In this experiment you will

• synthesize organomolybdenum complexes, namely [(h -Cp)Mo(CO)₃]₂, (h-Cp)Mo(CO)₃I, and (h-Cp)Mo(CO)₂(PPh₃)I according to the literature citation given below, and
• characterize the compounds by spectroscopic techniques.

 

No introduction is provided to you for this experiment. You are asked to write an appropriate introduction to this experiment. This introduction will be part of your report that you will turn in, and it will be graded.

1.   Synthesis of [(h-Cp)Mo(CO)₃]₂

In order to successfully synthesize the complexes, all reaction flasks and glassware must be assembled under N₂ or Ar and flame-dried prior to the addition of the chemicals. Assume that all solutions and solids are air-sensitive and must be handled in an inert atmosphere only unless specified otherwise.

The synthesis of this compound has been described as "somewhat of an art" and therefore attention must be paid to the details of the reaction. The reaction requires a refluxing period of three hours, therefore plan to set up your glassware ahead of time. Finding the proper glassware, assembling t, and flame-drying can easily use up one hour of time.

Synthesis.   Molybdenum hexacarbonyl (3.0 g, can be weighed quickly in air in a closed vial or weighing bottle) is suspended in dicyclopentadiene (20 mL, measured in air in a graduated cylinder) in a 50 mL round bottom flask fitted with a gas inlet, a condensor, and a magnetic stirbar. Oxygen is removed from the mixture by the freeze, pump, thaw cycle performed at least three times. The reaction mixture is kept under N₂ or Ar during the entire reaction time period. Gentle refluxing is accomplished by using a 100 mL heating mantle packed with glass wool to the top of the liquid in the 50 mL flask with a Variac power setting of 45%. This also causes the Mo(CO)₆ to sublime only onto the cool, exposed upper half of the flask and not into the condensor. Gently swirling the flask every 15 min washes the unreacted Mo(CO)₆ back into the solution and leads to relatively high yields of the product (70%).

The product may be isolated by pouring the room temperature reaction mixture into 40 mL of petroleum ether or hexane and filtering by gravity through filter paper. The residue is washed with petroleum ether or hexane and air dried. Unreacted Mo(CO)₆ is removed from the dried residue by sublimation (50 ^oC, 0.5mm). The purified product may be stored in a vial under N₂ at room temperature.

2.   Synthesis of (h-Cp)Mo(CO)₃I

Iodine is dissolved in 10 mL of chloroform in a 50 mL round bottom reaction flask fitted with a gas inlet, a magnetic stir bar, a dropping funnel, and a stopper. The solution is degassed and placed under an inert atmosphere. Into the dropping funnel is placed a solution of the cyclopentadienylmolybdenum tricarbonyl dimer prepared in part 1 above in degassed chloroform. This solution is added dropwise to the iodine solution with constant stirring. After the addition is complete, the reaction solution is stirred for an additional 15 min or until no further color change takes place. The reaction solution is mixed with an aqueous solution of sodium thiosulfate to remove excess iodine and the aqueous layer is separated and discarded. The chloroform layer is dried under vacuum and the reaction residue is then recrystallized from petroleum ether or hexane.

3.   Synthesis of (h-Cp)Mo(CO)₂(PPh₃)I

A 50 mL round bottom reaction flask is assembled under an inert atmosphere with a gas inlet, a stir bar, and a stopper. Into the flask is placed the (h-Cp)Mo(CO)₃I prepared in step 2 above and 20 mL of freshly distilled THF. Triphenylphosphine is added and the reaction mixture is warmed to reflux temperature under N₂ or Ar. Refluxing is continued for at least one hour, although a longer period of time may be needed. After cooling to room temperature, the solvent is removed under vacuum and the reaction residue is recrystallized from petroleum ether or hexane.

4.   Characterization of the Complexes

After synthesizing the complexes, you will characterize the complexes by spectroscopic methods already employed in this laboratory. After having employed a variety of techniques during the course of the laboratory, it is expected that you are capable of deciding which spectroscopic techniques are most adequate for the complete characterization of the complexes. Please note that you are asked to characterize the complexes unambiguously.

References

1. King, R. B. Organometallic Synthesis, vol. 1, Academic Press: New York, 1965, p. 110.

2. Lucas, R. C.; Walsh, K. A. J. Chem. Educ. 1987, 64, 265.

3. Abel, E. W.; Singh, A.; Wilkinson, G. J. Chem. Soc. 1960, 1321.