The structure of N,N′-bis(2-hydroxyethyl)ethane-1,2-diamine and its complexes with Zn(ii) and Cd(ii)

Abstract

The crystal structure of the nitrate salt of N,N′-bis(2-hydroxyethyl)-ethane-1,2-diamine (BHEEN), and its complex with Zn(II) and Cd(II) are reported. (H₂BHEEN)(NO₃)₂ packs in a layered structure with a herringbone pattern within each layer arising from H-bonding between amino and alcohol protons and NO₃⁻ counterions. In [Zn(BHEEN)₂]Cl₂, each ligand coordinates to Zn(II) through its two N-donors producing a distorted tetrahedral geometry at the metal centre. The two hydroxyethyl arms of each ligand are trans to each other and the crystals obtained contained a racemic mixture of the bis-trans-R,R and the bis-trans-S,S isomers. All four hydroxyl groups are H-bonded to chloride counter ions, creating a layered structure. Whilst distant from the metal ion (average 3.00 Å), the four O atoms of the pendent hydroxylamino groups appear positioned to interact with the metal. The orientation of the arms is preserved in a B3LYP gas phase calculation of the structure. An analysis using Bader's Atoms in Molecules indicates that the Zn–N bonds are predominantly ionic with some covalent character and that there is a weak interaction between the metal and the hydroxyl groups. Several other weak interactions including four O⋯HN, five O⋯HC and a H–H dihydrogen bond were identified. The Cd(II) complex of BHEEN crystallised as a dimer [(μ-Cl)₂(Cd(BHEEN)Cl)₂)] with two asymmetrically-bound bridging Cl⁻ ligands and a terminally-coordinated Cl⁻ on each metal ion. One hydroxyl group of each ligand is coordinated to the metal and the uncoordinated hydroxyl group is H-bonded to the H atom of the coordinated hydroxyl group of the second ligand in the complex. The ESI-MS spectrum shows the presence of di-cadmium complexes, but the most intense peaks are due to mono-cadmium complexes. The gas phase B3LYP structure of the dimer energy-minimises into two monomers and the longer bond between Cd(II) and bridging Cl⁻ breaks. Hence, dimerisation may be a consequence of the crystallisation process and the dimer may not be the predominant species in solution.