Phosphazenes
Monophosphazenes are a group of compounds of the general formula R3P=NR' where R and R' can be a variety of alkyl or substituted alkyl groups. Diphosphazenes have a general formula R3P=N-PR'2 and polyphosphazenes are polymers that can be described as R3P=N-(PR2=N)n-PR2. Polyphosphazenes are easily derivatized and have found use as biomedical, flame-resistant, sound-insulating, solvent-resistant, elastomeric as well as electrically conducting materials.Cyclic phosphazenes are quite common and are useful precursors to polyphosphazenes. Cyclic phosphazenes can be made by a variety of methods. Consider the synthesis of the cyclic triphosphazene below:
There are four important features about this triphosphazene to notice:
- As in benzene, the P=N bonds are not localized.
- The halides may be substituted (before or after polymerization) using Grignard reagents or other nucleophiles to afford a virtually limitless number of derivatives.
- These species lack hydrogens so we can not use 1H NMR to characterize them.
- This particular synthesis is a bit tricky and takes longer than our usual lab period.
In order to utilize an efficient and reliable synthesis and provide you with a material that can be characterized by 1H NMR as well as other nuclei, we will synthesize an N-substituted cyclodiphosphazane rather than a phosphazene. 1,1,1,3,3,3-hexachloro-2,4,-diisobutylcyclodiphosphaz(V)ane will be prepared according to the following unbalanced equation: 
Notice that this is similar to a cyclodiphosphazene, but lacks P=N bonds because of the additional substituents on P and N.
PCl5 and the product are moisture-sensitive solids that will decompose in moist air and evolve HCl fumes. Therefore, we will need to handle these solids in an inert atmosphere glovebox. We will also utilize a Schlenk line because the reaction mixture must be refluxed under a nitrogen atmosphere during the synthesis.
Multinuclear NMR Spectroscopy
We are all familiar with the concept of NMR spectroscopy from our organic and analytical laboratory courses. Until now, your experience has probably been limited to 1H and maybe 13C NMR. There are actually dozens of other nuclei that are also amenable to study using Nuclear Magnetic Resonance. Part of the joy of Inorganic Chemistry is that we need not limit ourselves to C and H in either syntheses or spectroscopy!NMR spectra of nuclei with a spin quantum number, I, of 1/2 are easiest for us to understand because they obey the same kind of coupling behavior we see for the most common I = 1/2 nucleus, 1H. Spins with I = 1 to 9/2 are also amenable to study and their interpretation is not all that much more difficult as you shall see in class.
Just because a nucleus has a non-zero spin does not automatically mean that we can obtain an NMR spectrum of that nucleus. There are at least four other factors we must consider:
- Isotopic Abundance. You can't see it if it isn't there. Some spin active nuclei such as 19F are 100% abundant (1H is 99.985%), but others such as 17O have such a low abundance (0.037%) that we can't expect to get much of a signal unless we isotopically enrich the sample. Consider: 13C is only 1.1% abundant -- that's one of the reasons we need to use a lot more sample and take more scans to obtain a 13C spectrum versus a 1H spectrum.
- Sensitivity. This is typically scaled to 1H = 1.0. There is only one (rather uncommon) isotope with a sensitivity >1.0; all other isotopes have sensitivities <1.0. The lower the sensititivity the greater the amount of time and sample it will take to get a signal. Some sensitivities are so low, such as 103Rh (100% abundant but only 0.000031 sensitivity), that obtaining a spectrum for the nucleus is generally impractical. However, the nucleus can still couple to other spin-active nuclei and provide useful information provided it has good abundance. In the case of rhodium, 103Rh coupling is easily observed in the 1H and 13C spectra and the JRhX can often be used to assign structures.
- Nuclear quadrapole. For spins greater than 1/2, the nuclear quadropole moment is usually larger and the line widths may become excessively large. This can sometimes be overcome by running the sample at low temperature.
