Polysulfates [SnO3n+1]2−: With the [S6O19]2− Anion,has the End been Reached?

The S₆O₁₉^2? ion was obtained both as rubidium and ammonium salt from the reaction of the respective sulfate with SO₃. It is the largest polysulfate ion known to date and exhibits a chain of six vertex‐connected [SO₄] tetrahedra. The unique compound was comprehensively characterized and the bonding within the anion was elucidated by theoretical investigations.     

The Electronic Structure of the Al3− Anion: Is it Aromatic?

Multiconfigurational high‐level electronic structure calculations show that the ${{\rm Al}{{- \hfill \atop 3\hfill}}}$ ring‐like cluster anion has three close low‐lying electronic states of different spin, all of them having strong multiconfigurational character. The aromaticity of the cluster has, therefore, been studied by means of total electron delocalization and normalized multicenter electron delocalization indices evaluated from the multiconfigurational wave functions of each state. The lowest‐lying singlet and triplet states are found to be highly aromatic, whereas the next lowest‐lying state, the quintet state, has much less, though non‐negligible, aromatic character.     

Superacidic or Not…︁? Synthesis,Characterisation, and Acidity of the Room‐Temperature Ionic Liquid [C(CH3)3]+ [Al2Br7]−

The room‐temperature ionic liquid (RT‐IL) [C(CH₃)₃]⁺ [Al₂Br₇]^? (m.p. 2 °C) was generated by bromide abstraction from tert‐butyl bromide with the Lewis acid aluminum bromide in the absence of solvent. The crystal structure of the tert‐butyl cation salt was determined by X‐ray diffraction. NMR, IR, and Raman spectroscopy, as well as quantum‐chemical and thermodynamic calculations, confirm the composition of this RT‐IL. Thus, one may consider this RT‐IL to be a readily accessible (and on a large scale) cationic Brønsted acid (protonated isobutene) with the potential for further reactivity. Based on the new absolute Brønsted acidity scale, we calculated an absolute pH_abs value of 171 for liquid bulk [C(CH₃)₃]⁺ [Al₂Br₇]^?. This value is about as acidic as 100 % sulfuric acid (pH_abs=171) and, thus, on the edge of superacidity.     

Evaluation of Electron Population Terms for 〈rSe−3〉4p, 〈rS−3〉3p,and 〈rO−3〉2p: How Do HOMO and LUMO Shrink or Expand Depending on Nuclear Charges?

Electron population terms are evaluated for N=Se, S, and O. Calculations are performed on HOMO and LUMO constructed by pure atomic 4p(Se), 3p(S), and 2p(O) orbitals, employing the 6-311+G(3d) and/or 6-311(++)G(3df,3pd) basis sets at the HF, MP2, and DFT (B3 LYP) levels. Se(4+), Se(2+), Se(0), and Se(2-) with the O(h) symmetry are called G(A: Se) and HSe(+), H(2)Se, and HSe(-) with the C(infinityh) or C(2v) symmetry are named G(B: Se), here [G(A+B: Se) in all]. HOMO and LUMO in G(A+B: N) (N=Se, S, and O) satisfy the conditions of the calculations for . The (4p), (3p), and (2p) values correlate well with the corresponding MO energies (epsilon(N)) for all calculation levels employed. Plots of (HOMO) and (LUMO) versus Q(N) (N=Se, S, and O) at the HF and MP2 levels are analyzed as two correlations. However, the plots at the DFT level can be analyzed as single correlation. A regression curve is assumed for the analysis. Behaviors of clarify how valence orbitals shrink or expand depending on Q(N). The applicability of is examined to establish a new method that enables us to analyze chemical shifts with the charge effect separately from others. A utility program derived from the Gaussian 03 (NMRANAL-NH03G) is applied to evaluate and examine the applicability to the NMR analysis.     

Multicenter Homogeneous Dendritic Catalysts: The Higher the Generation,the Better the Reactivity and Selectivity? – A Comparative Study of the Catalytic Efficiency of Dendrimeric [1,1′‐Binaphthalene]‐2,2′‐diol‐Derived Catalysts

The G0 and G1 generations of optically active, multicenter 1,1′‐binaphthalene‐based dendritic ligands 4 and 5 constructed on a rigid oligo(arylene) framework were prepared by divergent synthesis. Their corresponding aluminum complexes 1 and 2 , respectively, were shown to possess slightly better reactivity and enantioselectivity than those of a monomeric 1,1′‐binaphthalene catalyst 3 in the Diels–Alder reaction between cyclopentadiene and 3‐[(E)‐but‐2‐enoyl]‐oxazolidin‐2‐one.     

Orthoamides and Iminium Salts LXXII: N,N,N′,N′,N′′,N′′,N′′′,N′′′‐Octamethyl‐(but‐2‐yne)‐bis(amidinium) Tetrafluoroborate: The First Bis‐amidinium Salt of Acetylenedicarboxyclic Acid. A New Superelectrophile?

A method for the preparation of the first acetylenedicarboxamidinium salt from a bis‐orthoamide derivative of acetylenedicarboxyclic acid has been established. The salt reacted with cyclopentadiene and furan at room temperature to give bicyclic [4+2]‐cycloaddition products. The solid compounds were characterized by solution NMR spectroscopy and by single‐crystal X‐ray diffraction. Quantum‐chemical calculations of the isolated N,N,N′,N′,N′′,N′′,N′′′,N′′′‐octamethyl‐acetylene‐bis(carboxamidinium) ion showed very good agreement with the spectroscopic and diffraction data.     

Synthesis and Antimalarial‐Activity Evaluation of Tetraoxane?Triazine Hybrids and Spiro[piperidine‐4,3′‐tetraoxanes]

A series of tetraoxane? triazine hybrids and spiro[piperidine‐4,3′‐tetraoxanes] have been synthesized, and all the compounds were screened for in vitro antimalarial activity against chloroquine‐sensitive (D6) and chloroquine‐resistant (W2) strains of Plasmodium falciparum. Most of the spiro[piperidine‐4,3′‐tetraoxanes] exhibited moderate to good antimalarial activities, and two compounds have shown good antimalarial activity with IC₅₀ values in the range of 0.30 to 0.70 μM against both the strains with high selectivity index and no cytotoxicity towards mammalian kidney cell line.     

Linking Deltahedral Zintl Clusters with Conjugated Organic Building Blocks: Synthesis and Characterization of the Zintl Triad [R‐Ge9‐CHCH?CHCH‐Ge9‐R]4−

The accessibility of triads with deltahedral Zintl clusters in analogy to fullerene–linker–fullerene triads is another example for the close relationship between fullerenes and Zintl clusters. The compound {[K(2.2.2‐crypt)]₄[RGe₉‐CHCH CHCH‐Ge₉R]}(toluene)₂ (R=(2Z,4E)‐7‐amino‐5‐aza‐hepta‐2,4‐dien‐2‐yl), containing two deltahedral [Ge₉] clusters linked by a conjugated (1Z,3Z)‐buta‐1,3‐dien‐1,4‐diyl bridge, was synthesized through the reaction of 1,4‐bis(trimethylsilyl)butadiyne with K₄Ge₉ in ethylenediamine and crystallized after the addition of 2.2.2‐cryptand and toluene. The compound was characterized by single‐crystal structure analysis as well asNMR and IR spectroscopy.     

Mechanistic Insight into the NN Bond‐Cleavage of Azo‐Compounds that was Induced by an Al?Al‐bonded Compound [L2−AlII?AlIIL2−]

An α‐diimine‐stabilized Al? Al‐bonded compound [L^2?Al^II? Al^IIL^2?] (L=[{(2,6‐iPr₂C₆H₃)NC(Me)}₂]; 1 ) consists of dianionic α‐diimine ligands and sub‐valent Al²⁺ ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo‐compounds afforded phenylimido‐bridged products [L^?Al^III(μ₂‐NPh)(μ₂‐NAr)Al^IIIL^?] ( 2 – 4 ). During the reaction, the dianionic ligands and Al²⁺ ions were oxidized into monoanions and Al³⁺, respectively, whilst the [NAr]^2? imides were produced by the four‐electron reductive cleavage of the N?N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L^2?)₂Al^III₂(μ₂‐NPh)₂Na₂(thf)₄] ( 5 ). Interestingly, when asymmetric azo‐compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4 ). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet‐state monoanionic ligands could result from an exothermic singlet‐to‐triplet conversion during the reaction process.     

Is the HO4− Anion a Key Species in the Aqueous‐Phase Decomposition of Ozone?

The role of the HO₄^? anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O₃ + HO^? or HO₂ + O₂^?. The determination of the stability of HO₄^? in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short‐lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug‐cc‐pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic. We show that the HO₄^? anion has a planar structure in the gas phase, with a very large HOO? OO bond length (1.823 Å). In contrast, HO₄^? adopts a nonplanar configuration in aqueous solution, with huge geometrical changes (up to 0.232 Å for the HOO? OO bond length) with a very small energy cost. The formation of the HO₄^? anion is predicted to be endergonic by 5.53±1.44 and 2.14±0.37 kcal mol^?1 with respect to the O₃ + HO^? and HO₂ + O₂^? channels, respectively. Moreover, the combination of theoretical calculations with experimental free energies of solvation has allowed us to obtain accurate free energies for the main reactions involved in the aqueous decomposition of ozone. Thus, the oxygen transfer reaction (O₃ + OH^? → HO₂ + O₂^?) is endergonic by 3.39±1.80 kcal mol^?1, the electron transfer process (O₃ + O₂^? → O₃^? + O₂) is exergonic by 31.53±1.05 kcal mol^?1, supporting the chain‐carrier role of the superoxide ion, and the reaction O₃ + HO₂^? → OH + O₂^? + O₂ is exergonic by 12.78±1.15 kcal mol^?1, which is consistent with the fact that the addition of small amounts of HO₂^? (through H₂O₂) accelerates ozone decomposition in water. The combination of our results with previously reported thermokinetic data provides some insights into the potentially important role of the HO₄^? anion as a key reaction intermediate.     

Synthesis of 1H,7H,12bH‐Pyrano[3′,4′: 5,6]pyrano[3,4‐c][1]benzopyran‐1‐one via Domino Knoevenagel/Hetero‐Diels?Alder Reaction with Theoretical Investigations

The CuI‐catalyzed intramolecular oxa‐Diels? Alder reaction of 2‐(prop‐2‐yn‐1‐yloxy)benzaldehydes as unactivated terminal alkynes with 4‐hydroxy‐6‐methyl‐2H‐pyran‐2‐one is described. The reaction proceeds with remarkable chemoselectivity to yield pyranones 3 (Scheme 1). A theoretical investigation of the reaction in terms of HOMO? LUMO interactions in the gas phase is also reported. The reaction could be regarded as an inverse‐electron‐demand Diels? Alder cycloaddition. The theoretical results are in high agreement with the experimental evidences.     

Isocyanide in Biochemistry? A Theoretical Investigation of the Electronic Effects and Energetics of Cyanide Ligand Protonation in [FeFe]‐Hydrogenases

The presence of Fe‐bound cyanide ligands in the active site of the proton‐reducing enzymes [FeFe]‐hydrogenases has led to the hypothesis that such Brønsted–Lowry bases could be protonated during the catalytic cycle, thus implying that hydrogen isocyanide (HNC) might have a relevant role in such crucial microbial metabolic paths. We present a hybrid quantum mechanical/molecular mechanical (QM/MM) study of the energetics of CN^? protonation in the enzyme, and of the effects that cyanide protonation can have on [FeFe]‐hydrogenase active sites. A detailed analysis of the electronic properties of the models and of the energy profile associated with H₂ evolution clearly shows that such protonation is dysfunctional for the catalytic process. However, the inclusion of the protein matrix surrounding the active site in our QM/MM models allowed us to demonstrate that the amino acid environment was finely selected through evolution, specifically to lower the Brønsted–Lowry basicity of the cyanide ligands. In fact, the conserved hydrogen‐bonding network formed by these ligands and the neighboring amino acid residues is able to impede CN^? protonation, as shown by the fact that the isocyanide forms of [FeFe]‐hydrogenases do not correspond to stationary points on the enzyme QM/MM potential‐energy surface.     

Redox‐Triggered C?C Coupling of Diols and Alkynes: Synthesis of β,γ‐Unsaturated α‐Hydroxyketones and Furans by Ruthenium‐Catalyzed Hydrohydroxyalkylation

Direct ruthenium‐catalyzed C C coupling of alkynes and vicinal diols to form β,γ‐unsaturated ketones occurs with complete levels of regioselectivity and good to complete control over the alkene geometry. Exposure of the reaction products to substoichiometric quantities of p‐toluenesulfonic acid induces cyclodehydration to form tetrasubstituted furans. These alkyne‐diol hydrohydroxyalkylations contribute to a growing body of merged redox‐construction events that bypass the use of premetalated reagents and, hence, stoichiometric quantities of metallic by‐products.     

Which One is Bulkier: The 3,5‐Dimethylphenyl or the 2,6‐Dimethylphenyl Group? Development of Size‐Complementary Molecular and Macromolecular [2]Rotaxanes

We developed novel size‐complementary molecular and macromolecular rotaxanes using a 2,6‐dimethylphenyl terminal group as the axle‐end‐cap group in dibenzo‐24‐crown‐8‐ether (DB24C8)‐based rotaxanes, where the 2,6‐dimethylphenyl group was found to be less bulky than the 3,5‐dimethylphenyl group. A series of molecular and macromolecular [2]rotaxanes that bear a 2,6‐dimethylphenyl group as the axle‐end‐cap were synthesized using unsubstituted and fluorine‐substituted DB24C8. Base‐induced decomposition into their constituent components confirmed the occurrence of deslipping, which supports the size‐complementarity of these rotaxanes. The deslipping rate was independent of the axle length but dependent on the DB24C8 substituents. A kinetic study indicated the rate‐determining step was that in which the wheel is getting over the end‐cap group, and deslipping proceeded via a hopping‐over mechanism. Finally, the present deslipping behavior was applied to a stimulus‐degradable polymer as an example for the versatile utility of this concept in the context of stimulus‐responsive materials.