N-donor effects on carboxylate binding in mononuclear iron(II) complexes of a sterically hindered benzoate ligand

Using the sterically hindered 2,6-dimesitylbenzoate ligand Mes(2)ArCO(2)(-), a series of mononuclear Fe(II) carboxylate complexes has been obtained with the general formula (Mes(2)ArCO(2))(2)Fe(base)(2) (base = 1-methylimidazole (MeIm), pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-Lut), 2,6-lutidine (2,6-Lut), (base)(2) = N,N,N',N'-tetramethylethylenediamine (TMEDA)). For the monodentate base adducts, single-crystal IL-ray diffraction studies revealed several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the degree of alpha -substitution of the N-donors. Increasing alpha -substitution leads to the lengthening of the Fe-N bond, which in turn results in a change in carboxylate binding mode from eta (1) to eta (2). We surmise that this change is due to an electrostatic effect and is driven by increasing the Lewis acidity of the Fe center. Such a simple process for inducing carboxylate shifts could play a critical role in biological systems.