Protonation of γ-Butyrolactone and γ-Butyrolactam.

Invited for this month's cover is the group of Andreas Kornath at the Ludwig-Maximilian University of Munich (Germany). The cover picture shows the crystal structures of protonated γ-butyrolactone ([(CH2 )3 OCOH][AsF6 ]) and γ-butyrolactam ([(CH2 )3 NHCOH][AsF6 ]). Both salts were synthesized by reacting the neutral compounds in the superacidic system HF/AsF5 at low temperature. Interestingly, the X-ray structure analyses revealed not only structural parameters of the salts, but also the existent of C⋅⋅⋅F contacts in both species. Quantum chemical calculations were performed to investigate the nature of these contacts. Read the full text of their Full Paper at 10.1002/open.202000220.

Protonation of γ-Butyrolactone and γ-Butyrolactam.

γ-Butyrolactone and γ-butyrolactam were reacted in the superacidic systems XF/MF5 (X=H, D; M=As, Sb). Salts of the monoprotonated species of γ-butyrolactone were obtained in terms of [(CH2 )3 OCOH]+ [AsF6 ]- , [(CH2 )3 OCOH]+ [SbF6 ]- and [(CH2 )3 OCOD]+ [AsF6 ]- and the analogous lactam salts in terms of [(CH2 )3 NHCOH]+ [AsF6 ]- , [(CH2 )3 NHCOH]+ [SbF6 ]- and [(CH2 )3 NDCOD]+ [AsF6 ]- . The salts were characterized by low temperature Raman and infrared spectroscopy and for both protonated hexafluoridoarsenates, [(CH2 )3 OCOH]+ [AsF6 ]- and [(CH2 )3 NHCOH]+ [AsF6 ]- , single-crystal X-ray structure analyses were conducted. In addition to the experimental results, quantum chemical calculations were performed on the B3LYP/aug-cc-pVTZ level of theory. As in both crystal structures C⋅⋅⋅F contacts were observed, the nature of these contacts is discussed with Mapped Electrostatic Potential as a rate of strength.

Evidence of Phosphonium-Carbenium Dication Formation in a Superacid: Precursor to Fluorinated Phosphine Oxides.

Unambiguously confirmed by low-temperature in situ NMR experiments, X-ray diffraction and vibrational spectroscopy, phosphonium-carbenium superelectrophiles are shown to be generated in strong acidic conditions. Representing crucial intermediates, their exploitation allows for the synthesis of unprecedented fluorinated (cyclic) phosphine oxides.

Investigations on Squaric Acid in Superacidic Media.

The syntheses of [OC(COX)3 ][MF6 ] and [(COX)4 ][MF6 ]2 ⋅2 HF were carried out in superacidic media XF/MF5 (M=As, Sb; X=H, D). The degree of protonation is highly dependent on the stoichiometric ratio of the Lewis acid with regard to squaric acid. The salts of diprotonated squaric acid were characterized by Raman spectroscopy and, in the case of [(COH)4 ][MF6 ]2 ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH)4 ][AsF6 ] crystallizes in the monoclinic space group P21 /n with two formula units per unit cell. Analysis of the vibrational spectra was achieved with the support of quantum chemical calculations of the cation [(COH)4 ]2+ ⋅4 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH)3 ][AsF6 ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P21 /n with four formula units per unit cell. The protonation of squaric acid leads to a change of the carbon skeleton, which is discussed for the entire series, starting with the dianion of squaric acid and ending with the tetrahydroxy dication.