Phosphinate Analogues of IDA and NTA with Low Basicity of Nitrogen Atom: Acid-Base and Complexation Properties
Abstract
Analogues of iminodiacetic acid (H2IDA) and nitrilotriacetic acid (H3NTA) bearing two (hydroxomethyl) methylphosphinate or (2-carboxyethyl) methylphosphinate groups were synthesized. Their acid-base and coordination properties with divalent metal ions (Cu2+, Ni2+ and Zn2+) were studied by potentiometry.
The compounds exhibit very low basicity of the amino groups (pK(a) = 6.2-7.6) due to presence of two electron-withdrawing methylphosphinate groups and, consequently, a low stability of the complexes. In the case of IDA-analogues, the low complex stability results in precipitation of metal hydroxides in neutral region.
As expected, NTA-analogues form more stable complexes and, thus, they were also studied in the alkaline region. Presence of additional carboxylate in the 2-carboxyethylphosphinic acid groups results in the formation of dinuclear complexes.
Solid-state structures of two compounds were determined by X-ray diffraction analysis. The compounds are protonated on the nitrogen atom and the structures are stabilized by rich hydrogen bond network.