Structure and redox behavior of Ru(II)-diene complexes Enrique Mele ´ndez a, * , Vilmarı ´ Lo ´pez a , Thomas Concolino b , Arnold L. Rheingold b,1 a Department of Chemistry, University of Puerto Rico, P.O. Box 9019, Mayaguez, PR 00681, USA b Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA Received 8 April 2004; accepted 30 June 2004 Available online 4 August 2004 Abstract A series of Ru(acac) 2 (g 4 -diene) complexes containing cis- and trans-diene coordination have been investigated by cyclic voltam- metry to correlate structural bonding and conformation patterns of diene ligands with redox behaviors. The solid-state structure of Ru(acac) 2 (2,3-dimethyl-1,3-butadiene) has been determined by single crystal X-ray diffraction methods. Ru(acac) 2 (2,3-dimethyl-1,3- butadiene) crystallizes in the monoclinic space group C2/c with a = 12.368(2) A ˚ , b = 17.0600(2) A ˚ , c = 16.0110(2) A ˚ , b = 98.4405(10)° and V = 3341.38(10) A ˚ 3 for Z = 8. A structural comparison between several Ru-trans-g 4 -diene complexes and Ru-g 4 -1,3-cyclohexadiene revealed no difference in the Ru–C(diene) bond distances. However, through cyclic voltammetry exper- iments these species demonstrated different redox behavior, as function of the coordinated diene ligand. Ó 2004 Elsevier B.V. All rights reserved. Keywords: trans-g 4 -Diene coordination; cis-g 4 -Diene coordination Ru(II)-diene; Cyclic voltammetry; Solid-state structure 1. Introduction Tris(acetylacetonato)ruthenium(III), Ru(acac) 3 , has been a versatile precursor for many inorganic and or- ganometallic complexes [1–5]. In the past, our research group has reported the solid-state structure of a series of Ru(acac) 2 (diene) complexes in which for acyclic diene exhibited a rather unusual trans-g 4 -diene coordination (A), while for the cyclic diene, the cis-g 4 -diene coordina- tion (B) is observed due to geometric constrains [1,2,6]. Particularly interesting for the g 4 -trans-diene coordina- tion is that two diastereisomers, I and II (Fig. 1), are ob- served depending on which enantiophase is coordinated by Ru(acac) 2 [1,2,6]. NMR studies on the Ru-acyclic di- ene complexes have shown that both Ru-g 4 -trans-diene diastereisomers interconvert each other and have about the same thermodymanic stability in solution while the cis-diene coordination is only present in less than 10% at high temperature (85 °C) [2]. Thus, there is a thermo- dynamic preference for the trans-diene coordination over the cis when bound to Ru(acac) 2 . M M A B It is known that there exists a relationship between diene conformation and reactivity patterns when the diene is coordinated to metal centers. The remarkable differences between g 4 -cis- and trans-g 4 -diene coordina- 0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2004.06.058 * Corresponding author. Tel.: +1-787-8324040x2524; fax: +1-787- 2653849. E-mail address: emelendez@uprm.edu (E. Mele ´ndez). 1 Present address: Department of Chemistry, University of Cali- fornia, San Diego, La Jolla, CA 92093, USA. Journal of Organometallic Chemistry 689 (2004) 3082–3087 www.elsevier.com/locate/jorganchem