Modulation of the structure of octahedral 1,3-pdta-nickel(II) complex by introducing methyl substituents at the central 1,3-propanediamine carbon atom: Stereospecific formation and the crystal structure of [Mg(H2O)5Ni(2,2-diMe-1,3-pdta)]·1.5H2O

Abstract

The aim of the present study is to investigate how the modification in the structure of 1,3-propanediamine chain of 1,3-pdta (1,3-propanediamine-N,N,N’,N’-tetraacetate) ligand reflects on the structural characteristics of the corresponding metal complex. For this purpose, a hexadentate 2,2-dimethyl-1,3-propanediamine-N,N,N’,N’-tetraacetate (2,2-diMe-1,3-pdta) ligand, containing two methyl substituents at the central carbon atom of a 1,3-propanediamine, has been synthesized and used for the synthesis of [Mg(H2O)5Ni(2,2-diMe-1,3-pdta)]·1.5H2O complex. The complex was characterized by elemental microanalysis, infrared and electronic absorption spectroscopy and its crystal structure determined by a single-crystal X-ray diffraction analysis. The obtained structural parameters were compared to those previously reported in literature for the analogues 1,3-pdta- and 1,3-pndta-Ni(II) complexes (1,3-pndta is 1,3-pentanediamine-N,N,N’,N’-tetraacetate). Spectroscopic and crystallographic data showed that 2,2-diMe-1,3-pdta ligand is coordinated to the Ni(II) ion through 2 N and 4 O atoms forming an octahedral complex in which the six-membered 1,3-propanediamine chelate ring adopts a twist-boat conformation. This complex unit is further connected to the pentahydrated Mg(II) ion via the bridging axial carboxylate group thus forming an uncharged dinuclear complex with two stereogenic centers situated at chelated Ni(II) ion and one of the amine nitrogen atoms. Structural changes in 2,2-diMe-1,3-pdta-Ni(II) complex stimulated by the presence of geminal methyl side groups are discussed.