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A previous controversial discussion regarding the interpretation of Rydberg spectra of gaseous dimethylpiperazine (DMP) as showing the co-existence of a localized and delocalized mixed-valent DMP + radical cation is revisited. Here we show by high-level quantum-chemical calculations that an apparent barrier separating localized and delocalized DMP + minima in previous multi-reference configuration-interaction (MRCI) calculations and in some other previous computations were due to unphysical curve crossings of the reference wave functions. These discontinuities on the surface are removed in state-averaged MRCI calculations and with some other, orthogonal high-level approaches, which do not provide a barrier and thus no localized minimum. We then proceed to show that in the actually observed Rydberg state of neutral DMP the 3s-type Rydberg electron binds more strongly to a localized positive charge distribution, generating a localized DMP* Rydberg-state minimum, which is absent for the DMP + cation. This work presents a case where interactions of a Rydberg electron with the underlying cationic core alter molecular structure in a fundamental way. Previous theoretical interpretations of the Rydberg spectra of dimethylpiperazine (DMP) debated the existence of a localized minimum on the surface of the DMP+ cation. Here, the authors show a substantial influence of the Rydberg electron on the molecular structure, restoring the localized minimum.

A comparison of computed 19F NMR chemical shifts and experiment provides evidence for large specific solvent effects for fluoride‐type anions interacting with the σ*(C−H) orbitals in organic solvents like MeCN or CH2Cl2. We show this for systems ranging from the fluoride ion and the bifluoride ion [FHF]− to polyhalogen anions [ClFx]−. Discrepancies between computed and experimental shifts when using continuum solvent models like COSMO or force‐field‐based descriptions like the 3D‐RISM‐SCF model show specific orbital interactions that require a quantum‐mechanical treatment of the solvent molecules. This is confirmed by orbital analyses of the shielding constants, while less negatively charged fluorine atoms (e. g., in [EF4]−) do not require such quantum‐mechanical treatments to achieve reasonable accuracy. The larger 19F solvent shift of fluoride in MeCN compared to water is due to the larger coordination number in the former. These observations are due to unusually strong charge‐assisted C−H⋅⋅⋅F− hydrogen bonds, which manifest beyond some threshold negative natural charge on fluorine of ca. < −0.6 e. The interactions are accompanied by sizable free energies of solvation, in the order F−≫[FHF]−>[ClF2]−>[ClF4]−. COSMO‐RS solvation free energies tend to moderately underestimate those from the micro‐solvated cluster treatment. Red‐shifted and intense vibrational C−H stretching bands, potentially accessible in bulk solution, are further spectroscopic finger prints. Strong charge‐assisted CH⋅⋅⋅F hydrogen bonds to negatively charged fluorine atoms in organic solvents like acetonitrile lead to strong solvation and can have surprisingly large effects on the 19F NMR shifts of the solute anions. A proper quantum‐chemical treatment requires the use of microsolvated cluster models.

Dioxygen activation for the subsequent oxygenation of organic substrates that involves cheap and environmentally friendly chemical elements is at the cutting edge of chemical research. As silicon is a non-toxic and highly oxophilic element, the use of silylenes could be attractive for facile dioxygen activation to give dioxasiliranes with a SiO 2 –peroxo ring as versatile oxo-transfer reagents. However, the latter are elusive species, and have been generated and studied only in argon matrices at −233 °C. Recently, it was demonstrated that unstable silicon species can be isolated by applying the concept of donor–acceptor stabilization. We now report the first synthesis and crystallographic characterization of dioxasiliranes stabilized by N -heterocyclic carbenes that feature a three-membered SiO 2 –peroxide ring, isolable at room temperature. Unexpectedly, these can undergo internal oxygen transfer in toluene solution at ambient temperature to give a unique complex of cyclic sila-urea with C=O → Si=O interaction and the shortest Si=O double-bond distance reported to date. Converting dioxygen into more reactive species is extremely useful for direct oxygenation of organic compounds, but doing this with cheap and non-polluting elements is difficult. Now, a carbene-activated silylene has been shown to activate dioxygen, resulting in the isolation of elusive silicon–oxygen species at room temperature.

Bonds are at the very heart of chemistry. Although the order of carbon–carbon bonds only extends to triple bonds, metal–metal bond orders of up to five are known for stable compounds, particularly between chromium atoms. Carbometallation and especially carboalumination reactions of carbon–carbon double and triple bonds are a well established synthetic protocol in organometallic chemistry and organic synthesis. We now extend these reactions to compounds containing chromium–chromium quintuple bonds. Analogous reactivity patterns indicate that such quintuple bonds are not as exotic as previously assumed. Yet the particularities of these reactions reflect the specific nature of the high metal–metal bond orders. Extremely short quintuple bonds between chromium atoms have recently been discovered. Carboalumination reactions have now been performed to further investigate the properties of these unusual bonds, and show that they have interesting analogies to lower-order bonds, as well as revealing more about the nature of quintuple bonds.

