Coordination framework hosts consisting of 4-pyridyl-substituted carboxylic acid (PCA) dimers and 1D chains of Ni2+ and SCN-: a rational structural extension toward coordination framework hosts with large rectangular cavities

For the expansion of a rectangular cavity (RC) defined by two isonicotinic acid (isoH) dimers as bridging ligands and two SCN bridges, we conducted a structural extension based on the elongation of the bridging ligands by the replacement of isoH with longer 4-pyridyl-substituted carboxylic acid (PCA). For this purpose, the following three PCAs have been employed: trans-3-(4-pyridyl)propenoic acid (acrylH), 4-(4-pyridyl)benzoic acid (pybenH), and trans-3-(4-(4-pyridyl)phenyl)propenoic acid (pppeH). Self-assembly of Ni2+, SCN-, and each of four PCAs involving isoH, acrylH, pybenH, and pppeH in the presence of an aromatic guest gave four inclusion compounds formulated as [Ni(SCN)2(isoH)2].1/2(benz[a]anthracene) (1), [Ni(SCN)2(acrylH)2].1/2(benz[a]anthracene) (2), [Ni(SCN)2(pybenH)2].(pyrene) (3), and [Ni(SCN)2(pppeH)2](3/)(2).(benz[a]anthracene) (4). X-ray crystal structural determination of 1-4 revealed that the proposed structural extension was successful. Their crystal structures are layered structures of two-dimensional (2D) grid-type coordination frameworks (2D host layers) framed with bridging ligands of the corresponding PCA dimers and 1D chains consisting of Ni2+ ions and mu(1,3)-SCN- ions. The lengths of the PCA dimers are 12.269(5) A (isoH dimer), 16.890(4) A (acrylH dimer), 20.89(2) A (pybenH dimer), 25.387(3) A (pppeH dimer A), and 25.527(4) A (pppeH dimer B). Each 2D host layer has RCs defined by the two corresponding PCA dimers and the two SCN bridges. The dimensions of RCs are expanded in proportion to the increase in the lengths of the PCA dimers: 29.52 x 5.60-7.20 A2 (4) > 24.95 x 5.46-7.38 A2 (3) > 20.88 x 5.49-7.25 A2 (2) > 16.41 x 5.53-7.43 A2 (1). These expansions reflect the number of aromatic guests that can be included in RCs. RC of 1 include only one molecule of benz[a]anthracene, whereas RCs of 3 or 4 includes two molecules of pyrene or benz[a]anthracene, respectively. Comparison of the lengths between the PCA dimers and 4,4'-bipyridine-type ligands demonstrated that a design strategy-the preparation of a bridging ligand through self-assembly of two PCAs-is both efficient and particularly suitable for the preparation of very long bridging ligands.     

Precise measurement of positronium hyperfine splitting using the Zeeman effect

Positronium is an ideal system for the research of the quantum electrodynamics (QED) in bound state. The hyperfine splitting (HFS) of positronium, Δ_HFS, gives a good test of the bound state calculations and probes new physics beyond the Standard Model. A new method of QED calculations has revealed the discrepancy by 15 ppm (3.9σ) of Δ_HFS between the QED prediction and the experimental average. There would be possibility of new physics or common systematic uncertainties in the previous all experiments. We describe a new experiment to reduce possible systematic uncertainties and will provide an independent check of the discrepancy. We are now taking data and the current result of Δ_HFS?=?203.395 1 ±0.002 4 (stat., 12 ppm) ±0.001 9 (sys., 9.5 ppm) GHz has been obtained so far. A measurement with a precision of O(ppm) is expected within a year.     

Crystal structure and spin state of Fe0.5Co0.5(NCS)2(bpa)2

Fe(NCS)₂(bpa)₂, Co(NCS)₂(bpa)₂, and Fe_0.5Co_0.5(NCS)₂(bpa)₂ were synthesized. X-ray structural analysis revealed that the structure was the same among the three complexes. Although the metal-ligand distance in the mixed crystals was shorter than the distance in Fe(NCS)₂(bpa)₂ and was similar to that in Co(NCS)₂(bpa)₂, the spin state remained temperature-independent Fe^II high-spin state in ⁵⁷Fe M?ssbauer spectra.     

