Masaaki Nakamoto

High‐Power Electrochemical Energy Storage System Employing Stable Radical Pseudocapacitors

The development of electrical energy storage devices that can operate at high charge and discharge rates is fundamentally important, however although electrochemical capacitors (ECs) can charge and discharge at high rates, their electrochemical storage capacity remains an order of magnitude lower than that of conventional lithium-ion batteries. Novel pseudocapasitors are developed, based on the stable persilyl-susbtituted free radicals of the heavy group 14 elements, (tBu2 MeSi)3 E(.) [E=Si (1), Ge (2), and Sn (3)], as anode materials for energy storage system. Such systems showed a remarkable cycle stability without significant loss of power density, in comparison with similar characteristics of the known organic radical batteries, the dual carbon cell, and the electrochemical capacitor. Particularly important is that these novel electrochemical energy storage systems employing stable heavy group 14 element radicals are lithium-free. The electrochemical properties and structures of the reduced and oxidized species were studied by the cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and X-ray diffraction (XRD).

Stereomutation and apicophilicity of diastereomeric spirophosphoranes (10-P-5)

Abstract The apicophilicity for a series of monodentate groups were determined by deducing activation parameters of equilibration between diastereomeric spirophosphoranes bearing a Martin ligand and a modified Martin ligand. The equilibration was shown to be an intramolecular process and the order of apicophilicity of the groups turned out to be OMe≈H>COMe≈SMe>NMe2>Me>n-Bu on the basis of activation enthalpy. The involvement of π-conjugation type interactions was revealed by X-ray structures of symmetric analogs bearing COMe (acceptor) and NMe2 (donor) groups.

Stereochemistry of nucleophilic substitution reactions of sterically rigid phosphoranes

Abstract The stereomutation barriers of diastereomeric spirophosphoranes, [o- O(CF 3 )(CH 3 )C ∗ C 6 H 4 ][o- O(CF 3 ) 2 C 6 H 4 ] P ∗ X (X=OMe, SMe) were determined to be high enough to freeze the mutation at ambient temperatures [ ΔG ≠ (298K)=25.0, 30.4 kcal mol −1 ; respectively]. The nucleophilic substitution reaction of SMe compounds with alkyllithium reagents resulted in inversion of configuration, whereas that of OMe compounds gave various ratios of inversion and retention products depending on the stereochemistry of the diastereomeric reactant phosphoranes and solvent. However, the use of OCH 2 CH 2 NMe 2 as substituent lead to almost exclusive formation of the retention product regardless of these conditions.

Perfluoroaryltetrahedranes: Tetrahedranes with Extended σ−π Conjugation

The first stable aryl-substituted tetrahedrane derivatives 4-6 were synthesized by the reaction of tris(trimethylsilyl)tetrahedranyllithium with hexafluorobenzene or [(pentafluorophenyl)ethynyl]benzene in THF. Tetrahedranes having the fluoroaryl groups as electron-withdrawing substituents were found to be thermally stable up to 170 degrees C. X-ray analyses of 4 and 6 and UV-vis absorption spectra of 4-6 suggest sigma-pi conjugation between the strained tetrahedrane core and the benzene ring, which causes a considerable bathochromic shift.

Non-dissociative permutation of a stable hydrophosphorane and retention of stereochemistry of the electrophilic reaction of its conjugate phosphoranide base

Abstract The permutation of [o- OC ∗ (CH 3 )(CF 3 )C 6 H 4 ][o- OC(CF 3 ) 2 C 6 H 4 ]PH was found to be a non-dissociative process with activation parameters of ΔH≠=ca.24.5 kcal mol−1 and ΔS≠=ca.−4 eu. The reaction of its conjugate base, phosphoranide [o- OC ∗ (CH 3 )(CF 3 )C 6 H 4 ][o- OC(CF 3 ) 2 C 6 H 4 ]P − , with ethyl chloroacetate was found to proceed with full retention of configuration.

Isolation of Hypervalent Group-16 Radicals and Their Application in Organic-Radical Batteries

Using a newly prepared tridentate ligand, we isolated hypervalent sulfur and selenium radicals for the first time and characterized their structures. X-ray crystallography, electron spin resonance spectroscopy, and density functional theory calculations revealed a three-coordinate hypervalent structure. Utilizing the reversible redox reactions between hypervalent radicals and the corresponding anions bearing Li(+), we developed organic radical batteries with these compounds as cathode-active materials. Furthermore, an all-radical battery, with these compounds as the cathode and a silyl radical as the anode, was developed that exhibited a practical discharge potential of ∼ 1.8 V and stable cycle performance, demonstrating the potential of these materials for use in metal-free batteries that can replace conventional Li-ion batteries where Li is used in the metal form.

Sulfur-Substituted Tetrahedranes

The stable sulfur-substituted tetrahedrane derivatives 2-4 were synthesized by the reaction of tris(trimethylsilyl)tetrahedranyllithium 1 with diphenyl disulfide, bis(4-nitrophenyl) disulfide, and bis(2,4-dinitrophenyl) disulfide, respectively, and characterized by both NMR spectroscopy and X-ray crystallography. Phenylsulfonyltetrahedrane 5 was prepared by the reaction of 2 and m-chloroperbenzoic acid. The UV-vis absorption spectra of 2-4 suggested an interaction of the σ orbital of the tetrahedrane core and the lone-pair electrons on the sulfur atom, whereas no interaction for 5 was found. Thermal reactions of 2 and 5 are also reported; 2 underwent fragmentation into two acetylene molecules, whereas 5 gave the corresponding cyclobutadiene.

Electrochemical Properties and Computations of Stable Radicals of the Heavy Group 14 Elements (Si, Ge, and Sn)

A series of stable radicals centered on persilyl-substituted heavy Group 14 elements, (tBu(2)MeSi)(3)E(*) (E = Si, Ge, Sn), was studied by cyclic voltammetry in different solvents to determine their first oxidation and reduction potentials and to compare their ease of oxidation and reduction with known experimental and computed ionization energies (E(i)) and electron affinities (E(ea)), respectively. It has been observed that all of the first oxidation and reduction potentials for the three radicals studied are irreversible in o-dichlorobenzene (o-DCB), whereas the reduction waves are quasi-reversible in THF. A good correlation has been found between measured oxidation potentials and ionization energy values, but no correlation between reduction potentials and electron affinity values was found, probably due to kinetic and surface effects.

3Δ-1,2,3,4-Disilagermastannetene: the first cyclic germastannene

Abstract3Δ-1,2,3,4-Disilagermastannetene, the first cyclic germastannene, was quantitatively synthesized by the reaction of 1- and 2-disilagermirenes with the dichlorostannylene--dioxane complex in THF. The title germastannene is quite stable in the solution, which was explained by the appreciable π(Ge=Sn)-σ*(Si--Cl) orbitals mixing.

Photoisomerization of Perfluoroaryltetrahedranes to Perfluoroarylcyclobutadienes

Two perfluoroaryl-substituted cyclobutadiene derivatives, 6 and 7, were prepared as air- and moisture-sensitive red solids by the photochemical isomerization of the corresponding tetrahedranes (4 and 5, respectively). Remarkably, the 9,10-dicyanoanthracene-sensitized photochemical reaction of 4 also proceeded, giving 6, and the mechanism of this reaction is also discussed. The first aryl-substituted cyclobutadienes were characterized by spectroscopic data as well as by X-ray crystallography for 6, showing a distorted rectangular structure with extremely long C-C single bonds.