Selective scission of pyridine-boronium complexes: Mechanical generation of Bronsted bases and polymerization catalysts

Article abstract of DOI:10.1039/c0jm03619f

Coupling two pyridine-capped poly(methyl acrylate) (PMA) chains of varying molecular weights to bis(pentafluorophenyl)boron chloride afforded the first examples of boronium-based polymers. Ultrasonication of CH₃CN solutions of these polymers wi

Full text of DOI:10.1039/c0jm03619f

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PAPER
Selective scission of pyridine–boronium complexes: mechanical generation of
Brønsted bases and polymerization catalysts†
*
Kelly M. Wiggins, Todd W. Hudnall, Andrew G. Tennyson and Christopher W. Bielawski
Received 24th October 2010, Accepted 24th December 2010
DOI: 10.1039/c0jm03619f
Coupling two pyridine-capped poly(methyl acrylate) (PMA) chains of varying molecular weights to
bis(pentafluorophenyl)boron chloride afforded the first examples of boronium-based polymers.
Ultrasonication of CH3CN solutions of these polymers with number average molecular weights (Mn) >
40 kDa induced selective scission of a boron–pyridine bond, affording a two-fold reduction in Mn. The
liberated pyridine was used to effect a colorimetric change, via a stoichiometric Brønsted acid–base
reaction with an indicator, and to catalyze the polymerization of a-trifluoromethyl-2,2,2-trifluoroethyl
acrylate. No reduction in Mn, colorimetric response, or polymerization activity were observed (i) in the
absence of sonication, (ii) for polymers with Mn < 40 kDa, (iii) for a high molecular weight PMA (Mn ¼
110 kDa) containing a terminal boronium species, or (iv) when the boron–pyridine adduct was not
covalently linked to a PMA chain. Collectively, these results support the notion that the
aforementioned scission processes were induced by an applied mechanical force.
We8 and others9 have been interested in using ultrasound-
induced mechanochemistry to activate catalysts for facilitating
synthetic transformations, including olefin metathesis and
coupling reactions. All of the mechanoresponsive catalysts,
termed mechanocatalysts,9 reported to date feature transition
metals,8,9 which can be complicated by undesired changes in
coordination number and redox state leading to premature
decomposition.2,10 To circumvent these design limitations, we
sought to prepare a transition metal-free mechanocatalyst by
utilizing a p-block, rather than a d-block element. Lewis acid–
base pairs comprised of p-block elements are known to form
reversible bonds with tunable strengths,11 and thus may be well
suited for mechanoresponsive systems.
Introduction
Mastication and other methods of mechanical degradation on
the physical and chemical properties of polymeric materials have
been studied since the 1930s.1 More recently, ultrasound has
received significant attention for its ability to mechanically alter
the chemical structures, physical properties and functions of
polymeric materials.2,3 In this technique, solvodynamic shear
forces are created when microbubbles form in solution and then
collapse under sonication. The solvated polymer segments
nearest the collapsing bubbles move at higher velocities than
those farther away in a manner that forcibly elongates the
polymer chains.4 For polymers of sufficiently high molecular
weight, this tension may facilitate the selective activation of
chemical processes at predesignated, centrally located sites –
termed mechanophores.2,3
An attractive scaffold for p-block based mechanophores are
boroniums, which are tetrahedral, four-coordinate, formally
cationic boron compounds that feature two coordination sites
occupied by neutral ligands (e.g., pyridine) and two occupied by
s-bound anionic substituents. Boroniums have been known for
over half a century,12 and are the most stable of the known classes
of boron-based cations.13 Despite their high stabilities, these
species have received surprisingly little attention for use in
macromolecular applications. We hypothesized that the coordi-
nation of two pyridine-capped polymers to an electron-deficient
Recent advances in ultrasound-induced mechanochemistry
have enabled the targeted cleavage of predetermined scissile
bonds to release highly reactive moieties.2,3 For example, Moore
reported the generation of cyanoacrylate moieties via a mechan-
ically-facilitated electrocyclic ring-opening of cyclobutanes
embedded within polymer chains.5 Applications targeted for
these and other mechanically-activated systems range from self-
repairing materials and diagnostics6 to the establishment of
fundamentally new chemical reactions.7
diaryl boryl fragment would yield
a main group-based
mechanophore with boron–pyridine bonds capable of under-
going scission under stress. Herein we describe the synthesis of
the first boronium-based polymers and demonstrate the
mechanically-activated, selective bond-cleavage of a boron–
nitrogen bond therein, whereby the liberated pyridine effected
stoichiometric Brønsted acid–base reactions and catalyzed
anionic polymerizations.
