Accurate X-ray diffraction data required for proper evaluation of bond valence sums and global instability indexes: redetermination of the crystal structures of diamond-like Cu2CdSiS4 and Cu2HgSnS4 as a case study

Our calculations of the global instability index (G) values for some diamond-like materials with the general formula I₂–II–IV–VI₄ have indicated that the structures may be unstable or incorrectly determined. To compute the G value of a given compound, the bond valence sums (BVSs) must first be calculated using a crystal structure. Two examples of compounds with high G values, based on data from the literature, are the wurtz–stannite-type dicopper cadmium silicon tetrasulfide (Cu₂CdSiS₄) and the stannite-type dicopper mercury tin tetrasulfide (Cu₂HgSnS₄), which were first reported in 1967 and 1965, respectively. In the present study, Cu₂CdSiS₄ and Cu₂HgSnS₄ were prepared by solid-state synthesis at 1000 and 900 °C, respectively. The phase purity was assessed by powder X-ray diffraction. Optical diffuse reflectance UV/Vis/NIR spectroscopy was used to estimate the optical bandgaps of 2.52 and 0.83 eV for Cu₂CdSiS₄ and Cu₂HgSnS₄, respectively. The structures were solved and refined using single-crystal X-ray diffraction data. The structure type of Cu₂CdSiS₄ was confirmed, where Cd²⁺, Si⁴⁺ and two of the three crystallographically unique S²⁻ ions lie on a mirror plane. The structure type of Cu₂HgSnS₄ was also verified, where all ions lie on special positions. The S²⁻ ion resides on a mirror plane, the Cu⁺ ion is situated on a fourfold rotary inversion axis and both the Hg²⁺ and the Sn⁴⁺ ions are located on the intersection of a fourfold rotary inversion axis, a mirror plane and a twofold rotation axis. Using the crystal structures solved and refined here, the G values were reassessed and found to be in the range that indicates reasonable strain for a stable crystal structure. This work, together with some examples gathered from the literature, shows that accurate data collected on modern instrumentation should be used to reliably calculate BVSs and G values.     

Efficient synthesis of 3-hydroxymethylated cis- and trans-cyclobutane β-amino acids using an intramolecular photocycloaddition strategy

Uracil bearing a tethered allyl alcohol appendage at N1 undergoes a [2+2] photocycloaddition reaction to provide a single tricyclic adduct in high yield. This compound is transformed in one step into a cis-cyclobutane β-amino acid bearing a 3-hydroxymethyl group. Through appropriate functionalization and epimerization, the trans isomer is obtained therefrom in only three further steps.     

Preparation of Cyclobutene Acetals and Tricyclic Oxetanes through Photochemical Tandem and Cascade Reactions

We describe a photochemical reaction using two starting materials, a cyclopent‐2‐enone and an alkene, which are transformed in a controlled manner via the initial [2+2]‐photocycloaddition adducts into cyclobutene aldehydes (conveniently trapped as stable acetals) or unprecedented angular tricyclic 4:4:4 oxetane‐containing skeletons. These compounds are formed through tandem or triple cascade photochemical reaction processes, respectively. Small libraries of each compound class were prepared, thus suggesting that this photochemistry approach opens new opportunities for synthesis design and for widening molecular diversity.     

Synthesis, thermal reactivity and kinetics of substituted [(benzoyl)(phenylcarbamoyl)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

A range of 16 substituted benzoyl/arylcarbamoyl and benzoyl/arylthiocarbamoyl stabilised ylides have been prepared. They are found under conditions of flash vacuum pyrolysis to fragment giving a benzoyl ylide and aryl isocyanate or isothiocyanate accompanied in some cases by secondary pyrolysis products. Kinetic studies show the thiocarbamoyl ylides to react consistently faster than their carbamoyl analogues and substituent effects suggest a polar cyclic transition state, which involves attack by the benzoyl oxygen on the carbamoyl/thiocarbamoyl NH.     

