Modelling the use of lanthanides(III) as probes at calcium(II) binding sites in biological systems: Preparation and characterization of neodymium(III) and calcium(II) malonamato(-1) complexes

摘要

A bioinorganic approach into the problem of the isomorphous substitution of calcium(II) by lanthanide(III) ions in biological systems is discussed. Reactions of malonamic acid (H2malm) with Ca~(II) and Nd~(III) sources under similar conditions yielded the compounds [Ca(Hmalm) _2]_n (1), [Nd(Hmalm)_2(H_2O) _2]_n(NO_3)_n (2) and [Nd(Hmalm) _2(H_2O)_2]_nCl_n· 2_nH_2O (3·2nH_2O). Their X-ray crystal structure data show that the malonamate(-1) ligand presents two different ligation modes and coordinates through the two carboxylate and the amide-O atoms, thus bridging three Ca~(II) ions in 1 and two Nd_(III) ions in 2 and 3·2nH_2O. Complex 1 is a 3D coordination polymer based on neutral repeating units, whereas 2 and 3·2nH_2O are 1D coordination polymers based on the same cationic repeating unit. Hydrogen bonding interactions further stabilize the 3D framework structure of 1 and assemble the 1D chains of 2 and 3·2nH_2O into 3D networks. The three complexes were characterized spectroscopically (IR, far-IR, and Raman) and the thermal decomposition of 2 and 3·2nH_2O was monitored by TG/DTA and TG/DTG measurements. Variable-temperature magnetic susceptibility data for 2 are also reported. The bioinorganic chemistry relevance of our results is discussed.