1,720,977 research outputs found
Reactivity Studies of Heteroleptic Silylenes with N2O
No full textReaction of heteroleptic silylenes LSD( (L = PhC(NtBu)(2); X = PPh2 (1), NPh2 (2), NMe2 (3), OtBu (4)) with N2O resulted in the oxidized dimeric product [LSi(X)(mu-O)](2) (X = PPh2 (5), NPh2 (6), NMe2 (7), OtBu (8)), which contains a four -membered Si2O2 ring. Compounds 5-8 were characterized by spectroscopic and spectrometric techniques. The molecular structures of 5-8 were established by single-crystal X-ray structure analysis.Deutsche Forschungsgemeinschaft (DFG); Alexander von Humboldt Stiftun
Invariom modeling of ceftazidime pentahydrate: molecular properties from a 200 second synchrotron microcrystal experiment
No full textThe structure of ceftazidime pentahydrate, a third generation cephalosporin antibiotic, is reported. Data collection was carried out in a remarkably short time with synchrotron radiation and the latest detector technology, illustrating that single-crystal X-ray diffraction can be used as a technique for screening hundreds of compounds in a short amount of time. Structure refinement made use of invarioms, namely non-spherical scattering factors, which allow more information to be derived from a diffraction experiment. Properties that can be screened are bond-topological parameters, empirical hydrogen-bond energies, molecular dipole moments and electrostatic potentials.Deutsche Forschungsgemeinschaft DFG [DI 921/3-2
The generalized invariom database (GID)
No full textInvarioms are aspherical atomic scattering factors that enable structure refinement of more accurate and more precise geometries than refinements with the conventional independent atom model (IAM). The use of single-crystal X-ray diffraction data of a resolution better than sin/ = 0.6 angstrom 1 (or d = 0.83 angstrom) is recommended. The invariom scattering-factor database contains transferable pseudoatom parameters of the HansenCoppens multipole model and associated local atomic coordinate systems. Parameters were derived from geometry optimizations of suitable model compounds, whose IUPAC names are also contained in the database. Correct scattering-factor assignment and orientation reproduces molecular electron density to a good approximation. Molecular properties can hence be derived directly from the electron-density model. Coverage of chemical environments in the invariom database has been extended from the original amino acids, proteins and nucleic acid structures to many other environments encountered in organic chemistry. With over 2750 entries it now covers a wide sample of general organic chemistry involving the elements H, C, N and O, and to a lesser extent F, Si, S, P and Cl. With respect to the earlier version of the database, the main modification concerns scattering-factor notation. Modifications improve ease of use and success rates of automatic geometry-based scattering-factor assignment, especially in condensed hetero-aromatic ring systems, making the approach well suited to replace the IAM for structures of organic molecules
Selective functionalization of a bis-silylene
No full textFunctionalization of N-heterocyclic carbenes (NHCs) has an important influence on their stability, Lewis donor, and acceptor properties. In this study, we report on the selective functionalization of a four-membered N-heterocyclic bis-silylene (2,6-Ar2C6H3NSi:)(2) (1) (Ar = 2,4,6-iPr(3)C(6)H(2)) with mono-oxygen sources N2O and Me3NO. Treatment of 1 with N2O results in the selective formation of mono-silylene (2,6-Ar2C6H3NSi(OH)(2))(2,6-Ar2C6H3NSi:) (2) as a major product, along with a small amount of further oxidized product (2,6-Ar2C6H3NSi(OH)(2))(2) (3). Compound 2 is the first four-membered mono-silylene with a di-coordinate silicon atom.Deutsche Forschungsgemeinschaf
Selective Synthesis and Redox Sequence of a Heterobimetallic Nickel/Copper Complex of the Noninnocent Siamese-Twin Porphyrin
No full textThe Siamese-twin porphyrin (1H(4)) is a redox noninnocent pyrazole-expanded porphyrin with two equivalent dibasic {N-4} binding sites. It is now shown that its selective monometalation can be achieved to give the nickel(II) complex 1H(2)Ni with the second {N-4} site devoid of a metal ion. This intermediate is then cleanly converted to 1Ni(2) and to the first heterobimetallic Siamese-twin porphyrin 1CuNi. Structural characterization of 1H(2)Ni shows that it has the same helical structure previously seen for 1Cu(2), 1Ni(2), and free base 1H(6)(2+). Titration experiments suggest that the metal-devoid pocket of 1H(2)Ni can accommodate two additional protons, giving [1H(4)Ni](2+). Both bimetallic complexes 1Ni(2) and 1CuNi feature rich redox chemistry, similar to the recently reported 1Cu(2), including two chemically reversible oxidations at moderate potentials between -0.3 and +0.5 V (vs Cp2Fe/Cp2Fe+). The locus of these oxidations, in singly oxidized [1Ni(2)](+) and [1CuNi](2+) as well as twice oxidized [1CuNi](2+), has been experimentally derived from comparison of the electrochemical properties of the complete series of complexes 1Cu(2), 1Ni(2), and 1CuNi, and from electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS) (Ni and Cu K edges). All redox events are largely ligand-based, and in heterobimetallic 1CuNi, the first oxidation takes place within its Cu-subunit, while the second oxidation then occurs in its Ni-subunit. Adding pyridine to solutions of [1Ni(2)](+) and [1CuNi](2+) cleanly converts them to metal-oxidized redox isomers with axial EPR spectra typical for Nil having significant d(z)(21) character, reflecting close similarity with nickel complexes of common porphyrins. The possibility of selectively synthesizing heterobimetallic complexes 1MNi from a symmetric binucleating ligand scaffold, with the unusual situation of three distinct contiguous redox sites (M, Ni, and the porphyrin-like ligand), further expands the Siamese-twin porphyrin's potential to serve as an adjustable platform for multielectron redox processes in chemical catalysis and in electronic applications.Fonds der Chemischen Industrie; DOE; BER; NIH; NCRR; BMT
