CHE 450G: Practical Inorganic Chemistry

U of KY Dept of Chemistry

Synthesis and VT-NMR of Cp2TiS5

Introduction and Background

This is an integrated synthesis and measurements experiment. In the first part, you will synthesize the title compound using inert atmosphere techniques. In the second part, you will use variable temperature 1H NMR spectroscopy to study the dynamic nature of this compound and use computer analysis to extract the activation parameters of the exchange process.

Organometallic polysulfide complexes

Sulfur is remarkable for its variety of morphologies, forming linear and cyclic Sn species in both the elemental and ionic forms. While S8 is the most stable form of sulfur, rings containing 6-12, 18 and 20 are known. All of these are unstable with respect to n=8 at STP.

The ionic open-chain forms of these rings, Sn2- are called polysulfides or, when attached to transition metals, polysulfide ligands. A variety of such species have been prepared, including Cp2MoS3, Cp2HfS4 and Cp2TiS5 (Cp = cyclopentadienyl). More extensive background information on polysulfide complexes can be found in the required reading for this experiment and references therein.

In this experiment we will synthesize and characterize the polysulfide complex, Cp2TiS5. This complex contains two cyclopentadienyl rings; you may find out more about the bonding and nature of Cp rings by perusing Angelici Experiments 15 and 16 or any standard Inorganic textbook such as those on reserve.

The synthesis of Cp2TiS5 involves a two step "one pot" reaction. The following unbalanced equations detail the chemistry involved:

some chemical equations

The materials used in this synthesis are sensitive to air and water. In fact, the lithium reagant can spontaneously inflame in the presence of air or water. Therefore, it will be necessary to use a Schlenk flask and Schlenk line to perform this synthesis.

The structure of Cp2TiS5 is reminiscent of cyclohexane. In the structures below, notice that the S5 unit is closer to one cyclopentadienyl ring than the other. This creates two chemically and magnetically inequivalent Cp rings which appear as separate signals in the 1H NMR spectrum.

However, just to confuse matters a bit (and make this experiment more interesting), Cp2TiS5 undergoes a chair-chair rearrangement which effectively switches the polysulfide ligand from one side of the molecule to the other. As far as our NMR spectrometer is concerned, the two Cp rings have exchanged positions (even though they did not actually move). We've labeled the sulfurs to show you that this process is not merely rotation of the entire molecule by 180 degrees, but an inversion of the ring:

the structure of Cp2TiS5

Note: The Cp rings themselves freely rotate about the Ti-ring centroid with an exceedingly low energy barrier. Therefore, all five protons on the same ring act chemically equivalent even though some may be closer to the S5 in the static picture shown above.

This chair-chair inversion exchange process is slow at room temperature, but can be be conveniently studied over the range 20-120 degrees C using variable temperature 1H NMR.

Dynamic Exchange Processes

Classical kinetics can often be used to determine the rate constant and activation energy of a chemical reaction. In a typical study, changes in concentration of products and/or reactants versus time are monitored using any number of experimental techniques (IR, NMR and UV-VIS are the most common). This method works well for reactions that take place on the laboratory timescale (minutes to hours) where the rate constants for the reactions are typically 10-6 to 10-3 s-1.

This analysis becomes more complicated when we have to consider reversible reactions or systems that are at equilibrium. For example:

  1. If we are interested in the energy barrier to interconversion between two isomers but the two isomers can not be resolved or separated, then we can't use this approach (since their concentrations would be constant with time).

  2. If the rate of the reaction is very fast, we'd have an equilibrium mixture before we could even obtain the first measurement.

The prototypical example of such a system is the axial-equatorial interconversion of the chair form of cyclohexane. At room temperature the axial and equatorial protons are interchanged by a dynamic (fluxional) process in which the ring undergoes a "chair-chair" conformation change. As shown here, Ha and Hb are interchanged between axial and equatorial positions:

a chair-chair conformation change

To understand why this complicates our analysis remember that in the 1H NMR experiment we irradiate the protons to flip their nuclear spins and then wait as they give off this excess energy. The energy (frequency) of this relaxation is what we more commonly call the chemical shift of our proton. It takes time for our protons to relax to their nuclear ground states and this relaxation is governed by both the spin-lattice, T1, and spin-spin, T2, relaxation time.

Imagine that we irradiate a proton while it is in the equatorial position. Under normal circumstances, the proton would relax and we would detect it at a chemical shift characteristic of equatorial protons. However, if the molecule rearranges so that this proton is in the axial position when it relaxes, the chemical shift would be consistent with an axial proton.

To prevent exchange from occuring, all we have to do is cool the sample to a sufficiently low temperature. At -90 degrees C, the axial and equatorial protons of cyclohexane no longer interchange and are resolved as two separate resonances. But as we raise the temperature, the two peaks move together and broaden, indicating that there is some exchange (often called "slow exchange"). When the two peaks merge such that there is no distinguishable valley between them we say that the peaks have coalesced. As we raise the temperature still more the merged broad peak sharpens again. At this point, the lifetime of a species as axial or equatorial is much shorter than the timescale of the experiment (flipping the nuclear spin and observing the relaxation). We call such a system "fast on the NMR timescale" or the "high temperature limit".

cyclohexane VT-NMR

Figure 1. VT-NMR spectrum of cyclohexane-d11 (See Kegley, page 21). Note: All peaks are singlets instead of doublets because JH-D is small and 11 of the 12 protons are deuterated. In a non-deuterated sample, each peak would be a doublet with a Jax-eq of approximately 13 Hz.

In the background reading for this experiment you will find the equations that describe this chemical exchange behavior and how exchange leads to line broadening. Using these equations, and knowing the difference in frequency between the peaks at the slow exchange limit, we can extract the rate constants at each temperature directly from the observed peak shapes. These rate constants can then be used to calculate the activation parameters for the chair-chair interconversion using the Arrhenius and Eyring equations. The net result will be an understanding of the dynamic inversion process in Cp2TiS5.

Required Reading

Read the following BEFORE performing the synthesis part of the experiment:

  1. Errington, section 3.4. (syringe and cannula techniques)
  2. Aldrich Technical Bulletin AL-134, Handling Air-Sensitive Reagents.
  3. Aldrich Technical Bulletin AL-164, Handling Pyrophoric Reagents.
  4. Shaver, A.; McCall, J. M.; Marmolejo, G. Inorg. Synth.1990, 27, 59-65.
  5. Anything hyperlinked to the synthesis (selected readings from the Glassware Gallery).

Read the following BEFORE performing the NMR experiments:

  1. Kegley, S.E.; Pinhas, A. R. Problems and Solutions in Organometallic Chemistry, University Science Books: Mill Valley, California, 1986, pp 20-26.
  2. Gasparro, F. P.; Kolodny, N. H. J. Chem. Ed. 1977, 4, 258-261.

Optional but useful information (for motivated students):

  1. Cotton and Wilkinson, Chapter 13 (all you ever wanted to know about S, Se, Te, Po and more).
  2. Collman, Hegedus, Norton and Finke, pp 164-175 (Cp complexes).

Synthesis and Characterization

Equipment and Chemicals required

General Considerations

Synthesis Procedure, Day 1

This procedure has been adapted from Shaver, A.; McCall, J. M.; Marmolejo, G. Inorg. Synth. 1990, 27, 59-65.

Use a ring stand and clamp to position a 100 mL Schlenk flask above a magnetic stir plate. Place 0.250 g of elemental sulfur and a 1/2 inch magnetic stir bar in the Schlenk flask. Grease the lower joint of a 60 mL pressure-equalizing addition funnel and place it on the flask. Don't overdo the grease; use just enough to cover the top half of the joint but make sure there are no streaks. This will reduce the amount of grease that leaches into the reaction mixture.

Connect the Schlenk flask to the Schlenk line, and purge the apparatus with nitrogen for 5-10 minutes. Be sure the stopcock on the funnel is in the open position during the purge. Then, making sure the system is open to a bubbler, place a septum over the top of the addition funnel. Turn the nitrogen flow down to a bubble or two every few seconds.

Close the stopcock on the addition funnel and then make sure an instructor is around to watch you for the next step. Following the techniques outlined in Errington and the Technical Bulletins, add a stoichiometric amount (you calculate it!) of Super-Hydride to the addition funnel. Clean out the syringe needle immediately. Add the Super-Hydride solution to the sulfur dropwise. The resulting reaction is exothermic and effervescent (why?). When you are finished, be sure to close the stopcock on the addition funnel.

Place the required stoichiometric amount of Cp2TiCl2 (you calculate it!) and a 1/2 inch stirbar into a separate 100 mL Schlenk flask and then purge the flask with nitrogen for 5-10 minutes. Place a septum cap on the flask and then add approximately 30 mL of dry THF via cannula. Once the solid has completely dissolved, use a cannula to transfer this solution into the addition funnel on the flask containing the Super-Hydride/sulfur reaction mixture. Clean your cannula as soon as this step is complete.

Using the addition funnel, add the Cp2TiCl2 solution dropwise to the lithium polysulfide solution over a period of approximately 15 minutes. Let the reaction solution stir under nitrogen until the next lab period. Be sure to properly label your reaction.

Synthesis Procedure, Day 2

Remove the solvent from the reaction mixture using the Schlenk line. Remember to use a pre-trap to collect the solvent. Extract the solid residue with dichloromethane. Prepare a fritted funnel with a 1/2 inch pad of Celite which is damp with dichloromethane and then filter your extract through the Celite pad. Collect the filtrate and reduce it to dryness on a rotary evaporator. The (slightly smelly) product should be pure enough to use without recrystallization. Be sure to obtain a melting point and mass spectrum in addition to the required NMR data.

NMR study, Day 2

Due to the time required for this portion of the experiment, both lab teams may work together to collect one set of NMR data. If each team would like to collect their own data, talk to your instructors about arranging additional NMR time outside the regular class period.

Prepare an NMR sample of your Cp2TiS5 by placing approximately 3- 5 mg (don't bother to weigh it) in a clean, dry NMR tube. Add enough toluene-d8 to the tube so that the liquid level is approximately 2.5 inches high and cap the tube. Shake well to ensure a homogeneous solution. Be sure all the solid has dissolved; if not then consult your instructor.

Important: Please be sure you have done the appropriate background readings and read the NMR operating instructions before proceding.

  1. Log onto the instrument, obtain a room temperature NMR spectrum of your compound (don't forget the integrals) and print it out. Note: Reference your spectrum by setting the residual toluene methyl signal to 2.09 ppm. To do this, place a cursor over the center of the peak (which is a 5 line pattern with a very small JHD) and type rl(2.09p).

  2. Set up the spectrometer for high T operation by switching from air to nitrogen as the spinning gas. DO NOT DO THIS YOURSELF as you could contaminate the nitrogen supply for the entire building. Ask your TA or the person in charge of the NMR lab (John Layton) to assist you.

  3. Collect spectra at 45, 55, 65, 75, 85, 90, 95, 100, 110 and 120 degrees Celsius. The total run should take approximately 2.5 hours. To make efficient use of your lab time, one member of your lab team may can leave periodically to prepare other samples for the melting point and mass spectrum, but do not leave the spectrometer unattended.

  4. The NMR spectrometer has several "experiment spaces" that permit you to work on data from one experiment while another is running. To save time, you'll want to switch between experiment 1 and experiment 2 using the commands jexp1 and jexp2 during the run. To perform a run simply:

    1. Type temp='xx' su where xx is the temperature (in degrees C) that you want and then hit return. The temperature is displayed on the front console of the instrument. Once the desired temperature is achieved wait 5-10 minutes to allow for complete thermal equilibration. You'll find that the most efficient way to work is to set up the new temperature in one experiment and work up the data in the other experiment while the sample is thermally equilibrating.

    2. Once the sample has equilibrated, type ga and your spectrum will automatically be displayed. You can print it if you wish, but this is only necessary for the low T, coalescence and high T spectra.

    3. Save your spectrum by typing SVF('xxxx') where xxxx is a descriptive Unix-compliant (no spaces, slashes or other strange characters) filename that contains your intials and the temperature such as rt_75c. Write down the file name in your notebook for future reference! Now is a good time to switch to the other experiment, set up the next T for equilibration and then return to this experiment to work up your data (unless this is your first run, of course).

    4. Expand the spectrum so that only the two Cp peaks are on the screen. Make sure your expansion is wide enough to include a good baseline for the calculation.

    5. Type mark('reset'). Place a cursor on your first peak, type nl and hit return to center the cursor on the peak. Then press the Mark button to record its exact position for the lineshape calculation. Repeat for the second peak (if you have one).

    6. Click on the Main Menu, Analyze , Deconvolute, Use Mark, Fit and Result buttons. This will generate a listing of the calculated line shape and position for each peak. Record these numbers in your notebook. For the 80 degree run only, hit the Plot button to print out the peak and the calculated fit.

    7. Repeat.

  5. When you are finished with the spectrometer, type temp='n' su. This will turn off the VT heater. Once the probe temperature has been below 50 degrees C for five minutes, ask your TA or John Layton to switch the spinner from nitrogen to air.

    Approximation of Rate Constant from Linewidths, Post-Laboratory

    Using the method described in Gasparro and/or Kegley and the information you recorded in your notebook, generate an Arrhenius and Eyring plot from your data and determine the activation energy, Ea as well as delta G, H and S of activation for the inversion process. Be sure to report experimental uncertainties in your numbers! Comment on how these findings are consistent or inconsistent with possible mechanisms for the interconversion.

    Remember to discuss the synthesis and characterization in your lab report as well.

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