A 31P{1H} NMR Spectroscopic Study of Phosphorus-Donor Ligands

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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A 31P{1H} NMR Spectroscopic Study of Phosphorus-Donor Ligands and Their Transition Metal Complexes Ethan C. Cagle, Timothy R. Totsch, Mitzy A. Erdmann, and Gary M. Gray* Department of Chemistry, University of Alabama at Birmingham, Birmingham, Alabama 35294, United States S Supporting Information *

ABSTRACT: 31P{1H} nuclear magnetic resonance spectroscopy is a particularly useful tool for studying the reactions of P-donor ligands such as phosphines and phosphites with transition metals and other Lewis bases because the reactions take place on the nonbonding pair of electrons on the phosphorus. In addition, 31P has a 100% natural abundance and a high gyromagnetic ratio resulting in a high sensitivity that allows the spectra to be recorded using small amounts of sample. An activity that combines airsensitive synthesis of transition metal complexes of P-donor ligands and the characterization of these complexes with 31P{1H} NMR spectroscopy has been developed for an upper-division, laboratory-based inorganic course. This laboratory familiarizes students with this useful nucleus for NMR spectroscopy and allows them to study the factors that affect the 31P{1H} NMR chemical shifts of phosphorus-donor ligands and their transition metal complexes. Details of the activity, including procedure and pretest, are provided. Alternative methods and presentations are also proposed. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Problem Solving/Decision Making, Catalysis, Coordination Compounds, NMR Spectroscopy, Organometallics, Synthesis, Transition Elements

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addition, 31P{1H} NMR spectroscopy can be used to measure the purity of the ligands which is particularly important for phosphines, which are susceptible to oxidation, and phosphites, which are susceptible to hydrolysis. A laboratory activity that instructs students in the use and interpretation of 31P{1H} NMR spectroscopy and allows them to explore the effects of changes in the steric and electronic environment of the phosphorus nucleus on its 31P{1H} NMR chemical shift would expose students to basic approaches to catalyst characterization. In the laboratory activity described in this paper, students use 31P{1H} NMR spectroscopy to determine the chemical shifts of various P-donor ligands and their oxidation products. The ligands are then coordinated to two transition metal centers to determine the effects of metal coordination on the 31P{1H} NMR chemical shifts of the ligands. While this laboratory activity was initially developed for an Inorganic Chemistry Laboratory course, it could also be run in an Advanced Organic or Integrated Laboratory setting. This laboratory is novel in the range of P-donor ligands used and that the ligands are purposely oxidized so that students can determine the proficiency of their air-sensitive chemistry techniques.

INTRODUCTION Nuclear magnetic resonance (NMR) spectroscopy, a versatile method for tracking reaction progress and determining product identities, is widely used in research and industry. 31P{1H} NMR is a particularly useful tool for studying the reactions of P-donor ligands such as phosphines and phosphites with transition metals and other Lewis bases because the reactions involve the nonbonding pair of electrons on the phosphorus. In addition, the 31P isotope has a 100% natural abundance and a high gyromagnetic ratio allowing the spectra to be recorded using small amounts of sample. While many teaching laboratories have incorporated 31P{1H} NMR into their curriculum, the most recent examples are limited to the investigation of stereochemistry1 or of nucleophilic substitutions2 in organic chemistry laboratories. Those experiments designed to introduce 31P{1H} NMR spectroscopy in inorganic laboratories typically use expensive metals like platinum3−5 or ruthenium,6,7 coordinate organic ligands in addition to the Pdonor ligand,8−10 involve time-consuming syntheses,10,11 or use a narrow range of P-donor ligands.12−17 A major application for transition metal complexes of Pdonor ligands is as catalysts for many organic reactions used in research and industry.18 One method for tuning the properties of these catalysts is to vary the steric and electronic properties of the P-donor ligands in the catalysts. 31P{1H} NMR spectroscopy is often used to gain insight into the steric and electronic properties of the P-donor ligands in catalysts. In © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: September 11, 2017 Revised: March 12, 2018

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DOI: 10.1021/acs.jchemed.7b00673 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Scheme 1. General Reaction Scheme Depicting the Various Reactions That Take Place During This Experiment with 1 as the Starting Material

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temperature. The 31P{1H} NMR spectra are referenced to external 85% H3PO4 in a coaxial tube that also contains CDCl3.

SUMMARY OF PROCEDURE The pedagogic goal for this experiment, designed for upperlevel undergraduate inorganic chemistry students, is to become familiar with the basics of 31P{1H} NMR spectroscopy and to use this tool to understand how changes in the environment about the nucleus affect the 31P{1H} NMR chemical shift and 31 P−31P coupling. For the writeup of this laboratory activity, students use the combined results of all groups to produce a lab report or oral presentation.

Part I: Ligand Chemistry

Preparation of Ligand Sample. In this section, groups will prepare a pure sample of methyldiphenylphosphine (1) (0.050 mmol). To prepare the samples, all groups will flush both the sample bottle and a 5 mm NMR tube with nitrogen. The ligand sample is transferred to the NMR tube, and chloroform-d1 (0.6 mL) is added to the tube. The tube is capped, and then the tube is gently shaken. These samples are used to collect the 31P{1H} NMR spectrum of the ligand (Figure 1a). Oxidation of the Ligands. Two methods are employed to oxidize the ligands: reactions with (a) hydrogen peroxide or (b) elemental sulfur. The two products of these oxidations are depicted in Scheme 2. To prepare the oxidized samples, 1 (0.050 mmol) is dissolved in chloroform-d1 (1 mL) and allowed to react with the oxidizing agent (1 mL of 30% H2O2 or 0.010 g of S8). After 15 min, the reaction is complete: The organic (lower) layer of each reaction is passed through a pipet filter (which is prepared by placing a cotton plug in a disposable glass pipet and topping the plug with a small pad of Celite), the filtrate is collected in

Lab Experiment

Students work in groups of three to complete this 2-part activity. The groups are assigned letters A−D and assigned a specific laboratory procedure based on the group’s letter. Each group works with a different phosphorus-containing ligand for the majority of the laboratory. The experiment can be completed in either two 3 h laboratory periods or one 3 h lab period with modifications (SI). The workflow for the experiment is summarized in Scheme 1. For the sake of clarity, the experimental description will cover the procedure for one ligand. For other ligands used, please see the Additional Ligands section. NMR spectra are recorded on a Bruker DRX400 spectrometer. All solutions are prepared under a stream of N2 gas, and all NMR experiments are performed at room B




Figure 1. All 31P{1H} NMR spectra obtained by a student group. All other groups will perform a similar amount of work. Spectra are shown for the following: (a) pure 1, (b) 1 oxidized by H2O2, (c) cis-tetracarbonylmolybdenum(0) complex, (d) cis-tetracarbonylmolybdenum(0) (16.07 ppm) and trans-tetracarbonylmolybdenum(0) (29.12 ppm) complexes, and (e) cis-dichloropalladium(II) (26.32 ppm) and trans-dichloropalladium(II) (34.25 ppm) complexes.

Scheme 2. Reaction Scheme for the Oxidation of 1

Scheme 3. Reaction Scheme for the Complexation of 1 to Molybdenum and the HgCl2 Catalyzed Cis/Trans Isomerization

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Scheme 4. Reaction Scheme for the Complexation of 1 to Palladium and the Product Mixture of Cis and Trans Complexes

an NMR tube, and a 31P{1H} NMR spectrum is obtained (Figure 1b and Figure S6).

assigning each group a different ligand to use during the experiment and pooling their data at the end of the experiment, the students are exposed to the range of steric and electronic factors that influence the 31P{1H} NMR chemical shifts, and the coordination geometries of the transition metal complexes. The NMR spectra of the additional ligands are included in the Supporting Information.

