This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 7925–7927 7925

Cite this: Chem. Commun., 2012, 48, 7925–7927

Ring opening metathesis polymerization of an g4-benzene complex: a

direct synthesis of a polyacetylene with a regular pattern of p bound

metal fragmentsw

Paul D. Zeits, Tobias Fiedler and John A. Gladysz*

Received 23rd March 2012, Accepted 7th June 2012

DOI: 10.1039/c2cc32150e

The complex (g5-C5H5)Ir(g4-C6H6) reacts with Grubbs’ catalyst to

give a novel polyacetylene consisting of cyclopentadienyliridium

bound s-cis butadiene moieties separated by CQQQC linkages. A crystal

structure of the pentamethylcyclopentadienyl analog establishes

a strong structural analogy with norbornadiene, a classical

ROMP monomer.

The conjugated polymer polyacetylene has been the subject

of innumerable studies since MacDiarmid reported that its

conductance increases by ten orders of magnitude upon iodine

doping.1 However, it remains difficult to process due to its low

solubility. Monosubstituted acetylenes yield polyacetylenes

that are more soluble, but the 1,3-relationship of substituents

commonly induces twisting that inhibits conjugation.2

Grubbs established that monosubstituted cyclooctenes undergo

ring opening metathesis polymerization (ROMP)3 when treated

with Schrock-type catalysts,4 yielding less highly substituted

systems with improved conjugation. Feast examined the ROMP

of the cleverly designed cyclobutene A (Scheme 1), which gave a

soluble polymerB that after solution processing could be converted

by a retro-Diels–Alder reaction to the polyacetylene C.5

Schrock, Buchmeiser, Choi, and others have developed cyclo-

polymerizations of 1,6-heptadiynes that yield highly conjugated

soluble polyacetylenes.6

However, another approach to solubilizing polyacetylene

would entail the introduction of p bound transition metal

fragments. Since most transition metal complexes are redox

active, this could also provide new possibilities for hole or electron

delocalization. To our knowledge, the only transition metal

derivatives of polyacetylene involve ferrocenyl or ruthenocenyl

containing s substituents.7

ROMP reactions are facilitated when the cycloalkene is

strained as in A, and norbornene is a prototypical monomer.8

We were struck by the similarity between norbornene or

norbornadiene and one of the two principal resonance forms of

Z4-benzene complexes (F; Scheme 2, top). Although Z4-benzene

complexes are rare compared to Z6-adducts, they have been

known for some time9 and many are readily isolable.9–12 Hence,

we set out to attempt the ROMP of representative complexes.

This would generate a strikingly new class of functionalized

polyacetylenes, consisting of metal complexed s-cis butadiene

moieties separated by CQC linkages.

Two complexes were selected for this study. This first was the

previously reported eighteen valence electron cyclopentadienyl

iridium Z4-benzene complex (Z5-C5H5)Ir(Z4-C6H6) (1; Scheme 2,

bottom).10 This is easily synthesized by a double hydride abstraction

from the Z4-1,3-cyclohexadiene complex (Z5-C5H5)Ir(Z4-1,3-C6H8)

with Ph3C+ BF4

� to give the dicationic Z6-benzene complex

[(Z5-C5H5)Ir(Z6-C6H6)]

2+ 2BF4�, followed by reduction with

cobaltocene, (Z5-C5H5)2Co, to 1. The second was the new

pentamethylcyclopentadienyl analog (Z5-C5Me5)Ir(Z4-C6H6)

(2), similarly accessed by reduction of the known Z6-benzene

Scheme 1 One of several other ROMP approaches fo polyacetylene.

Scheme 2 Representations of Z4-benzene complexes (top), and syntheses

of iridium adducts (bottom).

Department of Chemistry, Texas A&M University, P.O. Box 30012,College Station, TX 77842-3012, USA.E-mail: gladysz@mail.chem.tamu.edu; Fax: (979)845-5629;Tel: (979)845-1399w Electronic supplementary information (ESI) available: Experimentalprocedures and additional figures. CCDC 873356. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c2cc32150e

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7926 Chem. Commun., 2012, 48, 7925–7927 This journal is c The Royal Society of Chemistry 2012

complex13 [(Z5-C5Me5)Ir(Z6-C6H6)]

2+ 2BF4� as detailed in

the supporting information.wSingle crystals of 2 could be obtained, and an X-ray crystal

structure was determined as described in the supporting

information.w Two independent molecules were found in the

unit cell. One is shown in Fig. 1, and key metrical parameters

for each are listed in the caption. The uncoordinated CQC

linkages (both 1.311(9) A) are considerably shorter than the

opposite coordinated C.�.�.�C linkages (1.416(9)–1.400(9) A), and

the uncoordinated CQ�C–�C bonds (1.477(9)–1.502(8) A) are

generally longer than the adjacent coordinated CQC–�C.�.�.��C

bonds (1.449(9)–1.481(9) A). These well precedented trends

are consistent with the above resonance formulations. Also,

the Z6-benzene ligand is clearly bent or folded. This can be

quantified by the angle between two least squares planes

(C–CQC–C and C.�.�.�C.�.�.�C.�.�.�C), and is 43.51 for both molecules.

The corresponding values in norbornene and norbornadiene are

68.5–65.51,14 and 45.3–42.71 in two other structurally characterized

Z4-benzene complexes.11

As shown in Scheme 3, 1 and Grubbs’ catalyst (10 mol%)

were combined in CD2Cl2 at room temperature. The slow

formation of a soluble polymer (poly-1) was easily monitored by1H and 13C NMR.15 The cyclopentadienyl and other resonances

of the monomer were cleanly replaced by new signals (C5H5,

s 5.14 - 5.22 (1H) and 76.6 - 77.4 (13C) ppm). When the

loading was increased to 20 mol%, 90% conversion was

attained after 6 d. When the same sample was kept at 40 1C

in a sealed tube, 4 93% conversion was attained after 2 d.

Grubbs’ second generation catalyst and Schrock’s catalyst

also effected the ROMP of 1, albeit at slower rates. No

reaction was observed with 2, in which the exo CQC face is

sterically shielded by the bulkier pentamethylcyclopentadienyl

ligand. It has been previously shown that Schrock type catalysts

preferentially add to the exo CQC face of norbornadienes and

7-oxanorbornadienes.16

When the polymerization of 1 was Z 90% complete, ethyl

vinyl ether was added to replace the ruthenium alkylidene

endgroup on the growing end of the polymer chain by a vinyl

endgroup,17 giving poly-10 (Scheme 3). Integration of the vinyl

protons versus the cyclopentadienyl protons typically showed

a degree of polymerization of 8–10,15 although values as high

as 20 could be attained. Poly-10 was moderately air sensitive,

blackening upon prolonged exposure.

The UV visible spectrum of 1 exhibited a featureless d–d

transition that tailed into the visible. However, that of poly-10

exhibited an additional shoulder near 360 nm.15 Analysis

analogous to those for other ROMP-generated polyacetylenes18

suggested an effective conjugation length of six-seven CQC

units. In this context, there are several isomers and tertiary

structures possible for poly-10. First, as with polynorbornene,8

the new CQC bonds may be Z or E, and within any dyad the

metal fragments may be syndiotactic or isotactic. Second, the

conformations of the CQC–C.�.�.�C linkages may be s-trans, as

depicted in the all E isomer G in Scheme 3 (bottom), or s-cis.

The latter would introduce markedly helical segments arising from

repulsive 1,7-carbon/carbon interactions. The closest molecular

analog to G of which we are aware would be the trans,trans,-

trans,trans,trans-1,10-diphenyldecapentaene complex H, in

which two syn ruthenium fragments are bound to s-cis butadiene

moietes separated by an E CQC linkage.19

The cyclic voltammogram of 1 (THF) exhibited only irreversible

oxidation and reduction processes, with feeble current during the

return scans.15 In the anodic direction, poly-10 exhibited four

oxidation peaks (0.13, 0.64, 0.96, 1.12 V), and five in the reversed

cathodic scan (0.52, �0.03, �0.75, �1.51, �2.11 V). When

drop-cast films of poly-10 were doped with iodine vapor, they

turned a lustrous brown. In a standard qualitative conductivity

test,20 interdigitated electrode arrays were coated with iodine

Fig. 1 Molecular structure of 2; thermal ellipsoid plot (50% probability

level) of one of the two independent molecules in the unit cell. Key bond

lengths and angles (molecules 1/2): Ir–C1 2.154(6)/2.137(6), Ir–C2 2.119(6)/

2.119(6), Ir–C3 2.104(6)/2.125(6), Ir–C4 2.153(6)/2.157(6), C1–C2 1.453(9)/

1.466(9), C2–C3 1.416(9)/1.400(9), C3–C4 1.449(9)/1.481(9),

C4–C5 1.485(8)/1.477(9), C5–C6 1.311(9)/1.311(9), C6–C1 1.484(8)/

1.502(8), C1–C2–C3 112.4(6)/113.0(6), C2–C3–C4 113.6(6)/113.9(5),

C3–C4–C5 116.8(6)/117.1(6), C4–C5–C6 113.9(6)/113.7(6), C5–C6–C1

115.1(6)/115.5(6), C6–C1–C2 116.4(5)/115.8(5).

Scheme 3 Syntheses of p complexed polyacetylenes.

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 7925–7927 7927

doped films (see Fig. 2). When potentials were applied, a

proportional increase in current was observed.15

In summary, we have established a facile entry into a

heretofore unavailable type of polyacetylene that is characterized

by a regular pattern of p complexed metal fragments. The system

accessed in this feasibility study features metal bound s-cis buta-

diene moieties that are separated by CQC linkages. However, the

ROMP of other types of p adducts of cyclic polyenes could lead to

a variety of substitution motifs. The steric, electronic, and solubility

properties of these polymers should be tunable by varying the

metal and/or ancillary ligands. Although the metal fragments affect

the electronic nature of the polyacetylene backbone, they introduce

new possibilities for hole or electron transport mechanisms. This

may lead to a large new class of conducting polymers with novel

and potentially useful properties.

Notes and references

1 H. Shirikawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang andA. J. Heeger, Chem. Commun., 1977, 578–580.

2 S. B. Clough, X. F. Sun, S. K. Tripathy and G. L. Baker, Macro-molecules, 1991, 24, 4264–4269.

3 C. B. Gorman, E. J. Ginsburg and R. H. Grubbs, J. Am. Chem.Soc., 1993, 115, 1397–1409.

4 R. R. Schrock, Dalton Trans., 2011, 40, 7484–7495.5 J. H. Edwards, W. J. Feast and D. C. Bott, Polymer, 1984, 25,395–398.

6 (a) H. H. Fox, M. O. Wolf, R. O’Dell, B. L. Lin, R. R. Schrock andM. S. Wrighton, J. Am. Chem. Soc., 1994, 116, 2827–2843;(b) S.-K. Choi, Y.-S. Gai, S.-H. Jin and H. K. Kim, Chem. Rev.,

2000, 100, 1645–1681; (c) M. R. Buchmeiser, C. Schmidt andD. Wang, Macromol. Chem. Phys., 2011, 232, 1999–2008 andreferences therein; (d) I. S. Lee, E.-H. Kang, H. Park and T.-L. Choi,Chem. Sci., 2012, 3, 761–765.

7 See the following lead references and citations therein:(a) M. Buchmeiser and R. R. Schrock, Macromolecules, 1995, 28,6642–6649; (b) M. R. Buchmeiser, N. Schuler, G. Kaltenhauser,K.-H. Ongania, I. Lagoja, K. Wurst and H. Schottenberger,Macromolecules, 1998, 31, 6642–6649; (c) M. Teraguchi andT. Masuda, J. Macromol. Sci., Part A: Pure Appl. Chem., 2003,40, 115–124; (d) C. K. W. Jim, A. Qin, F. Mahtab, J. W. Y. Lamand B. Z. Tang, Chem.–Asian J., 2011, 6, 2753–2761.

8 Handbook of Metathesis, ed. R. H. Grubbs, Wiley/VCH, Weinheim,2003, vol. 3, ch. 3.2 and 3.5.

9 E. L. Muetterties, J. R. Bleeke, E. J. Wucherer and T. A. Albright,Chem. Rev., 1982, 82, 499–525, and references therein.

10 J. Muller, P. E. Gaede and K. Qiao, J. Organomet. Chem., 1994,480, 213–220.

11 (a) C. Bianchini, K. G. Caulton, C. Chardon, O. Eisenstein,K. Folting, T. J. Johnson, A. Meli, M. Peruzzini, D. J. Rauscher,W. E. Streib and F. Vizza, J. Am. Chem. Soc., 1991, 113, 5127–5129;(b) G. Huttner and S. Lange, Acta Crystallogr., Sect. B: Struct.Crystallogr. Cryst. Chem., 1972, 28, 2049–2060.

12 Other representative examples: (a) R. C. Mills, S. Y. S. Wang,K. A. Abboud and J. M. Boncella, Inorg. Chem., 2001, 40,5077–5082 and references therein; (b) P. M. Budzelaar, N. N. P.Moonen, R. de Gelder, J. M. M. Smits and A. W. Gal, Chem.–Eur. J.,2000, 6, 2740–2747.

13 S. L. Grundy and P. M. Maitlis, J. Organomet. Chem., 1984, 272,265–282.

14 (a) J. Min, J. Benet-Buchholz and R. Boese, Chem. Commun., 1998,2751–2752; (b) J. Benet-Buchholz, T. Haumann and R. Boese,Chem. Commun., 1998, 2003–2004.

15 A figure is supplied in the supporting informationw.16 (a) G. C. Bazan, E. Khosravi, R. R. Schrock, W. J. Feast,

V. C. Gibson, M. B. O’Regan, J. K. Thomas and W. M. Davis,J. Am. Chem. Soc., 1990, 112, 8378–8387; (b) G. C. Bazan,J. H. Oskam, H.-N. Cho, L. Y. Park and R. R. Schrock, J. Am.Chem. Soc., 1991, 113, 6899–6907.

17 Z. Wu, S. T. Nguyen, R. H. Grubbs and J. W. Ziller, J. Am. Chem.Soc., 1995, 117, 5503–5511.

18 (a) R. R. Schrock, S. A. Krouse, K. Knoll, J. Feldman,J. S. Murdzek and D. C. Yang, J. Mol. Catal., 1988, 46,243–253; (b) E. J. Ginsburg, C. B. Gorman, S. R. Marder andR. H. Grubbs, J. Am. Chem. Soc., 1989, 111, 7621–7622.

19 H. Fukumoto and K. Mashima, Eur. J. Inorg. Chem., 2006,5006–5111.

20 (a) N. F. Sheppard, Jr., R. C. Tucker and C.Wu,Anal. Chem., 1993,65, 1199–1202; (b) C. E. Chidsey, B. J. Feldman, C. Lundgren andR. W. Murray, Anal. Chem., 1986, 58, 601–607; (c) K. Sakamoto,K. Aramaki and H. Nishihara, Chem. Lett., 1993, 659–662.

Fig. 2 Iodine doped film of poly-10 cast on an interdigitated

electrode.

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