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 g 4 -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 (g 5 -C 5 H 5 )Ir(g 4 -C 6 H 6 ) reacts with Grubbs’ catalyst to give a novel polyacetylene consisting of cyclopentadienyliridium bound s-cis butadiene moieties separated by C QC 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 polymer B 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 Z 4 -benzene complexes (F; Scheme 2, top). Although Z 4 -benzene complexes are rare compared to Z 6 -adducts, they have been known for some time 9 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 C Q C linkages. Two complexes were selected for this study. This first was the previously reported eighteen valence electron cyclopentadienyl iridium Z 4 -benzene complex ( Z 5 -C 5 H 5 )Ir(Z 4 -C 6 H 6 )( 1; Scheme 2, bottom). 10 This is easily synthesized by a double hydride abstraction from the Z 4 -1,3-cyclohexadiene complex (Z 5 -C 5 H 5 )Ir(Z 4 -1,3-C 6 H 8 ) with Ph 3 C + BF 4 À to give the dicationic Z 6 -benzene complex [(Z 5 -C 5 H 5 )Ir(Z 6 -C 6 H 6 )] 2+ 2BF 4 À , followed by reduction with cobaltocene, (Z 5 -C 5 H 5 ) 2 Co, to 1. The second was the new pentamethylcyclopentadienyl analog (Z 5 -C 5 Me 5 )Ir(Z 4 -C 6 H 6 ) (2), similarly accessed by reduction of the known Z 6 -benzene Scheme 1 One of several other ROMP approaches fo polyacetylene. Scheme 2 Representations of Z 4 -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)845-1399 w Electronic supplementary information (ESI) available: Experimental procedures and additional figures. CCDC 873356. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc32150e ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION