Electrochemical polymerization of iron(III) polypyridyl complexes through C-C coupling of redox non-innocent phenolato ligands

Article abstract of DOI:10.1021/acs.inorgchem.6b02378

Phenolato moieties impart redox flexibility to metal complexes due their accessible (oxidative) redox chemistry and have been proposed as functional ligand moieties in redox non-innocent ligand based transition metal catalysis. Here, the electro- and spectroelectrochemistry of phenolato based μ-oxodiiron(III) complexes [(L¹)Fe(μ-O)Fe(L¹)]²⁺ (1) and [(L²)Fe-(μ-O)Fe(L²)]²⁺ (2), where L¹ = 2-(((di(pyridin-2-yl)methyl)-(pyridin-2-ylmethyl)amino)methyl)phenol and L² = 3, 5-di-tert-butyl-2-(((di(pyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino)-methyl)phenol, is described. The electrochemical oxidation of 1 in dichloromethane results in aryl C-C coupling of phenoxyl radical ligand moieties to form tetra nuclear complexes, which undergo subsequent oxidation to form iron(III) phenolato based polymers (poly-1). The coupling is blocked by placing tert-butyl groups at para and ortho positions of phenol units (i.e., 2). Poly-1 shows two fully reversible redox processes in monomer free solution. Assignment of species observed during the electrochemical and chemical {(NH₄)₂[Ce^IV(NO₃)₆]} oxidation of 1 in acetonitrile is made by comparison with the UV-vis-NIR absorption and resonance micro-Raman spectroelectrochemistry of poly-1, and by DFT calculations, which confirms that oxidative coupling occurs in acetonitrile also. However, in contrast to that observed in dichloromethane, in acetonitrile, the oligomers formed are degraded in terms of a loss of the Fe(III)-O-Fe(III) bridge by protonation. The oxidative redox behavior of 1 and 2 is, therefore, dominated by the formation and reactivity of Fe(III) bound phenoxyl radicals, which considerably holds implications in regard to the design of phenolato based complexes for oxidation catalysis.

Full text of DOI:10.1021/acs.inorgchem.6b02378

Article
pubs.acs.org/IC
Electrochemical Polymerization of Iron(III) Polypyridyl Complexes
through C−C Coupling of Redox Non-innocent Phenolato Ligands
†
‡,§
,†
Duenpen Unjaroen, Marcel Swart,
and Wesley R. Browne*
†
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
‡
Institut de Química Computacional i Catal
̀
isi (IQCC), Departament de Química, Universitat de Girona, Campus Montilivi, 17003
Girona, Catalonia, Spain
§
ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
S Supporting Information
*
ABSTRACT: Phenolato moieties impart redox flexibility to
metal complexes due their accessible (oxidative) redox chemistry
and have been proposed as functional ligand moieties in redox
non-innocent ligand based transition metal catalysis. Here, the
electro- and spectroelectrochemistry of phenolato based μ-oxo-
1
1
2+
2
diiron(III) complexes [(L )Fe(μ-O)Fe(L )] (1) and [(L )Fe-
2
2+
1
(
(
μ-O)Fe(L )] (2), where L = 2-(((di(pyridin-2-yl)methyl)-
pyridin-2-ylmethyl)amino)methyl)phenol and L = 3,5-di-tert-
2
butyl-2-(((di(pyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino)-
methyl)phenol, is described. The electrochemical oxidation of 1
in dichloromethane results in aryl C−C coupling of phenoxyl
radical ligand moieties to form tetra nuclear complexes, which
undergo subsequent oxidation to form iron(III) phenolato based polymers (poly-1). The coupling is blocked by placing tert-butyl
groups at para and ortho positions of phenol units (i.e., 2). Poly-1 shows two fully reversible redox processes in monomer free
IV
solution. Assignment of species observed during the electrochemical and chemical {(NH ) [Ce (NO ) ]} oxidation of 1 in
4
2
3 6
acetonitrile is made by comparison with the UV−vis−NIR absorption and resonance micro-Raman spectroelectrochemistry of
poly-1, and by DFT calculations, which confirms that oxidative coupling occurs in acetonitrile also. However, in contrast to that
observed in dichloromethane, in acetonitrile, the oligomers formed are degraded in terms of a loss of the Fe(III)-O-Fe(III)
bridge by protonation. The oxidative redox behavior of 1 and 2 is, therefore, dominated by the formation and reactivity of Fe(III)
bound phenoxyl radicals, which considerably holds implications in regard to the design of phenolato based complexes for
oxidation catalysis.
INTRODUCTION
within metal complexes with the goal of balancing the redox
chemistry of the metal center with that of the ligand. The one-
electron oxidation of these metal-phenolato complexes leads to
species described as either a higher valent metal species
■
The redox chemistry of transition metal complexes has seen
rapid progress recently driven by both a desire to understand
the inner workings of bioinorganic systems and the necessity to
achieve sustainability in chemical processes, and especially
n+1
−
n •
(
M
L ) or ligand radicals (M L ) that are essentially
11,12
1
,2
electronic isomers of each other.
catalysis. These efforts have provided insight into mecha-
The study of metal-bound phenoxyl radicals is of special
nisms for many homogeneous catalytic and enzymatic
3
4
interest in the context of biological systems, not least the
reactions. The role played by redox non-innocent ligands in
13
1
,5
enzyme galactose oxidase (GO). The structure of the GO
copper(II) binding site, in its inactive form, contains two
imidazole rings (histidine residues), two phenol moieties
biological systems has been recognized for many decades,
and such an approach is increasingly seeing application in
homogeneous catalysis, not least because of the role of such
11,14
6
(tyrosine residues), and an acetato ligand.
In its active
ligands, e.g., tyrosine, in enabling two-electron redox processes
4
,7
form, the formal oxidation state of GO has been assigned as
copper(III); however, its spectroscopic properties suggest
assignment of a copper(II)-phenoxyl radical structure is more
using first-row transition metals.
8
,9
The phenolato moiety, in particular, imparts substantial
redox flexibility to metal complexes due to the accessible redox
chemistry of the phenolate unit itself. Phenols, such as tyrosine,
undergo one- and two-electron oxidations readily; however,
outside of the protective environment of an enzyme, they
equally readily undergo bimolecular coupling reactions to form
6
appropriate.
Inspired by phenolato based enzymes, phenolato based
11
Fe(II) complexes have been applied in oxidation catalysis, for
10
oligomeric and polymeric structures. Efforts to tame the
Received: September 29, 2016
redox chemistry of phenols has focused on their incorporation
©
XXXX American Chemical Society
A
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Figure 1. Structures of complexes 1 and 2.
Scheme 1. Mechanism for Oxidative Dimerization of 1 To Form a 4,4′-Bisphenolato Bridged-Bis-μ-oxo-diiron(III) Polypyridyl
Complex (3) That Subsequently Undergoes Coupling at the Other Phenolato Units To Form Oligomers (i.e., Poly-1)
example, with O for catechol C−C bond cleavage by
O-Fe(III) bridge in the binuclear complex and thereby results
in the formation of well-defined polymer films containing 4,4′-
bisphenolato bridged diiron complexes (poly-1). The polymer
formed undergoes highly reversible electrochemical oxidation
to singly and doubly oxidized states, in which the oxidation is
localized on the 4,4′-bisphenolato core and shows good stability
under ambient conditions. Comparison of spectroscopic data
obtained upon electrochemical and chemical oxidation of 1 in
acetonitrile with data obtained from thin films of poly-1
confirms that a similar oxidative coupling occurs in acetonitrile
also; however, the integrity of the Fe(III)-O-Fe(III) core of the
C−C coupled complexes (3) is compromised. Hence,
formation of polymer films (poly-1) by further C−C coupling
of 3 is precluded due to the instability of the oligomers formed
initially. The oxidatively driven C−C coupling is blocked
2
1
5,16
Velesumay et al.,
and in the μ-oxo-diiron(III) polypyridyl
1
7
complex (1), based on the ligand 2-(((di(pyridin-2-yl)-
methyl)(pyridin-2-ylmethyl)amino)methyl)phenol, in which
each of the Fe(III) centers is coordinated to a phenolato
ligand (Figure 1), applied over a decade ago by Feringa et al. to
the selective oxidation of benzyl alcohols to aldehydes with
1
8
H O . In the latter case, the activity of the complex was
2
2
ascribed tentatively to the involvement of the phenolato
moiety. However, the stability of phenol based ligands in
oxidation reactions, outside of the confines of an active site of
an enzyme, is uncertain and indeed coupling of oxidized
12,19,20
21
phenolato ligands,
as well as phenols, has been noted in
the literature already. Indeed, the oxidation of phenols
catalyzed by transition metal complexes is a key mechanistic
22
10
tool in oxidation chemistry.
effectively by the incorporation of tert-butyl groups at the
The challenge faced in biomimetic oxidation catalyst systems
is to balance the intrinsic reactivity of the oxidized ligand,
specifically radical dimerization, with the stabilization of
formally high metal oxidation states (Fe(IV)). In particular, it
is essential to avoid the oxidatively driven dimerization of the
phenol ligand.
Here, we report that electrochemical oxidation of 1 results in
sufficient phenoxyl radical character for aryl C−C coupling of
the phenol units to take place (Scheme 1). In dichloromethane,
the coupling between phenoxyl radical units occurs at both 200
and 293 K without compromising the integrity of the Fe(III)-
para and ortho positions of the phenol units (i.e., 2).
EXPERIMENTAL SECTION
■
Synthesis. The ligand 2-(((di(pyridin-2-yl)methyl)(pyridin-2-
1
1
ylmethyl)amino)methyl)phenol (HL ) and the complex [(L )Fe(μ-
1
O)Fe(L )](ClO ) (1) were prepared and isolated as previously
4
2
18
reported. The ligand 3,5-di-tert-butyl-2-(((di(pyridin-2-yl)methyl)-
2
(
[
pyridin-2-ylmethyl)amino)methyl)phenol (HL ) and the complex
2
2
(L )Fe(μ-O)Fe(L )](ClO ) (2) were prepared and isolated using an
4 2
18
analogous synthetic route to that reported for 1 and related
2
3
compounds. See the Supporting Information for details and
B
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characterization (Figures S1−S18). Commercially available chemicals
were used without further purification unless stated otherwise.
Solvents for electrochemical and spectroscopic measurements were
UVASOL (Merck) grade or better. For details for physical and
computational methods used, see the Supporting Information.
Caution! Perchlorate salts of metal complexes incorporating organic
ligands are potentially explosive. These compounds should be prepared in
small quantities and handled with suitable protective safeguards.
RESULTS AND DISCUSSION
■
[
(
Structure and Spectroscopic Characterization of
1
1
2
2
(L )Fe(μ-O)Fe(L )](ClO ) (1) and [(L )Fe(μ-O)Fe(L )]-
4 2
1
1
ClO ) (2). The ligand HL and the complex [(L )Fe(μ-
4
2
1
O)Fe(L )](ClO ) (1) have been reported previously together
Figure 2. UV/vis absorption spectra of 1 (black) and 2 (red), 0.075
mM in acetonitrile.
4
2
1
with single-crystal X-ray structural determination of [(L )Fe(μ-
1
18
2
O)Fe(L )](PF ) . HL and 2 were prepared by analogous
6
2
1
methods to HL and 1, respectively. Synthesis and detailed
FTIR and H NMR characterization of HL , HL , 1, and 2 are
charge-transfer transition (LMCT) based on resonance Raman
spectroscopy (Figures S21−S23) and DFT (Figures S25, S26
and Table S2). The absorption band at 343 nm is assigned to a
mixture of phenolato-to-Fe(III) and oxo-to-Fe(III) charge-
transfer transitions. The absorption spectrum of 1 in
acetonitrile is concentration independent (between 3.9 μM
and 1 mM, Figure S24), indicating that dissociation of a
mononuclear complex is highly unfavorable.
1
1
2
provided as Supporting Information. The ESI/MS spectrum of
1
1
2+
1
shows a base peak at m/z 445 ([(L )Fe(μ-O)Fe(L )] -
2
2
2+
(
(
ClO ) ) and of 2 at m/z 557 ([(L )Fe(μ-O)Fe(L )] -
ClO ) ) (Figures S15 and S16). In both cases, ESI/MS spectra
4 2
4
2
show no evidence for the presence of mononuclear species. The
1
H NMR spectra of both 1 and 2 show signals in the range of
−
5 to 40 ppm in CD CN (Figures S13 and S14), which,
3
24
Raman spectra recorded at λexc 561, 355, and 532 nm
Figures S21, S22, and S23) in dilute solution (0.25−0.5 mM)
together with broadening, and the magnetic susceptibility,
(
25
determined using Evans method, of 2.78 BM and 2.76 BM for
and 2, respectively (Figures S17 and S18), and the absence of
show resonance enhancement of some of the Raman bands of 1
and 2. The spectrum obtained at λexc 561 nm (Figure S20),
although resonant with an absorption assigned as a phenolato-
to-Fe(III) charge-transfer band, is similar to the Raman spectra
obtained at 1064 and 785 nm (Figures S19 and S20). At λexc
1
X-band EPR signals for both 1 and 2 at 77 K, is consistent with
antiferromagnetically coupled dinuclear iron(III) complexes.
