Highly polydentate ligands. Part 4. Crystal structures of neodymium(III) and erbium(III) complexes of 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioate(4–)

Abstract

The structures of calcium salts of the erbium(III) and neodymium(III) chelates of the calcium-selective octadentate ligand egta^4–[H₄egta = 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioic acid] have been determined, in order to identify the structural changes that occur in a calcium-selective binding site when lanthanides are substituted for calcium. Ca[Er(egta)(OH₂)]₂·12H₂O·(CH₃)₂CO (1) crystallizes in the monoclinic space group P2₁(Z= 2), with a= 12.710(2), b= 12.157(2), c= 17.765(3)Å, and β= 105.79(1)°R= 0.027, R′= 0.035. Ca[Nd(egta)(OH₂)]₂·9H₂O (2) crystallizes in the monoclinic space group P2₁/c(Z= 2), with a= 10.776(2), b= 18.218(4), c= 12.560(2)Å, and β= 112.14(1)°; R= 0.028, R′= 0.040. The full octadentate chelating capability of egta^4– is utilized in both chelates. In contrast to the eight-co-ordinate calcium ions in [Ca(egta)]^2–, the Er^III ions in (1) are nine-co-ordinate; the ninth co-ordination site is occupied by a water molecule. Of the three ligand atom types, the carboxylate oxygen atoms are bound at the shortest distance [Er–O(carboxylate)_av= 2.31(3)Å], followed by the ether oxygen atoms [Er–O(ether)_av= 2.42(5)Å], and the amine nitrogen atoms [Er–N_av= 2.57(4)Å]. The Nd^III ions in (2) are ten-co-ordinate; ten-co-ordination is achieved in the solid state by binding a water molecule, as well as a carboxylate oxygen atom from an adjacent complex ion The order of metal–Iigand bond distances observed for Nd^III is the same as that observed for Er^III Nd–O(carboxylate)_av= 2.46(1), Nd–O(ether)_av= 2.67(3), Nd–N_av= 2.81(7)Å.