Prof. Kazuyuki Tatsumi, Nagoya University, has received a CSJ Award for his contributions to the chemistry of transition metal chalcogenides that extends over to organometallics, inorganic materials, and to the interface between cluster chemistry and bio-inorganic chemistry. Early in his academic carrier, he was established as a theoretical chemist studying stereo-electronic properties and reactions characteristic of transition metal complexes by means of molecular orbital analyses.

The subjects were broad ranging from dioxygen complexes, metalloporphyrins, various organometallic reactions, and to lanthanide and actinide complexes. The theoretical background and concepts provided him with a basis and courage to challenge the synthesis of unprecedented transition metal chalcogenide complexes and clusters. Transition metal complexes with chalcogen ligands represent one of the most fruitful specialties in coordination chemistry, in view of their structural and electronic diversity. The significance of such chemistry is also well recognized in the area of metal clusters, metalloenzymes, and inorganic materials. However, development of transition metal chalcogenides has been hampered by lack of appropriate synthetic routes to desired compounds, due to propensity of meal chalcogenides to undergo uncontrollable disproportionation reactions associated with facile electron transfer processes, often resulting in ill-fated insoluble products. Tatsumi overcame this difficulty, and unfolded a new facet of transition metal thiolate chemistry. He also developed novel methods for the synthesis of a wide rage of dinuclear and cluster complexes, based on the newly discovered C-S(Se) bond cleaving reactions of early transition metal thiolates(selenolates) and the reactions at chalcogen ligands. His study encompasses chalcogen-based organo-element compounds, dynamic organometallic complexes, inorganic polymers, and clusters modeling the active sites of metalloenzymes. Major achievements by Tatsumi are itemized below, while they are closely inter-related.

1) Carbon-Sulfur Bond Cleavage and Reactions on Sulfur Ligands
Early study by Tatsumi dealt with C-S bond rupture of thiolates occurring at electron-deficient transition metal centers, such as Zr(IV), Nb(V), Ta(V), Mo(V), and W(V). These reactions are unique among metal-promoted C-S bond activation, because the well-established reductive fission mechanism cannot be invoked for the d0 and d1 metal complexes. The C-S bond cleaving reactions were utilized as new entries into early transition metal thio complexes. One such example is the synthesis of the first organometallic tris(thio) complexes of molybdenum and tungsten, (PPh4)[(η⁵-C₅Me₅)MS₃] (M = Mo, W). The tris(thio) complexes have widely been used as building blocks of various hetelometallic sulfide clusters, some of which exhibit intriguing non-linear optical properties. A similar synthetic approach led to the half-sandwich tungsten complex carrying three different chalcogenido (seleno, thio, and oxo) ligands, that expanded the scope of inorganic synthesis. Alternatively, the C-S bond formation was attained at the terminal thio ligands by the reactions with haloalkanes and alkynes. In particular, addition of alkynes to give dithiolene moieties is intriguing, which would provide a convenient method for preparation of the active sites of pterin-based molybdoenzymes.

2) Unusual Thiolates and Their Metal Complexes
Coordination chemistry of unusual thiolates have been developed, which include those having unsaturated carbon-carbon bonds, bulky substituents, and electronically-modulable substituents. Transition metal complexes of alkyne-, diyne-, and vinyl-thiolates were shown to facilitate the C-C bond formation between the unsaturated hydrocarbons, and the interconversion between "η¹-vinylthiolate" and "η³-thiallyl" coordination modes was observed. Study of the arylthiolate having trimethylsilyl substituents led to isolation of the first example of π-bonded arylthiolate complex, Mo{η⁵-SC₆H₃-2,6-(SiMe₃)₂}{η⁷-SC₆H₃-2,6-(SiMe₃)₂}. Although the unsymmetric π-sandwich structure is rigid at the NMR time scale, lability of the aryl-Mo π bond was manifested by the reaction with acetonitrile to give a molybdenum complex of η¹(S)-thiolate/η²-acetonitrile. On the other hand, use of bulky 2,6-dimesitylphenyl thiolate (SDmp) gave rise to a series of coordinatively unsaturated Fe(II), Ru(II), and Rh(I) complexes. The M-S bonds of these complexes exhibit unusual reactivity toward various organic/inorganic small molecules, and reversible dissociation/association of the anionic SDmp ligand was found to occur.

3) New Synthetic Routes to Transition Metal Chalcogenide Clusters and Activation of Small Molecules
Using the preformed thio complexes of Ta(V), Mo(VI), and W(V), Tatsumi investigated an extensive series of cluster forming reactions with organometallic/inorganic complexes of Fe, Ni, Cu, Ru, Rh, Ag, and Bi, to give thio-bridged heterometallic-dinuclear and cluster compounds showing interesting physicochemical properties. For instance, sulfido-bridged tungsten(VI)-ruthenium(II) dinuclear complexes activate alkynes, and promote the heterolytic cleavage of H₂ at room temperature under atmospheric pressure of H₂. Deuterium label experiments and theoretical analysis unraveled the mechanism of the H₂ activation and the reverse H₂ evolution processes. The analogous thio(oxo)-bridged dinuclear Ge-Ru complexes brought about yet another class of H₂ activation which involves heterolytic H₂ cleavage and the subsequent reaction of H₂ with the resulting μ-OH bridge to generate H₂O and hydride. Exploitation of new synthetic routes to metal chalcogenide clusters resulted in formation of a cyclic tricubane cluster [(η⁵-C₅Me₅)₂Mo₂Fe₂S₄]₃(μ-S₄)₃ via an efficient redox coupling reaction of the labile chlorides on a cubane precursor, (η⁵-C₅Me₅)₂Mo₂Fe₂S₄Cl₂, which was prepared by the FeCl₃-induced C-S bond breaking reaction of molybdenum t-butylthiolate. Interestingly, the Fe-Fe bond within each cubane is cleaved and three inter-cubane Fe-Fe bonds are formed, as the tricubane is aggregated into the tricubane.

4) Modelling Active Sites of Nitrogenase and Hydrogenase
The study of transition metal chalcogenides have evolved into synthesis of the cluster compounds modeling the active sites of nitrogenase and [NiFe]-hydrogenase. The Fe₈S₇ core of the P-cluster in nitrogenase is unique among the known ironsulfide clusters, and has been thought so unstable that it exists only in protein environments. Tatsumi discovered that this unusual Fe₈S₇ structure can be self-assembled from the reaction of Fe(II) bis-amide, tetramethylthiourea, 2, 4, 6-triisopropylbenzenethiol, and elemental sulfur in a specific molar ratio in non-polar solvents. Furthermore, the reactions of molybdenum precursors with appropriate iron complexes and sulfur reagents gave rise to a series of Fe/Mo/S clusters, and surprisingly an unsymmetric MoFe₅S₉(SH) was constructed in a self-assembly manner, the geometry of which bears a close resemblance to the FeMo-co structure. Tatsumi also accomplished the synthesis of a range of unprecedented thiolate-bridged Fe(CO)₃-Ni complexes from the preformed tetranuclear Fe₂Ni₂ precursor at low temperatures, while the reaction of Fe/CO/CN/thiolate complex with nickel/dithiocarbamate/bromide led to the thiolate-bridged dinuclear Fe(CO)₂(CN)₂-Ni complexes. These Fe-Ni complexes reproduce well the structural feature of the active sites of [NiFe] hydrogenase. The Fe₈S₇ and MoFe₅S₉(SH) clusters and the dinuclear Fe-Ni complexes are the first reliable structural models of the active sites of nitrogenase and [NiFe] hydrogenases, which would facilitate better understanding of the structure-function relationship of these metalloenzymes.

Prof. Tatsumi obtained his B. Sc. from Osaka University in 1971 majoring in chemical engineering, and completed his Ph. D. in 1976 studying on theoretical inorganic chemistry. He then moved to the USA, where he undertook postdoctoral research in the groups of M. Tsutsui at Texas A&M University and Roald Hoffmann at Cornell University where he learned beauty of chemistry. In 1982 he became assistant professor at Osaka University, where he began transition metal chalcogenide chemistry, and was promoted to associate professor in 1991. In 1994 he took up his current position as professor at Nagoya University, and was appointed as Director at Research Center for Materials Science in 2003. He also spent a period in 1985 as visiting professor at University of Helsinki, Finland, and in 1985 at EPFL, Switzerland. He was awarded a Inoue Prize for Science in 1998, and a Humboldt Research Award in 2004.