A modular approach to main-chain organometallic polymers

Article abstract of DOI:10.1021/ja054029k

A highly efficient route to a new class of organometallic polymers containing difunctional N-heterocyclic carbenes has been developed. Bis(imidazolium) halides and divalent group X metals were copolymerized to afford organometallic polymers in up to quant

Full text of DOI:10.1021/ja054029k

Published on Web 08/20/2005
A Modular Approach to Main-Chain Organometallic Polymers
Andrew J. Boydston, Kyle A. Williams, and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, Austin, Texas 78712
Received June 17, 2005; E-mail: bielawski@mail.cm.utexas.edu
Scheme 1 a
Main-chain conjugated organometallic polymers (COMPs) are
an interesting class of materials with desirable electronic and
mechanical properties.1 Unfortunately, access to these materials is
often challenged by factors such as the need for inert atmospheres
and anhydrous conditions, restrictive compatibilities of transition
metals with contemporary polymerization protocols, and limited
polymer stabilities and/or solubilities. Overcoming these restraints,
which have prevented COMPs from reaching their full potential,
requires development of new building blocks and polymerization
strategies. Although a variety of binding interactions have been
employed for integrating metals into organic polymers, Arduengo-
type N-heterocyclic carbenes2 (NHCs) remain an untapped resource.
This is surprising, considering that NHCs offer broad structural
diversity and generally display high affinities toward a wide range
of transition metals.3 We sought a versatile, direct, and simple route
to bis(NHC)s connected via conjugated arenes. With the availability
of these difunctional NHCs, we envisaged a new class of COMPs
could be readily accessed.
We targeted rigid benzimidazole-based arene spacers to inhibit
intramolecular metal chelation4 and to ensure regiocontrol over
metalation.5 Toward this end, commercially available 1,2,4,5-tetra-
aminobenzene (1), 3,3′-diaminobenzidine (3), and known dioxin-
tetraamine 56 were condensed with formic acid7 to provide the cor-
responding bis(imidazole)s. This cyclization protocol was con-
ducted under ambient atmosphere and provided products in high
purity (>95%) and high yields (>90%) after neutralization. As
shown in Scheme 1, direct access to the respective bis(imidazolium)
salts was accomplished via a single-step, 4-fold alkylation. Thus,
the bis(imidazole)s described above were each treated with NaH
in PhCH3 followed by addition of alkyl bromide. After 1 h at 110
°C, DMF was introduced as a cosolvent (50% v/v) which facilitated
dissolution of partially alkylated intermediates. After an additional
6 h at 110 °C, introduction of excess PhCH3 precipitated the desired
bis(imidazolium) bromides (2, 4, and 6) in good to excellent yields
(74-99%).
a Conditions: (a) HCO2H, 110 °C, 2-36 h. (b) (i) NaH (2.0 equiv), PhCH3,
110 °C; (ii) R-Br (6.0 equiv), 25 f 100 °C, 1 h.; (iii) DMF, 60 f 110 °C.
Overall yields are indicated.
Scheme 2
As shown in Scheme 2, polymerization and metal incorporation
was carried out via a modified Herrmann-Schwarz-Gardiner
method8 by treatment of bis(imidazolium) bromides 2, 4, or 6 with
Pd(OAc)2 in DMSO at 110 °C.9 After 1 h, the reaction solutions
were precipitated into an excess of MeOH or H2O which afforded
polymers 7-9 in high yields (84-99%).10 As expected, signals in
lecular weights (Mn’s), ranging from 8.0 × 103 to 1.1 × 105 Da,
dependent on the N-substituent and the bis(imidazole) structure.
Likewise, the thermal stabilities of the polymers exhibited similar
dependencies and began to degrade at 280-300 °C, as determined
by thermogravimetric analysis.
1
the H NMR spectra were relatively broad and shifted upfield in
comparison with those of their respective bis(imidazolium) salt
precursors. 13C NMR spectroscopy was used to verify Pd was
successfully incorporated into the backbones of the polymer chains.