Department of Chemistry and Biochemistry, The University of Texas at
Austin, Austin, TX, 78712, USA. E-mail: bielawski@cm.utexas.edu
† Electronic supplementary information (ESI) available: Additional
synthetic and characterization details. See DOI: 10.1039/c0jm03619f
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Table 1 Summary of molecular weight data for chain coupling and
scission experimentsa
Results and discussion
Mechanoresponsive polymer synthesis and analysis
[(1)(PyPM)2][Cl]
To access the targeted mechanoresponsive materials, pyridine-
capped PMA polymers8 of varying molecular weights (PyPM,
where M corresponds to the number average molecular weight
(Mn) in kDa of the PMA chain PM) were treated with 0.5 molar
equiv of bis(pentafluorophenyl)boron chloride (1–Cl).14 After
allowing the reaction to proceed for 12 h at room temperature,
the resultant polymer [(1)(PyPM)2][Cl] exhibited a Mn value twice
that of the starting polymers, as determined by gel permeation
chromatography (GPC) (see Step i in Scheme 1 and Table 1). As
Pre-Sonication
Post-Sonication
PyPM
Mn [kDa]
PDI
Mn [kDa]
PDI
Mn [kDa]
PDI
P7
6.8
11
21
25
66
1.2
1.2
1.2
1.4
1.4
13
23
41
48
1.2
1.1
1.2
1.3
1.3
13
23
21
24
62
1.3
1.2
1.2
1.4
1.4
P11
P21
P25
P66
120
a
Number and weight average molecular weights (Mn and Mw,
respectively) were determined by gel permeation chromatography
(GPC; eluent ¼ DMF, 0.1 M LiBr) and are reported relative to
polystyrene standards. Polydispersity indices (PDIs) were calculated
using the equation PDI ¼ Mw/Mn.
part of
a
series of control experiments (see below),
[(1)(PyPiv)2][Cl] and end-functionalized derivative [(2)(PyP110)]
were prepared by mixing 4-pyridinyl pivalate (PyPiv) with 0.5
molar equiv of 1 and treatment of PyP110 with 1.0 molar equiv of
tris(pentafluorophenyl)borane (2), respectively (structures not
shown).†
S1–S3).† Additionally, a significant upfield shift was observed in
the 11B NMR spectrum acquired for [(1)(PyPM)2][Cl] (1.48 ppm;
CDCl3) when compared to that of 1–Cl (59.1 ppm), consistent
with the rehybridization of the boron nucleus from sp2 in the free
borane to sp3 in the corresponding boronium salt.15
To gain additional evidence for pyridine coordination to the
1
boron center in the aforementioned polymers, H NMR spectra
of a low molecular weight PyPM (Mn ¼ 1.7 kDa) were acquired
before and after treatment with 1–Cl or 2. A downfield shift of
the pyridyl meta-protons from d ¼ 7.08 ppm in free PyPM to 7.55
ppm was observed upon formation of both [(1)(PyPM)2][Cl] and
[(2)(PyPM)] (CDCl3), consistent with the coordination of the
pyridine moiety to an electron-deficient Lewis acid (see Figures
Due to the centrally-positioned mechanophore in
[(1)(PyPM)2],2,3,8,9 we expected that sonication‡ of a solution of
this material would afford [(1)(PyPM)]+ and PyPM via boron–
pyridine bond scission (Step ii, Scheme 1). However, upon
concentration, the material produced in these experiments
resulted in no change in Mn, as determined by GPC, presumably
due to recoordination of the liberated pyridine to [(1)(PyPM)]+
(Step iii, see Figure S7 for additional details).† It has been
previously shown that HBF4 effectively traps liberated PyPM,8
hence we reasoned that the addition of a Brønsted acid to the
aforementioned reactions should sequester the PyPM as the
pyridinium [HPyPM]+ (Step iv). In support of this supposition,
we found that sonicating CH3CN solutions of [(1)(PyPM)2][Cl]
(where M ¼ 21, 25 and 66) and excess HBF4 in a Suslick16 cell
immersed in a 4 ꢀC ice bath for 4 h resulted in a two-fold
reduction of Mn (see Fig. 1A for M ¼ 66), consistent with the
anticipated scission a boron–pyridine bond (see Table 1).‡
To verify that HBF4 did not induce scission via protonation of
the pyridyl moieties, a series of control experiments were
performed in the absence of sonication (at both 4 and 25 ꢀC)
under otherwise identical conditions. Because no change in the
Mn values measured for the polymers analyzed occurred during
these experiments, we concluded that the aforementioned ultra-
sound induced reductions in molecular weight arose from
mechanically-induced boron–pyridine bond scission processes.
