A high molecular weight reversible coordination polymer of PdCl2 and 1,12-bis(diphenylphosphino)dodecane

Article abstract of DOI:10.1039/b301874a

1,12-Bis(diphenylphosphino)dodecane and PdCl₂ are shown to form linear supramolecular polymers in equilibrium with cyclic structures; high molecular weight material was obtained by melt polymerisation.

Full text of DOI:10.1039/b301874a

A high molecular weight reversible coordination polymer of PdCl2 and
1,12-bis(diphenylphosphino)dodecane†
Jos M. J. Paulusse and Rint P. Sijbesma*
Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, PO Box 513,
5600 MB Eindhoven, The Netherlands. E-mail: R.P.Sijbesma@tue.nl; Fax: +40-2472655; Tel: +40-2472655
Received (in Cambridge, UK) 19th February 2003, Accepted 9th May 2003
First published as an Advance Article on the web 29th May 2003
1,12-Bis(diphenylphosphino)dodecane and PdCl2 are shown
to form linear supramolecular polymers in equilibrium with
cyclic structures; high molecular weight material was
obtained by melt polymerisation.
species gave separate resonances in agreement with the reported
spectra.5b In addition, assignment of individual trans-peaks was
confirmed by diffusion ordered 31P-NMR7 (Fig. 1 and Table
1).
Diffusion coefficients of all peaks were normalized to the
diffusion coefficient of the trans-monomeric species. The
dimeric species have a slightly lower relative diffusion
coefficient (0.78) at a concentration of 0.2 M, whereas the
oligomeric species have a significantly lower diffusion coeffi-
cient than the monomeric cycle (0.19).
Relative diffusion coefficients of the monomeric and dimeric
cycles are independent of concentration (Table 1). The relative
diffusion coefficients of the trans-oligomers, however, decrease
from 0.58 at 0.025 M to 0.19 at 0.2 M. The average size of the
oligomeric species thus becomes larger as the concentration is
raised. This observation confirms the concentration dependent
size and the equilibrium nature of [PdCl2(dppdd)]n in chloro-
form-d.
A key feature of ring–chain equilibria is the existence of a
critical concentration, above which the partial concentration of
each individual cyclic species (and as a consequence the total
concentration of cycles) does not increase and linear polymers
are formed. Integration of the signals in the 31P-NMR spectra of
[PdCl2(dppdd)] in chloroform-d at a range of concentrations
shows that the partial concentration of monomeric trans-cycles
remains constant at 10.5 mM, above a critical concentration of
approximately 70 mM (Fig. 2). This result indicates that linear
polymeric [PdCl2(dppdd)] is formed at concentrations higher
than 70 mM. It also implies that under equilibrium conditions,
significant amounts of linear polymer can only be obtained at
In supramolecular polymers the monomers are connected by
reversible, non-covalent bonds, leading to unique properties.1
Coordinative interactions have been used to obtain self-
assembled structures with many different architectures ranging
from discrete structures such as nanotubes,2a cages,2b grids2c
and honeycombs2d to polymeric structures such as linear
polymers3a,b and coordination-block copolymers.3c Reversible
linear coordination polymers, however, have been studied in
much less detail.4
In a project aimed at exploring the use of coordination
polymers as catalysts, we have focused our attention on the
complexation
of
1,12-bis(diphenylphosphino)dodecane
(dppdd) with palladium dichloride, which has been studied
before by Hill et al.5a and by Gray et al.5b The latter authors
described solutions of [PdCl2(dppdd)] as an equilibrium system
consisting of cyclic monomers and oligomers. In reversible
systems, cyclic structures are generally in equilibrium with
linear polymeric species.6 This equilibrium is characterized by
a critical concentration, below which no polymer is present, and
above which the concentration of cycles is constant and only
linear polymer is formed. In view of the fundamental interest in
supramolecular coordination polymers, we have studied the
possibility of obtaining linear chains of [PdCl2(dppdd)]n with a
high molecular weight.
An equilibrium mixture of [PdCl2(dppdd)] (Scheme 1) was
obtained by stirring a solution of dppdd in chloroform-d with
palladium(II) dichloride.
The complex was characterized with 31P-NMR spectroscopy.
cis- and trans-coordinated monomeric, dimeric and oligomeric
Fig. 1 31P-NMR DOSY of a 0.2 M solution of [PdCl2(dppdd)] in
chloroform-d.
Table 1 Normalized diffusion coefficients of trans centers
Conc./M
Monomer
Dimer
Oligomer
0.025
0.050
0.075
0.100
0.200
1.0
1.0
1.0
1.0
1.0
0.74
0.77
0.85
0.86
0.78
0.58
0.54
0.52
0.42
0.19
Scheme 1 Cycles and polymers of [PdCl2(dppdd)], trans-species shown.
