26 Aug 2013

Synthesis and Migratory-Insertion Reactivity of CpMo(CO)3(CH3): Small-Scale Organometallic Preparations Utilizing Modern Glove-Box Techniques

Lab Experiment

Submitted by Matt Whited, Carleton College
Categories
Description: 

This laboratory experiment spans three weeks and introduces advanced undergraduates to modern small-scale synthesis techniques involving an inert-atmosphere glove box.  The robust syntheses transform [CpMo(CO3]2 into the methylated CpMo(CO)3(CH3) and examine the phosphine-induced migratory insertion to form various Cp-supported Mo(II) acetyl complexes.  At each step in the synthesis, a combination of IR and multinuclear (1H, 13C, and 31P) NMR spectroscopies allow students to assess the purity of their products and assign molecular geometry and stereochemistry using symmetry and group theory considerations.  We have also adapted the last step of the experiment to allow students to prepare previously unknown acetyl complexes using a variety of phosphines, and in two cases the products have been crystallographically characterized and published.

 

Here we attach a short overview similar to what might be published in J. Chem. Educ. or a related journal, student instructions, full synthetic details including IR and NMR spectra (for instructor reference), implementation notes, and a short handout introducing students to glove box theory and operation.

Learning Goals: 
  • Students should be able to explain basic principles relating to the operation of an inert-atmosphere glove box and will learn to adapt syntheses performed outside a glove box to run them easily and on small scale inside the box.
  • Students should be able to perform basic small-scale (10-100 mg) manipulations in an inert-atmosphere glove box, particularly those involving use of vacuum for filtrations and solvent removal.
  • Students should be able to apply knowledge of symmetry and group theory in order to rationalize and predict IR and NMR spectroscopic signatures for a series of Mo(II) complexes, paying particular attention to stereochemical implications.
Equipment needs: 
  • Inert-atmosphere glove box (N2 or Ar) equipped with a purge valve and vacuum feedthrough (some details regarding setup for vacuum are included as supplementary material here)
  • Infrared spectrometer and solution IR cells
  • NMR spectrometer capable of performing multinuclear (1H, 13C, 31P) experiments
Implementation Notes: 

More detailed notes are provided in a separate document, but here are a few highlights.  Of course it can be difficult to implement a lab experiment for many students with a single specialized piece of equipment like a glove box.  We have run this experiment with up to 16-18 students in a single term by splitting them into 4 sections (2 each for AM/PM) and having them perform the synthesis in groups of 2-3.  During a single lab period, 2 groups can easily perform the required synthesis and characterization by staggering their start times.  As outlined in the student handout attached, the lab can be completed in 3 weeks:

  • Week 1: Preparation of CpMo(CO)3(CH3)
  • Week 2: Purification and analysis of CpMo(CO)3(CH3), Begin preparation of CpMo(PR3)(CO)2(COCH3)
  • Week 3: Purification and analysis of CpMo(PR3)(CO)2(COCH3)

We have also included lab report guidelines that have worked well for our groups.

Time Required: 
3 weeks (3 laboratory meetings of 3-4 hours each)
Evaluation
Evaluation Methods: 

In addition to informal evaluation of lab skills during the weekly meetings, we assess student learning through semi-formal lab reports wherein synthetic and spectral data are reported in journal style and analyzed using knowledge of symmetry and group theory as well as a search of the relevant literature.

A generic rubric for how the points are split up on the lab report is as follows:

  • Analysis of data (40%)
  • Experimental section (35%)
  • Annotated spectra, including assignments (25%)
Evaluation Results: 

Student performance varies quite widely, as might be expected.  In general, the students show a solid understanding of how the glove box works and what kinds of experiments can be performed in a glove box, in addition to the care and special hazards associated with working in a glove box.

Student scores on laboratory reports trend mostly with the amount of time invested by the students in their preparation.  Students with the greatest number of citations for relevant papers from the primary literature also tend to invest the most time in careful reporting of experimental conditions and analysis of the products generated at each step.

Creative Commons License: 
Creative Commons Licence

Comments

Matt and Gretchen,

This is a great experiment!  I only wish I had an advanced class to do this with!

I am thinking to try this. Where do did you post the  full synthetic details including IR and NMR spectra (for instructor reference)?

Thank you. -Anna 

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