Triggered Metal Ion Release and Oxidation: Ferrocene as a Mechanophore in Polymers

Article abstract of DOI:10.1002/anie.201803524

The introduction of mechanophores into polymers makes it possible to transduce mechanical forces into chemical reactions that can be used to impart functions such as self-healing, catalytic activity, and mechanochromic response. Here, an example of mechanically induced metal ion release from a polymer is reported. Ferrocene (Fc) was incorporated as an iron ion releasing mechanophore into poly(methyl acrylate)s (PMAs) and polyurethanes (PUs). Sonication triggered the preferential cleavage of the polymers at the Fc units over other bonds, as shown by a kinetic study of the molar mass distribution of the cleaved Fc-containing and Fc-free reference polymers. The released and oxidized iron ions can be detected with KSCN to generate the red-colored [Fe(SCN)_n(H₂O)_6?n)]^(3?n)+ complex or reacted with K₄[Fe(CN)₆] to afford Prussian blue.

Full text of DOI:10.1002/anie.201803524

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
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Accepted Article
Title: Triggered metal ion release and oxidation: Ferrocene as new
mechanophore in polymers
Authors: Michela Di Giannantonio, Mathieu A. Ayer, Ester Verde-
Sesto, Marco Lattuada, Christoph Weder, and Katharina M.
Fromm
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803524
Angew. Chem. 10.1002/ange.201803524
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10.1002/anie.201803524
Angewandte Chemie International Edition
COMMUNICATION
Triggered metal ion release and oxidation: Ferrocene as new
mechanophore in polymers
Michela Di Giannantonio,[a] Mathieu A. Ayer,[b] Ester Verde-Sesto,*[b,c] Marco Lattuada,*[a,b] Christoph
Weder,*[b] and Katharina M. Fromm*[a]
Abstract: The introduction of mechanophores into polymers allows
transducing mechanical forces into chemical reactions that can be
used to impart functions such as self-healing, catalytic activity, or
mechanochromic response. Here, the first example of a mechani-
cally induced metal ion release from a polymer is reported. Ferroce-
ne (Fc) was incorporated as an Fe-ion releasing mechanophore into
poly(methyl acrylate)s (PMAs) and polyurethanes (PUs). Sonication
triggered the preferential cleavage of the polymers at the Fc units
over other bonds, as shown by a kinetic study of the molar mass
distribution of the cleaved Fc-containing and Fc-free reference
polymers. The released and oxidized Fe2+ ions can be detected with
KSCN to generate the red-colored [Fe(SCN)n(H2O)6-n)](3-n)+ or reacted
with K4[Fe(CN)6] to afford Prussian blue.
pyridine:Eu3+,[27a]
oxypyridine with a penta-(ethylene glycol) core, coordinated with
transition metal (Zn2+) and lanthanide (La3+) ions.[28]
and
2,6-bis-(1′-methylbenzimidazolyl)-4-
In general, the rate of chain scission upon mechanical stress
exerted e.g. by ultrasonication depends on the type of mechano-
phore incorporated in a polymer backbone. The pre-determined
breaking point in a polymer can be preferentially degraded if the
bond strength of the weak link is lower than that of all other
bonds in the polymer chain.[29] While there are studies on the
production of Fe nanoparticles from the decomposition of
ferrocene (Fc) during sonication, surprisingly, the Fc unit has so
far remained unexplored as
a mechanophore in stimuli-
responsive materials.[30] The enthalpy relative to the heterolytic
bond dissociation of this motif (40 kcal/mol)[31], is about half that
of typical covalent single bond values (80 kcal/mol for a C‒C
bond).[32] Its high susceptibility to a large number of organic
reactions[33] makes this sandwich complex useful in a wide range
of applications, such as molecular sensors,[34],[35] in asymmetrical
catalysis,[36] for electrochemical studies,[37] and biomedical
applications.[38] Relying on previous strategies for the design of
mechanically responsive polymers, we used single-electron
living radical polymerizations (SET-LRP)[33c],[8] to prepare
poly(methyl acrylate)s having one chain-centered ferrocene
The development of new stimuli-responsive polymers that have
the ability to change their chemical or physical properties upon
application of an external stimulus such as light,[1] temperature,[2]
pH,[3] mechanical force,[4] or redox potential is receiving
considerable attention.[5] A steadily growing number of polymeric
materials has been shown to respond in a useful way to
mechanical stress, allowing a wide range of chemical transfor-
mations that lead, for example, to
a
change of
color/fluorescence,[6] the formation of proton catalysts,[7] the
