Isoelectronic caesium compounds: the triphosphenide Cs[tBu3SiPPPSitBu3] and the enolate Cs[OCH=CH2

The caesium triphosphenide Cs[tBu3SiPPPSitBu3] was accessible from the reaction of CsF with the sodium triphosphenide Na[tBu3SiPPPSitBu3] in tetrahydrofuran at room temperature. In contrast to the preparation of tetrahydrofuran-solvated silanides M[SitBu3] (M = Li, Na, K), our efforts to synthesize the caesium silanide Cs[SitBu3] as a tetrahydrofuran complex failed. When tBu3SiBr was treated with an excess of caesium metal in tetrahydrofuran at room temperature, the caesium enolate Cs[OCH=CH2] and the supersilane tBu3SiH formed rather than the silanide Cs[SitBu3]. X-Ray quality crystals of the enolate Cs[OCH=CH2] (orthorhombic, Pnma) were obtained from tetrahydrofuran at ambient temperature. In contrast to the structures of its homologues M[tBu3SiPPPSitBu3] (M = Na, K), the caesium triphosphenide Cs[tBu3SiPPPSitBu3] features a polymer in the solid state (orthorhombic, Cmcm).     

Redox-active p-quinone-based Bis(pyrazol-1-yl)methane ligands: synthesis and coordination behaviour

The synthesis, structural characterisation and coordination behaviour of mono- and ditopic p-hydroquinone-based bis(pyrazol-1-yl)methane ligands is described (i.e., 2-(pz2CH)C6H3(OH)2 (2a), 2-(pz2CH)-6-(tBu)C6H2(OH)2 (2b), 2-(pz2CH)-6-(tBu)C6H2(OSiiPr3)(OH) (2c), 2,5-(pz2CH)2C6H2(OH)2 (4)). Ligands 2a, 2b and 4 can be oxidised to their p-benzoquinone state on a preparative scale (2a ox, 2b ox, 4 ox). An octahedral Ni II complex [trans-Ni(2c)2] and square-planar Pd II complexes [Pd2bCl2] and [Pd2b ox Cl2] have been prepared. In the two Pd II species, the ligands are coordinated only through their pyrazolyl rings. The fact that [Pd2bC12] and [Pd2b oxC12] are isolable compounds proves that redox transitions involving the p-quinone substituent are fully reversible. In [Pd2b oxCl2], the methine proton is highly acidic and can be abstracted with bases as weak as NEt(3). The resulting anion dimerises to give a dinuclear macrocyclic Pd II complex, which has been structurally characterised. The methylated ligand 2-(pz2CMe)C6H3O2 (11 ox) and its Pd II complex [Pd11 oxCl2] are base-stable. A new class of redox-active ligands is now available with the potential for applications both in catalysis and in materials science.     

Designed folding of pseudopeptides: the transformation of a configurationally driven preorganization into a stereoselective multicomponent macrocyclization reaction

The efficient synthesis of large-ring pseudopeptidic macrocycles through a multicomponent [2+2] reductive amination reaction is described. The reaction was entirely governed by the structural information contained in the corresponding open-chain pseudopeptidic bis(amidoamine) precursors, which have a rigid (R,R)-cyclohexane-1,2-diamine moiety. A remarkable match/mismatch relationship between the configurations of the chiral centers of the cyclic diamine and those of the peptidic frame was observed. The macrocyclic tetraimine intermediates have been studied in detail by NMR spectroscopy, circular dichroism (CD), and molecular modeling, and the results support the appropriate preorganization induced by the match combination of the chiral centers. We have also synthesized the corresponding open-chain bis(imine) model compounds. The structural studies (NMR spectroscopy, CD, modeling) of these systems showed an intrinsically lower reactivity of the mismatch combination, even when the product of the reaction was acyclic. In addition, a synergistic effect between the two chiral substructures for the correct folding of the molecules was observed. Finally, X-ray analysis of the HCl salt of one of the macrocycles showed an interesting pattern; the macrocyclic rings stack in columnar aggregates leaving large interstitial channels filled with water-solvated chloride anions.     

