Anionic chelate compounds

A compound of Formula I or a salt or solvate thereof:

• wherein:
• n is an integer of 1 to 4
• R1 is C1-3 alkyl or —(CH2)n+1—C(═O)—NR2R3;
• each X is —C(═O)—NR2R3 or —NH—C(═O)—R3;
• each R2 is hydrogen, C1-4 alkyl or an anionic substituent;
• each R3 is an anionic substituent optionally linked via a C1-4 alkylene;
• and,
• R4 is independently selected from the group comprising hydroxy, halo, amino, amido, C1-6 alkyl and C1-6 hydroxyalkyl and m is between 0-3, wherein each anionic substituent is independently selected from phosphate, phosphonate, hydroxycarboxylate, tetrazole, thiazolidindione, nitromethylsulfonylphenyl, 4-nitrothiophenol, nitromethylcarboxyphenyl, 2,4-dinitrophenol, or malonitrile.