Nonresonant Raman spectra of 1 and 2 (λexc 785 nm and λexc
1
064 nm) were recorded in solution (acetonitrile and acetone,
3
55 nm, in resonance with an absorption band assigned to an
−1
Figure S19) and in the solid state (Figure S20), whose peak
positions compare well with the computed IR spectra (Figure
S19). Complex 1 shows bands at 1595, 1562, 1475, 1455, 1279,
oxo-to-Fe(III) charge transfer, bands at 411 and 430 cm are
enhanced strongly (Figure S22), which are ascribed, tentatively,
to Fe-O-Fe modes that are observed typically between 380 and
−1
−1 27
1
205, 1154, 1114, 1025, 890, 602, 559, 431, 292, and 217 cm ,
540 cm . The computed IR spectrum shows these Fe-O-Fe
modes at 432 cm− (4 km/mol intensity) and 453 cm (35
1
−1
which compare closely with related complexes bearing pyridyl
III III
and phenolato ligands, e.g., [Fe Fe (bpbpmp)(CH CO ) ]-
km/mol intensity), respectively.
3
2 2
ClO (1591, 1564, 1478, 1451, 1276, 1274, 1151, 1112, 890,
The electronic structure of complex 1 is confirmed by
computational chemical data, which show that the spin ground
4
−1
7
95, 636, 612, and 335 cm , where bpbpmp is 2-bis[((2-
2
9
30
pyridylmethyl)-aminomethyl)-6-((2-hydroxybenzyl)(2-pyridyl-
state (at S12g/TZ2P with ZORA scalar relativistic
26
31
methyl))-aminomethyl]-4-methylphenol). The bands at 1279
corrections and COSMO solvation taken into account; see
and 602 cm− (found with DFT at 1251 and 582 cm , Figure
1
−1
the Supporting Information) corresponds to the antiferromag-
netically coupled Fe(III)-O-Fe(III) complex with other
S19) are assigned to the υ(CO) mode and υ(Fe-Ophenol
)
phenol
27
−1
mode, respectively, while the bands at 1595/1562 and 1475/
possible spin states higher in energy by at least 10 kcal·mol
1
455 cm− are largely in-plane bending modes of the pyridyl
1
(Table S1). The corresponding geometric structure is also in
good agreement with the X-ray structure (root-mean-square
deviation 0.2 Å), with similar iron−ligand bond distances
(mean absolute deviation 0.017 Å). Furthermore, the computed
IR spectrum shows peak positions consistent with the
nonresonant Raman spectra (Figure S19c). The TD-DFT
UV/vis absorption spectrum for 1 (Figure S25), obtained at
and phenolato moieties, respectively. Complex 2 shows overall
similar spectra, except for bands associated with Fe-O and
phenol based modes. Raman spectra of complex 2 in the solid
state at λexc 785 nm show the bands at 1601, 1469, 1442, 1413,
1
314, 1269, 1131, 836, 740, 606, 537 nm, which compares
−1
closely with 1. The bands at 1268 and 540 cm are assigned to
the υ(CO)phenol mode and υ(Fe-Ophenol) mode, respectively.
The absence of change between the solid state spectrum
30,32
(COSMO+ZORA)SAOP/TZ2P,
shows bands shifted to
longer wavelengths (ca. 140 nm) compared to the experimental
spectrum, and correspond to phenoxy-to-iron ligand-to-metal
(LMCT) transitions (Figure S25).
(
(
Figure S20) and those obtained in acetonitrile and acetone
Figure S19) indicates that the solid state structure is retained
in solution, in agreement with EPR and ESI-MS data.
Both 1 and 2 show visible absorption (ε (1) 4.1 × 10 M
Cyclic Voltammetry of 1 and 2 in Acetonitrile. The
cyclic voltammogram of 1 in acetonitrile at room temperature
displays three irreversible oxidation processes at 1.22, 1.40, and
1.57 V and one irreversible reduction at 0.37 V vs SCE on the
return cycle (Figure 3). The irreversible redox waves are
assigned to phenol centered processes by comparison with the
3
−1
5
35
−1
3
−1
−1
cm , ε (2) 4.7 × 10 M cm ) in acetonitrile and more
623
intense bands in the near-UV region (Figure 2). The
absorption spectra are consistent with those reported for
28
Fe(III)-phenolato complexes elsewhere and the longest
1
2
wavelength absorption is assigned to a phenolato-to-Fe(III)
voltammetry of HL and HL (see Figure S28), with the
C
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potential to 0.2 V, a nearly complete recovery of the initial
spectrum was obtained. Addition of [(NH ) Ce(NO ) ]
4
2
3 6
(
(
2
CAN) to 2 in acetonitrile resulted in similar spectral changes
Figure 6) consistent with one-electron oxidation only, i.e., to
+
, with absorption bands in the near-UV and visible region
similar to those observed by spectroelectrochemistry). The
original spectrum of 2 recovered rapidly, except for a small
decrease in the absorbance due to dilution.
Substantial changes in the vis−NIR absorption spectrum of 1
were observed in acetonitrile during the first positive cycle after
1
.31 and 1.72 V and on the return cycle, an irreversible
reduction at 0.81 V vs Ag/AgCl (Figure 5). The electro-
chemical oxidation of 1 at 1.4 V (number 2, red line in Figure
5
a,b) resulted in the appearance of the visible absorption band
Figure 3. Cyclic voltammetry of 1 (black) and 2 (red, current axis is
at 465 nm and NIR absorption at 850 nm. Oxidation at more
positive potential (number 3, blue line, Figure 5a,b) resulted in
a further increase in absorbance at 475 nm together with bands
at ca. 740 and 900 nm. Subsequent reduction (number 4, green
line, Figure 5a,b) resulted in a decrease in absorbance at 475
and 900 nm and the appearance of the news bands at 404 and
offset for clarity) (1.0 mM) in acetonitrile (0.1 M TBAPF ) at a glassy
6
carbon electrode (d = 3 mm) working electrode with a Pt counter
−1
electrode and an SCE reference electrode at 0.1 V s . The initial
potential in each case (*) was the open-circuit potential; initial scan
directions are indicated by arrows.
4
78 nm. Reduction at 0.81 V (number 1, cyan line, Figure 5c,d)
process at 0.37 V ascribed to the reduction of a product of a
follow-up chemical reaction between the oxidized phenol units.
In contrast, the voltammetry of 2 shows that two are
electrochemically reversible redox processes assigned to
phenolato centered oxidations at 1.04 and 1.23 V.
Thin-Layer Cyclic Voltammetry and Spectroelectro-
chemistry of 1 and 2 in Acetonitrile. In situ monitoring of
the changes that occur during cyclic voltammetry of 1 and 2
was carried out using an optically transparent thin-layer
resulted in complete loss of the absorbance at 470 nm and an
increase in absorbance at ca. 820 nm (Figure 5c,d). The new
irreversible oxidation processes at 0.89 and 1.37 V vs Ag/AgCl
appear in the second cycle. The first oxidation (number 2, pink
line, Figure 5c,d) resulted in the reappearance of the NIR
absorption band and the bands at 404 and 456 nm. Oxidation
at more positive potentials (number 3, navy line, Figure 5c,d)
showed a decrease in NIR absorption and an increase in
absorbance at 404 nm (vide supra). Overall, the oxidation of 1
in acetonitrile results in a permanent loss in the original visible
absorption band, and the spectroelectrochemistry indicates that
iron(III)-phenolato type complexes are no longer present in
solution after the second cycle. It should be noted that the
stability of the complex upon repeated cycles may be affected
by the decrease in pH through concomitant water oxidation at
the platinum working electrode.
(
OTTLE) cell, to characterize the products of the formed
after electrochemical oxidation. Thin-layer voltammetry of 2
shows that the first redox wave is reversible (chemically) but
that the second redox process is chemically irreversible despite
that the cyclic voltammetry at a GC (Figure 3 and Figure S29)
electrode shows that both redox waves are electrochemically
3
3
reversible. A cyclic voltammogram from 0.3 V to between the
first and second redox waves (Figure 4, left) shows the
reversible formation of singly oxidized species with strong
bands at 410 and 430 nm, weaker bands at 490 and 680 nm,
and isosbestic points maintained at 553 and 723 nm (Figure 4
right, vide infra). These bands are characteristic of a phenoxyl
radical cation and are consistent with the coincidence of the
Oxidation of 1 with Ceric Ammonium Nitrate (CAN).
The loss in absorbance after several cyclic voltammetric cycles
under thin-layer conditions could indicate precipitation of the
products from solution. Chemical oxidation of 1 with
[(NH ) Ce(NO ) ] (CAN) was followed by UV−vis−NIR
4
2
3 6
absorption and resonance Raman spectroscopy. As observed by
thin-layer spectroelectrochemistry, addition of 1 equiv of CAN
2
redox waves of 2 with those of HL . Upon returning the
Figure 4. Thin-layer voltammetry of 2 (5 mM) (left) at 0.01 V s−1 in an OTTLE cell in acetonitrile (0.1 M TBAPF ) (right) monitored in situ by
6
vis−NIR spectroscopy. The inset shows the change in absorbance at selected wavelengths as a function of applied potential. The initial (black) and
final (spectra) are shown as bold lines.
D
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Figure 5. Thin-layer cyclic voltammetry of 1 (5.0 mM) in acetonitrile (0.1 M TBAPF at 293 K) (a) first cycle, (c) second cycle and vis−NIR
6
absorption spectra recorded in situ during (b) first cycle, (d) second cycle. The spectra were recorded at the points in the voltammogram indicated
in (a) and (c).
Figure 6. UV/vis absorption spectra of (a) 1 (0.25 mM) in acetonitrile with 1 equiv of CAN, (c) 2 in acetonitrile (0.2 mM) with 1 equiv of CAN.
Time dependence of absorption of (b) 1 at 465, 785, and 850 nm with addition of 1 equiv of CAN, (d) 2 at 425, 488, and 625 nm with addition of 1
equiv of CAN.
to 1 in CH CN resulted in an immediate appearance of an
absorption band at 465 nm, which decayed over time,
concomitant with the appearance of new bands at 780 and
850 nm. Resonance Raman spectra recorded at 473 and 785
nm during the oxidation provide insight in the species
responsible for these absorption bands (vide infra).
3
E
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Figure 7. (a) Cyclic voltammetry of 1 (0.5 mM) in dichloromethane containing 0.1 M TBAPF at 293 K, (b) electrochemical position of poly-1 on a
6
−1
gold electrode by cyclic voltammetry (at 0.1 V s during repetitive cycles between 0.0 and 1.2 V vs Ag/AgCl).
+
•
Figure 8. Spin-density plots obtained at (COSMO+ZORA)S12g/TZ2P for (a) 1 and (b) 1 . See Table S1 for relative energies of spin states.
Cyclic Voltammetry of 1 in Dichloromethane. In stark
contrast to that observed in acetonitrile (vide supra), the cyclic
voltammetry of 1 in dichloromethane shows a single
irreversible oxidation at 0.81 V vs Ag/AgCl in the first sweep
toward positive potentials, followed by two reduction waves at
including platinum, gold, GC, and ITO electrodes (Figures
S30−S32).
The electrochemical oxidation of 1 and 2 can be viewed,
formally, as an Fe(III)-O-Fe(III) to Fe(III)-O-Fe(IV) and
subsequently to Fe(IV)-O-Fe(IV) process. However, the close
correspondence of the potentials for the first and second
0.55 and 0.34 V on the reverse sweep (Figure 7a). The second
1
oxidation of the ligand HL with that of 1 and 2 indicates,
cycle showed two new redox waves and a decrease in the
current of the oxidation wave at 0.81 V.
Repeated cyclic voltammetry of 1 between 0.0 and 1.2 V (vs
together with the oxidative phenol−phenol coupling observed,
that oxidation results primarily in a decrease in electron density
35
on the phenolate units and, in particular, the phenol C4
positions. Indeed, DFT calculations indicate that the oxidation
hardly affects the occupation of d-orbitals on the iron(III)
centers (whose occupations remain constant at ca. 4.6
Ag/AgCl, Figure 7b) results in a steady increase in current for
+
the redox waves assigned (vide infra) to poly-1/poly-1 and
+
2+
poly-1 /poly-1 at ca. 0.34 and 0.55 V, respectively; i.e., one of
the phenolato units in 1 undergoes a one-electron oxidation,
after which coupling with another phenoxyl radical complex
occurs in an overall four-electron ECCE process. The current
response at >0.81 V decreased over multiple cycles along with
an increase in current response of the redox waves at 0.34 and
+
•
electrons) for both 1 and 1 . Furthermore, in 1, the spin
density was observed mainly on the iron and adjacent ligating
atoms and (symmetrically) on the phenols (Figure 8a). After
+•
oxidation to the radical cation 1 , the spin density diminishes
on one phenol and increases at the other phenol (Figure 8b),
primarily at the C4 position as anticipated in Scheme 1.
0.55 V, consistent with a buildup of poly-1 at the electrode
(
vide supra). The decrease in current response at >1.0 V is in
Hence, the redox chemistry of 1 in dichloromethane can be
part due to depletion of the diffusion layer; however, stirring
between cycles to “refresh” the depleted diffusion layer resulted
in only a slight recovery of the current response from 1. This
indicates that film formation results in a reduction in
21b,36
rationalized as an ECCE mechanism,
by analogy with
34
dimethylaniline redox chemistry. The irreversible oxidation at
.81 V (vs Ag/AgCl) yields initially an iron bound phenoxyl
radical cation 1 , which undergoes dimerization via aryl C−C
coupling at the para position of the phenol unit to form H 3 ,
0
+
•
3
4
2
+
conductivity at the onset potential for oxidation of 1,
although the film continues to grow in thickness with each
cycle. Polymer film formation was observed in dichloromethane
at room temperature and at 200 K on a range of electrodes,
2
followed by double deprotonation to form 3 (Scheme 1). The
dimer 3 obtained is symmetrical and exhibits two reversible
one-electron oxidation waves at 0.34 and 0.55 V, which
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generate the radical cation 3+• and symmetric dication 32+,
respectively (Scheme 2).
supra). Oxidation of a poly-1-modified ITO electrode was
carried out by cyclic voltammetry with simultaneous acquisition
of vis−NIR absorption spectra (Figure 9, and Figure S34). The
Scheme 2. Oxidation of para-Bisphenoxy Bridge in 3 and
Poly-1 to Monocationic and Dicationic States
Figure 9. vis−NIR absorption spectroscopy and (inset) cyclic
voltammetry of a poly-1 film on an ITO slide (in dichloromethane
2
+
+•
The formation of the coupled tetramer H 3 from two 1 is
2
containing 0.1 M TBAPF ). Initial spectrum (black) and absorption
6
−1
slightly endothermic (+1.6 kcal·mol ), and the subsequent
spectra recorded during the cyclic voltammetry at the potentials
indicated by the red, blue, and gray arrows (see inset).
−
1
release of two protons is exothermic by 84.6 kcal·mol ,
excluding twice the solvation energy of a proton (−260.2 kcal·
−1
−1
37
mol in acetonitrile ) for a total of more than 600 kcal·mol
2
+
+
spectra obtained for the singly and doubly oxidized poly-12+
show some resemblance to the spectra obtained upon oxidation
of 1 in acetonitrile (vide supra). Electrochemical oxidation of
for the process of H 3 (solv) to 3(solv) + 2H (solv).
2
Cyclic Voltammetry at Poly-1-Modified Electrodes.
Poly-1-modified electrodes show two fully reversible oxidation
waves at 0.34 and 0.55 V (Figure S33a) in monomer free
solution. The cyclic voltammetry at the electrode modified with
poly-1 shows a linear dependence (Figure S33b) on the scan
+
poly-1 at 0.5 V to form poly-1 results in the appearance of
absorption bands at 430, 471 nm and an intense band at 870
nm. Further oxidation at 1.0 V to form poly-12+ resulted in the
disappearance of the NIR absorption band at 870 nm and the
appearance of a more intense visible band at 475 nm.
Subsequent reduction at 0.5 V resulted in a recovery of the
−1
rate between 0.1 and 1.0 V s , and E = Ep,c and Ip,a = Ip,c for
p,a
both redox waves, as expected for surface-confined redox
process. The non-Faradaic current shows a linear dependence
on the scan rate and is unaltered by repeated cyclic
voltammetry, further indicating that the film is not altered
significantly upon oxidation or reduction (vide infra). The
absence of a significant redox wave at 0.81 V on a poly-1-
modified electrode indicates that the degree of polymerization
is high, as the polymer chain ends (i.e., nondimerized phenolate
units) would be expected to show a reversible redox wave at a
potential similar to that of 2.
+
spectrum of poly-1 and at 0.0 V of the initial spectrum.
Resonance Raman Spectroscopy of Poly-1. The Raman
spectrum recorded from a poly-1-modified gold or ITO
electrode is devoid of signals from the complex due to the
limited thickness of the polymer layer and lack of resonance
enhancement. By contrast, polarization of the poly-1-modified
electrode at 0.5 V vs SCE in monomer free dichloromethane to
+
form poly-1 , which has a strong absorbance at 870 nm, results
Poly-1-modified electrodes prepared from solutions of 1 in
dichloromethane show fully reversible redox waves at 0.34 and
in the appearance of intense Raman scattering at λ 785 nm.
exc
The bands at 1618, 1556, 1478, 1377, and 1131 cm− observed
are identical to the rRaman spectrum of 1 obtained in
acetonitrile with 1 equiv of CAN (Figure 10). The intense band
at 1618 cm− is assigned, tentatively, to an aromatic CC
1
0.55 V (Figure S35) in monomer free acetonitrile. However, in
contrast to dichloromethane, a steady decrease in current was
observed, indicating desorption of poly-1 over repeated cycles
due either to ligand decomposition or to breakup of the μ-oxo
bridging unit (Figure S35). Nevertheless, the relative robust-
ness of poly-1 in monomer free acetonitrile indicates that the
lack of polymer film formation from 1 in acetonitrile (vide
supra) is due to the solubility and lack of stability of the
oligomers formed initially upon oxidation and not due to lack
of C−C bond formation.
1
−1
stretching mode (observed at 1592 cm in the computed IR
spectrum) and at 1556 cm− to a C−O stretching mode of the
1
−
1
phenoxy unit (found at 1579 cm in the computed IR
1
1,38
spectrum, Figure S19).
