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Summer Research Fellowship Programme of India's Science Academies

Synthesis of palladium complexes of chiral menthyl derived 1, 2, 4-triazole based N-heterocyclic carbene ligands

Suryakamal Sarma

Student, Department of Chemistry, B Borooah College, Guwahati 781007

Prof. Prasenjit Ghosh

Professor, Department of Chemistry, IIT Bombay, Mumbai 400076

Abstract

Two 1,2,4-triazole derived NHC-ligands 1,1’-(1S)-menthyl-1H-4,4’ethylidi-1,2,4-triazolium dibromide (1a) and 1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium dibromide (2a) were synthesised and characterised. Based on these, a series of bimetallic PEPPSI themed palladium complexes (1b) and (2b) namely, [1,1’-(1S)-menthyl-1H-4,4’-CH2(CH2)n-CH2-di-1,2,4-triazoline-5,5’-diylidene]Pd2Br4(pyridine)2 [n = 0 (1b) and n= 1 (2b)] were synthesised and characterised. The corresponding phosphine analogues of these bimetallic complexes (1c) and (2c) namely, [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’­-CH2(CH2)n-CH2-di-1,2,4 triazolium]Pd2Br4(PPh3)2 [n = 0 (1c) and n= 1 (2c)] were synthesised and characterised from the direct reaction of the respective PEPPSI themed palladium complexes ((1-2)b) with PPh3.

Keywords: NHC-ligand, bimetallic PEPPSI themed palladium, phosphine analogues

Abbreviations

Abbreviations
 NHCN- heterocyclic carbene
 PEPSSIPyridine enhanced precatalyst preparation stabilisation and initiation 
PdCl2 Palladium Chloride
K2CO3Potassium Carbonate
 KBrPotassium Bromide 
CuSO4Coppur Sulphate 
 NaSO4Sodium Sulfate 
NMR Nuclear Magnetic Resonance 
 Anal. Calcd.Analysis Calculated 
 CDCl3Deuterated Chloroform 

INTRODUCTION

Background

Enantiomerically pure compounds are very important for pharmaceutical and other uses which demands optical activity. Asymmetric synthesis is one of the most interesting and challenging method to prepare Enantiomerically pure compounds as one single molecule of chiral catalyst can transfer its chirality to produce thousands of new chiral molecules. Enantioselective reactions are the results of different possible diastereomeric reaction pathways and the efficiency of chiral transfer (enantiomeric excess) is depends on the steric and electronic factor of the transition states.

Statement of the Problems

Most of the asymmetric catalyst that have been developed so far are metal complexes with chiral ligands. These chiral ligands modify the reactivity and selectivity of the metal centre such a way that one of the enantiomers are formed in major amounts. The design of the suitable chiral ligands for a particular application remains formidable task as the mechanistic approach of the most of the ligands are not known. Nevertheless, for certain reactions such as Rh- catalysed hydrogenation and Pd-catalysed allylic substitution, the mechanisms are known. Moreover, useful general concepts have been developed during the last three decades that greatly facilitates the development of new chiral ligands, even in the absence of mechanistic explanations.

Objectives of the Research

  • To synthesise and characterise the two 1,2,4-triazole derived NHC-ligands.
  • To synthesise and characterise a series of bimetallic PEPSSI themed palladium complexes.
  • To synthesise and characterise corresponding phosphine analogues of these bimetallic complexes.

LITERATURE REVIEW

Information

N-heterocyclic carbenes (NHCs) are one of the most effective and versatile ligands in organometallic chemistry and related to the research fields of homogenous catalysis, supramolecular chemistry, material sciences and biomedical applications1,2,3,4. In development, NHCs are most observed ligands in carbene chemistry due to their reactivity with the additional features like electronic and steric tunability. The substitution of bulky group in the ring N-atom in NHC ligands gives additional impact in the metal coordination sphere which influences in the catalytic property and selectivity.5

1.png
    Synthesis of chiral alkyl bridged bis-1,2,4-triazole derived palladium complexes

    Ruthenium and palladium are the most utilised transition metals for synthesis of NHC-metal complexes with successful catalytic activity6. Palladium catalysed cross-coupling reactions that proceeds through insertion of metal into the N-C bond have been a successful area of research11,12. The strong σ-donation and variability of substituent in the coordination sphere of palladium in Pd-NHC complexes enhance the reactivity towards the oxidative addition and reductive eliminations steps7. The successful and commercially valuable cross-coupling reaction, known as Suzuki-Miyaura coupling, used NHC-Pd-PEPSSI (pyridine enhanced precatalyst preparation stabilisation and initiation) complex systems6.

    Summary

    In this context, we envisaged to synthesize new NHC-ligand systems and their subsequent bimetallic alkyl bridged NHC-based Palladium complexes and their characterisations. The triazole moiety, due to the ease of preparation and the ability of substitution of a wide range functional groups provides a good route for ligand design and synthesis8. Therefore, we choose 4-((1S,2S,5R)-2-isopropyl-5-methylcyclohexyl)-4H-1,2,4-triazole (B) as our precursor for the synthesis of the chiral ligands. At the time of our study we were aware of only a few reports9,10 of 1,2,4-triazole derived bimetallic palladium complexes based of alkyl bridged NHC-ligands and therefore we were interested to explore in this field.

    Also, the mixed NHC/phosphine ligands are well known for catalytic activity even higher11,12 than the NHC-Pd-PEPSSI complexes due to the simultaneous strong electron donation form the NHC-ligand and stabilisation from the bulky phosphine ligands. The synthesis of NHC/phosphine ligands are convenient by substituting the “loosely” bound pyridines by the phosphine ligands13,14. In this project we also report two bimetallic palladium complexes of mixed NHC/phosphine ligands obtained from the direct reaction of NHC-Pd-PEPSSI complexes with PPh3 ligands.

    METHODOLOGY

    Concepts

    All manipulations were carried out using standard Schlenk techniques. The (1R,2S,5R)-(-)-menthol were purchased from Sigma-Aldrich. Bruker 400 and 500 MHz NMR spectrometers were used for recording the 1H NMR, 13C{1H}NMR, and 31P{1H} NMR spectra. 1H NMR signals are labelled as singlet (s), doublet (d), triplet (t), broad (br) and multiplet (m). Thermo Finnigan FlashEA 1112 series elemental analyzer was used for the analysis of CHNS.

    Methods

    Synthesis of [(1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 4-methylbenzenesulfonate] (A)

    (1R,2S,5R)-(-)-menthol (5.01 g, 31.9 mmol) is stirred to dissolve in 15 mL pyridine at room temperature. p-tolunebenezenesulphonyl chloride (7.32 g, 38.4 mmol) is added to the reaction mixture at 0˚ C and allowed to stand for 6 hours. After completion of the reaction the product obtained was washed with water and then vacuum dried. The product obtained was recrystalised to get the pure product (A) as colourless crystals (9.02 g, 91%). 1H NMR (400 MHz, CDCl3) δ 7.78 (d, 3JHH = 8.2 Hz, 2H, (4)-CH3-C6H4), 7.32 (d, 3JHH = 8.0 Hz, 2H, (4)-CH3-C6H4), 4.39 (td, 3JHH = 10.8, 4.5 Hz, 1H, CH-O-C7H7O2S), 2.45-0.97 (m, 20H, CH3C6H9CH(CH3)2), 0.90 (d, 3H, 3JHH = 7 Hz, CH3C6H9CH(CH3)2) . Anal. Found for :C17H26O3S: C, 65.77; H, 8.44; S, 10.33.Calcd.: C, 65.90; H, 8.64; N, 10.41%.

    Synthesis of [4-((1S,2S,5R)-2-isopropyl-5-methylcyclohexyl)-4H-1,2,4-triazole] (B)

    1,2,4-Triazole (5.04 g, 72.9 mmol) is mixed with 30 mL dimethylformamide and then stirred to dissolve. Then 5 g sodium hydride is added portion wise and then allowed to stir for 20 minutes at 0˚ C and then the [(1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 4-methylbenzenesulfonate] (A) (10.1 g, 32.3 mmol) is added to the reaction and allowed to stir for 30 minutes in room temperature. After that, the reaction is placed in reflux for 24 hours. After completion of the reaction, ethylacetate (ca 70 mL) is added to reaction mixture and washed with water (12 x 50 ml) and then dried to get the crude product. This product is purified using column chromatography and it is done with 12%-14% Ethyl acetate/petroleum ether solution and vacuum dried to get the product (B) as white solid (2.42 g, 34%). 1H NMR (400 MHz, CDCl3) δ 8.14 (s, 1H, N-C(5)H-N), 7.9 (s, 1H, N-C(3)-H-N ), 4.77 (d, 3JHH = 2.4 Hz, 1H, CH3C6H9CH(CH3)2), 2.08 – 1.00 (m, 9H, CH3C6H9CH-(CH3)2 and CH3C6H9CH(CH3)2), 0.86 (d, 3JHH = 6.5 Hz, 3H, CH3C6H9CH(CH3)2), 0.82 (d, 3JHH = 6.6 Hz, 3H, CH3C6H9CH-(CH3)2), 0.77 (d, 3JHH = 6.6 Hz, 3H, CH3C6H9CH(CH3)2). Anal. Calcd. for C12H21N3: C, 69.52; H, 10.21. Found: C, 69.18; H, 10.86%.

    Synthesis of 1,1’-(1S)-menthyl-1H-4,4’ethylidi-1,2,4-triazolium dibromide (1a)

    A mixture of 1-(1S)-menthyl-1H-1,2,4-triazole (1.01 g, 4.84 mmol) (B) and 1,2-dibromoethane (0.46 g, 2.49 mmol) is heated at 120˚C for 8 hours. The residue obtained was washed with diethylether and vacuum dried to obtain the ligand 1,1’-(1S)-menthyl-1H-4,4’ethylidi-1,2,4-triazolium dibromide (1a) as white solid (0.77 g, 57%). 1H NMR (500 MHz, CDCl3) δ 11.4 (s, 2H, N-C(5)H-N), 9.8 (s, 2H, N-C(3)H-N), 5.6 (br, 4H, CH2CH2), 5.1 (br, 4H, CH3C6H9CH(CH3)2), 2.15-1.4 (m, 18H, CH3C6H9CH(CH3)2 & CH3C6H9CH(CH3)2), 0.99 (d, 3H, 3JHH = 9 Hz, CH3C6H9CH(CH3)2), 0.92 (d, 3H, 3JHH = 3.5 Hz, CH3C6H9CH(CH3)2), 0.82 (d, 3H, 3JHH = 6.5 Hz, CH3C6H9CH(CH3)2) . Anal. Calcd. for C26H46N6Br2 CHCl3: C, 44.92; H, 6.56; N, 11.64 Found: C, 45.98; H, 7.46; N, 11.16%.

    Synthesis of 1,1’-(1S)-menthyl-1H-4,4’propylidi-1,2,4-triazolium dibromide (2a)

    A mixture of 1-(1S)-menthyl-1H-1,2,4-triazole (1.01 g, 4.88 mmol) (B) and 1,3-dibromopropane (0.49 g, 2.48 mmol) is heated at 120˚C for 8 hours. The residue obtained was washed with diethylether and vacuum dried to obtain the ligand 1,1’-(1S)-menthyl-1H-4,4’propylidi-1,2,4-triazolium dibromide (2a) as white solid (1.06 g, 70%). 1H NMR (400 MHz, CDCl3) δ 11.25 (s, 2H, N-C(5)H-N), 9.64 (s, 2H, N-C(3)H-N), 5.13 (br, 2H, CH3C6H9CH(CH3)2), 5.12 (br, 2H, CH2), 4.96 (br, 2H, CH2), 3.09 (br, 2H, CH2), 2.29-1.39 (m, 18H, CH3C6H9CH(CH3)2 & CH3C6H9CH(CH3)2), 0.90 (d, 3H, 3JHH = 6.4 Hz, CH3C6H9CH(CH3)2), 0.87 (d, 3H, 3JHH = 6 Hz, CH3C6H9CH(CH3)2), 0.81 (d, 3H, 3JHH = 6.8 Hz, CH3C6H9CH(CH3)2).

    Synthesis of [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’ethylidi-1,2,4 triazolium]PdBr4(pyridine)2 (1b)

    A mixture of 1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’ethylidi-1,2,4 triazolium dibromide (1a) (0.23 g, 0.37 mmol), PdCl2 (0.13 g, 0.74 mmol), KBr (0.18 g, 1.49 mmol) and K2CO3 (0.21 g, 1.49 mmol) was refluxed in pyridine (5 mL, 63 mmol) for 16 hours. The reaction mixture was cooled to room temperature, diluted with EtOAc (ca. 70 mL) and then washed with aqueous CuSO4 solution (ca. 3 × 50 mL) and water (ca. 100 mL). The organic layer was separated, dried over Na2SO4 and finally vaccum dried to give the product (1b) as a yellow solid (0.34 g, 78 %). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 9.02 (d, 4H, 3JHH = 5 Hz, C6H5N), 8.39 (s, 2H, N-C(3)H-N), 7.82 (t, 2H, 3JHH = 1.6 Hz, C6H5N), 7.40 (t, 4H, 3JHH = 1.6 Hz, C6H5N), 5.71 (br, 2H, CH3C6H9CH(CH3)2), 5.66 (d, 2H, 2JHH = 10.8 Hz, CH2CH2), 5.55 (d, 2H, 2JHH = 2.8 Hz, CH2CH2), 2.55-1.25 (m, 18H, CH3C6H9CH(CH3)2 & CH3C6H9CH(CH3)2), 1.08 (d, 6H, 3JHH = 6.8 Hz, CH3C6H9CH(CH3)2), 0.84 (d, 6H, 3JHH = 6.4 Hz, CH3C6H9CH(CH3)2), 0.64 (d, 6H, 3JHH = 6.8 Hz, CH3C6H9CH(CH3)2). 13C {1H} NMR (CDCl3, 100 MHz, 25 °C): δ 155.1 (Pd−NCN), 152.4 (C6H5N), 143.0 (N-C(3)H-N), 138.3 (C6H5N), 124.8 (C6H5N), 61.7 (CH3C6H9CH(CH3)2), 47.7 (CH2), 47.1 (CH3C6H9CH(CH3)2), 41.0 (CH3C6H9CH(CH3)2), 35.2 (CH3C6H9CH(CH3)2), 28.9 (CH3C6H9CH(CH3)2), 25.9 (CH3C6H9CH(CH3)2), 23.4 (CH3C6H9CH(CH3)2), 23. (CH3C6H9CH(CH3)2), 22.3 (CH3C6H9CH(CH3)2), 19.8 (CH3C6H9CH(CH3)2).

    Synthesis of [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium]PdBr4(pyridine)2 (2b).

    A mixture of 1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium dibrimide (2a) (0.24 g, 0.39 mmol), PdCl2 (0.13 g, 0.74 mmol), KBr (0.19 g, 1.52 mmol) and K2CO3 (0.21 g, 1.50 mmol) was refluxed in pyridine (5 mL, 63 mmol) for 16 hours. The reaction mixture was cooled to room temperature, diluted with EtOAc (ca. 70 mL) and then washed with aqueous CuSO4 solution (ca. 3 × 50 mL) and water (ca. 100 mL). The organic layer was separated, dried over Na2SO3 and finally vaccum dried to give the product (2b) as a yellow solid (0.17 g, 60 %). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 9.02 (d, 4H, 3JHH = 1.2 Hz, C6H5N), 8.28 (s, 2H, N-C(3)H-N), 7.78 (t, 2H, 3JHH = Hz, C6H5N), 7.37 (t, 4H, 3JHH = 1.6 Hz, C6H5N), 5.71 (br, 2H, CH3C6H9CH(CH3)2), 5.01(m, 2H, CH2), 4.72 (m, 2H, CH2), 3.40 (br, 2H, CH2), 2.63-1.39 (m, 18H, CH3C6H9CH(CH3)2 & CH3C6H9CH(CH3)2, 1.27-0.97 (m, 6H, CH3C6H9CH(CH3)2), 0.89 (d, 6H, 3JHH = 7.6 Hz, CH3C6H9CH(CH3)2).

    Synthesis of [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’ethylidi-1,2,4 triazolium]PdBr4(PPh3)2 (1c)

    A mixture of [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’ethylidi-1,2,4 triazolium]PdBr4(pyridine)2 (1b) (0.18 g, 0.16 mmol) and PPh3 (0.11 g, 0.40 mmol) was stirred in CH2Cl2 (ca. 30 mL) at room temperature for 8 hours. The solvent was removed under vacuum to give the crude product as a yellow solid. The crude product was washed with CH3CN to give product (1c) as a yellow solid (0.16 g, 68 %). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.11 (s, 2H, N-C(3)H-N), 7.98-7.25 (br, 30H, C6H6), 5.38 (br, 2H, CH3C6H9CH(CH3)2), 4.93 (d, 2H, 2JHH = 9.6 Hz, CH2CH2), 4.12 (d, 2H, 2JHH = 9.6 Hz, CH2CH2), 2.02-0.81 (m, 18H, CH3C6H9CH(CH3)2 & CH3C6H9CH(CH3)2, 0.95 (d, 6H, 3JHH = Hz, CH3C6H9CH(CH3)2), 0.60 (d, 6H, 3JHH = 6.4 Hz, CH3C6H9CH(CH3)2). 0.31 (d, 6H, 3JHH = 6.4 Hz, CH3C6H9CH(CH3)2).

    Synthesis of [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium]PdBr4(PPh3)2 (2c)

    A mixture of [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium]PdBr4(pyridine)2 (2b) (0.25g, 0.41 mmol) and PPh3 (0.26 g, 1.01 mmol) in CH2Cl2 (ca 30 mL) was stirred overnight at room temperature. The solvent was removed under vacuum to give the crude product as a yellow solid. The crude product is washed with petroleum ether (3 x 15 mL). The crude product was purified by column chromatography using silica gel as a stationary phase and eluting it with a CHCl3/MeOH mixture (98 : 2 v/v), then vaccum dried to give product (2c) as an yellow solid (0.284 g, 87 %). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.86 (s, 2H, N-C(3)H-N), 7.43-7.27 (br, 30H, C6H6), 5.2 (br, 2H, CH3C6H9CH(CH3)2), 4.21-4.17 (m, 2H, CH2), 3.27 (s, 2H, CH2), 2.80 (br, 2H, CH2), 2.01-0.78 (m, 18H, CH3C6H9CH(CH3)2 & CH3C6H9CH(CH3)2), 0.95 (t, 6H, 3JHH = 6.5 Hz, CH3C6H9CH(CH3)2), 0.65 (t, 6H, 3JHH = 6.5 Hz, CH3C6H9CH(CH3)2). 13C {1H} NMR (CDCl3, 500 MHz, 25 °C): δ 141.2 (N-C(3)H-N), 131.6 (C6H5), 128.7 (C6H5), 128.6 (C6H5), 60.9 (CH3C6H9CH(CH3)2), 49.45 (CH2), 46.4-49.2 (CH3C6H9CH(CH3)2), 34.6 (CH3C6H9CH(CH3)2), 27.8 (CH3C6H9CH(CH3)2), 25.6 (CH3C6H9CH(CH3)2), 23.9 (CH3C6H9CH(CH3)2), 23.4 (CH3C6H9CH(CH3)2), 20.7 (CH3C6H9CH(CH3)2). 31P {1H} NMR (CDCl3, 202 MHz, 25 °C): δ 29.9 (Pd−PPh3).

    RESULTS AND DISCUSSION

    The mixed organohalido (NHC)Pd2Br2(R) type complexes (1b), (2b), (1c) and (2c) of a 1,2,4-triazole derived N-heterocyclic carbene ligand, were synthesised through a sequence of reactions (Scheme 1). In particular, the 1,2,4-triazole derived carbene ligand precursor 4-((1S,2S,5R)-2-isopropyl-5-methylcyclohexyl)-4H-1,2,4-triazole (B) was prepared from the substitution reaction of the (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl-4-methylbenzenesulfonate (A) and 1,2,4-triazole15 with 34% yield and characterised by 1H NMR and elemental analysis. After that the ligands 1,1’-(1S)-menthyl-1H-4,4’ethylidi-1,2,4-triazolium dibromide (1a) and 1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium dibromide (2a) were synthesised by the alkylation of (B) with 1,2-dibromoethane and 1,3-dibromopropane respectively15,16 in 70-80% yields and characterised by 1H NMR and elemental analysis. The characteristics downfield shift for the carbenoid (NCHN) peaks are observed at the 9.5-11 ppm region for both the compounds.

    The NHC-Pd-PEPSSI complexes [1,1’-(1S)-menthyl-1H-4,4’ethylene-di-1,2,4-triazoline-5,5’-diylidene]Pd2Br4(pyridine)2 (1b) and [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium]Pd2Br4 (pyridine)2 (2b) are prepared through the reaction of ligand (1a) and (2a) respectively with palladium (II) chloride, potassium bromide and pyridine using potassium carbonate as a base in yields 38% and 60% respectively. The complex (1b) is characterised by 1H NMR and 13C{1H} NMR and complex (2b) is characterised by 1H NMR. The downfield resonance peak of NCHN peak at the 9.5-11 ppm region is disappeared and characteristic pyridine-based peaks at 7.3-7.8 and 9.0 ppm is observed for both the complexes. The palladium bound characteristics carbon peak in the 13C resonance is observed at the 152.13-155.14 ppm for the complex (1b).

    Again, to study the reactivity of the complexes (1b) and (2b) the complexes are allowed to displace their pyridine substitution by σ-donating ligand PPh3 to give the complexes [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’ethylidi-1,2,4 triazolium]Pd2Br4(PPh3)2 (1c) and [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium]Pd2Br4(PPh3)2 (2c). The reaction is carried out by reaction of the complexes (1b) and (2b) respectively with triphenylphosphine using dichloromethane as a solvent. The yields of these reactions are found to be 68% and 87% respectively and are characterised by 1H NMR of the (1c) complex and 1H, 13C and 31P NMR of the (2c) complex.

    2.png
      1H NMR Spectrum in CDCl3 of A
      3.png
        The Expanded 1H NMR Spectrum in CDCl3 of A
        4_1.png
          1H NMR Spectrum in CDCl3 of B
          5.png
            The Expanded 1H NMR Spectrum in CDCl3 of B
            6.png
              The Elemental Analysis data of B
              7.png
                1H NMR Spectrum in CDCl3 of (1a)
                8.png
                  The Expanded 1H NMR Spectrum in CDCl3 of (1a)
                  9.png
                    1H NMR Spectrum in CDCl3 of (2a)
                    10.png
                      The Expanded 1H NMR Spectrum in CDCl3 of (2a)
                      11.png
                        1H NMR Spectrum in CDCl3 of (1b)
                        12.png
                          The Expanded 1H NMR Spectrum in CDCl3 of (1b)
                          13.png
                            13C NMR Spectrum in CDCl3 of (1b)
                            14.png
                              1H NMR Spectrum in CDCl3 of (2b)
                              15.png
                                The Expanded 1H NMR Spectrum in CDCl3 of (2b)
                                16.png
                                  1H NMR Spectrum in CDCl3 of (1c)
                                  17.png
                                    The Expanded 1H NMR Spectrum in CDCl3 of (1c)
                                    18.png
                                      1H NMR Spectrum in CDCl3 of (2c)
                                      19.png
                                        The Expanded 1H NMR Spectrum in CDCl3 of (2c)
                                        20.png
                                          31P NMR Spectrum in CDCl3 of (2c)

                                          CONCLUSION

                                          Two 1,2,4-triazole derived NHC-ligands 1,1’-(1S)-menthyl-1H-4,4’ethylidi-1,2,4-triazolium dibromide (1a) and 1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’propylidi-1,2,4 triazolium dibromide (2a) were synthesised and characterised. A series of bimetallic PEPPSI themed palladium complexes (1b) and (2b) namely, [1,1’-(1S)-menthyl-1H-4,4’-CH2(CH2)n-CH2-di-1,2,4-triazoline-5,5’-diylidene]Pd2Br4(pyridine)2 [n = 0 (1b) and n= 1 (2b)] were synthesised and characterised. The corresponding phosphine analogues of these bimetallic complexes (1c) and (2c) namely, [1,1’-((1S,2S,5R)-2-i-propyl-5-methylcyclohexyl)-1H-4,4’­-CH2(CH2)n-CH2-di-1,2,4 triazolium]Pd2Br4(PPh3)2 [n = 0 (1c) and n= 1 (2c)] were synthesized and characterised.

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                                          14. Kumar, A.; Katari, M.; Ghosh, P., Understanding the lability of a trans bound pyridine ligand in a saturated six-membered N-heterocyclic carbene based (NHC)PdCl2(pyridine) type complex: A case study. Polyhedron 2013, 52, 524-529.

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                                          ACKNOWLEDGEMENTS

                                          • Prof. Prasenjit Ghosh
                                          • Dr. A. P. Prakasham
                                          • Dr. Diganta Choudhury
                                          • Ms. Shreyata Dey
                                          • Ms. Jyoti Singh
                                          • Dr. Muthumari Subramanian
                                          • Lab mates and friends
                                          • Indian Academy of Science, Banglore
                                          • Department of Chemistry, Indian Institute of Technology, Bombay
                                          • Department of Chemistry, B Borooah College, Guwahati
                                          • Central Facility and NMR laboratory, Department of Chemistry, IIT Bombay

                                          I owe my deepest gratitude to all the teachers of Department of Chemistry, B Borooah College for their continuous support and encouragement.

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