Synthesis and reactivity
Chapter 11 from George Stanley's organometallics course, Ligand Substitution
this chapter covers ligand substitution reactions.
The powerpoint slides contain answers to some of the in-class exercises, so those are behind the "faculty only" wall. I share these with students after the class, but not before.
Everyone is more than welcome to edit the materials to suit their own uses, and I would appreciate being notified of any mistakes that are found.
Students completed a full lab report for this activity, in which they described their results, including specific responses to the questions in the handout. This was evaluated using the rubric in the instructor notes. The report is graded out of a total of 50 points.
Overall, students did well. The grade range for most students is 40/50 to 50/50. This rubric and set of questions is the result of iterating a similar experiment using a different organic transformation.
Students had some unexpected proposals for the structural details that might explain catalytic activity (or lack thereof). In most cases, I was fine with these being incorrect (or less likely to be correct), if they carried this logic through their response to question 3. If the students propose a new ligand that will address their hypothesis, great! We can go in the lab and test these with the next group. If a student proposes a reasonably priced ligand, I generally buy it for next year's group to try out.
Question 4 (chirality) prompted some vague responses. I need a more detailed prompt and discussion about this point. I think it is relevant since the word "chirality" immediately grabs the attention of many organic chemists and students who are interested in organic aspects of catalysis.
If I am pressed for time to discuss TON/TOF in class, students will generally ask more questions about how to calculate these values. I may incorporate Sibrina Collins' activities about TON/TOF in future (see above links) to address these in greater detail in the lecture course.
This LO describes a laboratory experiment in which students generate (in situ) an iron catalyst for the arylation of alkyl halides (Kumada coupling). Students pool data from the class to discern what features lead to successful catalyst systems. GC-MS or GC-FID may be used to quantify the catalytic performance of each system, and results may be expressed as % conversion, with TON/TOF values. Students gain experience proposing reasonable coordination complexes that may be formed from the catalyst precursors, and searching the literature/databases for related compounds/systems. Students write a full report, and address the questions listed in the handout.
To explore the use of transition metal complexes as homogeneous catalysts for an organic transformation. As a group, to identify the possible influence of spectator ligands on catalytic performance by pooling data. Specific technical and educational objectives are:
- Students will be able to operate the inert atmosphere glovebox and block reactor to conduct a (mostly) air-free reaction
- Students will be able to analyze gas chromatographic (GC) data to determine the percent conversion, TON, and TOF for a catalytic system
- Students will be able to use data from in situ experiments to propose a structure for an active catalyst
- Students will be able to draw conclusions about the relationship between steric and electronic parameters of ligands and catalyst performance
GC-MS or GC-FID (other detectors may be used, but I have not quantified using these).
I run this using a vial in an Al heating block, but an oil or water bath would work just as well.
Reagents and solvents are listed in the instructor notes (with CAS numbers).
I created this experiment as a way to introduce catalysis without the need to rationally prepare new compounds for each group. I would like to use the wisdom of the group to find out what tweaks are needed to adapt this experiment to other institutions/courses. If you would like to conduct this experiment with your students, it would be great if we could compile data from your group using the Google Spreadsheet link embedded in the instructor notes. Ultimately, I would like to publish this experiment in JCE or similar if it is well received. Although this experiment has only been run once using this organic transformation, a previous version using a hydrosilylation reaction was successfully employed for five years previous to this.
In the lecture, students will have covered the formation of coordination complexes, as well as types of ligands, electron counting, etc. I use catalysis as a motivation and starting point for several of my discussions throughout the semester, so while they will not have discussed this explicitly, they will have all of the components mentioned in this experiment. Typically, I will discuss catalysis and turnover one or two lectures after the in lab portion of this experiment, so students are already grappling with TON/TOF calculations. I do not cover GC interpretation explicitly in lecture, since all of our students will have spent a little time on this in the organic chemistry majors lab. Most questions can be handled with an impromptu discussion about data interpretation.
DISCLAIMER - The nature of the experiment (novel catalysts generated in situ) leads to some unpredictability. If you (or your students) aren't up for an element of surprise in your catalytic data, this may not be for you. I pitch the experiment to the students this way, and offer to conduct a different (but probably less exciting) experiment if they are concerned about the potential for poor/no catalytic activity. So far no one has taken me up on this offer.
Some discussions questions can be taken out and used for exams, quizzes or problem sets.
The instructor can develop a rubric to evaluate these questions based on their needs.
Monitoring student discussions, or grading student written responses based on implementation.
The set of questions in this literature discussion activity is intended to engage students in reading and interpreting scientific literature and to develop a clear and coherent understanding of agostic interactions. The activity is based on a paper by Dorsey & Gabbai (2008). The paper describes agostic interactions in a silicon-based compound (R3C-H→SiFR3). The set of questions in this literature discussion activity is appropriate for an upper division course in inorganic chemistry. The research described in the article ties together concepts of agostic interactions and their impact on the coordination geometry of a Lewis acidic species. The discussion activity includes guided questions for students to understand and determine the presence of agostic interactions experimentally and through computational methods. The activity has specific questions related to bonding, structure, synthesis, characterization, theoretical and computational methods used in the literature. The activity may require reviewing some secondary sources.
Students will be able to..
Define an agostic interaction and relate it to other types of bonding.
Describe how the agostic interaction affects the coordination geometry of a Lewis acidic atom.
Provide examples of how the presence of an agostic interaction can be determined experimentally and through computational methods.
Differentiate between computational methods in terms of the information they can provide.
Find related sources of information to aid in comprehension of the concepts in the article.
This literature discussion was developed at the VIPEr 2017 workshop at Franklin and Marshall College so it has not yet been implemented. The authors believed that implementation of this article is best for an inorganic course that is post-organic, post-spectroscopy. It could be helpful after a discussion of 3-center 2-electron bonding and/or Lewis acidity/basicity. As with all lit. discussion LOs, this article also provides a valuable experience in reading the literature, including an interpretation and analysis of the experimental section. There are many questions included in this activity and instructors may want to pick and choose these questions and adapt it to their class.
Students could be evaluated based on their participation in the in-class discussion or on their submitted written answers to assigned questions.
This LO has not been used in a class at this point. Evaluation results will be uploaded as it is used (by Spring 2018 at the latest).
This paper describes the synthesis of a stable compound of sodium and helium at very high pressures. The paper uses computational methods to predict likely compounds with helium, then describe a synthetic protocol to make the thermodynamically favored Na2He compound. The compound has a fluorite structure and is an electride with the delocalization of 2e- into the structure.
This paper would be appropriate after discussion of solid state structures and band theory.
The questions are divided into categories and have a wide range of levels.
Dong, X.; Oganov, A. R.; Goncharov, A. F.; Stavrou, E.; Lobanov, S.; Saleh, G.; Qian, G.-R.; Zhu, Q.; Gatti, C.; Deringer, V. L.; et al. A stable compound of helium and sodium at high pressure. Nature Chemistry 2017, 9 (5), 440–445 DOI: 10.1038/nchem.2716.
After reading and discussing this paper, students will be able to
- Describe the solid state structure of a novel compound using their knowledge of unit cells and ionic crystals
- Apply band theory to a specific material
- Describe how XRD is used to determine solid state structure
- Describe the bonding in an electride structure
- Apply periodic trends to compare/explain reactivity
The questions are divided into categories (comprehensive questions, atomic and molecular properties, solid state structure, electronic structure and other topics) that may or may not be appropriate for your class. To cover all of the questions, you will probably need at least two class periods. Adapt the assignment as you see fit.
CrystalMaker software can be used to visualize the compound. ICE model kits can also be used to build the compound using the template for a Heusler alloy.
Evaluation methods are at the discretion of the instructor. For example, you may ask students to provide written answers to the questions, evaluate whether they participated in class discussion, or ask students to present their answers to specific questions to the class.
In this literature discussion, students use a paper from the literature to explore the synthesis, structure, characterization (powder XRD, EDS and TEM) and energetics associated with the production of a metastable wurtzite CoS phase. Students also are asked define key terms and acronyms used in the paper; identify the goal of the experiments and determine if the authors met their goal. They examine the fundamental concepts around the key crystal structures available.
Preserving Both Anion and Cation Sublattice Features during a Nanocrystal Cation-Exchange Reaction: Synthesis of a Metastable Wurtzite-Type CoS and MnS
Powell, A.E., Hodges J.M., Schaak, R.E. J. Am. Chem. Soc. 2016, 138, 471-474.
There is an in class activitiy specifically written for this paper.
In answering these questions, a student will be able to…
define important scientific terms and acronyms associated with the paper;
describe the rocksalt, NiAs, wurtzite, and zinc blende in terms of anion packing and cation coordination;
differentiate between the structure types described in the paper;
explain the difference between thermodynamically stable and metastable phases and relate it to a free energy diagram; and
describe the structural and composition information obtained from EDS, powder XRD, and TEM experiments.
This learning object was created at the 2017 IONiC Workshop on VIPEr and Literature Discussion. It has not yet been used in class.
This was developed after the semester in which I teach this material. I look forward to using it next fall and I hope to post some evaluation data at that point.
This literature discussion is based on a paper describing the ligand-based reductive elimination of a diphosphine from a thorium compound (Organometallics, 2017, ASAP). The thorium compound contains two bidentate NHC ligands providing an opportunity to discuss the coordination of these ligands. The ligand-based reduction is very subtle and would be challenging for students to pick up without some guidance. The compound undergoing reductive elimination also presents an excellent introduction into magnetic nonequivalence and virtual coupling. In addition, the compounds presented in this paper provide the opportunity to do electron counting on f-block compounds.
Upon completing this LO students should be able to
- Use the CBC method to count electrons in the thorium compounds in this paper
- Describe the bonding interaction between a metal and a NHC ligand
- Discuss magnetic nonequivalency and virtual coupling
- Describe ligand-based reductive elimination and rationalize how it occurs in this system