Structural diversification of the coordination mode of divalent metals with 1,3-propanediaminetetraacetate (1,3-pdta): The missing crystal structure of the s-block metal complex [Sr2(1,3-pdta)(H2O)6]·H2O

This paper reports the synthesis and X-ray characteristics of the missing homonuclear s-block metal complex {[Sr2(1,3-pdta)(H2O)6]·H2O}n. In the title compound, the hexadentate 1,3-propanediaminetetraacetate (1,3-pdta) ligand joins to two Sr(II) centers via the diamine chain. Moreover, each Sr(II) is bridged through two carboxylate O atoms and a water molecule to two neighboring Sr(II) ions. The coordination sphere around each Sr(II) ion consists of one diamine nitrogen, four carboxylate oxygens and four water molecules. Comparison with the previously reported M(II)–1,3-pdta complexes reveals that increasing of the ion size results in the incorporation of water molecules into its first coordination sphere and consequent increase of the coordination number (C.N.) from six to seven or eight, while keeping the hexadentate coordination mode of the ligand. Further increase of the metal ion size leads to the loss of the chelating properties of the diamine and formation of a bis-tridentate complex. Associated with it is the change in the binding mode of the carboxylate groups. This forms the basis for classification of divalent metal 1,3-pdta complexes into five distinct structural classes. Additionally, in the present study X-ray powder diffraction and IR spectroscopy were used to distinguish the different structural types of M(II)–1,3-pdta complexes, including Ba[Ba(1,3-pdta)]·2H2O which has been used for their preparation.

Graphical abstractThe present study focuses on describing the coordination mode in a series of divalent metal complexes with the 1,3-pdta ligand as a function of metal ion size and structural function of the carboxylato ligands. With increasing ion size, a systematic transition is observed from C.N. six to seven and eight, up to the loss of the chelating properties of the diamine accompanied by an increase of the bridging function of carboxylate groups.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Synthesis and X-ray characteristics of the {[Sr2(1,3-pdta)(H2O)6]·H2O}n complex. ► X-ray diffraction and IR spectroscopy of M(II)-1,3-pdta complexes. ► The affects of the metal ion size on the ligand functionality. ► Structural correlation between five classes of different M(II)–1,3-pdta complexes.