- Relaxation time. Two factors govern the rate at which the excited nucleus relaxes its spin. Spin-lattice relaxation, T1, is the time it takes for a nucleus to relax. If the spin population has not re-equilibrated before we pulse again, then we will not have measured the full signal. Spin-spin relaxation, T2, refers to how long it takes for a set of aligned nuclei to lose their phase coherence. T1 is usually much greater than T2, so we normally only concern ourselves with T1.
The combination of these four factors governs whether a given nucleus will give a useful NMR spectrum. Some of the more common I = 1/2 nuclei used by chemists are:| Nucleus | Natural Abundance | Relative Sensitivity |
|---|
| 1H | 99.985 | 1.0 |
| 13C | 1.108 | 0.016 |
| 19F | 100 | 0.83 |
| 31P | 100 | 0.07 |
There are other nuclei that are readily used, but you will be asked to examine these in a problem set.
The most important aspect of multinuclear NMR is that all spin active nuclei can couple to each other and that the multiplicity of the coupling is given by 2nI + 1 where n = the number of equivalent nuclei that are being coupled to. For example, if a proton is adjacent to two equivalent protons, the resonance will appear as a triplet because 2nI + 1 = 2(2)(1/2) + 1 = 3. If the two adjacent nuclei were fluorines instead of hydrogens the resonance would still be triplet because I = 1/2 for flourine and 19F is 100% abundant. The only difference in these two cases would be the magnitude of the coupling constants, JHH versus JHF.
It is strongly recommended that you review NMR coupling from your previous courses before you attempt to analyze your NMR spectra. Either NMR book in the Reading List for this experiment should be a good refresher and make you more comfortable with the topic.
Decoupling
When we obtain an NMR spectrum all the spin active nuclei in the system will couple to each other. For example, a methyl group appears as a quartet in a 13C spectrum because both 13C and 1H have I = 1/2 and the coupling follows the 2nI + 1 rule described previously. For a standard 13C spectrum we call this condition proton-coupled although some use the perverse term, "undecoupled".We can suppress this coupling by using an electronic decoupler to selectively remove the coupling from a specific kind of nucleus during our data acquisition. Thus, if we proton decouple and take a 13C spectrum of our methyl group it would appear as a singlet because we've suppressed the JCH. The advantage of this technique is that we can simplify crowded spectra and reduce the amount of time required to obtain a good signal. The drawback is that we lose information about how many protons are attached to our carbon atom.
In this experiment you will obtain a very simple coupled and decoupled 31P spectrum to gain an appreciation for how decoupling works. We will then take this a step further and perform some 2-dimensional NMR experiments.
We are not limited to the heteronuclear decoupling experiments. We can also perform selective homonuclear decoupling. For example, suppose our 1H NMR spectrum has a multiplet that we suspect is coupled to a doublet. We can irradiate the multiplet with our homonuclear decoupler while collecting our NMR spectrum. If the doublet collapses to a singlet upon decoupling, then the multiplet had to have been coupled to the doublet. If not, then we know the doublet is not coupled to the multiplet.
Homonuclear decoupling is extremely easy to do and requires no more time than taking a regular NMR spectrum. For this reason it is a technique that should be in the repertoire of any synthetic chemist.
Two Dimensional NMR Spectra
For simple compounds it is usually easy to determine which NMR resonances (nuclei) are coupled to others and thereby establish the connectivity of the atoms. When the spectrum is complicated by overlapping resonances, has several couplings with the same magnitude or has non-first order behavior this task is much more difficult.A way around this difficulty is called COSY which is derived from the term Correlation Spectroscopy. The most common version of this is called H,H-COSY, meaning that the experiment is dealing with proton-proton (homonuclear) couplings. COSY spectra are obtained by examining coherence transfer, i.e. the transfer of magnetization between coupled spins, but the math behind it need not concern us.
The COSY experiment generates a square graph containing the normal one-dimensional proton NMR spectrum along each axis (dimension). Any nuclei that are coupled will generate a peak at the intersection of their positions on the graph. Since all nuclei correlate to themselves, the diagonal of the plot therefore contains the 1-D spectrum. Any cross peaks lying to either side of this diagonal indicate coupled nuclei. Since JAB and JBA are identical, the plot will be symmetric about the diagonal.
