The crystal structures of methyl prop‐2‐ynoate, dimethyl fumarate and their protonated species

Methyl prop‐2‐ynoate (C4H4O2) was investigated in the binary superacidic system HF/MF5 (M = Sb or As) and dimethyl fumarate (C6H8O4) in the superacidic system HF/SbF5, as well as HF/BF3. The starting materials methyl prop‐2‐ynoate and dimethyl fumarate were crystallized, the former for the first time. The protonated species of these esters, namely, (1‐methoxyprop‐2‐yn‐1‐ylidene)oxidanium hexafluoroarsenate, C4H5O2+ AsF6−, 1,4‐dimethoxy‐4‐oxobut‐2‐en‐1‐ylidene]oxidanium tetrafluoroborate bis(hydrogen fluoride), C6H9O4+ BF4− 2HF, and hemi{[1,4‐dimethoxy‐4‐oxidaniumylidenebut‐2‐en‐1‐ylidene]oxidanium} undecafluorodiantimonate, 0.5C6H10O42+ Sb2F11−, were characterized by single‐crystal X‐ray diffraction and Raman spectroscopy. The protonated species were recrystallized from anhydrous hydrogen fluoride. In the solid state of the monoprotonated species of methyl prop‐2‐ynoate and the diprotonated species of dimethyl fumarate, strong intramolecular O—H…F hydrogen bonds build a three‐dimensional network. The monoprotonated species of dimethyl fumarate builds chains by strong O—H…O hydrogen bonds between the cations. Methyl prop‐2‐ynoate and dimethyl fumarate have been crystallized, the former for the first time. The protonated species of both were isolated for the first time and identified by single‐crystal X‐ray diffraction and Raman spectroscopy.