17 Jul 2014

Synthesis, Characterization, and Computational Modeling of [Co(acacen)L2]+, an Inhibitor of Zinc Finger Proteins

Lab Experiment

Submitted by Elizabeth Bajema, Northwestern University
Categories
Description: 

In this experiment, students will synthesize a cobalt Schiff base complex with varying axial ligands ([Co(acacen)L2]+). They will characterize the complex using various techniques, and may perform computational modeling to predict spectroscopic properties.

Learning Goals: 

Students will be able to:

  • Synthesize a series of cobalt(III) complexes containing acetylacetonatoethylenediimine and ammine and imidazole derivatives.

  • Calculate the percent yield of the complexes synthesized.

  • Determine the steric and electronic effects of the axial ligands on the complex using UV-Vis spectroscopy.

  • Analyze multinuclear NMR spectra to determine the effect of axial ligands on the equatorial ligand.

  • Analyze the IR spectra to confirm the expected structure

  • Create a diagram that illustrates the effect of the axial ligands on the protons of the equatorial ligand.

  • Discuss the binding ability of the complex to biological molecules based on the primary literature.

  • Use Electronic Structure calculations at a minimum of the B3LYP/6-31G(d) level of theory using the PCM model for water to calculate the HOMO, LUMO, vibrational modes, NMR spectra, and UV-Vis spectra.  

  • Compare calculated values to experimental spectra.    

  • Communicate their findings in written and/or oral format.
Equipment needs: 

Standard glassware: Round bottom flask, Stir bar, Vacuum filtration system

Instrumentation: NMR, IR, UV-Vis

A computational package such as Gamess or Gaussian™ capable of electronic structure calculations, and a visualization package such as WebMO, GaussView™, Chem3D™, or Avogadro to visualize input and output files.

Implementation Notes: 

This lab lends itself to many adaptations:

1)   The inorganic chemistry instructor can collaborate with the organic laboratory instructor in the synthesis of the Schift base ligands.  The organic chemistry students will characterize the ligand using proton and carbon-13 NMR.

2)   Each student or pair of students will synthesize the ammine and an imidazole derivative complex.  Students will share the data to investigate effects of the trans ligands on the proton of the acacen ligand.

3)   Co-NMR can also be used to investigate the properties of the trans ligand.

Time Required: 
2-3 weeks. This activity can be done in two-weeks if the TA or the faculty synthesizes the ligand.
Evaluation
Evaluation Methods: 

Students will complete the syntheses of the desired compounds, obtain the required instrumental spectra, complete the electronic structure compounds.  Based on these results, students will write a lab report in the style of an inorganic chemistry paper to present the required analyses.  A rubric for the grading of the laboratory report is included. 

Evaluation Results: 

This learning object was developed for the Summer 2014 VIPER workshop, and has not yet been evaluated.

Creative Commons License: 
Creative Commons Licence

Comments

I am trying this lab with two of my students this semester, and they did the 4-methylimidazole synthesis.  They get a brownish red solution after stirring under air for 24 h.  They reduce the solvent by half (not quite sure how far to go), but when they add the ether it oils out.   Is there something we're doing wrong or should the oil get crunchy if we let it go long enough?   

As it turns out, someone has improved upon this synthesis since this LO was published, and has found a way to avoid this problem. The reaction that seems to work better involves formation of the Co(4MeIm)4 first, then addition of the acacen. The procedure is as follows:

  • Combine 1.0 eq cobalt bromide and 4.1 eq of 4MeIm, and stir in methanol for 5 min under N2 (reaction should turn purple).
  • Add 1.1 eq of the acacen and stir overnight open to air (reaction should turn brown over time).
  • To purify, make sure that there is enough solvent that the product is clearly in solution (reducing solvent won't necessary solve the problem if the solvent is reduced to the point that you are actually making a suspension rather than a solution). Then, slowly add ether (~1.5 mL at a time) while sloshing around. The pure product should precipitate out.

I hope this is helpful! I will update the LO documents soon to reflect these changes.

Thanks for your help Liz!  That is a great help--any estimation of the solvent volume or percent after reducing?  We managed the second time around to get an oil that did "crunchify" after stirring with ether.  Waiting for the lab report to see how pure it looks, but maybe someone can try the new method and report back!  

Another somewhat strange question that I had on discussing the results of this with one of my students.  Is it possible there is a step for adding the counterion that is left out of this synthesis?  One of the original articles has a BPh4- counter ion and I am wondering if the Br- alone (I am assuming that is the counterion in our case...) is not sufficiently big and maybe that is why we weren't getting nice precipitate.   

Hilary, to answer both your questions:

1) Just prior to recrytallizing your product, it turns out reducing the solvent concentration may not be necessary at all. It is important to make sure that your product is clearly in solution, not a suspension. Concentrating solvent too much may result in a suspension and therefore imperfect recrystallization.

2) We use only Br- counter ion in this synthesis, so nothing larger should be needed.

I have also now updated the protocols in the LOs to reflect the previously mentioned changes. All is up to date!

My inorganic class tried the modified protocol provided by Liz (thanks!).  My students found that use of 7N NH3 in methanol created an opaque purple solution (or very fine suspension) of the intermediate [Co(NH3)4]2+ ion prepared under nitrogen (note that we started with anhydrous CoBr2 dissolved in 15 mL MeOH rather than CoBr2.6H2O dissolved in 10 mL MeOH, so the waters of hydration may be relevant here in keeping the reaction mixture in solution).  Addition of 1.1 equivalents of H2acacen (or H2Meacacen) and exposure to air gave the expected brown product, but in our experience no diethyl ether was needed to precipitate the product - the product was simply filtered off.

 

Also, for the synthetic procedure, we found that the 4.1 equivalents of 7N NH3 indeed equated to 1 mL of this solution, but in our calculations (after doing the reaction as described) for how much 4-MeIm we should have used, we found that 4.1 equivalents was 0.58 g and not 0.43 g as described.  We were unable to precipitate the corresponding 4-MeIm products with the addition of diethyl ether as described in the procedure, but we have yet to confirm whether the addition of 0.15 g of more 4-MeIm to prepare the purple solution intermediate under nitrogen improves matters.

Hi everyone,

A pair of students in my inorganic lab class may work on this experiment over the next few weeks.  Is it likely that bromide is as small as you can go wtih the counteranion?  We have cobalt chloride on hand, but would need to order the bromide.

Thanks for any insights.

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