Nonheme oxoiron(iv) complexes of pentadentate N5 ligands: Spectroscopy, electrochemistry, and oxidative reactivity

Article abstract of DOI:10.1039/c2sc21318d

Oxoiron(iv) species have been found to act as the oxidants in the catalytic cycles of several mononuclear nonheme iron enzymes that activate dioxygen. To gain insight into the factors that govern the oxidative reactivity of such complexes, a series of five synthetic S = 1 [Fe^IV(O)(L _N5)]²⁺ complexes has been characterized with respect to their spectroscopic and electrochemical properties as well as their relative abilities to carry out oxo transfer and hydrogen atom abstraction. The FeO units in these five complexes are supported by neutral pentadentate ligands having a combination of pyridine and tertiary amine donors but with different ligand frameworks. Characterization of the five complexes by X-ray absorption spectroscopy reveals FeO bonds of ca. 1.65 A in length that give rise to the intense 1s → 3d pre-edge features indicative of iron centers with substantial deviation from centrosymmetry. Resonance Raman studies show that the five complexes exhibit ν(FeO) modes at 825-841 cm^-1. Spectropotentiometric experiments in acetonitrile with 0.1 M water reveal that the supporting pentadentate ligands modulate the E_1/2(iv/iii) redox potentials with values ranging from 0.83 to 1.23 V vs. Fc, providing the first electrochemical determination of the E_1/2(iv/iii) redox potentials for a series of oxoiron(iv) complexes. The 0.4 V difference in potential may arise from differences in the relative number of pyridine and tertiary amine donors on the L_N5 ligand and in the orientations of the pyridine donors relative to the FeO bond that are enforced by the ligand architecture. The rates of oxo-atom transfer (OAT) to thioanisole correlate linearly with the increase in the redox potentials, reflecting the relative electrophilicities of the oxoiron(iv) units. However this linear relationship does not extend to the rates of hydrogen-atom transfer (HAT) from 1,3-cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA), and benzyl alcohol, suggesting that the HAT reactions are not governed by thermodynamics alone. This study represents the first investigation to compare the electrochemical and oxidative properties of a series of S = 1 Fe^IVO complexes with different ligand frameworks and sheds some light on the complexities of the reactivity of the oxoiron(iv) unit.

Full text of DOI:10.1039/c2sc21318d

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Nonheme oxoiron(IV) complexes of pentadentate N5
ligands: spectroscopy, electrochemistry, and oxidative
reactivity†
Cite this: Chem. Sci., 2013, 4, 282
Dong Wang,a Kallol Ray,‡a Michael J. Collins,ab Erik R. Farquhar,a Jonathan R. Frisch,a
c
a
d
d
´
Laura Gomez, Timothy A. Jackson, Marion Kerscher, Arkadius Waleska,
d
c
a
*
*
*
Peter Comba, Miquel Costas and Lawrence Que, Jr.
Oxoiron(IV) species have been found to act as the oxidants in the catalytic cycles of several mononuclear
nonheme iron enzymes that activate dioxygen. To gain insight into the factors that govern the oxidative
reactivity of such complexes, a series of five synthetic S ¼ 1 [FeIV(O)(LN5)]2+ complexes has been
characterized with respect to their spectroscopic and electrochemical properties as well as their relative
abilities to carry out oxo transfer and hydrogen atom abstraction. The Fe]O units in these five
complexes are supported by neutral pentadentate ligands having a combination of pyridine and tertiary
amine donors but with different ligand frameworks. Characterization of the five complexes by X-ray
˚
absorption spectroscopy reveals Fe]O bonds of ca. 1.65 A in length that give rise to the intense 1s /
3d pre-edge features indicative of iron centers with substantial deviation from centrosymmetry.
Resonance Raman studies show that the five complexes exhibit n(Fe]O) modes at 825–841 cmꢀ1
.
Spectropotentiometric experiments in acetonitrile with 0.1
M water reveal that the supporting
pentadentate ligands modulate the E1/2(IV/III) redox potentials with values ranging from 0.83 to 1.23 V
vs. Fc, providing the first electrochemical determination of the E1/2(IV/III) redox potentials for a series of
oxoiron(IV) complexes. The 0.4 V difference in potential may arise from differences in the relative
number of pyridine and tertiary amine donors on the LN5 ligand and in the orientations of the pyridine
donors relative to the Fe]O bond that are enforced by the ligand architecture. The rates of oxo-atom
transfer (OAT) to thioanisole correlate linearly with the increase in the redox potentials, reflecting the
relative electrophilicities of the oxoiron(IV) units. However this linear relationship does not extend to the
rates of hydrogen-atom transfer (HAT) from 1,3-cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA),
and benzyl alcohol, suggesting that the HAT reactions are not governed by thermodynamics alone. This
study represents the first investigation to compare the electrochemical and oxidative properties of a
series of S ¼ 1 FeIV]O complexes with different ligand frameworks and sheds some light on the
complexities of the reactivity of the oxoiron(IV) unit.
Received 21st August 2012
Accepted 25th October 2012
DOI: 10.1039/c2sc21318d
www.rsc.org/chemicalscience
Introduction
aDepartment of Chemistry and Center for Metals in Biocatalysis, University of
Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA. E-mail: larryque@
chem.umn.edu
High valent iron-oxo intermediates are considered to be the
active species in the catalytic cycles of mononuclear nonheme
iron enzymes that activate dioxygen.1–3 Within the last ten years,
such species have been identied by rapid-freeze-quench tech-
niques in several 2-oxoglutarate-dependent enzymes, namely
TauD,4–7 prolyl 4-hydroxylase,8 and the halogenases CytC3 (ref.
