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 Mes₂ArCO₂^-, a series of mononuclear Fe(II) carboxylate complexes has been obtained with the general formula (Mes₂ArCO₂)₂Fe(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 X-ray diffraction studies revealed several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the degree of α-substitution of the N-donors. Increasing α-substitution leads to the lengthening of the Fe-N bond, which in turn results in a change in carboxylate binding mode from η¹ to η². 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.