Mimicry of the 2-His-1-Carboxylate Facial Triad Using Bulky N,N,O-Ligands: Non-Heme Iron Complexes Featuring a Single Facial Ligand and Easily Exchangeable Co-Ligands

Abstract

Mononuclear iron(II) complexes with facially coordinating N,N,O-ligands were synthesized as accurate structural mimics of the 2-His-1-carboxylate facial triad found in mononuclear non-heme iron enzymes. Mimicking of the facial triad is achieved by designing sterically demanding ligands providing two histidine-like benzimidazole moieties and a coordinating carboxylate or ester moiety. The new methyl-substituted analogue of the bis(benzimidazolyl)propionate ligand, BMBMeIP and its ester analogue, BMBMeIPnPr are designed to prevent the formation of previously reported coordinatively saturated FeL2 type complexes. The crystal structure of [Fe(BMBMeIPnPr)(OTf)2] shows a facial N,N,O-coordination that is very similar to the structure of the enzyme active sites. Both [Fe(BMBMeIPnPr)(OTf)2] and [Fe(BMBMeIP)(OTf)(MeCN)n] catalyze the epoxidation of olefins using H2O2, reaching turnover numbers up to 5.8 per iron. These results represent the first examples of iron complexes bearing a single facial and rigid N,N,O-coordinating ligand and three readily available coordination sites showing catalytic activity in oxidation reactions.