Decomposition of water (H2O) into hydrogen gas (H2) and oxygen gas (O2) results in storage of energy by238kJ/mol. The reverse reaction, that is, the combustion of hydrogen gas (H2) with oxygen gas (O2) , leads to the output of either heat or electricity. One of the most important applications of this promising new energy is its use in the fuel cells driving the cars. Moreover, the combustion of hydrogen energy dose not lead to the emission of carbon dioxide (CO2) which is especially notorious as a greenhouse gas causing the global heating on the earth. Therefore, hydrogen energy is now considered as a clean energy to be used in our future energy resource.
In this context, it is considerably important for us chemists to develop highly efficient catalysts to lower the activation barriers for both the oxidation and reduction of water into oxygen and hydrogen gases, respectively. It must be also noted that, unfortunately, the human beings have been unsuccessful thus far in developing the catalysts that are applicable to the actual use.
In order to solve the above-mentioned energy issues, a large variety of multinuclear metal complexes have been developed and tested in our group (multinuclear metal complexes denote molecular compounds involving two or more metal ions) .
As an extended project, the solar energy conversion based on decomposition of water into hydrogen and oxygen gases has also been investigated for a long time in our group. After KS has moved to Kyushu University, the group started to investigate the catalytic functions of particular multinuclear metal complexes in electrochemical oxygen reduction or alcohol oxidation in view of their applications to the fuel cells.
In each system, metal complexes are required to mediate a multi-electron process in the catalysis. In other words, multinuclear metal complexes are considered as promising candidates of catalysts for such multi-electron one-step processes.
Artificial photochemical water splitting and some of the important results obtained in our group
In contrast with artificial systems, biological systems indeed involve highly efficient catalysts which are classified as 'metal complexes' (colloidal Pt or Pt nanoparticles are defined as heterogeneous catalysts, while molecular catalysts containing metal ions are defined as 'complex catalysts') .
Although nitrogen gas is extremely inert, the nitrogenase enzyme, existing in some bacteria of the root nodules in plants, converts nitrogen gas (N2) to ammonia (NH3) under the ambient temperature and pressure. At the active site of the enzyme, multinuclear Fe7Mo complexes are known to catalyze the nitrogen fixation. However, it must be noted here that the actual reaction mechanism for this process still remains unclarified.
On the other hand, the oxygen evolution process in the photosynthesis of green plants is conducted under the aid of tetranuclear manganese complex catalysts. Generation of hydrogen gas (H2) from protons (H+) and electrons (e-) as well as the reverse process is known to be promoted by the FeFe- or FeNi-containing enzymes, called hydrogenases. Attempts to develop efficient catalysts by mimicking the attractive features of such enzymes have been extensively carried out by many researchers by now, even though no one has been successful in the development of catalysts which could be employed in the actual industrial processes.
However, it must be emphasized here that the highly efficient catalytic conversion processes in biological systems are all based on metal complexes, which must be given as a result of Darwin's revolutionary theory. In contrast with heterogeneous catalysts, catalytic activities of metal complexes can be rationally controlled by modifying the electronic structures, stereochemistry, and reactivity of the complexes. Thus, the multinuclear metal complexes have unlimited possibilities for the development of our important future technologies.
Typical metal complexes containing enzymes
Some of the important results obtained thus far in our group are discussed below.
We first discovered the fact that the dinuclear Pt (II) compounds having a strong Pt-Pt interaction possess higher activity compared to the mononuclear compounds (Sakai et al., J. Mol. Catal., 1993) .
Many other 'non-platinum-containing' transition metal complexes have been tested and confirmed to be inactive at all towards the reduction of water into hydrogen gas. We believe this is interpreted in terms of the good match of affinity between the Pt and H atoms, as in the case for the electrochemical reduction of water at the Pt electrode surfaces. A large variety of new mononuclear and multinuclear platinum (II) complexes have been prepared and their H2-evolving activities have been evaluated to clarify the structure-activity relationship. We think the clarification of such relationships may lead to the rational design of catalysts with higher activity.
Furthermore, during the course of these studies, a photosensitizing complex has been tethered to an H2-evolving complex to develop a 'photo-hydrogen-evolving molecular device' which possesses two functionalities within a molecule. After our on-going research since 1991, we have recently succeeded in the development of the first example of such a complex and its functionality has bee characterized in detail (Ozawa, Haga, Sakai, J. Am. Chem. Soc., 2006) .
Thus, we think various important topics still remain unexplored in the research area of metal complexes. At the moment, we are paying great attention to the chemistry correlated with the following keywords: multinuclear metal complexes, multi-electron one-step processes, hydrogen energy, solar energy conversion, fuel cells, etc.
In addition to these important fields of chemistry, some of the metal complexes have attracted considerable attention due to their pharmaceutical applications. As an example of such activities, attempts have been made in our group to develop new 'cisplatin-derived' anticancer drugs, under the collaboration with a company.
Our research projects are listed below.