Elisabeth Irran

Hollow Ferrocenyl Coordination Polymer Microspheres with Micropores in Shells Prepared by Ostwald Ripening

Hollow microspheres with pores in their shells have received much attention owing to their hierarchically porous structures and advanced applications in electrochemical capacitive energy storage, hydrogen storage, drug delivery, sensing, and catalysis. For example, Lou et al. reported that hollow SnO2 nanospheres with nanoporous shells showed high reversible charge capacity and good cycling performance. Zhu et al. investigated the drug-delivery properties of hollow silica spheres with mesoporous shells and found that the hollow microspheres were able to store significantly more molecules with higher release rates than conventional mesoporous silica. Template synthesis is one of the most-used strategies to prepare hierarchically hollow microspheres, especially for pores inside the shells. Braun and co-workers have prepared hollow ZnS microspheres with mesoporous shells using dual templates assembled by lyotropic liquid crystals on the surfaces of silica or polystyrene colloidal templates. Liu et al. have produced organic–inorganic hybrid hollow nanospheres with microwindows on the shells templated by tricopolymer aggregates. The template method is general to prepare hollow microspheres with pores in the shells, but expensive and tedious post-treatment processes, such as solvent extraction, thermal pyrolysis, or chemical etching, and resultant fragile frameworks, limit or even impair its applicability. 3, 4] As a result, it remains an important challenge to develop a convenient and template-free method to prepare hollow microspheres with porous shells. Porous coordination polymers are highly ordered porous multifunctional materials prepared by linking metal ions or metal oxide clusters with multidentate organic ligands without any additional template. Construction of shells of hollow materials with porous coordination polymers is an especially promising approach to design hollow microspheres with porous shells through a template-free method and to endow materials with multifunctionality, such as electric, magnetic, and optical properties. Herein, we report the formation of hollow coordination polymer microspheres with microporous shells by a one-pot solvothermal reaction without any additional template; the shells are constructed of iron-based ferrocenyl coordination polymers. We confirm that the Ostwald ripening mechanism is responsible for the formation of hollow cavities with controllable size. Hollow iron-based ferrocenyl coordination polymer microspheres (Fe-Fc-HCPS) were synthesized by a solvothermal reaction of FeCl3·6H2O with 1,1’-ferrocenedicarboxylic acid (H2FcDC) in N,N-dimethyl formamide (DMF; Figure 1a). The precipitate was collected by centrifugation and washed several times with DMF and CHCl3. The reaction temperature, reaction time, and molar ratio of reactants play important roles in the formation of hollow spherical particles. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and optical microscopy (OPM) were

Caboxamycin, a new antibiotic of the benzoxazole family produced by the deep-sea strain Streptomyces sp. NTK 937*

Caboxamycin, a new benzoxazole antibiotic, was detected by HPLC-diode array screening in extracts of the marine strain Streptomyces sp. NTK 937, which was isolated from deep-sea sediment collected in the Canary Basin. The structure of caboxamycin was determined by mass spectrometry, NMR experiments and X-ray analysis. It showed inhibitory activity against Gram-positive bacteria, selected human tumor cell lines and the enzyme phosphodiesterase.

Electron-Rich N-Heterocyclic Silylene (NHSi)–Iron Complexes: Synthesis, Structures, and Catalytic Ability of an Isolable Hydridosilylene–Iron Complex

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(←:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(←:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(←:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.

An Ylide-like Phosphasilene and Striking Formation of a 4π-Electron, Resonance-Stabilized 2,4-Disila-1,3-diphosphacyclobutadiene

The first N-donor-stabilized phosphasilene LSi(SiMe(3))═PSiMe(3) (L = PhC(NtBu)(2)) has been synthesized in 87% yield through 1,2-silyl migration of the (Me(3)Si)(2)P-substituted, N-heterocyclic silylene [LSi-P(SiMe(3))(2)]. Remarkably, the latter reacts with dichlorotriphenylphosphorane Ph(3)PCl(2) to give the unprecedented 4π-electron Si(2)P(2)-cycloheterobutadiene [(LSi)(2)P(2)] with two-coordinate phosphorus atoms. The striking molecular structures as well as the (29)Si and (31)P NMR spectroscopic features of both products indicate the presence of zwitterionic Si═P bonds which is also in accordance with results by DFT calculations.

Synthesis and unexpected coordination of a silicon(II)-based SiCSi pincerlike arene to palladium.

