Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline or Benzimidazole Ligands

Article abstract of DOI:10.1021/acs.inorgchem.5b00449

We have previously investigated cis/trans isomerization processes in phenoxido-bridged mixed-valent Fe^IIFe^III complexes that contain either one aniline or one anilide ligand. In this work, we compare the properties of similar complexes bearing one terminal protic ligand, either aniline or 1H-benzimidazole. Whatever the ligand, ¹H NMR spectroscopy clearly evidences that the complexes are present in CH₃CN as a mixture of cis- and trans-isomers in a close to 1:1 ratio. We show here that addition of NEt₃ indeed allows the deprotonation of these ligands, the resulting complexes bearing either anilide or benzimidazolide that are coordinated to the ferric site. The latter are singular examples of a high-spin ferric ion coordinated to a benzimidazolide ligand. Whereas the trans-isomer of the anilide complex is the overwhelming species, benzimidazolide species are mixtures of cis- and trans-isomers in equal proportions. Moreover, cyclic voltammametry studies show that Fe^IIIFe^III complexes with 1H-benzimidazole are more stable than their aniline counterparts, whereas the reverse is observed for the deprotonated species. (Graph Presented).

Full text of DOI:10.1021/acs.inorgchem.5b00449

Article
pubs.acs.org/IC
Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline
or Benzimidazole Ligands
†
,#
†,▽
́ ́
Angelique Troussier, Patrick Dubourdeaux, Martin Clem
ancey,†,§
†,‡
†
Eric Goure,
́
Michael
̈
Carboni,
†
,‡
†,‡
∥,⊥
∥,⊥
Nathalie Gon, Ramachandran Balasubramanian, Colette Lebrun, Jacques Pec
Genevieve Blondin,* and Jean-Marc Latour*
̀
́
aut,
,
†,‡
,†
†
‡
§
∥
CEA, iRTSV-LCBM-pmb, 17 rue des Martyrs, 38054 Grenoble, France
CNRS, iRTSV-LCBM-pmb, 17 rue des Martyrs, 38054 Grenoble, France
Univ. Grenoble Alpes, iRTSV-LCBM-pmb, 17 rue des Martyrs, 38054 Grenoble, France
CEA, INAC-SCIB-RICC, 38054 Grenoble, France
⊥
Univ. Grenoble Alpes, INAC-SCIB-RICC, 38054 Grenoble, France
S Supporting Information
*
ABSTRACT: We have previously investigated cis/trans
isomerization processes in phenoxido-bridged mixed-valent
II III
Fe Fe complexes that contain either one aniline or one
anilide ligand. In this work, we compare the properties of
similar complexes bearing one terminal protic ligand, either
1
aniline or 1H-benzimidazole. Whatever the ligand, H NMR
spectroscopy clearly evidences that the complexes are present
in CH CN as a mixture of cis- and trans-isomers in a close to
3
1
:1 ratio. We show here that addition of NEt indeed allows
3
the deprotonation of these ligands, the resulting complexes
bearing either anilide or benzimidazolide that are coordinated
to the ferric site. The latter are singular examples of a high-spin
ferric ion coordinated to a benzimidazolide ligand. Whereas the trans-isomer of the anilide complex is the overwhelming species,
benzimidazolide species are mixtures of cis- and trans-isomers in equal proportions. Moreover, cyclic voltammametry studies
III III
show that Fe Fe complexes with 1H-benzimidazole are more stable than their aniline counterparts, whereas the reverse is
observed for the deprotonated species.
1
0
INTRODUCTION
sequential synthesis allow the variation of the complexing
branch of each metal (see Chart 1). In this work, three
complexes analogous to 1 and further denoted as 5, 6, and 7
were investigated (Scheme 2). The protic ligand is either the
aniline group in 5 as in 1 or 1H-benzimidazole in 6 and 7.
When embedded in a bridging ligand, the protonation state of
■
Intervalence charge transfer (IVCT) has attracted continuous
interest among inorganic chemists for several decades. In most
cases, it occurs through a ligand bridging two metals in different
oxidation states, these metals being identical or not.
1
2
−5
We
recently reported that such an IVCT can be induced in a mixed-
II,III
valent Fe2
1H-benzimidazole was shown to influence the metal−metal
complex 1 by deprotonation of an aniline ligand
11−13
6
interactions.
1H-Benzimidazole groups may also act as
bound to the ferrous site, as illustrated in Scheme 1.
A similar process has been described only in an
heterodinuclear RuOs complex and in the semimet forms of
the dioxygen carrier hemerythrin, albeit in the latter case the
system switches between a ferrous pentacoordinate site and a
14,15
terminal ligands in dinuclear iron complexes.
that the protonation state of one histidine in the Rieske protein
is responsible for the shuttle of the iron−sulfur protein between
the two cytochromes in the bacterial cytochrome bc complex.
The choice of 1H-benzimidazole was dictated here by the
remote position of the exchangeable proton compared to that
of aniline. The other branch of the binucleating ligand is either
the N,N-bis((pyridin-2-yl)methyl)amine (BPA) in 6 as in 1 or
the N,N-bis((1-methylbenzimidazol-2-yl)methyl)amine
BMBA) in 5 and 7. In addition, the two iron ions are bridged
by the dicarboxylato ligand m-phenylenedipropionate. Com-
It is proposed
7
16
8
hydroxo-bound hexacoordinate ferric center. The thermody-
namic and kinetic aspects of the deprotonation of 1 were
investigated by spectroscopic and electrochemical techniques. A
thorough electrochemical study of its redox properties and H/
D labeling experiments have revealed that the deprotonation-
induced intervalence transfer is indeed a concerted proton−
(
9
electron transfer. Owing to the peculiarity of this system, it
appeared of interest to assess how the redox potentials of the
iron centers and the acidity of the protic ligand influence this
reaction. The modularity of the ligand constitution and its
Received: February 27, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00449
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Inorganic Chemistry
Article
Scheme 1. Valence Inversion in the cis- and trans-Isomers of the Fe FeIII Complex 1 Resulting from Deprotonation of the
II
Aniline Ligand
a
Scheme 2. Six Dicationic Mixed-Valent Fe2II,III Complexes
−7 Investigated in This Work
Chart 1. Scheme of the Ligands Used in This Work
2
triethylamine indeed leads to the deprotonation of the aniline
ligand in 5 or the 1H-benzimidazole ligand in 4, 6, and 7, the
resulting anilide, or benzimidazolide being coordinated to the
a
The subscripts refer to the labels of the complexes.
