Equilibria and structure of the lanthanide(III)-2-hydroxy-1,3-diaminopropane-N,N,N',N'-tetraacetate complexes: formation of alkoxo-bridged dimers in solid state and solution.
The complex formed between 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid (H4L-OH) and Nd3+ at pH 7.5 was found to be a dinuclear dimer in the solid state by X-ray crystallography. In the complex K4[Nd2(L-O)2(H2O)2].14H2O each ligand is coordinated to both Nd3+ atoms with an iminodiacetate group (the Nd3+-Nd3+ distance is 3.9283(8) A). The alcoholic OH groups are deprotonated, and the alkoxo oxygens are coordinated to both Nd3+ in a bridging position. The Nd3+ ions are nine-coordinated with one water molecule per Nd(III) ion in the inner sphere. The complex K4[Nd2(L-O)2(H2O)2].14H2O has an inversion center, and the space group is P1. Two of the K+ counterions are six-coordinated, while the other two K+ ions are eight-coordinated; polar polymeric water-K+ layers are formed between the apolar ligand layers via the bridging water molecules. The dinuclear dimer complexes are also present in aqueous solution. The proton relaxivities of the Gd3+ complex decrease with the increase of pH, and at pH > 6, the low relaxivity values indicate the probable absence of H2O in the inner sphere and the predominance of the eight-coordinated dimer species [Gd2(L-O)2].4- The results of ESI-TOF MS studies of the complexes of La3+, Nd3+, and Lu3+ proved the formation of dinuclear dimers in dilute (0.25 mM) solutions. pH-potentiometric titrations indicate the formation of complexes with 1:1 (Ln(L-OH)-, Ln(HL-OH), and Ln2(L-O)24-) and 2:1 (Ln2(L-O)+) metal-to-ligand ratios. The stability constants of the Ln(L-OH)- species increase from La3+ (log K = 10.19) to Lu3+ (log K = 14.08). The alcoholic OH group of the Ln(L-OH)- species dissociates at unusually low pH values. The pH range of dissociation shifts to lower and lower pH's with the increasing atomic number of the lanthanides. This pH range is about 4-7 for the La3+ complex and 1-4 for the Lu3+ complex. The results of 1D and 2D 1H and 13C NMR studies of the La3+ complex, the number and multiplicity of signals, and the values of coupling constants are in agreement with the dinuclear dimer structure of the complex in solution.
Tircsó
Gyula
G
Department of Chemistry, Laboratory for X-ray Diffraction, Department of Inorganic and Analytical Chemistry, University of Debrecen, Egyetem tér 1, Debrecen H-4010, Hungary.
Bényei
Attila
A
Brücher
Ernõ
E
Kis
Anita
A
Kiraly
Róbert
R