Effect of immobilization on catalytic characteristics of saturated Pd-N-heterocyclic carbenes in Mizoroki–Heck reactions O ¨ zge Aksın a , Hayati Tu ¨ rkmen b , Levent Artok a, * , Bekir C ¸ etinkaya b , Chaoying Ni c , Orhan Bu ¨yu ¨kgu ¨ngo ¨r d , Erhan O ¨ zkal a a Department of Chemistry, Faculty of Science, Izmir Institute of Technology, Urla 35430 Izmir, Turkey b Department of Chemistry, Faculty of Science, Ege University, Bornova 35100, Izmir, Turkey c Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA d Department of Physics, Faculty of Science and Art, Ondokuz Mayıs University, Kurupelit 55139, Samsun, Turkey Received 13 December 2005; received in revised form 27 February 2006; accepted 8 March 2006 Available online 14 March 2006 Abstract A saturated Pd-N-heterocyclic complex was immobilized on an amorphous silica. The complex itself is of very high thermal stability. However, TEM observations, hot filtration, reusability, and poisoning tests all revealed that the complex acted only as a precatalyst to highly active Pd species in Mizoroki–Heck reactions when immobilized. The complex appears more stable when used under homoge- neous reaction conditions. The immobilized complex afforded high turnover numbers, 10 4 –10 5 . The higher turnover frequencies were realized at the lower Pd concentrations, which is a characteristic property of ligand-free Pd catalyzed reactions. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Mizoroki–Heck reaction; C–C coupling; Heterocyclic carbene; Immobilized palladium; Heterogeneous catalyst, Palladium leaching 1. Introduction Since their first introduction by O ¨ fele and Wanzlick [1], N-heterocyclic carbenes (NHCs) have become universally accepted ligands in organometallic and inorganic coordina- tion chemistries. Due to their being strong r-donors and low p-acceptors, NHCs have even been proposes as replacements for the more commonly used phosphane ligands in palladium catalyzed coupling reactions, because they resemble organophosphanes [2]. Furthermore, NHCs are easily acquired, non-toxic and exhibit high thermal sta- bility when compared with phosphanes [3]. These proper- ties have been exploited in a broad range of catalytic applications [4]. As a catalyst, Pd is very important in the pharmaceutical industry; however, because of its expense and toxicity, it is particularly important to reduce both its loss and presence in the product solution, respectively. To this end, immobi- lization of Pd-catalysts onto a support allows heteroge- neous catalyst with decreased leaching of Pd to the solution. When an NHC is complexed with a metal, it exhibits extraordinary thermal stability, making it a rea- sonable candidate for immobilization. Likewise, due to the stability of the metal complex, it is presumed that the metal will remain in its immobilized state when anchored to a support prior to reaction making it be less likely to leach into solution [3c,5]. According to the proposed reaction mechanism for cou- pling reactions involving haloarenes, the oxidative Pd addi- tion to haloarenes is believed to be the rate-determining step with the activity of catalyst being determined by the electron density on palladium. Therefore, in the paper pre- sented here, the use of saturated NHCs was preferred because of their electron-donating ability as compared with their unsaturated counterparts. In contrast to view that metals of NHCs are less likely to leach into solution, results will be shown here which 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.03.012 * Corresponding author. Tel.: +90 232 7507529; fax: +90 232 7507509. E-mail address: leventartok@iyte.edu.tr (L. Artok). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 3027–3036