Journal of Organometallic Chemistry 896 (2019) 162e167 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem Synthesis and investigation of catalytic activity of phenylene e And biphenylene bridged bimetallic Palladium-PEPPSI complexes _ * _ € Fatma Imik, Sedat Yas¸ar , Ismail Ozdemir Inonü€ University, Faculty of Science and Letter, Department of Chemistry, 44280, Malatya, Turkey article info abstract Article history: A series of phenylene and biphenylene-bridged N-heterocyclic carbene (NHC) and their bimetallic NHC Received 4 February 2019 ePd-PEPPSI complexes were reported and fully characterized by 1H NMR, 13C NMR, HRMS, FTIR, and Received in revised form elemental analysis. The N tags with different electronic and steric properties were used on the synthesis 21 May 2019 of bridged bimetallic NHCePd-PEPPSI (PEPPSI: Pyridine Enhanced Precatalyst Preparation Stabilization Accepted 14 June 2019 Initiation) complexes. The catalytic properties of the bridged bimetallic NHCePd-PEPPSI complexes were Available online 18 June 2019 investigated in the different CeC coupling reaction such as Suzuki-Miyaura reactions of aryl bromides with arylboronic acid, the Heck coupling reactions of aryl bromide with styrene and in the Sonogashira Keywords: e Bimetallic N-heterocyclic carbene complex reactions of aryl bromides with phenylacetylene. Generally, all bridged bimetallic NHC Pd-PEPPSI e Palladium complexes showed good catalytic activity in the C C coupling reactions. CeC bond formation © 2019 Elsevier B.V. All rights reserved. Sonogashira Heck Suzuki 1. Introduction fine chemicals in the field of biologically active compounds, natural products etc [6]. There are many reports in the field of homoge- After the first isolation of stable free N-heterocyclic carbene neous catalysts for CeC cross-coupling reactions which are palla- (NHC) by Arduengo, NHC have been an important ligand class in the dium catalyzed [7]. When these studies are examined, it is seen that design of homogeneous and heterogeneous catalysts due to their monometallic PdeNHC complexes are very common. However, bi- strong s-donor bond character. The use of different electronically or polymetallic PdeNHC complexes have been reported in recent and sterically NHC lead to stronger and stable bonds with a metal years. Among these reports, a large number of bidentate bis-NHC center, which facilitates the catalytic performances [1], compared ligands such as flexible alkyl [8], ether [9], benzyl [10], methyl- to classical ligands [2]. Recently, benzimidazolium ligands were pyridine [11], picolly [12] and rigid linkers [13] and their Pd- used to the synthesis of many different metaleNHCecomplex complexes have been examined. instead of imidazolium-based ligands because of the donation of To better understand the catalytic behavior of bimetallic palla- extra stability to the metal NHC bond by benzimidazole ring [3]. dium complexes, we reported the synthesis and characterisation of The NHC complexes are preferred complexes in the many organic bridged bis-benzimidazole based NHC and their bimetallic Pd- transformations reaction as a catalyst due to their unique catalytic PEPPSI complexes and we implemented its activities on Suzuki- features [4]. PdeNHC complexes have a privileged place within Miyaura, Heck and Sonogashira CeC cross-coupling reactions. NHC complexes. PdeNHC complexes have gained enormous rele- vance in various CeC cross-coupling reactions such as Heck, Stille, 2. Results and discussions Suzuki, Sonogashira, and BuchwaldÀHartwig reactions for the synthesis of the different arylated product [5]. Many fine chemicals 2.1. Synthesis and characterisation could be synthesized much more efficiently than before by this methodology. Arylated products are important for the synthesis of The preparation of bridged bis-N-heterocyclic carbene pre- cursors from different N tags was presented in Scheme 1. These bis- benzimidazole NHC precursors were fully characterized by 1H, 13C * Corresponding author. NMR and elemental analyses. The reaction of N-substituted benz- 0 0 E-mail address: [email protected] (S. Yas¸ ar). imidazole with p-xylyl dibromide or 2,2 -Bis(bromomethyl)-1,1 - https://doi.org/10.1016/j.jorganchem.2019.06.019 0022-328X/© 2019 Elsevier B.V. All rights reserved. _ F. Imik et al. / Journal of Organometallic Chemistry 896 (2019) 162e167 163 Scheme 1. Synthesis of bridged NHC salts (1a-c) and their bimetallic Pd-PEPPSI complexes (2a-c). biphenyl in N,N-dimethylformamide (DMF) resulted in phenylene bromides with phenylboronic acid, the Heck reaction of aryl bro- or biphennylene bridged bis-benzimidazolium salts (1aec) mides with styrene and the Sonogashira reaction of aryl bromides (78e82%). The structure of bridged N-substituted benzimidazole with phenylacetylene. salts (1a-c) was clearly confirmed by 1H and 13C spectra, where the The Suzuki-Miyaura reaction