| Abstract
| - The extent of the steric hindrance of the (C6)NH2 group on metal ion binding at N7 in the adenine residue is evaluated via the stabilities of M(DMBI)2+ complexes. It is further concluded that the (C4)NH2 group in MABI can participate to some extent in chelate formation due to its relatively high basicity. The basicity of the amino group in the adenine residue is much lower.
- The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+,Zn2+, or Cd2+ (=M2+) and 1-methyl-4-aminobenzimidazole (MABI) or 1,4-dimethylbenzimidazole (DMBI) weredetermined by potentiometric pH titrations in aqueous solution (25 °C; I = 0.5 M, NaNO3). Some of the stabilityconstants were also measured by UV spectrophotometry. The acidity constants of the species H2(MABI)2+ andH(DMBI)+ were determined by the same methods, some twice. Comparison of the stability constants of theM(MABI)2+ and M(DMBI)2+ complexes with those calculated from logversus pstraight-line plots,which were established previously for sterically unhindered benzimidazole-type ligands (=L), reveals that thestabilities of the M(MABI)2+ and M(DMBI)2+ complexes are significantly reduced due to steric effects of the C4substituents on metal ion binding at N3. This effect is more pronounced in the M(DMBI)2+ complexes. Consideringthe steric equivalence of methyl and (noncoordinating) amino groups (as they occur in adenines), it is concludedthat the same extent of steric inhibition by the (C6)NH2 group is to be expected on metal ion binding at N7 withadenine derivatives. The basicity of the amino group in MABI is significantly higher than in its correspondingadenine derivative. Indeed, it is concluded that in the M(MABI)2+ complexes chelate formation involving theamino group occurs to some extent. The formation degrees of these “closed” species are calculated; they vary forthe complexes of Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ between about 50 and 90%. The stability of theM(MABI)2+ and M(DMBI)2+ complexes with the alkaline earth ions is very low but unaffected by the C4substituent; this probably indicates that in these instances outersphere complexes (with a water molecule betweenN3 and the metal ion) are formed.
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