For example, the 13C NMR spectrum of 8a exhibited a resonance
at 172 ppm which was diagnostic of an imidazol-2-ylidene-Pd
bond.4,11,12 As summarized in Table 1, absorption maxima for the
polymer systems were clustered around 320 nm, and molar ab-
sorptivities (normalized per repeat unit) as high as 8.4 × 104 M-1
cm-1 were observed. Analysis of the polymers by gel permeation
chromatography13 (GPC) indicated their polydispersities were
typical of step-growth polymerizations with number-average mo-
Copolymerization of 4a with PdCl2 in the presence of NaOAc14
(2.0 equiv) successfully produced a polymeric material with
comparable molecular weight and yield as when prepared with Pd-
(OAc)2. No reaction was observed when 4a was treated with NaOAc
in DMSO at 110 °C in the absence of metal. These results prompted
us to explore the incorporation of other group X metals under
similar reaction conditions. As shown in Scheme 2 and summarized
in Table 1, copolymerizations of the bis(imidazolium) salts with
PtCl2/NaOAc afforded materials with remarkably high molecular
weights; for example, the Mn of 7c was found to be 1.8 × 106 Da.
9
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J. AM. CHEM. SOC. 2005, 127, 12496-12497
10.1021/ja054029k CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Physical Characteristics of Polymers 7-9
together via conjugated, difunctional NHCs. The polymerizations
did not require anhydrous conditions or inert atmospheres, and the
entire process, from commercial materials to polymeric products,
was accomplished in three simple manipulations with minimal tech-
nical difficulty. The physical properties of the polymers were found
to be dependent on both the transition metal and the modular struc-
tural characteristics of the NHC, and could be further modified
through post-polymerization ligation. Control over both molecular
weight and end-groups was also demonstrated through the inclusion
of chain transfer agents during polymerization. Combined with the
ability to prepare polymers with high molecular weights, these mod-
ular building blocks should open new opportunities in polymer syn-
thesis, conductive polymers, nonlinear optics, and electronic devices.
Full investigation of the electronic and physical characteristics of
the polymers presented herein will be reported in due course.
polymer
yield (%)a
Mn
×
103 Dab
Mw/Mn
Td
(°
C)c
λ
max (nm)e
Log(ꢀ e,f
)
7a
7b
7c
7d
8a
8b
9a
9b
90
84
99
76
85
84
98
93
18.4
1.9
1.8
1.3
1.7
2.0
1.4
1.5
2.6
291
316
314
329
322
308
308
323
329
4.72
4.72
4.45
4.51
4.80
4.93
4.63
4.92
d
103
1760
766
26.1
8.00
106
294
295
284
d
299
d
408
a Isolated yield. b Determined by GPC relative to polystyrene standards in
DMF. c The decomposition temperature (Td) is defined as the temperature at
which 10% weight loss occurs determined by thermogravimetric analysis under
N2, rate ) 10 °C/min. d Not determined. e Determined in DMF under ambient
conditions. f Per repeat unit.
Scheme 3
Acknowledgment. We thank the University of Texas at Austin
and the U.S. Army Research Office (W911NF-05-1-0430) for their
generous financial support.
Supporting Information Available: Detailed experimental pro-
cedures and characterization of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Table 2. Experimental and Theoretical Molecular Weight Data
for 11
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a
b
polymer
Xn,theo
Xn,NMR
Mn,NMR
(
×
103 Da)b
Mn,GPC (×
103 Da)c
11a
11b
11c
10
20
40
8.9
23
47
6.4
17
34
4.8
8.5
11
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a Theoretical number average of monomer units per polymer chain given by
(2[4b]o/[10]o). b Determined by 1H NMR spectroscopy. c Determined by GPC
relative to polystyrene standards in DMF.
