New pentamethylene-bridged bis-imidazolium dication ligands and its palladium(II) complexes: Synthesis, characterization, and catalysis Manoj Trivedi a,⇑ , R. Nagarajan a , Nigam P. Rath b,⇑ , Abhinav Kumar c , Kieran C. Molloy d , Gabriele Kociok-Köhn d a Department of Chemistry, University of Delhi, Delhi 110007, India b Department of Chemistry and Biochemistry, Centre for Nanoscience, University of Missouri-St. Louis, One University Boulevard, St. Louis, MO 63121-4499, USA c Department of Chemistry, Faculty of Science, Lucknow University, Lucknow 226007, India d Department of Chemistry, University of Bath, Bath BA2 7AY, UK article info Article history: Received 26 August 2011 Accepted 27 October 2011 Available online 12 November 2011 Keywords: Imidazolium dication ligand Palladium complex Crystal structure Weak interaction Catalysis abstract Two new pentamethylene-bridged bis-imidazolium dication ligands (L 1 = 1,5-bis(1-(4-cyanophenyl)- imidazolium-1-yl)pentane dibromide) and L 2 = 1,5-bis(1-(4-methoxyphenyl)-imidazolium-1-yl)pentane dibromide) bearing CPI (1-(4-cyanophenyl)-imidazole) and MPI (1-(4-methoxyphenyl)-1H-imidazole)- functionality have been prepared via the reaction of 1,5-dibromopentane with a substituted imidazole derivative. The imidazolium ligand L 1 and L 2 on reaction with [PdBr 2 (C 6 H 5 CN) 2 ] led to the formation of neutral bis-imidazolium dication palladium(II) complexes [L 1 ] [PdBr 4 ][1] (L 1 = 1,5-bis(1-(4-cyanopheny- limidazolium-1-yl)pentane) and [L 2 ] [PdBr 4 ] [2] (L 2 = 1,5-bis(1-(4-methoxyphenyl) imidazolium- 1-yl)pentane), respectively. The new ligands as well as their palladium(II) complexes, has been characterized by elemental analysis, electronic, IR, 1 H and 13 C NMR, FAB–MS and ESI–MS spectroscopy. The molecular structure of the representative complex [L 2 ] [PdBr 4 ][2] have been determined by single crystal X-ray analysis. Complex 2 exhibits the strong intra- and inter-molecular C–HX (X = Br, O, p) weak interactions, which plays an important role in stabilizing the crystal packing. Furthermore, the C–HO interactions in 2 lead to a single-helical motif. The complexes 1 and 2 exhibited good activity in a model Suzuki–Miyaura coupling reaction. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction N-heterocyclic carbenes (NHCs) are an extremely useful and versatile class of ligands for catalyst design. After the pioneering reports by Öfele and Wanzlick [1,2], and the early studies by Lappert [3–8] on the coordination to late transition-metal com- plexes, NHC chemistry remained quiescent for more than twenty years, until Arduengo pointed out the idea that NHCs could be sta- ble enough for crystallographic characterization [9]. In 1995, Herrmann took the story a step forward, with the use of NHCs in the preparation of the first NHC-based homogeneous catalysts [10]. Since then, many research groups have provided a large num- ber of NHC-based catalysts for a wide variety of reactions, and many reviews covering aspects such as preparation [11–13], stability [14], stereoelectronic properties [15–17], coordination strategies [13,18,19], and catalytic applications [20–26] have been entirely devoted to this type of ligand. The concept of two NHC groups separated by various spacer such as pyridine, benzene, amido, ether, and pentamethylene functionalities have been explored by several groups, yielding Pd(II) pincer complexes containing various alkyl/aryl groups [27–35]. These ligands are highly ‘‘tunable’’ and active precatalysts for C–C coupling reactions and exhibit excellent thermal stability and resistance to degrada- tion reactions [27,30,31,33,36,24]. Ag(I) (NHC) complexes have been shown to be facile reagents for the transmetalation of a variety of functionalized NHC ligands to Pd(II) [37–42]. They are readily accessed through the reaction of an imidazolium salt with Ag 2 O by the method of Wang and Lin [43]. In recent years the number of crystallographically characterized Pd(II) (NHC) com- plexes has increased considerably with a rich structural diversity revealed, especially when halide ions are present in the compound [44–46]. Keeping these facts in mind, herein, we present synthetic, spectral characterization of two new pentamethylene-bridged bis-imidazolium dication ligands (L 1 = 1,5-bis(1-(4-cyanophenyl)- imidazolium-1-yl)pentane dibromide) and L 2 = 1,5-bis(1-(4- methoxyphenyl)-imidazolium-1-yl)pentane dibromide) bearing CPI (1-(4-cyanophenyl)-imidazole) and MPI (1-(4-methoxyphenyl)- 1H-imidazole)-functionality which are potential precursors to 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.10.065 ⇑ Corresponding authors. Tel.: +91 (0) 9811730475 (M. Trivedi); +314 516 5333 (N.P. Rath). E-mail addresses: manojtri@gmail.com (M. Trivedi), rathn@umsl.edu (N.P. Rath). Inorganica Chimica Acta 383 (2012) 118–124 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica