A strong hydride donating, acid stable and reusable 1,4-dihydropyridine for selective aldimine and aldehyde reductions.

A 1,4-dihydropyridine derivative, lacking carbonyl groups and containing bulky aryl substituents, was synthesized and found to have a high hydride donating ability, acid resistance and reusability. Thermodynamic parameters for electron and hydride transfer in the redox system comprising the 1,4-dihydropyridine and its corresponding pyridinium ion were determined. In addition, studies showed that the 1,4-dihydropyridine with steric hindrance can be used to promote efficient, boron trifluoride catalyzed selective reduction reactions of aldimines and aldehydes under mild conditions.

Molecular and Spin Structures of a Through-Space Conjugated Triradical System.

A through-space conjugated multi-phenalenyl triradical 1 a has been prepared and characterized. Partial occupancy of doubly degenerate molecular orbitals in 1 a leads to Jahn-Teller distortion, creating a molecular skeleton in which C2v doublet states are lower in energy than undistorted D3h doublet and quartet states. Triradical 1 a exists in an acute form in the solution state, whereas it adopts a preferred obtuse form in the solid state. The results of the investigation show that these two distorted forms are reversibly interconverted by thermal energy.

Spin-Spin Interactions in One-Dimensional Assemblies of a Cumulene-Based Singlet Biradical.

The synthesis of phenalenyl-endcapped [5]cumulene as a cumulene-based singlet biradical and the spin correlation changes of one-dimensional aggregates are described. The high propensity for self-aggregation of phenalenyl rings and the introduction of bulky substituents into the appropriate positions led to the formation of a one-dimensional chain assembly. Single-crystal X-ray structural analysis indicated that the bond length alternation of the cumulene chain increased with decreasing temperature, along with improved overlapping of the phenalenyl rings. Variable-temperature Raman spectroscopy and magnetic susceptibility measurements revealed that a localized spin pair within the molecule decouples at low temperatures, and a continuum spin system involving intra- and intermolecular spin-spin interactions emerges in the one-dimensional chain.

Stable Singlet Biradicals of Rare-Earth-Fused Diporphyrin-Triple-Decker Complexes with Low Energy Gaps and Multi-Redox States.

Dinuclear rare-earth (TbIII , YIII ) triple-decker complexes with a fused diporphyrin (FP) and two tetraphenylporphyrin (TPP) ligands were synthesized in neutral, dianionic, and diprotonated forms. The neutral forms have large open shell biradical character, as determined from the temperature dependency of the magnetic susceptibility measurements and theoretical calculations. The coupling value (J=-1.4 kcal mol-1 , -487 cm-1 ) of the radical centers in the neutral form of the YIII complex indicates weak pairing interactions. Theoretical calculations on the neutral form reveal a significant biradical character (y=68 %). Furthermore, the TbIII complex exhibits multi-redox states, having more than eight clear peaks in the voltammetry curves. The optical (3700 nm, 0.33 eV) and electrochemical measurements (3400 nm, 0.36 eV) indicate that the neutral form has very small HOMO-LUMO energy gap. Despite the large biradical character, the neutral complexes are thermally stable and do not decompose on heating at 120 °C. These complexes with unique characteristics are promising candidates for use in molecular electronics, optics, and spintronics.

Solvent viscosity-dependent isomerization equilibrium of tetramethoxy-substituted bianthrone.

The designed and synthesized tetramethoxy-substituted bianthrone exists as an equilibrium mixture of the folded and twisted conformers in solution. The introduction of methoxy groups into the 3,3',6,6'-positions of bianthrone reduces the energy difference between the conformers and results in a bistable system. The survey of its equilibrium constant based on the optical absorption and variable temperature quantitative NMR spectroscopic studies in various solvents revealed that the equilibration of the isomerization reaction was found to be most sensitive to solvent viscosity among various solvent parameters. In the statistical thermodynamic approach combined with DFT calculations, the twisted conformations with various torsional angles are populated by thermal fluctuations on the highly anharmonic potential surface, and their ensemble average depends on solvent viscosity because a viscous drag of the solvent controls the degree of fluctuation. Such a statistical population does not occur in the folded conformer because of the harmonic potential surface with respect to the torsional rotation. This difference may result in the solvent viscosity dependence of the isomerization equilibrium constant, that is, the energy difference between the conformers.

Donor-Donor'-Acceptor Triads Based on [3.3]Paracyclophane with a 1,4-Dithiafulvene Donor and a Cyanomethylene Acceptor: Synthesis, Structure, and Electrochemical and Photophysical Properties.

Donor-donor'-acceptor triads (1, 2), based on [3.3]paracyclophane ([3.3]PCP) as a bridge, with electron-donating properties (D') using 1,4-dithiafulvene (DTF; TTF half unit) as a donor and dicyanomethylene (DCM; TCNE half unit) or an ethoxycarbonyl-cyanomethylene (ECM) as an acceptor were designed and synthesized. The pulse radiolysis study of 1 a in 1,2-dichloroethane allowed the clear assignment of the absorption bands of the DTF radical cation (1 a.+ ), whereas the absorption bands due to the DCM radical anion could not be observed by γ-ray radiolysis in 2-methyltetrahydrofuran rigid glass at 77 K. Electrochemical oxidation of 1 a first generates the DTF radical cation (1 a.+ ), the absorption bands of which are in agreement with those observed by a pulse radiolysis study, followed by dication (1 a2+ ). The ESR spectrum of 1 a.+ showed a symmetrical signal with fine structure and an ESR simulation predicted that the spin of 1 a.+ is delocalized over S and C atoms of the DTF moiety and the central C atom of the trimethylene bridge bearing the DTF moiety. Pulse radiolysis, ESR, and electrochemical studies indicate that the DTF radical cation of 1 a.+ is more stable than that of 6.+ , and the latter shows a strong tendency to dimerize. This result indicates that the [3.3]PCP moiety as a bridge can stabilize the DTF radical cation more than the 1,3-diphenylpropane moiety because of kinetic stability due to its rigid structure and the weak electronic interaction of DTF and DCM moieties through [3.3]PCP.

Radical Cation of an Oligoarylamine Having a Nitroxide Radical Substituent: A Coexistent Molecular System of Localized and Delocalized Spins.

A trimer derivative of oligotriarylamine bearing a nitroxide radical substituent as a localized spin center {N,N-bis[4-(di-4-anisylamino)phenyl]-N-[3-tert-butyl-5-(N-tert-butyl-N-oxylamino)phenyl]amine (1)} was characterized by electrochemical, spectroelectrochemical, and electron paramagnetic resonance spectroscopic measurements. The first and second oxidations of 1 occurred from the triamine moiety, leaving the nitroxide radical moiety intact. The delocalized polaronic state in the triamine moiety was generated by one-electron oxidation of 1, indicating the coexistence of localized and delocalized spins on 1+, where an intramolecular antiferromagnetic interaction was detected.

Intramolecular Interaction, Photoisomerization, and Mechanical C-C Bond Dissociation of 1,2-Di(9-anthryl)benzene and Its Photoisomer: A Fundamental Moiety of Anthracene-Based π-Cluster Molecules.

We report variable and unique properties of 1,2-di(9-anthryl)benzene 1 as a fundamental moiety of anthracene-based π-cluster molecules. Due to a through-space π-conjugation between anthracene units, excimer emission at room temperature and charge delocalized state in radical cation state of 1 could be observed. Photoirradiation to 1 afforded an intramolecular [4 + 4] cyclized anthracene dimer 1' having a high strain energy with long C-C bond that exceeded 1.68 Å, resulting in C-C bond dissociation by simple mechanical grinding.

Isolation of a hydrogen-bonded complex based on the anthranol/anthroxyl pair: formation of a hydrogen-atom self-exchange system.

A hydrogen-bonded complex was successfully isolated as crystals from the anthranol/anthroxyl pair in the self-exchange proton-coupled electron transfer (PCET) reaction. The anthroxyl radical was stabilized by the introduction of a 9-anthryl group at the carbon atom at the 10-position. The hydrogen-bonded complex with anthranol self-assembled by π-π stacking to form a one-dimensional chain in the crystal. The conformation around the hydrogen bond was similar to that of the theoretically predicted PCET activated complex of the phenol/phenoxyl pair. X-ray crystal analyses revealed the self-exchange of a hydrogen atom via the hydrogen bond, indicating the activation of the self-exchange PCET reaction between anthranol and anthroxyl. Magnetic measurements revealed that magnetic ordering inside the one-dimensional chain caused the inactivation of the self-exchange reaction.

