30 Jun 2016

Electrochemical and Carbonyl Frequencies to Explain Ligand Non-Innocence in Organometallic Pincer Complexes

Literature Discussion

Submitted by R Bryan Sears, Emmanuel College
Categories
Description: 

In this literature discussion, students read an Inorganic Chemistry paper (doi: 10.1021/ic503062w) about diarylamido-based PNZ pincer ligands and their Ni, Pd, and Rh complexes. Specifically, this paper uses IR and E1/2 potentials to demonstrate that the redox events occur not on the metal center but on the pincer ligands.  For these non-innocent ligands,  the electron donating ability of the pincer ligand towards the metal is more strongly influenced by the donors directly attached to the metal (phosphorus or nitrogen substituents) while the oxidation potential is more affected by the substituents on the diarylamine backbone.  This paper also provides x-ray crystallography data, NMR spectra (including J-coupling information), and a wealth of synthetic information.  This LO was created for the 2016 TUES Viper Workshop on organometallic chemistry.

AttachmentSize
File Student Handout23.44 KB
Learning Goals: 

In answering these questions, a student will…

●      Employ textual clues to define chemical terms such as pincer ligands

●      Apply CBC rules to count electrons for pincer-ligand containing complexes

●      Relate v(CO) stretching frequencies to electron donating abilities of ligands

●      Integrate prior knowledge of periodic trends and electrochemical data from the paper to refine their definition of non-innocent ligands.

●      Correlate electrochemical potential to the “electron richness” of the complex

Implementation Notes: 

Students should read the paper and complete the reading guide before the literature discussion. 

 We hope that instructors will mix and match questions that are appropriate to their classes.  In particular, instructors may want to be selective among the in-depth questions 5-19 depending on the desired emphasis.

 Summary:

Questions 1-4 assess scientific reading competency and foundational concepts, question 5-11 address fundamental inorganic topics related to changing electron density on the metal, wheras questions 12-19 require deeper discussion of ligand non-innocence and experimental methods to determine difference in electron richness.

Note: we envision question 4 being divided up among multiple groups with each group getting one of the rows.  Then, the instructor should highlight the fact that all complexes had the same values.

 

Time Required: 
1 class period
Evaluation
Evaluation Methods: 

This LO has not been implemented, however, we recommend a few options for evaluating student learning:

● implement as in class group work, collect and grade all questions

● have students complete the literature discussion questions before lecture, then ask them to modify their answers in another pen color as the in-class discussion goes through each questions

●  hold a discussion lecture for the literature questions; then for the following lecture period begin class with a quiz that uses a slightly modified problem analogous to question #6 or #8 where a comparison between two different complexes from the paper is proposed, students are asked to summarize differences in their experimental values of CO frequency and potential and chemical reasoning for these differences. 

Evaluation Results: 

This LO was created for the 2016 TUES workshop and has not yet been tested in the classroom.

Creative Commons License: 
Creative Commons Licence

Comments

I used this LO in my senior-level discussion-based organometallics course in Fall 2016. Although I did not collect student responses, I gave the students the reading guide to help them prepare for class. This allowed the discussion to be aimed/developed in a non-random way. In future years, I would recommend assigning it as preclass homework, letting students use their notes during class, and then collecting it after the discussion, to give something to grade or evaluate student performance.

the student presenters followed this reading guide fairly closely and it led to a good discussion of the content of the paper.

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