Reversible mechanochemistry of a PdII coordination polymer

Article abstract of DOI:10.1002/anie.200460040

Breaking up and making up: The ultrasonic cleavage of high-molecular-weight linear coordination polymers of phosphane telechelic polytetrahydrofuran and palladium dichloride in dilute solution is a reversible process (see picture). Sonication for 1 h led

Full text of DOI:10.1002/anie.200460040

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Ultrasound-induced chain scission in dissolved PdII coordination
polymers is highly selective for the dissociation of phosphane ligands
from palladium and therefore reversible. The original molecular weight
of the polymer is restored after sonication. See the Communication by
J. M. J. Paulusse and R. P. Sijbesma on the following pages for more
details.
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DOI: 10.1002/anie.200460040
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Coordination Polymers
anhydride[10] with subsequent termination by the diphenyl-
À1
¯
¯
¯
phosphane anion (Mn = 7300 gmol ; Mw/Mn = 1.11). Linear
coordination polymers 2 were obtained by stirring a solution
in toluene of ligand 1 with a small excess of palladium
dichloride for 2 days (Scheme 1).
Reversible Mechanochemistry of a PdII
Coordination Polymer**
Size-exclusion chromatographic (SEC) analysis of the
equilibrated samples of 2 at different concentrations (Fig-
ure 1a) shows the presence of high-molecular-weight mate-
Jos M. J. Paulusse and Rint P. Sijbesma*
5
À1
The design and synthesis of appropriate ligands is a well-
established, but sometimes cumbersome method for control-
ling the coordination chemistry of transition-metal com-
plexes. Manipulation of the coordination sphere by mechan-
ical means is an exciting, yet barely explored alternative,[1]
which may open up novel avenues in coordination chemistry
and catalysis. Although mechanical forces act on even the
smallest molecules, the effect of extensional forces is most
pronounced in macromolecular species. If these forces act on
a coordination polymer[2] of sufficient length, the noncovalent
bonds may be broken.[3] Reversibility[4,5] of the coordinative
interactions could then result in the continuous reformation
of polymer chains until a steady-state degree of polymer-
ization is reached. Irreversible disruption of covalent bonds is
known to occur in polymers subject to strong shear forces;[6] it
is the basis of some processing technologies, such as the
mastication of natural rubber,[7] and even causes undesired
degradation of macromolecules in chromatography.[8] Ultra-
sonication is one of the most efficient methods to induce
breakage in polymer chains in homogeneous solution.[9] The
shear forces required for chain scission are provided by the
collapse of cavitation bubbles created by ultrasound.
rial[11]
(Mtop = 2.50 ” 105 gmolÀ1,
Mw = 1.70 ” 10 gmol ,
¯
based on polystyrene standards), as well as peaks at 10500
and 23000 gmolÀ1, which decrease in intensity relative to the
peak for the high-molecular-weight material with increasing
concentration. The low-molecular-weight peaks correspond
well with the molecular weights of cyclic monomers and
dimers (7500 and 15000 gmolÀ1). The molecular-weight
distributions in Figure 1a show that even at the lowest
concentration high-molecular-weight material is present,
and the data are in line with an equilibrium between cyclic
and linear complexes with a critical polymerization concen-
tration[12] of approximately 0.15 mm.[13]
In linear coordination polymers the degree of polymer-
ization is strongly influenced by any stoichiometric imbalance
between the bifunctional ligand and the metal, or by the
presence of monofunctional ligands.[4c,5] Indeed, the addition
of 1-dodecyl(diphenyl)phosphane (as a palladium dichloride
complex to preserve the metal to ligand stoichiometry) to 2
led to a decrease in the molecular weight after equilibration,
which confirms the linear nature of the polymer (Figure 1b).
The ultrasound-induced scission of coordination polymer
2 was studied by using a sonication probe in toluene under
argon. A 1.5 mm solution, which had been equilibrated for
3 days, was sonicated for 1 h at a constant temperature of
208C. Samples were taken during the experiment and
analyzed by SEC. The results of the sonication experiment
Herein, we report on the reversible sonochemistry of
high-molecular-weight coordination polymers in dilute solu-
tion. The polymeric diphosphane 1 was prepared by the
cationic polymerization of tetrahydrofuran initiated by triflic
Scheme 1. Synthesis of the polymeric diphosphane ligand 1 and coordination polymer 2. DTBP=2,6-di-tert-butylpyridine, Tf=trifluoromethanesul-
fonyl.
¯
are shown in Figure 2a. After sonication for 1 h, the Mw had
[*]J. M. J. Paulusse, R. P. Sijbesma
decreased from 1.70 ” 105 gmolÀ1 to 1.02 ” 105 gmolÀ1.
Laboratory of Macromolecular and Organic Chemistry
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Fax: (+31)40-2472-655
1H NMR, 13C NMR, and 31P NMR spectroscopy showed
that no chemical degradation of complex 2 had occurred.
Furthermore, when the sample was left for 1 day, the original
molecular-weight distribution was restored nearly quantita-
tively. To test the reversibility of the sonochemical degrada-
tion, a 1.5 mm solution was sonicated for 1 h and left to
equilibrate for 23 h. This procedure was repeated five times.
In Figure 2b, the weight-averaged molecular weight of the
sample after each sonication and equilibration period is
shown, and there is no sign of irreversible polymer degrada-
E-mail: r.p.sijbesma@tue.nl
[**]This work was supported by the Netherlands Research School
Combination—Catalysis. The authors thank M. W. A. Kuijpers and
Dr. M. F. Kemmere for helpful discussions and assistance with
sonication experiments. We thank Dr. K. Pieterse for preparing
graphics.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460040
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tion. The reversibility of the effects of sonication shows that
no covalent bonds are broken in the process, and that the
mechanical degradation is highly selective for the weaker
palladium–phosphane coordination bonds.
Although the temperature was kept constant at 208C
during all sonication experiments, we wanted to confirm that
the decrease in molecular weight is indeed brought about by
the mechanical action of ultrasound, and not by a trivial
heating effect. Therefore, an equilibrated solution of 2 was
heated at 808C for 3 h and then analyzed at room temper-
ature by SEC. Heating resulted in a much smaller decrease in
Mw than sonication, from 1.70 ” 105 to 1.50 ” 105 gmolÀ1.
¯
Moreover, heating led to an increase in lower-molecular-
weight material over the whole molecular-weight range,
particularly in monomeric cyclic products, whereas sonication
led to an increase in polymeric material of a more specific
length (after 5 min of sonication the greatest increase was
found at M = 1.90 ” 105 gmolÀ1).
In a second control experiment, a solution of complex 2
was equilibrated for 3 days at 0.35 mm. This sample was
subsequently concentrated swiftly by partially evaporating
the solvent to give a 1.5 mm solution with a molecular-weight
distribution corresponding to that of a less concentrated
solution. Part of this sample was sonicated and showed a large
decrease in molecular weight, whereas the molecular-weight
distribution of the rest of the sample shifted towards higher
molecular weight (Figure 3). This experiment shows that
sonication moves the composition of the solution away from
thermodynamic equilibrium.
Figure 1. a) Molecular-weight distributions of 2 equilibrated in toluene
at different concentrations; b) molecular-weight distributions of 2
(1.5 mm in toluene) equilibrated in the presence of different quantities
of [PdCl2(dodecyl(diphenyl)phosphane)2].
Figure 3. Molecular-weight evolution of equilibrating and sonicated
samples of solutions 1.5 mm in 2 of non-equilibrium composition (see
text for details).
Ultrasonic chain scission is a nonrandom process that acts
preferentially on longer chains.[7,9] In solutions of 2, this
phenomenon is evidenced by the complete disappearance of
the highest-molecular-weight polymers after short sonication
times (Figure 2a). Several observations suggest a mechanism
by which cyclic polymers are formed. For a decrease in
molecular weight of any linear polymer, either additional
chain ends or rings have to be formed (or a combination of
both). Ligand exchange in palladium(ii)–phosphane com-
plexes is known to occur predominantly through an associa-
tive mechanism and is therefore strongly accelerated by trace
Figure 2. Ultrasound-induced decrease in the molecular weight of 2
(1.5 mm in toluene): a) molecular-weight distribution of samples taken
during sonication; b) weight-averaged molecular weight (Mw) during a
reversibility test consisting of five cycles of sonication (1 h) followed
by equilibration (23 h).
