Oligomerization of N‐Heterocyclic Silylene into Zwitterionic Silenes

N‐Heterocyclic carbenes (NHC's) are known to serve as efficient substrates for the stabilization of various transient species possessing low‐valent Group 14 elements and for the generation of double E=C bonds. Herein, we report that the thermal tri‐ and tetramerizations of pyridoannulated silylene 1 lead to the formation of remarkably stable silenes 2 and 3 featuring zwitterionic distribution of electron density. Co‐oligomerization of 1 and its germanium analogue gives a related tetrameric product 4 containing low‐valent germanium atom stabilized by binding with the partial carbene‐character C atom. Bonding situations in 2 – 4 are best described as silene or germene with the significant zwitterionic distribution of electron density. The singlet diradical electronic state of 2 is 10 kcal mol^?1 higher than the ground state configuration.     

Selected Least Studied but not Forgotten Bioluminescent Systems

Bioluminescence is a form of chemiluminescence generated by luminous organisms. Luminous taxa have currently been reported from about 800 genera and probably over 10 000 species in the world. On the other hand, their bioluminescent systems, including chemical structures of luciferins/chromophores and the genes encoding luciferases/photoproteins, have been elucidated from only a few taxonomic groups, for example beetles, bacteria, dinoflagellates, ostracods and some cnidarians. Research efforts to understand unknown bioluminescence systems are being conducted around the world, and recently, for example, novel luciferin structures of luminous enchytraeid potworms and fungi were identified by the authors. In this study, we review the current status and perspectives, in the context of postgenomic era, of most likely novel but less‐revealed bioluminescence systems of ten selected organisms: earthworm, parchment tubeworm, fireworm, scaleworm, limpet, millipede, brittle star, acorn worms, tunicate and shark, which indeed are the next focus of our international collaboration.     

Biocompatible Ir(III) Complexes as Oxygen Sensors for Phosphorescence Lifetime Imaging

Synthesis of biocompatible near infrared phosphorescent complexes and their application in bioimaging as triplet oxygen sensors in live systems are still challenging areas of organometallic chemistry. We have designed and synthetized four novel iridium [Ir(N^C)₂(N^N)]⁺ complexes (N^C–benzothienyl-phenanthridine based cyclometalated ligand; N^N–pyridin-phenanthroimidazol diimine chelate), decorated with oligo(ethylene glycol) groups to impart these emitters’ solubility in aqueous media, biocompatibility, and to shield them from interaction with bio-environment. These substances were fully characterized using NMR spectroscopy and ESI mass-spectrometry. The complexes exhibited excitation close to the biological “window of transparency”, NIR emission at 730 nm, and quantum yields up to 12% in water. The compounds with higher degree of the chromophore shielding possess low toxicity, bleaching stability, absence of sensitivity to variations of pH, serum, and complex concentrations. The properties of these probes as oxygen sensors for biological systems have been studied by using phosphorescence lifetime imaging experiments in different cell cultures. The results showed essential lifetime response onto variations in oxygen concentration (2.0–2.3 μs under normoxia and 2.8–3.0 μs under hypoxia conditions) in complete agreement with the calibration curves obtained “in cuvette”. The data obtained indicate that these emitters can be used as semi-quantitative oxygen sensors in biological systems.     

Ab initio X‐ray structural characterization of an inclusion compound with a compositionally disordered chiral guest: no prior knowledge of the crystal composition

The crystal structure and absolute configuration of a molecular host/guest/impurity inclusion complex were established unequivocally in spite of our having no prior knowledge of its chemical composition. The host (4R,5R)‐4,5‐bis(hydroxydiphenylmethyl)‐2,2‐dimethyl‐1,3‐dioxolane, (I), displays expected conformational features. The crystal‐disordered chiral guest 4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one, (II), is present in the crystal 85.1 (4)% of the time. It shares a common site with 4a‐hydroperoxymethyl‐4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one, (III), present 14.9 (4)% of the time, which is the product of autoxidation of (II). This minor peroxide impurity was isolated, and the results of nuclear magnetic resonance, mass spectrometry, and X‐ray fluorescence studies are consistent with the proposed structure of (III). The complete structure was therefore determined to be (4R,5R)‐4,5‐bis(hydroxydiphenylmethyl)‐2,2‐dimethyl‐1,3‐dioxolane–4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one–4a‐hydroperoxymethyl‐4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one (1/0.85/0.15), C₃₁H₃₀O₄·0.85C₁₀H₁₄O·0.15C₁₀H₁₄O₃, (IV). There are host–host, host–guest, and host–impurity hydrogen‐bonding interactions of types S and D in the solid state. We believe that the crystals of (IV) were originally prepared to establish the chirality of the guest (II) by means of X‐ray diffraction analysis of host/guest crystals obtained in the course of chiral resolution during cocrystallization of (II) with (I). In spite of the absence of `heavy' elements, the absolute configurations of all anomeric centres in the structure are assigned as R based on resonant scattering effects.     

