Mononuclear iron(ii) complexes containing a tripodal and macrocyclic nitrogen ligand: synthesis, reactivity and application in cyclohexane oxidation catalysis

Abstract

Two novel tripodal ligands L¹ and L² based on a tris(methylpyridyl)amine (TPA) motif have been prepared and reacted with two different iron(II) salts. The ligand L¹ contains a bis(amino-phenyl)-TPA group whereas the macrocyclic ligand L² displays two different coordinating cores, namely TPA and pyridine–dicarboxamide. The resulting mononuclear complexes 1–4 have been characterized in the solid state and in solution by spectroscopic and electrochemical methods. All complexes are high spin and mainly pentacoordinated. X-ray diffraction analyses of the crystals of complexes 2 and 3 demonstrate that the coordination sphere of the iron(II) centre adopts either a distorted bipyramidal–trigonal or square pyramidal geometry. In the absence of an exogenous substrate, oxidation of complex 2 by H₂O₂ induces an intramolecular aromatic hydroxylation, as shown by the X-ray structure of the resulting dinuclear complex 2′. Catalytic studies in the presence of a substrate (cyclohexane) show that the reaction process is strongly impacted by the macrocyclic topology of the ligand and the nature of the counter-ion.