Catalytic Water Oxidation with Cobalt and Copper Complexes

Abstract

In this thesis, the catalytic water oxidation process, which is a pivotal step in the production of solar fuels, has been investigated using various metal catalysts. This thesis consists of two sub-topics, which are (1) Photocatalytic Production of Hydrogen Peroxide from Water and Oxygen with p-Benzoquinone Derivatives and an Earth-Abundant Cobalt Complex and (2) Catalytic Water Oxidation by a Copper(II)-Oxyl Complex. Solar-driven production of H2O2 is a necessary approach to address both the energy and environmental crises. However, photocatalytic production of H2O2 from H2O and O2 with use of a homogeneous catalyst has so far required a noble metal-based photocatalyst such as Ru(II) complexes. In the chapter 1, this is the first time to develop the photocatalytic production of H2O2 from H2O and O2 without any sacrificial electron donor using a p-benzoquinone derivative as an organic photoredox catalyst and an earth-abundant cobalt(III) complex with a tetraamido macrocyclic (TAML) ligand ([(TAML)CoIII]–) as a water oxidation catalyst in acetonitrile (MeCN). Photocatalytic production of H2O2 resulted from the combination of the [(TAML)CoIII)- mediated photocatalytic oxidation of H2O by DDQ– and the photocatalytic oxidation of DDQH2 by O2. The maximum turnover number (TON) based on [(TAML)CoIII]– was 1,900 that is the largest ever reported. [(TAML•+)CoIII(OH)]– was formed as a key intermediate; This Co-OH species functions as a powerful oxidant that influences the oxidation of water under dark. On the other hand, a copper(III) complex with a tetraamido macrocyclic ligand ([(TAML)CuIII]–) was oxidized by cerium(IV) ammonium nitrate (CAN) in trifluoroethanol (TFE) to generate a protonated copper(II)-oxyl TAML radical cation complex ([(TAMLH•2+)CuII(O•–)]–), which was assigned by UV-vis spectral changes. Four equivalents of [(TAMLH•2+)CuII(O•–)]– reacts with H2O to produce O2 via nucleophilic attack of H2O to the O•– moiety of [(TAMLH•2+)CuII(O•–)]–. This suggests [(TAMLH•2+)CuII(O•–)]– acts as a key intermediate. Moreover, [(TAML)CuIII]– has been shown to act as a catalyst for water oxidation for water oxidation induced by DDQ to create O2 and [(TAMLH•2+)CuII(O•–)]– as a critical intermediate, which allows H2O to be converted to O2. ;본 학위논문에서는 다양한 금속 촉매를 이용하여 태양광 연료 생산의 중추적인 단계인 물 산화 촉매에 대해 조사하였다. 본 학위논문은 (1) p-벤조퀴논 유도체와 지구상에 풍부한 코발트 복합체를 이용한 물과 산소로부터 광촉매적 과산화수소의 생성 및 (2) 구리(II)-옥실 화합물에 의한 촉매적 물 산화 반응의 두 하위 주제로 구성되어 있다. 우선, p-벤조퀴논 유도체와 코발트(III) 화합물인 [(TAML)CoIII]-를 이용한 시스템을 개발하였다. 1장에서는 아세토니트릴(MeCN) 용매 하에서 p-벤조퀴논 유도체를 유기 광산화-환원 촉매로 사용하고, 물 산화 촉매로서 테트라아미도 거대고리(TAML) 리간드를 갖는 지구에 풍부한 코발트(III) 화합물 ([(TAML)CoIII]-)을 사용하여 희생-전자공여체 없이 H2O와 O2로부터 H2O2의 광촉매적 생성을 처음으로 규명하였다. 2장에서는 테트라아미도 거대고리 리간드를 포함하는 구리(III) 화합물 ([(TAML)CuIII]-)은 트리플루오로에탄올(TFE)에서 Ce(IV)-질산암모늄 (CAN)에 의해 산화되어 양성자화된 구리(II)-옥실 TAML 라디칼 양이온 ([(TAMLH•2+)CuII(O•–)]–)을 생성하였다. 또한 [(TAML)CuIII]–는 2,3-디클로로-5,6-디시아노-p-벤조퀴논(DDQ)에 의한 물의 광구동 산화 반응에서 물 산화 촉매로 작용하여 H2O를 O2로 산화시킬 수 있음을 확인할 수 있었으며, 이 때 [(TAMLH•2+)CuII(O•–)]– 종이 활성 중간체임을 제안하였다.