An insight into the extraction of transition metal ions by picolinamides associated with intramolecular hydrogen bonding and rotational isomerization†
The clear connection between molecular structures of N-substituted picolinamides and extraction behaviour has been rationalized by highlighting the relationship between intramolecular hydrogen bonding and rotational isomerism. To this aim aromatic pyridine-2,6-dicarboxamides 1a–1c with N-substitution and their analogues 3a and 3b containing intramolecular hydrogen bonds were designed and synthesized. The results from the liquid–liquid extraction towards some representative transition metal picrates including Ag+, Hg2+, Pb2+, Cd2+, Zn2+, Cu2+, Co2+ and Ni2+ salts demonstrated that the higher selectivity and efficiency towards Hg2+ (88.6–95.4%) over other metal cations stem mainly from N-substitution via disruption of intramolecular H-bonding. X-ray structural analysis, and ordinary and variable-temperature proton and carbon NMR experiments provided supporting information for expounding the difference in extraction ability among these ligands, particularly the importance of N-substitution that leads to the formation of rotamers affecting the extraction process.