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C-H bond oxidation catalyzed by an imine-based iron complex: A mechanistic insight

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. 1H 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 dat

This work was also partially supported by Università di Roma La Sapienza- Area Supporto alla Ricerca (Progetti di Ricerca 2014 and Avvio alla Ricerca 2014). M.C. acknowledges financial support from ERC-2009-StG-239910 (to M.C.), the Spanish Ministry of Science (Nos. CTQ2012-37420-C02-01/BQU and CSD2010-00065, to M.C.) and Generalitat de Catalunya (ICREA Academia Award and No. SGR-862

© Inorganic Chemistry, 2015, vol. 54, núm. 21, p. 10141-10152

American Chemical Society (ACS)

Autor: Olivo, Giorgio
Nardi, Martina
Vidal Sánchez, Diego
Barbieri, Alessia
Lapi, Andrea
Gómez Martín, Laura
Lanzalunga, Osvaldo
Costas Salgueiro, Miquel
Di Stefano, Stefano
Data: 2015
Resum: 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. 1H 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 dat
This work was also partially supported by Università di Roma La Sapienza- Area Supporto alla Ricerca (Progetti di Ricerca 2014 and Avvio alla Ricerca 2014). M.C. acknowledges financial support from ERC-2009-StG-239910 (to M.C.), the Spanish Ministry of Science (Nos. CTQ2012-37420-C02-01/BQU and CSD2010-00065, to M.C.) and Generalitat de Catalunya (ICREA Academia Award and No. SGR-862
Format: application/pdf
ISSN: 0020-1669 (versió paper)
1520-510X (versió electrònica)
Accés al document: http://hdl.handle.net/10256/12247
Llenguatge: eng
Editor: American Chemical Society (ACS)
Col·lecció: MICINN/PN 2010-2016/CSD2010-00065
MINECO/PN 2013-2015/CTQ2012-37420-C02-01
AGAUR/2014-2016/2014 SGR-862
Reproducció digital del document publicat a: http://dx.doi.org/10.1021/acs.inorgchem.5b01500
Articles publicats (D-Q)
info:eu-repo/grantAgreement/EC/FP7/239910
És part de: © Inorganic Chemistry, 2015, vol. 54, núm. 21, p. 10141-10152
Drets: Tots els drets reservats
Matèria: Oxidació
Oxidation
Ferro -- Oxidació
Iron -- Oxidation
Catalitzadors
Catalysts
Títol: C-H bond oxidation catalyzed by an imine-based iron complex: A mechanistic insight
Tipus: info:eu-repo/semantics/article
Repositori: DUGiDocs

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