Mikio Uruichi

Dehydrocoupling reactions of borane-secondary and -primary amine adducts catalyzed by group-6 carbonyl complexes: formation of aminoboranes and borazines.

Photoirradiation of a solution of BH(3).NHR(2) (1a: R = Me, 1b: R = 1/2C(4)H(8), 1c: R = 1/2C(5)H(10), 1f: R = Et) containing a catalytic amount of a group-6 metal carbonyl complex, [M(CO)(6)] (M = Cr, Mo, W), led to dehydrogenative B-N covalent bond formation to produce aminoborane dimers, [BH(2)NR(2)](2) (2a-c, f), in high yield. During these reactions a borane sigma complex, [M(CO)(5)(eta(1)-BH(3).NHR(2))] (3), was detected by NMR spectroscopy. Similar catalytic dehydrogenation of bulkier amineboranes, BH(3).NH(i)Pr(2) (1d) and BH(3).NHCy(2) (1e, Cy = cyclo-C(6)H(11)), afforded monomeric products BH(2) horizontal lineNR(2) (4d, e). The reaction mechanism of the dehydrocoupling was investigated by DFT calculations. On the basis of the computational study, we propose that the catalytic dehydrogenation reactions proceed via an intramolecular pathway and that the active catalyst is [Cr(CO)(4)]. The reaction follows a stepwise mechanism involving NH and BH activation. Dehydrocoupling of borane-primary amine adducts BH(3).NH(2)R (1g: R = Me, 1h: R = Et, 1i: R = (t)Bu) gave borazine derivatives [BHNR](3) (5g-i).

Alternating covalent bonding interactions in a one-dimensional chain of a phenalenyl-based singlet biradical molecule having Kekulé structures.

A novel naphthoquinoid singlet biradical (2a) stabilized by phenalenyl rings is prepared by a multistep procedure and is investigated in terms of covalent bonding interactions. The molecule 2a gives single crystals, in which a 1D chain is formed with a very short π-π contact at the overlapping phenalenyl rings. The unpaired electrons in 2a are involved in covalent bonding interactions not only within the molecule but also between the molecules in the 1D chain, and a linear conjugation is made of the alternating intra- and intermolecular covalent bonding interactions through conventional π-conjugation and multicenter bonding, respectively. The linear conjugation causes a lower-energy shift of the optical transition band in the crystal, but the transition energy is higher than that of the benzoquinoid singlet biradical (1a). This optical behavior and the magnetic susceptibility measurements reveal that the intermolecular covalent bonding interaction in the 1D chain of 2a is greater in strength than the intramolecular one, despite the fact that a fully conjugated Kekulé structure can be drawn for 2a.

Resonance balance shift in stacks of delocalized singlet biradicals.