Similarly, the polarization of the
electrode at 0.85 V to form poly-12+ (which shows strong
absorbance at 475 nm) resulted in the appearance of intense
Raman scattering at λexc 488 nm, with bands at 1608, 1523,
1349, 1343, 1263, and 1139 cm . Again, these bands are
virtually identical to those observed in the Raman spectrum
(λexc 473 nm) of 1 with 1 equiv of CAN (vide supra). The
−1
vis−NIR Absorption Spectroelectrochemistry of Poly-
1
. The vis−NIR absorption spectrum of poly-1 shows a weak
band in the visible region similar to that of 1 in solution (vide
G
DOI: 10.1021/acs.inorgchem.6b02378
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
Figure 10. Resonance Raman spectra of (red line) a poly-1 -modified gold electrode at λ 785 nm, (black line) 1 in CH CN with 1 equiv of CAN
exc
3
2+
at λexc 785 nm, (blue line) a poly-1 -modified gold electrode at λ 488 nm, (gray line) 1 in CH CN with 1 equiv of CAN at λ 473 nm. Identical
exc
3
exc
+
spectra were obtained for poly-1 on ITO.
bands at 1608 and 1523 cm−1 are ascribed to CC and C−O
opportunity for controlling oxidative coupling of phenols using
complexation with Fe(III). It should be noted that, despite that
it does not undergo C−C coupling, complex 2 shows similar
reactivity in the oxidation of alcohols as is observed for complex
1. Future studies will explore the relevance of oxidative
phenol−phenol coupling chemistry under conditions in which
catalysis with H O is carried out.
38
stretching modes, respectively.
In the case of oxidation with CAN, it is tempting, considering
the time dependence of the changes in absorbance and the
similarity of the initial absorbance to that of the one-electron
oxidized form of 2 (Figure 4), to assign the absorption band at
4
65 nm to the one-electron oxidized form of 1. However,
comparison of the Raman spectrum of 1 with CAN recorded at
73 nm with that recorded at 488 nm for poly-1 (Figure 10)
2
2
4
ASSOCIATED CONTENT
Supporting Information
■
confirms that the formation of the two-electron oxidized 4,4′-
*
S
2
+
bisphenolato species 3 from 1 is extremely rapid. The
subsequent changes, i.e., the decrease in intensity and
appearance of the NIR absorption bands, are due to reduction
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02378.
of the primary oxidation product to the monocationic species
Details of computational data, experimental methods and
syntheses, spectroscopic and electrochemical data (PDF)
+
(
3 ).
CONCLUSIONS
■
AUTHOR INFORMATION
Corresponding Author
E-mail: w.r.browne@rug.nl.
■
In the present study, we show that the electrochemical
oxidation of a phenol based Fe(III) complex, used earlier in
*
18
alcohol oxidations, leads to surprisingly well-defined reactivity
of the iron(III) bound phenoxyl radical generated. Of relevance
to catalysis is that, in acetonitrile, despite the quite complex
spectroelectrochemical changes observed, one-electron oxida-
tion with Ce(IV) leads to essentially instantaneous C−C bond
ORCID
Marcel Swart: 0000-0002-8174-8488
Wesley R. Browne: 0000-0001-5063-6961
Notes
2+
The authors declare no competing financial interest.
formation, followed by further oxidation (to form 3 , and
related oligomers) and then slower reduction to the radical
cation and then further reduction. The reduced C−C coupled
tetra nuclear complex formed appears similar to the original
complex spectroscopically. The introduction of tert-butyl
groups at the para position of the phenol moieties leads to a
dramatic increase in stability and precludes phenol C−C bond
formation. The data indicate that, for complexes bearing phenol
units, similar processes may occur and present challenges to
interpretation of spectroscopic analyses under reaction
conditions. Notwithstanding this, the formation of well-defined
iron containing polymers on electrode surfaces presents an
ACKNOWLEDGMENTS
■
The European Research Council (StG, no. 279549, D.U.,
W.R.B.) and the Ministry of Education, Culture and Science
(Gravity program 024.001.035, W.R.B.), the Ministerio de
Economia y Competitividad (MINECO, projects CTQ2014-
59212-P and CTQ2015-70851-ERC, M.S.), the DIUE of the
Generalitat de Catalunya (project 2014SGR1202, M.S.), and
the European Fund for Regional Development (FEDER,
UNGI10-4E-801, M.S.) are acknowledged for financial support.
A. van Dam (ERIBA, University of Groningen) is thanked for
H
DOI: 10.1021/acs.inorgchem.6b02378
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
assistance with a recording of ESI-MS spectra. L. Kortekaas
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4
33, 227−239. (c) Ito, N.; Phillips, S. E.; Stevens, C.; Ogel, Z. B.;
(
University of Groningen) is acknowledged for discussion. This
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Thioether Bond revealed by a 1.7 Å Crystal Structure of Galactose
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work was performed in the framework of the COST action
CM1305 “Explicit Control Over Spin-states in Technology and
Biochemistry (ECOSTBio)” (with support of an STSM to
W.R.B., COST-STSM-CM1305-29045).
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