Chloro(g 5 -dihydropentalenyl)bis(triphenylphosphine)ruthenium(II): synthesis, structural characterization and catalytic activity in the dimerization of phenylacetylene Rein U. Kirss a, * , Richard D. Ernst b , Atta M. Arif b a Department of Chemistry, Northeastern University, 102 Hurtig Hall, Boston MA, 02115, USA b Department of Chemistry, University of Utah, 315 S. 1400 E., Salt Lake City, UT 84112, USA Received 27 June 2003; accepted 19 September 2003 Abstract Reactions between HRuCl(PPh 3 ) 3 and 1,3- or 1,5-cyclooctadiene yield the 1,2-dihydropentalenyl complex (g 5 - C 8 H 9 )Ru(PPh 3 ) 2 Cl through a series of steps including olefin insertion and electrocyclization. The reaction is accompanied by the loss of two equivalents of hydrogen. The product crystallizes in the monoclinic space group P  1 (No. 2). (g 5 -C 8 H 9 )Ru(PPh 3 ) 2 Cl catalyzes the dimerization of phenylacetylene to a 2:1 mixture of Z:E 1,4-diphenyl-1-buten-3-yne. Comparison of the catalytic activity of (g 5 -C 8 H 9 )Ru(PPh 3 ) 2 Cl with (g 5 -C 5 H 5 )Ru(PPh 3 ) 2 Cl, (g 5 -C 5 Me 5 )Ru(PPh 3 )H 3 and {g 5 -HB(pz) 3 }Ru(PPh 3 ) 2 Cl suggests that the more electron-rich g 5 ligands favor formation of the Z isomer. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Electrocyclic reactions; Ruthenium compounds; Dimerization of alkynes 1. Introduction Flash vacuum pyrolysis (FVP) of (g 5 -pentamethylcyclopentadienyl)-(g 5 -cyclooctadienyl)ruthenium, ({g 5 -C 5 Me 5 } {g 5 -C 8 H 11 }Ru) results in electrocyclic ring closure of the C 8 H 13 ligand, forming (g 5 -pentamethylcyclopentadienyl) (g 5 -1,2-dihydropentalenyl)ruthenium ({g 5 -C 5 Me 5 }{g 5 -C 8 H 9 }Ru). The reaction is accompanied by the loss of one equivalent of H 2 (Eq. (1)) [1]. The potential synthetic utility of ruthenium-mediated electrocyclic reactions in the construction of [3.n.0] fused ring systems can be better realized if conditions can be identified under which the reaction occurs in solution at lower temperature. Ru Ru H 2 350˚C ð1Þ The relatively mild conditions for the electrocyclic reaction of 1,4-pentadiene [2] (Eq. (2)) suggested that HRuCl(PPh 3 ) 3 might be a versatile reagent for the construction of fused [3.n.0] carbocyclic ring systems from cyclic Journal of Organometallic Chemistry 689 (2004) 419–428 www.elsevier.com/locate/jorganchem * Corresponding author. Tel.: +1-617-373-4513; fax: +1-617-373-8795. E-mail address: rkirss@neu.edu (R.U. Kirss). 0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2003.09.052