¯
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Scheme 2. Proposed elementary steps upon sonication of 2.
amounts of free phosphane.[14] Because sonicated solutions of
2 equilibrate at the same rate as non-equilibrium solutions
obtained by other means (e.g. by rapidly increasing the
concentration), the presence of significant amounts of free
phosphane end groups after sonication may be excluded, thus
suggesting that the newly formed polymer is mainly cyclic. We
propose that the phosphane chain ends formed by cleavage of
the polymer react rapidly with a palladium complex either
intramolecularly or intermolecularly (Scheme 2). In each
case, upon substitution at the palladium center a new reactive
phosphane end group is formed and a chain reaction is
initiated which terminates when a free phosphane reacts with
a coordinatively unsaturated palladium(ii) chain end. How-
ever, an intermolecular reaction merely results in redistrib-
ution to give a statistical (most probable) chain-length
distribution and does not change the average molecular
weight of the polymer. On the other hand, intramolecular
attack of a phosphane end group results in the splitting off of a
cyclic compound and a net decrease in molecular weight, as
observed for sonicated solutions of 2.
The results presented herein demonstrate that ultrasound
may be used to mechanically induce ligand dissociation from
transition-metal complexes, thus making it a novel method for
studying and controlling the chemistry of coordination
complexes. The creation of vacant coordination sites in
combination with the complete reversibility of the process
point the way to the application of this principle in transition-
metal catalysis. Present efforts are focused on identifying the
polymer architecture and experimental conditions required
for optimal effectiveness of the sonochemical process, and on
studying the reactivity changes induced by ligand dissociation.
[1] K. S. Suslick, Adv. Organomet. Chem. 1986, 25, 73.
[2] R. D. Archer, Coord. Chem. Rev. 1993, 128, 49.
[3] Ultrasound has been used to decrease the size of micellar
aggregates; see, for example: J. Massey, K. N. Power, I. Manners,
M. A. Winnik, J. Am. Chem. Soc. 1998, 120, 9533.
[4] a) U. Michelsen, C. A. Hunter, Angew. Chem. 2000, 112, 780;
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[5] J. M. J. Paulusse, R. P. Sijbesma, Chem. Commun. 2003, 1494.
[6] a) J. A. Odell, A. Keller, Y. Rabin, J. Chem. Phys. 1988, 88, 4022;
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[7] A. Casale, R. S. Porter, Polymer stress reactions, Academic Press,
London, 1978.
[8] H. G. Barth, F. J. Carlin, J. Liq. Chromatogr. 1984, 7, 1717.
[9] a) T. J. Mason, J. P. Lorimer, Applied Sonochemistry, 1st ed.,
Wiley-VCH, Weinheim, 2002; b) K. S. Suslick, G. J. Price, Annu.
Rev. Mater. Sci. 1999, 29, 295.
[10] M. F. Dubreuil, N. G. Farcy, E. J. Goethals, Macromol. Rapid
Commun. 1999, 20, 383.
[11] For equilibrated solutions of pure 2, the top of the high-
molecular-weight peak shifts towards higher molecular-weight
values with increasing concentration and reaches 2.50 ”
105 gmolÀ1 at 1.5 mm, which corresponds to
a degree of
polymerization of 27 units. At even higher concentrations Mtop
does not shift further, although the fraction of cyclic monomers
is lower. Based on the assumption that 2 contains a small amount
of monophosphane impurities, such as monofunctionalized
polymer chains or chains with one oxidized phosphane group,
which act as chain ends, the observed ceiling for the concen-
tration-dependent degree of polymerization corresponds to
approximately 3.5% impurity in the polymeric ligands.
[12] a) H. Jacobsen, W. H. Stockmayer, J. Chem. Phys. 1950, 18, 1600;
b) G. Ercolani, M. Luigi, P. Mencarelli, S. Roelens, J. Am. Chem.
Soc. 1993, 115, 3901.
Received: March 18, 2004 [Z460040]
[13] Ligand-exchange rates in 2 are slow on the SEC timescale. A
lifetime of approximately 13 h was found for the closely related
system reported in reference [4].
Keywords: coordination polymers · macrocycles · phosphanes ·
supramolecular chemistry · ultrasound
[14] A. W. Verstuyft, L. W. Cary, J. H. Nelson, Inorg. Chem. 1976, 15,
3161.
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