Complex formation of the anti-inflammatory drugs tenoxicam and piroxicam with Fe(III) in methanol and acetone

A spectrophotometric study of the chemical coordination of two nonsteroidal anti-inflammatory drugs (tenoxicam and piroxicam) with Fe(III) has been undertaken in methanol in order to determine the stoichiometry and formation constants of the formed complexes, 1 : 1 and 1 : 2 stoichiometric ratios [Fe(III) : oxicam]. The formation constants determined with the aid of the program SQUAD, at 293 K in methanol, are: Fe(tenox), log 11 = 4.71 0.03; Fe(tenox)2, log 12 = 8.57 0.04; Fe(pirox), log 11 = 5.54 0.09; Fe(pirox)2, log 12 = 9.75 0.08. However, the study in acetone gave rise to complexes with completely different stoichiometric ratios with polynuclear species [dimers of Fe(III)]. The formation constants determined with the aid of program SQUAD, at 293 K in acetone, are: Fe2(tenox), log 21 =9.04 0.03; Fe2(tenox)2, log 22 = 14.75 0.06; Fe2(tenox)3, log 23 = 18.45 0.07; Fe2(pirox), log 21 = 9.06 0.30; Fe2(pirox)2, log 22 = 14.77 0.09; Fe2(tenox)3, log 23 = 18.48 0.40. Besides the well-known effect of the solvent on thermodynamic constants, the stoichiometry seems to be strongly driven by the nature of the solvent.