Novel CO2-soluble pyridine derivatives and the extraction of heavy metals into Sc-CO2

New CO2-soluble, 4-aminopyridine derivatives (3a–f) for use as chelating agents were designed and synthesized from the parent compound 4-aminopyridine (1) by simple chemical modifications and tested for solubility and metal ion extraction ability in supercritical CO2 (Sc-CO2). All of the newly prepared derivatives showed much higher solubility in Sc-CO2 than the corresponding 4-aminopyridine parent compound, and good or moderate extraction efficiencies for several metal ions (Cu2+, Co2+, Zn2+, and Sr2+) in 50–90% yield under different conditions, such as temperature, ligand:co-ligand ratio, extraction time, and CO2 pressure. Interestingly, it was observed that N-(4-pyridyl)pentadecafluorooctanamide (3f) with the ‘CO2-philic’ fluoroalkyl moiety had the lowest CO2 solubility among the new amide-group-containing pyridine derivatives (3a–f), possibly owing to stronger self-interaction. The highest extraction efficiency, especially for copper ions (91%), was achieved with N-hexyl-N-(4-pyridyl)octanamide (3e) including a tertiary amide group with only nonfluorous alkyl chains. It is proposed that a tertiary amide group can be considered as a new ‘CO2-philic’ moiety.