Salts of the two-coordinate homoleptic manganese(I) dialkyl anion [Mn{C(SiMe3)3}2](-) with quenched orbital magnetism.

The reduction of Mn{C(SiMe3)3}2 with KC8 in the presence of crown ethers yielded the d(6), Mn(I) salts [K2(18-crown-6)3][Mn{C(SiMe3)3}2]2 and [K(15-crown-5)2][Mn{C(SiMe3)3}2], that have near-linear manganese coordination but almost completely quenched orbital magnetism as a result of 4s-3dz(2) orbital mixing which affords a non-degenerate ground state.

Substituent effects in formally quintuple-bonded ArCrCrAr compounds (Ar = terphenyl) and related species.

The effects of different terphenyl ligand substituents on the quintuple Cr-Cr bonding in arylchromium(I) dimers stabilized by bulky terphenyl ligands (Ar) were investigated. A series of complexes, ArCrCrAr (1-4; Ar = C6H2-2,6-(C6H3-2,6-iPr2)2-4-X, where X = H, SiMe3, OMe, and F), was synthesized and structurally characterized. Their X-ray crystal structures display similar trans-bent C(ipso)CrCrC(ipso) cores with short Cr-Cr distances that range from 1.8077(7) to 1.8351(4) A. There also weaker Cr-C interactions [2.294(1)-2.322(2) A] involving an C(ipso) of one of the flanking aryl rings. The data show that the changes induced in the Cr-Cr bond length by the different substituents X in the para positions of the central aryl ring of the terphenyl ligand are probably a result of packing rather than electronic effects. This is in agreement with density functional theory (DFT) calculations, which predict that the model compounds (4-XC6H4)CrCr(C6H4-4-X) (X = H, SiMe3, OMe, and F) have similar geometries in the gas phase. Magnetic measurements in the temperature range of 2-300 K revealed temperature-independent paramagnetism in 1-4. UV-visible and NMR spectroscopic data indicated that the metal-metal-bonded solid-state structures of 1-4 are retained in solution. Reduction of (4-F3CAr')CrCl (4-F3CAr' = C6H2-2,6-(C6H3-2,6-iPr2)2-4-CF3) with KC8 gave non-Cr-Cr-bonded fluorine-bridged dimer {(4-F3CAr')Cr(mu-F)(THF)}2 (5) as a result of activation of the CF3 moiety. The monomeric, two-coordinate complexes [(3,5-iPr2Ar*)Cr(L)] (6, L = THF; 7, L = PMe3; 3,5-iPr2Ar* = C6H1-2,6-(C6H-2,4,6-iPr3)2-3,5-iPr2) were obtained with use of the larger 3,5-Pri2-Ar* ligand, which prevents Cr-Cr bond formation. Their structures contain almost linearly coordinated CrI atoms, with high-spin 3d5 configurations. The addition of toluene to a mixture of (3,5-iPr2Ar*)CrCl and KC8 gave the unusual dinuclear benzyl complex [(3,5-iPr2Ar*)Cr(eta3:eta6-CH2Ph)Cr(Ar*-1-H-3,5-iPr2)] (8), in which a C-H bond from a toluene methyl group was activated. The electronic structures of 5-8 have been analyzed with the aid of DFT calculations.

Thermodynamic, spectroscopic, and computational evidence for the irreversible conversion of beta- to alpha-endosulfan.

Previous studies have provided unequivocal evidence for the symmetry of beta-endosulfan and the corresponding asymmetry of alpha-endosulfan; the conversion of beta-endosulfan to alpha-endosulfan was identified. In this study, evidence from differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments combined with computational chemistry calculations was used to propose a molecular mechanism for the corresponding conformational changes that occur in this process. DSC and NMR data of mixtures indicated that both isomers can influence the conformer populations in the solid, solution, and vapor phase. Computational chemistry demonstrated that the relative S==O configuration between alpha- and beta-isomers can be the intermediate state through which the conformations of alpha- and beta-isomers affect each other. Furthermore, calculations for mixtures indicated that the asymmetrical conformation of the sulfite in alpha-endosulfan can induce asymmetry in beta-endosulfan, and conversion to alpha-endosulfan occurs from this transition state.

Nematode-antagonistic trichothecenes from Fusarium equiseti.

A strain of the fungus Fusarium equiseti isolated from soybean cyst nematode secretes nematode-antagonistic compounds. Bioassay-guided fractionation of an extract of the culture broth was undertaken to identify the compounds. Fractions were assayed for activity against a root-knot nematode (Meloidogyne incognita), a plant pathogen that attacks the roots of numerous plant species. Two trichothecene compounds were isolated that inhibited egg hatch and immobilized second-stage juveniles of this nematode: 4,15-diacetoxy-12,13-epoxy-3,7-dihydroxytrichothec-9-en-8-one (4,15-diacetylnivalenol) and 4,15-diacetoxy-12,13-epoxy-trichothec-9-en-3-ol (diacetoxyscirpenol). This is the first published report of these compounds affecting plant-parasitic nematodes.

Atranones A-G, from the toxigenic mold Stachybotrys chartarum.

Atranones A-G have been isolated from the toxigenic fungus Stachybotrys chartarum. These compounds contain several unusual features including an enol-lactone as part of a 3,7-dioxabicyclo[3.3.0]octane-2-one ring system fused to an 11-membered ring. Two new dolabellane diterpenes, related in structure to the atranones were also isolated, which suggests a diterpenoid origin for the C24 atranones.

Synthesis and magnetic and transport properties of Sr6V9S22O2: "AM2S5" phases revisited.

The compound Sr6V9S22O2 was prepared from SrS, sulfur, vanadium metal, and V2O5 at 950 degrees C in an evacuated quartz tube. The compound is rhombohedral, R3, with a = 8.7538(6) A, c = 34.934(3) A, and Z = 3, and shows strong preferred orientation in its XRD profiles (00l) due to the layered nature of the structure. The compound contains charged CdI2 type VS2 layers of formula [V7S14]4- separated by [Sr6(VOS3)2(S2)]4+ layers. The latter has VOS3(3-) tetrahedra and S2(2-) disulfide units linked by Sr2+ ions. Magnetic susceptibility and four-probe resistivity studies show essentially temperature-independent paramagnetism above 80 K and small gap semiconductor behavior, respectively. The compound has a positive Hall coefficient at room temperature. The relationship among Sr6V9S22O2, "SrV2S5" (J. Solid State Chem. 1996, 126, 189), and other AM2S5 phases is discussed.

Memnobotrins and memnoconols: novel metabolites from Memnoniella echinata.

Four novel metabolites have been isolated from a rice culture of Memnoniella echinata (JS6308) by solvent extraction and radial silica chromatography. The structures were elucidated by spectroscopic techniques, and the absolute stereochemistry of memnobotrin A determined by X-ray crystallography.