III
Fe ion.
plexes 2, 3, and 4 were also synthesized for the sake of
comparison (Scheme 2). We will show here that addition of
We have recently evidenced by a combination of NMR,
Mo
̈
ssbauer, and optical spectroscopies supported by DFT
B
DOI: 10.1021/acs.inorgchem.5b00449
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Inorganic Chemistry
Article
II
calculations that the aniline coordinated to the Fe ion in 1 and
III
the anilide coordinated to the Fe in 1′ may be in trans or cis
position with respect to the bridging phenoxide (see Scheme
1
7
1
). Moreover, the cis/trans ratios are different for the two
complexes, ca. 65:35 in 1 and ca. 15:85 in 1′, indicating that the
trans-isomer of 1 is slightly more acidic than the cis-isomer. We
will show here that similar isomers are present for complexes
5−7 as for the corresponding deprotonated species 6′ and 7′.
RESULTS
■
Synthesis of the Ligands and Complexes. The ligands
used in the present work (Chart 1) were synthesized according
to the already published procedures that were adapted for
inserting the desired N,N-bis((pyridin-2-yl)methyl)amine, N,N-
bis((1-methylbenzimidazol-2-yl)methyl)amine, N,N-bis((1H-
benzimidazol-2-yl)methyl)amine, N-((pyridin-2-yl)methyl)-N-
(
(2-aminophenyl)methyl)amine, and N-((pyridin-2-yl)-
methyl)-N-((1H-benzimidazol-2-yl)methyl)amine branches
(
2
see Supporting Information, Schemes S1 and S2). Complexes
1
8,19
−7 were prepared by the usual procedures.
They were
characterized by elemental analyses and ElectroSpray Ioniza-
tion−Mass Spectroscopy (ESI-MS) and other spectroscopic
techniques (see below).
X-ray Structures of 3(ClO4)2 and 4(ClO ) ·2CH CN.
4 2
3
Figure 1 displays the X-ray structure of the two dicationic
complexes 3 and 4. The crystal parameters are listed in
Supporting Information, Table S1 and a selection of distances
and angles in Supporting Information, Table S2. In both
complexes, the two iron ions are bridged by the oxygen atom of
the phenoxide of the deprotonated ditopic ligand and by the
2
−
two carboxylates of m-phenylenedipropionate (mpdp ). The
coordination spheres of the two metal ions are completed by
three nitrogen atoms from one tertiary amine and two imines
from the pyridine or the 1H- or 1-methylbenzimidazole groups.
The overall N O coordination around Fe1 and Fe2
3
3
corresponds to a rhombically distorted octahedron. The
metal−ligand bond lengths compare well with those
determined on similar complexes with either four pyridine
2
2
Figure 1. ORTEP drawing of the dicationic complexes 3 (part A)
and 4 (part B) with thermal ellipsoids drawn at the 50% probability
level. Only the hydrogen atoms bound to the nitrogen atoms of the
2+
20
groups as in [(bpmp)Fe (μ-OPr) ]
methylimidazole groups as in [(bimp)Fe (μ-O CPh) ] .
or with four N-
2
2
2+
21
II
III
1
H-benzimidazole rings in 4 are shown. The Fe and Fe ions are
drawn in green and pink, respectively.
2
2
2
The presence of the two perchlorate anions per diiron complex
II III
is in agreement with a mixed-valent Fe Fe composition. Table
Table 1. Fe−O1 and Average Fe−Ligand Bond Lengths (in
1
compares the Fe−O1 and the average Fe−ligand distances in
Å) in 3 and 4
the two complexes. In both systems, they are shorter for Fe2
than for Fe1, indicating that Fe1 is the ferrous site, while Fe2 is
the ferric one. It must be noticed that in 3, the Fe ion is in the
Fe1−O1
average Fe1−L
Fe2−O1
average Fe2−L
III
3
4
2.127(5)
2.157
2.122
1.944(5)
2.064
2.069
II
BPA cavity, while the Fe ion is coordinated by the BMBA
2.0745(18)
1.9588(17)
group. On the contrary, the BPA cavity in 4 is occupied by the
II
III
Fe ion, the Fe being bound to the two 1H-benzimidazole
groups. In addition, these groups are H-bonded with the
perchlorate anions (see Supporting Information Figure S1),
giving a slight benzimidazolide character to the ligands of Fe2
in 4, further stabilizing its +III oxidation state. The valence
locations are thus inverted in 3 and 4. Because of the better
donating character of the 1H- or 1-methylbenzimidazole
compared to that of the pyridine, the valence location in 3 is
counterintuitive and may result from solid state effects (see
below).