Part II: Coordination Chemistry Preparation of cis-Mo(CO)4Lx and Its Isomerization Product

As this is an inorganic teaching laboratory, it is important to include ligand complexation to a transition metal center to determine coordination geometries and the potential binding modes (cis or trans) of the ligands. After obtaining the 31P{1H} NMR of the ligand sample, the group will add tetracarbonyl(norbornadiene)molybdenum(0) (0.045 mmol) to the NMR tube. The tube is shaken for several minutes, and a new 31 1 P{ H} NMR spectrum is obtained of the cis-tetracarbonylbis(methyldiphenylphosphine)molybdenum(0) complex (Figure 1c). After the spectrum is obtained, several crystals of mercury chloride are added to the tube, and the contents are shaken. The NMR tube is then sealed and wrapped in aluminum foil and stored until the next laboratory period. During this time, the complex forms an equilibrium mixture (Scheme 3) of cisand trans-tetracarbonylbis(methyldiphenylphosphine)molybdenum(0) complexes. The 31P{1H} spectrum is obtained for the mixture during the second week of the lab (Figure 1d). If the lab must be completed in one period, the isomerization portion can be omitted. Preparation of Pd(Cl)2Lx. During the second week of the laboratory, each group will prepare another free ligand sample using the procedure described in the Preparation of Ligand Sample section with the addition of a few drops of dry dimethyl sulfoxide to increase solubility. After the 31P{1H} NMR spectrum of the ligand sample is obtained (to ensure ligand purity), dichloro(1,5-cyclooctadiene)palladium(II) (0.052 mmol) is added to the NMR tube, and the tube is shaken for several minutes. The 31P{1H} NMR spectrum of the newly formed palladium complex (Scheme 4) is then obtained (Figure 1e).

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HAZARDS All chemicals should be handled in the fume hood. Eye protection and gloves should be worn at all times, and care should be taken to avoid contact with the material. All chemicals must be handled as stated in the information available on their safety data sheets. Chloroform-d1 is toxic and is a cancer suspect agent. Hydrogen peroxide (30%) is a strong oxidizer and harmful if it is swallowed or inhaled, or if it comes in contact with the skin. Compounds 1, 2, and 3 are eye, skin, and mucus membrane irritants. The toxicities of tetracarbonyl(norbornadiene)molybdenum(0) and dichloro(1,5cyclooctadiene)palladium(II) are not known.

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LIMITATIONS AND ALTERNATE PRESENTATION It is understood by the authors that not all universities have access to NMR spectrometers with multinuclear probes. If this is the case, to implement this lab, the authors recommend utilizing partnerships with other institutions that have the needed instrumentation. Further, advances in technology have allowed for dedicated 31P or tunable probes in benchtop NMR units. If needed, the authors can provide the appropriate spectra upon request.

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RESULTS AND DISCUSSION The results of these experiments have been highly reproducible over two sections of 15−24 students each year during the past five years. All students are able to complete the experiment in a single 3 h laboratory period, with exception of the isomerization step, and the synthesis and 31P{1H} NMR spectroscopic characterization of the dichloropalladium complex. Each year, students were able to produce and isolate the oxidized phosphorus species as well as show proficiency with airsensitive techniques by producing pure samples of the ligands.

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ADDITIONAL LIGANDS Other ligands included in this experiment include the following: 1,2-bis(diphenylphosphino)ethane (2), triethylphosphite (3), and a 1:1 mixture of 1 and 3. Ligand 2 was chosen due to its electronic similarities to 1, as both contain two aryl groups and an alkyl group on the phosphorus. However, 2, unlike 1, is chelating, which will allow students to see the effect that this has on both the 31P{1H} chemical shifts of the free and coordinated ligands and the coordination geometry of the ligand to metal centers. Compound 3 was chosen specifically for its electronic properties, being both a better π-acceptor and a poorer σ-donor than 1. The introduction of 3 introduces students to the wide range of chemical shifts that are possible in 31 1 P{ H} NMR spectroscopy. The 1:1 mixture of 1 and 3 allows students to probe the relative reactivity of the two ligands and provides a more in-depth look into the steric and electronic factors affecting the equilibrium between the cis and trans isomers of the tetracarbonylmolybdenum(0) complexes. By

