Equilibrium and structure of the Al(iii)-ethylenediamine-N,N′-bis(3-hydroxy-2-propionate) (EDBHP) complex. A multi-method study by potentiometry, NMR, ESI MS and X-ray diffraction

Abstract

The equilibrium and structure of the complex formed by Al(III) and ethylenediamine-N,N′-bis(3-hydroxy-2-propionate) (EDBHP²⁻) have been studied using pH-potentiometry, ¹H and ²⁷Al NMR, ESI MS and single crystal X-ray diffraction methods. The EDBHP ligand is a strong Al-binder in aqueous solution for pH between 4 and 8 and for c_Al = c_EDBHP ≥ 0.1 mmol dm⁻³. The dominating complex identified by ESI MS and potentiometry is a neutral dimer, Al₂L₂(OH)₂, with logβ₂₂₋₂ = 14.16 ± 0.03. In the solid Al₂(EDBHP)₂(OH)₂·2H₂O the Al(III) ions are connected through a double hydroxo bridge. Both four-dentate organic ligands are coordinated terminally through two carboxylate groups and two N-donors forming three five-membered chelate rings. The hydroxyl groups of the ligand EDBHP remain protonated and are not coordinated to the aluminium ions. The structure and composition of the dimer are very likely the same in solution and the solid state.