ppm domain. They are reminiscent of those reported for
analogous mixed-valent Fe Fe species with high-spin iron
II III
ions. The first notable feature is the observed full spectral
range: 340, 370, 365, 510, 435, and 380 ppm for complexes 2 to
7, respectively. With the exception of complex 5, the relatively
narrow spectral range (390 ± 50 ppm) is similar to that
2
+
20
observed in [(bpmp)Fe (μ-OPr) ] (390 ppm) and
2
2
2+
21
[(bimp)Fe (μ-OAc) ] (350 ppm) and indicates that the
2
2
iron valences are delocalized (at least partly) on the NMR time
scale. It has been previously demonstrated that the equatorial
methylene protons of the binucleating ligands are detected
1
1
H NMR Studies of Complexes 2−7. H NMR spectra
were recorded on CD CN solutions of complexes 2−5 and 7;
3
2+
20
for complex 6, a mixture of CD CN and d -DMF was used to
above 150 ppm in [(bpmp)Fe (μ-OPr) ]
and [(bimp)-
3
7
2
2
2+
21
ensure complete dissolution. They are shown in Supporting
Fe (μ-OAc) ] . Six such protons are present in 2, but only
2
2
Information, Figure S2, and Figure 2 focuses on the 480−130
three peaks are detected at 338, 199, and 170 ppm. This is fully
C
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Inorganic Chemistry
Article
spectroscopy. Mo
̈
ssbauer spectra were recorded at 80 K on
powder samples for complexes 2, 3, and 5 and on solutions for
complexes 4−7 (see Supporting Information, Figures S5 and
S6, respectively). DMF was added for the complete dissolution
of 6. All spectra can be successfully simulated by assuming two
doublets of equal area. The nuclear parameters are listed in
Table 2. The isomer shift and quadrupole splitting values are
Table 2. Isomer Shift (δ) and Quadrupole Splitting (ΔE )
Q
Values Used to Reproduce the 80 K Mo
Complexes 2−7
̈
ssbauer Spectra of
a
1
Figure 2. Four hundred eighty to 130 ppm domain of the H NMR
b
complex
δ (mm/s)
ΔEQ (mm/s)
Γfwhm (mm/s)
spectra of complexes 2−5 and 7 recorded in CD CN solutions and in
3
c
FeII
2
3
4
5
5
6
7
1.16
0.46
1.16
0.46
1.12
0.47
1.07
0.43
1.16
0.47
1.16
0.46
1.16
0.46
2.69
0.31
2.90
0.33
2.64
0.51
2.19
0.37
2.60
0.42
2.60
0.44
2.68
0.36
0.30
0.32
0.33
0.39
−0.53
0.41
−0.71
0.34
0.56
0.36
0.50
0.37
0.54
0.36
the CD CN/d -DMF solution for complex 6.
3
7
III
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
c
II
consistent with the delocalization of the iron valences. For 3
and 4, owing to the asymmetry of the ligand, the methylene
groups are nonequivalent, and indeed, six peaks are observed
above 140 ppm that can be associated with the equatorial
methylene protons.
III
II
d
c
III
II
III
II
By contrast, the larger spectral width observed for 5 is similar
d
d
d
6
,17
to that observed in 1 (ca. 550 ppm)
and in [(L-Bn)Fe (μ -
2 2
III
II
mpdp)(S)]2 (HL-Bn = 2-(N,N-bis((pyridin-2-yl)methyl)-
aminomethyl)-6-(N-((pyridin-2-yl)methyl)-N-((benzyl)-
aminomethyl)-4-methylphenol, S = CH CN or H O, 605
+
III
II
3
2
1
8,19
ppm),
indicating localized iron valences. The spectrum of 5
is indeed strongly reminiscent of that recorded in the ≈65:35
cis/trans isomer mixture of 1 (see Supporting Information
III
a
b
Each iron site accounts for 50% of the total area. The positive and
negative signs correspond to Lorentzian and pseudo-Voigt profiles,
respectively. Recorded on a powder sample. Recorded on frozen
1
7
Figure S3). It is worth noticing that an intense and narrow
peak is detected at −17 ppm for 5. A similar signal was
observed at −19 ppm for 1 that was indeed characteristic of the
c
d
solutions.
1
cis-isomer. Hence, the H NMR spectrum of 5 reveals the
similar to that observed for similar complexes and are fully
consistent with a Fe Fe composition, the two iron ions being
in the high-spin state. The iron valences are thus localized at
low temperatures. These simulations do not evidence the
presence of cis/trans mixtures for complexes 5−7. The two
isomers are expected to differ mainly in the quadrupole splitting
coexistence of two isomers that, by analogy with 1, we assign to
the cis and trans coordination of the aniline with respect to the
phenoxido bridge, the aniline being bound to the Fe site in 5.
Since the methylene protons in the trans-isomer of 1 are
associated with the narrowest peaks (see Supporting
Information, Figure S3), the same hypothesis applied to 5
allows one to estimate the cis/trans ratio to approximately
II III
II
II
17
of the Fe site as in 1. However, the large line width of the
ferrous site (Γ
of such a mixture.
≥ 0.50 mm/s) in 5−7 may hide the presence
fwhm
5
5:45. Thus, the cis-isomer is the major species in the two
aniline complexes 1 and 5.
Electrochemical Studies of Complexes 2−7. Cyclic
voltammograms were recorded on millimolar CH CN solutions
of complexes 2−7. They are shown in Supporting Information,
Figure S7. They all present two quasi-reversible waves
associated with the metal centered one-electron reduction
and one-electron oxidation of the mixed valence complexes
generating the corresponding diferrous and diferric species
For complexes 6 and 7, the high number of peaks detected
above 130 ppm also suggests the presence of two isomers, the
H-benzimidazole ligand being cis or trans to the bridging
phenoxide. The isomer ratio is close to 75:25 for 6 and to 60:40
for 7, but we have no indication so far to assert if it corresponds
to the cis/trans or trans/cis ratio. As mentioned above, the
spectral ranges suggest that 6 and 7 have delocalized iron
valences, the delocalization being more important in 7 than in
3
1
(
Chart 2).
6
. The same trend is observed when comparing 5 and 1 that
Chart 2
have the same nonprotic triamine branches, respectively.
Remarkably, these H NMR studies thus show that complexes
1
1
−7 exhibit a complete range of electronic (de)localization of
the Fe valences depending on the various combinations of
amine ligands (see below).
UV−Visible Studies of Complexes 2−7. The UV−visible
absorption spectra were recorded for complexes 2−7 and are
shown in Supporting Information, Figure S4. As expected, the
The values of the redox potentials are listed in Table 3. The
two redox processes are separated by ≈700 mV, a value
III
six complexes present a phenoxide-to-Fe charge transfer band
−1
−1
15,21,23,24
in the 500−600 nm region (ε = 645−1365 M cm ).
Mossbauer Studies of Complexes 2−7. To further
characterize the complexes and to get more insights into the
coordination spheres of the iron sites, we turned to Mossbauer
commonly determined for similar complexes.
The
1
̈
isomers detected by H NMR for complexes 5−7 appear to
behave identically in cyclovoltammetry. The values for complex
2
+
̈
2 are higher than those of [(bimp)Fe (μ-OAc) ] and lower
2 2
D
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Table 3. Redox Potentials (in V vs SCE) in Increasing Order
for Complexes 1−7 and Two Related Complexes
ligands on the tertiary
a
Ered1/2 Eox1/2
complex
amines
ref
[
(bimp)Fe (μ-
MeIm2
MeIm2
−0.22 0.44 21
2
OAc)2]2+
2
7
4
3
5
6
MeBzIm2 MeBzIm2
−0.18 0.52 this
work
MeBzIm2 Py/HBzIm −0.15 0.54 this
work
Py2
HBzIm2
−0.14 0.58 this
work
Py2
MeBzIm2
−0.12 0.58 this
work
MeBzIm2 Py/PhNH2 −0.09 0.62 this
work
Py2
Py2
Py2
Py/HBzIm −0.09 0.63 this
work
[(bpmp)Fe (μ-
Py2
−0.01 0.69 20
2
OPr)2]2+
1
Py/PhNH2 0.00
0.70
9
a
The ligands are connected to the tertiary amines by a CH group at
2
position 2.
2
+
than those of [(bpmp)Fe (μ-OPr) ] . As expected, values for
2
2
3
are in between those of 2 and those of [(bpmp)Fe (μ-
2
2
2+
OPr) ] . This ordering is in agreement with the lower
donating character of pyridine compared to that of 1-
methylbenzimidazole. Comparison of these redox potentials
for pairs of complexes confirms the expected trends and allows
one to quantify them. Substituting the BPA complexing arm by
BMBA leads to a decrease of the redox potentials in 5 and 7
compared to those in 1 and 6 (ca. −80 and −60 mV,
respectively). Similarly, replacing the aniline by the 1H-
benzimidazole facilitates oxidation according to the ca. 80 and
Figure 3. Titrations of 5 (part A) and of 7 (part B) in CH CN by
3
NEt . The dependences of the absorption at 728 nm for 5 (dark blue
3
squares) and at 528 nm for 7 (mauve dots) on the number of
equivalents of the base are reproduced in part C. The best fits (solid
lines, see text) obtained according to equation S2 (Supporting
Information) lead to Kapp = 12 ± 4 and 8.5 ± 1 for 5 and 7,
respectively.
60 mV, respective, potential decrease measured in 6 and 7
versus 1 and 5.
Deprotonation Studies of Complexes 4−7. The
deprotonation by triethylamine was investigated for complexes
4
−7 that contain a protic ligand, aniline for 5, and 1H-
1
in 4′, 6′, and 7′. This will be further confirmed by H NMR
benzimidazole for 4, 6, and 7. The process was evidenced first
by UV−visible spectroscopy and cyclic voltammetry and further
studied by H NMR and Mo
insight into the possible formation of isomers as for 1. The
deprotonated complexes will be denoted 4′−7′.
studies (see below). We propose this intense 530 nm
III
1
absorption to correspond to a benzimidazolide-to-Fe charge
̈
ssbauer spectroscopies to get an
transfer transition.
The variations of the absorption with the added equivalents
of base can be satisfyingly reproduced by assuming the presence
of two chromophores, the first corresponding to the acid form,
and the second to the basic one, that are in equilibrium owing
to the presence of NEt and of its conjugated acid Et NH (see
Chart 3). The simulations lead to Kapp = 4.8 ± 1, 12 ± 4, 7.5 ±
UV−Visible Studies of Complexes 4′−7′. Figure 3
displays the titrations of 5 and 7 by NEt , while those of 4 and
3
6
are reproduced in Supporting Information, Figure S8.
+
3
3
Addition of NEt induces an increase in the absorption that
3
maximizes at titration completion at 728 nm for 5′ (ε = 4650
7
28
−
1
−1
−1
−1
M
cm ), 532 nm for 4′ (ε = 3355 M cm ), and 528
5
32
−1 −1 −1
Chart 3. Definition of the Kapp Constant Associated with the
Deprotonation of 4−7 by NEt3
nm for 6′ (ε = 3330 M cm ) and 7′ (ε = 3875 M
528
528
−
1
cm ). These four transformations occur with at least one
isosbestic point, in agreement in every case with the presence of
only two chromophores. The titration of 5 is reminiscent of
6
II III
that of 1, suggesting that 5′ is the mixed-valent Fe Fe
complex with the anilide coordinated to the ferric site. As in
1
, deprotonation of the aniline in 5 must have induced a
valence inversion (see Scheme 1). The similarity of the
absorption spectra recorded at titration completion for 4, 6, and
7
1, and 8.5 ± 1 for complexes 4 to 7, respectively (see
Supporting Information for the theoretical expressions).
Complexes 1 and 5 bearing an aniline ligand are more acidic
(Kapp = 34 for 1) than complexes 4, 6, and 7 containing the
1H-benzimidazole ligand. These latter complexes present very
similar Kapp constants, suggesting that they are associated with
strongly suggests that the addition of NEt leads to the
3
17
deprotonation of the 1H-benzimidazole. As the anilide ligand is
III
coordinated to the Fe ion in 1′ and 5′, it is reasonable to
propose that the generated benzimidazolide binds the ferric ion
E
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Inorganic Chemistry
Article
the same reaction, the deprotonation by NEt of the 1H-
benzimidazole ligand.
