10.1021/ic5014347.s001 William D. Kerber William D. Kerber Kaitlyn A. Perez Kaitlyn A. Perez Chuqiao Ren Chuqiao Ren Maxime A. Siegler Maxime A. Siegler Speciation of Ferric Phenoxide Intermediates during the Reduction of Iron(III)−μ-Oxo Dimers by Hydroquinone American Chemical Society 2014 29H proton inventory experiments TPA potentiometric titration data formation pH oxo log β Ferric Phenoxide Intermediates 2014-11-03 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Speciation_of_Ferric_Phenoxide_Intermediates_during_the_Reduction_of_Iron_III_Oxo_Dimers_by_Hydroquinone/2240176 The aqueous speciation of iron­(III)–tris­(pyridylmethyl)­amine (TPA) complexes was determined from potentiometric titration data, and the overall formation constants (β) for relevant species were calculated. At pH < 3 the mononuclear complex [Fe­(TPA)]<sup>+3</sup>(aq) predominates (log β = 10.75(15). Above pH 3 Fe<sup>3+</sup>–OH<sub>2</sub> hydrolysis produces the μ-oxo dimer [Fe<sub>2</sub>(μ-O)­(TPA)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>+4</sup> (<b>1a</b>; log β = 19.91(12)). This species is a diprotic acid with the conjugate bases [Fe<sub>2</sub>(μ-O)­(TPA)<sub>2</sub>(H<sub>2</sub>O)­(OH)]<sup>+3</sup> (<b>1b</b>; log β = 15.53(6)) and [Fe<sub>2</sub>(μ-O)­(TPA)<sub>2</sub>(OH)<sub>2</sub>]<sup>+2</sup> (<b>1c</b>; log β = 10.27(7)). The p<i>K</i><sub>a</sub>s of <b>1a</b> are 4.38(14) and 5.26(9). Compounds <b>1a</b>–<b>c</b> quantitatively oxidize hydroquinone to benzoquinone with concomitant formation of 2 equiv of Fe­(II). Kinetic and spectroscopic data at pH 5.6 are consistent with rapid equilibrium formation of a diiron­(III)–phenoxide intermediate followed by rate-controlling electron transfer. The equilibrium constant for the formation of the intermediate complex is 25(3) M<sup>–1</sup>, and the rate constant for its decomposition is 0.56(9) s<sup>–1</sup>. A kinetic isotope effect of <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 1.5 was determined from proton inventory experiments in mixed H/D media. The μ-oxo–diiron­(III) phenoxide intermediate is hydrolyzed in a pH dependent process to form a mononuclear iron­(III)–phenoxide, which complicates the kinetics by introducing a fractional dependence on total iron­(III) concentration in the pH range 4.1–5.2. The pH-dependent cleavage of μ-oxo–diiron­(III)–phenoxides was investigated with phenol, a redox-inert proxy for hydroquinone. The addition of phenol to <b>1</b> facilitates acidic cleavage of the μ-oxo dimer to form [Fe­(TPA)­(OPh)­(H<sub>2</sub>O)]<sup>+2</sup>, which becomes the dominant iron­(III)–phenoxide as the pH decreases to 4. The 2-naphtholate analogue of this intermediate, [Fe­(TPA)­(2-naphtholate)­(OCH<sub>3</sub>)]­ClO<sub>4</sub> (<b>6</b>), was characterized by single-crystal X-ray diffraction (C<sub>29</sub>H<sub>28</sub>FeN<sub>4</sub>O<sub>2</sub>,ClO<sub>4</sub>; <i>P</i>2<sub>1</sub>; <i>a</i> = 13.2646(2) Å, <i>b</i> = 15.2234(3) Å, <i>c</i> = 13.7942(3) Å; <i>Z</i> = 4).