Molecular Orbital (MO) Theory

NOTE: For safety precautions, the Schlenk line safety should be reviewed prior to conducting the experiments. Glassware should be inspected for star cracks before using. Care should be taken to ensure that O₂ is not condensed in the Schlenk line trap if using liquid N₂. At liquid N₂ temperature, O₂ condenses and is explosive in the presence of organic solvents. If it is suspected that O₂ has been condensed or a blue liquid is observed in the cold trap, leave the trap cold under dynamic vacuum. Do NOT remove the liquid N₂ trap or turn off the vacuum pump. Over time the liquid O₂ will evaporate into the pump; it is only safe to remove the liquid N₂ trap once all of the O₂ has evaporated. For more information, see the "Synthesis of a Ti(III) Metallocene Using Schlenk line Technique" video.¹

1. Setup of the Schlenk Line for the Synthesis of Ni(dppf)Cl₂ and Pd(dppf)Cl₂

NOTE: For a more detailed procedure, please review the "Schlenk Lines Transfer of Solvent" video in the Essentials of Organic Chemistry series).

1. Close the pressure release valve.
2. Turn on the N₂ gas and the vacuum pump.
3. As the Schlenk line vacuum reaches its minimum pressure, prepare the cold trap with either liquid N₂ or dry ice/acetone.
4. Assemble the cold trap.

2. Synthesis of Ni(dppf)Cl₂ (Figure 5) under Anaerobic/Inert Conditions

Note While the synthesis of Ni(dppf)Cl₂ can be conducted in aerobic conditions, higher yields are obtained when conducted in anaerobic conditions.

1. Add 550 mg dppf (1 mmol) and 40 mL of isopropanol to a three-neck flask.
   Note dppf can be purchased from Sigma Aldrich or synthesized using methods found in the literature.²
2. Fit the center neck of the three-neck flask with a condenser and a vacuum adapter. Fit the two remaining necks with 1 glass stopper and 1 rubber septum.
3. Degas the solution by bubbling N₂ gas through the solvent for 15 min. Use the vacuum adapter at the top of the condenser as the "vent."
4. Connect the vacuum adapter at the top of the condenser to N₂ using the Schlenk line.
5. Begin heating the three-neck flask in a water bath set to 90 °C.
6. In a single neck round bottom flask, add 237 mg NiCl₂·6H₂O (1 mmol) to 4 mL of a 2:1 mixture of isopropanol (reagent grade) and methanol (reagent grade). Sonicate the resulting mixture until all of the Ni salt has dissolved (about 1 min).
   NOTE: If a Sonicator is not available, gently heat the mixture in a water bath.
7. Degas the Ni solution by bubbling N₂ gas through the mixture for 5 min.
8. Add the NiCl₂·6H₂O solution to the three-neck round bottom flask via cannula transfer.
9. Allow the reaction to reflux for 2 h at 90 °C.
10. Allow the reaction to cool in an ice bath. Isolate the resulting green precipitate by vacuum filtration through a fritted funnel.
11. Wash the product with 10 mL of cold isopropanol, followed by 10 mL of hexanes.
12. Allow the product to air dry before preparing the NMR sample.
13. Take a ¹H NMR of the product in chloroform-d.
14. If the ¹H NMR is indicative of a paramagnetic species, prepare an NMR for the Evans method, following the instructions in step 4.

Figure 5. Synthesis of Ni(dppf)Cl₂.

3. Synthesis of Pd(dppf)Cl₂ (Figure 6)¹

NOTE: Use standard Schlenk line techniques for the synthesis of Pd(dppf)Cl₂ (see the "Synthesis of a Ti(III) Metallocene Using Schlenk line Technique" video).

Note While the synthesis of Pd(dppf)Cl₂ can be conducted in aerobic conditions, higher yields are obtained when conducted in anaerobic conditions.

1. Add 550 mg (1 mmol) dppf and 383 mg (1 mmol) bis(benzonitrile)palladium(II) chloride to a Schlenk flask and prepare the Schlenk flask for the cannula transfer of solvent.
2. Add 20 mL of degassed toluene to the Schlenk flask via cannula transfer.
3. Allow the reaction to stir for at least 12 h at room temperature.
4. Isolate the resulting orange precipitate by vacuum filtration through a fritted funnel.
5. Wash the product with toluene (10 mL), followed by hexanes (10 mL).
6. Allow the product to air dry before preparing the NMR sample.
7. Take a ¹H NMR of the product in chloroform-d.
8. If the ¹H NMR is indicative of a paramagnetic species, prepare an NMR for the Evans method following the instructions outlined in step 4.

Figure 6. Synthesis of Pd(dppf)Cl₂.

4. Preparation of the Evans Method Sample

NOTE: For a more detailed procedure, please refer to the "Evans method" video.

1. In a scintillation vial, prepare a 50:1 (volume:volume) solution of chloroform-d:trifluorotoluene. Pipette 2 mL of deuterated solvent, and to this add 40 µL of trifluorotoluene. Cap the vial.
   NOTE: In this example, we will be using ¹⁹F NMR to observe the shift of the F signal in trifluorotoluene in the presence of the paramagnetic species.
2. With this solution, prepare the capillary insert.
3. Weigh 10-15 mg of the paramagnetic sample into a new scintillation vial and note the mass.
4. Pipette ~ 600 µL of the prepared solvent mixture into the vial containing the paramagnetic species. Note the mass. Make sure that the solid completely dissolves.
5. In a standard NMR tube, carefully drop the capillary insert at an angle, to ensure it does not break.
6. Pipette the solution containing the paramagnetic species into the NMR tube.
7. Acquire and save a standard ¹⁹F NMR spectrum.
8. Note the temperature of the probe.
9. Note the radiofrequency.