2 Dec 2016

Methane activation by a tungsten allyl

Literature Discussion

Submitted by Chip Nataro, Lafayette College
Categories
Description: 

The literature discussion is based on a paper by Legzdins (Organometallics, 2017, 36, 26). In this work, the C-H activation of methane by a [Cp*W(NO)(allyl)(alkyl)] compound is described. The paper is extremely well written and approachable for undergraduates, although the initial length and large quantity of experimental data might be a bit intimidating at first. The problem of using methane is a signifiant real world problem and as such should provide an interesting context to talk about this paper. The bonding of NO and allyl ligands is discussed as are a number of reactions in the process of converting methane to a larger ketone. These include C-H activation at a d0 compound (so it is not oxidative addition), CO insertion and an internal nucleohilic attack. Electron counting is an important component of this exercise. There is a large amount of spectroscopic data in the paper, but this LO only briefly examines the relationship between IR vibrations and electron density at the metal center and coupling to spin 1/2 nuclei that are less than 100% naturally abundant.

AttachmentSize
File Questions for students16.87 KB
Learning Goals: 

Upon completing this LO students should be able to

  1. Describe why the activation of methane is a significant problem that needs to be addressed
  2. Use the CBC method to count electrons in the tungsten compounds in this paper
  3. Describe the bonding in compounds with linear NO and η3-allyl ligands
  4. Outline the steps for the C-H activation of methane by this tungsten complex including a description as to why the C-H activation is not an oxidative addition
  5. Explain 183W satellites
Implementation Notes: 

Students should read the paper before coming to class. Although there are a lot of questions in the LO, if the students have done a good job reading the paper I would anticipate that they can get through them all. Certainly some of the questions can be left out, or perhaps only provide the students with a few of them before class. In particular, question 1 is about the big picture problem of methane transportation, and would likely be good for the students to do some research into this area before talking about the paper in class.

Time Required: 
50 minutes or so
Evaluation
Evaluation Methods: 

This paper was presented late in the fall semester and as such I was unable to use it in class. However, I will likely use it as the basis for my final. As a discussion I would envision collecting the answers to the questions that the students come up with jointly in class. I would also envision some component of their grade being based on participation. 

Evaluation Results: 

None currently

Creative Commons License: 
Creative Commons Licence

Comments

I used this LO as the final oral exam for my senior-level organometallics class in Fall 2016. Students were given approximately one week to read and study the paper, followed by a 1 hour oral exam in my office. The students summarized the paper for me in as much time as they needed (10-30 min) and then I asked them questions from this LO that they had not covered already. As such, we did not cover all the material. I will give my general impression of student answers here, but won't give away the answers in my comment.

2) 4/5 were able to come up with the answer on their own before class. All could draw the Lewis structure and understand it. We discussed how it was really counted using CBC notation. We had NOT covered NO explicitly in class though it was in their reading.

3/4) 5/5 got electron counting correcton the first try, in fact most used the electron count of the complexes in order to derive the electron count and CBC notation for NO.

5) Students generally struggled with how allyl binds to a metal (we did not fully go through the MO picture in class or in my office). 4/5 had to be led to the answer. Having the haptotropic shift diagram in the paper was helpful for discussion.

7)  3/5 students mentioned this specifically during their oral exam, 1 more discussed it with me during Q&Q

8) 3/5 students were able to reason out a mechanism on their own. 2 more fully understood it after a discussion.

9) 1/5 mentioned this explicity, but 5/5 were able to describe the isomers with some guidance or leading questions. The students felt the argument in the paper was hand wavey.

11) 4/5 students were able to explain this mechanism in detail; 1/5 needed some assistance to get to the right answer.

13) Students who had seen this in prior courses (2/5) understood this fine. 2/5 had not seen it before but were led to understanding through the conversation.

14) 2/5 students had difficulty with this question while 2/5 had no difficulty. 1/5 did not discuss it.

 

I also used portions of this LO as my final exam. I provided the students with the paper prior to the exam. I then gave them select parts of it to refer to on the exam (in particular structures and schemes). Overall the students really seemed to like this paper and having it on the exam (in total it was worth 40/180 points). My class only had 5 students, and 4 of the 5 made comments to me that they really enjoyed being able to read and understand a paper. They thought it build on what we had learned in class very well.

The one problem that they struggled with came as quite a surprise. They all got the Lewis structure of NO wrong. Four out of five drew the structure of NO+. I didn't expect this because they were taught to count using the CBC method and we never talked about the ionic method of treating nitrosyl ligands as NO+. This was particularly disappointing given the fact that I asked them to draw the Lewis structure and fill in an MO diagram for NO on consecutive questions. It should have been painfully evident that there is an unpaired electron in NO. The final student did correctly NO as a radical. Unfortunately, that student chose to add an unpaired electron to what otherwise would have been a good structure for NO+. Regrettably this left oxygen with an expanded octet.

The other question the students struggled with was one in which they were asked to identify which orbitals on the NO ligand were interacting with the tungsten based on the MO diagram. I knew they would have a hard time with this because we had not ever really looked at the bonding in an NO in quite this way. Four of the five students had correctly identified the linear NO ligand as an X3 (forgive the lack of a bar).  They all correctly identified that the SOMO would interact with the tungsten, but they all then thought it would be the two filled π orbitals and not one of the filled π and the empty π* orbitals. The logic behind converting LZ to X2 (which in the case of a linear nitrosyl is really converting LXZ to X3), was a little much for them to grasp on the exam. I think it would have made an interesting discussion in class. 

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