Role of Metal Ions and Hydrogen Bond Acceptors in the Tautomeric Equilibrium of Nitro-9[(Alkylamino)Amino]-Acridine Drugs

3-nitro-9-[2-(dialkylamino)ethyl)]aminoacridines (alkyl = methyl or ethyl) have been used as ligands towards platinum(If). The end product is a complex in which the acridine acts as a tridentate ligand contributing the two exocyclic nitrogen atoms and one of the two peri carbons. The metallation takes place predominantly at the peri position of the unsubstituted ring. The coordinated acridine is in the imino tautomeric form although, in the free state, it occurs exclusively in the amino form (both in the solid state and in solution). The imino tautomer is considered to be the biologically active form. In the platinated species the N(10)H of the acridine can be involved in strong hydrogen bonding with a chloride ion leading to formation of an association complex, the formation constant has been found to be 1.4+/-10(3) M(-1). The N(10)H...CI interaction can influence the tautomeric equilibrium of the acridine dye also in the uncoordinated species, however, the shift in favor of the imino tautomer is not complete.