Previous reports have shown that extending polymer chain
length increases the rate of mechanophore activation, and that
a minimum molecular weight of 20–40 kDa is required for acti-
vation to occur.2,3,8,9 To determine if similar behavior was
‡ General sonication conditions: All sonications were performed on
solutions with a final volume of 10 mL in a Suslick cell under
a positive pressure of argon. Pulsed ultrasound (1.0 s on, 1.0 s off) was
supplied at 23% power (10.1 W cm2) for 4 h. The external temperature
was maintained at 4 ꢀC aided by the use of a cold room and ice–water
bath. The internal temperatures of the sonicated solutions were
monitored via a thermocouple and measured at #9 ꢀC.
Scheme 1 Synthesis of boronium-based mechanoresponsive reagents
and chain scission in response to sonication. Conditions: i) CH2Cl2, room
temperature, 12 h; ii) sonication‡ for 4 h at 4 ꢀC of [(1)(PyPM)2][Cl] (10
mg) in CH3CN (10 mL) and iii) re-coordination of PyPM to boron or iv)
trapping of PyPM with 20 equiv HBF4 to form [HPyPM][BF4].
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Fig. 1 Representative gel permeation chromatograms (GPCs) and kinetic data for the ultrasound-induced mechanical scission of [(1)(PyPM)2][Cl]. (A)
GPC traces of [(1)(PyP66)2][Cl] before (red) and after (blue) sonication, with PyP66 (black) overlaid for reference.‡ (B) Aliquots were removed over the
course of the sonication of a solution of [(1)(PyP66)2][Cl] at timed intervals and analyzed by GPC; the gradual reduction in peak intensity measured at
retention time ¼ 19.11 min ((t ¼ 0, topmost red; t ¼ 4–16 min, fading red) was accompanied by the growth of a new peak measured at retention time ¼
19.87 min (t ¼ 30–240 min, darkening blue). All aliquots analyzed consisted of an initial polymer concentration of 4.0 mg mLꢁ1. (C) First-order rate plot
for the mechanical scission of [(1)(PyPM)2][Cl], where M ¼ 66 (black), 25 (red), 21 (blue), 11 (orange) and 7 (purple). I0 and I correspond to the RI signal
intensity for each polymer taken at the peak retention times before sonication and at each time-point t afterward, respectively (cf., Table S2). (D) Plot of
scission rates determined in (C) as a function of the number average molecular weight (Mn) of [(1)(PyPM)2][Cl]. For C and D, the data points were
calculated from the average of three separate experiments. The error bars shown are the standard deviation.