9) and SyrB2,10 as well as the pterin-dependent phenylalanine11
and tyrosine hydroxylases.12 These intermediates have been
bDepartment of Chemistry, Viterbo University, La Crosse, WI 54601, USA
c
´
Departament de Quımica, Universitat de Girona, Campus de Montilivi, E-17071
Girona, Spain. E-mail: miquel.costas@udg.edu
d
¨
Universitat Heidelberg, Anorganisch-Chemisches Institut, INF 270, D-69120
Heidelberg, Germany. E-mail: peter.comba@aci.uni-heidelberg.de
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, experimentally observed and calculated ESI-MS spectra for 3,
additional cyclic voltammetry traces for 1a–5a, EPR and ESI-MS data for 1b–5b,
additional details of inter-complex oxo-atom transfer reactions, plots showing
the determination of pseudo rst order and second order rate constants for
reactions of 1–5 and the KIE values, and details of the EXAFS analyses. See DOI:
10.1039/c2sc21318d
¨
characterized by Mossbauer spectroscopy as having high-spin
iron(IV) centers. Furthermore, the presence of a terminal Fe]O
unit in the TauD,6,7 CytC3,13 and SyrB2 (ref. 10) intermediates
has been established by resonance Raman and/or Extended X-
ray Absorption Fine Structure (EXAFS) methods.
¨
‡ Current address: Humboldt-Universitat zu Berlin, Department of Chemistry,
Brook-Taylor Str. 2, 12489 Berlin, Germany.
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Recent biomimetic efforts have identied a large number of furthermore deuterium kinetic isotope effects (KIEs) much
mononuclear nonheme oxoiron(IV) complexes.14–17 Most of these larger than the semiclassical limit were observed. Character-
complexes have been spectroscopically characterized to have S ¼ 1 izing the redox properties of these synthetic complexes and
ground states and the FeIV]O units are supported by a variety of correlating them with their reactivity towards substrates would
polydentate ligand frameworks with mainly N donors, including thus be of great interest, and the present paper focuses on 1–5
macrocyclic cyclam,18–22 bicyclic bispidine,23–26 and tripodal ligand (Scheme 1), a series of oxoiron(IV) complexes with pentadentate
motifs.27–29 Only a few complexes have been shown experimentally supporting ligands.
to have S ¼ 2 spin states.30–34 Crystal structures of ve complexes
The determination of the FeIV/III reduction potentials for the
have now been reported, namely [FeIV(O)(TMC)(NCCH3)]2+,19 [FeIV- oxoiron(IV) complexes can provide quantitative insight into the
(O)(TMC-py)]2+,35 [FeIV(O)(N4Py)]2+ (1),36 [FeIV(O)(TMG3tren)]2+,37 thermodynamic factors that govern their differing oxidizing
and [FeIV(O)(H3buea)]ꢀ (ref. 32) (TMC ¼ 1,4,8,11-tetramethyl- abilities. However, the electrochemical behavior of these
1,4,8,11-tetraazacyclotetradecane; TMC-py ¼ 1,4,8-trimethyl-11- complexes exhibits some complexity that has not been straight-
(20-pyridylmethyl)-1,4,8,11-tetraaza-cyclotetradecane; TMG3tren ¼ forward to interpret. For example, in the cyclic voltammetry (CV)
1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine); of 1 in dry MeCN, the cathodic scan exhibited a wave at ꢀ0.44 V
H3buea, 1,1,1-tris[(N0-tert-butylureaylato)-N-ethyl]amine tri- vs. Fc+/0
,
presumably reecting the reduction of 1, while the
42
anion; for N4Py, see Scheme 1). With this range of complexes, a anodic scan showed multiple poorly dened peaks of much
question of signicant interest is how the reactivity of the FeIV] lower intensity, suggesting the formation of a mixture of species
O unit is modulated by the various ligands that support it. For upon reduction.43 The Ep,c value observed for 1 is in fact more
example, [FeIV(O)(TMC)(NCCH3)]2+ can only oxidize substrates negative than the E1/2 values associated with the FeIII/II couples
with C–H bond dissociation energies (BDE) of <80 kcal molꢀ1
,
of the corresponding iron(II) complexes [FeII(N4Py)(NCCH3)]2+
but replacing the axial CH3CN ligand with anions increases the (E1/2 ¼ 0.61 V vs. Fc+/0) and [FeII(N4Py)(OH2)]2+ (Ep,c ¼ 0.15 V vs.
rates of these reactions.38 On the other hand, some oxoiron(IV) Fc+/0 44–46,85 and would appear incommensurate with the reported
)
complexes with combinations of N-heterocycle and amine ability of 1 to oxidize cyclohexane.39,47 This apparent contradic-
donors have been found to attack even the strong C–H bonds of tion led us to investigate the electrochemical properties of 1
cyclohexane.28,29,39–41 For several of these complexes, an inverse under different conditions.
linear correlation was obtained when log k02 values (the second
The irreversibility of the electrochemistry of 1 in CH3CN may
order rate constant normalized on per hydrogen atom basis) derive from the expected high basicity of its one-electron
were plotted versus the substrate C–H bond strength, suggesting reduced counterpart and its instability in the absence of a
that the reactions proceed by hydrogen atom transfer; proton donor. Indeed there is only one well characterized
example of an oxoiron(III) complex to date,48 and the oxide
ligand in this complex is stabilized by several hydrogen bonding
groups designed into the polydentate supporting ligand.
Moreover, similar complications have been reported in the
electrochemistry of RuIV]O complexes and reversible cyclic
voltammetric waves for the RuIV/III couples can be observed only
in acidic aqueous solution.49–54 To circumvent this likely
problem, we carried out electrochemical experiments in
‘wet’ CH3CN (CH3CN with added water) and used spec-
tropotentiometry to elicit the Nernstian oxidation of the FeIII–
OH complex 1b to 1 at a signicantly more positive potential
(E1/2(IV/III)) of 0.90 V vs. Fc+/0 46 In this work, we report a detailed
.
comparison of complexes 1–5 with respect to their spectro-
scopic and electrochemical properties, and compare these
properties with their rates of oxo-atom transfer (OAT) and
hydrogen-atom transfer (HAT) to gain insights into what factors
affect the reactivity of the FeIV]O unit.
