9 Aug 2012

Alkyne Semihydrogenation with Niobium(II) Imido Complexes: Exploring An Unconventional Hydrogenation Mechanism

Literature Discussion

Submitted by Matt Whited, Carleton College
Categories
Description: 

This is a literature discussion based on an interesting Bergman/Arnold paper utilizing d2 niobium imido complexes for the semihydrogenation of arylalkynes to Z-alkenes.  The mechanism is quite unusual, and I found it to be an interesting paper to discuss after we had talked about the classical hydrogenation mechanisms (typically observed for late transition metals).  The students should come into the discussion understanding fundamental reaction mechanisms (including σ-bond metathesis), and it's helpful if they are somewhat familiar with mono- and dihydride mechanisms for hydrogenation.

We used this paper and questions for in-class presentation and discussion, but they could be easily adapted for a problem set or exam question (if you want to have students reading primary literature for an exam).

Learning Goals: 
  • The primary goal is to help students read and pull out key points from a medium-length (4 pg.) mechanistic organometallic paper.
  • Students should be able to correlate observed physical properties and reactivity with structure, bonding, and oxidation state.
  • Students should be able to identify key steps in several possible catalytic cycles and evaluate which cycles are most reasonable based on a variety of circumstantial experimental evidence.
  • Students should be able to critically evaluate mechanistic evidence presented in a paper and present their findings to the class for discussion.
Implementation Notes: 

We used this paper and set of questions for a literature presentation, where my entire organometallics class (12 students) read the paper in preparation for a presentation by 4 students.  This helped the students to process the information enough ahead of time for them to have a useful discussion about the chemistry.  I gave these guiding questions to the presenters ahead of time and handed them out to the entire class for the presentation.

Many of the questions have answers that are provided directly or indirectly in the article, but they have been designed to provoke some discussion.  We found it very useful to have the presenters ready to go at the white board.  All in all, I found this to be a very useful introduction to the primary literature, as it was the first time that the students were required to read, reflect, and present without my direct involvement (I did not offer much input at all during the presentation).

Also note: I did not include the isocyanide bit in the original version of this assignment, but it was so interesting to the students that I decided to include a "Further Discussion" question here along with fairly extensive notes about the influence of metal-ligand bonding on CO and CNR stretching frequencies.  I'm happy to hear any comments you have!

Time Required: 
30-40 minutes (if paper has already been read)
Evaluation
Evaluation Methods: 

Student performance (in the presentation) and learning (from the paper) was assessed informally, since this assignment involved group discussion.  I primarily evaluated the extent to which the presenters had carefully read the paper and invoked previous topics from the class in explaining their findings.  For the non-presenters, evaluation was based primarily on participation, though this was somewhat less important since everyone in the class would have an opportunity to present at least once during the term.

Evaluation Results: 

This assignment has only been used once, but the presenters did a great job of picking out the important details in the paper and honing in on some of the confusing things (e.g., the CNR stretching frequency increasing on coordination to Nb and certain arguments about the mechanism of H-H bond cleavage).  I found that the presenting students required surprisingly little guidance from me (aside from the leading questions), since the leading questions were largely addressed directly in the paper.

Creative Commons License: 
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