title: The mechanism of stereospecific C-H oxidation by Fe(Pytacn) complexes: bioinspired non-heme iron catalysts containing cis-labile exchangeable sites.
abstract: A detailed mechanistic study of the hydroxylation of alkane C-H bonds using H2O2 by a family of mononuclear non heme iron catalysts with the formula [Fe(II)(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 α and γ 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 C-H 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 C-H hydroxylation is performed by highly electrophilic [Fe(V)(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the C-H bond, followed by rebound of the hydroxyl ligand. The [Fe(V)(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 α substituent of the pyridine, and this reactivity behavior is rationalized by computational methods.
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC), Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Spain.
authors.forename: Irene
authors.initials: I
authors.lastname: Prat
authors.forename: Anna
authors.initials: A
authors.lastname: Company
authors.forename: Verònica
authors.initials: V
authors.lastname: Postils
authors.forename: Xavi
authors.initials: X
authors.lastname: Ribas
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
authors.forename: Josep M
authors.initials: JM
authors.lastname: Luis
authors.forename: Miquel
authors.initials: M
authors.lastname: Costas
mesh_terms: D000473:Alkanes / Q000737:chemistry; D001372:Aza Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D005296:Ferrous Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000737:chemistry; D006900:Hydroxylation; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011725:Pyridines / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: C585088:1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane; D000473:Alkanes; D001372:Aza Compounds; D005296:Ferrous Compounds; D008024:Ligands; D011725:Pyridines; D006418:Heme; D006861:Hydrogen Peroxide; D007501:Iron; C023666:pyridine