Results and discussion
A. Spectroscopic characterization of oxoiron(IV) complexes
Oxoiron(IV) complexes 1–5 of the pentadentate LN5 ligands
shown in Scheme 1 were obtained by treatment of the iron(II)
precursors 1a–5a with excess solid PhIO in CH3CN at 25 ꢁC.
Aer maximum formation of 1–5 (monitored by UV-vis spec-
troscopy), excess PhIO was removed by ltration. These
complexes exhibit pale green or yellowish-green chromophores
that derive from near-IR bands arising from ligand eld
Scheme
1
Structures of oxoiron(IV) complexes 1–5 studied in this work.
Numerical designations 1a–5a refer to corresponding [FeII(LN5)(NCCH3)]2+
complexes, while 1b–5b refer to corresponding [FeIII(OH)(LN5)]2+ species.
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transitions of an S ¼ 1 FeIV]O center (Table 1, Fig. 1),23,24,39 as
assigned for 1 and [FeIV(O)(TMC)(NCCH3)]2+ on the basis of a
detailed magnetic circular dichroism (MCD) analysis.55,56
A
similar near-IR chromophore is observed for 3 (Fig. 1), which is
reported for the rst time in this study, and its formation is
supported by the appearance of a dominant ion at m/z 546 in its
electropspray ionization mass spectrometry (ESI-MS) spectrum
with the isotope distribution pattern predicted for the formu-
lation {[Fe(O)(Me2TACN-Py2)](CF3SO3)}+ (Fig. S1†).
Table 1 also lists resonance Raman data for 1–5. These
complexes exhibit a Raman feature at ca. 830 cmꢀ1 upon
406.7 nm excitation that can be assigned to the Fe]O stretch-
ing mode on the basis of the large 18O shi elicited upon oxygen
atom exchange with H218O. The n(Fe]O)’s range from 841 cmꢀ1
for 1 to 825 cmꢀ1 for 5, with observed 18O downshis fully
consistent with that expected for a diatomic Fe]O oscillator.
X-ray absorption spectroscopic (XAS) data collected for 1–5
are also compiled in Table 1. All ve complexes exhibit rst
inection points in the rising Fe K-edge at 7124(1) eV, consis-
tent with those obtained for other S ¼ 1 oxoiron(IV) complexes in
earlier studies.21,57,58 A single prominent pre-edge transition
centered at 7114.2 ꢂ 0.3 eV is observed for each complex, which
is assigned to a 1s / 3d transition. This feature has a
normalized pre-edge intensity of 32–34 units for 1–4 and 45
units for 5. The much larger pre-edge intensity of 5 presumably
reects a greater distortion from centrosymmetry at the iron
center of 5 relative to 1–4, as suggested by density functional
theory (DFT) calculations.59
Fig. 1 Electronic spectra of oxoiron(IV) complexes 1 (black dashed line), 2 (black
solid line), 3 (red dashed line), 4 (red solid line) and 5 (blue solid line) recorded in
CH3CN at room temperature.
The data for the series of [FeIV(O)(LN5)] complexes presented
above show that their spectroscopic properties are modulated
by the nature of the supporting pentadentate ligand. The UV-vis
spectra of all ve complexes exhibit a principal feature at 696–
740 nm, with a weaker band near 900 nm for 2–5 (Table 1).
When the near-IR band of [FeIV(O)(TMC)(NCCH3)]2+ (lmax
824 nm) is included in the series, there is a trend in the absor-
bance maxima that reects the length of the equatorial Fe–N
bonds. Complex 1, with a 4-pyridine equatorial ligand set, exhibits
a band at 696 nm at the low end of the series and has an average
Fe–Neq bond length of 1.96 A, while [FeIV(O)(TMC)(NCCH3)]2+,
36
˚
with a 4-tertiary-amine ligand set, has a band at 824 nm at the
EXAFS analysis of 1, 3, 4, and 5 (Fig. 2 and Tables 1 and S1†)
high end of the range and has an average Fe–Neq bond length of
2.09 A.19 Complexes 2–5 have combinations of equatorial pyridine
˚
˚
reveals a short Fe–O distance at 1.62–1.64 A associated with the
Fe]O unit, which is comparable in length to the Fe]O
and tertiary amine ligands and average Fe–Neq bonds of inter-
mediate lengths; consequently, their principal near IR features
(740–745 nm) fall in between the two extremes. Therefore, the
longer the Fe–N bond becomes, the weaker of an effect of the N
donor can exert on the d-orbital splitting, the lower the energy of
the ligand eld transitions.
distances found in the crystal structures of complexes 1 (1.639
A), [FeIV(O)(TMC)(NCCH3)]2+ (1.646 A), [FeIV(O)(TMC-py)]2+
36
19
˚
˚
[FeIV(O)(TMG3tren)]2+
(1.661
and
35
37
˚
A),
˚
A),
(1.667
[FeIV(O)(H3buea)]ꢀ (1.680 A). In addition, 1 and 3 exhibit a
32
˚
˚
shell of nitrogen scatterers at 1.96 and 2.00 A, respectively,
assignable to ligated pyridine and tertiary amine donors. On the
other hand, ts to 4 and 5 require two subshells of N scatterers
The accumulated characterization data suggest that 5 is the
most distinct in properties of the ve complexes presented here.
While the EXAFS ts reported here do not reveal any signicant
differences among the ve complexes, the X-ray absorption near
edge structure (XANES)’data in Table 1 show that 5 has a pre-edge
˚
at ca. 1.98 and 2.17 A, consistent with structures derived from
DFT calculations for 4 and 5 and reecting the constraints
imposed by the bispidine ligand framework.59
Table 1 Spectroscopic properties of 1–5
XAS pre-edge
lmax (nm) in MeCN
n(Fe]O)
XAS sample
purity
Area
(normalized)
XAS edge
energy (eV)
f
(3, Mꢀ1 cmꢀ1
)
(cmꢀ1) [D18O]
Energy (eV)
r(Fe–X) (A)
˚
1
2
3
4
5
696 (400)a
841 [ꢀ35]
835 [ꢀ39]
839e [ꢀ35]
840 [ꢀ35]
825 [ꢀ34]
90%
90%
85%
74%
85%
31 (34)
29 (32)
29 (34)
24 (32)
38 (45)
7114.4
7114.1
7114.1
7114.5
7114.0
7124.0
7123.7
7124.7
7124.4
7123.4
1O @ 1.64 4N @ 1.96
1O @ 1.67 4N @ 2.00
1O @ 1.63 4N @ 2.00
1O @ 1.64 3N @ 1.98 2N @ 2.15
1O @ 1.62 4N @ 1.97 1N @ 2.18
739 (400), 900 (sh)a
740 (340), 900 (200)b
730 (400), 916 (sh)c,d
730 (380), 896 (sh)c
a
From ref. 39. b Molar extinction coefficients estimated from thioanisole oxidation data. c From ref. 23. d From ref. 24. e Value obtained from the
midpoint of a Fermi doublet at 831 and 847 cmꢀ1 for the 16O sample. f EXAFS analysis of 2 reported in ref. 57; ts for the other complexes listed in
Table S1.† The typical uncertainty for rst-shell distances by EXAFS analysis is ꢂ0.02 A.
˚
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Table 2 Summary of electrochemical results for 1–5a
E1/2(III/II) (DE) in Ep,c observed in E1/2(IV/III) in CH3CN
Complex dry CH3CNb (V)
dry CH3CNc (V)
with 0.1 M waterd (V)
1
2
3
4
5
0.61 (0.08)
0.69 (0.08)
0.55 (0.07)
0.72 (0.11)
0.77 (0.14)
ꢀ0.53
ꢀ0.43
ꢀ0.58
ꢀ0.48
ꢀ0.32
0.90
1.07
0.83
0.98e
1.23
a
b
All potentials are referred to Fc+/0
.
Obtained by cyclic voltammetry
c
with a scan rate ¼ 0.1 V sꢀ1 for 1 mM solutions of 1a–5a. Obtained
by cyclic voltammetry with a scan rate ¼ 0.1 V sꢀ1 for 1 mM solutions
d
e
of 1–5. Obtained by spectropotentiometry. Estimated based on the
correlation shown in Fig. 4.
conditions, the Ep,c values of the complexes were found to fall in
the range of ꢀ0.30 to ꢀ0.60 V vs. Fc+/0 and decrease in the
following order: 5 > 2 > 4 > 1 > 3.
We also tried to obtain the FeIV/III potentials of the complexes
by electrochemical oxidation of iron(II) precursors 1a–5a in
CH3CN. In dry solvent, oxidation of the iron(II) complexes gave
rise to quasi-reversible CVs (Fig. S3†), with E1/2 values that did
not depend signicantly on scan rate (0.01–0.25 V sꢀ1) or
temperature. These features were found by coulometry to result
from one-electron processes corresponding to the
[Fe(LN5)(NCCH3)]3+/2+ couples, as previously reported for 1a.46
Further scans to higher potentials up to +1.25 V under the dry
solvent conditions did not elicit any other features in the CVs
that could be correlated to the formation of oxoiron(IV) species.
This result should not be surprising, as there is no source of the
oxygen atom that is needed to stabilize the iron(IV) oxidation
state under the solvent conditions used. Subsequent cyclic
voltammetric experiments carried out on 1a in CH3CN solutions
containing 0.1 M H2O did not change the outcome, which is
likely due to slow ligand exchange between bound solvent
and water at the iron(III) stage relative to the CV time scale
(Scheme 2).46,60,85
Fig. 2 Unfiltered EXAFS spectra (dotted lines) and corresponding best fits (solid
lines) of 1, 3, 4, and 5. The fits shown are given in bold italics in Table S1 in the ESI.†
area much larger than those of the others (45 normalized units
versus 32–34), indicating that it has a geometry with the largest
distortion from centrosymmetry. In addition, Table 1 shows that
5 has a n(Fe]O) value of 825 cmꢀ1, which is lower than the
835 cmꢀ1 value found for 2 and the ꢃ840 cmꢀ1 value associated
with 1, 3, and 4, suggesting that 5 has a somewhat weaker Fe]O
bond than the rest. As will be seen in the subsequent sections,
these differences can be related to our observations on their
electrochemical potentials and oxidative reactivity.
However high yields of FeIV]O complexes 2, 3, and 5 could be
obtained by bulk electrolytic oxidation of 2a, 3a, and 5a at
applied potentials of $1.3 V in CH3CN with added water. The
rate of oxidation depended on the concentration of water, and no
electrochemical generation of the oxoiron(IV) species occurred in
dry CH3CN. The bulk electrolytic oxidation required 30 minutes
to complete when carried out in CH3CN with 0.1 M water. For
each complex, chronocoulometry gave approximately the
number of coulombs (within 5–10%) expected for a two-electron
redox reaction. On the other hand, for some undetermined
reason, bulk electrolysis of 4a only afforded a 40% yield of 4.
B. Electrochemistry
To complement the spectroscopic results presented in the
preceding section, electrochemistry experiments were carried
out on 1–5 to assess their redox properties. As previously
reported, cyclic voltammetric experiments on 1 carried out in
dry CH3CN showed irreversible behavior, where a reduction
wave (Ep,c) was observed only at a rather negative potential of
ꢀ0.53 V vs. Fc+/0 at 25 C with no accompanying feature in the
ꢁ
oxidative scan.42,43,46,60 Analogous observations have also been
previously described for [FeIV(O)(TMC)(NCCH3)]2+ by Nam
and Fukuzumi.42,43 The reduction wave was assigned to the
FeIV(O)/FeIII(O) reduction and its electrochemically irreversible
nature was demonstrated to arise from slow electron transfer.
For 2–5, irreversible CV behavior was observed in CH3CN as
well, with all complexes showing broad reduction waves at
similarly negative potentials (Table 2, Fig. S2†). Under identical
Scheme 2
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spectra for the formation of 2, 3 and 5 are shown in Fig. 3. A plot
of the absorbance at 740 nm for 2 as a function of the applied
potential shown in Fig. 3A inset shows a sigmoidal shape that
can be t well with the Nernst equation to give a midpoint
potential E1/2 of +1.07(1) V vs. Fc+/0 for the FeIV/III couple.
Similarly good ts to corresponding data for 3 (Fig. 3B inset)
and 5 (Fig. 3C inset) gave respective E1/2 values of +0.83(1) V
and +1.23(1) V vs. Fc+/0. The corresponding experiment for 4
proved unsuccessful, as it was not possible to obtain 4 in
high yield by electrochemical oxidation. For 1, 2, 3, and 5, the
E1/2(IV/III) values were independent of the concentration of H2O
(up to 5 M) added into the CH3CN solutions.
In contrast, bulk electrolytic reduction under the same
solvent conditions takes a much longer time at any given
applied potential than the corresponding oxidative process.
This illustrates the complexity in the electrochemical behavior
of nonheme FeIV]O complexes that has been observed previ-
ously but is not well understood42,43,46,60,66 and may be related to
the requirement for a proton in the reduction (Scheme 2).
Similar difficulties have been reported in the electrochemistry
of RuIV]O complexes in organic solvents, which were alleviated
in aqueous solution.49,50,54,67,68 Despite these electrochemical
complexities, the spectropotentiometric information we have
obtained for a series of FeIV]O complexes in this work never-
theless provides quite useful chemical insight (vide infra).
The FeIV/III potentials measured by spectropotentiometry for
the four complexes are over 1.2 V more positive than corre-
sponding Ep,c values described above in CV experiments in dry
acetonitrile. Nevertheless, they exhibit the same decreasing
order: 5 > 2 > 1 > 3. In fact, when the E1/2(IV/III) values obtained by
spectropotentiometry are plotted against the Ep,c values
obtained by CV, a linear correlation can be observed (Fig. 4,
lled circles), demonstrating that both measurements are
sensitive to the same ligand-specic effects on the potential.
This plot thus allows us to estimate the E1/2(IV/III) value of 4 to
be +0.98 V based on its Ep,c value measured in dry CH3CN.
The plot shown in Fig. 4 emphasizes that the nature of the
Fig. 3 Difference spectra derived from spectropotentiometric titrations for
complexes 2 (A), 3 (B), and 5 (C) at 25 ꢁC in CH3CN containing 0.1 M H2O. The iron
concentration used for all three experiments was 0.25 mM. For each panel, the
increase in the absorbance at ꢃ740 nm is plotted as a function of the applied
potential to show a sigmoidal shape that can be fit well with the Nernst equation.
The bulk electrolytic oxidation could be followed spectro-
photometrically in a spectroelectrochemical cell (Fig. 3). Like pentadentate ligand can have a profound effect on the redox
previous results reported for 1,46 these experiments revealed a
two-step process to generate the oxoiron(IV) complex. In the rst
step, an intense near-UV band was generated around 350 nm,46
a chromophore we assign to the hydroxo-to-iron(III) charge
transfer transition of the corresponding FeIII–OH complex
derived from the one-electron oxidation of the iron(II)
precursor.44,61–65 Electron paramagnetic resonance (EPR) studies
of chemically oxidized (with tris(4-bromophenyl)aminium
perchlorate) solutions of 1a–5a under the same solvent condi-
tions showed signals associated with the formation of high-spin
iron(III) species (Fig. S4†). In the second step of the bulk elec-
trolytic oxidation, the near-UV chromophore of the FeIII–OH
complex decayed concomitant with the appearance of the
characteristic near-IR band of the oxoiron(IV) complex, and
isosbestic points were observed for this transformation (Fig. 3).
Fig. 4 Plot of Ep,c (black circles) and n(Fe]O) values (blue squares) versus E1/2(IV/III)
The conversion of [FeIII(OH)(LN5)]2+ species to [FeIV-
vs. Fc+/0 for the FeIV]O complexes in this study (data from Tables 2 and 1,
(O)(LN5)]2+ was then monitored in a stepwise fashion as a
respectively). The red line represents the best linear fit for the Ep,c vs. E1/2(IV/III)
function of applied potential. Difference spectra vs. those of the
correlation with the red dot indicating an estimation of the E1/2(IV/III) value for 4
[FeIII(OH)(LN5)]2+ complexes were calculated, and representative from the linear correlation.
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potential of the FeIV]O unit. Despite the fact that all ve was observed over the entire course of the reactions, and second
ligands are neutral in charge and consist only of combinations order rate constants were extracted from the dependence of the
of tertiary amine and pyridine donors, there is a difference of pseudo-rst-order rate constants on thioanisole concentration
0.3–0.4 V in the potentials for the complexes in the series. The (Table 3, Fig. S5†). From these results, a reactivity order of 5 > 2 >
main difference between the three complexes at the bottom half 4 > 1 > 3 was obtained. Furthermore, when the logarithms of the
of this range (i.e. 1, 3, and 4) and the two at the top (i.e. 2 and 5) second order rate constants were plotted versus the E1/2(IV/III)
is the presence of at least one pyridine donor whose plane is values obtained from spectropotentiometric experiments in
oriented perpendicular to the Fe]O unit. In fact 5 has two such CH3CN containing 0.1 M H2O, an excellent linear correlation
pyridine donors perpendicular to the Fe]O unit and exhibits was observed (Fig. 5), demonstrating that the FeIV/III potentials
the highest potential among the complexes in the series. In reect the relative electrophilicities of the Fe]O units in these
contrast, 4, which is an isomer of 5 with no perpendicular complexes.
pyridine rings, has a considerably lower potential that is
The thioanisole oxidation results of the ve ferryl complexes
comparable to those of 1 and 3. In an earlier DFT study of the were further corroborated by a study of the oxo-transfer abilities
possible isomers of 2, we noted the greater instability of isomers of 1–5 to the various iron(II) precursors 1a–5a at 25 ꢁC. Previ-
with a larger number of perpendicular pyridine donors,36
a
ously Nam and co-workers42 established a reactivity order of 2 >
notion supported by the experimental observations on 5. The 1 > [FeIV(O)(TMC)(NCCH3)]2+ in their investigation of the reac-
difference in stability has been attributed to steric effects of the tions of an oxoiron(IV) complex of one ligand and the iron(II)
a-H on the perpendicular pyridine donor, resulting in a longer complex of a different ligand.42 The present study extends the
calculated Fe–N bond distance. Also shown in Fig. 4 is a plot of series to include 3, 4, and 5. Thus, addition of 3a (1.5 mM) to an
the n(Fe]O) values observed for the ve complexes by reso- equimolar solution of 1 in CH3CN resulted in the disappearance
nance Raman spectroscopy relative to their E1/2(IV/III) values. of the 695 nm band of 1 and the concomitant quantitative
Interestingly, 2 and 5, the complexes with the higher potentials
exhibit lower n(Fe]O) frequencies that presumably reect
weaker, and perhaps more reactive, Fe]O bonds engendered
by the interactions of the pentadentate ligands with the FeIV]O
unit. This study provides for the rst time the E1/2(IV/III) poten-
tials of ve related oxoiron(IV) complexes directly determined by
electrochemical methods. Complexes 1–5 thus represent an
interesting series with which to probe the oxidative reactivity of
the FeIV]O unit.
C. Oxidative reactivity
Table 3 compares kinetic data for the ve complexes with
respect to their rates of oxo-atom transfer and H-atom abstrac-
tion. In one set of experiments, the oxidation of thioanisole by
1–5 was investigated in CH3CN solutions at ꢀ10 ꢁC, resulting in
about a 90% yield of methyl phenyl sulfoxide (Table 3). The
Fig. 5 Plots of the logarithms of the second-order rate constants for the
oxidation of thioanisole at ꢀ10 ꢁC (filled black squares), CHD at ꢀ40 ꢁC (filled red
progress of oxo-transfer could be monitored by the loss of the
characteristic near-IR bands of the oxoiron(IV) complexes. For 1
and 3, formation 1a and 3a could also be conveniently moni-
circles), PhCH2OH at 25 ꢁC (open blue triangles) and DHA at 25 ꢁC (open black
diamonds) in CH3CN vs. the E1/2(IV/III) value measured by spectropotentiometry
for oxoiron(IV) complexes 1–5. The red solid line is defined by the rates of thio-
tored by the appearance of their characteristic intense visible
anisole oxidation by 1–5, while the red dashed lines are defined by the rates of H-
chromophores at 458 nm (3 ꢃ 4000 Mꢀ1 cmꢀ1
)
and 455 nm
69
atom abstraction from various substrates by 1–3. The red dashed lines show that
the H-atom abstraction rates associated with 4 and 5 are about an order of
magnitude lower than predicted.
(3 ꢃ 2600 Mꢀ1 cmꢀ1), respectively. Thus 1–5 are quite effective at
oxo-transfer to thioanisole. Pseudo-rst-order kinetic behavior
Table 3 Kinetics results for oxoiron(IV) complexes 1–5
1
2
3
4
5
Complex
E1/2(IV/III) vs. Fc+/0 in CH3CN with 0.1 M water
0.90
1.07
0.83
(0.98)a
1.23
k2,c Mꢀ1 sꢀ1 (% yield of
CHD (ꢀ40 ꢁC)
DHA (25 ꢁC)
0.070(3) (89%)
18(1) [20]b
0.10(1) (80%)
0.014 (90%)
0.96(3) (99%)
100(6) [16]
0.86(2) (83%)
0.33 (93%)
0.027(1) (92%)
7.4(1) [31]
0.042(1) (80%)
0.004 (90%)
0.014(1) (88%)
1.1(1) [13]
0.013(1) (85%)
0.024 (75%)
0.37(2) (89%)
40(3) [12]
0.33(2) (90%)
2.4 (95%)
oxidation product) [KIE]d
PhCH2OH (25 ꢁC)
PhSMe (ꢀ10 ꢁC)
Value estimated from the correlation in Fig. 4. From ref. 47. See Fig. S9–S12.† d See Fig. S12 and S13.†
a
b
c
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formation of the near-IR features of 3 (Fig. S6†), indicating that 1 HAT reactions of cumylperoxyl radical versus the redox poten-
is more oxidizing than 3. The other oxoiron(IV) complexes, 2, 4, tials of the substrates.70 The general trend observed for reac-
and 5, were also found to convert 3a to 3 quantitatively, while the tions with phosphines and suldes showed an increase in OAT
addition of 1a, 2a, 4a, or 5a to 3 did not result in the respective rates with a decrease in the oxidation potential of the substrate.
formation of 1, 2, 4, or 5. On the other hand, 5 was found to HAT rates from N,N-dimethylanilines show a similar trend, but
transfer its oxo atom to 1a, 2a, 3a, and 4a. Because of the simi- they plateaued with the introduction of electron donating
larity of the near IR features of 2 and 5, the reaction between 5 substituents, suggesting some mechanistic complexity.
