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.

GRAPHICAL ABSTRACT

Keywords:

• Aminophosphinate
• phosphinic acid
• dissociation constant
• complex stability
• potentiometry

Additional information

Funding

Support from the Grant Agency of the Czech Republic (No. P207/10/P153) and from Long-Term Research Plan of the Ministry of Education of the Czech Republic (No. MSM0021620857) is acknowledged. This work was carried out in the framework of COST TD1004, TD1007 and CM1006 (MSMT no: LD 13012) Actions.