Mechanochemistry of Cationic Cobaltocenium Mechanophore

Article abstract of DOI:10.1021/jacs.1c05233

Recent research on the mechanochemistry of metallocene mechanophores has shed light on the force-responsiveness of these thermally and chemically stable organometallic compounds. In this work, we report a combination of experimental and computational stud

Full text of DOI:10.1021/jacs.1c05233

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Mechanochemistry of Cationic Cobaltocenium Mechanophore
Yujin Cha, Tianyu Zhu, Ye Sha, Huina Lin, JiHyeon Hwang, Matthew Seraydarian, Stephen L. Craig,
and Chuanbing Tang*
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ABSTRACT: Recent research on the mechanochemistry of metal-
locene mechanophores has shed light on the force-responsiveness of
these thermally and chemically stable organometallic compounds. In
this work, we report a combination of experimental and computa-
tional studies on the mechanochemistry of main-chain cobaltoce-
nium-containing polymers. Ester derivatives of the cationic
cobaltocenium, though isoelectronic to neutral ferrocene, are unstable
in the nonmechanical control experimental conditions that were
accommodated by their ferrocene analogs. Replacing the electron withdrawing C-ester linkages with electron-donating C-alkyls
conferred the necessary stability and enabled the mechanochemistry of the cobaltocenium to be assessed. Despite their high bond
dissociation energy, cobaltocenium mechanophores are found to be selective sites of main chain scission under sonomechanical
activation. Computational CoGEF calculations suggest that the presence of a counterion to cobaltocenium plays a vital role by
promoting a peeling mechanism of dissociation in conjunction with the initial slipping.
mol),34−38 while the charged nature and the pairing of
counterion also provide cobaltocenium with properties that
1. INTRODUCTION
Mechanophores, mechanically labile building blocks, continue
have been used for applications of functional polyelectrolytes,
to emerge as critical components in mechano-responsive
including biomedicine and membranes.39−47 The potential of
polymers. Mechanical degradation of polymers causes the
cationic metallocenium mechanophores increases the likely
reduction in molecular weight, viscosity, and mechanical
significance of mechanistic considerations relative to their
uncharged analogues: (1) potential contributions from the
counterion to the mechanism of chain scission at the metal−
ligand bond; (2) the anticipated greater influence of Cp
substituents on metallocenium stability, especially electron-
donating versus electron-withdrawing groups. The rich
mechanochemistry of neutral metallocenes motivated us to
explore cobaltocenium as a mechanophore.
Herein we report the design, synthesis, and mechanochem-
istry of main-chain cobaltocenium-containing polymers. The
cobaltocenium is integrated into a polyoctene-like backbone
with the aid of metathesis chemistry. Sonication was employed
to carry out the mechanochemistry coupled with mechanistic
trapping of scission intermediates. The results indicate that
cobaltocenium possesses some unique mechano-responsive
behaviors, different from the shearing mechanism of neutral
metallocenes. Strikingly, computational models suggest that
mechanical distortion of the complex facilitates access of the
strength. The emergence of novel mechanophores has enabled
new opportunities to open up mechanochemistry.1−5 Several
types of mechanophores have been investigated, including
those that change color,6−9 activate latent catalysts,10,11 release
small molecules,12,13 empower self-strengthening materi-
als,14−17 and prepare conjugated polymers.18
Perhaps the classic mechanochemical response, however,
remains as chain scission. Often, sites of preferential scission
involve functional groups that have a low bond dissociation
energy (BDE), such as peroxide,19 diazo,20 disulfide,21 and
thioether,22 or even carbon−carbon bonds whose homolytic
scission leads to the formation of persistent radicals.23 It is now
appreciated that mechanochemical bond scission can be quite
mechanistically complex,14,24,25 and that simple bond dissoci-
ation energy is an imperfect correlate of mechanical lability.26
Recently, the mechanochemistry of metallocene-containing
polymers has been explored.27−30 Neutral metallocenes (e.g.,
ferrocene and ruthenocene) have been shown to act as highly
selective sites of mechanical scission, despite the thermody-
namically and chemically inert character associated with the
high BDE of the metal-cyclopentadienyl (Cp) bond.
Received: May 20, 2021
Published: July 20, 2021
Cobaltocenium, which is isoelectronic to neutral ferrocene,
represents a type of charged metallocene (or metallocenium)
with high thermal and chemical stability.31−33 The metal−
ligand BDE of Co-Cp is reported to be significantly higher
than that of ferrocene (102−195 kcal/mol vs 73−91 kcal/
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counterion to the exposed cobalt center, which in turns shifts
the extent of the mechanical coupling.
concentration of 2.5 mg/mL was exposed to an acoustic field
with a power of 8.7 W/cm2 and 50% duty cycle (1 s on and 1 s
off) in an ice bath for 2 h and then characterized by 1H and 19
F
2. RESULTS
NMR. To our surprise, in contrast to the ruthenocene and
ferrocene analogs, the cobaltocenium end group is not stable
under these sonication conditions (Figure S1). The chemical
instability remains when the dithioester CTA is replaced by a
tertiary nitrile (Figure S2).
2.1. Design Principle of Cobaltocenium Mechano-
phore. Assessing the mechanochemical reactivity of mecha-
nophores requires that they be chemically stable under the
conditions where mechanochemistry is performed. Typically,
“non-mechanical” control experiments verify the necessary
stability, but of course this needs not always be the case. Here,
cobaltocenium derivatives and polymers were initially prepared
following prior studies of the mechanochemistry of neutral
ferrocene and ruthenocene (Figure 1).27,28 A cobaltocenium
We hypothesized that the lack of stability might result from
the presence of the ester linkage. Electron-withdrawing groups
(EWG) (Figure 1, 2−1 and 2−2) destabilize cobaltocenium
due to the combination of reduced electron density on the Cp
rings and the positively charged metal center.45,46 We therefore
replaced the C-ester linkage with a triazole, which serves as a
weak electron-donating group (EDG) to the cobaltocenium
Cp ligands (Figure 1, 2−3). It was achieved by a click reaction
between ethynyl cobaltocenium and the azide-modified chain
transfer agent (Scheme S2).48 As desired, this low molecular
weight, electron-rich derivative showed no evidence of
structural change by 1H NMR in the sonication controls
(Figure S3). The stability holds in an end-functionalized
polymer control. Cobaltocenium triazole-linked PMMA was
prepared by RAFT polymerization (Mn = 53,000 g/mol, Đ =
1.03; see Table 1, P2, Scheme S2). As with the small molecular
control, no evidence of change in either the cobaltocenium or
−
PF6 counterion is observed under sonication (Figure S4).
Direct comparisons of small molecules and polymers
containing cobaltocenium substituted with EDGs suggest
that the EWGs contribute the decomposition of cobaltoce-
nium. Though the less stable cobaltocenium under the
sonication conditions does not rule out its possible mechanical
susceptibility, it must be avoided in the structures to
unambiguously explore the sole effect of mechanical stress.
This finding suggests a useful consideration for the future
design of cationic metallocenium as mechanophores.
2.2. Synthesis of Main-Chain Cobaltocenium-Con-
taining Polymers. Following the initial investigations, a class
of main-chain cobaltocenium-containing polymers was con-
ceptualized using a simple polyoctene-like chain as the
framework. This design considers (1) alkyl group as an EDG
and (2) the exclusion of functional groups to eliminate
Figure 1. Structural design of metallocenes as mechanophores:
comparison between neutral metallocenes (ferrocene and rutheno-
cene) and cationic cobaltocenium.
ester-linked dithioester was prepared as a chain transfer agent
(CTA) and employed in the reversible addition-fragmentation
chain-transfer (RAFT) polymerization of methyl methacrylate,
affording cobaltocenium ester end-labeled poly(methyl meth-
acrylate) (PMMA) (Mn= 77 000 Da, Đ = 1.02) (Table 1, P1,
Scheme S1). The use of end-functionalized polymer serves as a
control for mechanical activation under pulsed ultrasonication
of polymer solutions, as mechanical forces are focused on the
center of the polymer chain. The solution of cobaltocenium
end-labeled PMMA in dichloromethane (DCM) at a
Table 1. Cobaltocenium-Containing Polymers Used in This Study
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unnecessary complications. A combination of ring-closing
metathesis (RCM) and ring-opening metathesis polymer-
ization (ROMP) was executed toward the synthesis of multiple
cobaltocenium-labeled polymers. Multimechanophore labeling
is more sensitive than a single mechanophore in mechano-
chemistry.49 First, a diene compound 1 was prepared from
lithium 1-butene-4-cyclopentadienide and CoBr2 (Scheme 1
synthesized by copolymerization of 2 and 5-methoxycyclooc-
tene in a similar approach to previous reports.27,28
2.3. Mechanochemistry of Cobaltocenium Polymers.
1,1′-Dibutenyl cobaltocenium 1 was stable under prolonged
sonication, further confirming the effect of EDGs (Figure S5).
Sonication of P3 was then conducted in DCM at a
concentration of 2.5 mg/mL. Two-hour sonication led to a
decrease in molecular weight from 67,000 g/mol to 46,000 g/
mol (Figure 2C). The susceptibility of cobaltocenium was
further characterized by the change in 1H NMR. The Cp peaks
from cobaltocenium significantly reduced with the emergence
of new peaks corresponding to the formation of terminal
cyclopentadiene after the sonication (Figure 2B and Figure
Scheme 1. Synthesis of Cationic Cobaltocenophane and
Main-Chain Cobaltocenium-Containing Copolymers
−
S7). In addition, 19F NMR also showed the decrease of PF6 ,
indicating the destruction of cobaltocenium (Figure S6). These
results demonstrated that cobaltocenium is the preferential site
of scission in the polymer chain.
We anticipated that the scission of cobaltocenium would
resemble the heterolytic dissociation we observed previously in
ferrocene and ruthenocene,27,28 through which Co would
maintain its +3 oxidation state in the free cation or its half-
sandwich (Figure 2A). Following the earlier work on ferrocene,
we employed 1,10-phenanthroline as a trap of the dissociated
metal ions. P3 was dissolved to a final concentration of 2.5
mg/mL in a 3.0 mM solution of 1,10-phenanthroline in DCM.
and the Supporting Information). Cyclic cobaltocenium
monomer 2 was prepared by RCM of 1. Cobaltocenium-
containing polymer P3 (Mn = 67,000 g/mol, Đ = 1.15) was
Figure 2. (A) Schematic illustration of cobaltocenium dissociation under ultrasonication for 2 h with a concentration of 2.5 mg/mL; (B) 1H NMR
spectra of P3 before and after sonication; (C) GPC traces of P3 before and after sonication; (D) UV−vis absorption spectra of P3 without 1,10-
phenanthroline before and after sonication; (E) UV−vis absorption spectra of 1,10-phenanthroline, [Co(phenanthroline)3]3+, and P3 with 1,10-
phenanthroline before and after sonication.
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Figure 3. (A) Side view and (B) top view for the structural evolution of cobaltocenium (top panel) and ferrocene (bottom panel) model
compounds with different stretching distances.
The solution was then subject to the same sonication
conditions described above. The UV−vis absorption was
compared before and after sonication. The results revealed the
appearance of an absorption peak at 325 nm (Figure 2E, solid
red line), which is assigned to the ligand to metal charge
transfer of [Co(phenanthroline)3]3+. By contrast, sonication
without 1,10-phenanthroline did not show any new absorption
peaks (Figure 2D). In order to further confirm the assignment,
we independently prepared [Co(phenanthroline)3]3+ and
compared its UV−vis absorption spectrum to the sonication
product.50 As shown in Figure 2E (purple dashed line), the
peak at 325 nm is similar to that of the product of polymer
sonication. This result supports that the dissociation of
cobaltocenium is heterolytic.
The presence of a counterion differentiates cobaltocenium
from neutral metallocenes. To further understand the
stretching and scission of cobaltocenium mechanophore,
DFT calculations on 1,1′-diethyl cobaltocenium hexafluor-
ophosphate were carried out using the constrained geometry
simulating external force (CoGEF) method, which is often
employed, with important caveats, to model mechanochemis-
try.51,52 To mimic the elongation of the polymer backbone, the
end-to-end distance between the carbon atoms of the two CH3
groups of this model mechanophore was varied (Figure S8). As
shown in Figure 3B, the Cp rings are aligned in a slightly
staggered geometry at the fully relaxed state (the stretching
distance is considered as 0). As the cobaltocenium is stretched
by 2 Å, the two Cp rings rotate to orient two ethyl groups in
the antiposition. With a further increase in end-to-end
distance, the initially aligned two Cp rings start to slip so
that the Co metal atom is no longer normal to the Cp
centroids, resulting in a sharper increase of the system energy.
This slipping of the parallel Cp ligands well resembles that of
ferrocene.27 However, further displacement results in dramat-
ically different structural evolutions from what has been
observed in ferrocene. As shown in Figure 3A, the simulated
stretching of cobaltocenium leads to an onset of peeling
motion in the Cp rings at approximately 3.8 Å. In comparison,
ferrocene is still under the slipping process. Further stretching
of this structure results in the elongation of the Co-Cp bond
and further opens the peeled structure with an increased
potential. The full dissociation of Cp takes place when the
stretching distance reaches 6.5 Å, producing Cp− and
[CpCo]2+PF6 . This dissociation mechanism is strikingly
−
different from that of ferrocene. As shown in Figures 3A,B,
ferrocene undergoes a chain slipping mechanism, but
cobaltocenium has a mechanism of first slipping and then
peeling.
One should acknowledge the limit of the CoGEF
calculations, which characterize the chain scission behavior at
essentially 0 K. In real cases, thermally assisted reactions would
occur long before the force reaches the CoGEF calculated
maximum. Even the specific thermally assisted pathway might
differ from that calculated by CoGEF. Nevertheless, the
calculations provide insights into the likely dissociation
mechanism and reveal significant differences in cobaltocenium
scission relative to its ferrocene analog. The energy increases
until it reaches the first potential peak at 3.7−3.8 Å because of
the distorted bond angle and bond distance with the onset of
the peeling process (Figure 4A, blue symbol). When the
stretching distance of cobaltocenium reaches 3.8 Å, the
CoGEF potential abruptly decreases. A second decrease in
potential energy occurs at 5.6 Å. Neither transition is observed
in ferrocene dissociation, which proceeds through a smooth,
single-step process (Figure S9). In addition, the maximum
relative energy prior to dissociation (Emax) of cobaltocenium is
lower than that of ferrocene. Furthermore, under the same
pulling point and attachments of the model compounds, the
maximum force (Fmax) of cobaltocenium calculated over the
stretching is significantly lower than that of ferrocene. Fmax is
2.73 and 3.18 nN for cobaltocenium and ferrocene,
respectively (Figure S10).
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dissociation of cobaltocenium through a peeling-like pathway
rather than slipping. Given that the calculated Emax and Fmax for
cobaltocenium are lower than those of ferrocene, it can be
concluded that the counterion facilitates the bond dissociation,
making cobaltocenium a more sensitive mechanophore.
To further examine the mechanism of bond cleavage, a
molecular electrostatic potential (ESP) was used to depict the
charge distribution during dissociation.54 The representative
ESP maps of the cobaltocenium model compound are shown
as a function of stretching distance in Figure 4B. Before chain
elongation, the charge distribution was biased on cobaltoce-
−
nium and PF6 . At an added stretch of 3.8 Å, where the Cp is
peeled off from the sandwich structure, the negative charge on
Figure 4. (A) CoGEF potential (system energy, blue symbol) and
metal-ion (Co−P) distance (red symbol) as a function of the change
in end-to-end distance and (B) representative electrostatic potential
map of cobaltocenium as a function of the change in end-to-end
distance (the contour surface is described based on van der Waals
surface with an electron density of 0.001 e/Bohr3 as proposed by
Bader53).
−
PF6 starts to diminish. With the decreased distance between
−
cobalt and PF6 , the charge separation is reduced through
delocalization, stabilizing the system. After a single ligand
dissociation is complete, the half-sandwich cobaltocenium
holds a more positive charge, and the negative charge densities
are distributed on PF6 and dissociated Cp−. The ESP results
−
support the heterolytic dissociation of cobaltocenium
mechanophore, resulting in separated charge densities after
dissociation.
We speculated that this difference in dissociation might be a
result of the presence of a counterion. To examine the effect of
the counterion during chain stretching, the distance of cobalt
to PF6− was measured and compared to the CoGEF potential.
As shown in Figure 4A (red symbol), the distance between
cobalt(III) and phosphorus (Co−P) dramatically decreases at
the energetic transition points (local maxima at 3.8 and 5.6 Å)
noted above. We also calculated the distance of the metal
center to the nearest fluoride (Co−F) (Figure S11), which
shows a similar trend to that observed for Co−P. At these
We further explored how a change in counterion affects the
calculated dissociation of cobaltocenium using a smaller Cl−
anion. The addition of Cl− also contributes to a peeling-like
−
dissociation mechanism, similar to PF6 (Figure S12). The
−
interaction of cobalt(III) with Cl− is stronger than with PF6 ;
Co(III) is considered to be a hard acid, which favors
interactions with a harder base like Cl− relative to the softer
55
−
PF6 . Consistent with the expectation of a stronger
interaction, the use of Cl− results in Cp dissociation at a
shorter stretching distance (6.1 Å) and lower Emax (75.3 kcal/
−
points, PF6 inserts between the two Cp rings (Figure 3B),
facilitating the peeling of the sandwich structure. This
observation can be explained by the coordinated interplay of
stretching and Coulombic attraction. The stretching of
cobaltocenium makes Cp rings slip, providing space for the
−
mol) than computed for PF6 (Figure S13).
3. DISCUSSION
−
PF6 anion to approach the cobalt cation, which is otherwise
Experimentally, both end group analysis and metal ion
trapping indicate that the dissociation of cobaltocenium
hindered by two Cp rings. This attraction guides the
Figure 5. Proposed mechanisms of dissociation of mechanophores: (A) ferrocene, (B) ansa-ferrocenophane, and (C) cobaltocenium.
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mechanophore is a net heterolytic process, in which the
counterion in cobaltocenium might stabilize the dissociated
cobalt ions even in the state of a partial half-sandwich
structure. Though conceived differently, it was previously
observed that the addition of external salts (tetrabutylammo-
nium bromide) promotes the chain scission of ferrocene by
stabilizing charge separation and/or anionic ligand exchange.27
The insights into dissociation mechanisms between
cobaltocenium and ferrocene could be further gained by
probing the change of coordination states of ligands to the
metal center during stretching. We plotted the distances
between Co and each carbon atom on one of the ligands versus
the stretching distance. As shown in Figures S14 and S15, in
the beginning of cobaltocenium, the coordination is η5.
However, once the Cp is peeled off at 3.8 Å, the coordination
changes to η1 based on the dramatic change of the Co−C
distance. In the case of ferrocene, the coordination changes
from η5 to η2 during the slipping process. We ascribe this
difference in coordination to the increased coordination/
charge donation provided by the associating counterion, which
pushes the dissociating Cp to form the η1 rather than η2 state.
Different dissociative mechanisms, based on slipping and
peeling, have been previously reported in metallocene
mechanophores.27,28 For instance, ferrocene and ruthenocene
undergo slipping of Cp against an external force, leading to
complete dissociation (Figure 5A). Very recently a peeling
process was observed for ferrocenophane mechanophores in
which Cp ligands are bridged by an alkyl tether.30 The bridge
serves as a conformational lock that restricts rotation along the
Cp-Fe-Cp axle, inhibiting slipping and forcing the Cp to be
peeled away from the nascent sandwich structure (Figure 5B).
Here, an unbridged cobaltocenium mechanophore undergoes
peeling-like dissociation not because of restricted conforma-
tions but due to the interaction between cobalt cation and
counterion. The insertion of the counterion distorts the
sandwich structure, resulting in a partial tilting of the Cp and
contributions from peeling-type pathways (Figure 5C).
Specifically, the partial shearing at the beginning opens the
space to facilitate the interaction of the counterion with the
metal center. This creates a CoGEF reaction path that differs
from both ferrocene/ruthenocene and ferrocenophane.
Materials, characterization methods, sonication test,
synthetic schemes, NMR spectra, and calculation details
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Chuanbing Tang − Department of Chemistry and
Biochemistry, University of South Carolina, Columbia, South
Carolina 29208, United States; orcid.org/0000-0002-
0242-8241; Email: tang4@mailbox.sc.edu
Authors
Yujin Cha − Department of Chemistry and Biochemistry,
University of South Carolina, Columbia, South Carolina
29208, United States
Tianyu Zhu − Department of Chemistry and Biochemistry,
University of South Carolina, Columbia, South Carolina
29208, United States; orcid.org/0000-0001-9115-6462
Ye Sha − Department of Chemistry and Biochemistry,
University of South Carolina, Columbia, South Carolina
29208, United States; orcid.org/0000-0003-3338-1228
Huina Lin − Department of Chemistry and Biochemistry,
University of South Carolina, Columbia, South Carolina
29208, United States
JiHyeon Hwang − Department of Chemistry and
Biochemistry, University of South Carolina, Columbia, South
Carolina 29208, United States
Matthew Seraydarian − Department of Chemistry and
Biochemistry, University of South Carolina, Columbia, South
Carolina 29208, United States
Stephen L. Craig − Department of Chemistry, Duke
University, Durham, North Carolina 27708, United States;
orcid.org/0000-0002-8810-0369
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c05233
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Science Foundation
(NSF) under Grant CHE-1904016. The partial support from
the NSF EPSCoR Program under Grant OIA-1655740 is
acknowledged.
4. CONCLUSIONS
In summary, we explored cationic cobaltocenium as a
mechanophore and prepared main-chain cobaltocenium-
containing copolymers via RCM and ROMP. It was discovered
that cobaltocenium must be attached with EDGs, while EWGs
would destabilize the mechanophore chemically, significantly
different from ferrocene and ruthenocene. Although the
cobaltocenium possesses high thermodynamic stability, it
showed chain scission under an acoustic field. In contrast to
neutral metallocenes in the absence of counterions, cobalto-
cenium has a different mechanism of chain scission, starting
from the initial slipping followed by the peeling in the presence
of the counterion, confirmed by the CoGEF computational
studies. This new finding further expands the fascinating
mechanochemistry of metallocenes.
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