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

A family of iron complexes with the general formula [FeII( 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 α-position of the pyridine, R′ corresponds to the group at the γ-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, 1H 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 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 [FeII(H,R′Pytacn)(CD 3CN)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 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 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. © 2013 American Chemical Society.