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理工学研究科 分子物質化学専攻
学 位 の 種 類
博士（理学）
学 位 記 番 号
理工博 第 208 号
学位授与の日付
平成 28 年 9 月 30 日
課程・論文の別
学位規則第４条第 1 項該当
学位論文題名
Synthesis of Trinuclear Ruthenium Catalysts for Photocatalytic
Oxygenation of Organic Substrates in Water Media
光増感性三核錯体による水中における有機物の触媒的酸素化反応
（英文）
論文審査委員
主査 准教授 稲垣 昭子
委員 教 授 野村 琴広
委員 准教授 久冨木 志郎
委員 教 授 穐田 宗隆(東京工業大学)
【論文の内容の要旨】
Summary
Utilization of clean and inexhaustible sunlight energy toward molecular
transformation is attracting a great deal of attention, since it provides a new pathway for
synthesis of various materials without consuming limited oil resources. Nowadays,
many researchers devote much of their effort into development of highly active
molecular catalysts, which can produce dihydrogen or dioxygen from abundantly
present water, because there is an urgent need to resolve future energy problems and
realize artificial photosynthesis.
Although only little attention has been paid to apply these systems for organic
syntheses, continuous pursuit of organic chemists for developing green reactions has led
to the rapid expansion in the use of photoredox catalysts. Our group has been
concentrated on utilization of light for catalytic transformation of organic compounds
using photoactive of molecular catalysts.
Previously, we developed a pyridine-substituted 2,2-bipyrimidine ligand
(6-pyridyl-2,2-bipyrimidine, pybpm) and synthesized a dinuclear ruthenium catalyst,
[Ru(bpy)2(pybpm)Ru(bpy) (OH2)]4+, linked by the pybpm ligand. The dinuclear
ruthenium catalyst was investigated for sulfide oxygenation, and was found to show
higher activity than the mixture of mononuclear ruthenium components. Based on these
results, we extended this work to trinuclear ruthenium cluster catalysts having stronger
electronic conjugation between the metals.
Here, we report synthesis of trinuclear catalysts containing two ruthenium
photosensitizers and one ruthenium reaction center, which are linked by conjugated
pybpm and/or bpm ligands. Electronic absorption spectra revealed that an addition of
Ru photosensitizer into dinuclear complexes can extend the absorption range in the
visible region up to 800 nm.
Photocatlytic oxygenation of organic sulfides (thioanisole) are tested. Reaction was
conducted with 1/1000/500 ratio of catalysts /CoIII oxidant /substrate in degassed
phosphate buffer (pH 6.8) aqueous solution. Di- and trinuclear catalysts showed much
higher performance while compared to the corresponding mononuclear system.
Interestingly, the trinuclear catalyst exhibited highest quantum efficiency of 10 % in
contrast with di- (8.6 %) and mononuclear (1.6 %) catalysts.
Photocatalytic oxygenation was investigated with alkenes such as styrene,
α-Me-styrene, stilbene, and cis-2-octene. The results showed that alkenes are converted
to corresponding aldehydes or ketones via stepwise oxygenation of each C=C double
bond, resulting in the C-C bond cleavage. Oxygenation of aliphatic alkene, such as
cis-2-octene, was also successful which indicates wide scope of applicable substrates to
the photocatalytic system.
Isotope labeling experiment has been conducted using H218O as a reaction solvent.
The molecular mass number of the product was detected by GC-MS, which clearly
indicated water as the oxygen source of reaction. The pH-dependent electrochemical
data (Pourbaix diagram) of trinuclear catalyst was plotted in the range of pH 4-10. The
plot revealed the generation of high-valent RuV=O species formed at low oxidation
potential around 1.2 V in neutral condition (pH~7). The reaction between the substrates
and RuV=O was proposed to gives an oxygenated product (sub=O).
Effect of electron donating group (EDG) and withdrawing group (EWG) on
bridging ligands has been investigated. Trinuclear ruthenium complexes having
5,5’-dimethyl- and 5,5’-dibromo-2,2’-bipyrimidines were synthesized to assess the
electronic effect of the substituents toward catalytic activities. Photophysical and
electrochemical properties of the complexes are compared with the corresponding
non-substituted complex.
Comparison of the catalytic oxygenation toward sulfide and α-methyl styrene
revealed the highest activity of the Br-substituted catalyst. Correlation of activity with
the photophysical and electrochemical activities are thoroughly discussed in the thesis.