J. Chem. Sci. Ó (2020) 132:51 Indian Academy of Sciences https://doi.org/10.1007/s12039-020-1754-y Sadhana(0123456789().,-volV)FT3](0123456789().,-volV) REGULAR ARTICLE A new PEPPSI type N-heterocyclic carbene palladium(II) complex and its efficiency as a catalyst for Mizoroki-Heck cross-coupling reactions in water DHRUBAJIT BORAHa,b , BISWAJIT SAHAc,d , BIPUL SARMAe and PANKAJ DASa,* aDepartment of Chemistry, Dibrugarh University, Dibrugarh, Assam 786004, India bDepartment of Chemistry, N. N. Saikia College, Titabar, Assam 785630, India cAdvanced Materials Group, Materials Sciences and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat, Assam 785006, India dAcademy of Scientific and Innovative Research (AcSIR), CSIR-NEIST Campus, Jorhat, Assam 785006, India eDepartment of Chemical Sciences, Tezpur University, Napaam, Tezpur, Assam 784028, India E-mail: [email protected] MS received 29 August 2019; revised 7 November 2019; accepted 10 November 2019 Abstract. A new air and moisture stable PEPPSI (PEPPSI: pyridine-enhanced pre-catalyst preparation, stabilisation, and initiation) themed palladium N-heterocyclic carbene (NHC) complex [Pd(L)Br2(Py)] (1) [L: 2-flurobenzyl)-1-(4-methoxyphenyl)-1H-imidazolline-2-ylidene] was synthesized and characterized. The structure of complex 1 was determined by X-ray single-crystal analysis. The palladium center in 1 adopted a square planar geometry with carbene and pyridine ligands occupying the mutual trans position. The complex 1 was employed to catalyze the Mizoroki-Heck cross-coupling reactions of aryl bromides/iodides with styrene in water. To the best of our knowledge, this is the first report where a Pd-PEPPSI catalyst was successfully employed in aqueous-phase Mizoroki-Heck reaction. Good to excellent yields of cross-coupling products were obtained with a range of representative aryl bromides/iodides under relatively mild conditions (100 °C, 1 mol% of 1). Keywords. N-heterocyclic carbene; Palladium; PEPPSI; Mizoroki-Heck reaction; aqueous media. 1. Introduction due to their strong r-donating and poor p-accepting properties, they can produce many stable complexes N-heterocyclic carbenes (NHCs) are among the most compared to the analogous phosphine-based systems.5 intriguing classes of ligands that have generated In the past few years, Pd-NHC systems have been numerous breakthroughs in the field of organometal- increasingly used as catalysts for various types of 1 lic chemistry and homogeneous catalysis. In recent cross-coupling reactions. Since the first report of the years, NHCs have been viewed as a sustainable utilization of Pd-NHC system in Mizoroki-Heck alternative to phosphines in many Pd-catalyzed reaction by Herrmann’s group,6 a large number of reactions, including carbon-carbon and carbon-het- efficient catalysts bearing NHC are documented for 2 eroatom bond formation reactions. The main catalyzing Heck reaction. Despite remarkable pro- advantages of the NHC systems over phosphines lie gress that has been made in this field, to date, only a in their trouble-free syntheses, easy handling prop- few catalytic systems are effective to carry out the erty, non-toxic behaviour, air and moisture reaction in neat water.7 It may be noted that water is stable properties, and tuneable catalytic activity via always considered as a potentially benign solvent for altering the stereo-electronic property of the pendant organic synthesis. In this prospect, the development 3,4 groups attached to the imidazole moiety. Moreover, of a robust NHC-Pd based catalytic system for Heck *For correspondence Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-020-1754-y) contains supplementary material, which is available to authorized users. 51 Page 2 of 10 J. Chem. Sci. (2020) 132:51 reaction in water is of tremendous significance. 2. Experimental Among various NHC systems, the air-stable and user- friendly pyridine stabilized complexes, so-called 2.1 Materials and methods PEPPSI complexes (PEPPSI: pyridine-enhanced pre- catalyst preparation, stabilisation, and initiation) All reactions and manipulations were carried out under air atmosphere unless otherwise stated. The imidazole precur- developed by Organ and co-workers8 have got sors (4-methoxyphenyl)-1H-imidazole and PdCl2 were numerous attention as pre-catalysts for various purchased from TCI and Sigma-Aldrich, respectively. All organic reactions. The NHC ligand, because of its solvents, substrates for catalysis and other chemicals were strong trans directing property, is expected to create a purchased from various commercial firms like TCI, Acros vacant site for substrate binding by easily releasing Organics, and Merck. The NMR spectrum for the imida- the labile pyridine ligand trans to NHC and