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).