Kinetic Trapping Effects in Amphiphilic Iron(II) Spin Crossover Compounds.

In this work, the synthesis of four new iron complexes with a Schiff base-like amphiphilic equatorial ligand (L) and dimethylaminopyridine (dmap) or 4,4'-bipyridine (bipy) as axial ligands is reported. Three of the complexes ([FeL(dmap)2] 1, [FeL(dmap)(MeOH)0.5]·MeOH 2, and [FeL(bipy)] n 3 have an iron(II) center, and two of those with an N4O2 coordination sphere (1 and 3) are spin crossover active. Both exhibit a thermal hysteresis (10 K with T1/2 = 131 K for 3 and 23 K with T1/2 = 161 K for 1) where the width depends on the velocity used for the measurement. Additionally, in both cases, the high spin state is trapped by rapid cooling, and a TTIESST was determined to be 121 K (1) and 101 K (3). Single crystals suitable for X-ray structure analysis were obtained for the three different complexes with dmap as axial ligand (1S, 2, and [µ-O-{FeL}2(dmap)] 4). The complex 1S has two dmap molecules in axial position, while the other two structures were obtained for a complex where a dmap ligand is exchanged by methanol and one where the iron(II) center is oxidized to iron(III) to form a dinuclear µ-O-complex. All three complexes were obtained under similar reaction conditions in the presence/absence of oxygen, and all three structures show the formation of lipid layer-like arrangements in the packing.

Carbazole-, Aspidofractinine-, and Aspidocarpamine-Type Alkaloids from Pleiocarpa pycnantha.

Three new alkaloids, janetinine (1a), pleiokomenine A (2), and huncaniterine B (3a), and 13 known compounds, pleiomutinine (3b), huncaniterine A (3c), 1-carbomethoxy-ß-carboline (4), evoxanthine (5), deformyltalbotine acid lactone (6), pleiocarpamine (7), N4-methyl-10-hydroxygeissoschizol (8), spegatrine (9), neosarpagine (10), aspidofractinine (11), N1-methylkopsinin (12), pleiocarpine (13), and N1-methylkopsinin- N4-oxide (14), were isolated from the stem bark of Pleiocarpa pycnantha. Janetinine (1a) is a carbazole alkaloid; in pleiokomenine A (2), two aspidofractinine-type alkaloids are bridged by a methylene unit in an unprecedented way, and huncaniterine B (3a) is a pleiocarpamine-aspidofractinine-type dimer. The structures and relative configurations of these compounds were elucidated on the basis of NMR and MS analyses. Their absolute configurations were defined by means of experimental and calculated ECD data, and additionally, the structures of 5 and 13 were determined by single crystal X-ray diffraction. Compounds 1a, 2, 3b, 4, 6, 9, and 12 displayed cancer chemopreventive properties through either quinone reductase induction ( CD = 30.7, 30.2, 29.9, 43.5, and 36.7 µM for 1a, 4, 6, 9, and 12, respectively) and/or NF-κB inhibition with IC50 values of 13.1, 8.4, 9.4, and 8.8 µM for 2, 3b, 6, and 12, respectively.

Synthesis and characterization of Lewis base stabilized mono- and di-organo aluminum radicals.

Two cyclic (alkyl)(amino)carbene (cAAC) stabilized mononuclear neutral radicals of aluminum have been synthesized. They contain an ethyl [(cAAC)2AlClEt (1)] and as well a diethyl group [(cAAC)2AlEt2 (2)], and have been prepared from the reduction of EtAlCl2 and Et2AlCl, respectively, with KC8. Compounds 1 and 2 are monoradicals, which were confirmed by EPR measurements to have the spin located on the carbene carbon of one of the cAAC ligands.

Protonation of Ferrocene: A Low-Temperature X-ray Diffraction Study of [Cp2 FeH](PF6 ) Reveals an Iron-Bound Hydrido Ligand.

Ferrocene, Cp2 Fe, is quantitatively protonated in a mixture of liquid HF/PF5 to yield [Cp2 FeH](PF6 ), which was characterized by 1 H/13 C NMR and 57 Fe Mössbauer spectroscopy as well as single-crystal X-ray diffraction analysis. X-ray diffraction analysis at 100 K revealed a disordered, iron-coordinated hydrido ligand, which was unambiguously located by aspherical atom refinement at 100 K, and by analyzing the non-disordered crystal structure at 30 K, revealing a non-agostic structure.

Formation of a 1,4-diamino-2,3-disila-1,3-butadiene derivative.

A 1,4-diamino-2,3-disila-1,3-butadiene derivative of composition (Me2-cAAC)2(Si2Cl2) (Me2-cAAC = :C(CMe2)2(CH2)N-2,6-iPr2C6H3) was synthesized by reduction of the Me2-cAAC:SiCl4 adduct with KC8. This compound is stable at 0 °C for 3 months in an inert atmosphere. Theoretical studies reveal that the silicon atoms exhibit pyramidal coordination, where the Cl-Si-Si-Cl dihedral angle is twisted by 43.3° (calcd 45.9°). The two silicon-carbon bonds are intermediates between single and double Si-C bonds due to twisting of the C-Si-Si-C dihedral angle (163.6°).

Paeciloside A, a new antimicrobial and cytotoxic polyketide from Paecilomyces sp. strain CAFT156.

Paeciloside A (1) and eight known compounds, acremoauxin A (2), farinosones A (3) and B (4), 1,5-dideoxy-3-C-methyl-arabitol (5), ergosterol, ergosterol peroxide, cerebroside C, and adenosine, were isolated from cultures of Paecilomyces sp. CAFT156, an endophytic fungus occurring in Enantia chlorantha Oliv (Annonaceae) leaves. The structure of the new compound 1 was elucidated using MS, UV, 1D and 2D NMR experiments, while its absolute configuration was established by subsequent single-crystal X-ray diffraction analysis using copper Kα radiation and invariom nonspherical scattering factors. Paeciloside A (1) and compounds 2, 4, and 5 displayed inhibitory effects on two gram-positive bacteria, Bacillus subtilis and Staphylococcus aureus at a concentration of 40 µg per paper disk and moderate cytotoxicity towards brine shrimp larvae (Artemia salina). This study presents the first report on an endophytic fungus isolated from E. chlorantha Oliv and its natural products.

Electron densities of three B12 vitamins.

The electron densities of the three natural B(12)-vitamins, two of them being essential cofactors for animal life, were determined in a procedure combining high-order X-ray data collection at low to very low temperatures with high-level density functional calculations. In a series of extensive experimental attempts, a high-order data set of adenosylcobalamin (AdoCbl) could be collected to a resolution of sin theta/lambda = 1.00 A(-1) at 25 K. This modification contains only minor disorder at the solvent bulk. For methylcobalamin (MeCbl), only a severely disordered modification was found (sin theta/lambda = 1.00 A(-1), 100 K, measured with synchrotron radiation). The already published data set of cyanocobalamin (CNCbl) (sin theta/lambda = 1.25 A(-1), 100 K) was reintegrated to guarantee similar treatment of the three compounds and cut to sin theta/lambda = 1.11 A(-1) to obtain a higher degree of completeness and redundancy. On the basis of these accurate experimental geometries of AdoCbl, MeCbl, and CNCbl, state-of-the-art density functional calculations, single-point calculations, and geometry optimizations were performed on model compounds at the BP86/TZVP level of theory to evaluate the electronic differences of the three compounds. AdoCbl and MeCbl are known to undergo different reaction paths in the body. Thus, the focus was directed toward the characterization of the dative Co-C(ax) and Co-N(ax) bonds, which were quantifed by topological parameters, including energy densities; the source function including local source; and the electron localizability indicator (ELI-D), respectively. The source function reveals the existence of delocalized interactions between the corrin macrocycle and the axial ligands. The ELI-D indicates unsaturated Co-C(ax) bonding basins for the two biochemically active cofactors, but not for CNCbl, where a population of 2.2e is found. This may be related to significant pi-backbonding, which is supported by the delocalization index, delta, of 0.15 between the Co atom and the N atom of the cyano ligand. Considering all results, the inherent electronic differences between AdoCbl and MeCbl are found to be small thus, supporting earlier findings that the interaction with the protein site mainly controls the type of Co-C(ax) bond cleavage.

A new aspect of specific radiation damage: hydrogen abstraction from organic molecules.

Radiation damage is one of the major impediments in obtaining high-resolution structural information utilizing ionizing radiation. From electron microscopy it is known that electron irradiation of biological samples results in the formation of molecular hydrogen. In the present work radiation-induced structural changes of the polypeptide cyclosporine A were observed at a temperature of 100 K. Bond length changes are thought to be due to radiation-induced hydrogen abstraction which chemically modifies the molecules in an irreversible way. The resulting formation of molecular hydrogen might explain the observed increase of the crystal mosaicity, which has also been reported in many previous radiation damage studies.

Dipole moment enhancement in molecular crystals from X-ray diffraction data.

Although reliable determination of the molecular dipole moment from experimental charge density analyses on molecular crystals is a challenging undertaking, these values are becoming increasingly common experimental results. We collate all known experimental determinations and use this database to identify broad trends in the dipole moment enhancements implied by these measurements as well as outliers for which enhancements are pronounced. Compelling evidence emerges that molecular dipole moments from X-ray diffraction data can provide a wealth of information on the change in the molecular charge distribution that results from crystal formation. Most importantly, these experiments are unrivalled in their potential to provide this information in such detail and deserve to be exploited to a much greater extent. The considerable number of experimental determinations now available has enabled us to pinpoint those studies that merit further attention, either because they point unequivocally to a considerable enhancement in the crystal (of 50 % or more), or because the experimental determinations suggest enhancements of 100 % or more--much larger than independent theoretical estimates. In both cases further detailed experimental and theoretical studies are indicated.