A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C−X (X=Cl,Br) Bond Activation

The geminal frustrated Lewis pair tBu₂PCH₂B(Fxyl)₂ ( 1 ; Fxyl=3,5‐(CF₃)₂C₆H₃) is accessible in 65 % yield from tBu₂PCH₂Li and (Fxyl)₂BF. According to NMR spectroscopy and X‐ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P ??? B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H₂, EtMe₂SiH, CO₂/CS₂, Ph₂CO, and H₃CCN. Terminal alkynes react with heterolysis of the C?H bond. Haloboranes give cyclic adducts with strong P?BX₃ and weak R₃B?X bonds. Unprecedented transformations leading to zwitterionic XP/BCX₃ adducts occur on treatment of 1 with CCl₄ or CBr₄ in Et₂O. In less polar solvents (C₆H₆, n‐pentane), XP/BCX₃ adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph₂CO with EtMe₂SiH.     

Using Concept Maps and Interpretive Essays for Assessment in Mathematics

This study explored the use of student-constructed concept maps in conjunction with written interpretive essays as an additional method of assessment in three undergraduate mathematics courses. The primary objectives of this study were to evaluate the benefits of using concept maps and written essays to assess the “connectedness” of students' knowledge; to measure the correlation between students' scores on the concept maps and written essays, course exams, and final grade; and to document students' perception of the effect of this approach on their mathematical knowledge. Results indicated that concept maps, when combined with written essays, are viable tools for assessing students' organization of mathematical knowledge. In addition, students perceive this approach as enhancing their mathematical knowledge.     

Kinetic study of azidopentaammine cobalt(III) complex as photoinitiator and termination agent of N‐vinylpyrrolidone radical polymerization

N‐Vinylpyrrolidone polymerization photoinitiated at 365 and 546 nm by azidopentaammine cobalt(III) {[Co(NH₃)₅N₃]²⁺} was investigated at room temperature in an argon atmosphere. By excitation into the ligand to metal charge transfer (LMCT), the cobalt complex showed an efficient photoredox process leading to the formation of a cobalt(II) and an azide radical (N, Φ_photoredox = 0.24). The same process was found to occur by excitation into the ligand field band with a low but not negligible quantum yield (Φ_photoredox = 0.016). Two different domains were clearly present when the plot of the rate of polymerization as a function of the cobalt(III) complex was studied; for [Co(III)] < 2.0 × 10⁻⁴ M, the termination step mainly involved a mutual annihilation of growing radicals whereas an oxidative termination was present in the range of 2.0 × 10⁻⁴ M < [Co(III)] < 1.0 × 10⁻³ M. Within the former domain the rate of polymerization (R_p ) varied with the first power of the monomer concentration and with the square root of the absorbed light intensity while for the latter domain the R_p was proportional to the monomer concentration and absorbed light intensity. Further investigations using the viscosity‐average molecular weight data allowed us to corroborate the proposed polymerization mechanism. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3997–4005, 2000     

(4S,5S)‐4‐[(1R)‐1,2‐Di­hydroxy­ethyl]‐5‐tri­decyl‐1,3‐oxazolidin‐2‐one

The title compound, C₁₈H₃₅NO₄, is a new bioactive amphiphilic lipid with a cis‐substituted 1,3‐oxazolidin‐2‐one head group. In the crystal structure, the mol­ecules form intercalating bilayers in which the oxazolidinone head groups are joined together by hydrogen bonds into chains.     

