Takuya Kuwabara

Curved Oligophenylenes as Donors in Shape‐Persistent Donor–Acceptor Macrocycles with Solvatofluorochromic Properties

Many optoelectronic organic materials are based on donor-acceptor (D-A) systems with heteroatom-containing electron donors. Herein, we introduce a new molecular design for all-carbon curved oligoparaphenylenes as donors, which results in the generation of unique shape-persistent D-A macrocycles. Two types of acceptor-inserted cycloparaphenylenes were synthesized. These macrocycles display positive solvatofluorochromic properties owing to their D-A characteristics, which were confirmed by theoretical and electrochemical studies.

Synthesis, Structures, and Electronic Properties of Triple- and Double-Decker Ruthenocenes Incorporated by a Group 14 Metallole Dianion Ligand

The neutral triple-decker ruthenocenes and anionic ruthenocene bearing a stannole dianion were successfully synthesized by the reactions of dilithiostannoles with [Cp*RuCl]4. This is the first example of a transition-metal complex bearing a group 14 metallole dianion with μ-η(5):η(5) coordination mode. These complexes were fully characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis. In the complexes, each of the ruthenium atoms is coordinated by the stannole ring in an η(5)-fashion. The aromaticity of the stannole dianion moieties is retained judging from no C-C bond alternation in the stannole rings. CH/π interaction was found in the packing structure of the SiMe3 derivative, which leads to a well-ordered column-like structure. The oxidation wave of the triple-decker complex was observed at -0.43 V (vs ferrocene), which reveals that the triple-decker type heavy ruthenocene is oxidized more easily than the ferrocene. Comparison of the oxidation potential between the triple-decker complex and decamethylruthenocene (Cp*2Ru, Cp* = η(5)-C5Me5) reveals that a stannole ligand functions as an electron-donating ligand much stronger than the conventional electron-rich Cp* ligand.

(η4-Butadiene)Sn(0) Complexes: A New Approach for Zero-Valent p-Block Elements Utilizing a Butadiene as a 4π-Electron Donor

Research on zero-valent p-block elements is a recent hot topic in synthetic and theoretical chemistry because of their novel electronic states having two lone pairs in both the s- and p-orbitals. It is considered that σ-donating ligands bearing large substituents are essential to stabilize these species. Herein, we propose a new approach using butadiene as a 4π-electron donor to stabilize zero-valent group 14 elements. During our study to explore the coordination chemistry of stannacyclopentadienyl ligands, unexpected products, in which the tin atom is coordinated by a butadiene in a η(4)-fashion, were obtained. Because butadiene is a neutral 4π-electron donating ligand, the formal oxidation number of the tin atoms of the products should be zero, which is supported by X-ray diffraction analysis and theoretical calculations. A mechanism for the formation of the products is also described.

Diversity of the Structures in a Distannene Complex and its Reduction to Generate a Six‐Membered Ti2Sn4 Ring Complex

In contrast to olefin complexes, their congeners of heavier elements display various coordination modes, and their complexes may be present as bis(metallylene) complexes, with side-on coordination, as metallacyclopropanes, or as π complexes. In the course of our studies on the reactivity of dilithiostannoles towards transition-metal reagents, three-membered TiSn2 and six-membered Ti2 Sn4 ring complexes were obtained. According to its geometric parameters, NMR analysis, and theoretical calculations, the TiSn2 complex cannot be categorized into any of these previously described bonding modes. Therefore, a novel resonance structure has been proposed for a complex that has a delocalized σ-orbital over the TiSn2 ring to understand its electronic structure. The mechanism for the formation of the Ti2 Sn4 ring complex and its EPR spectrum are also discussed.

Facile Synthesis of Dibenzopentalene Dianions and Their Application as New π-Extended Ligands

Reduction of phenyl(silyl)ethynes with potassium followed by quenching with iodine gave dibenzopentalenes in moderate yields. The intermediates of the reactions, dipotassium dibenzopentalenides, were isolated. The first dibenzopentalene-transition-metal complex was successfully synthesized. The ruthenium atoms are located above the six-membered rings. However, X-ray diffraction analysis and theoretical calculations revealed that the aromatic nature of the five-membered rings was retained. The cyclic voltammetry of the Ru complex revealed two oxidation waves with relatively large separation.

Unexpected formation of Ru2Sn2 bicyclic four-membered ring complexes with butterfly and inverse-sandwich structures.

Reactions of tetraethyldilithiostannole 1 with [Cp*RuCl]4 afforded not η(5)-stannole dianion complexes but two novel bis(stannylene)-bridged dinuclear ruthenium complexes, which have butterfly and inverse-sandwich structures, respectively, depending on the stoichiometry of [Cp*RuCl]4 toward dilithiostannole. The redox behavior between the two complexes is found to be reversible. The molecular structures were determined by X-ray diffraction analysis. The Ru-Ru bond of the butterfly complex is 2.3428(6) Å, which is the shortest among those of dinuclear ruthenium complexes having Cp or Cp*Ru units. Theoretical calculations revealed that the very short Ru-Ru bond is due to the presence of one σ bond between the ruthenium atoms and two three-centered bonds delocalized over the two Ru2Sn rings.

1,1'-Di-tert-butyl-2,2',3,3',4,4',5,5'-octa-ethyl-1,1'-bis-tannole.

The title compound, [Sn2(C4H9)2(C12H20)2], has two 1-stannacyclopentadiene skeletons related by inversion symmetry located at the mid-point of the Sn—Sn bond [2.7682 (2) Å]. Thus, the asymmetric unit comprises one half-molecule. The planarity of the stannacyclopentadiene ring is illustrated by the dihedral angle of 0.3 (1)°, defined by the C4 and C—Sn—C planes. To avoid steric repulsion, the two stannole rings are oriented in an anti fashion through the Sn—Sn bond. These structural features are similar to those of other bistannoles.