Nagao Azuma

Magnetic properties of verdazyl free radicals. VII. π‐Bridged verdazyl biradicals with triplet ground state

Magnetic susceptibilities of two π‐bridged verdazyl biradical solids have been measured in the temperature range 1.8−300°K. Paramagnetic Curie constants of 1.0±0.1 and 0.90±0.05 K·emu/mol have been found in 1,4‐bis(1,5‐diphenyl‐3‐verdazyl)benzene (P‐verdazyl) and 1,3‐bis(1,5‐diphenyl‐3‐verdazyl)benzene (M‐verdazyl), respectively. Both Curie constants suggest the presence of the triplet ground state. The intramolecular positive exchange integrals, J/k, of these biradicals are much greater than 300°K. The susceptibility of P‐verdazyl exhibits a maximum at 19±1°K, which implies a negative exchange interaction between the triplets, its magnitude being estimated to be about 7°K.

Spectroscopic studies on copper(II) complexes of chiral cyclens: [CuN4Cl] chromophores varying from square pyramidal to trigonal bipyramidal stereochemistry

A series of five copper(II) complexes with four geometrically isomeric ligands of 1,4,7,10-tetrabenzyl-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane and with (2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane has been investigated by means of ESR, CD and MCD in solution. X-ray studies reported separately had shown that the [CuN4Cl] chromophores were distorted to varying extents from square pyramidal to trigonal bipyramidal geometry. The stability and rigidity of the molecular structure have enabled us to carry out a complementary study of the crystalline state and solution chemistry. The ground state Kramers doublets varied from 3dx2−y2 to 3dz2 type with increasing distortion around the copper ion. The orbital sequence of the 3d1 positive hole, dx2−y2<dz2<dyz,xz<dxy, has been obtained for the C4v complexes. The 3dxy orbital is also the highest in energy for the complex with the 3dz2 type ground state doublet. This complex exhibited superhyperfine splitting in the gz region due to the chlorine nucleus, which implies the principal z axis of the g tensor is along the CuCl bond. The sign of the copper hyperfine tensors was determined by means of Swalens treatment. The changing values of the orbital coefficients decreasing for the 3dx2−z2 and increasing for the 3dz2, 3dxz and 3dyz in the ground state doublets have been attributed to concomitant elongation of the in-plane CuN bond and reduction of the out-of-plane CuCl bond length with the increasing distortion around the copper ions.

Ideal enantiomeric resolution by recrystallization of a racemic compound (part 4): Relationship between enantiomeric resolution phenomenon and crystal properties

A new example of a racemate showing unusual enantiomeric resolution phenomenon, in which simple recrystallization of the racemate leads to remarkable enantiomeric enrichment of either enantiomer up to 100% ee in the mother liquor, has been found. This compound is (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]-ethyl]dimethylsulfonium p-nitrobenzenesulfonate [EtOCH2CH(OH)CH2OC6H4NHCOCH2CH2SMe2+O2NC6H4SO3−] [(±)-SN]. By repeating recrystallization of (±)-SN and the resulting deposited crystals successively and collecting the resulting enantiomerically enriched mother liquors with the same chirality sense, highly efficient enantiomeric resolution of the racemate into its separate enantiomers has been accomplished. The relationship between the occurrence of this enantiomeric resolution phenomenon and the crystal properties has been investigated with respect to SN and its aryl- and alkylsulfonate derivatives. The mode of enantiomeric resolution of (±)-SN was similar to that of para-substituted benzenesulfonate derivatives (±)-ST (4-MeC6H4SO3−) and (±)-SC (4-ClC6H4SO3−) previously reported, whereas the unsubstituted derivative (±)-SB (C6H5SO3−) and alkysulfonate derivatives (±)-SO (n-C8H17SO3−) and (±)-SM (CH3SO3−) did not show such an enantiomeric resolution phenomenon. The crystalline form of the former racemates that underwent the enantiomeric resolution was racemic compounds, while the latter were mixed crystals (solid solutions) composed of the respective optical antipodes. Chirality 9:220–224, 1997.

A novel synthesis of alkylbismuthonium salts and their reaction with some nucleophiles. First X-ray structural analysis of a stabilized alkylbismuthonium tetrafluoroborate

Abstract Treatment of triarylbismuth difluorides 1 with silyl enol ethers 2 in the presence of BF 3 ·OEt 2 gives (2-oxoalkyl)triarylbismuthonium tetrafluoroborates 3 as a stable crystalline solid, the bismuth centre of which has been found by X-ray analysis to possess a tetragonal geometry. The 2-oxoalkyl moiety of compound 3 couples with enolates and thiolates to afford the corresponding triketones 4 and α-thioketones 6 , respectively.

Polymorphism of Compound Existing as Both Racemic Compound Crystals and Mixed Crystals of Enantiomers

Abstract By recrystallization of certain racemic compound (ST), racemic compound crystals, mixed crystals of two enantiomers, or a mixture of these two types of crystals were obtained, depending on the recrystallization conditions employed. Their crystal forms were determined by X-ray diffraction of the single crystals and powdered materials. The space group and lattice parameters of the mixed crystals were identical with those of the enantiomeric crystals. This polymorphism of ST crystals would be ascribed to the small difference in the thermodynamic stability between the enantiomeric and racemic crystals. The physicochemical properties of ST are described, too.

Syntheses and properties of genuine organic magnetic conductors: (1:1) and (1:2) salts of ethyl-pyridinium-substituted verdazyl radicals with TCNQ

Abstract Four kinds of (1:1) and (1:2) salts of 3-(4- and 3-ethyl-pyridinium)-1,5-diphenylverdazyl radical cations with tetracyanoquinodimethanide (TCNQ) anion ([p-EtPyDV]+[TCNQ]− (1), [m-EtPyDV]+[TCNQ]− (2), [p-EtPyDV]+[TCNQ]2− (3), and [m-EtPyDV]+[TCNQ]2− (4)) have been prepared, and the magnetic susceptibilities (χM) measured between 1.8 and 300 K. The values of χMT at 300 K are 0.437, 0.383, 0.620, and 0.482 K emu mol−1 for the salts (1), (2), (3), and (4), respectively, and decrease by lowering the temperature. The χM of the salt (2) follows the Curie–Weiss law with a Curie constant of 0.374 K emu mol−1 and a negative Weiss constant of −2.4±0.2 K in the temperature region 1.8–300 K, indicating disappearance of a net magnetic moment due to the formation of strong spin pairs in TCNQ anion molecules. The χM of the salt (3) shows a broad maximum at T max = 118 ± 2 K and can be well explained by a singlet–triplet equilibrium model with 2 J / k B = - 190 K The χM of the salt (4) shows a broad maximum at T max = 9.3 ± 0.5 K The χM can be well reproduced by the sum of the contributions from: (i) a one-dimensional (1D) Heisenberg antiferromagnetic alternating-chain system with 2 J / k B = - 15.2 K (alternation parameter α=J2/J1=0.4); and (ii) the dimer system with 2 J / k B = - 741 K The magnetic property of (4) was discussed based on the results obtained by crystal structure analysis of (4). On the other hand, the susceptibility of the salt (1) showed anomalous magnetic behavior that cannot be explained by a simple model. The (1:1) salts (1) and (2) are insulators. On the other hand, the pressed pellet conductivity (σ) of the (1:2) salts (3) and (4) at 20°C was σ=6.9×10−2 and 1.5×10−3 S cm−1 with activation energy EA=0.072 and 0.25 eV, respectively, showing the property as semiconductor. The two (1:2) salts (3) and (4) are new, genuine organic magnetic semiconductors.

Spectroscopic characterization of coaxially and π—π stacked binuclear copper(II) complexes of ω,ω′-bis(pyridine-2-carboxamido)alkanes in acidic solution

Abstract Using electron spin resonance (ESR) and visible spectroscopies, coaxially stacked binuclear copper(II) complexes of ω,ω′-bis(pyridine-2-carboxamido)alkanes in acidic aqueous solution (pH 3.8–5.0), H2peda (alkane = ethane), H2ppda (alkane = propane), and H2pbda (alkane = butane), were characterized. trans-1,2-Bis(pyridine-2-carboxamido)cyclohexane or trans-H2pcyhda (alkane = trans-1,2-cyclohexane) did not provide coaxially stacked binuclear complex, because of the lower conformational freedom at the HNCH-CHNH bond. The coaxially stacked structure causes π—π stacking between the pyridyl rings. The conformation of the propanediyl chain is gauche—gauche, while the ethanediyl and butanediyl chains contain an eclipsed conformation. The driving force for the formation of these binuclear complexes in acidic solution is intramolecular π—π stacking interaction between the pyridyl rings in concert with hydrophobic interaction. Although H2ppda gave crystals of a binuclear complex with many lattice water molecules, H2peda and H2pbda ligands provided crystals of copper(II) polymer complexes from the respective acidic solution. The trans-H2pcyhda ligand forms a copper(II) binuclear complex with open-chain structure in acidic solution. Crystal structures of the polymer complexes are also presented.

Crystal structure and ESR study of a nitrodochromium(V) complex

Abstract The crystal structure of the nitridochromium(V) complex [CrN(salen)] (H2salen = N,N′-bis(salicylidene)ethylenediamine) has been determined. This complex crystallizes in the monoclinic system, space group P21/c, a = 9.575(1), b = 12.322(3), c = 12.999(2) A, β = 103.57(1)°, V = 1490.8(5) A 3 , Z = 4; final R2 = 0.059 (R = 0.054), wR2 = 0.082 (wR = 0.044) for 1935 observed reflections (I⩾1.5σ(l)). The coordination geometry is square pyramidal with an N2O2 donor set (LN202) in the basal plane and the nitride ion at the apex. The UV-Vis spectra of the complex have been re-measured in several solvents and exhibited a d-d band maximum around 540 nm (ϵ ≈200 M−1cm−1). This is the first example of an observed d-d band among the nitridochromium(V) complexes isolated so far. The ESR spectra have been also re-measured in more detail. The ESR parameters have been compared with those of the [CrN(LN4)] complexes and a remarkable nephelauxetic effect of the 3d sub-shell in comparison with the 3s sub-shell is indicated for the nitridochromium(V) complexes.

Molecular and crystal structures of trans-[VOCl2(H2O)2] hydrogen bonded adducts with 15-crown-5 and benzo-15-crown-5 ether

Abstract The crystal structures of molecular adducts of vanadyl complex of trans-[VOCl2(H2O)2] with 15-crown-5 (15C5), benzo-15-crown-5 (B15C5) have been studied by X-ray diffraction at ambient temperature. The adduct of the same complex with 18-crown-6 (18C6) recrystallized from a solvent different from that previously reported, has been also re-refined and the unresolved chloro position has been resolved into two disordered positions. The ClVCl bond angles in the 15C5 and B15C5 adduct are 143° and 145°, respectively, which are smaller than the OVO bond angles of 153° and 154°. There appear eclipsed type H2O-VCl2 conformations in all the adducts. These observations can be ascribed to the steric repulsion between the chloro ligands and aqua hydrogen atoms as well as to the coulombic repulsion between the chloro and oxido ligand. The crystals are composed of adduct polymers with 1 : 1 stoichiometry in which the chromophore and crown ether alternate along the polymer axis. The polymers are formed by hydrogen bonding between the aqua ligand and the oxygen atoms on the crown ring. The crown ethers adopt invariably all-gauche CC conformation. The OC bonds are of trans conformation except for a gauche OC bond in 15C5. ESR parameters of trans-[VOCl2(H2O)2] observed for the 15C5 adduct powder are also presented.

Nephelauxetic series of in-plane ligands for square-pyramidal nitridochromium(V) complexes as studied by means of ESR spectroscopy

Abstract The crystal structure of the nitridochromium(V) complex [CrN(salophen)]·MeCN [H2salophen=N,N′-bis(salicylidene)-o-phenylenediamine] has been determined. The coordination geometry of this complex, similar to those of all the nitridochromium(V) complexes investigated so far, is square pyramidal with the nitride ion occupying the apex. The ESR parameters of the nitridochromium(V) complexes isolated so far were compared with one another to show an enormous expansion of the 3d electron cloud due to the strong CrN triple bonding. Moderate covalency in the in-plane bonds was also concluded. The nephelauxetic series of three types of the basal ligands is deduced to be salen≈salophen>bpb≈mebpb≈dmebpb>oep≈ttp (in nephlauxetic ratio) for the high-oxidation nitridochromium(V), H2salen=N,N′-bis(salicylidene)ethane-1,2-diamine, H2bpb=1,2-bis(pyridine-2-carboxamido)benzene, H2mebpb=1,2-bis(pyridine-2-carboxamido)-4-methylbenzene, H2dmbpb=1,2-bis(pyridine-2-carboxamido-4,5-dimethylbenzene, H2oep=2,3,7,8,12,13,17,18-octaethylporphyrin and H2ttp=5,10,15,20-tetra-p-tolylporphyrin.