Remote Control of the Synthesis of a [2]Rotaxane and its Shuttling via Metal-Ion Translocation

Remote control in an eight-component network commanded both the synthesis and shuttling of a [2]rotaxane via metal-ion translocation, the latter being easily monitored by distinct colorimetric and fluorimetric signals. Addition of zinc(II) ions to the red colored copper-ion relay station rapidly liberated copper(I) ions and afforded the corresponding zinc complex that was visualized by a bright sky blue fluorescence at 460 nm. In a mixture of all eight components of the network, the liberated copper(I) ions were translocated to a macrocycle that catalyzed formation of a rotaxane by a double-click reaction of acetylenic and diazide compounds. The shuttling frequency in the copper-loaded [2]rotaxane was determined to k₂₉₈=30 kHz (ΔH^≠=62.3±0.6 kJ mol⁻¹, ΔS^≠=50.1±5.1 J mol⁻¹ K⁻¹, ΔG^≠₂₉₈=47.4 kJ mol⁻¹). Removal of zinc(II) ions from the mixture reversed the system back generating the metal-free rotaxane. Further alternate addition and removal of Zn²⁺ reversibly controlled the shuttling mode of the rotaxane in this eight-component network where the ion translocation status was monitored by the naked eye.     

A validated model for a pin-slot clearance joint

The numerical modeling of joints with a certain amount of clearance and a subsequent validation of the model are important for accurate multibody simulations. For such validated modeling, not only the kinematic constraints, but also the contact models, are important. If a joint has no clearance, it is assumed to be ideal. However, in real applications, there is frequently some clearance in the joints. Adding clearance and kinematic conditions to a pin-slot joint significantly increases the number of kinematic and contact parameters. Consequently, the resulting kinematics and the contact forces can vary significantly with regard to the selection of those parameters. This research covers the development of a validated model for a pin-slot clearance joint. Different kinematic constraints and contact models are discussed. The presented model is an experimentally validated one for a pin-slot clearance joint that is commonly used in safety-critical applications like electrical circuit breakers. Special attention is given to the Hertz, Kelvin–Voigt, Johnson, and Lankarani–Nikravesh contact models. When comparing different contact models within numerical approaches and comparing the results with experimental data, significant differences in the results were observed. With a validated model of a pin-slot clearance joint, a physically consistent numerical simulation was obtained.     

A Comparative Study of (tBu3Si)SiX2SiX2(SitBu3) (X?=?Cl, Br, I) Including the Result of the X-Ray Structure Analysis of (tBu3Si)SiI2SiI2(SitBu3)

Abstract  

The tetraiodotetrasilane (tBu₃Si)SiI₂SiI₂(SitBu₃) can be prepared from precursor (tBu₃Si)SiH₂SiH₂(SitBu₃). When (tBu₃Si)SiH₂SiH₂(SitBu₃) was treated with an excess of iodine at 120 °C, (tBu₃Si)SiI₂SiI₂(SitBu₃) was formed. X-ray quality crystals of (tBu₃Si)SiI₂SiI₂(SitBu₃) were grown from benzene at ambient temperature. The tetraiodotetrasilane (tBu₃Si)SiI₂SiI₂(SitBu₃) crystallizes in the monoclinic space group C2/c, a = 10.0110(10) ?, b = 13.9130(10) ?, c = 25.422(2) ?, β = 99.072(4)°, V = 3496.6(5) ?³, Z = 4, d _calcd = 1.829 g cm³; R ₁ = 0.0844, wR ₂ = 0.1854 for 3,017 reflections with I > 2σ(I). X-ray-crystallographic data show that the bromo and iodo derivatives (tBu₃Si)SiX₂SiX₂(SitBu₃) (X = Br, I) are isomorphous. The solid-state structure of (tBu₃Si)SiI₂SiI₂(SitBu₃) as well as those of (tBu₃Si)SiX₂SiX₂(SitBu₃) (X = Cl, Br) reveals a staggered conformation which adopts a trans-orientation of the supersilyl substituent. Unequal dihedral angles as found in (tBu₃Si)SiX₂SiX₂(SitBu₃) (X = Cl, Br, I) indicate that these compounds are sterically overcrowded.