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1. Chemistry of Stable Compounds with a Silicon-Silicon Triple Bond
Unsaturated hydrocarbons, alkenes, and alkynes are among the most fundamental classes of organic compounds. The analogues of alkenes R2E=ER2 and alkynes RE≡ER of the heavier Group 14 elements (E = Si-Pb) were originally thought to be synthetically inaccessible. The synthesis of the first stable disilene, tetramesityldisilene Mes2Si=SiMes2, was achieved by West and co-workers in 1981. Since then, a molecule with a silicon-silicon triple bond, RSi≡SiR, has been an important target, but countless attempts to synthesize such a molecule had failed. Sekiguchi designed a bulky and electropositive R group that could sterically and electronically stabilize a disilyne and, using this group, he was able to synthesize the first isolable disilyne RSi≡SiR (R = SiiPr[CH(SiMe3)2]2) in 2004. To understand the nature of the p-bonding of the silicon-silicon triple bond, he also investigated the reactivity of the disilyne toward a variety of reactants, such as alkenes, alkynes, RLi (R = Me, tBu), alkali metals, nitriles, silyl cyanides, amines, hydroboranes, 1,3,4,5-tetramethylimidazol-2-ylidene, and 4-dimethylaminopyridine, which has opened a new field of unsaturated heavier Group 14 element chemistry. He has also synthesized an unsymmetrically substituted disilyne.
2. Dilithiosilane and Disilenides and Their Application in the Synthesis of Heavy Alkenes
Until Sekiguchi developed a general synthesis, there were few known unsymmetrical R2E=ER′2 or heteronuclear R2E=E′R2 (E, E′ heavier Group 14 elements) dimetallenes. Their preparation was made possible by reductive cleavage of silacyclopropene and germacyclopropene, respectively, to dilithiosilanes (R2SiLi2) and dilithiogermanes (R2GeLi2), whose condensation with dihalosilanes, germanes, and stannanes gave easy access to a variety of dimetallenes. This has led to a considerable increase in our knowledge of the structure and reactivity of heteronuclear dimetallenes. He was also able to synthesize a disilenyllithium, a silicon version of vinyllithium, as an isolable compound. The disilenyl anions have been shown to be very useful synthetic blocks for the preparation of a variety of unsaturated silicon compounds.
3. Chemistry of Heavy Analogues of Carbenium Ions: Si-, Ge-, and Sn-Centered Cations
The synthesis of the analogues of carbenium ions of the heavier Group 14 elements, R3E+ (E = Si, Ge, Sn), was one of the long-standing goals in organometallic chemistry. The synthetic approaches, which were used for stable carbenium ions in organic chemistry, proved to be rather inefficient in the synthesis of the heavier Group 14 cations because of their high electrophilicity and large size, which leads to their intrinsic kinetic instability. Sekiguchifs novel approach to structurally characterized long-lived free cations of Group 14 elements involved the preparation of cyclopropene analogues with rings composed entirely of silicon and germanium atoms. These were converted to fully delocalized trisila- and trigermacyclopropenylium ions, respectively. In addition to the heavier aromatic compounds with a 2π-electron system, he also achieved the synthesis of free Si and Ge cations with homoaromatic and bishomoaromatic natures. The all-Si version of cyclobutenylium ion, homocyclotrisilenylium ion, was prepared by the demethylation of cyclotrisilene followed by ring expansion.
4. Chemistry of Isolable Si-, Ge-, and Sn-Centered Radicals
Long-lived carbon-centered radicals have been known for nearly a century, while silicon-centered analogues were reported to be only transient species. No isolable Si-centered radical was known until Sekiguchifs work. In 2002, he synthesized the cyclotetrasilenyl radical, a silicon version of the cyclobutenyl radical, as the first stable silyl radical. The synthesis of this silyl radical was accomplished by the one-electron reduction of its cationic cyclotetrasilenylium ion precursor. A second class of stable unconjugated silyl, germyl, and stannyl radicals was accessed by oxidation of tris(silyl)silyl, germyl, and stannyl anions. Sekiguchi also opened the chemistry of oligoradicals with two (or more) heavier Group 14 element radical centers, such as meta-disilaquinodimethane, which has a triplet ground state.
5. Chemistry of Anion- and Cation-radicals of the Heavier Group 14 Elements
Sekiguchi also developed the chemistry of isolable anion-radicals and cation-radicals of the heavier Group 14 elements. The first stable disilene anion-radical was isolated as a solvent-separated ion pair by the one-electron reduction of the highly sterically congested disilene. The anion radical of distannene was also prepared. The synthesis of a stable silylene anion-radical was achieved by the two-electron reduction of the disilene followed by the addition of a crown ether. The disilene cation-radical was also prepared as a stable compound upon the one-electron oxidation of the disilene with trityl cation. The disilyne anion-radical was also isolated by the reaction of disilyne with potassium graphite.
6. Chemistry of Heavy Analogues of the Cyclobutadiene Dianion and Cyclopentadienide, and Their Complexations with Transition Metals
The first heavy analogue of cyclobutadiene dianion, tetrasilacyclobutadiene dianion, was synthesized and the structure was characterized by X-ray crystallography, showing that the Si4 ring is markedly puckered with the K+ ions, above and below, dihapto-coordinated to the ring. Sekiguchi also synthesized the first analogue of cyclopentadienide ion incorporating three Si atoms in the ring. The trisilacyclopentadiene derivative, featuring three skeletal Si atoms and a Si=Si-C=C double-bond system, underwent reductive cleavage of the Si-Si bond, forming the trisilacyclopentadienide ion, which was isolated as the lithium salt. The ability of these heavy analogues of the cyclobutadiene dianion and cyclopentadienide to form complexes with transition metals has also been developed.
These outstanding achievements in the chemistry of low-coordinate organosilicon and heavier Group 14 elements place Sekiguchi as one of the key figures in the field, and he has become one of the most prominent scientists in main group element chemistry as well as organometallic chemistry. He undoubtedly deserves The Chemical Society of Japan Award 2011.
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