Synthesis, characterization and catalytic behaviour of ansa-zirconocene complexes containing tetraphenylcyclopentadienyl rings: X-ray crystal structures of [Zr{Me 2 Si(g 5 -C 5 Ph 4 )(g 5 -C 5 H 3 R)}Cl 2 ] (R = H, Bu t ) Santiago Go ´ mez-Ruiz a,b , Dorian Polo-Cero ´n a , Sanjiv Prashar a, * , Mariano Fajardo a , Vı ´ ctor L. Cruz c , Javier Ramos c , Evamarie Hey-Hawkins b a Departamento de Quı ´mica Inorga ´ nica y Analı ´tica, E.S.C.E.T., Universidad Rey Juan Carlos, 28933 Mo ´ stoles (Madrid), Spain b Institut fu ¨ r Anorganische Chemie der Universita ¨ t Leipzig, Johannisallee 29, D-04103 Leipzig, Germany c Instituto de Estructura de la Materia, CSIC, Serrano, 113bis, E-28006, Madrid, Spain Received 29 August 2007; received in revised form 2 November 2007; accepted 19 November 2007 Available online 5 December 2007 Abstract The ansa-bis(cyclopentadiene) compounds, Me 2 Si(C 5 HPh 4 )(C 5 H 4 R) (R=H (2); Bu t (3)), have been prepared by the reaction of C 5 HPh 4 (SiMe 2 Cl) (1) with Na(C 5 H 5 ) or Li(C 5 H 4 Bu t ), respectively, and transformed to the di-lithium derivatives, Li 2 {Me 2 - Si(C 5 Ph 4 )(C 5 H 3 R)} (R = H (4); Bu t (5)), by the action of n-butyllithium. The ansa-zirconocene complexes, [Zr{Me 2 Si(g 5 -C 5 Ph 4 )(g 5 - C 5 H 3 R)}Cl 2 ] (R = H (6); Bu t (7)), were synthesized from the reaction of ZrCl 4 with 4 or 5, respectively. Compounds 6 and 7 have been tested in the polymerization of ethylene and compared with their methyl-substituted analogues, [Zr{Me 2 Si(g 5 -C 5 Me 4 )(g 5 -C 5 H 3 R)}Cl 2 ] (R = H (8); Bu t (9)). Whilst 8 and 9 are catalytically active, the tetraphenyl-substituted complexes 6 and 7 proved to be inactive in the polymerization of ethylene. This phenomenon has been explained by DFT calculations based on the reaction intermediates in the poly- merization processes involving 6 and 7, which showed that the extraction of a methyl group from the zirconocene complex to form the cationic active specie is endothermic and therefore unfavourable. Ó 2007 Elsevier B.V. All rights reserved. Keywords: ansa-Metallocene; Zirconium; Tetraphenylcyclopentadienyl; Polymerization; DFT calculations 1. Introduction Taking into account that metallocene complexes are amongst those most studied in the field of organometallic chemistry [1], it is therefore surprising that only a handful of compounds containing tetraphenylcyclopentadienyl or pentaphenylcyclopentadienyl ligands have been reported [2]. These reports have shown that the presence of these bulky cyclopentadienyl ligands greatly reduce the reactivity of their complexes, when compared with the unsubstituted metallocene analogues [2]. Solid-state and solution studies have allowed a greater understanding of the influence of the phenyl groups and of the interaction between and within the cyclopentadienyl rings of the metallocene complex [2,3]. Further work dealing with the stability and conformation of bis(tetraphenylcyclopentadienyl) metal complexes, such as [M(g 5 -C 5 HPh 4 ) 2 ] (M = V, Cr, Co, and Ni), has also been carried out although the bent metal- locene conformation was never observed [2]. Nevertheless, in the case of the group four metals, the bent metallocene conformation was easily accessible although the reactivity of these complexes was limited [2,4]. In particular, for zir- conium, only a few metallocene complexes incorporating the tetraphenylcyclopentadienyl have been described [4,5]. 0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2007.11.054 * Corresponding author. Tel.: +34 914887186; fax: +34 914888143. E-mail address: sanjiv.prashar@urjc.es (S. Prashar). www.elsevier.com/locate/jorganchem Available online at www.sciencedirect.com Journal of Organometallic Chemistry 693 (2008) 601–610