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

Abstract

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 (H 2malm) 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] n Cl n ·2 nH 2O ( 3·2 nH 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 3·2 nH 2O. Complex 1 is a 3D coordination polymer based on neutral repeating units, whereas 2 and 3·2 nH 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 3·2 nH 2O into 3D networks. The three complexes were characterized spectroscopically (IR, far-IR, and Raman) and the thermal decomposition of 2 and 3·2 nH 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.