The X-ray Structures of Sulfones

Abstract  

The X-rays structures of diphenyl sulfone 1, bis(3-aminophenyl) sulfone 2, bis(4-nitrophenyl) sulfone 3, 2-nitrophenylphenyl sulfone 4, benzylphenzyl sulfone 5, 4-nitrobenzylphenyl sulfone 6, 4-methylphenylbenzyl sulfone 7, dibenzyl sulfone 8, benzylisopropyl sulfone 9, and methylacetate,ethyl sulfone 10, are reported. B3PW91 DFT calculations were performed on selected sulfones for comparison. The S = O distances in these compounds range from 1.392(5) to 1.463(3) Å and S–C bond distances range from 1.743(7) to 1.790(3) Å. The C–S–C bond angles vary from 101.1(3)° to 106.80(14)° and the O–S–O bond angles vary from 116.7(2)° to 120.61(8)°. Using data obtained here and comparison with published structural data no evidence was found for para substituents on aryl groups influencing S–O bond distances, however, small changes on S–C bond distances were noted. Additionally, if the sulfur is flanked by an alkyl group the S–C distance is slightly larger than when it is flanked by an aryl group. Most of the sulfone compounds studied here display intermolecular soft hydrogen bonding interactions which influence their solid state packing. Furthermore, in dialkyl sulfone compounds these interactions appear to be strong enough to overcome conformational barriers calculated to be ca. 6 kJ mol⁻¹.     

Moderating the acidity of Pb(II)-water complexes through the coordination of nonaqueous ligands: a computational study

The metal dication Pb(II) is known to promote catalytic cleavage of the sugar-phosphate backbone in tRNA. The mechanism proposed to achieve this step requires that the [Pb(II)OH(-)](+) moiety act as a nucleophile and alter the local acidity of surrounding water molecules. MP2 calculations investigating the effect that nonaqueous bases have on the stability of dihydrated-Pb(II) show that the height and position of the proton-transfer barrier are sensitive to the presence of a single N- or O-coordinating "spectator" ligand and that, with the addition of two ligands coordinated directly to the Pb(II) center, the equilibrium for the hydrolysis reaction can shift to the left, thus making the Pb(II)-hydrate complex more stable than the Pb(II)-hydroxide complex. The calculations reveal a good correlation between the gas-phase basicities of nonaqueous ligands coordinated to the metal center and the barriers to proton transfer in [Pb(H(2)O)(2)](2+). In terms of the Pb(II)-induced hydrolysis of tRNA, these results indicate that the coordination of [Pb(II)-OH(-)](+) to uracil and cytosine in tRNA increases the basicity of the hydroxyl group and promotes nucleophilic attack of H(+).     

Intrinsic Folding Proclivities in Cyclic β‐Peptide Building Blocks: Configuration and Heteroatom Effects Analyzed by Conformer‐Selective Spectroscopy and Quantum Chemistry

This work describes the use of conformer‐selective laser spectroscopy following supersonic expansion to probe the local folding proclivities of four‐membered ring cyclic β‐amino acid building blocks. Emphasis is placed on stereochemical effects as well as on the structural changes induced by the replacement of a carbon atom of the cycle by a nitrogen atom. The amide A IR spectra are obtained and interpreted with the help of quantum chemistry structure calculations. Results provide evidence that the building block with a trans‐substituted cyclobutane ring has a predilection to form strong C8 hydrogen bonds. Nitrogen‐atom substitution in the ring induces the formation of the hydrazino turn, with a related but distinct hydrogen‐bonding network: the structure is best viewed as a bifurcated C8/C5 bond with the N heteroatom lone electron pair playing a significant acceptor role, which supports recent observations on the hydrazino turn structure in solution. Surprisingly, this study shows that the cis‐substituted cyclobutane ring derivative also gives rise predominantly to a C8 hydrogen bond, although weaker than in the two former cases, a feature that is not often encountered for this building block.     

Very high stereoselectivity in organocatalyzed desymmetrizing aldol reactions of 3-substituted cyclobutanones

N-Phenylsulfonyl (S)-proline catalyzes the direct aldol reaction of 3-substituted cyclobutanones and aryl aldehydes in good yield and with excellent diastereoselectivity and enantioselectivity. This desymmetrization process provides highly functionalized cyclobutanones with control over three contiguous stereogenic centers.