is one of the most efficient and NCHN hydrogens have low field shifts at 10.47, 11.61, 10.96 ppm, awarded methods for synthesis of arylated products which are respectively in the 1H NMR spectra and the NCHN carbons have also highly valuable chemicals in many areas, under mild reaction low field at 153.6, 143.2, 160.2 ppm, respectively in the 13C NMR condition from non-toxic comprehensive starting materials. The spectra. Results are similar to those previously reported [14]. In Suzuki-Miyaura reactions of aryl halides with boronic acid cata- addition, in the FTIR, the n(CN) stretching frequency peaks of the lyzed by Pd-PEPPSI complexes was first reported by Organ and co- bridged bis-N-heterocyclic carbene precursors were observed at workers [15]. After that, the different type of Pd-PEPPSI complexes À 1560, 1556 and 1549 cm 1, respectively for 1a-c. were tested on Suzuki-Miyaura reactions [16]. The preparation of bimetallic NHCePd-PEPPSI complexes was We tested the catalytic activity of 2a-c in Suzuki-Miyaura re- straightforward, starting from bis-NHC precursors 1a-c. In a pres- actions when performed at room temperature in 2-propanol/ sure tube, the reaction of 1a-c with PdCl2 and excess of K2CO3 in the water (1:1 v/v) in the presence of K2CO3 by using an equi- presence of pyridine allowed the bimetallic PEPPSI-Pd-NHC com- molecular amount of aryl halides (1 mmol) and boronic acid plexes (2aec), which can be purified by filtration on a short silica (1.1 mmol). The addition of water to CeC cross-coupling reactions pad to afford an air- and moisture-stable light-yellow solid in as co-solvent may have significant profits. According to the liter- 60e79% yield, respectively (Scheme 1). The structure of 2aec was ature [17], high polarity of water can have an accelerating effect on confirmed by NMR, HRMS, IR spectroscopy and elemental analyses the oxidative addition of aryl halides as well as increase solubility techniques. The C2 carbene carbon atoms of 2a-c come to resonance of the base. In addition, the addition of water may be causing the in 13C NMR spectra at d ¼ 164.7, 163.7 and 164.7 ppm, respectively. displace of labile ligands such as especially iodide and bromide The n(CN) stretching frequency peaks of the NHCePd-PEPPSI com- from the metal center. The CeC cross-coupling reaction in pure À plexes was observed at 1407, 1413 and 1412 cm 1 in the FTIR, water is one of the major challenges. However, the low solubility respectively for 2a-c. The shifts in the stretching frequency of n(CN) of some substrates in water causing the use of aqueous solution as are expected because of the electron flow from NHC to palladium a solvent in reactions. Thus, several Suzuki-Miyaura reactions metal. Unfortunately, we were not able to get a single crystal to were performed in aqueous mixtures of DMF, i-PrOH, EtOH or perform X-ray diffraction of 2a-c complexes. CH3CN. Herein, we choose the i-PrOH/water aqueous solutions due to our experience in this solvent mixture. In general, the complex 2a-c showed good catalytic activity and afforded high 2.2. Catalysis yields with all aryl bromides but with 4-chloroacetophenone and 4-chloroanisol afforded very low yields (Table 1, entries 6,7). The In order to demonstrate the catalytic activity of the bimetallic higher yields were obtained with electron drawing group PEPPSI-Pd-NHC complexes, we decided to try three different model substituted aryl bromide such as 4-bromoacetophenone and 4- reactions to demonstrate the usability of complexes in different bromoaldehyde (Table 1, entries 1,2,3). From the results types of CeC cross-coupling reactions: the Suzuki-Miyaura of aryl _ 164 F. Imik et al. / Journal of Organometallic Chemistry 896 (2019) 162e167 Table 1 Suzuki-Miyaura cross-coupling reactions of phenylboronic acid catalyzed by 2a-c complexesa.2 Entry Product [catalyst]/yield (%)b 2a 2b 2c 1 80 86 83 2c 95 80 93 3 95 90 93 4 79 65 83 5 77 71 91 6d 25 10 18 7d,e 75 45 77 a .Reaction condition: 0.5 mol٪ 2a-c, 1 mmol p-R-C6H4Br, 1.1 mmol phenylboronic acid, 1.0 mmol K2CO3, rt, 1 h b Yields obtained by GC analysis using undecane as internal standard. c Reaction carried out 30 min. d Aryl chloride was used. e Reaction carried out at 80 C. presented, it can be seen that our catalytic results in Suzuki- the discovery of Sonogashira reaction, many modifications have Miyaura reaction better than previously reported results been made to on Sonogashira reaction to catch optimum yield by although in those reported case the reactions proceeded in higher removing addition of Cu-salts [26], an addition of several additives temperatures, catalyst loading and reaction times [18]. Among the [27], and using a different type of ligand [28]. Due to ligand plays a complex 2a-c,complex2c has a better catalytic activity in Suzuki- key role which generally stabilizes the active nano-palladium Miyaura reactions.