ic5014347_si_001.pdf (2.27 MB)
Speciation of Ferric Phenoxide Intermediates during the Reduction of Iron(III)−μ-Oxo Dimers by Hydroquinone
journal contribution
posted on 2014-11-03, 00:00 authored by William D. Kerber, Kaitlyn A. Perez, Chuqiao Ren, Maxime
A. SieglerThe 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)]+3(aq) predominates (log β = 10.75(15). Above pH 3 Fe3+–OH2 hydrolysis produces the μ-oxo
dimer [Fe2(μ-O)(TPA)2(H2O)2]+4 (1a; log β = 19.91(12)).
This species is a diprotic acid with the conjugate bases [Fe2(μ-O)(TPA)2(H2O)(OH)]+3 (1b; log β = 15.53(6)) and [Fe2(μ-O)(TPA)2(OH)2]+2 (1c; log β
= 10.27(7)). The pKas of 1a are 4.38(14) and 5.26(9). Compounds 1a–c 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–1, and the rate
constant for its decomposition is 0.56(9) s–1. A
kinetic isotope effect of kH/kD = 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 1 facilitates acidic cleavage
of the μ-oxo dimer to form [Fe(TPA)(OPh)(H2O)]+2, which becomes the dominant iron(III)–phenoxide as
the pH decreases to 4. The 2-naphtholate analogue of this intermediate,
[Fe(TPA)(2-naphtholate)(OCH3)]ClO4 (6), was characterized by single-crystal X-ray diffraction (C29H28FeN4O2,ClO4; P21; a = 13.2646(2) Å, b = 15.2234(3) Å, c = 13.7942(3) Å; Z = 4).