A bidentate ligand with appended hydrogen bond donors: Synthesis and structure of four-coordinate metal complexes with bis[(tert-butyl)aminocarbonyl]-1,2-diaminoethane

The secondary coordination sphere in the active sites of metalloproteins is instrumental in controlling function. Non-covalent interactions, such as hydrogen bonds, are often used to create unique microenvironments around the metal ion(s). To duplicate these effects in synthetic complexes, first row transition metal complexes of the ligand bis[(tert-butyl)aminocarbonyl]-1,2-diaminoethane (H₄1) have been synthesized and structurally characterized. Twofold deprotonation of H₄1 affords the dianionic, bidentate ligand [H₂1]^2-, which forms the four-coordinate complexes [M^II(H₂1)₂]^2-. X-ray diffraction studies on [M^II(H₂1)₂]^2- (M=Fe, Co, Zn) show that these complexes have pseudo-tetrahedral coordination geometry. H-bond donors from [H₂1]^2- surround the metal centers in each complexes, and metrical data suggest that N-H⋯M H-bonds are present in these complexes. The square-planar [Ni^II(H₂1)]^2- has H-bond donors that span both faces of the coordination plane, forming intramolecular H-bonds between the [H₂1]^2- ligands.