1,3-Diarylimidazolidin-2-ylidene (NHC) complexes of Pd(II): Electronic effects on cross-coupling reactions and thermal decompositions Hayati Tu ¨rkmen, Bekir C ¸ etinkaya * Department of Chemistry, Ege University, EU Kampusu, 35100 Bornova-Izmir, Turkey Received 13 December 2005; received in revised form 7 May 2006; accepted 12 May 2006 Available online 20 May 2006 Abstract The synthesis of 1,3-diarylimidazolidin-2-ylidene (NHC) precursor, 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride, (3b) has been extended to the electronically and sterically modified NHC precursors 3a (X = H), 3c (X = Br) and 3e (X = Cl) in order to inves- tigate the electronic effect of a p-substituent (X) on cross-coupling catalysts. Complexes of the type PdCl 2 (NHC) 2 (5), PdCl 2 (NHC)(PPh 3 ) (6) and [RhCl(NHC)(cod)] (7) were prepared from 3 or 4d (1,3-bis(2,4-dimethylphenyl)-2-trichloromethylimidazolidin). Initial decompo- sition temperatures of the complexes 5 and 6 were determined by TGA. In situ formed complexes from Pd(OAc) 2 and 3 as well as the preformed complexes 5 and 6 have been tested as catalysts in coupling of phenylboronic acid with 4-haloacetophenones. The electron donating ability of NHCs derived from 3 was assessed by measuring C–O frequencies in the respective [RhCl(NHC)(CO) 2 ] complex 8 which was prepared by replacement of cod ligand of 7 with CO. An interesting correlation between the electron-donating nature of the aryl substituent and catalytic activity and also initial decomposition temperature of the complexes 5 and 6 was observed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Palladium; N-heterocyclic carbenes; Electronic effects; Cross-coupling; Thermal gravimetric analysis 1. Introduction Recently, N-heterocyclic carbene (NHC) ligands are receiving intensive attention in the fields of coordination chemistry and homogeneous catalysis because organome- tallic compounds containing NHC ligands exhibit high reactivities. NHCs have appeared as sterically and electron- ically tunable ligands which make stable coordinative bonds with transition metals [1–3]. Dramatic improve- ments in C–C coupling activity have been observed with complexes (A, M = palladium; L = ligands other than NHC; X = Me) using five-membered NHCs as ligands. Since then, a number of five-membered NHC ligands with different substituents have been studied [4,5]. X N N X ML n A It was found that ligands bearing two bulky Aryl groups (Ar = Mes or 2,6-di-i-propylphenyl) promote the reaction of unactivated aryl chlorides [6,7]. The ligands may afford coordinatively unsaturated mono- or dicarbene ligated complexes and accelerate the catalytic steps, that is, oxida- tive addition, transmetalation and reductive elimination [8]. The steric bulk of the ligand is believed to aid the reduc- tive elimination of product, while electron richness of the ligand imparts a high catalytic performance via an oxida- tive addition step [7]. Since Hermann’s discovery in 1995 [9] many structural modifications, including steric tuning 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.05.020 * Corresponding author. Tel.: +90 232 3881092; fax: +90 232 38881036. E-mail address: bekir.cetinkaya@ege.edu.tr (B. C ¸ etinkaya). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 3749–3759