experiment 39

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Name: Jiani He ID: 26380107 Performance date: 11/21/2023 Lab partner: Tran Ahn Tuan Nguyen Reports due date: 12/01/2023 NMR Investigation of Molecular Fluxionality: Synthesis of Allylpalladium Complexes

Abstract: Pd complex [PdCl(C 3 H 5 ) 2 ], the intensities on the IR spectrum are weaker in some bands, and there is a weak sp3 C-H stretching band by comparison of literature IR spectrum in SDBS [3] . The wavenumbers are similar to the literature wavenumbers, which indicates some of the products were synthesized successfully although the percentage yield was not presented. The overall results are satisfactory. Introduction: In this experiment, Pd complex was prepared and characterized by Infrared and H NMR spectroscopy. In general, Lewis bases can act as ligands in coordination compounds, and form bonds with central metal atom. The ability to form bonds depends on the properties of ligands and metals. Some ligands can interact with the central metal atom and form bonds with one or more atoms in the π system in the structure. This kind of ligands is in the field of organometallic chemistry, and hapticity is introduced at this time. The hapticity of a ligand relates to η i , and i represents the number of atoms bonded to the central atom [4] . In the second part of this experiment, a dimeric palladium complex containing the allyl cation is prepared based on the reaction equation PdCl 2 +2H 2 C=CHCH 2 Br → [PdCl(C 3 H 5 ) 2 ] experimental: 100 mg of palladium (II) chloride, 3.00 mL of glacial acetic acid, and 3.00 mL of water were added to a 25 mL round bottom flask containing a stir bar. The mixture was heated to 100 ℃ for 15 minutes under reflux. Once the solid dissolved, the temperature was decreased to 60 ℃ before the addition of 500 μL allyl bromine and kept for one hour. 3.00 mL of DCM was added, and the reaction mixture was transferred to a second flask, and then dried with anhydrous magnesium sulfate. Next, the suction filtration was used. to separate the product and liquid. The mass was weighed, and its infrared spectrum was obtained. Data: Table 1: Amount of Reagents Chemical Amount Moles (mmol) PdCl 2 100.9 mg 0.569 Acetic Acid 3.00 mL 52.5 Allyl Bromide 1.00 mL 5.79

Based on the reaction equation PdCl 2 +2H 2 C=CHCH 2 Br → [PdCl(C 3 H 5 ) 2 ], the limiting reagent is PdCl 3 , and the stoichiometric ratio of reagent and product is 2: 1. Therefore, the theoretical yield is calculated as m=0.5(0.615 mmol) × 365.89 mg/mmol=109.74 mg. There’s no percentage of yield present. Table 2. Infrared Spectrum Analysis of [PdCl(C 3 H 5 ) 2 ] Wavenumber (cm -1 ) Intensity Mode [3] 2900 weak V sp3(C-H) 1381.0 & 1258.0 Medium v(C=C) 1012 & 1080 Medium δ sy (C-H) 862&790 Medium δ oop (C-H Table 3. 1 H NMR Spectrum Analysis of [PdCl(C 3 H 5 ) 2 ] in CDCl 3 Chemical Shift (ppm) Integration Multplicity J (Hz) Proton Identity 5.42 1.00 Septet 3 J AB =5, 3 J Ac =15 A 4.20, 4.19 2.00 Doublet 3 J AB =5 B 3.09, 3.06 2.00 Doublet 3 J Ac =15 C Calculation for J-coupling constant in 500 MHz 1 H NMR: 3 J AB =500 × (4.20-4.19)=5Hz 3 J AC =500 × (3.09-3.06)=15Hz Based on the Figure 1, signal at 5.42 ppm is assigned as the CH group with proton A. From the textbook, trans position has a larger coupling constant [10] . Therefore, the signal at 3.07 ppm with a coupling constant is assigned as proton C. The other signal is assigned to proton B, which is on the cis position to proton A.

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Figure 1. Assignment of Protons in the Allyl Group [1] Table 4. Temperature Variation 1 H NMR Spectrum Analysis of [PdCl(C 3 H 5 ) 2 ] in DMSO Temperature ( ℃ ) Shift (ppm) Intensity Shape Proton 25 4.40, 3.48 Medium Singlet B, C 35 4.40, 3.48 Strong Singlet B, C 45 4.40, 3.48 Medium Singlet B, C 55 4.40, 3.48 Weak Singlet B, C 65 No significant signal observed Results: Anhydrous palladium (II) chloride was dissolved in the acetic acid with heat and stirring. Once the temperature reached 60 ℃ , the reaction was keeping for one hour. Brown precipitate instead of orange- yellow powder was obtained. Rotary evaporation was applied to evaporate the solvent to reduce the volume and the color was changed to dark brown, which indicates the experiment was successful. Mass of product was not measured and percentage yield was not obtained. By analyzing the infrared spectrum, there is a weak peak which contained sp3 CH stretching vibration. By comparing the literature spectrum from SDBS [3] , the wavenumbers were similar although some peaks are weaker than peaks in the literature spectrum. 1 H NMR operation was not performed, and the spectra were analyzed with the spectra provided on Moodle site.

Discussion: In this experiment, allylpalladium complex was synthesized. Palladium is a very good metal catalyst for many organic reactions. One example of a palladium-catalyzed organic reaction is the reduction of alkynes. Palladium is used as the catalyst and helps the addition of hydrogen atom on the carbons of alkynes, and the eliminate the triple bond into single bond efficiently [7] . Ligands with π system in the structure can coordinate to the transition metal atom with their π networks, and the π system helps to form the bond between metal and carbon. To increase the covalency in the bonds, there are more than one interaction mode for one single π system with ligand, which leads to different hapticities of the ligand. As mentioned in the introduction, the hapticity of a ligand relates to η i , and i represents the number of atoms bonded to the central atom [4] . The allyl cation is involved in this experiment can be either η 1 or η 3 , and cyclopentadienyl (Cp) anion can be also η 1 or η 3 . The bonding modes are shown in Figure 2. Fluxionality is the interconversion between different coordination modes of one complex. and this Based on the experiment, palladium complexes are considered labile. If they are inert, molecular fluxionality is not expected, since the metal-ligand interactions will be too strong. The derivatives of Cp also have fluxional behaviors according to the literature. Figure 3. Binding Modes of Allyl [6] To break the Pd-dimer, dimethyl sulfoxide (DMSO) is used, and the new dative bond forms between the soft base sulfur and the soft acid palladium. As mentioned before in the first part of the experiment of hard-soft acid-base theory, it is more favorable to form bond between two similar hardness species [4] . For palladium itself, it has a large atomic radius of 163 pm [8] , and therefore, it is considered as soft as well. In this part, mass of product was not recorded. The dimeric product was characterized with IR spectroscopy. However, there is a small peak around 3000 cm -1 , which indicates the sp3 C-H stretching

vibration, therefore, the possibility of ally bromide might exist. By comparison of other bands with SDBS literature spectrum [3] , the wavenumbers are similar, but the intensities for some bands are not strong enough. The reason of causing this error might come from the solvent in the product have not been evaporated completely or there might still be unreacted starting materials. When dissolving the product in deuterated chloroform for obtaining a 1 H NMR spectrum with three signals at 5.42, 4.20 and 3.07 ppm, respectively. The signal at 5.42 ppm is a septet, which indicates the hydrogen in the middle position in the allyl group. As Figure 1 shows, there are three inequivalent protons, and this proton is assigned as H A . This signal has complex splitting pattern and coupling constants with other two signals would be determined. The signal at 4.20 ppm is a doublet, which indicates the hydrogen is on the position B, and it is cis to H A . The coupling constant of H A and H B was calculated as 5 Hz, which is in the range of 3 J coupling between cis hydrogens [10] . For the rest signal at 3.07 ppm, which indicates the H C is trans to H A with a coupling constant of 15 Hz. To dissolve the product in a coordinating solvent dimethyl sulfoxide (DMSO), dative bond forms between sulfur and palladium, which are soft acid and soft base, respectively. Temperature variation 1 H NMR spectrum was obtained by dissolving the product in DMSO. Based on the Figure 1, H A has four neighbors and signal appears a quintet. With the temperature increases, fluxion frequency also increases, the intensities of other two singlet signals become weaker. The reason might be after fluxion, the double bond characteristics will become weaker and broader, and therefore, the coupling characteristic and even for the peak intensities also become weaker. Conclusion: In the palladium complex product, allyl ligand with π system was the starting material. The complex was synthesized from palladium (II) chloride and allyl bromide, with the percentage yield was not obtained. Since the reaction was under heating condition, fluxional behaviors of allyl ligand occurred, and the structure can vary from different possible hapticities. In 1H NMR spectrum when the sample was dissolved in CDCl3, the three bond coupling constants were determined as 5 Hz and 15 Hz in cis and trans positions. If the sample is dissolved in DMSO, the intensities of the signals would be decreased with the increase of temperature, because the fluxional behaviors of the allyl ligand become more sufficiently. No melting point was determined at this time. 1. Account for the multiplicities in the 1 H NMR spectra of both products.

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From the figure shown below, when dissolved product in CDCl 3 solution, protons B and C both couple to proton A separately, and two doublets were resulted. Since proton B is cis to proton A, the coupling constant should be smaller, which is 5 Hz. The other proton is trans to proton A, which has a coupling constant of 15 Hz. For the signal for proton A, a septet represents it . 2. Write a mechanism showing the fluxionality of the monomer in DMSO. Be sure to show how the A and B protons can interconverted 3. Why does the DMSO-d 6 spectrum change with temperature? When dissolved the product in DMSO, molecular fluxionality behavior is observed with allyl cation. Then proton B and C is equivalent on the spectrum. Moreover, the signal of proton A is a quintet, since there

are four neighbor protons. With increasing the temperature, the intensities of signals became weaker and broader since the fluxionality became more efficiently, and there might be no more coupling exist. 4. Provide reasoned arguments as to whether a Pd-Pd bond is present in the dimer. Pd-Pd bond does not exist based on electronic repulsion. Because the two chlorine atoms between them indicate there is a relatively large interatomic distance. References 1. Szafran, Zvi, et al. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience . John Wiley & Sons, 1991. 2. Bora, T.; Singh, M. M. Sulphoxide Complexes of Ruthenium. Transition Met. Chem. 1978, pp. 27-31. 3. Spectral Database for Organic Compounds,SDBS. No. 50264 (n.d.). Retrieved November 19, 2020, from https://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi 4. Miessler, Gary L., et al. Inorganic Chemistry . Pearson Education South Asia Pte Ltd, 2018. 5. Ambidentate Ligands: Facts, Summary & Definition: Chemistry Revision. (n.d.). Retrieved November 20, 2020, from https://alevelchemistry.co.uk/definition/ambidentate-ligands/ 6. Access from online MarvinJS: https://marvinjs-demo.chemaxon.com/latest/index.html 7. Libretexts. (2020, August 11). 9.6: Reduction of Alkynes. Retrieved November 20, 2020, from https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:_Organic_Chemistry_(McMurry)/ 09:_Alkynes_-_An_Introduction_to_Organic_Synthesis/9.06:_Reduction_of_Alkynes 8. Palladium. (n.d.). Retrieved November 20, 2020, from https://chemglobe.org/ptoe/_/46.php Davision, A.; Rakita, P. E. Inorganic Chemisitry . 1970 , 9, 289.