Structural correlation and metal ion movement in stable pentaammineruthenium(III)-hypoxanthine complexes

The crystal and molecular structures of 7-((Hyp)(NH3)SR~)C13.3Hz0 and 9-((7-MeHyp)(NH3)5R~)C13 are reported. Crystals of both compounds belong to the orthorhombic space group Pnma with unit cell dimensions: a = 11.465 (3) A, b = 6.820 (2) A, and c = 22.520 (9) A for the former; a = 11.146 (4) A, b = 6.835 (2) A, and c = 21.490 (9) A for the latter. The hypoxanthine complex was prepared under neutral to mildly acidic conditions with deoxyinosine as the initial ligand. Chromatographic analysis of the final product showed the 7-coordinated complex to be the only monomeric species present, which verifies N(7) as the preferred coordination site for (NH3)SR~11*'1' ions with inosine and guanosine ligands. When hypoxanthine or 7-methylhypoxanthine were employed as ligand, the 3- and 9-coordinated isomers formed. Under acidic conditions the (NH&RulI1 was observed to undergo linkage isomerization from the N(3) to the N(9) positions with a rate of 1.25 X lo4 s-I at 37 OC, whereas in neutral media this rate was depressed to 2.2 X lod s-l, due to proton ionization from the N(1) site. Activation parameters in acidic media are AH' = 90 kJ/mol and AS* = -31 J/(mol K). An empirical correlation is found between ApK, values of various ruthenium(II1)-purine, -pyrimidine, and -imidazole complexes and 1 /12, where r is the distance between the metal ion and the deprotonation site. A similar correlation is observed between 1/12 and the decrease in the Ru(II1) reduction potential, AE, on deprotonation of the complexes. A free energy correlation is also noted between the ApK, and AE values for these complexes. The coordination of transition-metal ions to nucleotides, nucleic acids, and their constituent bases has assumed an increasing importance in recent years due to the mutagenicity' and anticancer activity of a number of transition-metal complexes.24 In par- ticular, recent studies have shown that a range of ruthenium complexes are mutagenics7 and several also show promise in the development of anticancer pharmaceutical^.^^^ The pentaammineruthenium(II1)-hypoxanthine compounds treated here were initially prepared in an effort to establish the differing physical and chemical effects of a metal ion firmly coordinated at various sites on a purine ring. The effects of varying the coordination site between the N(3), N(7), and N(9) positions on (1) the ligand to metal charge-transfer bands, (2) the acidity of the ligand, and (3) the reduction potential of the metal ion have since been reported.1° In order to assign the various linkage isomers unequivocally, it was also necessary to firmly establish the molecular structures of at least two of the three possible isomers. Moreover, since deoxyinosine, which was employed in the synthesis of the N(7)-coordinated hypoxanthine complex, provides an excellent model for guanosine ligands, the structure determination of this complex coupled with a correspondence in spectroscopic and chemical properties makes it possible to verify the coordination site of pentaammineruthenium(II1) on various guanosine ligands and on these ligands in nucleic acids. In previous investigations it had been shown that the N(3)- bound isomer undergoes linkage isomerization to the N(9)-co- ordinated form in acidic media.IO The work reported here quantitates the rate of this isomerization and further shows that the same process occurs (with a significantly dimiminished rate) even at neutral pH. Finally, the structural parameters made available from this and other studies of pentaammineruthenium(II1) complexes with purine, pyrimidine, and imidazole ligands provide the basis for several empirical linear free energy correlations. These rela- tionships allow the prediction of the acidic properties of the ligand and the reduction potential of the metal or can be used to estimate the distance between the metal coordination and proton ionization sites in new complexes.