17 Nov 2007

Manganese Carbonyl experiment

Lab Experiment

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Description: 
This experiment has been modified and expanded from the J. Chem. Ed. article linked below (J. Chem. Ed., 1988, 65, 1020) and includes four syntheses, that of dppm, and then three Mn complexes of dppm.  Students must select appropriate characterization methods for their molecule, chosen from a list of available instrumentation including NMR (1H, 13C, 31P), IR, UV-Vis, melting point and optical rotation.  Students are graded on yield and purity;  there is time during the semester to repeat work for improved yield.  Students write a brief abstract/discussion (1/2-1 page) and then a full experimental report including their characterization data, written up as if it were part of a publication.
Learning Goals: 

A student will use liquid ammonia as a solvent

A student will carry out a carbonyl substitution reaction under an inert atmosphere

A student will write up synthetic results in an experimental format

Equipment needs: 

Triphenylphospine
NH4Br
Triphenylphosphite
Hydrazine monohydrate
Sodium Sulfate
1-propanol           
Hexanes
Methylene Chloride
Ether
Toluene
MnBr(CO)5
[NO][BF4]               
Sodium metal               

Glassware/Equipment required
24/40 Gas Inlet Adapter
100 mL Schlenk Flask with 24/40 joint
Glass-covered Stirbar        
Syringe filled with grease
Dry Ice Condensor with 24/40 joint
Oil Bubbler               
Blast shield (recommended in case of vigorous liquid ammonia boiling during reagent addition;  careful work behind hood sash is ok)

Implementation Notes: 
My junior level laboratory course is a synthesis and characterization course. Students select 6 syntheses (from a total of 12 in the lab manual) during an 8 week period and carry out their preparation.  This laboratory is in 2 parts, synthesis of dppm in liquid ammonia in one week (can be omitted, just use commercial dppm), and synthesis of Mn complexes, done over a 2 week period.
Time Required: 
Two or three 3-4 hour laboratory periods
Evaluation
Evaluation Methods: 

Students write a brief abstract/discussion (~1 page) and then a full experimental report including their characterization data, written up as if it were part of a publication. Students are graded on yield and purity;  there is time during the semester to repeat work for improved yield. A grading rubric is attached.

 

 

Evaluation Results: 

The synthesis of dppm is not particularly challenging but careful addition of the reagents (as specificed in the student notes) is important. Students should obtain a 31P, and 1H NMR spectrum of dppm, as well as melting point.  IR and UV-Vis are significantly less informative and can be omitted. For the Mn carbonyl complexes, 1H NMR is complicated, and  typically only required for the fac-complex for completeness. However, IR is very informative.  For the writeup, I like to see some discussion of analysis of the MO diagram to explain the rearrangement of the complex upon oxidation/reduction.  Students have no problems acquiring the IR (and the NMR if they choose to do it) but usually gloss over the MO analysis.

 Typical yields:  dppm yields are low, less than 40%, and it often must be recrystallized before use.  yields of the first two Mn complexes are in the 40-80% range, and for the third, about 20-40%.  Yields are generally much higher for the Mn complexes when using commerical dppm.

Creative Commons License: 
Creative Commons Licence

Comments

Why do you need a glass-covered stirbar?

Sodium in liquid ammonia is stable, for a while.  But, if it sees a tiny bit of iron, that catalyzes the reaction to form sodamide.  And, prolonged exposure of the Teflon stir bars results in etching of the coating to reveal the tasty iron center.  So, if you are just demonstrating sodium in a ammonia to show the color, a regular stir bar will be fine.  Or, just don't stir it.  But, to do the reaction, you really do need to stir it to ensure all the solids dissolve.

by the way, this experiment is a good candidate for CV.  You can investigate the electrochemical oxidation/reduction of 2/2'/3 and compare it to theoretical and experimental results.  The CV is described well in the J Chem Educ article.

 That is why I decided to  link this LO to a more recent CV LO.

Adam

I have also updated this experiment to contain a Gaussian calculation to investigate the MOs of the Mn complexes.  You need to use a fairly high level of theory to get the "right" answer;  small basis sets give an incorrect prediction for the relative stability of 2/2'/3.  I will post an update after this year.

Adam,

I have a student in my inorganic class who has chosen to make a tungsten carbonyl complex with dppm as her independent project.  We looked around the literature and decided to adapt a dppe prep using lithium diphenylphosphide, and reacting it with DCM in THF.  Judging by the fact that you go through all the trouble of doing the reaction with solvated electrons in liquid ammonia, I feel like we're missing something.  Does our plan look doomed to failure?

 

to be honest, I've pulled the liquid ammonia portion of the experiment for the past few years.  I make it available for students who want to try it.  I didn't put it in because it was the most efficient or easy way necessarily to make dpm, but I wanted to include some main group synthesis in my course, and I was using liquid ammonia for my research then.  I really don't know phosphine chemistry well at all, but I don't see why LiPPh2 wouldn't work with DCM.  Something is just behind the tip of my brain that makes me think there is a reason why you need to go through all this to make it work, but it would be in the JCS 1962 paper linked above.  Check that article and see what it says.  I know there is something different about CH2Cl2 vs dichloroethane, but I can't remember now.

Good luck, and post back to let us know how it works!

Adam

I forgot to give an update as to how this experiment went when I tried it with a student of mine last fall. 

So, it seems that the liquid ammonia is pretty important for the success of this experiment.  We had little trouble making lithium diphenylphosphide; we came up with a neat way of activating lithium metal by sonicating it in a THF solution of triphenylphosphine (it works great for Grignard reagents, too).  Though the deep red/brown solution that we obtained after the lithium had been consumed did react with DCM to make something, it was a complete mess, and the 1H-NMR didn't show anything that could be construed as the methylene group in dppm.  

Just my luck, I found a tank of ammonia in the back of our stockroom last week.

Spring 2019 update: BrMn(CO)5 was unavailable this spring (unless you wanted to spend $600/g), so I substituted this experiment at the last minute for the synthesis of (arene)Mo(CO)3 and Mo(CO)3(PR3)3. Since I hadn't done that experiment before it didn't work that well but 3 of my students got great results and I like how it worked out. I'll post that experiment when I have it better worked out.

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