Speciation of Ferric Phenoxide Intermediates during the Reduction of Iron(III)−μ-Oxo Dimers by Hydroquinone

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).