† Electronic supplementary information (ESI) available: experimental
details, SEC results and NMR spectra. See http://www.rsc.org/suppdata/cc/
b3/b301874a/
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Much higher molecular weights were observed in SEC traces
of polymer obtained by melt polymerization. This material was
prepared by heating [PdCl2(dppdd)]n under argon at 185 °C for
15 min. The yellow solid melted and became a bright orange
viscous oil. Upon cooling, the oil became a brown brittle solid.
1H-NMR and 31P-NMR did not show any evidence of
decomposition. Remarkably, the 31P-NMR spectrum of freshly
prepared solutions of this material indicated that all phosphines
in the melt polymer are trans-coordinated, and no monomeric
cycles are detected. The SEC-trace of this polymer (Fig. 3)
shows a broad distribution and proves the presence of polymers
of up to 500 units (M ≈ 3.6 3 105 g mol21).
Due to the dynamic nature of the system and the low
concentration at which the SEC analysis was performed, this
number represents a lower limit of the DP’s (degrees of
polymerisation) that are actually present in the melt polymer.
SEC analysis of melt polymer prepared with 1%, 5%, 10% and
50% of a monofunctional phosphine, 1-(diphenylphosphino)do-
decane, as endcapper shows molecular weights in good
agreement with calculated DP’s (see Supplementary Informa-
tion†).
In summary, we have given experimental evidence for the
formation of high molecular weight coordination polymer in a
dynamic system. Established theories6 of ring–chain equilibria
predict that this high molecular weight material is linear, and
not cyclic, as was assumed in previous studies5 of this system.
Although the present system is sufficiently stable to be analyzed
in dilute solution, exact stoichiometry is crucial for the
observation of high DP’s for two reasons: excess ligand or
excess metal will reduce the degree of polymerization in the
same way as an imbalance in stoichiometry reduces the DP in
condensation polymerization,9 while an excess of ligand will
also drastically reduce the lifetime of the polymer.
Fig. 2 Partial concentration of trans monomeric cycle. [PdCl2(dppdd)]1.
high concentrations, because at any concentration above the
critical concentration, 70 mM of the product remains in a cyclic
form, of which approximately one seventh is cyclic monomer.
Therefore, the isolation of polymeric [PdCl2(dppdd)]n was
attempted by slow evaporation of a dichloromethane solution.
After several days, a highly viscous orange oil separated from
solution, which was isolated by removal of residual solvent.
31P-NMR of this product showed that it contained approx-
imately 5% cyclic trans monomer, to give a calculated content
of 34% cyclic and 61% linear polymeric material.
For characterization and for applications in dilute solution, it
is necessary that the linear polymer has a sufficient lifetime
before equilibration results in the formation of cycles. The
kinetics of cyclization of a freshly prepared solution of
polymeric [PdCl2(dppdd)]n were determined at 298 K in
chloroform-d at a concentration of 15 mM by monitoring signal
intensities in the 31P-NMR spectrum as a function of time. An
excellent fit (R2 = 0.998) to the data was obtained assuming
first-order kinetics with a t of 774 min for the formation of the
monomeric cycle. A small imbalance in stoichiometry was
found to have a strong effect on the lifetime of the polymer.
Addition of 0.5 mol% dppdd to the coordination polymer
resulted in a decrease of t to 47 min. Addition of other
phosphines such as triphenylphosphine or 1-(diphenylphosphi-
no)dodecane resulted in a similar reduction of polymer lifetime,
in line with the known effect of excess ligand on exchange
processes in Pd–phosphine complexes.8
For future work directed towards the use of polymeric Pd–
phosphine complexes at low concentration, a reduction of the
critical concentration will be sought by using longer bi-
functional phosphorus ligands.
The work in this article was financially supported by the
Netherlands Research School Combination
(NRSCC).
– Catalysis
Notes and references
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exclusion chromatography (SEC) for the analysis of molecular
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(Fig. 3). In samples that were measured immediately after
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assigned to oligomeric cycles by comparison to chromatograms
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Fig. 3 SEC-trace of [PdCl2(dppdd)]n obtained by evaporation and melt
polymerisation.
9 P. J. Flory, Principles of Polymer Chemistry, Cornell University Press,
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