Chelate ligands can be important for steering the reactivity of transition-metal complexes. Among multidentate ligands, pincer arenes, consisting of a central aromatic backbone linked to two-electron Lewis donor atoms (E) by different spacers, are particularly attractive because of their feasible structural tuning with many choices of donor groups. 2] Recently, the chemistry of pincer-ligated transition-metal complexes underwent rapid developments and has been used for several intriguing chemical transformations. 3] The coordination chemistry of the most common pincer ligands such as NCN-, PCP-, and SCS-type ligands toward transition metals in A (Figure 1) has been explored extensively. Pincer arenes

Formation of a donor-stabilized tetrasilacyclobutadiene dication by a Lewis acid assisted reaction of an N-heterocyclic chloro silylene.

The first donor-stabilized tetrasilacyclobutadiene dication species has been synthesized and fully characterized. Its unexpected formation occurs by the Lewis acid assisted reaction of the N-heterocyclic chloro silylene [L(Si:)Cl] (L = PhC(NtBu)(2); amidinate) with Cp*ZrCl(3) (Cp* = pentamethylcyclopentadienyl) in the molar ratio of 3:2. Remarkably, the four-membered Si(4) core consists of two N-donor stabilized silylium subunits and two silylene-like moieties. The dicationic charge is somewhat delocalized on the Si(4) core, which is supported by DFT calculations.

The Elusive Silyliumylidene [ClSi:]+ and Silathionium [ClSiS]+ Cations Stabilized by Bis(Iminophosphorane) Chelate Ligand†

Silylenes, the silicon analogues of singlet carbenes, are highly reactive compounds with dicoordinate divalent silicon atoms. Parent silylene and its derivatives R2SiD with small organic groups R represent reactive intermediates, which have been investigated in the gas-phase, in diluted solutions, and in frozen rare-gas matrices at low temperatures. Likewise, dichlorosilylene (DSiCl2) is an elusive divalent silicon species, which plays a particular role in the Siemens process, in the chemical vapor deposition of thin silicon films, and in dry etching of silicon wafers by elemental chlorine, as well as in the plasma etching of silicon and silicon dioxide interfaces. Although synthesis and reactivity of gaseous DSiCl2 has been investigated since 1964, studies on its reactivity have been limited to the gas phase and matrix-isolation systems at low temperatures (77 K), because it polymerizes readily to (SiCl2)n at higher temperatures. [3] Since 1994, the concept of donor–acceptor stabilization has been very successfully applied to the synthesis of several types of isolable cyclic and acyclic silylenes. Recent progress includes the striking synthesis of stable H2Si: complexes, reported by Rivard, Robinson, and their respective co-workers. In 2009, the research groups of Roesky and Filippou showed that dihalosilylenes DSiX2 (X = Cl, Br) can be stabilized by Nheterocyclic carbenes (NHCs) to form isolable NHC!SiX2 complexes A (Scheme 1). The latter represent long-sought convenient dihalosilicon(II) precursors. Another challenge is the synthesis of isolable divalent silicon cations, that is, silyliumylidene cations ([RSiD]; R = H, halogen, organo groups). Remarkably, by utilizing suitable thermodynamic and/or kinetic stabilization, the first isolable silyliumylidene cations RSi (R = pentaalkylcyclopentadienyl, b-diketiminate), which bear bulky monovalent substituents R with additional donor sites, could be synthesized. Other types of silyliumylidenes would be very attractive for employment as versatile building blocks and Lewis acid catalysts. Accordingly, monochlorosilyliumylidene [ClSiD] appears to be a very promising silyliumylidene precursor, because the chlorine atom could be replaced by suitable nucleophiles R to pave the way to other types of silyliumylidene derivatives [RSiD]. However, [ClSiD] can only be generated by gas-phase synthesis, for example, by hollow cathode discharge of SiCl4 diluted in a helium atmosphere, and can only be detected by infrared spectroscopy and mass spectrometry in the gas phase under unusual experimental conditions. Very recently, Reid, Roesky, Stalke, and their respective co-workers synthesized the cationic chlorogermyliumylidene and chlorostannyliumylidene complexes B through a Lewis base mediated autoionization of GeCl2 and SnCl2 in the presence of a neutral tridentate donor ligand (Scheme 1). In 1996, Cooks and co-workers reported a mass spectrometry study of bis(pyridine)-supported [ClSiD]. However, an isolable complex that bears the chlorosilyliumylidene cation is still unknown. The formation of B prompted us to develop a Lewis base stabilized chlorosilyliumylidene [ClSiD] by employing a bis(ylide) donor ligand. Recently, we have shown that a bis(phosphorus ylide) can serve as an effective donor to stabilize highly Lewis acidic Si sites (see carbocyclic silylene C in Scheme 1). Herein, we report the synthesis of the first isolable chlorosilyliumylidene species 2 stabilized by the bis(iminophosphorane) chelate ligand 1. Moreover, the reactivity of 2 toward elemental sulfur, which leads to the unprecedented chlorosilathionium complex 3 (Scheme 2), is presented. Because of the two N=PnBu3 ylide moieties, which are bonded at positions 1 and 8 of the naphthalene ring, the neutral ligand 1 behaves both as a very strong Brçnsted and Lewis base (Scheme 2). Thus, treatment of trichlorosilane Scheme 1. NHC-stabilized dihalosilylenes A, chlorogermyliumylidenes and their tin analogues B, and the bis(phosphorus ylide)-stabilized silylenes C.

