title: C-H Bond Oxidation Catalyzed by an Imine-Based Iron Complex: A Mechanistic Insight.
abstract: A family of imine-based nonheme iron(II) complexes (LX)2Fe(OTf)2 has been prepared, characterized, and employed as C-H oxidation catalysts. Ligands LX (X = 1, 2, 3, and 4) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe(OTf)2 in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (L1)2Fe(OTf)(ClO4) and (L3)2Fe(OTf)2 are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. (1)H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (LX)2Fe(OTf)2 efficiently catalyze the oxidation of alkyl C-H bonds employing H2O2 as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C-H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C-H bonds. Isotopic labeling analyses show that H2O2 is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O-O cleavage, without the assistance of a cis-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H2O2 at (LX)2Fe(OTf)2 differs from that occurring in biomimetic iron catalysts described to date.
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza" and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Rome, Italy.|Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona , Campus de Montilivi, 17071 Girona, Spain.
authors.forename: Giorgio
authors.initials: G
authors.lastname: Olivo
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza" and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Rome, Italy.
authors.forename: Martina
authors.initials: M
authors.lastname: Nardi
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona , Campus de Montilivi, 17071 Girona, Spain.
authors.forename: Diego
authors.initials: D
authors.lastname: Vìdal
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza" and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Rome, Italy.
authors.forename: Alessia
authors.initials: A
authors.lastname: Barbieri
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza" and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Rome, Italy.|Consortium of Chemical Catalysis and Reactivity, CIRCC Interuniversity , Via Celso Ulpiani 27, 70126 Bari, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Lapi
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona , Campus de Montilivi, 17071 Girona, Spain.|Serveis Tècnics de Recerca (STR), Universitat de Girona , Parc Cientı́fic i Tecnològic, E-17003 Girona, Spain.
authors.forename: Laura
authors.initials: L
authors.lastname: Gómez
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza" and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Rome, Italy.|Consortium of Chemical Catalysis and Reactivity, CIRCC Interuniversity , Via Celso Ulpiani 27, 70126 Bari, Italy.
authors.forename: Osvaldo
authors.initials: O
authors.lastname: Lanzalunga
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona , Campus de Montilivi, 17071 Girona, Spain.
authors.forename: Miquel
authors.initials: M
authors.lastname: Costas
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza" and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Rome, Italy.|Consortium of Chemical Catalysis and Reactivity, CIRCC Interuniversity , Via Celso Ulpiani 27, 70126 Bari, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Di Stefano