thus zolium salt L was recorded in CD3OD, while the complex would enhance the catalytic activities. In fact, there was performed in CDCl3 with tetramethylsilane as an exist several recent reports on the utility of PEPPSI- internal standard and operating on a Bruker Avance 400 Pd-NHC complexes in Negishi,8 Suzuki,9 C-N MHz NMR spectrometer. High-resolution mass spectra coupling,10 Sonogashira,11 C-H bond activation,12 were recorded with an Agilent 6550 iFunnel Q-TOF MS aryl amination,13 etc. Moreover, heterogeneous NHC- system. The GCMS spectra of the catalytic products were Pd based systems have also been explored for the performed in an Agilent GC Model 7820A with a mass detector model 5975 series. aqueous phase cross-coupling reaction like Suzuki- Miyaura reaction. To cite an example, recently Choudhury et al., have reported an efficient co-ordi- 2.2 Procedure for preparation of 3-(2- nation polymer anchored Pd-NHC system and their flurobenzyl)-1-(4-methoxyphenyl)-1H-imidazolium application as a promising catalyst in aqueous phase bromide (L) Suzuki reaction.9 However, their uses in Mizoroki- Heck coupling are limited. To our knowledge, only (4-methoxyphenyl)-1H-imidazole (174 mg, 1 mmol) and three catalytic systems (Figure 1) are known where 2-fluorobenzyl bromide (1.89 mg, 10 mmol) were taken in a PEPPSI-Pd-NHC complexes are employed in Mizor- round bottom flask (50 mL) and the mixture was dissolved 14 oki-Heck reaction to date. However, uses of envi- in 5 mL CH3CN and then the resulting mixture was heated ronmentally unfavourable reaction media like dioxane at 80 °C for 48 h. The reaction mixture was allowed to come (Shen),14a DMF (Crudden and Lin),14b,c along with to room temperature and diethyl ether was added to obtain a high reaction temperatures (*140 °C) are the major precipitate. The solution was filtered and the solid residue was washed with diethyl ether thrice and then dried in limitations of those systems. To the best of our vacuum. The procedure yielded 268 mg (74%) of the pro- knowledge, there is no literature precedent available 1 duct as a white powder. H NMR (CD3OD, 400 MHz, d, pertaining to the utility of Pd-PEPPSI-NHC com- ppm): 9.60 (s, 1H, NCHN), 7.79 (t, J = 1.6 Hz, 1H, CH=CH, plexes in the Mizoroki-Heck reaction in water, imidazole), 7.78 (s, CH=CH, imidazole), 7,63-7.59 (m, 3H, although such systems have been employed in aque- Ar), 7.53-7.47 (m, 1H, Ar), 7.31-7.20 (m, 2H, Ar), 7.16- ous media for other cross-coupling reactions like 7.12 (m, 2H, Ar), 5.61 (s, 2H, benzyl CH2), 3.30 (s, 3H, 15 11 13 Suzuki, Sonogashira, etc. Herein, we report the OCH3), C (CD3OD, 100 MHz, d, ppm) 163.72, 162.50, 3 3 synthesis and characterization of a new PEPPSI-Pd- 161.26, 136.67, 133.14 (d, JC,F = 8.4 Hz), 132.47 (d, JC,F = 4 NHC complex and its catalytic application of 2.9 Hz), 129.21, 126.38 (d, JC,F = 3.6 Hz), 125.09, Mizoroki-Heck reaction in water. 124.30,123.73, 122.79,122.15, 117.14, 116.93, 116.43, Figure 1. PEPPSI-Pd-NHC precatalysts for Mizoroki-Heck reaction. J. Chem. Sci. (2020) 132:51 Page 3 of 10 51 56.34, 49.63. HR-MS: [M-Br]? = 283. 16, calculated: 0.0343; wR2: 0.0580; Reflections collected: 69825; Unique 283.12. reflections: 4807; Observed reflections: 3472. Complex 1a (CCDC no: 1863543), Empirical Formula: C10H10Br2N2Pd; MW (g/mol): 424.42; Crystal size: 2.3 Procedure for the preparation of PEPPSI-Pd- 0.27x0.21x0.11; Colour: Yellow; Crystal system: Triclinic; Space group: P1; Cell length (in A˚ ): a, 5.656(3); b: NHC (1) complex 7.125(3); c: 7.784(4); Cell angles (in 8), a: 79.958(4); b: 88.681(4); c: 88.681(4); Cell volume, 307.3(3); Cell density A 50 mL flask equipped with a magnetic stir bar was - [g/cm3]: 2.293; T (K): 100K; l (mm 1): 4.308; GoF: 1.145; charged with imidazolium salt L (218 mg, 0.6 mmol), R1: 0.076; wR2: 0.2361; Reflections collected: 9344; PdCl (89 mg, 0.5 mmol), K CO (207 mg, 1.5 mmol), 2 2 3 Unique reflections: 1590; Observed reflections: 1464. excess KBr, pyridine (5 mL). The mixture was allowed to stir at 80 °C for 48 h and then cooled to room temperature and the solvent was removed under vacuum. The residue 2.5 General procedure for the catalytic reaction was dissolved in dichloromethane (DCM) and purified by column chromatography, eluting with DCM/hexane (7:3). A round-bottomed flask (50 mL) equipped with a condenser Complexes 1a and 1 were recovered from the first and second and a magnetic stirring bar was charged with aryl halide fraction of the solvent respectively. Complex 1: Yield 265 (1 mmol), styrene (1.5 mmol), K CO (3 mmol) and 1 mg, (70%); yellow powder; 1H NMR (CDCl , 400 MHz, d, 2 3 3 (1 mol%) in water (3 mL) were allowed to stir at 100 °Cin ppm) 8.87 (d, J = 8 Hz, 2H, py) 7.90 (t, J = 7.6 Hz, 2H, Py), air.