Herein, [NEt3Me]2[PdCl4] is reported as a highly active catalyst for the mercury‐free hydrochlorination of acetylene to vinyl chloride, resulting from the combination of the bichloride‐based ionic liquid [NEt3Me][Cl(HCl)n] with PdCl2. Replacing gaseous HCl with the bichloride shifts the reaction in the liquid phase increasing the process safety by pressure reduction and achieves a turnover frequency of TOF = 110 molVCM h−1 molPdCl2−1 matching the productivity of state‐of‐the‐art heterogeneous systems. Additionally, [NEt3Me]2[PdCl4] shows remarkable long‐term stability and can be re‐used over ten reaction cycles (200 h in total) without any problems due to its resistance to reduction by acetylene and coking as revealed by kinetic, theoretical, and spectroscopic investigations. Finally, initial catalytic studies demonstrate promising outcomes for applications of the bichloride‐based ionic liquids with other noble metals, like platinum, if disproportionation phenomena, as observed for gold, do not occur. Combining the bichloride‐based ionic liquid [NEt3Me][Cl(HCl)n] with PdCl2 results in the formation of the highly active, mercury‐free catalyst [NEt3Me]2[PdCl4] for acetylene hydrochlorination to vinyl chloride. Replacing HCl with the bichloride improves the process safety and achieves a competitive turnover frequency of 110 molVCM h−1 molPdCl2−1. The system is stable and re‐usable, with a potential for other applications.

Hydrogen-substituted silylium ions are long-sought reactive species. We report a protolysis strategy that chemoselectively cleaves either an Si–C(sp²) or an Si–H bond using a carborane acid to access the full series of [CHB11H₅Br₆]⁻-stabilized R₂SiH⁺, RSiH₂⁺, and SiH₃⁺ cations, where bulky tert-butyl groups at the silicon atom (R = tBu) were crucial to avoid substituent redistribution.The crystallographically characterized molecular structures of [CHB11H₅Br₆]⁻-stabilized tBu₂HSi⁺ and tBuH₂Si⁺ feature pyramidalization at the silicon atom, in accordance with that of tBu₃Si⁺[CHB11H₅Br₆]⁻. Conversely, the silicon atom in the H₃Si⁺ cation adopts a trigonal-planar structure and is stabilized by two counteranions. This solid-state structure resembles that of the corresponding Brønsted acid.

Certain oxygen-tolerant hydrogenases contain a unique [4Fe-3S] cluster near the catalytic site, but the role of this cofactor is not fully understood. Crystallographic, spectroscopic and computational data now provide evidence for redox-dependent transformations of this cluster, potentially explaining how specialized hydrogenases can safely reduce inhibitory O 2 . Hydrogenases catalyze the reversible oxidation of H 2 into protons and electrons and are usually readily inactivated by O 2 . However, a subgroup of the [NiFe] hydrogenases, including the membrane-bound [NiFe] hydrogenase from Ralstonia eutropha, has evolved remarkable tolerance toward O 2 that enables their host organisms to utilize H 2 as an energy source at high O 2 . This feature is crucially based on a unique six cysteine-coordinated [4Fe-3S] cluster located close to the catalytic center, whose properties were investigated in this study using a multidisciplinary approach. The [4Fe-3S] cluster undergoes redox-dependent reversible transformations, namely iron swapping between a sulfide and a peptide amide N. Moreover, our investigations unraveled the redox-dependent and reversible occurence of an oxygen ligand located at a different iron. This ligand is hydrogen bonded to a conserved histidine that is essential for H 2 oxidation at high O 2 . We propose that these transformations, reminiscent of those of the P-cluster of nitrogenase, enable the consecutive transfer of two electrons within a physiological potential range.

