylic oxidation using oxygen gas. The oxidation of 3,5-di-tert-butylcatechol to the corresponding ortho-quinone with oxygen gas using the copper complex proved that it can be considered as a model of catecholase. The dimeric Ru complex and the dinuclear Mn and Fe complexes proved to catalyse water oxidation when employing stoichiometric amounts of the oxidant [Ru(bpy)3](PF6)3. Furthermore, using [Ru(bpy)2(deeb)](PF6)2 as photosensitiser together with Na2S2O8 as sacrificial electron acceptor in aqueous phosphate buffer at pH = 7.2, photochemical water oxidation was demonstrated. The bimetallic dyads however, did not show catalytic activity for the oxidation of water.

Water Oxidation by Single-Site Ruthenium Complexes : Using Ligands as Redox and Proton Transfer Mediators

Water Oxidation by Single-Site Ruthenium Complexes : Using Ligands as Redox and Proton Transfer Mediators

Dinuclear manganese complexes for water oxidation : evaluation of electronic effects and catalytic activity

During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2(II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.

Light-Induced Water Oxidation by a Ru complex Containing a Bio-Inspired Ligand

The new Ru complex 8 containing the bio-inspired ligand 7 was successfully synthesized and characterized. Complex 8 efficiently catalyzes water oxidation using Ce(IV) and Ru(III) as chemical oxidants. More importantly, this complex has a sufficiently low overpotential to utilize ruthenium polypyridyl-type complexes as photosensitizers.

Photosystem II Like Water Oxidation Mechanism in a Bioinspired Tetranuclear Manganese Complex

The synthesis of Mn-based catalysts to mimic the structural and catalytic properties of the oxygen-evolving complex in photosystem II is a long-standing goal for researchers. An interesting result in this field came with the synthesis of a Mn complex that enables water oxidation driven by the mild single-electron oxidant [Ru(bpy)3](3+). On the basis of hybrid density functional calculations, we herein propose a water oxidation mechanism for this bioinspired Mn catalyst, where the crucial O-O bond formation proceeds from the formal Mn4(IV,IV,IV,V) state by direct coupling of a Mn(IV)-bound terminal oxyl radical and a di-Mn bridging oxo group, a mechanism quite similar to the presently leading suggestion for the natural system. Of importance here is that the designed ligand is shown to be redox-active and can therefore store redox equivalents during the catalytic transitions, thereby alleviating the redox processes at the Mn centers.

Synthesis and Catalytic Water Oxidation Activities of Ruthenium Complexes Containing Neutral Ligands

Two dinuclear and one mononuclear ruthenium complexes containing neutral polypyridyl ligands have been synthesised as pre-water oxidation catalysts and characterised by (1)H and (13)C NMR spectroscopy and ESI-MS. Their catalytic water oxidation properties in the presence of [Ce(NH(4))(2)(NO(3))(6)] (Ce(IV)) as oxidant at pH 1.0 have been investigated. At low concentrations of Ce(IV) (5 mM), high turnover numbers of up to 4500 have been achieved. An (18)O-labelling experiment established that both O atoms in the evolved O(2) originate from water. Combined electrochemical study and electrospray ionisation mass spectrometric analysis suggest that ligand exchange between coordinated 4-picoline and free water produces Ru aquo species as the real water oxidation catalysts.