Cobalt(III) complexes of monodentate N9-bound adeninate (ade-), [Co(ade-kappaN9)Cl(en)2]+ (en = 1,2-diaminoethane): syntheses, crystal structures, and protonation behaviors of the geometrical isomers

In acidic aqueous solution, a cobalt(III) complex containing monodentate N(9)-bound adeninate (ade(-)), cis-Co(ade-kappaN(9))Cl(en)(2)]Cl (cis-1]Cl), underwent protonation to the adeninate moiety without geometrical isomerization or decomposition of the Co(III) coordination sphere, and complexes of cis-CoCl(Hade)(en)(2)]Cl(2) (cis-2]Cl(2)) and cis-Co(H(2)ade)Cl(en)(2)]Cl(3) (cis-3]Cl(3)) could be isolated. The pK(a) values of the Hade and H(2)ade(+) complexes are 6.03(1) and 2.53(12), respectively, at 20 degrees C in 0.1 M aqueous NaCl. The single-crystal X-ray analyses of cis-2]Cl(2).0.5H(2)O and cis-3]Cl(2)(BF(4)).H(2)O revealed that protonation took place first at the adeninate N(7) and then at the N(1) atoms to form adenine tautomer (7H-Hade-kappaN(9)) and cationic adeninium (1H,7H-H(2)ade(+)-kappaN(9)) complexes, respectively. On the other hand, addition of NaOH to an aqueous solution of cis-1]Cl afforded a mixture of geometrical isomers of the hydroxo-adeninato complex, cis- and trans-Co(ade-kappaN(9))(OH)(en)(2)](+). The trans-isomer of chloro-adeninato complex trans-Co(ade-kappaN(9))Cl(en)(2)]BF(4) (trans-1]BF(4)) was synthesized by a reaction of cis-2](BF(4))(2) and sodium methoxide in methanol. This isomer in acidic aqueous solution was also stable toward isomerization, affording the corresponding adenine tautomer and adeninium complexes (pK(a) = 5.21(1) and 2.48(9), respectively, at 20 degrees C in 0.1 M aqueous NaCl). The protonated product of trans-Co(7H-Hade-kappaN(9))Cl(en)(2)](BF(4))(2).H(2)O (trans-2](BF(4))(2).H(2)O) could also be characterized by X-ray analysis. Furthermore, the hydrogen-bonding interactions of the adeninate/adenine tautomer complexes cis-1]BF(4), cis-2](BF(4))(2), and trans-2](BF(4))(2) with 1-cyclohexyluracil in acetonitrile-d(3) were investigated by (1)H NMR spectroscopy. The crystal structure of trans-Co(ade)(H(2)O)(en)(2)]HPO(4).3H(2)O, which was obtained by a reaction of trans-Co(ade)(OH)(en)(2)]BF(4) and NaH(2)PO(4), was also determined.