Main group chemistry

9 Jun 2019

Advanced Inorganic Chemistry

Submitted by Weiwei Xie, Louisiana State University

9 Jun 2019

Triphenylphosphine: Transformations of a Common Ligand

Submitted by Bradley Wile, Ohio Northern University

Evaluation Methods:

This lab report is graded using the attached rubric (see faculty files).

Evaluation Results:

Over the last four iterations of this lab, the average total score was ~42/50 (n = 21). Students are generally good at recognizing that a redox process is occurring, though some struggle with this realization. Most students generate a Lewis structure with a dative bond, though some do not use the MO diagram to infer a reasonable direction for the dative interaction. I typically work through this with the students, asking them questions like "which orbitals have electrons?" and "what orbitals are interacting in your Lewis depiction?" This has been a good introduction to these synthetic and instrumental methods, and gives the lab partners an opportunity to divide up their responsibilities.

Description:

This experiment tasks students with preparing triphenylphosphine sulfide, and the corresponding I₂ adduct, then characterizing these products using common instrumental methods. Students are asked to consider MOs and tie this to their Lewis bonding depiction for the final product. This discussion is supported by WebMO calculations and tied to the experimental data obtained by the student.

If you would like to use this lab, please complete the feedback form (faculty files) and let me know how you adapt it. I would like to publish this procedure (eventually), and I am open to collaborative projects to get this to the best final form.

Course Level:

Upper Division

Prerequisites:

Organic Chemistry

Topics Covered:

Main group chemistry

Learning Goals:

After completing this lab report, students should be able to:

Construct an MO diagram for I₂, and relate this to the bonding in the Ph₃PS-I₂ complex
Using MO theory as a basis, decide on the best Lewis representation for Ph₃PS-I₂
Discover the wealth of bonding modes within main group species
Identify changes in the observable spectra for P(III) and P(V) compounds
Search and reference the primary chemical literature using correct ACS reference formatting

Subdiscipline:

Main Group Chemistry

Corequisites:

No Corequisites

Equipment needs:

This experiment is run using our in house instumentation including:

NMR spectrometer capable of acquiring ¹H and ³¹P spectra
IR spectrometer
UV-vis spectrometer (we acquire data on a Spec200 that works just fine for this)
GC-MS (optional)

These spectra are provided as faculty files. If you do not have any of these capabilities, the spectra may be given to students as a handout.

Additionally, the experiment will require use of round-bottomed flasks, condensers, beakers, scintillation vials, hot plates, and gravity filtration apparatus (stemless if hot filtration required). Solvents and reagents are typically already present in the department, or may be purchased at reasonable cost.

Implementation Notes:

I use this lab as the first experiment of the semester, and begin the first week's activity after the introduction and lab safety discussion.

Prior to running the experiment, I prepare approximately one batch of each product (Ph₃PS and Ph₃PS-I₂) in case of a laboratory mishap. The products are indefinitely stable under ambient conditions.

I do not describe the reaction as a redox process, or suggest a bond order (i.e. I try to write the formula for Ph₃PS with an ambiguous bond order, as shown here).

Depending on the age of your bottle of Ph₃P, you may spot a small quantity of Ph₃P=O in the ³¹P spectrum (small peak around 30 ppm in the included spectrum). This may be an opportunity to discuss connections to biochemistry or atmospheric oxidation, or ask students to draw Lewis depictions of these species.

I teach my students how to manually run their own NMR spectra using TopSpin at this point (they have previously learned ¹H and ¹³C using the autosampler). I typically discuss the differences between ³¹P{¹H} and ³¹P (non decoupled) spectra at this time. Note that the lab handout has some instructions specific to the Bruker software that may be updated if you use a different spectrometer.

Literature articles describing the crystal structure of the final adduct (and related I₂ species) are linked here. I have not typically gone into great detail about this, as the assembled I₂ ribbons can confuse the students that are just putting the basic concepts together.

Time Required:

Two full 3 hour labs, and approximately 1 additional hour (first week). If characterization is done outside of normal lab hours, this could be accomplished in one full 3 hour lab and one additional hour.

Web Resources:

Crystal structure of the triphenylphosphine sulfide-iodine addition complex J. Phys. Chem. 1968,72(5), 1561-1565

A new assembly of diiodine molecules at the triphenylphosphine sulfide template J. Chem. Soc., Dalton Trans., 1999, 3069-3073

Crystal structure of triphenylphosphine sulfide diiodine J. Chem. Soc., Dalton Trans., 1998, 1289-1292