Inorganic Chemistry
Article
sphere reactivity of FeIVaqO . Direct comparison with several
2+
ACKNOWLEDGMENTS
■
other compounds in Table 1 for which we have calculated the
value of A places this reactivity of Fe aqO (A = 17.2) above
that for Fe(cp) (15.4) and Fe(Cp)(C H CH OH) (15.4),
We are grateful to Dr. Stanbury. for a gift of tris(phen-
anthroline)osmium(II) triflate. This research was supported by
the U.S. Department of Energy, Office of Science, Basic Energy
Sciences, Division of Chemical Sciences, Geosciences, and
Biosciences through the Ames Laboratory. The Ames
Laboratory is operated for the U.S. Department of Energy by
Iowa State University under Contract DE-AC02-07CH11358.
IV
2+
+
+
2
5
4
2
•
−
close to ABTS (19.1), and below that for the strong oxidants
IrCl62 (20.4) and Os(phen)3 (22.7).
−
3+
0
Even though individual values of E and k cannot be
obtained from the available data, reasonable limits can be
estimated. In view of the large rate constants, one is tempted to
2
2
2
+/+
suggest that the reduction potential for Fe O
is not less
aq
REFERENCES
than 0.89 V, the highest among the potentials for outer-sphere
reductants in Table 1. However, the observation that all of the
reactions proceeded to completion does not guarantee large
equilibrium constants, i.e., large E for Fe O . The observed
outcome is determined by the very nature of these reactions
which become irreversible owing to the rapid removal of
initially formed Fe O by protonation in eq 2. We therefore
calculate the lower limit for the potential by making the most
unfavorable assumption, i.e., that the back reaction between
Fe O and IrCl has a diffusion controlled rate constant (k
10 M s ), which leads to an equilibrium constant for the
■
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0
2+/+
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aq
(
(
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aq
(
(
6
(
III
+
2−
aq
6
1
0
−1 −1
=
IV
2+
3−
−4
Fe O /IrCl
reaction of 10 and the lower limit for
E (Fe O ) of ≥0.65 V. A much greater potential is
suggested by further analysis described below.
aq
6
0
2+/+
aq
1988, 110, 1229−1231.
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18,60
OSET has recently been documented
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reactions of macrocyclic and acyclic ferryl(IV) complexes with
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large reorganization energies λ, as shown in Table 2 for three
such compounds. To enable direct comparison with our work,
(
1
(
60
we fitted the literature data to Marcus cross relationship and
obtained the self-exchange rate constants, also shown in Table
. The two sets of derived parameters are mutually related so
(
2
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that the rate constant decreases as the reorganization energy
grows larger. Greater flexibility of the ligands and greater
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reorganization energy, so that the smallest λ and largest k are
associated with the reaction of (TMC)Fe O , the complex
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with its coordination sphere containing five unidentate ligands
will have a larger reorganization energy than (TMC)Fe O , i.
(
22
IV 2+
(
IV
2+
2
(
IV 2+
Langston, M. C.; Green, M. T. J. Am. Chem. Soc. 2014, 136, 9124−
9131.
e. k22 < 10−
5
M
−1 −1
s , from which E(Fe O ) > 1.3 V. This
large potential and fast kinetics of OSET are remarkable for a
2+/+
aq
(
18) Fukuzumi, S. Coord. Chem. Rev. 2013, 257, 1564−1575.
+
reaction producing Fe(H O) O , a thermodynamically unstable
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2
5
and unusual isomer of doubly deprotonated aqua-iron(III). In
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IV
2+/+
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transfer define Fe
O
aq
as an extremely potent and versatile
inorganic oxidant.
(
(
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23) Bataineh, H.; Pestovsky, O.; Bakac, A. Chem. Sci. 2012, 3, 1594−
ASSOCIATED CONTENT
Supporting Information
■
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Tables S1−S4, Figures S1−S13 (PDF)
(
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2
5, 4108−4114.
AUTHOR INFORMATION
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■
*
1
(
4
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Notes
The authors declare no competing financial interest.
E
Inorg. Chem. XXXX, XXX, XXX−XXX