Malte Hesse

Well-Defined Stibonic and Tellurinic Acids

The first stibonic acids RSb(O)(OH)2 [1] and tellurinic acids RTe(O)(OH) (R = aryl, alkyl) were extensively investigated more than 90 years ago in the context of pharmacological studies on arsonic acids RAs(O)(OH)2 and the closely related remedies atoxyl and salvarsan, marking the beginning of modern chemotherapy. Unlike their lighter Group 15 and 16 congeners, all hitherto described stibonic and tellurinic acids are ill-defined, amorphous, high-melting compounds that are poorly soluble in most organic solvents. Molecular weight determinations and Sb M ssbauer spectroscopic studies confirm a high degree of aggregation and a trigonalbipyramidal structure for PhSb(O)(OH)2. By contrast, all phosphonic and arsonic acids RE(O)(OH)2 (E = P, As), as well as sulfinic and seleninic acids RE(O)(OH) (E = S, Se), are well-defined molecular compounds with tetrahedrally coordinated central atoms E, polar (formal) E=O double bonds and E OH groups that are usually involved in intermolecular hydrogen bonding in the solid state. Aggregation was also observed for related triarylantimony oxides and diaryltellurium oxides, which exist in two distinctively different structures, namely as asymmetric dimers, for example (Ph3SbO)2 [6] and (Ph2TeO)2 , [7] and as one-dimensional polymers, for example, (Ph3SbO)n [8] and (pAns2TeO)n (Ans = MeOC6H4). [9]

Reactivity Differences between α,β-Unsaturated Carbonyls and Hydrazones Investigated by Experimental and Theoretical Electron Density and Electron Localizability Analyses

It is still a challenge to predict a compounds reactivity from its ground-state electronic nature although Bader-type topological analyses of the electron density (ED) and electron localizability indicator (ELI) give detailed and useful information on electron concentration and electron-pair localization, respectively. Both ED and ELI can be obtained from theoretical calculations as well as high-resolution X-ray diffraction experiments. Besides ED and ELI descriptors, the delocalization index is used here; it is likewise derived from theoretical calculations as well as from experimental X-ray results, but in the latter case, demonstrated here for the first time. We investigate α,β-unsaturated carbonyl and hydrazone compounds because resonance exhibited by these compounds in the electronic ground-state determines their reactive behavior. The degree of resonance as well as the reactivity contrast are quantified with the electronic descriptors. Moreover, competitive mesomeric substituent effects are studied using the two biologically important compounds acrolein and acrylamide. The reactivity differences predicted from the analyses are in line with the known reactivity of these compounds in organic synthesis. Hence, the capability of the ED and ELI for rationalizing and predicting different and competing substituent effects with respect to reactivity is demonstrated.

Electron Density of Corannulene from Synchrotron Data at 12 K, Comparison with Fullerenes

The electron density of corannulene, C20H10, was derived from a high-resolution synchrotron data set (sinθ /λ = 1.11 Å−1) measured at 12 K and from an ab-initio geometry optimization on the B3LYP/6-31G* level. A full topological analysis yielded atomic and bond-topological properties which were compared between experimental and theoretical findings and, as far as steric congruences exist, with corresponding fragments of the fullerene C70. For the four different types of C-C bonds in corannulene, a rather close bond-order range between 1.3 and 1.8 was found indicating a considerable delocalization in this molecule. As was already found earlier in fullerene cages, the deformation density on the C-C bonds is not arranged symmetrically. There is more density located outside than inside the corannulene bowl so that in total, charge accumulation is shifted to the outer surface of the molecule. The electrostatic potential suggests an H...π stacking in the crystal which directs the relative orientation of the two crystallographically independent corannulene molecules. The positively charged rim region of one molecule is oriented almost perpendicular to the negative potential region at the bottom of a second molecule. Graphical Abstract Electron Density of Corannulene from Synchrotron Data at 12 K, Comparison with Fullerenes

Oxygen Transfer from an Intramolecularly Coordinated Diaryltellurium Oxide to Acetonitrile. Formation and Combined AIM and ELI-D Analysis of a Novel Diaryltellurium Acetimidate

The reaction of the intramolecularly coordinated diaryltellurium(IV) oxide (8-Me2NC10H6)2TeO with acetonitrile proceeds with oxygen transfer and gives rise to the formation of the novel zwitterionic diaryltelluronium(IV) acetimidate (8-Me2NC10H6)2TeNC(O)CH3 (1) in 57% yield. Hydrolysis of 1 with hydrochloric acid affords acetamide and the previously known diarylhydroxytelluronium(IV) chloride [(8-Me2NC10H6)2Te(OH)]Cl.

Linear MgCp*2 vs Bent CaCp*2: London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions

In the family of metallocenes, MgCp*2 (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*2 is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp*2 suggests that the bending must have an intramolecular origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp2 and MCp*2 gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*2. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp*2 and CaCp*2 were determined which facilitated the identification and characterization of intermolecular pseudo-pregostic interactions, C-H···Ca, in the CaCp*2 crystal structure.