Reactivity of Molecular Dioxygen towards a Series of Isostructural Dichloroiron(III) Complexes with Tripodal Tetraamine Ligands: General Access to μ‐Oxodiiron(III) Complexes and Effect of α‐Fluorination on the Reaction Kinetics

Abstract

We have synthesized the mono, di-, and tri-alpha-fluoro ligands in the tris(2-pyridylmethyl)amine (TPA) series, namely, FTPA, F(2)TPA and F(3)TPA, respectively. Fluorination at the alpha-position of these nitrogen-containing tripods shifts the oxidation potential of the ligand by 45-70 mV per added fluorine atom. The crystal structures of the dichloroiron(II) complexes with FTPA and F(2)TPA reveal that the iron center lies in a distorted octahedral geometry comparable to that already found in TPAFeCl(2). All spectroscopic data indicate that the geometry is retained in solution. These three isostructural complexes all react with molecular dioxygen to yield stable mu-oxodiiron(III) complexes. Crystal structure analyses are reported for each of these three mu-oxo compounds. With TPA, a symmetrical structure is obtained for a dicationic compound with the tripod coordinated in the kappa(4)N coordination mode. With FTPA, the compound is a neutral mu-oxodiiron(III) complex with a kappa(3)N coordination mode of the ligand. Oxygenation of the F(2)TPA complex gave a neutral unsymmetrical compound, the structure of which is reminiscent of that already found with the trifluorinated ligand. On reduction, all mu-oxodiiron(III) complexes revert to the starting iron(II) species. The oxygenation reaction parallels the well-known formation of mu-oxo derivatives from dioxygen in the chemistry of porphyrins reported almost three decades ago. The striking feature of the series of iron(II) precursors is the effect of the ligand on the kinetics of oxygenation of the complexes. Whereas the parent complex undergoes 90 % conversion over 40 h, the monofluorinated ligand provides a complex that has fully reacted after 30 h, whereas the reaction time for the complex with the difluorinated ligand is only 10 h. Analysis of the spectroscopic data reveals that formation of the mu-oxo complexes proceeds in two distinct reversible kinetic steps with k(1) approximately 10 k(2). For TPAFeCl(2) and FTPAFeCl(2) only small variations in the k(1) and k(2) values are observed. By contrast, F(2)TPAFeCl(2) exhibits k(1) and k(2) values that are ten times higher. These differences in kinetics are interpreted in the light of structural and electronic effects, especially the Lewis acidity at the metal center. Our results suggest coordination of dioxygen as an initial step in the process leading to formation of mu-oxodiiron(III) compounds, by contrast with an unlikely outer-sphere reduction of dioxygen, which generally occurs at negative potentials.

Asymmetry and steric hindrance in tripodal ligands: Reaching the limit for octahedral geometry with the newly synthesized [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod in FeCl2 complexes

“…A sharp signal observed at d = 7.6 ppm (D m1/2 = 38 Hz) was assigned to the c protons of the coordinated pyridyl arms. Qualitatively, the spectrum differs from the well-defined ones obtained from FeCl 2 complexes in which the ligand coordinates in a tetradentate way [15][16][17]21,[23][24][25]. Quantitatively, the measured values of the midintensity width (D m1/2 ) of the b-protons stand above those generally observed for complexes with distorted octahedral geometry (40 < D m1/2 < 130 Hz): this strongly supports the occurrence of a trigonal bipyramidal geometry around the metal centre [14,21,23,24].…”

“…A dark brown solid could be isolated, and its UV-Vis spectrum was reminiscent of that obtained upon oxygenation of the F 2 TPAFeCl 2 complex [15], with absorptions at k = 263 nm (e = 11 000 mol À1 L cm À1 ) and k = 339 nm with e = 2800 mol À1 L cm À1 , suggesting the presence of a l-oxo diferric segment. This dark brown compound was dissolved in acetonitrile, and layering the solution with diethyl ether afforded single crystals of the unsymmetrical l-oxo The ORTEP diagram is displayed in Fig.…”

“…2. Simple l-oxo diferric compounds in which the metals are bridged by one oxygen atom only are well known with TPA ligands [15,26,27]. In the symmetrical [j 4 -N (TPAFeCl) 2 -O] 2+ dication, p interaction between two adjacent pyridines of distinct TPA ligands stabilize the structure.…”

“…The trans-equatorial distortion parameter q was defined in the past as a distortion indicator within series of FeCl 2 complexes [15] (see the Supporting Information for details). In the present case, q = 67°2, which is a moderate distortion, in line with elongated metal to ligand distances.…”

“…Keeping in mind that the presence of a vacant coordination site is a key point with respect to reactivity studies, and that coordination of 0020-1693/$ -see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.04.015 molecular oxygen should proceed smoothly in order to be fully under control [14][15][16][17], we decided to prepare the fully unsymmetrical FBrTPA ligand, [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine depicted in Scheme 1, and to characterize the corresponding FeCl 2 complex with hope that modulation of the steric hindrance of the ligand would enhance the flexibility at the metal site.…”