or aryl group containing 1 to 22 carbon atoms, more particularly 1 to 3 carbon atoms, or the group ???X???OH, R3 and R4 independently of one another have the same meaning as R2 or stand for R1CO???O???X???, X is a linear or branched alkylene group containing 2 to 22 carbon atoms optionally interrupted by 1 to 10 oxygen atoms and Z??? is a charge-equalizing anion, more especially halide, methosulf