Crystalline guanine adducts of natural and synthetic trioxacarcins suggest a common biological mechanism and reveal a basis for the instability of trioxacarcin A
No full textX-ray crystallographic characterization of products derived from natural and fully synthetic trioxacarcins, molecules with potent antiproliferative effects, illuminates aspects of their reactivity and mechanism of action. Incubation of the fully synthetic trioxacarcin analog 3, which lacks one of the carbohydrate residues present in the natural product trioxacarcin A (1) as well as oxygenation at C2 and C4 yet retains potent antiproliferative effects, with the self-complimentary duplex oligonucleotide d(AACCGGTT) led to production of a crystalline covalent guanine adduct (6). Adduct 6 is closely analogous to gutingimycin (2), the previously reported guanine adduct derived from incubation of natural trioxacarcin A (1) with duplex DNA, suggesting that 3 and 1 likely share a common basis of cytotoxicity. In addition, we isolated a novel, dark-red crystalline guanine adduct (7) from incubation of trioxacarcin A itself with the self-complimentary duplex oligonucleotide d(CGTATACG). Crystallographic analysis suggests that 7 is an anthraquinone derivative, which we propose arises by a sequence of guanosine alkylation within duplex DNA, depurination, base-catalyzed elimination of the trioxacarcinose A carbohydrate residue, and oxidative rearrangement to form an anthraquinone. We believe that this heretofore unrecognized chemical instability of natural trioxacarcins may explain why trioxacarcin analogs lacking C4 oxygenation exhibit superior chemical stabilities yet, as evidenced by structure 3, retain a capacity to form lesions with duplex DNA. (C) 2014 Elsevier Ltd. All rights reserved.Deutsche Forschungsgemeinschaft DFG [921/3-2
A Super-Reduced Diferrous [2Fe-2S] Cluster
No full textA biomimetic [2Fe-2S] cluster has been isolated in the fully reduced diferrous form and characterized by X-ray diffraction. This completes a consistent series of synthetic analogues of protein-bound [2Fe-2S](z) redox centers (z = 2+, 1+, 0) with identical capping ligands. Fe-57 Mossbauer data of the extremely oxidation-sensitive complex compare well with those of the very few reports of all-ferrous ferredoxins and Rieske centers; they confirm the S-T = 0 ground state and establish a lower limit for the exchange coupling, -J >= 30 cm(-1)
Donor–acceptor stabilized silaformyl chloride
No full textGhadwal R, Azhakar R, Roesky HW, et al. Donor–acceptor stabilized silaformyl chloride. Chemical Communications. 2012;48(66): 8186.Formyl chloride (H(Cl)C[double bond, length as m-dash]O) is unstable at room temperature and decomposes to HCl and CO. Silicon analogue of formyl chloride, silaformyl chloride IPr·SiH(Cl)[double bond, length as m-dash]O·B(C6F5)3 (3) (IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene), was stabilized by Lewis donor–acceptor ligands. Compound 3 is not only the first stable acyclic silacarbonyl compound but also the first silacarbonyl halide reported so far
Neutral Pentacoordinate Silicon Fluorides Derived from Amidinate, Guanidinate, and Triazapentadienate Ligands and Base-Induced Disproportionation of Si<sub>2</sub>Cl<sub>6</sub> to Stable Silylenes
No full textPentacoordinate silicon fluorides (LSiF3)-Si-1 (2a), (LSiF3)-Si-2 (2b), and ((LSiF2)-Si-3)(2) (2c)(2) based on amidinate (L-1 = PhC((NBu)-Bu-t)(2)), guanidinate (L-2 = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate), and triazapentadienate = (L-3 = NC(NMe2)NC-(NMe2)NAr, Ar = 2,6-(Pr2C6H3)-Pr-i) ligands were prepared by fluorination of the corresponding chlorosilanes (LSiCl3)-Si-1 (1a), (LSiCl3)-Si-2 (1b), and (LSiCl2)-Si-3 (1c) with Me3SnF at ambient temperature. Compounds 1b. 1c, 2a, 2b, and (2c)(2) were characterized by H-1, C-13, F-19, and Si-29 NMR spectroscopic studies. Molecular structures of 1b, 1c, 2a, and (2c)(2) were determined by single crystal X-ray structural analysis. Invariom refinement involving non-spherical scattering factors of the Hansen-Coppens multipole model was performed for 1b. Compound (LSiF2)-Si-3 (2c) is dimeric both in the solid state and in solution, whereas its chloro-analogue lc is monomeric. The attempted synthesis of diamidinatotetrachlorodisilane by reaction of lithium amidinate with Si2Cl6 led to the formation of the silane (1a) and the silylene (LSiCl)-Si-1 (3). Reaction of Si2Cl6 with N-heterocyclic carbenes (NHC) afforded NHC adducts of dichlorosilylene and SiCl4. A one pot method for the preparation of base-stabilized silylenes from Si2Cl6 is discussed.Deutsche Forschungsgemeinschaf
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