Synthesis of an Oxygen-Carrying Cobalt(II) Complex

1. Synthesis of Inactive [Co(salen)]₂

1. Charge a 250 mL 3-neck round-bottom flask with 120 mL of 95% EtOH and 2.20 g (0.192 mL, 0.018 mol) of salicylaldehyde.
2. Fit the center neck with a condenser connected to N₂. Fit the other two necks with a rubber septum and an addition funnel fitted with a rubber septum.
3. Stir the reaction in a water bath and heat the solution to reflux (80 °C).
4. Add ethylene diamine (0.52 g, 0.58 mL, 0.0087 mol) via syringe through the round-bottom flask septum.
5. In a 50 mL round-bottom flask, prepare a solution of Co(OAc)₂·4H₂O (2.17 g, 0.0087 mol) in 15 mL of distilled water. Heat the solution in the same water bath containing the 3-neck flask to ensure that all of the cobalt acetate dissolves.
6. Add the cobalt acetate solution to the addition funnel.
7. Degas the cobalt acetate solution by bubbling N₂ through the liquid in the addition funnel for 10 min (see "Synthesis of a Ti(III) Metallocene Using Schlenk Line Technique" video for a more detailed procedure on purging liquids). The condenser N₂ adapter may need to be closed to allow the N₂ to bubble through the cobalt acetate solution.
   NOTE: Never heat a closed system! Make sure to vent the system during degassing.
8. Slowly add the cobalt(II) acetate solution (~ 1 drop/s), while vigorously stirring the ethanol mixture. Without sufficient stirring, a chunky precipitate will form that can jam the stir bar.
9. Once all of the cobalt acetate has been added, stir the reaction at reflux for 1 h.
10. Turn off the hot plate and remove the 3-neck round-bottom flask from the water bath.
11. Remove the condenser and addition funnel from the flask. Submerge the flask in an ice bath to facilitate precipitation of the [Co(salen)]₂.
12. Filter the solution under vacuum to isolate the solid and wash the resulting red solid with cold ethanol.
13. Isolate the solid. Calculate the yield of the reaction and collect an IR of the [Co(salen)]₂. Make sure that the [Co(salen)]₂ is dry before using it in the O₂ uptake reaction.

2. Apparatus Setup for O₂ Uptake (Figure 5)¹

Note: It is very important that the system does not leak. A leak in the system will lead to a lower than expected Co:O₂ ratio.

1. Connect a needle to an O₂ (ultra-high purity) gas cylinder with Tygon tubing. Gently bubble O₂ through 5 mL of DMSO for at least 10 min.
2. While the DMSO is being saturated with O₂, fit the two ends of a graduated 10 mL glass pipette with Tygon tubing (each 1.5 ft in length).
3. Attach a glass funnel to one of the Tygon tubing pieces.
4. Clamp the glass pipette and the funnel to a ring stand so that the funnel is facing up and the tubing forms a U shape ( Figure 5).
5. Fill the pipette and funnel with mineral oil. Add the oil through the funnel, making sure that the oil also fills the tubing connected to the pipette. Continue to add the oil until the funnel is filled about half way up the funnel. Don't let the oil get too close to the top of the funnel, as the O₂ bubbling through the funnel can cause splashing if the funnel is too full.
6. To the open end of the tubing, attach a side-arm test tube (test tube A).
7. Add 50 mg (0.077 mmol) of the inactive [Co(salen)]₂ to the side-arm test tube A connected to the glass pipette.
8. Add 2 mL of the DMSO saturated with O₂ to a 3 mL test tube (test tube B).
9. Use a pair of tweezers to gently lower test tube B into test tube A, being careful not to spill any of the DMSO. It is important to not expose the [Co(salen)]₂ to the DMSO at this point.
10. Seal test tube A with a rubber septum. Wire the septum to prevent leaks.
11. Insert the needle connected to the O₂ gas tank into the septum and purge the system with O₂ for 10 min.
12. Remove the O₂ needle and grease the top of the rubber septum to prevent leaks.
13. Some of the pressure within the setup may need to be released to get oil into the glass pipette. To do this, insert a free needle into the rubber septum on test tube A. Cover the opening with a finger and slowly release the pressure within the setup. Do not forget to cover the new hole with grease to prevent leaks.
14. Move the glass pipette and funnel so that the oil levels line up in both pieces of glassware.
15. Record the volume level of oil within the glass pipette.

Figure 5. O₂ uptake apparatus setup.

3. O₂ Uptake Reaction

1. Add the DMSO to the solid [Co(salen)]₂ by gently tipping the test tubes, making sure that none of the solution enters the side-arm of test tube A.
2. Once all of the DMSO has been added, hold the top of the test tube and gently mix the solution by shaking the test tube back and forth.
   NOTE: Do not use an up and down shaking motion. Banging the two test tubes together too violently can lead to the breaking of test tube A.
3. Continue to gently shake the test tubes by hand until the oil level in the pipette stops rising (about 15-20 min).
4. Once O₂ consumption ceases, move the pipette and funnel so that the oil levels line up.
5. Record the new volume level of the oil in the glass pipette. The volume difference is the volume of O₂ consumed during the reaction at atmospheric (1 atm) pressure.
6. Record the temperature of the room.

4. O₂ Liberation from [Co(salen)]₂ - O₂ Adduct

1. Transfer the resulting DMSO solution from step 3 to a 15 mL centrifuge tube.
2. Fill a second test tube with an equivalent amount of water.
3. Insert the test tubes across from each other into a centrifuge.
4. Centrifuge the sample for at least 15 min. The resulting solid pellet quality improves with increasing centrifuge time.
5. Gently remove the test tube with the [Co(salen)]₂-O₂ adduct sample, so not to disturb the pellet.
6. Carefully decant the DMSO solution above the pellet.
7. Holding the centrifuge tube at a 45 ° angle with the pellet facing up, slowly add 1 mL of CHCl₃ with a pipette, by allowing the solution to drip down the side of the centrifuge tube. Take extreme care to not disturb the solid [Co(salen)]₂-O₂ adduct.
8. Observe any physical changes that occur.