Iron complexes of chiral phenol-oxazoline ligands: Structural studies and oxidation catalysis

Abstract

Iron complexes of two ligands, HphoxCOOH and HphoxiPr, have been synthesized and characterized by crystal structure analyses. The complexes (HNEt3)2[Fe(phoxCOO)2](ClO4) and [Fe(phoxiPr)3] are reported. Reactions of the ligands rac-HphoxCOOH and rac- HphoxiPr with iron(II) or iron(III) perchlorate result in the formation of iron(III) complexes with pseudo-octahedral geometry around the metal center. The iron complex obtained from rac-HphoxCOOH crystallized in the centrosymmetric space group Cmca. The two ligands are bound in a tridentate manner generating a meridional coordination with both dianionic ligands on a metal center having the same chirality; due to the center of symmetry the complex with opposite chirality is also present. The complex (HNEt3)2[Fe(phox- COO)2](ClO4) is the first accurate structural model of the iron complex of a siderophore analog commonly observed in mycobactins. The three didentate ligands in the complex [Fe(phoxiPr)3] are bound with like atoms in a meridional manner to the metal center. The metal ion is surrounded by two ligands of the same chirality and one ligand of opposite chirality (ie. RRS or SSR); due to the presence of a center of symmetry both isomers are present in the crystal structure. The complex (HNEt3)2[Fe(phoxCOO)2](ClO4) shows promising activity in the oxidation of alkanes, such as toluene, ethylbenzene and cumene, while the complex [Fe(phoxiPr)3] does not show any catalytic activity in alkane oxidations under the conditions tested. The complex (HNEt3)2[Fe(phoxCOO)2](ClO4) is reasonably efficient in the conversion of H2O2 to oxidation products.