Kagetoshi Yamamoto

Electronic structure of a stable phenalenyl radical in crystalline state as studied by SQUID measurements, cw-ESR, and 13C CP/MAS NMR spectroscopy

Abstract A neat single crystal of phenalenyl radical was obtained for the first time by introducing the substituent group of tert -butyl X-ray structure analysis of the crystal of 2,5,8-tri- tert -butylphenalenyl( 2 ) showed that the two planar radicals face each other with inversion symmetry and are dimerized in the monoclinic crystal. This is supported by the magnetic susceptibility(χ) measurement of the crystalline solid state of 2 The observed temperature dependence of χT was reproduced by assuming the thermal equilibrium between a singlet ground state and excited triplet state of the dimer (2 J/k B = −2000K) with traces of paramagnetic impurity molecules (0.3mol%). The electronic structure of the radical dimer in the crystalline state has been identified also by single-crystal ESR and 13 C CP/MAS NMR measurements. The origin of the large antiferromagnetic interaction is discussed in terms of both the experimentally determined π-spin density distribution of monomer radicals and π MO calculations.

A simplest isolable monocyclic thiepin. Synthesis, structure, and thermal stability of 2,7-di-tert-butylthiepin

Abstract As an example of a simplest isolable monocyclic thiepin, 2,7-di-tert-butylthiepin (2) has been synthesized from 2,6-di-tert-butylthiopyrylium salt, and the thermal properties together with the X-ray crystal structure of (2) have been examined.

Effective exchange integrals and chemical indices for a phenalenyl radical dimeric pair

Abstract UHF, spin-polarized DFT, and CASSCF calculations have been performed for a 2,5,8-tri- t -butylphenalenyl radical dimeric pair and its simplified model to elucidate the characteristics of chemical bonding and the origin of strong antiferromagnetic coupling. The results show that the SOMO–SOMO overlap interaction is responsible for the strong antiferromagnetic interaction and intermediate covalent-bonding between the phenalenyl radicals. The t -butyl groups in the three β-positions hardly affect the magnetic coupling, simply playing steric hindrances in a crystalline state. Chemical indices can explain the bonding character and the efficiency of the electron correlation for the dimeric pair.

Tetra-tert-butyl-as-indaceno[1,2,3-cd:6,7,8-c′d′]diphenalene: a four-stage amphoteric redox system ☆

Tetra-tert-butyl-as-indaceno[1,2,3-cd:6,7,8-c′d′]diphenalene (TTB-as-IDPL) was prepared and found to behave as a four-stage amphoteric redox compound. The properties of its five redox states were investigated.

Electronic-spin and columnar crystal structures of stable 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl radical

Abstract 1,3-Diazaphenalenyl is a typical example of the isoelectronic mode of heteroatomic modification for phenalenyl. Recently, we have designed and isolated 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl (6) as the first example of stable azaphenalenyl. For further elucidation of the electronic and solid-state structures of 6, 15N atoms incorporated 1,3-diazaphenalenyl 7 was designed. New nitration reaction by K15NO3 of 2,7-di-tert-butylnaphthalene has enabled us to accomplish effective introduction of 15N atoms and synthesis of 7. The spin structure of 1,3-(15N2)diazaphenalenyl 7 was unequivocally determined by the ratio of the hfcc for 14N and 15N (A14N/A15N=0.292/0.409=0.714), which was equal to the ratio of each gyromagnetic ratio (γ14N/γ15N=0.713). Under an air atmosphere, the radical 7 decomposes slowly, but most of it remains unchanged for weeks. By comparing the spin densities of 7 with those of parent phenalenyl radical 2, it appears that the spin densities of the 1- and 3-positions decrease appreciably, while those of the 4-, 6-, 7-, and 9-positions increase. The radical 7 forms the syn-dimer with gable structure in the crystal, and the dimer stacks in a columnar structure motif.

Electronic Structure of a Stable Phenalenyl Radical as Studied by ESR/ENDOR, Paramagnetic NMR Spectroscopy and SQUID Measurements

Abstract We have studied magnetic properties and electronic structures of a novel persistent neutral radical, 2,5,8-tri-tert-butylphenalenyl(2), which is the first example of alternant hydrocarbon π-radical isolated in the crystalline state. The observed magnetic susceptibilities of polycrystalline sample 2 were reproduced by assuming the thermal equilibrium between singlet ground state and excited triplet state of the dimer (2J/k B = −2×103 K) with traces of paramagnetic impurity molecules (0.3mol%). The π-spin density distribution of radical 2 was determined by liquid-phase cw-ESR, 1H-ENDOR/TRIPLE, and paramagnetic 1H-NMR measurements. The origin of the large inter-molecular antiferromagnetic interaction in the crystal 2 was explained by an occurrence of effective SOMO-SOMO overlap between the two radicals on the six equivalent α-positions with large positive spin densities of the carbon sites.

Lewis acid-promoted cyclization of heteroatom-substituted enynes

Lewis acid-promoted cyclizations of heteroatom-substituted enynes have been examined. The reaction of enynes and bearing silicon substituents on an alkyne afforded the halogenated five-membered gamma-lactones and gamma-lactams as the main products. The reaction of substrates and having 2-phosphonoacrylate instead of malonate also gave halogenated five-membered cyclic compounds and as the major products. The cyclized products are highly substituted and potentially useful for further synthetic transformations.

The first non-Kekulé polynuclear aromatic high-spin hydrocarbon: Generation of a triangulene derivative and band structure calculation of triangulene-based high-spin hydrocarbons

An attempt has been made to generate a triangulene derivative (3), the first non-Kekule polynuclear aromatic hydrocarbon. By oxidizing a corresponding precursor in a toluene solution by p-chloranil, the desired ground-state triplet species in the rigid matrix was identified by ESR spectroscopy. The spin Hamiltonian parameters were determined by spectral simulation as S = 1, g = 2.003, |D/hc| = 0.0073 cm -1 , |E/hc| 0 cm -1 . The mean inter-spin distance was estimated to be 5.64 A by a point-dipolar approximation, being in consistent with the spin-delocalized nature of triangulene 3. It has turned out that 3 is stable enough to give the triplet ESR spectra, but the partial exposure of reactive carbon sites with the largest spin density causes the intermolecular chemical-bond formations. The band structure calculations have been carried out for triangulene-based high-spin polymeric systems, demonstrating the generation of a flat π-nonbonding crystal orbital, which delocalizes on the whole system. The flat band with N-fold degeneracy accommodates N ferromagnetically exchange-coupled spins in the ground state.

A synthesis of a doubly-bridged [24] annulene

Abstract Reductive coupling of 1, 6-bis(2-formylvinyl)cyclohepta-1, 3, 5-triene with a low-valent titanium reagent afforded a paratropic bismethano [24] annulene.

The chemistry of phenalenium systems XXXIII. The Dibenzo[de;thi]naphthacenyl dication and the dianon

Abstract The dibenzo[ de ; hi ]naphthacenyl dication ( 4 ) and dianion ( 5 ) have been generated from a mixture of the corresponding precursor hydrocarbons. The pmr spectra of 4 and 5 indicated that the positive and the negative charges were found to be fully delocalized over the molecules, respectively, consistent with the C 2v -symmetry structures. The pmr chemical shifts of 4 correlate well with the Huckel charge densities.