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 ​
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