Design and synthesis of new stable fluorenyl-based radicals.

Organic neutral radicals have long fascinated chemists with a fundamental understanding of structure-reactivity relationships in organic reactions and with applications as new functional materials. However, the elusive nature of these radicals makes the synthesis, isolation, and characterization very challenging. In this work, the synthesis of three long-lived, fluorenyl-based radicals are reported. The geometry and electronic structures of these radicals were systematically investigated with a combination of various experimental methods, besides density functional theory (DFT) calculations, which include X-ray crystallographic analysis, electron spin resonance (ESR), electron nuclear double resonance (ENDOR), cyclic voltammetry, and UV-vis-NIR measurements. Their half-life periods (τ(1/2)) in air-saturated solution under ambient conditions were also determined. Surprisingly, all three radicals showed remarkable stabilities: τ(1/2) = 7, 3.5, and 43 days.

Synthesis and characterization of quarteranthene: elucidating the characteristics of the edge state of graphene nanoribbons at the molecular level.

The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) that originates from electron-electron correlation in an edge-localized π-state, are investigated by preparing and characterizing quarteranthene molecules. The molecular geometry that was determined from the X-ray analysis is consistent with a zigzag-edge-localized structure of unpaired electrons. The localized electrons are responsible for the peculiar magnetic (room-temperature ferromagnetic correlation), optical (the lowest-lying doubly excited state), and chemical (peroxide bond formation) behaviors. On the basis of these distinguishing properties and a careful consideration of the valence bonding, insight into the edge state of ZGNRs can be gained.

Tunable Solid-State Thermochromism: Alkyl Chain Length-Dependent Conformational Isomerization of Bianthrones.

The introduction of linear alkoxy chains into bianthrone promoted the aggregation of the minor twisted conformer and made it compete with the major folded conformer in the solid-state. Rapid evaporation approach to the methoxy- and n-butoxy-substituted derivatives provided powder mixtures of folded and twisted conformers as metastable solids. Upon heating the powders, different lengths of alkyl chains converged to different conformers via solid-state isomerization reaction with an apparent color change. Monitoring by FT-IR spectroscopy and powder XRD ensured that these isomerization reactions involve only conformational isomerization without pyrolysis. In addition, single crystal X-ray structure analysis revealed that twisted conformers with longer alkyl chains are stabilized in the solid-state by the synergistic effect of their inter-chain hydrophobic interactions and π-π stacking interactions.

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.

"Marking" the nitrogen atoms of phenyl-(2-pyridyl)-(3-pyridyl)-(4-pyridyl)-methane. Synthesis and absolute configuration of the corresponding tris(pyridine N-oxide).

To "mark" the nitrogen atoms in phenyl-(2-pyridyl)-(3-pyridyl)-(4-pyridyl)methane (1), we have synthesized the corresponding tris(pyridine N-oxide) 2 by oxidation of 1 with m-chloroperbenzoic acid. The nitrogen atoms of 2 are unequivocally determined by the X-ray crystal analysis of a single crystal of rac-2 whereas the nitrogen atoms cannot be assigned at all in the case of rac-1. N-Oxide 2 can be resolved by chiral high-performance liquid chromatography under similar conditions to those used for the resolution of 1. The calculated circular dichroism (CD) curve for (R)-2 on the basis of time-dependent density functional theory reproduces the experimental spectra very well to suggest that the second-eluted fraction ([CD(+)283]-2) is the R isomer, namely (R)-[CD(+)283]-2. The independent absolute configuration determinations for 1 and 2 are in keeping with the chemical correlation between the two compounds by oxidation of (R)-1 into (R)-2.

Biradicaloid Behavior of a Twisted Double Bond.

A compound with a highly twisted C═C bond in the intermediate bond dissociation region was isolated, and its biradicaloid nature was experimentally demonstrated. A bianthrone derivative substituted with long-chain alkoxy groups was synthesized, and its twisted conformer was assembled into crystals through hydrophobic intermolecular interactions of the introduced alkoxy chains. X-ray crystallography revealed that the C═C bond connecting the two anthrone rings has a long bond length (1.429 Å) and a large torsional angle (57.33°). In addition, ESR studies revealed that the twisted bianthrone has biradicaloid character with a small singlet-triplet energy gap of 23.8 kJ mol-1. The thermally excited triplet species were observed in the ESR spectrum as distinct fine structures along with a ΔMS = ±2 forbidden transition.

Synthesis and characterization of teranthene: a singlet biradical polycyclic aromatic hydrocarbon having Kekulé structures.

A teranthene derivative has been successfully isolated in a crystalline form for the first time. Geometrical considerations and physical property investigations indicate that the molecule possesses prominent biradical character in the ground state.

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.

para-Phenylene-bridged spirobi(triarylamine) dimer with four perpendicularly linked redox-active π systems.

para-Phenylene-bridged spirobi(triarylamine) dimer 2, in which π conjugation through four redox-active triarylamine subunits is partially segregated by the unique perpendicular conformation, was prepared and characterized by structural, electrochemical, and spectroscopic methods. Quantum chemical calculations (DFT and CASSCF) predicted that the frontier molecular orbitals of 2 are virtually fourfold degenerate, so that the oxidized states of 2 can give intriguing electronic and magnetic properties. In fact, the continuous-wave ESR spectroscopy of radical cation 2(*+) showed that the unpaired electron was trapped in the inner two redox-active dianisylamine subunits, and moreover was fully delocalized over them. Magnetic susceptibility measurements and pulsed ESR spectroscopy of the isolated salts of 2, which can be prepared by treatment with SbCl(5), revealed that the generated tetracation 2(4+) decomposed mainly into a mixture of 1) a decomposed tetra(radical cation) consisting of a tri(radical cation) moiety and a trianisylamine radical cation moiety (≈75%) and 2) a diamagnetic quinoid dication in a tetraanisyl-p-phenylendiamine moiety and two trianisylamine radical cation moieties (≈25%). Furthermore, the spin-quartet state of the tri(radical cation) moiety in the decomposed tetra(radical cation) was found to be in the ground state lying 30 cal mol(-1) below the competing spin-doublet state.

An extremely simple dibenzopentalene synthesis from 2-bromo-1-ethynylbenzenes using nickel(0) complexes: construction of its derivatives with various functionalities.

Nice and easy: A very simple synthesis for dibenzopentalenes, which starts from 1-bromo-2-ethynylbenzenes, has been developed. It uses Ni(0) complexes (see scheme), from which a relatively stable Ni(II) complex as an important intermediate has been isolated. Dibenzopentalenes with various functional groups can be prepared by the procedure, and their electronic properties are consistent with theoretical calculations.An extremely simple dibenzopentalene synthesis from readily available 2-bromo-1-ethynylbenzenes using a nickel(0) complex is described. Although the yields are moderate, the formation of three C-C bonds in a single process and the high availability of the starting materials are important advantages of this reaction. The corresponding aryl-nickel(II) complex as an important intermediate was isolated as relatively stable crystals, and the structure was confirmed by X-ray crystallographic analysis. The high stability of this complex should play a key role in this reaction. The reaction is applicable to the preparation of dibenzopentalenes bearing various functional groups. Their electronic properties are consistent with theoretical calculations. The cyclic voltammograms of these compounds reveal highly amphoteric redox properties. In particular, the electron-donating property of a tetramethoxy derivative is greater than that of oligothiophenes and dibenzodithiophenes and almost comparable to that of pentacene.

An N-substituted aza[14]metacyclophane tetracation: a spin-quintet tetraradical with four para-phenylenediamine-based semi-quinone moieties.

A fully dianisylaminophenylated aza[1(4)]metacyclophane has been synthesized that exhibits four reversible two-electron oxidation processes, and its dicationic and tetracationic species have been found to be in spin-triplet and spin-quintet states.