Analysis

This experiment introduces upper-level undergraduate inorganic chemistry students to 31P{1H} NMR spectroscopy as an alternative to 13C{1H} and 1H NMR spectroscopy for reaction monitoring and product characterization. Students used the combined results of all student groups (Figure 1 shows the results of one student group) to produce their lab report or oral presentation. The data was used to compare the effects of varying the phosphorus substituents on the signal observed in the 31P{1H} NMR spectrum, as well as D




electronic and steric variations on the chemical shift of the P resonance.

the effects of ligand oxidation. These comparisons are summarized in Figure 2. Students were able to determine if

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SUMMARY This laboratory activity taught students the importance of understanding how changes around an NMR active nucleus can affect the chemical shift. This was achieved using the 31P{1H} NMR spectra of common phosphorus-containing ligands and of their organometallic complexes with tetracarbonylmolybdenum(0) and dichloropalladium(II). Students were able to use these spectra to determine the purity of the ligand sample, the potential species present in the sample based on chemical shifts, the ability of a complex to isomerize, and the different shifts caused by ligand coordination to a metal center and chalcogens. This lab led to an increase in the students’ perceived confidence to assess and interpret 31P{1H} NMR spectra while also increasing their assessed knowledge.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00673. Detailed instructor notes on preparing for and carrying out this activity, the experimental outline provided to students in the laboratory manual, and the pretest (PDF, DOCX)

Figure 2. Possible comparisons students could make on the basis of the experimental results.

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any oxidized ligand is present in the molybdenum complex as well as the palladium complex. Also, students saw a mixture of peaks in the 1:1 mixture of 3 and 1 representing cisMo(CO)4(3)2, cis-Mo(CO)4(1)2, and Mo(CO)4(1)(3); integrations were used to determine the product ratios. Further, students were able to analyze the composition of the isomerization products from the mercury(II) chloride isomerizations. Using integrations, students determined the mixture of cis and trans products of the molybdenum complexes and determined the properties required for a ligand to be able to undergo isomerization. The students also observed that there was no isomerization in the Mo(CO)4(2) complex because the ligand was unable to span the trans positions of the complex. The pedagogic goal for this experiment was for students to become familiar with the basics of 31P{1H} NMR spectroscopy and to become familiar with how changes in the environment about an NMR active nucleus affect the NMR resonance. Achievement of this goal was assessed using a student selfassessment and test in a pre/post-test format (SI). Over two laboratory sections (n = 41 students), 0% of students felt prepared to interpret 31P{1H} NMR in the pretest, compared to 70% of students who felt prepared on the post-test. This increase in perceived learning is paired with improved performance from the pretest to the post-test. On the pretest, students averaged 35% correct on questions related to 31P{1H} NMR; this percentage increased significantly to 60% on the post-test. These confidence and learning gain trends also followed for 1H and 13C{1H} NMR; however, they were not as significant as the 31P{1H} NMR results. There is still room for improvement of the post-test scores. As the electronic and steric contributions to the 31P{1H} NMR resonance are significantly intertwined, in the future, the authors intend to pair this laboratory activity with an X-ray crystallographic activity. The pairing of these two activities will allow the students to better understand the contributions of both

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ethan C. Cagle: 0000-0003-1235-6074 Mitzy A. Erdmann: 0000-0001-9090-8730 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge the support of the NSF Cooperative Agreement EPS-1158862. Special thanks go directly to Nancy Abney, Julia Austin, and Kelly Carter for teaching the CIRTL courses in which this activity was developed. E.C.C. would also like to thank NSF Alabama EPSCoR for the Graduate Research Scholars Program (GRSP) award and the UAB Department of Chemistry for support of this project.

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A 31P{1H} NMR Spectroscopic Study of Phosphorus-Donor Ligands

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