Assessing the Impact of Electronic and Steric Tuning of the Ligand in the Spin State and Catalytic Oxidation Ability of the Fe<sup>II</sup>(Pytacn) Family of Complexes
2013-08-19T00:00:00Z (GMT) by
A family of iron complexes with the general formula [Fe<sup>II</sup>(<sup>R,R</sup>′Pytacn)(X)<sub>2</sub>]<sup><i>n</i>+</sup> is described, where <sup>R</sup><i><sup>,</sup></i><sup>R</sup>′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 α-position of the pyridine, R′ corresponds to the group at the γ-position, and X denotes CH<sub>3</sub>CN or CF<sub>3</sub>SO<sub>3</sub>. 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, <sup>1</sup>H NMR and UV–vis spectroscopies, and magnetic susceptibility measurements. The electronic properties of the substituent in the γ-position of the pyridine ring (R′) modulate the strength of the ligand field, as shown by magnetic susceptibility measurements in CD<sub>3</sub>CN solution, which provide a direct indication of the population of the magnetically active high-spin <i>S</i> = 2 ferrous state. Indeed, a series of complexes [Fe<sup>II</sup>(<sup>H,R</sup>′Pytacn)(CD<sub>3</sub>CN)<sub>2</sub>]<sup>2+</sup> exist as mixtures of high-spin (<i>S</i> = 2) and low-spin (<i>S</i> = 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 α-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 CC oxidation reactions with H<sub>2</sub>O<sub>2</sub>. 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 <i>cis</i>-dihydroxylation is enhanced for complexes with R = Me, F, or Cl. Isotopic labeling studies in the epoxidation and <i>cis</i>-dihydroxylation reactions show that R has a definitive role in dictating the origin of the oxygen atom that is transferred in the epoxidation reaction.