Oxonium and sulfonium salts

A salt (I) having cations selected from the group consisting of

• where
• R1′ denotes CN and
• R2′ to R4′ each, independently of one another, have the meaning
• hydrogen,
• straight-chain or branched alkyl having 1-20 C atoms,
• straight-chain or branched alkenyl having 2-20 C atoms and one or more double bonds,
• straight-chain or branched alkynyl having 2-20 C atoms and one or more triple bonds,
• dialkylamino containing alkyl groups having 1-4 C atoms, which, however, is not bonded to the heteroatom of the heterocycle,
• saturated, partially unsaturated or fully unsaturated cycloalkyl having 3-7 C atoms,
• which may be substituted by alkyl groups having 1-6 C atoms, or
• aryl-C1-C6-alkyl,
• where R2′ and R4′ may be partially or fully substituted by F,
• where R2′ and R3′ may be partially or fully substituted by halogens or partially by NO2 or CN, and
• where, in R2′ to R4′, one or two non-adjacent carbon atoms which are not bonded directly to the heteroatom may be replaced by atoms and/or atom groups selected from the group consisting of —O—, —S—, —S(O)— and —SO2—,
• and
• anions selected from the group consisting of [PFx(CyF2y+1−zHz)6−x]− anions, where 2<x<5, 1<y<8 and 0<z<2y+1,
• or anions selected from the group consisting of [(BFn(CN)4−n]− anions, where n=0, 1, 2 or 3,
• or anions selected from the group consisting of [(Rf1SO2)2N]− anions, where Rf1 denotes F or perfluorinated straight-chain or branched alkyl having 1-20 C atoms, perfluorinated straight-chain or branched alkenyl having 2-20 C atoms and one or more double bonds, perfluorinated straight-chain or branched alkynyl having 2-20 C atoms and one or more triple bonds, perfluorinated, saturated, partially unsaturated or fully unsaturated cycloalkyl having 3-7 C atoms, which may be substituted by perfluoroalkyl groups,
• or anions selected from the group consisting of [BFwRf24−w]− anions, where w=0, 1, 2 or 3, where Rf2 denotes perfluorinated straight-chain or branched alkyl having 1-20 C atoms, perfluorinated straight-chain or branched alkenyl having 2-20 C atoms and one or more double bonds, perfluorinated straight-chain or branched alkynyl having 2-20 C atoms and one or more triple bonds, perfluorinated, saturated, partially unsaturated or fully unsaturated cycloalkyl having 3-7 C atoms, which may be substituted by perfluoroalkyl groups, where Rf1 or Rf2 may in each case be identical or different,
• where a pair of Rf1 or a pair of Rf2 may be connected to one another by single or double bonds,
• and where, in Rf1 or Rf2, one or two non-adjacent carbon atoms which are not in the α-position to the heteroatom may be replaced by atoms and/or atom groups selected from the group consisting of —O—, —SO2— and —NR′— or by the end group —SO2X′, where R′=non-, partially- or per-fluorinated C1- to C6-alkyl, C3- to C7-cycloalkyl, unsubstituted or substituted phenyl, or an unsubstituted or substituted heterocycle, and X′=F, Cl or Br.