The Metal-Arene Complex
This lab combines 7.6 mmol of Mo(CO)6 and 72 mmol of mesitylene to produce (Mesitylene) tricarbonylmolybdenum. The product, [1,3,5-C6H3(CH3)3]Mo(CO)3, is produced by boiling the two reactants for 30 minutes in an inert nitrogen atmosphere. The resulting crystals are then filtered and purified by dissolving in dichloromethane and precipitating out using hexane. An IR spectrum is then taken of the product.
The proposal of attempting to make metal-arene complexes such as [1,3,5-C6H3(CH3)3]Mo(CO)3 stemmed from the similarities of the cyclopentadiene rings used to make ferrocene and benzene rings. Both rings have a 6-electron pi system. The first correctly identified complex made with two benzene rings and a structure similar to ferrocene was bis(benzene)chromium in 1955. The creation of this complex led to the synthesis of many other bis(benzene) compounds with transition metals.
The equipment needed for this experiment is as following: 250 mL 3-necked flask (with two stoppers), condenser, stir bar, hot plate, Buchner funnel for filtering, and an IR spectrometer. The chemicals needed are: 2.0 g of Mo(CO)6, 10 mL mesitylene, about 50 mL of hexane, and 20 mL of dichloromethane.
1. Set up the apparatus (a condenser in the middle neck of the 3-necked flask with no water and stoppers on each side. One stopper must have a hole that can be covered up).
2. Weigh out 2.0 g (2.002 g) of Mo(CO)6 and put into the 250 mL 3 necked flask along with 10 mL of mesitylene and a stir bar.
3. Purge the flask for 5 minutes by inserting the tube with nitrogen through the top of the condenser and leaving the hole in the one stopper open.
4. With the nitrogen still on, but the hole covered, boil the solution for 30 minutes. The solution goes from clear to a clear yellow green color.
5. Allow the solution to cool to room temperature, then add 15 mL of hexane.
6. Suction filter and rinse with 5 mL of hexane. There should be yellow crystals along with tiny black dots of the molybdenum.
7. Purify the crystals by dissolving in a small beaker with dichloromethane and a stir bar. If the crystals will not dissolve, a small amount of heat can be added.
8. Filter the solution. Take the filtrate (the yellow solution left in the filtering flask) and add 25 mL of hexane. The product should precipitate out. If the product doesn’t look like shiny, yellow crystals, it can be re-dissolved in the dichloromethane to purify further.
9. Wash the crystals with 4 mL of hexane twice. Allow drying under suction.
10. Take an IR of the product.
I ran an IR after the first purification of the product. The rationale for this was because I had white crystals with only a few black dots and after two purifications, I had only bright yellow crystals left (with the yellow crystals being the product that I should have used for the IR). The IR indicates that the white powder is in fact the product. The spectrum shows the most pertinent peaks at 2000 cm-1. The peak corresponds to the C-O stretching vibrations due to the metal bound carbon monoxide. The IR spectra are attached.
The explanation for having two peaks at 2000 cm-1 is quite interesting. The peaks of metal carbonyls have to do with the number of ligands present in the complex and how the ligands stretch. The two types of stretches exhibited are: symmetric or anti-symmetric. Pertaining to a complex with only two ligands, such as the one produced in this lab, a symmetric stretch is when both ligands either stretch or contract identically. An anti-symmetric stretch is when either one ligand stretches and the other contracts or vice versa. One of the two peaks corresponds to the symmetric stretch and the other to the two anti-symmetric stretches.
I honestly think that the product is supposed to be yellow in color. It even mentions in the lab, “Suction filter off the yellow [1,3,5-C6H3(CH3)3]Mo(CO)3…” (pg 166). The rest of the class had white crystals and their solution while boiling had turned black instead of the yellow-greenish color I had. I’m thinking that maybe that the solutions turned black because the flasks were not completely purged with nitrogen in the beginning and the atmospheres were not inert. (I could be terribly wrong about this, but the lab says the product is yellow, which is what mine was.)
This lab combines 7.6 mmol of Mo(CO)6 and 72 mmol of mesitylene to produce (Mesitylene)tricarbonylmolybdenum. The product, [1,3,5-C6H3(CH3)3]Mo(CO)3 , turns out to be yellow crystals once purified. The IR taken indeed confirmed that the desired product was obtained.
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