Synthesis, Electrochemistry, and Optical Properties of Highly Conjugated Alkynyl-Ferrocenes and -Biferrocenes

Article abstract of DOI:10.1021/acs.organomet.1c00098

Sonogashira reactions are utilized herein to react iodo-ferrocenes and -biferrocenes with terminal alkyne ligands, functionalized with both pyridine and thioanisole groups. High-yielding reactions generate both monoalkynyl and dialkynyl derivatives, the r

Full text of DOI:10.1021/acs.organomet.1c00098

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Article
Synthesis, Electrochemistry, and Optical Properties of Highly
Conjugated Alkynyl-Ferrocenes and -Biferrocenes
Troy L. R. Bennett, Luke A. Wilkinson,* Jasmine M. A. Lok, Robert C. P. O’Toole,
and Nicholas J. Long*
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ABSTRACT: Sonogashira reactions are utilized herein to react iodo-
ferrocenes and -biferrocenes with terminal alkyne ligands, functionalized
with both pyridine and thioanisole groups. High-yielding reactions
generate both monoalkynyl and dialkynyl derivatives, the ratio of which
can be altered through changes in the reaction stoichiometry. This
methodology allowed us to synthesize a large family of derivatives,
comprising four symmetrical derivatives (3xx, where x represents a phenyl-
substituted terminal alkyne) and six less-studied asymmetrical derivatives
(3xy, where x and y represent two different phenyl-substituted terminal
alkynes), as well as a number of their biferrocenyl analogues (6x, 7xx, and
7xy), including the first known examples of asymmetrically disubstituted
biferrocenes. We examined the electrochemical behavior of all the systems in solution through the use of cyclic voltammetry and
demonstrate that these highly conjugated alkynyl ligands exert delicate redox control over the central ferrocene motif. We also note
that these substituents display some control over the mixed-valence character present in biferrocene monocations, with thioanisole
substituents imparting almost an order of magnitude higher Kc than their pyridyl analogues, and asymmetric systems displaying rare
characteristic properties of mixed-valence isomers. The electronic structure of these systems was further elucidated through a
combination of UV/vis spectroscopy and density functional theory calculations. Our methodology provides a facile and adaptable
route toward the isolation of a number of novel ferrocene and biferrocene derivatives. From our perspective, the asymmetric nature
of these systems, along with the delicate and predictable redox control that these ligands exert on the central ferrocene unit(s), could
lead to applications in molecular electronics, where these properties have previously shown promise in the fabrication of diodes and
rectifiers, as well as in the synthesis of donor-π-acceptor systems.
onto a ferrocene center, we can create a robust, highly
conductive structure with the ability to rotate around a central
node. These attractive properties have led to the application of
these systems in the fabrication of highly ordered macro-
molecular assemblies and more prominently in the design and
fabrication of electronic materials and single-molecule
wires.12−20 However, to date, most studies of ferrocene in
these contexts have focused on the synthesis and study of small
families of monosubstituted and symmetrically disubstituted
systems, potentially owing to the relative ease of their synthesis
and purification.21−28 Although impressive, these studies have
often been limited in scope and ignore the potential that
asymmetry could impart on the electronic and structural
properties of these species. For instance, research in the field of
INTRODUCTION
■
The desire to introduce a well-defined redox center, such as
ferrocene, into a wide-ranging array of molecules, has led to the
development of a rich library of new synthetic methods.1,2 This
abundance of techniques has resulted in ferrocene derivatives
being incorporated into a wide variety of fields including
catalysis, materials science, medicine, and a large number of
different electrochemical applications.3−9 One relatively recent
addition to this synthetic tool kit was the synthesis and
isolation of 1,1′-diiodoferrocene in high yields, which has been
demonstrated within our group, through the use of an
“oxidative purification” methodology, as well as by Nijhuis et
al. who utilized a sublimation-focused route.10,11 Both of these
methods generate the highly versatile synthon and its
biferrocene equivalent (1,1‴-diiodobiferrocene) in high yields
and with a high degree of purity. The realization of these
synthons has opened up the use of classical cross-coupling
methodologies. In particular, Sonogashira cross-couplings of
iodoferrocenes present a valuable opportunity for the synthesis
of highly conjugated redox-active molecules through reactions
with terminal alkynes. Through substitution of alkynyl moieties
Received: February 18, 2021
Published: April 12, 2021
https://doi.org/10.1021/acs.organomet.1c00098
© 2021 American Chemical Society
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Scheme 1. Synthetic Pathway Used in the Assembly of Ferrocene and Biferrocene Compounds Disubstituted in Each Case
a
with Two Ethynyl-Arene Substituents
a
“Step 1” denotes the formation of the ethynyl-arene ligands, “step 2” denotes the initial reactions with 1,1′-diiodoferrocene and 1,1‴-
diiodobiferrocene to form a statistical mixture of mono- and disubstituted ferrocenes, and “step 3” shows subsequent reactions that were utilized to
introduce asymmetry across the central ferrocenyl units.
molecular electronics has provided a number of examples
where the asymmetric design has been applied in the
fabrication of systems for molecular rectification and has also
been demonstrated as an effective tool for tailoring different
ends of a molecule to different electrode materials.29−33 In an
effort to bridge this gap between synthesis and design, we
constructed an adaptable synthetic route that could be applied
in the isolation of a large number of new asymmetric ferrocenyl
derivatives. For this, we selected four different “ligands” with
subtly different chemical, and by extension, electronic
structures, these being 4-ethynylpyridine (a), 3-ethynylpyr-
idine (b); 4-ethynylthioanisole (c), and 3-ethynylthioanisole
(d). Herein, we demonstrate a facile and scalable synthesis of a
series of novel symmetric and asymmetric alkynyl-ferrocenes
and -biferrocenes. We examined the properties of these
systems through a combination of electrochemistry, UV/vis,
and computational methods, demonstrating a predictable
redox tunability within the family, as well as uncovering rare
examples of mixed-valence isomers.
column chromatography) and the converse is true when
aiming for the symmetrical doubly substituted products (3xx).
Compounds 2x can then be taken forward in an additional
coupling step to obtain the asymmetric disubstituted products
(e.g., 3xy). Conveniently, multiple derivatives can be obtained
in a one-pot reaction of a monosubstituted ferrocene (2x) with
two different alkynes, although this leads to a statistical mixture
of products, as is seen in the synthesis of 3bc and 3cd (see
Supporting Information). Purification is largely facile; however,
the pyridyl-based analogues require separation on a short
alumina(V) column due to their propensity to streak on silica.
For this reason, it was beneficial to prepare the asymmetric
variants in a stepwise manner, starting from the thioanisole
precursors (2c or 2d) in order to maximize yield and ease of
purification. For comparative purposes, the ferrocenyl
derivatives containing only one arm [i.e., CpFe(C5H4CCAr)
(4) where Ar = 4-pyridyl or 4-thioanisole] were synthesized
from ethynyl-ferrocene according to Scheme 1, step 2 to
generate 4a and 4c, respectively. This methodology was
extended to the synthesis of a select number of biferrocenyl
derivatives through reaction of 1,1‴-diiodobiferrocene (5)
with both (a) and (c). This, once again, produced a mixture of
monosubstituted (6x) and disubstituted (7xx) products, which
could be isolated through the use of column chromatography.
The yields of the biferrocene reactions were somewhat reduced
in comparison to their monoferrocene counterparts, which we
attribute to decreased reactivity of the biferrocenyl centers.
Molecules (6x) were then substituted with different alkynes to
produce asymmetric biferrocene derivatives (7xy), which
would have been difficult to isolate through currently available
biferrocene functionalization methods.34 All compounds have
been characterized by mass spectrometry and their bulk purity
was determined through elemental analysis where possible.