Recently we succeeded in the isolation of delocalized singlet biradicals utilizing the spin-delocalizing character of the phenalenyl radical. We demonstrated that the singlet biradical 1 has strong spin–spin interactions between molecules through the overlap of phenalenyl rings in the onedimensional (1D) chain even though the closed-shell Kekul structure 1 can be drawn as a resonance contributor (Scheme 1). Huang and Kertesz gave further insight into the spin–spin interactions from a theroretical point of view and showed that the spin–spin interaction between the molecules was predicted to be stronger than that within the molecule. These experimental and theoretical findings are associated with very fundamental issues: Do delocalized singlet biradicals actually have open-shell character? Are the electrons coupled within a molecule involved in covalent bonding between molecules? In this study we will demonstrate that intraand intermolecular spin–spin interactions strongly correlate and can be altered in magnitude by an applied external field. Our proposal is based on the experimentally determined molecular structure of 2, a temperature-dependent reflection spectrum of 2, and a pressuredependent reflection spectrum of 1. Methyl groups at the 2and 10-positions in 2, where the frontier molecular orbital has very small coefficients, are expected to alter the distance between the overlapping phenalenyl rings with respect to the analogous separation in 1, and as a result, the magnitude of the intermolecular spin–spin interaction should be affected. The synthesis of 2 is outlined in Scheme 2. The 3,10and 3,11-bis(bromomethyl) compounds 3 were synthesized according to the previously reported procedures. The individual isomers were not isolated because both were expected to lead to the single compound 2. Bis(2-methylpropionic acid) derivatives 5 were obtained in three steps by standard methods. Friedel–Crafts cyclization of the acyl chloride derivatives of 5 with AlCl3 afforded diketones 6. These were reduced with NaBH4 and subsequently dehydrated with a catalytic amount of p-toluenesulfonic acid to afford the dihydro compounds 8. Dehydrogenation of 8 with p-chloranil afforded the hydrocarbon 2 as green prisms. Compound 2 was found to be stable in the solid state at room temperature. The small HOMO–LUMO gap of 2, which is an essential factor for a singlet biradical electronic structure, was confirmed by electrochemical and optical methods. The cyclic voltammogram of 2 shows four reversible redox waves: E 2 = + 0.51, E 1 =+ 0.11, E red 1 = 1.09, and E 2 = 1.62 (V vs. ferrocene/ferrocenium couple (Fc/Fc), see Figure S1 in the Supporting Information), which led to an electrochemical HOMO–LUMO gap of 1.20 eV. The electronic absorption spectrum of 2 in CH2Cl2 shows an intense low-energy band at 756 nm (13200 cm = 1.64 eV, e= 115000, f= 0.605, see Scheme 1. Resonance structures of 1 and 2. The arrows in the biradical structure represent antiparallel spins.

Charge-transfer salts of M(mnt)2 (M=Ni, Pd, Pt, Au) with BDNT: ferromagnetic interactions in conductive (BDNT)2–[Ni(mnt)2]

Charge-transfer salts between BDNT and M(mnt)2 (M=Ni, Pd, Pt, and Au) have been prepared by mixing BDNT–(SbCl6)2 and (TBA)2 –[M(mnt)2 ] and by electrochemical oxidation of BDNT in TBA–[M(mnt)2 ] solution. Almost all powdery samples of (1:1) salts show high electrical conductivity. The valence of M(mnt)2 was determined to be –1 by the CN stretching mode, which means that the valence of BDNT is +0.5, +1, or +2. Their crystal structures are not known except for the low-conductive BDNT–[Au(mnt)2 ]2 , in which Au(mnt)2 forms dimers which are arranged in crisscross stacks. Among these compounds, (BDNT)2–[Ni(mnt)2 ] exhibits a ferromagnetic interaction with |J|=3.4 K. It is concluded from the EPR experiment that [Ni(mnt)2 ]– is responsible for the ferromagnetic interaction.

Synthesis, crystal structure, and physical properties of sterically unprotected hydrocarbon radicals.

We have prepared and isolated neutral polycyclic hydrocarbon radicals. A butyl-substituted radical gave single crystals, in which a π-dimeric structure, not a σ-bonded dimer, was observed, even though steric protection was absent. Thermodynamic stabilization due to the highly spin-delocalized structure contributes effectively to the suppression of σ-bond formation.

Infrared and Raman Study of the Charge-Ordered State in the Vicinity of the Superconducting State in the Organic Conductor β-(meso-DMBEDT-TTF)2PF6

We present spectroscopic evidence for charge ordering in β-( meso -DMBEDT-TTF) 2 PF 6 below ∼70 K. Its infrared and Raman spectra change abruptly at ∼70 K. The amplitude of the charge order was estimated to be 0.5 from the splitting of the infrared-active C=C stretching mode. The coexistence of both high-temperature and low-temperature signals was observed in a narrow temperature range (∼4 K) near the phase transition temperature. The pressure and temperature phase diagram was obtained in the vicinity of the superconducting phase. The checkerboard-type charge-order phase is not directly adjacent to the superconducting phase, but the superconducting phase is adjacent to the insulating phase where the short-range ordered domain of charge order is growing. The coexistent region expands significantly when hydrostatic pressure is applied. In the coexistent region, the crystal is inhomogeneous not just on a macroscopic scale, but also on a mesoscopic scale.

