The Mechanism of Stereospecific CH Oxidation by Fe(Pytacn) Complexes: Bioinspired Non-Heme Iron Catalysts Containing cis-Labile Exchangeable Sites

A detailed mechanistic study of the hydroxylation of alkane CH bonds using H2O2 by a family of mononuclear non heme iron catalysts with the formula [FeII(CF3SO3)2(L)] is described, in which L is a tetradentate ligand containing a triazacyclononane tripod and a pyridine ring bearing different substituents at the and positions, which tune the electronic or steric properties of the corresponding iron complexes. Two inequivalent cis-labile exchangeable sites, occupied by triflate ions, complete the octahedral iron coordination sphere. The CH hydroxylation mediated by this family of complexes takes place with retention of configuration. Oxygen atoms from water are incorporated into hydroxylated products and the extent of this incorporation depends in a systematic manner on the nature of the catalyst, and the substrate. Mechanistic probes and isotopic analyses, in combination with detailed density functional theory (DFT) calculations, provide strong evidence that CH hydroxylation is performed by highly electrophilic [FeV(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the CH bond, followed by rebound of the hydroxyl ligand. The [FeV(O)(OH)L] species can exist in two tautomeric forms, differing in the position of oxo and hydroxide ligands. Isotopic-labeling analysis shows that the relative reactivities of the two tautomeric forms are sensitively affected by the substituent of the pyridine, and this reactivity behavior is rationalized by computational methods.