Table 4. Oxidation Potential (in V vs SCE) in Increasing
3
Order for Complexes 1′ and 4′−7′
Electrochemical Studies of Complexes 4′−7′. Figure 4
a
Eox1/2
complex
ligands on the tertiary amines
ref
reproduces the variations of the cyclovoltammograms of 5 and
5′
1′
4′
7′
6′
MeBzIm2
Py2
Py/PhNH−
Py/PhNH−
HBzIm/BzIm−
Py/BzIm−
0.16
0.24
this work
9
b
Py2
0.34
this work
this work
this work
MeBzIm2
Py2
0.38
0.46
Py/BzIm−
a
The ligands are connected to the tertiary amines by a CH group at
2
b
position 2. Value corresponding to the average of the anodic and
cathodic peak values. The shape of the cyclovoltammograms indicates
a slow electron transfer at the electrode.
suggesting that the deprotonated diferrous species are unstable
9
and immediately (re)protonate as observed for 1′.
1
H NMR Studies of Complexes 4′−7′. To further
characterize the deprotonated complexes 4′−7′, we turned to
1
H NMR spectroscopy that was the single technique able to
differentiate the isomers in the parent complexes 5−7. The
II III
spectra of 4′−7′ are consistent with mixed-valent Fe Fe
species with high-spin ferrous and ferric ions. The spectrum
of 5′ is very similar to that of 1′ (see Supporting Information,
Figure S10). It expands over ca. 760 ppm, from 524 to −238
ppm, indicating that the iron valences are localized on the
NMR time scale. In addition, two signals are detected at −208
and −238 ppm. They are the signatures of the two ortho
III
6,17
protons of the anilide ring coordinated to the Fe site. The
close resemblance of the spectra of 5′ and 1′ strongly suggests
that the trans-isomer is also the overwhelming species for 5′:
the anilide in trans position versus the phenoxido bridge is
strongly favored. In the present case, the cis−trans isomer-
ization reaction in 5′ is too fast to be monitored, and the
proportion of the cis-isomer is too low to allow the
identification of its NMR signature.
1
Figure 5A reproduces the evolution of the H NMR spectra
of a CD CN solution of 4 in the 540−80 ppm domain upon
3
the addition of NEt (see Supporting Information Figure S11
Figure 4. Dependence of the cyclic voltammograms of 5 (part A) and
3
for the 145 to −30 ppm domain of the spectra). The spectrum
recorded at completion (see spectrum (g) in Figure 5A)
expands over ca. 515 ppm from 490 to −23 ppm. This suggests
that the iron valences are localized in 4′ on the NMR time
scale. We propose that the localization of the iron valences is
induced by the deprotonation, the generated benzimidazolide
being coordinated to the ferric ion. The variation of the
position of six peaks attributed to equatorial or axial H nuclei of
methylene groups (signals labeled A−D, F, and J in Figure 5A)
of the dinucleating ligand in 4 are shown in Figure 6A. These
six curves can be perfectly reproduced assuming a unique
constant Kapp (see Chart 3). The best fit leads to Kapp, 4 = 10 ±
6. This value also leads to a satisfying reproduction of the
variations of four other signals (see Supporting Information,
Figure S12). This value is consistent albeit larger with that
determined from UV−visible studies. From the analogy of the
7
(part B) upon the addition of increasing amounts of NEt (0, 0.5,
3
1.0, 1.5, and 2.1 equiv for 5 and 0 to 0.94 equiv by 0.1 for 7). The
unfilled green arrow indicates the scan direction. The 0.87 V peak
corresponds to the oxidation of excess NEt3.
7
upon the addition of NEt . The analogous experiments with 4
3
and 6 are shown in Supporting Information, Figure S9. The
main feature is the decrease of the wave associated with the
oxidation process to the benefit of a new quasi-reversible wave
detected at a lower potential. This new process may be
associated with the one-electron oxidation of the deprotonated
complexes 4′−7′ into diferric species (Chart 4). The associated
redox potentials are listed in Table 4.
Chart 4
II
III +
spectrum of 4′ with that of [Fe (L)(μ-O P(OPh) ) Fe ]
2
2 2
(
(
H L = 2-(N,N-((pyridin-2-yl)methyl)aminomethyl)-6-(N-
(pyridin-2-yl)methyl)-N-((2-hydroxyphenyl)methyl)amino-
2
methyl)-4-methylphenol), we tentatively assign the most
For 5, with an aniline ligand, the deprotonation leads to a
downfield shifted A−C signals in 4′ to the equatorial H
0
.46 V decrease of the oxidation potential that is identical to
that observed for 1. For 4, 6, and 7, this decrease is smaller, ca.
.2 V. By contrast, for all complexes, the reduction to the
diferrous state is essentially unaffected by the pH increase,
protons of the methylene groups located on the ferric side,
−
signal A originating from N−CH −PhO . The overall low
2
0
number of resonances is inconsistent with the expected
coexistence in solution of the cis- and the trans-isomers
F
DOI: 10.1021/acs.inorgchem.5b00449
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Dependence of a selection of NMR signals of 4 (part A) and
1
Figure 5. Evolution of the H NMR spectra in the 550−80 ppm
7
(part B) upon the addition of NEt . The labels of the signals refer to
3
domain upon addition of increasing amounts of NEt to CD CN
solutions of 4 (part A) and 7 (part B). The numbers of added
equivalents of base are for 4 (a) 0, (b) 0.30, (c) 0.90, (d) 1.50, (e)
.80, (f) 2.26, and (g) 3.01 and for 7 (a) 0, (b) 0.21, (c) 0.41, (d) 0.62,
3
3
that in Figure 5. The solid and dotted lines reproduce the best fits
obtained according to equation S10 (Supporting Information) with
Kapp = 10 ± 6 for 4 and 6.1 ± 1.5 for 7. In part B, the closed circles and
solid lines are associated with the signals of isomer 1 and the closed
squares and dotted lines to those of isomer 2.
1
(
e) 0.82, (f) 1.03, (g) 1.54, (h) 2.06, (i) 4.12, and (j) 6.18.
which should result from the deprotonation of a single 1H-
benzimidazole group. This contradiction can be resolved if one
considers that the two isomers differ only by a single proton
bound to the cis- or trans-benzimidazole group. It is conceivable
that this proton may exchange between the 1H-benzimidazole
and the benzimidazolide groups rapidly on the NMR time scale,
thus averaging the number of peaks (see Supporting
Information, Scheme S3).
vs 4′. This suggests that 6′ and 7′ exist in solution as cis- and
trans-isomers with distinctive NMR signatures. Assuming that
the two most downfield signals of 6′ and 7′ located above 450
ppm are associated with the same proton but in different
isomers, we estimate the isomer 1/isomer 2 ratio to be close to
30:70 in 6′ and 40:60 in 7′. The presence of d -DMF for the
7
complete dissolution of 6 leads to peak broadening, thus
preventing a thorough analysis of the titration experiment.
Consequently, we will focus here on the titration of 7. By
analogy with 4′, the equatorial H nuclei of methylene groups of
the binucleating ligand on the ferric side in 7′ would
correspond to signals A1−C1 (respectively A2−C2) for isomer
Similar NMR titration experiments were performed on 6 and
. The full-scale spectra are shown in Supporting Information,
7
Figures S13 and S14, respectively. Figure 5B focuses on the
5
20−80 ppm window for the titration of 7. As previously
observed for the titration of 4, the spectra at completion expand
over a large width, ca. 515 ppm for the CD CN/d -DMF
1 (respectively isomer 2), with signals A1 and A2
−
corresponding to the N−CH −PhO group. Figure 6B
3
7
2
solution of 6′ and ca. 550 pm for the CD CN solution of 7′.
reproduces the evolution of the peak position for 12 signals,
6 for each isomer. These evolutions indicate that the isomer
ratio remains constant during titration. Accordingly, the
3
The deprotonated species thus present iron valences localized
on the NMR time scale. As for 4′, the localization of the iron
valences would originate from the deprotonation, and we
simultaneous fit of the 12 curves in Figure 6B led to Kapp, 7
=
III
propose that the benzimidazolide is coordinated to the Fe ion
6.1 ± 1.5, in agreement with the above UV−visible
investigations (see Supporting Information for theoretical
equations). This analysis is further confirmed by the fit of the
variation of the peak position of other protons (see Supporting
Information, Figure S15). A definite assignment of a set of
in 6′ and 7′. The comparison between the spectrum (g) of 4′ in
Figure 5A on the one hand and spectrum (j) of 7′ in Figure 5B
or spectrum (i) of 6′ in Supporting Information, Figure S14 on
the other hand evidences a doubling of the signals in 7′ and 6′
G
DOI: 10.1021/acs.inorgchem.5b00449
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
resonances to the cis- or trans-isomer is not possible since the
benzimidazolide group lacks typical signatures as found in
and confirms the presence in solution of two forms as in 1.
Changing the protic ligand from aniline to 1H-benzimidazole
leads to interesting modification of the redox behavior.
Two Forms Are the cis- and trans-Isomers. The NMR
spectra recorded on solutions of complexes 5−7 are typical of
6
,17
10
anilide
or phenoxide terminal ligands in the trans-isomer
II III
of Fe Fe complexes.
Mo
̈
ssbauer Studies of Complexes 4′−7′. NMR spec-
II III
troscopy allows the distinction of the two isomers in 7′ and 6′,
mixed-valent Fe Fe complexes with high-spin metal ions. The
and the identification of the single one in 5′ as the trans-isomer.
number of peaks is inconsistent with a single complex present
in solution. We have previously demonstrated for complex 1
that the two forms in equilibrium do not correspond either to
valence isomers or to substitution of the aniline by an
acetonitrile or a water molecule but to isomers that differ in
the cis or trans coordination of the aniline ligand bound to the
̈
To further characterize these isomers, we recorded Mossbauer
spectra on solutions at the end of the titration for the four
̈
complexes 4−7. 80 K Mossbauer spectra of 4′−7′ are
reproduced in Supporting Information, Figure S16. These
four spectra can be simulated with two doublets of equal area.
The nuclear parameters are listed in Table 5. The isomer shift
II
Fe with respect to the phenoxido bridge. The similarity
between spectra of 1 and 5 allows one to conclude that
analogous isomers exist for 5. The same arguments apply to the
1H-benzimidazole analogues, and consequently, the two forms
detected for 6 and 7 also correspond to the cis- and trans-
isomers, the isomerization involving the 1H-benzimidazole
ligand. Addition of NEt3 allows the generation of the
deprotonated species 6′ and 7′ that clearly present a
coordinated benzimidazolide. The in depth analysis of the
NMR titration of 7 allows the determination of the equilibrium
constant associated with the deprotonation of the two forms by
Table 5. Isomer Shift (δ) and Quadrupole Splitting (ΔE )
Q
Values Used to Reproduce the 80 K Mo
̈
ssbauer Spectra of
a
Solutions of Complexes 4′−7′
b
Complex
FeII
δ (mm/s)
ΔEQ (mm/s)
Γfwhm (mm/s)
4′
5′
6′
7′
1.13
0.52
1.21
0.51
1.07
0.46
1.17
0.46
2.51
1.72
2.90
1.58
2.34
0.52
2.83
0.51
−0.54
0.36
III
Fe
Fe
Fe
Fe
Fe
Fe
Fe
II
−0.36
0.38
III
II
0.58
NEt . No significant difference between them was evidenced,
3
III
II
0.50
strongly suggesting that the resulting two forms of the
deprotonated complex 7′ are also the cis- and trans-isomers.
Pyridine vs 1-Methylbenzimidazole. As indicated above,
substitution of BPA by BMBA has little or no influence on the
isomer ratio either in the protonated species, 1 vs 5 and 6 vs 7,
or in the deprotonated ones, 1′ vs 5′ and 6′ vs 7′.
Consequently, the acid strengths of 5 and 7 are nearly identical
to that of 1 and 6, respectively. The 0.4 difference in the pKa
unit for 1 and 5 determined from the UV−visible titrations is
just slightly above the uncertainty of the measurements. Cyclic
voltammetry studies performed on 5−7 and previously on 1
show that this substitution leads to a stabilization of the +III
iron oxidation state. The same ca. 70 mV decrease in the redox
0.40
III
0.41
a
b
Each iron site accounts for 50% of the total area. The positive and
negative signs correspond to Lorentzian and pseudo-Voigt profiles,
respectively.
values are perfectly consistent with a mixed-valent composition
II
III
with Fe and Fe ions in the high-spin state, in agreement with
1
H NMR studies. To the best of our knowledge, deprotonation
of 1H-imidazole coordinated to Fe usually leads to low-spin
III
ferric imidazolide species with the exception of [Fe(Ph-
+
III
Himap)2] where the Fe remains high-spin in [Fe(Ph-
Himap)(Ph-imap)] (Ph-Himap = 4-(2-hydroxyphenylimino-
II III
III III
potential of the Fe Fe /Fe Fe couple is observed when
comparing 1′ with 5′, 6′ with 7′, and 3 with 2. This is fully
consistent with the better donating character of the 1-
methylbenzimidazole ligand compared to that of the pyridine.
Aniline vs 1H-Benzimidazole. Substituting the aniline
group by the 1H-benzimidazole also leads to a stabilization of
the +III oxidation state. Indeed, both the reduction and
oxidation waves of the protonated complexes 1 and 5 are
shifted by ca. 75 mV toward more negative potentials in 6 and 7
(see Table 3). Upon deprotonation, the potential of the
oxidation couple is displaced toward lower values for these four
complexes. It is worth noticing that the shift is more important
for the aniline-based complexes 1 and 5 (ca. 460 mV) than for
1H-benzimidazole-based species 6 and 7 (ca. 160 mV).
Accordingly, the oxidation processes of 6′ and 7′ are detected
at potentials ca. 220 mV higher than those of 1′ and 5′.
Consequently, the bisferric complexes are more favored with
anilide than with benzimidazolide ligand. In addition, the ca.
25
methyl)-2-phenylimidazole). The main difference between
complexes 4′−7′ is the quadrupole splitting value of the ferric
II/III
site. We and others have previously evidenced that [Fe (μ-
III 2+/3+
−
OR)(μ-O X) Fe ]
complexes (RO = alkoxido or
2
2
−
phenoxido, XO2 = carboxylato or phosphato) containing an
additional negatively charged ligand on the ferric site in trans
position with respect to the phenoxido or alkoxido bridge
III
exhibit unusually large quadrupole splitting values for the Fe
6
,17,18,26−30
ion.
Smaller values are measured for cis-iso-
1
0,17,26,27,31,32
mers.
Accordingly, the values listed in Table 5
suggest that a single isomer is present at low temperature for
4
′−7′. It is the trans-isomer for 4′ and 5′, while it is the cis-
isomer for 6′ and 7′. This conclusion is fully consistent with
NMR studies for 5′. By contrast, for 6′ and 7′, two isomers
were detected by NMR; it is thus possible that one isomer is
favored during the sample cooling required for Mo
experiments.
̈
ssbauer
1
60 mV drop of the oxidation process in 6 and 7 upon
DISCUSSION
■
deprotonation is smaller than the ca. 230 and 250 mV decrease
The study of complex 1 enabled us to evidence a pH-controlled
valence inversion involving first-row transition metal ions and
two isomerization processes in dinuclear complexes. Mod-
ifications of the binucleating ligand allow the synthesis of a
observed in the dinuclear complexes [(bpp)FeS Fe-
2
2−
33,34
−
(dtbp)]
(bpp = phenyl-benzimidazol-benzimidazolide,
2−
Pr
dtbp = 2,2′-dithiolide-1,1′-biphenyl) and [Fe S ( bbim)-
2
2
Pr
2−35
Pr
( bbimH)]
( bbimH = 4,4-bis(benzimidazol-2-yl)-
2
II III
series of mixed-valent Fe Fe complexes. NMR spectroscopy
reveals delocalization of the iron valences to different extents
heptane) where the metal ions are in a tetrahedral environment.
It is roughly half the one that has been estimated from the
H
DOI: 10.1021/acs.inorgchem.5b00449
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
0
conversion of mononuclear high-spin ferrous complexes with
three imidazole ligands into mononuclear low-spin ferric
complexes with three imidazolides (ca. 300 mV per proton
detection as described previously. The elemental analyses have been
performed in the Centre Regional de Mesures Physiques pour l’Ouest
University of Rennes 1, France) and at the Service de Microanalyse of
the Departement de Chimie Moleculaire (University of Grenoble-
́
(
2
3,36,37
́
́
release).
In the latter case, this difference may originate
III
Alpes, France). Mass spectra were obtained in the electrospray
ionization mode on a LCQ-Finnigan Thermoquest spectrometer
equipped with an octupolar analyzer and an ion trap. Electronic
absorption spectra were recorded in acetonitrile (0.1−0.2 mM
solutions with 1 cm optical length cuvettes) on a Varian Cary 50
from the high-spin state of the Fe center in 6′ and 7′.
Titrations monitored by UV−visible absorption spectroscopy
have shown similar acidity for the three complexes 4, 6, and 7.
+
38
Using pK (Et NH ) = 18.6 in CH CN and assuming a 7.5
a
3
3
pH unit decrease of the pK in water compared to that in
spectrometer. Mossbauer spectra were recorded on 2−4 mM solutions
̈
a
3
9
57
acetonitrile, one gets a value of 10.3 ± 0.1 for the pK of these
of nonenriched complexes except for 4 and 4′ that were fully Fe-
enriched. The experiments were performed using a horizontal
transmission cryostat (Oxford Instruments) and a 50 mCi source of
a
three complexes in water. The 1H-benzimidazole ligand is less
acidic by ca. 7 and 10 pK units in the mixed-valent complexes
a
57
6
Pr
Pr
−
35
2− 33,34
Co(Rh) as previously described. All velocity scales and isomer shifts
[
Fe S ( bbim)( bbimH)] and [(bpp)FeS Fe(dtbp)] ,
2 2 2
are referred to metallic iron foil at room temperature. Analysis of the
data was performed with the software WMOSS4Mossbauer Spectral
Analysis Software; www.wmoss.org, 2012.
respectively. It is also more basic, however, to a weaker extent
than the imidazole ligand in the reduced Rieske protein. The
pK -value compares well with that of the 1H-imidazole ring
determined for the microperoxidase 11 complex and for the
three successive deprotonations of mononuclear ferric com-
plexes bearing three imidazoles. It is also close to that
determined for [Co (en) (OH)(ImH)] and similar com-
plexes. Titration of 5 leads to Kapp = 12 and thus to a pKa
value of 5 in water close to 10.0. This value is just slightly above
16
a
Diffraction data were taken using an Oxford-Diffraction XCalibur S
kappa geometry diffractometer (Mo−Kα radiation, graphite mono-
chromator, λ = 0.71073 Å). To prevent evaporation of cocrystallized
solvent molecules, the crystals were coated with light hydrocarbon oil,
and the data were collected at 150 K. The cell parameters were
obtained with intensities detected on three batches of 5 frames. The
crystal−detector distance was 4.5 cm. The number of settings and
frames has been established taking in consideration the Laue symmetry
40
23
III
2+
2
4
1
9
that previously determined for 1 (9.6). Surprisingly, the four
45
of the cell by CrysAlisPro Oxford-diffraction software. Four hunded
complexes 4−7 present similar acid strengths, whereas aniline is
ten for 3 and 498 for 4 narrow data were collected for 1° increments in
1
3
4 orders of magnitude less acid than 1H-benzimidazole (pK =
a
42
ω with a 320 s exposure time for 3 and 50 s for 4. Unique intensities
45
0.6 for aniline and 16.4 for 1H-benzimidazole in DMSO). It
detected on all frames using the Oxford-diffraction Red program
is well-known that coordination of a protic molecule to a
transition metal ion increases its acidity by several orders of
were used to refine the values of the cell parameters. The substantial
redundancy in data allows empirical absorption corrections to be
applied using the ABSPACK Oxford-diffraction program for both
II
2+
magnitude. For instance, the pK s of [Fe (OH ) ] and
a
2 6
4
5
III
3+
compounds. Space groups were determined from systematic
absences, and they were confirmed by the successful solution of the
structure. The structures were solved by a charge flipping method
[
Fe (OH ) ] have been determined to 9.5 and 2.2,
2 6
43
respectively. Aniline is expected to present a behavior similar
to that of water owing to the exchangeable proton bound in
both cases to the atom coordinating the metal ion. This acidity
enhancement is expected to be weaker for 1H-benzimidazole
46
using SUPERFLIP software. All non-hydrogen atoms were found by
2
47
and refined on F . Hydrogen atoms were found by difference Fourier
syntheses and refined for 3 and 4, except for the hydrogen atoms of
the acetonitrile methyl group which were fixed in ideal position and
refined with a riding model.
44
owing to the remote exchangeable proton. Serendipitously,
this dissimilarity in the nature of the exchangeable proton leads
to comparable acid strengths in the mixed-valent complexes
investigated here.
Electrochemical measurements were carried out using a PAR model
273 potentiostat. All electrochemical experiments were run under an
argon atmosphere in a glovebox, using a standard three-electrode
electrochemical cell. All potentials were referred to a Ag/AgCl-
CONCLUSIONS
■
(saturated) reference electrode in an acetonitrile/0.1 M tetra-n-
This work has allowed a comparison of the properties of aniline
and 1H-benzimidazole as terminal ligands to an iron ion.
Interestingly, 1H-benzimidazole stabilizes the Fe Fe redox
butylammonium perchlorate (TBAP) electrolyte. The potential of the
+
regular ferrocene/ferrocenium (Fc/Fc ) redox couple used as an
III III
internal standard was 0.36 V under our experimental conditions. The
working electrodes were platinum disks polished with 1 μm diamond
paste that were 5 mm in diameter for cyclic voltammetry experiments.
state by 75 mV with respect to aniline. However, in the
deprotonated forms, the reverse is observed since anilide
III III
stabilizes the Fe Fe redox state by 230 mV with respect to
1
H-benzimidazolide. This inversion results from a larger
ASSOCIATED CONTENT
■
III III
stabilization of the Fe Fe redox state by deprotonation of
aniline (460 mV) with respect to 1H-benzimidazole (160 mV).
This difference can have two origins: (i) the delocalization of
the charge in benzimidazolide, which will lessen the interaction
with the Fe ion and (ii) the ability of the anilide to engage in a
six-membered ring by coordination to Fe , whereas 1H-
*
S
Supporting Information
Description of the syntheses of the ligands and complexes;
1
crystallographic data of 3 and 4; and additional H NMR, UV−
III
̈
visible, and Mossbauer spectra and cyclovoltammograms. The
III
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.5b00449.
benzimidazolide is restricted to a less flexible five-membered
ring.
AUTHOR INFORMATION
■
EXPERIMENTAL SECTION
General Methods. Materials. Acetonitrile for spectroscopic
measurements was distilled under argon over CaH and stored within
Corresponding Authors
*(G.B.) E-mail: genevieve.blondin@cea.fr.
*(J.-M.L.) E-mail: jean-marc.latour@cea.fr.
■
2
an inert atmosphere glovebox. All other reagents were of reagent grade
Present Addresses
(E.G.) Univ. Grenoble Alpes, DCM/CIRE, 38041 Grenoble
Cedex 9, France.
quality and used as received.
#
Physicochemical Studies. 1H NMR spectra were recorded in
deuterated acetonitrile on a Bruker AC 200 with a 5 mm indirect
I
DOI: 10.1021/acs.inorgchem.5b00449
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
▽(
M.C.) CEA, ICSM/LHYS, Site de Marcoule, 30207 Bagnols
Article
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
̈
The Agence Nationale de la Recherche (Grants ANR 2010
BLAN 703 and ANR-11-LABX-0003-01) and the Reg
Rhone-Alpes (Grants CIBLE 07 016335 and CIBLE 08
19180) are gratefully acknowledged for their financial support.
1907−1914.
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DOI: 10.1021/acs.inorgchem.5b00449
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