FIELD : medicine; pharmaceuticals.SUBSTANCE : invention generally relates to a pharmaceutically acceptable salt of the compound of formula (IV), wherein the salt contains a halide, phosphate, sulphate, trifluoroacetate, toluenesulphonate, acetate, mesylate (methanesulphonic) or benzoate anion; R1 is phenyl which is optionally substituted with halogen or C1-6 alkyl; R3, R4, R5 and R6 can be identical or different and separately represent hydrogen or halogen; m is equal to 3; n equals 1. Pharmaceutically acceptable salt of the compound of formula (IV) is obtained by reacting a compound of formula (II), where each of R2, identical or different, is hydrogen or tert-butoxycarbonyl, provided that at least one of R2 is tert-butoxycarbonyl with an acid to form a pharmaceutically acceptable salt of the compound of formula (IV). Invention also relates to a method of producing a pharmaceutically acceptable salt of a compound of formula (V) comprising reacting a pharmaceutically acceptable salt of the compound of formula (IV) with a fluoride source in the presence of ammonium, carbonate or bicarbonate salt to form a pharmaceutically acceptable salt of the compound of formula (V). Invention relates to salt of formula (VI) and (VII), where X is formate or ascorbate, respectively.EFFECT : compounds of formula (VI) and (VII) are intended for preparing a pharmaceutical composition for visualizing heart failure in a subject.47 cl, 7 tbl, 10 dwg, 32 ex