Calcium binding capacity of carboxylic acids with an ethereal function. Crystal and molecular structure of calcium ethylenedioxydiacetate

Abstract

The influence of structural parameters on the calcium binding capacity of acyclic dicarboxylic ethers of general formula HO₂C–(CH₂)_n–O–(CH₂)_m–O–(CH₂)_n–CO₂H (n= 1 or 2, m= 1–3 respectively) and HO₂C–(CH₂)_n–O–(CH₂)_n–CO₂H (n= 1 or 2) has been investigated. A good building efficiency is displayed by oxydiacetic (oda) and ethylenedioxydiacetic (edoda) acids or their sodium salts: these are the only compounds examined capable of forming five-membered chelate rings. No increase of calcium sequestering capacity is observed going from the tridentate oda to the tetradentate edoda ligand. The crystal and molecular structure of Ca(edoda)·3H₂O has been determined by three-dimensional X-ray analysis, using Patterson and Fourier methods and refined by block-diagonal least squares to R 0.030 for 1 792 independent observations. Crystals are triclinic, space group P with Z= 1 in a unit cell of dimensions a= 8.560(2), b= 10.171(2), c= 7.851(2)Å. α= 101.5(1), β= 115.5(1), γ= 109.5(1)°, The crystal structure is built up of {Ca(edoda)·3H₂O}₂ dimers lying on centres of symmetry of the crystal lattice and linked together by hydrogen bonding involving the water molecules. The eight-fold co-ordination of the calcium atom is approximately dodecahedral. The Ca–O bond distances range from 2.378(2) to 2.645(2)Å. The values of the internal rotation angles of the seven single bonds of the edoda ligand are typical of trans and gauche conformations.