formula i comprising the desired x u2212 anion preferably this ion exchange is carried out by treatment with hcl advantageously in ethyl acetate other solvents could be used but some are capable of leading to the formation of by products the conditions for deprotection of the compounds of formula ii disclosed above are particularly advantageous in that they make it possible to achieve a compound of formula i without introducing metal impurities during this step or forming organic impurities according to one variant of the invention it may be envisioned to purify the compound of formula ii by means other than filtration on a support capable of retaining metals for instance by crystallization from an appropriate solvent according to this variant the compound of formula ii is subsequently deprotected using any deprotection means not involving the use of metal compounds in particular using a quinone in particular ddq preferably under the conditions disclosed above another subject of the invention is therefore a process for preparing compounds corresponding to formula i described above characterized in that it comprises at least one step for deprotection of the r u2014 cz u2014 group of the amine of the phenothiazine ring of the compound of formula ii using deprotection means not involving the use of metal compounds the expression u201c deprotection means not involving the use of metal compounds u201d is intended to mean the use of non metal reactants and of solvents not comprising metal residues preferably lt 0 01 ppm in reactors not comprising any metal parts for instance.