and 2a was monitored instead by ESI-MS, which is shown in Although the Fukuzumi study focused on the effects of
Fig. S7.† Mixing equimolar amounts of 2a and 5 resulted in a substrate redox potential, and not of the oxidant, the conclu-
decrease in the signals corresponding to [FeII(BnT- sions reached may be regarded as coincident with ours,
PEN)(CF3SO3)]+ (m/z ¼ 628.12) and {[FeIV(O)(BP2)](CF3SO3)}+ showing that reaction rates generally correlate with the elec-
(m/z ¼ 736.01) with a concomitant increase in the signals cor- trochemical driving force of the reaction. In another study,
responding to {[FeIV(O)(BnTPEN)](CF3SO3)}+ (m/z ¼ 644.04) and Sastri et al. correlated redox potentials with OAT and HAT
[FeII(BP2)(CF3SO3)]+ (m/z ¼ 720.11). In contrast, no oxo-transfer reaction rates of a series of [FeIV(O)(TMC)(X)] complexes (X ¼
ꢀ
was observed in the reverse reaction between 2 and 5a. The oxo- NCCH3, ꢀO2CCF3, ꢀN3, or SR).38 It was found that the OAT
transfer ability of 4 was found to be signicantly lower than that rates increased with the electrophilicity of the oxoiron(IV)
of its structural isomer 5, 4 being able to transfer its oxo group center, as reected by the FeIV/III potential observed by cyclic
only to 1a (Fig. S8†) and 3a, but not to 2a. So 3a is the universal voltammetry, which decreased as the axial ligand became more
oxo-atom acceptor in the series, while 5 is the most effective oxo- basic. In contrast, the HAT rates for the same series showed the
transfer donor. The intermetal oxo-transfer experiments thus opposite trend, where reaction rates increased with the basicity
conrm the reactivity order of 5 > 2 > 4 > 1 > 3 that was observed of the axial ligand. Clearly the trends observed in the present
for the oxidation of thioanisole. Therefore the relative OAT study do not follow those found in the TMC study. While both
reactivities of the FeIV(O) complexes appear to be independent of sets of OAT rates increase with higher FeIV/III potential, the HAT
the nature of the oxygen atom acceptor (i.e. thioanisole and rates have opposite trends. It should be noted that the TMC
[FeII(LN5)]2+), despite their having fundamentally distinct series entails a systematic change of only the ligand trans to the
chemical structures.
oxo group, while the present study involves changes in the
For comparison, the hydrogen atom abstraction rates of 1–5 ligands cis to the oxo group. These two sets of data suggest that
were also investigated with three substrates: 1,3-cyclohexadiene the factors governing the rate of H-atom abstraction are likely to
(CHD), 9,10-dihydroanthracene (DHA), and benzyl alcohol be complex and demand further scrutiny.
(Fig. S9–S13†). An earlier study of 1 and 2 found linear corre-
Unlike oxo-transfer, which is a 2-eꢀ oxidation process, H-
lations in plots of the logarithms of the second order oxidation atom abstraction generally involves rate determining transfer of
rate constants (normalized on a per hydrogen basis) for a large one electron coupled with a proton. Applying the Bordwell–
number of hydrocarbon substrates versus their DC–H values (81– Polanyi relationship to metal–oxo complexes, Mayer71–73 has
99 kcal molꢀ1),39,47 consistent with C–H bond cleavage by the argued that the rate of H-atom abstraction by metal–oxo
FeIV]O unit as the rate determining step. In addition, large complexes correlates linearly with the strength of the MO–H
kinetic isotope effects (KIE) were found for the cleavage of C–H bond (DO–H) that is formed. The DO–H value in turn is deter-
bonds of a number of hydrocarbon substrates. For this study, mined by the 1-eꢀ Mn+/(nꢀ1)+ (M ¼ transition metal ion) redox
large KIE values for the oxidation of DHA were observed for all potential and the pKa of the nascent O–H bond; thus the latter is
ve complexes (Table 3, Fig. S12 and S13†), demonstrating that an additional factor that can be modulated by the nature of the
C–H bond cleavage is a signicant component of the rate pentadentate supporting ligands, which vary in structure and in
determining step. Furthermore all KIE values were greater than the number of amine and pyridine donors. Unfortunately, the
the semi-classical limit of 7 (Table 3), suggesting that hydrogen pKa values of the FeIIIO–H species are not straightforward to
tunnelling likely plays a role in the H-atom abstraction step.
measure. On the assumption that Mayer’s approach applies for
Interestingly, the linear correlation observed for the log k2 the ve complexes in this study, the lower than expected C–H
values for thioanisole oxidation versus E1/2(IV/III) values does not bond cleaving reactivity of 4 and 5 may result from their cor-
extend to rates for C–H bond cleavage by 1–5 (Fig. 5, Table 3). responding FeIIIO–H species having pKa’s that are lower than
While the respective log k02 values associated with 1–3 for the those of 1–3.
oxidation of CHD, DHA, and benzyl alcohol respectively fall on
Alternatively, the deviation of the C–H bond cleaving rates
lines that are approximately parallel to the line obtained for thi- from what is expected by considering only the electrophilicity of
oanisole oxidation by all ve complexes, the bispidine complexes the oxoiron(IV) complex may arise from differences in electronic
4 and 5 deviate from this pattern and exhibit H-atom abstraction structure engendered by the different pentadentate ligands.