A comparison of computed 19 F NMR chemical shifts and experiment provides evidence for large specific solvent effects for fluoride‐type anions interacting with the σ*(C−H) orbitals in organic solvents like MeCN or CH 2 Cl 2 . We show this for systems ranging from the fluoride ion and the bifluoride ion [FHF] − to polyhalogen anions [ClF x ] − . Discrepancies between computed and experimental shifts when using continuum solvent models like COSMO or force‐field‐based descriptions like the 3D‐RISM‐SCF model show specific orbital interactions that require a quantum‐mechanical treatment of the solvent molecules. This is confirmed by orbital analyses of the shielding constants, while less negatively charged fluorine atoms (e. g., in [EF 4 ] − ) do not require such quantum‐mechanical treatments to achieve reasonable accuracy. The larger 19 F solvent shift of fluoride in MeCN compared to water is due to the larger coordination number in the former. These observations are due to unusually strong charge‐assisted C−H⋅⋅⋅F − hydrogen bonds, which manifest beyond some threshold negative natural charge on fluorine of ca. < −0.6 e. The interactions are accompanied by sizable free energies of solvation, in the order F − ≫[FHF] − >[ClF 2 ] − >[ClF 4 ] − . COSMO‐RS solvation free energies tend to moderately underestimate those from the micro‐solvated cluster treatment. Red‐shifted and intense vibrational C−H stretching bands, potentially accessible in bulk solution, are further spectroscopic finger prints.

TeX3[Al(ORF)4] (X=Cl, Br, I; RF=C(CF3)3) were synthesized by the reaction of Ag[Al(ORF)4] and TeX4 or the reaction of AuX, Ag[Al(ORF)4], and elemental tellurium in liquid SO2. The compounds were characterized by 125Te NMR in solution and by X‐ray diffraction, Raman, and IR spectroscopy in the solid state. The vibrational spectra and the crystal structure show very weak secondary interactions, indicating “pseudo gas phase conditions” in the condensed phase. The observed trend of the 125Te NMR chemical shifts along the [TeX3]+ series follows neither the monotonous decrease known as “normal halogen dependence” nor the increase known as “inverse halogen dependence”. By relativistic two‐component calculations based on the ZORA approach, we find that this “abnormal halogen dependence” results from an interplay of relativistic and solvent effects, where non‐negligible scalar relativistic effects and intermediate‐sized spin‐orbit effects compensate to some extent. The reasons for these trends are evaluated in the context of the Te s‐orbital character of the TeX bonds and compared with the halogen dependence(s) within the isoelectronic [SeX3]+ and PX3 series and related trihalomethyl [CX3]+ cations. TeX3[Al(ORF)4] have been synthesized and characterized by 125Te NMR in solution and by X‐ray diffraction, Raman, and IR spectroscopy in the solid state (X=Cl, Br, I; RF=C(CF3)3). The vibrational analysis shows a very weak contact of [TeX3]+ to the anion. The observed “abnormal halogen dependence” of the 125Te NMR chemical shifts results from an interplay of relativistic and solvent effects.

Hydrogenases catalyze the reversible oxidation of H(2) into protons and electrons and are usually readily inactivated by O(2). However, a subgroup of the [NiFe] hydrogenases, including the membrane-bound [NiFe] hydrogenase from Ralstonia eutropha, has evolved remarkable tolerance toward O(2) that enables their host organisms to utilize H(2) as an energy source at high O(2). This feature is crucially based on a unique six cysteine-coordinated [4Fe-3S] cluster located close to the catalytic center, whose properties were investigated in this study using a multidisciplinary approach. The [4Fe-3S] cluster undergoes redox-dependent reversible transformations, namely iron swapping between a sulfide and a peptide amide N. Moreover, our investigations unraveled the redox-dependent and reversible occurence of an oxygen ligand located at a different iron. This ligand is hydrogen bonded to a conserved histidine that is essential for H(2) oxidation at high O(2). We propose that these transformations, reminiscent of those of the P-cluster of nitrogenase, enable the consecutive transfer of two electrons within a physiological potential range.Hydrogenases catalyze the reversible oxidation of H(2) into protons and electrons and are usually readily inactivated by O(2). However, a subgroup of the [NiFe] hydrogenases, including the membrane-bound [NiFe] hydrogenase from Ralstonia eutropha, has evolved remarkable tolerance toward O(2) that enables their host organisms to utilize H(2) as an energy source at high O(2). This feature is crucially based on a unique six cysteine-coordinated [4Fe-3S] cluster located close to the catalytic center, whose properties were investigated in this study using a multidisciplinary approach. The [4Fe-3S] cluster undergoes redox-dependent reversible transformations, namely iron swapping between a sulfide and a peptide amide N. Moreover, our investigations unraveled the redox-dependent and reversible occurence of an oxygen ligand located at a different iron. This ligand is hydrogen bonded to a conserved histidine that is essential for H(2) oxidation at high O(2). We propose that these transformations, reminiscent of those of the P-cluster of nitrogenase, enable the consecutive transfer of two electrons within a physiological potential range.