Highly Selective Iron-Catalyzed Synthesis of Alkenes by the Reduction of Alkynes

Herein, the iron-catalyzed reduction of a variety of alkynes with silanes as a reductant has been examined. With a straightforward catalyst system composed of diiron nonacarbonyl and tributyl phosphane, excellent yields and chemoselectivities (>99%) were obtained for the formation of the corresponding alkenes. After studying the reaction conditions, and the scope and limitations of the reaction, several attempts were undertaken to shed light on the reaction mechanism.

Facile and efficient reduction of ketones in the presence of zinc catalysts modified by phenol ligands.

In the present study, the zinc-catalyzed hydrosilylation of various ketones to give their corresponding alcohols has been examined in detail. Diethyl zinc that can be modified by easily accessible phenol ligands allows the efficient reduction of various aryl and alkyl ketones. By using a practical in situ catalyst, excellent turnover frequencies up to 1000 h(-1) and a broad functional group tolerance were achieved.

Trimerization of Alkali Dicyanamides M[N(CN)2] and Formation of Tricyanomelaminates M3[C6N9] (M=K, Rb) in the Melt: Crystal Structure Determination of Three Polymorphs of K[N(CN)2], Two of Rb[N(CN)2], and One of K3[C6N9] and Rb3[C6N9] from X‐ray Powder Diffractometry

The alkali dicyanamides M[N(CN)2] (M=K, Rb) were synthesized through ion exchange, and the corresponding tricyanomelaminates M3[C6N9] were obtained by heating the respective dicyanamides. The thermal behavior of the dicyanamides and their reaction to form the tricyanomelaminates were investigated by temperature-dependent X-ray powder diffractometry and thermoanalytical measurements. Potassium dicyanamide K[N(CN)2] was found to undergo four phase transitions: At 136 °C the low-temperature modification α-K[N(CN)2] transforms to β-K[N(CN)2], and at 187 °C the latter transforms to the high-temperature modification γ-K[N(CN)2], which melts at 232 °C. Above 310 °C the dicyanamide ions [N(CN)2]− trimerize and the resulting tricyanomelaminate K3[C6N9] solidifies. Two modifications of rubidium dicyanamide have been identified: Even at −25 °C, the α form slowly transforms to β-Rb[N(CN)2] within weeks. Rb[N(CN)2] has a melting point of 190 °C. Above 260 °C the dicyanamide ions [N(CN)2]− of the rubidium salt trimerize in the melt and the tricyanomelaminate Rb3[C6N9] solidifies. The crystal structures of all phases were determined by powder diffraction methods and were refined by the Rietveld method. α-K[N(CN)2] (Pbcm, a=836.52(1), b=646.90(1), c=721.27(1) pm, Z=4), γ-K[N(CN)2] (Pnma, a=855.40(3), b=387.80(1), c=1252.73(4) pm, Z=4), and β-Rb[N(CN)2] (C2/c, a=1381.56(2), b=1000.02(1), c=1443.28(2) pm, β=116.8963(6)°, Z=16) represent new structure types. The crystal structure of β-K[N(CN)2] (P21/n, a=726.92(1), b=1596.34(2), c=387.037(5) pm, β=111.8782(6)°, Z=4) is similar but not isotypic to the structure of α-Na[N(CN)2]. α-Rb[N(CN)2] (Pbcm, a=856.09(1), b=661.711(7), c=765.067(9) pm, Z=4) is isotypic with α-K[N(CN)2]. The alkali dicyanamides contain the bent planar anion [N(CN)2]− of approximate symmetry C2v (average bond lengths: C−Nbridge 133, C−Nterm 113 pm; average angles N-C-N 170°, C-N-C 120°). K3[C6N9] (P21/c, a=373.82(1), b=1192.48(5), c=2500.4(1) pm, β=101.406(3)°, Z=4) and Rb3[C6N9] (P21/c, a=389.93(2), b=1226.06(6), c=2547.5(1) pm, β=98.741(5)°, Z=4) are isotypic and they contain the planar cyclic anion [C6N9]3−. Although structurally related, Na3[C6N9] is not isotypic with the tricyanomelaminates M3[C6N9] (M=K, Rb).