Photocatalytic Water Reduction with Cyclometalated Transition Metal Complexes

Abstract

The global energetic and environmental context requires that alternative fuels, such as molecular hydrogen (H2), should be provided replacing fossil fuels, oil, coal and natural gas. With such objectives, there is a number of issues to be addressed. The most important one is that the source of energy should be renewable and eco-friendly. Renewable energies such as sunlight and wind are considered so far, however, they are intermittent and regional concentrated. In this context, converting those energies to another storable energy form is essential. H2 as a fuel has a number of merits compared to other sources of energy: i) high gravimetric energy density of 122 kJ/g, which is 2.7 times higher than gasoline (44.5 kJ/g) and 5.4 times higher than methanol (22.7 kJ/g), ii) clean fuel without emissing CO2 or other carbon-based byproducts upon combustion, and iii) easier to store and transport than electricity. Currently the dominant technology for direct production of H2 is steam reforming (~96 %) which intrinsically have CO2 emission problem. In this regards, reducing of water into its constituent elements to produce H2 using limitless solar energy by photon-induced catalysis represents a promising solution. The critical issue on photocatalytic water reduction is to mimic highly-efficient photosynthesis system in nature, which are facilitated by homogeneous catalysts. There have been great efforts to evaluate the so-called artificial photosynthesis system with molecular organometallic complexes as homogeneous photocatalysts. Homogeneous photocatalysis system comprising molecular catalysts is distinguished from heterogeneous system, and obviously possess an important issue to understand the catalytic process as a prerequisite for further improvement of photocatalytic water reduction system. In this regard, this work describes following four topics: i) investigate electronic and steric effect on Ir(III) photosensitizers photocatalytic activity, ii) developing robust Ir(III) photosensitizers with arylsilyl substituents for photocatalytic water reduction, iii) developing highly efficient electron reservoir Pt(II) water reduction catalyst for photocatalytic water reduction, and iv) developing tethered photosensitizer and water reduction catalyst with Ir-Pt bimetallic molecular device for efficient photocatalytic water reduction. Photochemical process of photocatalytic water reduction starts from the photoexcitation of a photosensitizer. In chapter II, a series of Ir(III) photosensitizers with different electronic and size effects on the N^N ligands. Structural integrity of the complexes were assured by introducing sulfone- or thioether-substituted N^N ligands. Systemic study on the substituent effect was carried out to investigate the structure-photocatalytic activity of the Ir(III) photosensitizers with the view to improve the photocatalytic activity of Ir(III) photosensitizers. In chapter III, a series of [Ir(III)(C^N)2(N^N)]+ complexes having bulky/protective arylsilyl substituent are reported with the aim of developing new photocatalytic water reduction systems with highly improved durability. Turnover number (TON) exceeding 17000 was achieved by using the Ir(III) photosensitizer in cooperation with triethylamine as a sacrificial reagent and colloidal Pt as a water reduction catalyst. Another issue on photocatalytic water reduction is to develop highly efficient molecular water reduction catalyst. In chapter IV, a water reduction catalyst with arylsilyl substituent is reported with high photocatalytic activity. Expanded -system of arylsilyl substituent renders an electron reservoir characteristic to the Pt(II) water reduction catalyst, thus gives robustness upon multi-electron reduction and high catalytic activity. TON upto 500000 was achieved by using the Pt(II) water reduction catalyst with N,N´-dimethylaniline as a sacrificial reagent and arylsilyl-substituted Ir(III) complex as a photosensitizer. Photocatalytic water reducing molecular device comprising Ir-Pt bimetallic complex was studied in Chapter V. The enhanced photocatalytic water reduction kinetics, as well as the robustness of Ir-Pt bimetallic complex are examined systematically in conjunction with a multi-component system as a control.