Terpsichorean movements of pentaammineruthenium on pyrimidine and isocytosine ligands

Pentaammineruthenium moves on ambidentate nitrogen heterocycles by both rotation and linkage isomerization, which may affect the biological activity of potential ruthenium metallopharmaceuticals. The rapid rotation rates of [(NH3)5RuIII] coordinated to the exocyclic nitrogens of isocytosine (ICyt) and 6-methylisocytosine (6MeICyt) have been determined by 1H NMR. Since these rotamers can be stabilized by hydrogen bonding between the coordinated ammines and the N1 and N3 endocyclic nitrogens, rotamerization is under pH control. Spectrophotometrically (UV-vis) measured pKa values for the two endocyclic sites for the ICyt complex are 2.78 and 9.98, and for 6MeICyt are 3.06 and 10.21, which are probably weighted averages for ionization from N3 and N1, respectively. Activation parameters for the rotamerizations were determined by variable-temperature NMR at pKa1 < pH < pKa2 for the complexes with (ICyt-kappa N2)-, (6MeICyt kappa N2)-, and 2AmPym kappa N2. For [(6MeICyt kappa N2)(-)-(NH3)5RuIII]2+, delta H* = 1.6 kcal/mol, delta S* = -37 cal/mol K, and Ea = 2.2 kcal/mol. Due to strong RuIII-N pi-bonding, the activation enthalpies are approximately 10 kcal lower than the expected values for the free ligands. Rotameric structure is correlated with pKa values, pH-dependent reduction potentials, and 1H NMR parameters. Linkage isomers of [(2AmPym)(NH3)5Ru]n+ are reported in which RuII is coordinated to the endocyclic nitrogen (N1) and RuIII to the exocyclic nitrogen (N2). The rate constant for the kappa N2-->kappa N1 isomerization as part of an ECE mechanism is 3.9 s-1 at pH 3. The pH dependence of the acid-catalyzed hydrolysis of [(2AmPym kappa N1)(NH3)5Ru]2+ is determined.     

1H NMR, EPR, UV-Vis, and Electrochemical Studies of Imidazole Complexes of Ru(III). Crystal Structures of cis-[(Im)(2)(NH(3))(4)Ru(III)]Br(3) and [(1MeIm)(6)Ru(II)]Cl(2).2H(2)O

Comparisons of the spectroscopic properties of a number of Ru(III) complexes of imidazole ligands provide methods of distinguishing between various types of bonding that can occur in proteins and nucleic acids. In particular, EPR and (1)H NMR parameters arising from the paramagnetism of Ru(III) should aid in determining binding sites of Ru(III) drugs in macromolecules. Electrochemical studies on several imidazole complexes of ruthenium suggest that imidazole may serve as a significant pi-acceptor ligand in the presence of anionic ligands. Crystal structures are reported on two active immunosuppressant complexes. cis-[(Im)(2)(NH(3))(4)Ru(III)]Br(3) crystallizes in the triclinic space group P&onemacr; (No. 2) with the cell parameters a = 8.961(2) ?, b = 12.677(3) ?, c = 7.630(2) ?, alpha = 98.03(2) degrees, beta = 100.68(2) degrees, gamma = 81.59(2) degrees, and Z = 2 (R = 0.044). [(1MeIm)(6)Ru(II)]Cl(2).2H(2)O crystallizes in the monoclinic space group P2(1)/n (No. 14) with the cell parameters a = 7.994(2) ?, b = 13.173(4) ?, c = 14.904(2) ?, beta = 97.89(1) degrees, and Z = 2 (R = 0.052). The average Ru(II)-N bond distance is 2.106(8) ?.