This synthesis was aided throughout by excellent resolution
within the 1H NMR spectra, whereby all compounds show the
SYNTHESIS
■
Synthesis of each compound was achieved via an iterative
Sonogashira coupling process, using a methodology that has
previously found success in our group.34 The arylbromide
[ArBr, where Ar = 4-pyridyl (a), 3-pyridyl (b), 4-thioanisole
(c), or 3-thioanisole (d)] was coupled to trimethylsilylacety-
lene and subsequently deprotected to yield the terminal
alkynes in high yields. From here, it was possible to couple the
functionalized alkynes to 1,1′-diiodoferrocene (1). In this step
(Scheme 1, step 2), access to both the monosubstituted
products (2x) and the symmetrically disubstituted products
(3xx) can be achieved through careful control of reaction
stoichiometry. To preferentially obtain the monosubstituted
products (2x), an excess of diiodoferrocenes must be
employed (this is easily recovered after the reaction through
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expected number of pseudo-triplet signals associated with the
ferrocene Cp protons at 4.0−4.8 ppm. For the symmetrical
monoferrocene species (3aa−dd), these present as two
pseudo-triplets, whereas four distinct pseudo-triplets are
observed for the asymmetric variants (e.g., 3ac). When
extending this analysis to the biferrocene family, once again,
very distinct profiles can be observed in the H NMR spectra
with the symmetrical biferrocenes producing four pseudo-
triplets and the asymmetric systems producing eight (Figure
1).
Table 2. CV Data Obtained for Complexes 6x and 7xx in
Solution
a
[Fc2]
Ep
64
415
13
378
3
322
4
347
49
391
−21
330
Ec
E1/2
ΔE
ipa/ipc ΔE1E2
Kc
105.12
6a
−23
279
−71
295
−57
230
−66
241
−56
287
−103
253
44
347
−29
337
−27
276
−31
294
−34
336
−62
291
87
136
84
83
60
92
70
106
105
109
82
1.15
0.95
0.51
0.62
1.04
0.95
303
366
303
325
370
353
106.19
105.12
105.50
106.26
105.97
1
6c
7aa*
7ab*
7ac
7cc
78
a
0.1 M NnBu4PF6 in CH2Cl2 taken at 100 mV s−1 with a glassy carbon
working electrode and a Pt wire as reference and counter electrodes.
All potentials (mV) are reported vs Fc/Fc+ and have been corrected
for iRs with values obtained from AC impedance measurements.
system; a fact that is nicely evidenced by comparing the
potentials of, for example, 3cc and 4c. Additionally, the
substituents on the arene were found to exert a significant
influence on the overall potential and reversibility of the redox
process. The molecules that incorporate only thioanisole
groups (Xc/d) undergo redox chemistry that is both
chemically (ipa/ipc ≈ 1) and electrochemically reversible (ΔE
≈ 65 mV over multiple scan rates), with diffusion-controlled
electron transfer rates (ip ∝ ν1/2).37 Changes in redox potential
can be observed between the 3-(3dd) and 4-substituted (3cc)
thioanisole compounds, with 3-thioanisoles occurring at a
higher potential than their 4-substituted analogues. A rationale
for this stems from the ability of the thiomethyl moiety to
donate electrons into the π-system and stabilize a positive
charge. A qualitative analysis of the relevant resonance forms
indicates that the positive charge is delocalized more effectively
in the presence of the 4-thioanisole than the 3-thioanisole, thus
reflecting the trend observed in the data. A similar, yet
opposite trend is observed for the series containing only
pyridyl groups (3a/b). The 3-pyridyl moiety is able to stabilize
the positive charge to a greater degree than the corresponding
4-pyridyls, with the analogue containing both groups (3ab)
displaying an intermediate potential. The trend in the redox
potentials of this series reflects those in both the pyridyl- and
thioanisole-based analogues, such that a continuum of
potentials can be observed across the whole series of molecules
(Figure 2). Thus, while the net effect of the arylalkynes is
Figure 1. Comparative 1H NMR spectra of different compounds,
illustrating various Cp-H environments that arise from both
asymmetry and the inclusion of multiple ferrocene units.
ELECTROCHEMISTRY
■
Cyclic voltammetry (CV) experiments were performed on all
molecules in dichloromethane solutions with 0.1 M NnBu4PF6
as the supporting electrolyte, and the relevant data are reported
in Table 1 (monoferrocenes) and Table 2 (biferrocenes).
Looking first at the monoferrocene compounds, these all
displayed a characteristic one-electron redox process, associ-
ated with the ferrocene motif, at potentials, which are more
positive than that of standard ferrocene.35,36 This is a
consequence of the ethynylarene substituents, which withdraw
electron density from the redox-center by extending the π-
Table 1. CV Data Obtained for Complexes 3xx and 4x in
a
Solution
[Fc]
Ep
Ec
E1/2
ΔE
ipa/ipc
3aa
3ab
3ac
3ad
3bb
3bc
3bd
3cc
3cd
3dd
4a
336
319
247
284
296
246
268
196
196
237
195
135
264
249
182
213
209
172
188
130
130
173
127
69
300
284
215
249
253
209
228
163
163
205
161
102
72
70
65
71
87
74
80
66
66
64
68
66
1.23
1.06
1.08
1.11
1.05
1.02
0.89
0.93
0.86
1.00
0.98
0.94
4c
a
0.1 M NnBu4PF6 in CH2Cl2 taken at 100 mV s−1 with a glassy carbon
working electrode and a Pt wire as reference and counter electrodes.
All potentials (mV) are reported vs Fc/Fc+ and have been corrected
for iRs with values obtained from AC impedance measurements.
Figure 2. Redox potentials (mV) of compounds 3xx (solid bar) and
4x (striped bar).
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Figure 3. FMOs for a number of compounds that illustrate the differences in the formation of the HOMOs and LUMOs that result from the
addition of an additional ferrocene motif and also from introducing asymmetry across the ferrocene unit.
electron-withdrawing, the redox properties of the ferrocene can
be tuned further by varying the electron-donating/-with-
drawing properties of the arene. Turning now to examine the
biferrocene systems 6x and 7xx/xy. All show two distinct redox
processes and we attribute these redox features to the
sequential oxidation of the two Fe2+/Fe3+ couples present in
each system.21 Compounds 7aa and 7cc are fully symmetrical
systems so the separation between the two redox processes
(ΔE1/2) can be attributed to charge stabilization via electronic
coupling between the two ferrocene centers in the mixed-
valence ion (e.g., [FeII−FeIII]+). The stability of this mixed-
valence ion can be inferred from the comproportionation
constant (KC) for the equilibrium between the mixed-valence
ion, and its neutral and doubly oxidized forms, and can be
derived from the Nernst equation, according to eq 1.
Using the potentials obtained from the CV of 7aa and 7cc,
we can determine Kc as 1.3 × 105 and 9.3 × 105, respectively,
suggesting that they belong to class III of the Robin and Day
series. For compounds 6x, 7ab, and 7ac, these calculations
become more complicated due to the asymmetric structure of
the molecules. The monoferrocenes described previously
(3xx/xy and 4x) clearly show that the redox properties of an
individual ferrocene center can be influenced by substituents
on the Cp ring. Therefore, it is incorrect to consider these
biferrocene units as simple mixed-valence systems, rather they
are rare examples of mixed-valence isomers.38 In such systems,
ΔE1/2 is no longer purely a consequence of coupling between
redox centers, but is also affected by the intrinsic potential
difference between the two independent redox sites.39
A
consequence of this is that ΔE1/2 values obtained from the CV
of mixed-valence isomers can provide an overestimation of the
electronic coupling present within the system. To illustrate
Kc = 10(F·ΔE E /2.303·RT)
1
2
(1)
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this, the redox profile of biferrocene 7ac can be compared to
those of monoferrocenes 4a and 4c. 4a is oxidized at 161 mV,
while 4c is oxidized at 102 mV, which gives ΔE4a/4c = 59 mV.
ΔE1/2 of 7ac is 370 mV and approximately 59 mV of that must
come from the intrinsic potential difference between the two
independent ferrocene centers. This means that only around
311 mV (=370−59) can be attributed to stabilization via
electronic coupling. Gratifyingly, this value of 311 mV is
intermediate between those of the analogous symmetrical
biferrocenes 7aa (303 mV) and 7cc (353 mV). While care
must be taken to compare “like-for-like” systems,34 Kc is often
used as an indicator of coupling strength in mixed-valence
systems. However, due to the redox asymmetry present in 6x,
7ab, and 7ac, Kc cannot be employed in this way. A more in-
depth investigation into the electronics of these mixed-valence
isomers is currently underway in our laboratory.
that also contributes to the overall signals below 350 nm. In
most cases, the HOMO and LUMO are somewhat delocalized
across the whole molecule, although in pyridyl-based analogues
there is a clear bias toward a ferrocene-based HOMO and a
LUMO situated on the ethynylpyridine. In addition to this, the
FMOs of those systems containing both a pyridine and a
thioanisole substituent seem to show particular favorability for
the HOMO to be localized on the thioanisole fragment and the
LUMO to be localized on the pyridyl fragment as is
particularly evident in the FMOs of 3ac and 7ac (Figure 3).
When considering the biferrocene systems, the UV/vis analysis
demonstrates that these show similar transitions to their
monoferrocene analogues, with the primary difference being a
general increase in their intensity. The TD-DFT data suggests
that the majority of these transitions are dominated by
complex MLCT processes (see e.g., Table S14). Finally, it is
also worth noting that an analysis of the FMO energies of these
systems generated a trend in the energy of the HOMOs, which
mirrored that which was observed in the redox potentials of
our systems as reported above (Figure S83), suggesting that a
predictable redox tunability exists within this family. Energy
level diagrams, iso-surface depictions of the FMOs, and a list of
predicted transitions, with their full assignments, for each
compound, are provided in the Supporting Information.
INVESTIGATING THE ELECTRONIC STRUCTURE
■
To investigate the electronic structure of this series, a
combination of theoretical calculations and UV/vis spectros-
copy was employed. For this, the most stable ground-state
geometry of each compound was determined by density
functional theory (DFT) calculations in the gas phase, using
the B3LYP functional and the 6-311+G(d,p) basis set. In
addition, time-dependent DFT (TD-DFT) calculations, used
to model the UV/vis spectra, employed the aforementioned
optimized geometries and were performed using a dichloro-
methane conductor-like solvent polarization continuum model
(CPCM) with the PBE1 functional and the 6-31+G(d,p) basis
set to generate results that could be compared to the
experimental spectra obtained in the same solvent (see
Supporting Information). A few initial conclusions could be
drawn from the experimental UV/vis spectra. For instance, it
has previously been reported that unsubstituted ferrocene
displays a transition at approximately 442 nm, which relates to
two overlapping spin-allowed, Laporte forbidden transitions.40
We see that, in all cases, our introduction of alkynyl ligands
causes a bathochromic shift of this peak to higher wavelengths
(between 445 and 460 nm), coinciding with previous work in
the literature, which suggests that inclusion of an alkyne
substituent acts to decrease the highest occupied molecular
orbital (HOMO)−lowest unoccupied molecular orbital
(LUMO) gap.41−43 This is clearly demonstrated when
comparing molecules 4a and 4c, where the more electron-
withdrawing pyridyl causes a larger shift, though the trend
becomes far less distinct upon introduction of a second alkynyl
substituent. Trends relating to other ferrocene-associated
transitions are harder to quantify as these are often obscured
by large intense absorptions that occur between 250 and 325
nm. Analysis of the frontier molecular orbital (FMO) energies
and their iso-surfaces as determined by DFT calculations,
alongside the predicted absorption spectra (as modeled by
TD-DFT calculations) provided further useful insights into the
nature of these optical transitions. Our calculations suggest
that the lower energy signals that appear around 450 nm are
primarily metal-to-ligand charge transfer (MLCT) in character,
while those around 350 nm result from transitions between
orbitals that are more delocalized across the whole molecule
and are essentially π → π* in character. The absorption bands
observed at lower wavelengths can be assigned to higher-
energy π → π* transitions, though it should be noted that
those molecules containing two different arylalkyne substitu-
ents display a significant degree of interligand charge transfer
CONCLUSIONS
■
Herein, we have shown a facile synthesis of a large library of
novel, highly conjugated, alkynyl-substituted ferrocene and
biferrocene derivatives that have been fully characterized.
Cyclic voltammetry was used to examine the electronic
structure of these systems and it was shown that the differing
substitution patterns impart fine redox control over the central
ferrocene unit(s). The biferrocene molecules display subtly
different mixed-valence character, with 7ac, and likely 7ab,
displaying the characteristics of rare mixed-valence isomers. A
combination of UV/vis spectroscopy and theoretical calcu-
lations, derived from DFT, was used to further elucidate the
electronic structure of these compounds, providing clear
justification for the redox tuning as a function of HOMO
energy, occurring in a highly predictable manner. This analysis
also allowed us to analyze the optical transitions observed for
these compounds, noting a variety of complex MLCT
processes, with those asymmetric systems containing both
pyridyl and thioanisole fragments displaying more interligand
charge transfer characteristics than their symmetrically
substituted counterparts. Broadly speaking, this work demon-
strates the ease and practicality of introducing asymmetry
across a redox-center and how this can be used to subtly alter
said center’s electronic structure. Asymmetric molecules have
previously shown promise in the field of molecular electronics
in devising new systems for diodes, rectifiers, and in the tuning
of optical band gaps.44−51 The stepwise nature of our synthesis
provides a routine method of incorporating an organometallic
redox-active fragment into these designs and also provides an
opportunity to study how their properties can be changed by
externally controlling the ferrocene redox processes through
the use of electrochemical gating.52−55 In addition, this fine
control of electronic structure provides a further parameter for
the predictable tuning of FMO energies, which has large
implications in both electronic and thermal conductivity as
both are highly dependent on the alignment of the energy
levels of electrodes and analytes.56−61 We are currently
undertaking further work to examine how these molecules
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00098.
■
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AUTHOR INFORMATION
Corresponding Authors
■
Luke A. Wilkinson − Department of Chemistry, University of
York, York YO10 5DD, U.K.; orcid.org/0000-0002-
8550-3226; Email: luke.a.wilkinson@york.ac.uk
Nicholas J. Long − Department of Chemistry, Imperial College
London, London W12 0BZ, U.K.; orcid.org/0000-0002-
8298-938X; Email: n.long@imperial.ac.uk
(12) Vasdev, R. A. S.; Findlay, J. A.; Garden, A. L.; Crowley, J. D.
Redox Active [Pd2L4]4+cages Constructed from Rotationally
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2019, 55, 7506−7509.
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Prescimone, A.; Mayor, M. Deltoid versus Rhomboid: Controlling the
Shape of Bis-Ferrocene Macrocycles by the Bulkiness of the
Substituents. Organometallics 2017, 36, 858−866.
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Ferrocene-Alkynyl Conjugated Molecular Wires: Synthesis, Charac-
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Troy L. R. Bennett − Department of Chemistry, Imperial
College London, London W12 0BZ, U.K.
Jasmine M. A. Lok − Department of Chemistry, University of
York, York YO10 5DD, U.K.
Robert C. P. O’Toole − Department of Chemistry, University
of York, York YO10 5DD, U.K.
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge funding from the EPSRC (L.A.W.
for EP/N032977/2 and TLRB for studentship 2033057), and
N.J.L. is grateful for a Royal Society Wolfson Research Merit
Award. L.A.W. is also grateful to the Leverhulme Trust for an
Early Career Fellowship ECF-2019-134. This article is
dedicated to Professor Wolfgang Kaim on the occasion of
his 70th birthday, and to acknowledge his outstanding
contributions to inorganic coordination chemistry and the
understanding of mixed-valency and electron transfer.
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