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Creation of Novel Silicon Compounds with Unique Electronic Properties
Prof. Mitsuo Kira, Tohoku University, has made a remarkable contribution in the creation of a number of silicon and related group-14 element compounds having unusual bonding and structure, discovery of their unique properties and reactions, and elucidation of their origin. The following are his major achievements:
(1) Creation of the first stable dialkylsilylene that is least electronically perturbed. Whereas silylenes are usually short-lived reactive intermediates, Kira and his collaborators have succeeded in the synthesis of a stable cyclic dialkylsilylene that is sterically protected by a unique helmet-like ligand. Although several stable silylenes have been synthesized, they are substituted by two amino groups and stabilized by strong electron donation from
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Prof. Mitsuo Kira
Department of Chemistry, Graduate School of Science,
Tohoku University
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filled pπ-type orbitals of the nitrogen atoms to the vacant pπ-orbital of the divalent atom, and hence, the electronic structure of the silylenes is strongly modified from that of the parent silylene. Kira et al. have elucidated the structural characteristics of the dialkylsilylene by X-ray crystallography, UV and NMR spectroscopies, and theoretical calculations and concluded his silylene is sterically well-protected from the dimerization but the least electronically perturbed. By using the stable dialkylsilylene, they found various novel reactions that had never been observed for transient silylenes. They elucidated the unique electronic structure and reactivity of the lowest singlet excited state and the radical anion of the dialkylsilylene, which were generated as novel electronic systems. The synthesis of a series of stable silylene, germylene, and stannylene with the same helmet substituents was achieved by his group to allow the systematic study of the element dependence of the structure and reactivity of the divalent group-14 element compounds.
(2) Creation of unique Si=Si doubly-bonded compounds (disilenes) including spiropentasiladiene and trisilaallene. Since the first synthesis of a stable disilene in 1981, much attention has been focused on the chemistry of silicon-based unsaturated compounds. However, the type of stable disilenes whose structures were determined by X-ray crystallography had been limited for a long time only to acyclic disilenes with at least two aromatic substituents. Kira is a pioneer of the chemistry of stable disilenes having sterically bulky trialkylsilyl substituents and took the initiative in the global expansion of the chemistry of stable disilenes. As the first isolable disilenes without aromatic substituents, Kira et al. synthesized various kinds of stable tetrakis(trialkylsilyl)disilenes. His group achieved the synthesis of a cyclotetrasilene and a cyclotrisilene as the first stable cyclic Si=Si compounds and discovered the interesting interconversion between the cyclotetrasilene and the corresponding tetrasilabicyclo[1.1.0]butane. His group succeeded in the isolation of a stable spiropentasiladiene and revealed the effective spiroconjugation in this system using UV-vis and 29Si NMR spectroscopy as well as theoretical calculations. Since carbon-based spiropentadienes have not been obtained yet as stable compounds, the results are especially valuable. Kira et al. have achieved the synthesis of fascinating trisilaallene and its germanium congeners by applying the chemistry of the stable dialkylsilylene and germylene above mentioned. Remarkably, the trisilaallene is the first isolable compound with a formal sp-hybridized silicon atom. In contrast to linear carbon allenes having two mutually perpendicular π bonds, the trisilaallene skeleton was found to be significantly bent and remarkably fluxional; the facile rotation of the central silicon atom in the trisilaallene around the axis through the two terminal silicon atoms was observed even in the solid state. The trisilaallene may be a useful starting point for the synthesis of novel silicon-based materials with a variety of electronic structures.
(3) Creation of a 1,3-disilabicyclo[1.1.0]butane as the first isomer with an inverted bridge Si-Si σ bond. Although theoretical studies predict the existence of two isomers having a short (banana bond) and a long bridge σ bond (inverted s bond) isomers, no experimental evidence for the isomer with an inverted bridge bond had been obtained. The first 1,3-disilabicyclo[1.1.0]butane derivative was synthesized by the reaction of adamantanone with a 1,2-dilithiodisilane that was derived from a stable tetrakis(trialkylsilyl)disilene developed by his group. The disilabicyclobutane was characterized by X-ray crystallography and UV-vis and NMR spectroscopies as the first bicyclo[1.1.0]butane derivative with an inverted bridge σ bond.
(4) Development of the chemistry of disilene-transition metal complexes. A number of studies have been directed towards the chemistry of alkene transition metal complexes because of their importance in the catalytic organic synthesis. In contrast, there have been reported a limited number of studies on transition metal complexes of disilenes. Kira et al. have developed a new chemistry of disilene transition metal complexes. They have synthesized the first 14-electron palladium η2-disilene complex that has the strongest π-complex character among the known disilene complexes. They have succeeded also in the isolation of (E)- and (Z)-isomers of disilene complexes and revealed a unique E,Z-isomerization through the corresponding silylsilylene complex intermediate.
(5) Creation of a cross-conjugated compound with an Si=C double bond. The first 4-silatriafulvene derivative was synthesized by his group as a stable compound. Its unique structural characteristics and reactivities due to the less-polar Si=C double bond were revealed both experimentally and theoretically.
The scientific interests of Kira are not limited to unsaturated silicon and related compounds but extended to rather wide aspects of the chemistry of heavier group-14 elements. Among his contributions to the other aspects are helical regulation of linear oligosilanes and polysilanes achieved by the introduction of terminal chiral groups, theoretical studies of the mechanisms for 1,3-silyl migrations in allylsilanes and formylmethylsilanes, elucidation of the intramolecular charge-transfer fluorescence of aryldisilanes, and control of photochemical reactions using a non-resonant two-photon excitation method.
Kira's contributions are not confined to the synthesis and characterization of the novel compounds but also demonstrated by the success in bringing out the unique and important features in the bonding, structure, and reactions that are hidden in the compounds. His outstanding achievements are always inspiring the chemistry of not only silicon and heavier group-14 elements but also the entire main-group elements, and hence, are highly admired internationally and deserve the Chemical Society of Japan Award.
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