Palladium N-heterocyclic carbene complexes: Synthesis, characterization and catalytic properties in amination Ismail Özdemir a, * , Serpil Demir a , Onur S ßahin b , Orhan Büyükgüngör b , Bekir Çetinkaya c a Inönü University, Faculty of Science and Art, Department of Chemistry, 44280 Malatya, Turkey b OndokuzMayıs University, Department of Physics, 55139 Samsun, Turkey c Ege University, Faculty of Science, Department of Chemistry, 35100 Bornova- _ Izmir, Turkey article info Article history: Received 7 November 2009 Received in revised form 12 February 2010 Accepted 9 March 2010 Available online 15 March 2010 Keywords: N-heterocyclic carbene Palladium catalysis Amination Triarylamines Amines abstract Transmetalation has proved to be a promising procedure to obtain NHC–metal complexes, which typi- cally involves treatment of the imidazolium salt with Ag 2 O to form the Ag–NHC complex, followed by transmetalation to a species such as [PdCl 2 (CH 3 CN) 2 ] gave the palladium complex in which the N-hetero- cyclic carbene was bound to the metal center. New bis(NHC)–Pd complexes were synthesized and char- acterized by elemental analysis, 1 H NMR, 13 C NMR, and IR spectroscopy. The crystal and molecular structure of the trans-dichlorobis{1-(2,3,5,6-tetramethylbenzyl)-3-(2,3,4,5,6-pentamethylbenzyl)imidazoli- din-2-yliden}palladium(II) complex was determined by single-crystal X-ray diffraction. The activity of the Pd(II) complexes in the coupling reaction of anilines or amines with bromobenzene was investigated. A preliminary catalytic study showed that these bis(NHC)–Pd complexes were highly active in the direct synthesis of triarylamines and secondary amines in a single step. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The N-aryl amination (Buchwald–Hartwig reaction) has at- tracted enormous interest over the last 20 years and belongs now- adays to an indispensable set of cross-coupling reactions, which finds its application in all areas of organic chemistry, ranging from the laboratory bench, the synthesis of pharmaceutical fine chemi- cals and the production of bulk chemicals [1,2]. The most research groups are focused on the development of palladium complexes with newly designed ligand systems in order to improve their efficiency, scope, and applicability as catalyst [2]. Up to now, there have been several categories of ligands listed as the most frequently used: phosphine, palladacycle, metalcontaining biden- tate phosphine, N-heterocyclic carbenes (NHCs) [3]. Nucleophilic N-heterocyclic carbenes(NHC) have emerged as a class of highly useful ligands in organometallic chemistry during the last 15 years [4]. Two main scientific achievements may have comprised the ini- tial spur to this development: (i) the first use by Herrmann and co- workers of NHC complexes in catalysis [5] and (ii) the preparation of the Grubbs’ second generation catalyst and related catalysts [6]. Among numerous successful applications of NHC ligands are highly efficient Pd catalyzed amination, Heck olefin arylation, and cou- pling reactions of arenes [7]. To perform a coupling reaction, two approaches are available to generate a catalytically active species. One approach employs a Pd(0) source combined with an ancillary ligand to generate the catalyst in situ [8] and the second uses a precatalyst, mostly Pd(II) complexes [9], which will be activated in the reaction mixture. We have been strongly interested in the efficiency of Pd–NHC complexes in the catalytic C–N coupling reac- tion. Here we decided to examine their efficiency in the N-arylation of anilines and amines to form triarylamines and secondary amines, respectively. 2. Results and discussion 2.1. Synthesis and characterization of Pd–NHC complexes Transmetalation has proved to be a promising procedure to ob- tain NHC–metal complexes [10], which typically involves treat- ment of the imidazolium salt with Ag 2 O to form the Ag–NHC complex, followed by transmetalation to a species such as [PdCl 2 (CH 3 CN) 2 ] to give the Pd–NHC complex (Scheme 1). They were isolated as a pale yellow solid. Complexes are stable to air and moisture and very soluble in dichloromethane and chloroform, but insoluble in diethyl ether and hexane. The crude products recrystallized from dichloromethane:diethyl ether (1:2) at room temperature afforded the corresponding crystals. The six new com- plexes were characterized by 1 H NMR, 13 C NMR, IR and elemental analysis techniques which support the proposed structures. The products appear to be spectroscopically pure, and exhibit signals slightly upfield in comparison with the parent carbene precursors (1); as expected, the C 2 –H signal is absent. Correspondingly, the 13 C 0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2010.03.013 * Corresponding author. E-mail address: iozdemir@inonu.edu.tr (I. Özdemir). Journal of Organometallic Chemistry 695 (2010) 1555–1560 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem