Tuning the coordination mode in mononuclear manganese complexes by changing the steric bulk of the carboxylates

Abstract

Three mononuclear manganese complexes of the tridentate ligand biap [ = N,N-bis(2-ethyl-5-methylimidazol-4-ylmethyl)aminopropane] with various carboxylates as co-ligands have been synthesized and structurally characterised by X-ray diffraction. All three complexes can be described by the general formula [Mn^II(biap)(RCOO)₂(H₂O)_n] (n = 0, 1). The manganese(II) ion in [Mn^II(biap)(C₆H₅COO)₂] (1) is five coordinated by three nitrogen donors from biap and two oxygen donors of monodentate benzoate molecules. This monodentate binding mode is confirmed by IR analysis. In the compound [Mn^II(biap)((C₆H₅)₂CHCOO)₂(H₂O)] (2), the manganese ion is coordinated in a distorted octahedron with an additional water molecule as the sixth ligand. This aqua ligand forms strong intramolecular hydrogen bonds to both carboxylate groups as is reflected in the IR. Finally the manganese ion in the complex [Mn^II(biap)((C₆H₅)₃CCH₂COO)₂] (3) is again five coordinated with triphenylpropionate as coordinating counter ion. In 3 both carboxylates are bound monodentately, but one shows a semi coordination of the free oxygen to the manganese center (Mn–O = 3.169 Å). Both carboxylates are involved in hydrogen bonding albeit to a different degree. This asymmetric binding mode is reflected in the IR spectrum, which shows two pairs of carboxylate vibrations. The structural differences of the three complexes are expressed by the very different X-band EPR spectra of the solid compounds. In solution, however, all three complexes are most likely octahedrally coordinated as indicated by their similar EPR spectra in frozen MeOH–EtOH solution.