Polymorphism of dinitro[tris(2‐aminoethyl)amine]cobalt(III) chloride

Three polymorphs of bis(nitrito‐κN)[tris(2‐aminoethyl)amine‐κ⁴N,N′,N′′,N′′′]cobalt(III) chloride, [Co(NO₂)₂(C₆H₁₈N₄)]Cl, have been structurally characterized in the 100–300 K temperature range. Two orthorhombic polymorphs are related by a solid‐state enantiotropic order–disorder k2 phase transition at ca 152 K. The third, monoclinic, polymorph crystallizes as a nonmerohedral twin. In the structure of the high‐temperature (300 K) orthorhombic polymorph, the Co^III complex cation resides on a crystallographic mirror plane, whereas the Cl⁻ anion occupies a crystallographic twofold axis. In the unit cell of the monoclinic polymorph, the cationic Co^III complex is in a general position, whose charge is balanced by two halves of two Cl⁻ anions, each residing on a crystallographic twofold axis.     

The Chemical Basis of Fungal Bioluminescence

Many species of fungi naturally produce light, a phenomenon known as bioluminescence, however, the fungal substrates used in the chemical reactions that produce light have not been reported. We identified the fungal compound luciferin 3‐hydroxyhispidin, which is biosynthesized by oxidation of the precursor hispidin, a known fungal and plant secondary metabolite. The fungal luciferin does not share structural similarity with the other eight known luciferins. Furthermore, it was shown that 3‐hydroxyhispidin leads to bioluminescence in extracts from four diverse genera of luminous fungi, thus suggesting a common biochemical mechanism for fungal bioluminescence.     

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations

Rhodium(I) carbenes were generated from propargylic alcohol derivatives as the result of a dehydrative indole annulation. Depending on the choice of the electron‐withdrawing group on the aniline nitrogen nucleophile, either a cyclopropanation product or dimerization product was obtained chemoselectively. Intramolecular hydroamidation occurred for the same type of propargylic alcohol derivatives when other transition‐metal catalysts were employed.     

An example of the refinement of positional disorder modeled as compositional disorder in [5‐(2‐formylphenyl)‐10,15,20‐triphenylporphyrinato]nickel(II)

The title compound, [Ni(C₄₅H₂₈N₄O)], crystallizes in the space group I2d and resides on a crystallographic fourfold rotoinversion axis with only a quarter of the complex in the asymmetric unit. The complex displays positional disorder as the one aldehyde group on the ligand can be located at four different positions. It was necessary to model this as compositional disorder to obtain a correct model and refinement. The practical approach to the refinement is explained.     

A 1,2,4‐diazaphospholane complex of rhodium

The crystal structure of a prospective olefin catalyst, namely {2‐[1‐acetyl‐5‐(2‐hydroxy­phenyl)‐4‐phenyl‐1,2,4‐di­aza­phospholan‐3‐yl]­phenyl acetate‐κP}chloro­(η⁴‐cyclo­octa‐1,5‐diene)rhodium(I) di­chloro­methane solvate, [RhCl(C₈H₁₂)(C₂₄H₂₃N₂O₄P)]·CH₂Cl₂, has been determined at 173 K. The five‐membered heterocycle of the phosphine ligand is in a slightly distorted twist conformation. An intramolecular N1—H1⃛Cl1 hydrogen bond contributes to the adopted conformation and may additionally participate in secondary interactions with substrates during catalysis.