Structure of two novel binuclear complexes of EuIII containing bridging DL- and L-α-alaninehydroxamic acids

Abstract

Two novel binuclear complexes of Eu^III with bridging DL- and L-α-alaninehydroxamic acids of the formulae [Eu(DL-α-Alaha)(H₂O)₆]₂(ClO₄)₆ and [Eu(L-α-AlahaH)(H₂O)₆]₂(ClO₄)₈ (further denoted as 1 and 2) were synthesized from an aqueous solution at pH≈4.5. Their crystal structures were determined by X-ray diffraction with the final R=0.036 and 0.043 for compounds 1 and 2, respectively. The structures were solved by direct methods and refined by full-matrix least-squares using SHELXTL-PC programs, on 4332F₀ values for complex 1 and 5728 for complex 2, with anisotropic temperature factors for the non-hydrogen atoms. Positions of hydrogen atoms were taken from the ΔF synthesis and refined with isotropic temperature factors. The title compounds crystallize in the P2₁/n (1) and C2 (2) space groups, with the unit cell parameters as follows: 1 a=10.347(1), b=11.289(2), c=16.968(3) Å, β=95.37(1)°; 2 a=19.912(3), b=8.620(2), c=15.881(3) Å, β=107.37(3)°. The two structures consist of dimer units formed by two NO bridging groups of the aminohydroxamic acid molecules. In addition, the two metal ions are chelated by the CO and NO groups, forming five-membered rings. The bridging oxygen atoms in the complexes are not the carbonyl oxygens but rather the hydroxamic ones. Centrosymmetric dimers are formed in compound 1, whereas in the case of the compound with the L-handed ligand the dimer possesses twofold axes. Additionally, two molecules of HClO₄ are present in the structure of 2. Hydrogen bonds are created between N and O sites of the ligands (Alaha, H₂O molecules). A structural effect of the ligand chirality was found and the Eu–Eua distances were determined: these are equal to 4.114 and 4.079 Åfor compounds 1 and 2, respectively. These results are, to our knowledge, the first ones obtained for lanthanide complexes with aminohydroxamic acids from X-ray data.