Rapid additions of bulky tert-butyldimethylsilyl cyanide to hindered ketones promoted by heterogeneous tin ion-exchanged montmorillonite catalyst

Tin ion-exchanged montmorillonite (Sn-Mont) was found to be a powerful heterogeneous catalyst for the cyanosilylation of various ketones including congested ones with a bulky cyanide source, tert-butyldimethylsilyl cyanide (TBDMSCN), giving the corresponding cyanohydrin tert-butyldimethylsilyl ethers in good (85%) to excellent (>98%) yields at room temperature. Compared to the previously reported catalysts, Sn-Mont is easy to prepare, environmentally benign, nontoxic, noncorrosive, and recyclable.     

Chiral linker. Part 3: Synthesis and evaluation of aryl substituted m-hydrobenzoins as solid supported open chain chiral auxiliaries for the diastereoselective reduction of α-keto esters

Five partly novel aryl substituted m-hydrobenzoins were synthesized and the corresponding desymmetrized hydrobenzoin ethers evaluated as open chain chiral auxiliaries in the L-Selectride^® mediated stereoselective reduction of phenylglyoxylates, resulting in de values of up to 91%. Two optimized auxiliary structures were immobilized on commercially available Wang-resin and applied as a reusable solid supported chiral auxiliary in the same type of reaction.     

Spectral measurement of weak THz waves with quantum Hall detectors

A terahertz (THz) microspectroscope is developed, in which the frequency of extremely weak THz radiation is resolved by scanning the magnetic field for a quantum Hall detector. The electron density of the detectors is controlled by the back-gate biasing, so that the detector sensitivity is calibrated over a spectral range studied. Reliable spectral measurements with a spectral resolution of 1.2 cm⁻¹ has been made with a sensitivity better than 10 femtowatt level over 1 s integration time.     

Formation of chiral charge-transfer complex with axially chiral 1,1′-bi-2-naphthol and viologen derivatives

By using three types of viologen derivatives, we synthesized chiral charge-transfer (CT) complexes with an axially chiral 1,1′-bi-2-naphthol molecule and successfully controlled the crystal structure and inclusion ability of the third component by changing the viologens.     

Microwave and millimeter-wave spectroscopy of the open-shell van der Waals complex Ar-HO2

Pure rotational transitions of a rare gas atom-reactive open-shell triatom van der Waals complex Ar-HO2 have been observed by Fourier transform microwave spectroscopy. The transitions observed are of a type with K(a) = 0 and 1. Furthermore, by monitoring the change of the free induction decay signal of the a-type transitions, b-type transitions have been observed by a double resonance technique in the region 18-49 GHz. All these transitions provide us precise molecular constants. The r0 structure of Ar-HO2 has been determined by fixing the structure of the HO2 monomer. The determined structure is planar and almost T shaped, where the argon atom is slightly shifted to the hydrogen atom of HO2. The experimental data supplemented by high-level ab initio calculations indicate that the van der Waals bond of Ar-HO2 is relatively rigid. On the other hand, effects on the unpaired electron distribution by the complex formation are found to be fairly small, since the fine and hyperfine constants of Ar-HO2 are well explained by those of the HO2 monomer.     

Reversible and stable redox behavior of a Pt(II) bis(dithiobenzoate)-type complex attributed to rotaxane-based stabilization

A three-dimensionally insulated Pt(II) bis(dithiobenzoate) complex with a [1]rotaxane structure was synthesized using permethylated cyclodextrins. We have confirmed macrocyclic insulation is necessary to yield the Pt(II) bis(dithiobenzoate) complex with high selectivity. Furthermore, the formed Pt(II) complex showed highly stable and reversible two-step redox behavior in cyclic voltammetry due to the three-dimensional insulation.