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.