In acidic aqueous solution, a cobalt(III) complex containing monodentate N(9)-bound adeninate (ade-), cis-[Co-(ade-κ N 9)Cl(en)2]Cl (cis-Cl), underwent protonation to the adeninate moiety without geometrical isomerization or decomposition of the CoIII coordination sphere, and complexes of cis-[CoCl(Hade)(en) 2]Cl2 (cis-Cl2) and cis-[Co-(H 2ade)Cl(en)2]Cl3 (cis-Cl3) could be isolated. The pKa values of the Hade and H2ade + complexes are 6.03(1) and 2.53(12), respectively, at 20 °C in 0.1 M aqueous NaCl. The single-crystal X-ray analyses of cis-Cl 2·0.5H2O and cis-Cl2(BF 4)·H2O revealed that protonation took place first at the adeninate N(7) and then at the N(1) atoms to form adenine tautomer (7H-Hade-κ N9) and cationic adeninium (1H,7H-H 2ade+-κ N9) complexes, respectively. On the other hand, addition of NaOH to an aqueous solution of cis-Cl afforded a mixture of geometrical isomers of the hydroxo-adeninato complex, cis- and trans-[Co(ade-κ N9)(OH)(en)2]+. The trans-isomer of chloro-adeninato complex trans-[Co(ade-κ N 9)Cl(en)2]BF4 (trans-BF4) was synthesized by a reaction of cis-(BF4)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 (pKa = 5.21(1) and 2.48(9), respectively, at 20 °C in 0.1 M aqueous NaCl). The protonated product of trans-[Co(7H-Hade-κ N 9)Cl(en)2](BF4)2·H 2O (trans-(BF4)2·H2O) could also be characterized by X-ray analysis. Furthermore, the hydrogen-bonding interactions of the adeninate/adenine tautomer complexes cis-BF4, cis-(BF4)2, and trans-(BF4)2 with 1-cyclohexyluracil in acetonitrile-d3 were investigated by 1H NMR spectroscopy. The crystal structure of trans-[Co(ade)(H 2O)(en)2]HPO4·3H2O, which was obtained by a reaction of trans-[Co(ade)(OH)(en)2]BF4 and NaH2PO4, was also determined.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry