1. Experimental

a. MoO₂dtc₂

- The reaction starts with the reaction of sodium molybdate (Na₂MoO₄* 2H₂O) with Nadtc*3 H₂O in a solution of sodium acetate. Because the solution is basic at this point, the product does not precipitate.

The addition of the 1 M HCl (How can this solution be prepared from concentrated HCl? Does the exact concentration matter here?) should be done slowly. The pH should not be allowed to drop below pH=5.5, because the compound starts to decompose then (How can you detect that?).

- The crude product is filtered, washed with 95% ethanol and dry diethylether, and then dried in vacuo.

- The recystallization should be done from warm toluene (=50-60 ^oC and not boiling toluene!). If the temperature is too high, the product will decompose.


b. MoOdtc₂

- The synthesis of compound B has entirely be conducted under inert gas, because the product is extremely air sensitive (color change from pink to dark purple). The purple color originates from the Mo(V) compound. (Mo₂O₃dtc₄)..

- The reaction starts with the reaction of sodium molybdate (Na₂MoO₄) with Nadtc in the presence of sodium dithionite (Na₂S₂O₄). Initially a dark purple precipitate is formed that will turn pink after about two hours if the reaction conducted properly.

- It might make sense to start this reaction first because it usually takes two hours. The use of deaerated water is advisable since it speeds up the reaction (Why?).

- The filtration has to be performed using a Schlenk frit. The final product has to be stored under inert gas.


2. Characterization

a. FTIR spectrum (YH 6076, YH 1033)

- Acquired using ATR setup (>600 cm^-1) and as Nujol mull in between CsI plates (<600 cm^-1)
- Read the chapter on how to acquire infrared spectra (Appendix A and B, p. 129-138)

b. EPR spectrum for compound B only (in dry dichloromethane, use an EPR tube not the NMR tube)

- Read the chapter on how to acquire EPR spectra (Appendix D, p. 147-151)


3. Catalysis experiment

The student will have to prepare dry DMF for this part of the experiment as well as dry the molecular sieve.

You will have to set up three reactions:

a. using MoO₂dtc₂ as catalyst without molecular sieve
b. using MoO₂dtc₂ as catalyst with molecular sieve
c. using MoOdtc₂ as catalyst without a molecular sieve
d. blank (only benzoin, no catalyst, no molecular sieve)

The reactions should be run using 0.05 mmol of the catalyst, 1 mmol of benzoin and 25 mL of dry DMF. The round-bottomed flask should be closed using drying tubes with anhydrous (granular) CaCl₂. Take one sample two hours into the reaction and the other one after approximately 24 hours. It is important that the setups are reasonably identical in terms of flask size and availability of oxygen.

When using a drying agent like MgSO₄, it is crucial to use a small amount. Why?

To determine the efficiency of the catalyst, GC samples are prepared of the individual products from the reaction. The samples should be dissolved in solution of camphor in ethyl acetate (5 mg/mL). The concentration of the product in the solution should be 2-5 mg/mL. For the calibration, several standards with have to be prepared with different concentration ratios of benzoin to benzil. It might be a good idea to prepare one set of standards for the group working on this particular project at a given point in time. Please inform the instructor the meeting prior the GC sample submission that you need GC vials.

It might be a good idea to use the rotary evaporator to remove the solvent from the samples.


4. Hints to questions

ad 1: How many 'ligands' do you have in those compounds? Which coordination figures are possible for this number of ligands?

ad 4: Write a balanced equation for the reaction that you are doing in the lab. What are the side products of the reaction?


5. Problems to be considered

a. What is the function of sodium acetate in the synthesis of compound A?

b. If the synthesis of compound B is not properly carried out, a dark-pink to purple compound is observed. How can this be explained?

c. What is the product of the oxidation of S₂O₄^2- in the synthesis of compound B?

d. Why is it important to prepare GC sample in a concentration range of 2-5 mg/mL?

e. Why is it advisable to degass the CDCl₃ when acquiring the NMR spectrum for compound B?

f. We used to use diethyl ether instead of dichloromethane for the workup. Which problem poses this?