As with Pd, molecular weights were found to be dependent on the
organic moiety; however, the transition metal obviously played a
dominant role. Unfortunately, no polymeric materials were obtained
with Ni(OAc)2‚4H2O, which may reflect the reduced affinity of
NHCs for Ni.15
Since the metal centers in the main chain of the polymers are
coordinatively unsaturated, we envisioned altering the physical
properties of the macromolecules via a post-polymerization modi-
fication.16 For example, treatment of 8a, which is virtually insoluble
in THF,17 with 2 equiv of PCy3 or PPh3 resulted in rapid dissolution.
GPC and 1H NMR analysis of the phosphine-bound polymer
confirmed the macromolecular structure was not compromised.
Phosphine ligation to Pd was confirmed by 31P NMR spectroscopy
(δ ) 25 ppm).18
(9) Polymerizations proceeded at temperatures as low as 50 °C, although
longer reaction periods were required (3 h).
(10) Prolonged standing of 7b in H2O or 10% HCl (aq) did not affect
spectroscopic or molecular weight characteristics.
(11) Frøseth, M.; Netland, K. A.; Tornroos, K. W.; Dhindsa, A.; Tilset, M. J.
Chem. Soc., Dalton Trans. 2005, 1664.
(12) Based on analogous small molecule reactions (cf. 8), polymer end groups
were assumed to be MX2; however, due to their high molecular weights,
conclusive evidence was not obtainable using 1H or 13C NMR spectro-
scopy.
(13) Mn’s are reported relative to polystyrene standards in DMF.
(14) The use of stronger bases (e.g., NaOH, Et3N) gave inferior yields of
polymer.
Precise control of molecular weight is essential for tailoring or
fine-tuning the physical characteristics of polymeric materials.19
Chain transfer agents (CTAs) have been used extensively to control
molecular weights of polymers obtained from equilibrium poly-
merizations.20 Since coordination of NHCs to Group X metals is
dynamic,21 inclusion of monofunctional NHCs as CTAs during these
polymerizations should provide an effective handle for controlling
molecular weight. As shown in Scheme 3, copolymerization of 4b
with Pd(OAc)2 in the presence of various amounts of N,N′-
dibenzylbenzimidazolium bromide (10)22 provided the correspond-
ing end-functionalized polymers 11. Integration of the benzyl end-
groups (PhCH2- δ ) 6.2 ppm) with respect to the butyl groups
(ArNCH2- δ ) 4.8 ppm) along the polymeric backbone indicated
the molecular weights of the polymers were in excellent agreement
with their theoretical values based on complete incorporation of
the CTA (Table 2).
(15) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N. J. Am. Chem.
Soc. 2002, 124, 15188.
(16) Boaen, N. K.; Hillmyer, M. A. Chem. Soc. ReV. 2005, 34, 267.
(17) Polymers 7-9 exhibit good solubility in highly polar aprotic solvents (e.g.,
DMF, DMSO, and NMP).
(18) This is consistent with the reported 31P chemical shift for (PCy3)2PdCl2
(δ ) 21.2 ppm); see: Ru¨egger, H. Magn. Reson. Chem. 2004, 42, 814.
(19) Odian, G. Principles of Polymerization, 3rd ed.; Wiley-Interscience: New
York, 1991.
(20) Goethals, E. J. Telechelic Polymers: Synthesis and Applications; CRC
Press: Boca Raton, FL, 1989.
(21) (a) Gsto¨ttmayr, C. W. K.; Bo¨hm, V. P. W.; Herdtweck, E.; Grosche, M.;
Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1363. (b) Titcomb,
L. R.; Caddick, S.; Cloke, F. G. N.; Wilsona, D. J.; McKerrecherb, D.
Chem. Commun. 2001, 1388.
(22) Claramunt, R. M.; Elguero, J.; Meco, T. J. Heterocycl. Chem. 1983, 20,
1244.
In conclusion, we have developed a highly efficient route to a
new class of organometallic polymers with metal centers linked
JA054029K
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