Metallic and Mott Insulating Spin-Frustrated Antiferromagnetic States in Ionic Fullerene Complexes with a Two-Dimensional Hexagonal C60.− Packing Motif

(MDABCO(+))(C60(·-))(TPC) (1), in which MDABCO(+) is N-methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene-based quasi-2D metal and contains closely packed hexagonal fullerene layers with interfullerene center-to-center distances of 10.07 Å at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m0 =6.7). The latter indicates a narrow-band nature, which is consistent with the calculated bandwidth of 0.10-0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ(+))(C60(·-))(TPC) (2) with a hexagonal arrangement of C60(·-) was obtained by increasing the interfullerene distance with the bulkier N-methylquinuclidinium cations (MQ(+)) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center-to-center distances in the layers (10.124, 10.155, and 10.177 Å at 250 K). Unit-cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1, 2 exhibits a nonmetallic spin-frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is -27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2, and thus a Mott-Hubbard insulating state with antiferromagnetically frustrated spins results.

Reversible iodine absorption of nonporous coordination polymer Cu(TCNQ)

Polycrystalline powders of Cu(TCNQ) (TCNQ = 7,7′,8,8′-tetracyanoquinodimethane) absorb iodine to form Cu(TCNQ)I4 upon solid grinding with iodine or immersion in a hexane solution of iodine. Of the two polymorphs of Cu(TCNQ), phase II Cu(TCNQ) exhibits a much slower iodine-absorption rate than that of phase I Cu(TCNQ) in the liquid-phase reaction, whereas the solid grinding reaction results in efficient absorption for both phases. The valence state of the iodine-containing salt is [Cu+I−(TCNQ0)](I2)1.5, where the copper ion is coordinated with an iodide anion and neutral TCNQ. The salt is a semiconductor (σRT = 3 × 10−3 S cm−1, compaction pellet) with an electrical conductivity one order lower than that of Cu(TCNQ). The salt releases iodine by annealing to regenerate the original phases of Cu(TCNQ) via an intermediate Cu(TCNQ)I state. A solid-state reaction of TCNQ, CuI, and iodine also produces the iodine-containing salt. The iodine absorption–desorption mechanism of Cu(TCNQ) differs from that of alkali-TCNQ salts that we reported previously.

Reversible iodine absorption by alkali-TCNQ salts with associated changes in physical properties

Alkali metal salts of 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) reversibly absorb iodine forming the ternary salts M(TCNQ)I (M = Li, Na, K) and M2(TCNQ)3I2 (M = Rb). The ternary salts are also obtained by solid-state reactions of TCNQ with alkali iodides. These salts are paramagnetic and have high electrical conductivities, ∼10−1 S cm−1, for compacted pellets, whereas the alkali metal salts of TCNQ are diamagnetic insulators. The ternary salts further absorb iodine to give over-doped salts M(TCNQ)In (n ≈ 6, M = Na, K), which gradually release iodine to give M(TCNQ)I. In contrast, the solid-state reaction of F4TCNQ and sodium iodide produces Na(F4TCNQ), which does not exhibit iodine absorption.

Raman Spectra of (Me2-DCNQI)2CuxLi1-x(0≤x ≤1): The Evidence for Charge Separation at Room Temperature in a One-Dimensional Conductor Having a Quarter-Filled Band

Raman spectra of (Me 2 -DCNQI) 2 Cu x Li 1- x (0≤ x ≤1) have been measured at room temperature, 200 K, 100 K and 5 K. The observed Raman bands are assigned on the basis of the vibrational analysis done by Lunardi and Pecile. The Raman band assigned to the a g v R8 fundamental mode (quinoid C=N stretching) shifts downward with an increase of x and exhibits a remarkable split for 0≤ x ≤0.29. This frequency shift is attributable to the change of a charge density on a Me 2 -DCNQI molecule. From the split of the v R8 Raman band, it is concluded that the charge densities on Me 2 -DCNQI molecules are not equivalent even at room temperature for 0≤ x ≤0.29.