Metal–phenoxyalkanoic acid interactions. Part 28. Crystal and molecular structures of the sodium, ammonium, and magnesium complexes of (o-phenylenedioxy)diacetic acid

Abstract

The crystal structures of three compounds involving (o-phenylenedioxy)diacetic acid (H₂L) and sodium, ammonium, and magnesium ions have been determined by X-ray diffraction. [{Na₄L₂(H₂O)₈}_n](5) is triclinic, with a= 7.206(1), b= 14.306(5), c= 14.545(4)Å, α= 101.86(2), β= 92.19(2), γ= 90.32(2)°, Z= 2, and space group P. Ionic [NH₄]₂[L](6) is orthorhombic, with a= 11.833(3), b= 9.199(2), c= 10.932(1)Å, Z= 4, and space group Pbcn. [MgL(H₂O)₃]·4H₂O (7) is monoclinic, with a= 29.17(1), b= 6.758(2), c= 17.347(8)Å, β= 108.99(3)°, Z= 8, and space group C2/c. (5), (6), and (7) were refined to residuals 0.044, 0.033, and 0.071 for 3 034, 1 024, and 1 448 observed reflections respectively. Complex (5) has a polymeric structure comprising four independent and different sodium centres, one five-, two six-, and one seven-co-ordinate. Each planar L^2– ligand is bonded to a sodium ion via its four ‘inner’ oxygen donor atoms, with two waters in the axial sites of a pentagonal bipyramid. However, in one case the fifth pentagonal planar site is occupied by a bridging water molecule while for the other there is no water in this site. One of the axial waters in each also forms bridges to the third and fourth sodiums, together with a carboxyl oxygen of each acid ligand. The co-ordination about these two sodium ions is completed by the remaining water ligands, including two which bridge the centres and another which forms a polymer link. Stereochemistry is distorted octahedral in one, distorted square pyramidal in the other, with a Na–O range and mean of 2.304(2)–2.693(4)Å and 2.420(2)Å respectively. In contrast, complex (6) is discretely ionic with the ammonium ion located outside the ionophore cavity of L^2– but hydrogen bonded to the carboxylate oxygens (2.89–3.13 Å). Complex (7) is isostructural and isomorphous with the zinc(II) and cobalt(II) complexes of L^2–. It is monomeric with pentagonal-bipyramidal stereochemistry about Mg, comprising unequal bonds to the four ‘inner’ oxygens of the ligand [Mg–O(ether)(mean) 2.417(5)Å, Mg–O(carboxylate)(mean) 2.126(5)Å] and three to water molecules [two axial, 2.048(6)Å(mean); one equatorial, 2.052(5)Å].