DUGi

Assessing the Impact of Electronic and Steric Tuning of the Ligand in the Spin State and Catalytic Oxidation Ability of the Fe-II(Pytacn) Family of Complexes

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A family of iron complexes with the general formula [Fe-II((R,R)’Pytacn)(X)(2)](n+) is described, where (R,R)’Pytacn is the tetradentate ligand 1-[(4-R’-6-R-2-pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, R refers to the group at the alpha-position of the pyridine, R’ corresponds to the group at the gamma-position, and X denotes CH3CN or CF3SO3. Herein, we study the influence of the pyridine substituents R and R’ on the electronic properties of the coordinated iron center by a combination of structural and spectroscopic characterization using X-ray diffraction, H-1 NMR and UV-vis spectroscopies, and magnetic susceptibility measurements. The electronic properties of the substituent in the gamma-position of the pyridine ring (R’) modulate the strength of the ligand field, as shown by magnetic susceptibility measurements in CD3CN solution, which provide a direct indication of the population of the magnetically active high-spin S = 2 ferrous state. Indeed, a series of complexes [Fe-II((H,R)’Pytacn)(CD3CN)(2)](2+) exist as mixtures of high-spin (S = 2) and low-spin (S = 0) complexes, and their effective magnetic moment directly correlates with the electron-releasing ability of R’. On the other hand, the substitution of the hydrogen atom in the alpha-position of the pyridine by a methyl, chlorine, or fluorine group favors the high-spin state. The whole family of complexes has been assayed in catalytic C H and C=C oxidation reactions with H2O2. These catalysts exhibit excellent efficiency in the stereospecific hydroxylation of alkanes and in the oxidation of olefins. Remarkably, R’-substituents have little influence on the efficiency and chemoselectivity of the catalytic activity of the complexes, but the selectivity toward olefin cis-dihydroxylation is enhanced for complexes with R = Me, F, or Cl. Isotopic labeling studies in the epoxidation and cis-dihydroxylation reactions show that R has a definitive role in dictating the origin of the oxygen atom that is transferred in the epoxidation reaction

Financial support for this work was provided by the Spanish Ministry of Science (Project CTQ2012-37420-C02-01/BQU and Consolider Ingenio Ingenio/CSD2010-00065 to M.C. and X.R. and project CTQ2012-32436 to T.P.) and the European Research Council (ERC-2009-StG-239910 to M.C.). M.C. and X.R. acknowledge Generalitat de Catalunya for ICREA-Academia Awards and 2009-SGR637. A.C. acknowledges the European Commission for a Career Integration Grant (FP7-PEOPLE-2011-CIG-303522). The Spanish Ministry of Science is acknowledged for a FPU PhD grant to I.P. and for a Ramon y Cajal contract to A.C. We thank Catexel for a generous gift of tritosyl-1,4,7-triazacyclononane. X.R. is grateful for financial support from INNPLANTA project IPN-2011-0059-PCT-4200 0-ACT1

American Chemical Society