Brønsted-acidic fluoroalkyl phosphonates

Compounds of the formula I

• [Kt]z+z[RfP(O)(OH)O]− I,
• where
• Rf corresponds to the formula CnF(2n+1)−(m+k)HmXk,
• X denotes Cl, Br or I,
• n denotes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12,
• m denotes 0, 1, 2, 3 or 4,
• k denotes 0, 1 or 2,
• with the proviso that, if n denotes 1, m denotes 0, 1 or 2 and/or k denotes 0, 1 or 2 and
• [Kt]z+ denotes a singly, doubly, triply or multiply positively charged metal atom.