title: 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.
abstract: 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 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, (1)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 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 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.
authors.affiliation: Grup de Química Bioinorgànica i Supramolecular (QBIS), Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus Montilivi, E17071 Girona, Catalonia, Spain.
authors.forename: Irene
authors.initials: I
authors.lastname: Prat
authors.forename: Anna
authors.initials: A
authors.lastname: Company
authors.forename: Teresa
authors.initials: T
authors.lastname: Corona
authors.forename: Teodor
authors.initials: T
authors.lastname: Parella
authors.forename: Xavi
authors.initials: X
authors.lastname: Ribas
authors.forename: Miquel
authors.initials: M
authors.lastname: Costas
mesh_terms: D002384:Catalysis; D018360:Crystallography, X-Ray; D004583:Electrons; D005296:Ferrous Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction
chemical_list: D005296:Ferrous Compounds; D006573:Heterocyclic Compounds, 1-Ring; D008024:Ligands