Development of bio-inspired chelates with hydrogen bond donors: synthesis and structure of monomeric metal acetate complexes with intramolecular hydrogen bonds

Abstract

The synthesis of the new multidentate tripodal compounds bis[(N′-tert-butylureido)-N-ethyl]-2-pyridylmethylamine (H₄1) and bis[(N′-tert-butylureido)-N-ethyl]-N-methylamine (H₄2) is described. These ligands contain two urea-ethylene arms that when deprotonated and bound to a metal ion, position two hydrogen bond donors near the metal center so that formation of intramolecular H-bonds with other coordinated species is possible. The complexes K[M^II(H₂1)(η¹-OAc)] and K[M^II(H₂2)(η²-OAc)] (M^II = Co and Fe) have been isolated and characterized. The multidentate ligands mediate the mode of acetate coordination: the [M^II(H₂1)(η¹-OAc)]⁻ complexes have a monodentate coordination, whereas in [M^II(H₂2)(η²-OAc)]⁻ the acetate binds in a bidentate manner. X-Ray diffraction studies reveal that all complexes have trigonal bipyramidal coordination geometries with one of the acetate oxygen atoms being nearly trans to the amine nitrogen donor. All the complexes have relatively long M–O bond lengths in comparison with other M^II–OAc complexes characterized previously. The complexes exhibit intramolecular H-bonds from the urea NH groups to one of the coordinated oxygen atoms of the acetate, causing a lengthening of the M^II–OAc bond distances. The NH⋯O bond lengths and vibrational energies in these acetate complexes support weaker H-bonds than the H-bonding interactions in similar ligand complexes with hydroxo ligands.