generation of acids[7-8] and bases,[9] or the release of small
molecules.[10] Applications of such mechanoresponsive
polymeric materials include sensors,[11] self-healing materials,[12]
drug delivery,[13] tissue engineering,[14] and catalysis.[15],[16] They
involve mechanophores such as spiropyran,[17] azo,[18]
epoxide,[19] β-lactam,[20] dioxetane,[21] or rhodamine moieties,[22]
among others.[23] Several examples involve non-covalent
(supramolecular) motifs,[24] or coordination bonds.[25] Sijbesma
and coworkers demonstrated the reversible mechanically
induced dissociation of a Pd-phosphane bond embedded in a
linear polymer using ultrasound.[26] This result triggered the
investigation of other mechanoresponsive polymers containing
e.g. Ag-[26] and Ru-carbene complexes[15] for catalysis.[9, 15, 26-27]
Further examples include 2,6-bis(1′-methylbenzimidazolyl)-
motif (Fc-PMA). We further exploited
a polycondensation
reaction to synthesize polyurethanes,[39] with multiple,
statistically distributed ferrocene moieties along the backbone
(Fc-PU).[40] The mechanoresponse of these materials was
studied by monitoring the decrease of the molecular weight of
these polymers, induced by ultrasonication of dilute solutions.[18,
23e, 26, 41]
A mathematical model was used to describe the
scission kinetics, while the release and oxidation of Fe2+ ions
upon ultrasonication was demonstrated by complexation with
KSCN or K4[Fe(CN)6] to afford the red-colored [Fe(SCN)n(H2O)6-
n)](3-n)+ (n = 1-6) or Prussian blue, respectively.
[a]
M. di Giannantonio, Prof. Dr. M. Lattuada, Prof. Dr. K. M. Fromm
Department of Chemistry
University of Fribourg
Chemin du Musée 9, CH-1700 Fribourg
E-mail: Katharina.fromm@unifr.ch
Dr. M. A. Ayer, Dr. E. Verde-Sesto, Prof. Dr. C. Weder
Adolphe Merkle Institute
University of Fribourg
Chemin des Verdiers 4, CH-1700 Fribourg
Dr. E. Verde-Sesto
Polymat, Joxe Mari Korta Center
University of the Basque Country UPV/EHU
Acda Tolosa 72, E-20018 Donostia-San Sebastian
Scheme 1. Syntheses of a) the ferrocene-containing poly(methyl acrylate)
(Fc-PMA) and b) the ferrocene-free poly(methyl acrylate) reference (ref-PMA).
[b]
[c]
For a first proof of principle, a Fc-containing difunctional initiator
(Scheme 1a) or, for reference purposes, the monofunctional Fc-
free initiator methyl α-bromoisobutyrate (Scheme 1b), were
used to prepare Fc-PMA and ref-PMA with number-average
molecular weights (Mn) of 133 and 119 kDa respectively, and a
narrow dispersity of Đ ~ 1.1 (Table 1 and Figure S20).[8, 17, 23a, 42]
FT-IR spectra (Figures S1-S2) confirmed the successful
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201803524
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indicated (in min); c) KSCN (dissolved in a 1:1 v/v mixture of MeOH and THF,
0.53 M) was added to the Fc-PMA solution before and after sonication, to
show the formation of the red complex FeSCN2+ after sonication and the
consequent breaking of ferrocene with the releasing of Fe ions. Fc-PMA
solutions (2 mg/mL in THF) before (pale yellow on the left) and after 90 min
sonication (red complex on the right).
preparation of all polymers (with and without ferrocene), which,
together with the monomers and initiator, were also fully
1
characterized by H NMR and 13C NMR spectroscopy (Figures
S3-S22), size exclusion chromatography (SEC) (Figures S23-
S25), and elemental analysis (see SI).
Further reference experiments with ferrocene and a short Fc-
PMA2 with a Mn of ca. 40 kDa show that these species do
neither dissociate nor release metal ions upon sonication,
consistent with the fact that their Mn is below the limiting
molecular weight Mlim for chain scission of PMA during
ultrasonication (see SI, Figures S25-S26).[43],[41b]
Upon ultrasonication at 0°C under Ar (see SI for details), the Mn
of Fc-PMA, determined by SEC, decreases much faster than in
the case of ref-PMA, notably during the first minutes of the
experiment (Figure 1a). A bimodal profile is observed in the
SEC traces (Figure 1b), showing a decrease of the most
frequent species in Fc-PMA at an elution time of ca. 12 min
(associated with an Mn of 133 kDa) in favor of a peak at an
elution time of ca. 12.7 min (Mn = 52 kDa) (Figure 1)a. This
indicates that the chains are cleaved preferentially at their
centers, so that Mn is roughly cut in half, similar to the previously
reported PMA samples containing oxime sulphonate or
benzocyclobutene as mechanophores.[8, 12, 17] For Fc-PMA, this
could be interpreted as preferential cleavage at the ferrocene
entity versus the C‒C bonds of the remainder of the polymer
backbone, as the Fc-unit is placed in the middle of the polymer.
With ref-PMA (Figure S23), the result was similar; although the
chain scission rate was significantly lower (Figure 1a).
To prove the ultrasound-induced cleavage of Fc-PMA at the Fc
unit itself (and not at e.g. the ester function), KSCN was added
in air to the sonicated solutions, yielding the typical red Fe3+
complex [Fe(SCN)n(H2O)6-n)](3-n)+ (n = 1-6) (Figure 2c).[44] Similar
experiments have been performed also with Fc-2, Fc-PMA2 and
Fc-PMA3 (with one Fc at the chain end, see SI) as further
controls. In none of these cases was a color change detected
during and after sonication (Figures S47-S49).
The detection of Fe3+ with KSCN after sonication of Fc-PMA
indicates that an oxidative release of the metal ions took place.
The cleavage of the Fc unit may occur either homolytically[45] or
heterolytically.[46] Oxidation of the iron ion can take place in a
half-sandwich type complex or after its release into the solution.
While thiocyanate is able to act as terminal ligand on
monocyclopentadienyl complexes of Fe,[47] the UV-vis spectrum
confirms the presence of [Fe(SCN)n(H2O)6-n)](3-n)+ (n = 1-6)
(Figure S30).[44]
To gain insight in the kinetics of the ultrasound-induced chain
scission of the polymers investigated, we analyzed the data with
a recently developed kinetic model (see SI for details, Figures
S33-S45) that allows a distinction between specific chain
scission (i.e. cleavage of the mechanophore) and random chain
cleavage at other positions.[48] Considering that the polymer size
influences the scission rate (which increases with the molecular
weight), the model assumes that the Mn of the polymer
decreases by following a pseudo first order kinetic equation, with
effective kinetic constants that change with time (see SI), on
account of the fact that the polymer chains become shorter
during the experiment. The mean values of these constants are
compiled in Table 1. Figure 1a shows the results of the model
calculations (black and red curves) compared to the
experimental evolution of the ratio of the number-average
molecular weight (Mn(t)) and the initial average molecular weight
(Mn) upon ultrasonication of solutions of ref-PMA (black
squares) and Fc-PMA (red squares). The data describe the
evolution of the entire population of polymer chains in the
system, based on a previously tested approach.[49] For the ref-
PMA, the average rate constant of k2 = 0.0086 min-1, derived
from the initial slope of the curve, corresponds thus to the
unspecific chain scission events. For the Fc-PMA, two average
rate constants of k1 = 0.058 (associated with the ferrocene
dissociation) and k2 = 0.0064 min-1 (corresponding to unspecific
chain scission) were found (Table 1), hence the ferrocene
cleavage is about 10 times more probable than the unspecific
chain scission. The model also permits to evaluate the fraction
of ferrocene groups cleaved during the sonication process,
which is about 52% in the case of Fc-PMA and less than 4% in
the case of Fc-PMA2 polymers.
Figure 1. a) Decrease of molecular weight of ref-PMA (black) and Fc-PMA
(red) as a function of time (Mn(t)). Results are shown as averages of three
experiments (red and black squares) with error bars representing standard
deviation at each data point. The inset shows a magnification of the data
recorded in the first 15 min; b) Size exclusion chromatography (SEC) traces
revealing an increase of the elution time (concomitant with a decrease of the
molecular weight) upon ultrasonication of a Fc-PMA solution for the time
a
A shoulder appearing after 50 min of sonication at an elution time of ca. 13.5
min (Mn = 33 kDa) seems to be associated with the repeated (non-specific)
cleavage of fragments formed by the first cleavage of the population of chains
with the highest molecular weight (shoulder at an elution time around 11.2 min
corresponding to an Mn of 283 kDa) (details about the correlation between
elution time and polymer mass are provided in the Supporting Information).
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COMMUNICATION
In order to increase the number of metal ion release events per
polymer chain, we also investigated polyurethanes, obtained by
a polycondensation reaction (Scheme 2), with several ferrocene
moieties randomly distributed in the polymer backbone.[39-40]
Thus, Fc-PU (Mn =100 kDa, Đ = 2.03) with an average ferrocene
content of 6/chain (3 mol-% Fc), was synthesized and subjected
to an ultrasonication experiment. The obtained results were
compared with a ferrocene-free ref-PU of similar molecular
weight and dispersity (see SI, Figure S21).
reference experiments are available in the Supporting
information.
We applied a similar kinetic model to the molecular weight data
extracted from the ultrasound-induced chain scission
experiments of PU as with Fc-PMA (Figure S46). The larger
number of Fc moieties in the Fc-PU, and their random
distribution along the chains, increase their probability of being
cut by subsequent scission events. The molecular weight
decrease observed upon ultrasonication of ref-PU, void of Fc
moieties, can be described by a single rate constant of k2 =
0.0002 min-1, ascribed to the non-specific bond cleavage. This is
much lower than for the non-specific bond cleavage in ref-PMA.
For Fc-PU, the model reveals the overlay of two types of chain
scission events, that are characterized by average rate
constants of k1 = 1.65 and k2 = 0.00085 min-1, which can be
associated with the dissociation of the (weaker) ferrocene
mechanophore and unspecific chain scission, respectively. The
difference between the values of k2 for ref-PU and Fc-PU can be
explained by the fact that the constants depend on the initial
molecular weight of the polymer and its constitution. It is
important to highlight that the value of k1 determines the initial
slope of the molecular weight evolution as function of time, as
well as the final consumption as ferrocene moieties. The k1
value for Fc-PU is thus ca. three orders of magnitude higher
than the unspecific rate of scission k2 (Table 1), and confirms the
role of Fc groups as effective mechanophores. A comparison of
the chain scission data for Fc-PMA and Fc-PU shows that the
scission rate constants associated with the cleavage of the Fc
moieties are much larger in PU than in PMA, and always much
larger than that of non-specific scission rate constants in both
polymers (Table 1). This can be accounted for by the larger
number of Fc units in Fc-PU that are each preferentially cleaved
with respect to the other bonds in the polymer backbone as well
as the chemical differences between PMA and PU. The above
model also permits to estimate that about 25% of the Fc
moieties are cleaved at the end of sonication in the Fc-PU
polymer, which is largely confirmed by a titration experiment
(see SI).
Scheme 2. Synthesis of ferrocene-containing polyurethane (Fc-PU) and
ferrocene-free polyurethane (ref-PU) using 1,1’-di(hydroxymethyl)ferrocene
(Fc-2) as bifunctional monomer that copolymerize with polytetrahydrofuran
(PTHF), methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BDO)
(see SI). (MA = methylacrylate; Me6TREN = tris[2-(dimethylamino)ethyl]amine;
DBTDL = dibutyltin dilaurate).
The scission of Fc-PU upon sonication is indicated by a shift of a
broad band observed in the SEC trace (Figure 2a). Since each
Fc-PU chain contains several ferrocene mechanophores that
are statistically distributed along the backbone, the breaking of a
Fc entity can also lead to longer fragments that can then be
cleaved again, even multiple times. As a result, the molecular
weight distribution remains quite broad, with a wide range of
species present. For the ferrocene-free ref-PU, the elution peak
shifts as well, however slower, and the peak width is decreasing,
likely indicating that the polymer breaks into more regular
fragments (Figure S27). The same behaviour was observed for
similar polyurethanes featuring azo motifs as mechanophores.[50]
a
Table 1. Description, fitting parameters and cleavage rate constants of the
sonicated ferrocene-containing and ferrocene-free polyurethanes Fc-PU and
ref-PU and the ferrocene-containing and ferrocene-free poly(methyl acrylate)s,
Fc-PMA, ref-PMA1, and ref-PMA2.
a
Polymer
Mn
Đa,b
Fc nbrc
Cleavage rate
(kDa)
constantsd(min-1)
e
b
k1
k2
Fc-PU
ref-PU
Fc-PMA
ref-PMA
100
119
133
129
2.03
1.60
1.10
1.08
6
0
1
0
1.6548
-
0.0583
-
0.00085
0.0002
0.0064
0.0086
Figure 2. a) SEC traces of Fc-PU revealing an increase of the elution time
(decrease of the molecular weight,; b) KSCN (dissolved in a 1:1 v/v mixture of
MeOH and THF) was added to the solution of Fc-PU (0.75 mg/mL in THF),
before (left, uncolored solution) and after sonication (right, red solution), to
show the formation after sonication of the red complex Fe[SCN]2+ (due to the
ferrocene breaking and the consequent release of Fe ions).
Fc-PMA2
43.6
1.12
1
0.0144
0.00098
aDetermined by size-exclusion chromatography (SEC). bThe dispersity (Đ) is
determined from the ratio of Mw/Mn. cAverage number of ferrocene moieties
per chain calculated from the measured Mn and the composition of the
reaction feed.[39] dObtained by modeling the Mn decrease as shown by SEC
data after the sonication experiments (0.75 mg/mL in THF, 10.4 W/cm2, 0 °C).
eThe k2 values of Fc-PU and ref-PU are lower than those for PMA and Fc-PU
of similar molecular weight. This may be related to the fact that the PMA
contains higher substituted carbon atoms in the chain, as well as ester
functions instead of urethane groups, which are expected to break more easily
than the bonds in PU.
As a result of the multiple breaking of the chains and the release
of more than one Fe-ion per polymer chain,
a lower
concentration of Fc-PU (0.75 mg/mL) as compared to Fc-PMA
solution (2 mg/mL) was sufficient to detect the oxidative release
of iron as red thiocyanate complex after sonication (Figure 2b).
Again, the formation of the red complex [Fe(SCN)n(H2O)6-n)](3-n)+
(n = 1-6) was proven by UV-vis spectroscopy (Figure S31).[44]
Videos of the Fc-PU color-change and the corresponding
Thus, ultrasonication can selectively cleave Fc-containing
polymers at the ferrocene moieties, triggering thus oxidative
metal ion release that can be evidenced by the formation of the
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are under way to determine the scission mechanism of the
metallocene and the use of such a metal-release on demand
system for applications in catalysis.
red thiocyanate complex (Figures 1c, 2b and supporting
movies). Control experiments with KSCN containing solutions of
Fc-PU, Fc-PMA and Fc-2 in THF (2 mg/mL) that were heated to
reflux for 24 hours without sonication did not lead to color
changes, proving that the ferrocene scission is not thermally
induced.
Acknowledgements
Metal ion release was further proven by a possible application,
namely the generation of the dark blue pigment Prussian blue on
demand, which is used for example in medical applications for
the absorption of toxic thallium ions or radioactive cesium.[51] To
achieve this, we added K4[Fe(CN)6] (0.0414 M) to a solution of
Fc-PU2 containing 12% of ferrocene (0.75 g/mol, Mn = 46 kDa,
Đ = 2.6), which was prepared by a similar procedure as the Fc-
PU with 3% Fc, described above (Figure S15 and Figure S25).
During the sonication of Fc-PU2, we observed the formation of a
blue colloidal solution (Prussian blue), which confirms the
oxidative release of iron ions (Figure 3 (right)). A control
experiment with ref-PU or Fc with K4[Fe(CN)6] under sonication
did not yield the blue pigment (Figures S50 and S51), nor did
Fc-PU2 in presence of hexacyanoferrate(II) without sonication
(Figure 3a). Although the Mn of Fc-PU2 of 46 kDa is likely close
to the limit of Mn allowing to study the molecular weight decrease
upon sonication, it was nevertheless shown to work as a further
example of color change after the oxidative iron ion release.
This work was supported by the National Center of Competence
in Research “Bio-Inspired Materials” (Project 141849) of the
Swiss National Science Foundation, the Fribourg Center for
Nanomaterials (FriMat), the Adolphe Merkle Foundation, and the
University of Fribourg. The research leading to these results has
received funding from the European Research Council under the
European Union's Seventh Framework Programme (FP7/2007-
2013)/ERC grant agreement n° 291490-MERESPO.
The authors thank Prof. Bernd Giese for fruitful discussion.
Keywords: Polymer chemistry, stimuli-responsive polymers,
mechanophores, organometallic chemistry.
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In conclusion, we have used for the first time ferrocene
derivatives as mechanophores with the ability to break and to
release the central metal under mechanical stress in solution.
Two competitive scission events were observed in Fc-containing
polymers, the cleavage of the Fc mechanophore (k1) and the
one of random bonds (k2). The scission for Fc is at least 10
times more favored than unspecific cleavage. It is thus
conceivable to use also other metallocenes as mechanophores
for the selective metal ion release.[52] Furthermore, the triggered
oxidative release of the metal ion from ferrocene can be used to
induce a color change and/or the formation of a pigment for e.g.
analytical and medical applications. It is envisageable to use this
unique type of polymer scission also in catalysis, as iron ions are
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.
formation of Cp , considering that during sonication experiments
radical species can also be easily generated.[8, 46] Further studies
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This work focuses on ferrocene derivatives as new mechanophores embedded in
polyurethane and poly(methyl acrylate), able to selectively break under mechanical
stress in solution, releasing iron ions in solution.
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