exhibited by the aforementioned boronium-based polymers, the
rate of the change in molecular weight was measured for all of
[(1)(PyPM)2][Cl] (Fig. 1A–C) and plotted versus the initial Mn
(Fig. 1D). The kinetics of the observed loss of the peak at the
initial Mn were determined by GPC analysis of aliquots removed
at timed intervals during the sonication of [(1)(PyPM)2][Cl].‡ For
the polymers exceeding the minimum molecular-weight
threshold (Mn > 40 kDa), the refractive index (RI) signal at the
peak retention time associated with the initial Mn steadily
decreased over 4 h and was accompanied by growth of the signal
at the peak retention time associated with [(1)(PyPM)]+ and
[HPyPM]+ (for M ¼ 66, see Fig. 1B). Plotting ꢁln(I/I0) versus
time, where I0 and I correspond to the RI signal at the peak
retention time for [(1)(PyPM)2][Cl] before sonication and at each
time-point analyzed, respectively, enabled determination of the
rate of chain scission.2,3,8 The polymers above the molecular
weight threshold, [(1)(PyP21)2][Cl], [(1)(PyP25)2][Cl] and
[(1)(PyP66)2][Cl], were measured to exhibit first-order rates of
0.80, 0.52, and 1.98 ꢂ 10ꢁ2 minꢁ1, respectively.x In contrast, no
scission was observed for the polymers below the threshold,
[(1)(PyP7)2][Cl] and [(1)(PyP11)2][Cl] (Fig. 1C and 1D).
reaction with an acid–base indicator. We have previously shown
that 4-[(4-anilinophenyl)azo]-benzenesulfonic acid (3) in its
protonated form ([3H][BF4]) was viable for the qualitative
detection of sonochemically-liberated PyPM, and afforded
a deep purple to yellow colorimetric response in the presence of 1
equiv of PyPM.8 Employing the same indicator (see Fig. 2A),
a
purple solution was obtained upon mixing 10 mM
[(1)(PyPM)2][Cl] with 10 mM [3H][BF4] in 10 mL CH3CN. This
solution was subsequently transferred to a Suslick cell, where
mechanical chain scission was induced as described above.
Sonication of the higher molecular weight polymers (PyPM; M ¼
21, 25 and 66) afforded clear, yellow solutions, consistent with
the deprotonation of [3H][BF4] by free PyPM (for M ¼ 66, see
Fig. 2B).‡ These yellow solutions returned to their original
purple color upon the addition of 1 equiv of HBF4, which sug-
gested to us that the observed colorimetric response was due to
deprotonation of the indicator and not sonochemical degrada-
tion. In the absence of sonication, the same solutions remained
purple, indicating that thermally-induced or proton-facilitated
ligand displacement did not occur. To confirm that this Brønsted
acid–base reaction originated via
a mechanically-induced
process, similar colorimetric experiments were performed via
sonication of: (i) polymers below the minimum molecular weight
threshold required for mechanical activation, [(1)(PyP7)2][Cl]
and [(1)(PyP11)2][Cl] (see Fig. 2C for M ¼ 11); (ii) the end-capped
derivative [(2)(PyP110)] (Fig. 2D); (iii) and a mechanophore
model complex [(1)(PyPiv)2][Cl] (in the presence of 90 kDa
PMA) (Fig. 2E). No color change was observed during any of
these control experiments. Collectively, these results support the
sonochemically-induced mechanical chain scission of
[(1)(PyPM)2][Cl] at the boron–pyridine bond.
Qualitative colorimetric detection of PyPM
To confirm that bond scission was occurring at the anticipated
boron–pyridine linkage, we performed a series of colorimetric
experiments whereby the liberated PyPM was detected via
x The magnitude of the chain scission rates measured for analogous
polymers containing pyridine-palladium complexes8 was approximately
double that of the polymers described herein.
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Fig. 2 (A) Indicator system employed for the qualitative, colorimetric detection of free PyPM. (B–E) Results of colorimetric indicator experiments for
[(1)(PyP66)2][Cl] and selected control experiments. The sonication experiments‡ were performed for 4 h at 4 ꢀC on a 10 mL CH3CN solution containing 1
equiv of [3H][BF4] with respect to 10 mM analyte: (B) [(1)(PyP66)2][Cl], (C) [(1)(PyP11)2][Cl], (D) [(2)(PyP110)], (E) [(1)(PyPiv)2][Cl] in the presence of 90
kDa PMA. The solution containing [(1)(PyP66)2][Cl] changed from purple before sonication (before) to yellow afterwards (after) indicating the
formation of free PyP66 whereas no color change was observed during any of the control experiments.
Mechanocatalyzed anionic polymerization reactions
Because the sonication of [(1)(PyPM)2][Cl] generated PyPM
in situ, we reasoned that this species could be utilized to facilitate
base-catalyzed reactions.8 Mechanoresponsive materials are
ideally suited for self-healing applications, wherein the initiation
of polymerization reactions may induce repair processes;6,17
Scheme 2 Pyridine catalyzed polymerization of 4 by PyP66 liberated
therefore, we focused on polymerizations that are initiated by
from [(1)(PyP66)2][Cl] occurred only under ultrasound (see text).
pyridine. Unfortunately most known pyridine-initiated poly-
merizations (e.g., ring-opening polymerizations) require deriva-
tives with functional groups that enhance nucleophilicity, high
experiments. Similarly, no polymer was obtained from a solution
monomer concentrations, high catalyst loadings, and/or elevated
of [(1)(PyP66)2][Cl] and 4 in CH3CN in the absence of sonication,
temperatures,18 which were not conducive to the constraints
representing negligible thermal background formation of p(4)
inherent to the sonochemical conditions necessary to effect
(cf., Table S4).† Collectively, these results suggested to us that
pyridine liberation (i.e., micromolar mechanophore concentra-
the polymerization of 4 to p(4) was activated by mechanical force
tion and low temperatures). Recently, Willson reported that
and was catalyzed by the release of PyP66 from [(1)(PyP66)2][Cl]
pyridine catalyzes the anionic polymerization of a-tri-
fluoromethyl-2,2,2-trifluoroethyl acrylate (4), which was found
to proceed at ꢁ78 ꢀC and at low pyridine loadings (< 0.5 wt%).19
during sonication.
Sonication of [(1)(PyP66)2][Cl] and 4 in CH3CN for 4 h,‡ fol-
ꢀ
Conclusion
lowed by stirring at 4 C for 20 h to allow for sufficient chain-
In conclusion, we have synthesized a mechanoresponsive reagent
growth, resulted in formation of the desired polymer p(4) (Mn ¼
comprised of an electron-deficient boron atom coordinated to
21 kDa, PDI ¼ 1.5) in 49% yield with respect to monomer
two pyridine-capped poly(methyl acrylate)s, effectively
consumption (Scheme 2).{ As negative controls, sub-threshold
producing the first known boronium-based polymer. Ultra-
sound-induced stress facilitated the selective cleavage of a boron–
pyridine bond in polymers of sufficient molecular weight,
whereby the liberated pyridine moiety effected stoichiometric
Brønsted acid–base reactivity and catalyzed an anionic poly-
merization reaction. Comprehensive control experiments
confirmed that chain cleavage was exclusively the result of
ultrasonically-supplied mechanical force. Collectively, our find-
ings demonstrate that mechanophores derived from p-block
elements can afford mechanoresponsive materials that exhibit
dynamic structures and catalytic functions akin to their d-block
analogues. The Lewis acid–base mechanophore and method-
ology described herein are expected to preclude complications
that can arise from organic (i.e. irreversible bond formation/
[(1)(PyP7)2][Cl], end-capped [(2)(PyP110)], and small molecule
analogue [(1)(PyPiv)2][Cl] in the presence of 90 kDa PMA were
individually subjected to sonication under the same conditions;
polymer formation was not observed for any of these
{ The unoptimized polymerization conditions employed may have led to
early termination that curtailed monomer consumption. For example,
using PyP66 as the pyridine source and similar conditions to those
employed for the sonication reactions described above resulted in the
formation of p(4) (Mn ¼ 30 kDa; PDI ¼ 1.3) in 50% yield with respect
to monomer consumption (see Table S4). Additionally, the liberated
boron species did not appear to promote or hamper the polymerization
reaction, as evidenced by the similar molecular weights and yields
obtained for polymers grown independently using pyridine as an
initiator.
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We are grateful to the Army Research Office (grant No.
W911NF-07-1-0409) and the Robert A. Welch Foundation
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