rates that are about an order of magnitude below what are pre- According to the two-state reactivity (TSR) concept developed by
dicted by these lines. Clearly, the oxo-transfer and H-atom Shaik,74–78 the rate of H-atom abstraction by an S ¼ 1 FeIV]O
abstraction reactions are not governed by a common set of factors. complex is governed by a more reactive low lying S ¼ 2 excited
Our observations can be compared with the results of state that becomes populated as the reaction traverses along the
two previous studies. In one investigation, Fukuzumi and co- reaction coordinate. DFT calculations on the [FeIV(O)(TMC)(X)]
workers studied the dependence of rate constants in OAT and series showed that the energy gap between the S ¼ 1 ground
288 | Chem. Sci., 2013, 4, 282–291
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state and the S ¼ 2 excited state decreased with increasing energies) and the question of whether these reactions are truly
basicity of the axial ligand, thereby making the least electro- concerted or instead are dominated by electron transfer or
philic complex in the series the most reactive for H-atom proton transfer. In fact, the observation that 4 and 5 have lower
abstraction.78 To determine whether the TSR rationale can KIEs for DHA oxidation than the other three complexes (Table 3)
explain the reactivity behavior of 1–5 requires a measurement of may suggest a HAT mechanism for the two bispidine complexes
the zero eld splitting (ZFS) of the S ¼ 1 ground states of the ve that is somewhat different from that of the other three
complexes. The zero eld splitting reects the extent of mixing complexes. Clearly, more studies are needed to clarify the
between the triplet ground state and the quintet excited state via mechanistic picture.
spin-orbit coupling, which can be estimated by determining the
In summary, we have characterized the structural and
zero eld splitting parameter D of each complex by high-eld spectroscopic properties of ve synthetic S ¼ 1 oxoiron(IV)
Mossbauer spectroscopy (with an uncertainly of ꢂ2–3 cmꢀ1) or, complexes 1–5 supported by pentadentate ligands with combi-
¨
with higher precision, by high frequency EPR spectroscopy nations of pyridine and amine donors. We have also measured
(ꢂ0.05 cmꢀ1).79 Carrying out such measurements on these their FeIV/III redox potentials by spectropotentiometry,
complexes would clearly be helpful.
providing for the rst time electrochemical determination of
Stereoelectronic effects may also play a role in modulating the E1/2(IV/III) redox potentials for a series of oxoiron(IV)
the H-atom abstraction rates of 1–5. From a consideration of the complexes. We have compared their redox properties with
frontier molecular orbitals (FMOs) of the S ¼ 1 oxoiron(IV) unit, respect to their abilities to carry out oxo-atom transfer and H-
it has been postulated that the attack of the substrate C–H bond atom abstraction. While the measured oxo-transfer rates to
by the oxoiron(IV) unit mainly involves the interaction between thioanisole nicely correlate with redox potential, this is not the
the p* MOs of the oxoiron(IV) unit and the target substrate C–H case for H-atom abstraction, demonstrating that the latter
bond, requiring an Fe]O–HC angle of 120ꢁ.56,80,81 On the other transformation is governed by factors more complex than for
hand, consideration of the FMOs of the S ¼ 2 oxoiron(IV) unit oxo-atom transfer.
suggests that C–H bond attack can also involve the s* MO that
would ideally require an Fe]O–HC angle of 180ꢁ. Based on a
Nuclear Resonance Vibrational Spectroscopy (NRVS) study of
Acknowledgements
1,82 Solomon postulated that the a-H-atoms on the pyridine This work was supported by grants from the US National
ligands present a barrier for the p-approach of substrate C–H Institutes of Health (GM-33162 to LQ and postdoctoral fellow-
bonds towards the oxoiron(IV) unit that would decrease the rate ship GM-75700 to TAJ), the US National Science Foundation
of H-atom abstraction. This notion was supported by the (CHE1058248 to LQ), the German Science Foundation (DFG to
observed higher reactivity of an S ¼ 1 oxoiron(IV) complex PC), MCYT of Spain, (CTQ2009-08464/BQU to MC and PhD
related to 1, where two of the pyridine donors were replaced grant to LG) and European Research Council (ERC-29910 to
with less bulky carboxylates.83 While similar considerations MC), and Generalitat de Catalunya (Icrea Academia to MC and
`
should apply to the other complexes, such stereoelectronic 2009SGR637). We thank Dr Yuming Zhou for his assistance in
factors may also be inuenced by ligand topology, a variety of obtaining stopped ow kinetic data for the oxidation of DHA at
which is represented by the pentadentate ligands that support room temperature. XAS data were collected on beamlines X9B
the Fe]O units in 1–5 (Scheme 1). In addition, the greater and X3B at the National Synchrotron Light Source (NSLS),
rigidity of the bicyclic bispidine ligand framework may impose Brookhaven National Laboratory. NSLS is supported by the U.S.
additional constraints to the reactivity of the Fe]O unit that Department of Energy, Office of Science, Office of Basic Energy
lead to the lower than expected reactivity of 4 and 5. Particularly Sciences, under Contract no. DE-AC02-98CH10886. NSLS
germane to the above discussion are two papers by Nam and beamlines X9B and X3B were supported by the National Insti-
co-workers. In one paper was compared the oxidative reactivity tute for Biomedical Imaging and Bioengineering under P41-EB-
of the topological isomers of [FeIV(O)(BQCN)(NCCH3)]2+ 001979. We thank Dr Alexander Ignatiev and Dr Sandeep Rekhi
(BQCN
¼
N,N0-dimethyl-N,N0-bis(8-quinolyl)cyclohexane-1,2- for technical assistance with our XAS experiments.
diamine) where the cis-a isomer was found to be much more
reactive than the cis-b isomer.84 In another paper a comparison
of [FeIV(O)(TMC)(NCCH3)]2+ and [FeIV(O)(TBC)(NCCH3)]2+
(TBC ¼ 1,4,8,11-tetrabenzylcyclam) was reported,22 where the
oxidative properties of the relatively unreactive TMC complex
were found to be enhanced 100-fold by the substitution of the
methyl groups by benzyl groups, demonstrating that simple
substitutions can have profound effects on reactivity.
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