By examining the cross peaks and simply extrapolating to the x and y axis, one can quickly tell which peaks are coupled. You can find a short description of the basis for COSY as well as some examples in the Required Reading for the NMR portion of this experiment (pp 160-163 of Abraham et. al.).
This technique can also be applied to look for heteronuclear coupling, the most common of which is H,C-COSY. In more general usage this method is called HETCOR for Heteronuclear Correlation Spectroscopy. HETCOR works on the same principles as COSY and the interpretation of the spectra is similar except that the diagonal is no longer observed. HETCOR is a useful way of determining, for example, which carbon resonance corresponds to which proton resonance. You should look at the examples on pages 163-167 of Abraham before collecting your data in this experiment.
Equipment and Chemicals required
- Glove box with balance (discuss plans with the other lab team)
- A magnetic stir plate and ring stand.
- One 100 mL Schlenk flask.
- One Schlenk line in a fume hood (both teams use the same line).
- One condenser.
- One nitrogen inlet adapter.
- One 60 mL addition funnel.
- Heating mantle and cord.
- A Variac transformer.
- Carbon tetrachloride (provided).
- Dry, distilled isobutyl amine (provided).
- Phosphorous pentachloride, PCl5 (will be in the glovebox with a balance).
- Stirbars, spatulas etc.
General Considerations
- CAUTION: Carbon tetrachloride is a suspected carcinogen. Phosphorous pentachloride and isobutyl amine are severe eye, skin and mucous membrane irritants. All materials used in this experiment should be handled in a fume hood.
- In an effort to more closely simulate "real" research, you will only be told the amount of one starting material to use. You must determine the correct amount of the remaining reagents based on the unbalanced chemical reactions provided in the Introduction (i.e. you need to balance the reaction and then calculate the stoichiometric amounts required). DO THIS BEFORE YOU COME TO LAB. Preparation is a good lab technique, and lab technique is part of your grade in this course.
NMR study, Day 1
NOTICE: You are going to do this experiment "backward". On Day 1 you will start your synthesis and be provided with a sample to so you can start your NMR studies right away. While one team works on the NMR, the other can set up their synthesis. On Day 2 you will work up your product and obtain additional characterization data. If you prefer to analyze your own sample on Day 2, then you will probably require more than 2 lab periods.Prepare an NMR sample of your phosphazane (you can do this in air if you work quickly) by placing approximately 25 mgs of the material (don't bother to weigh it) in a clean, dry NMR tube. Add enough C6D6 (dried immediately prior to use by passage through a short plug of freshly dried alumina in a pipette) to the tube so that the liquid level is approximately 2.5 inches high. Cap the tube and 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 proceeding.
1H NMR spectrum
Log onto the instrument and type jexp1 to switch to the first experiment workspace. Obtain a 1H NMR spectrum of your compound (don't forget the integrals) and print it out. Note: Reference your spectrum by setting the residual C6D5H signal to 7.15 ppm. To do this, place a cursor over the center of the peak and type nl rl(7.15p). Plot your spectrum and make sure you get a line list that includes all the peaks (you will need this to determine the coupling constants).You will probably want to print out enlargements of some of the smaller peaks. You can do these on separate sheets or ask your TA how to plot an inset spectrum on your original one.
H,H-COSY NMR spectrum
Print out the COSY instruction sheet from the Departmental NMR page. Type jexp2 to switch to the second experiment workspace and then type mp(1,2) to import your data parameters from the first experiment. Set the number of transients in your proton experiment to 4 (i.e. nt=4) and narrow your window to include only the peaks that belong to your compound as described in the instruction sheet. Collect your data and print out the 2D spectrum.Phosphorous NMR spectra
Type jexp3 to switch to the third experiment workspace. To run a 31P NMR spectrum we have to tune the probe to phosphorous. There is an instruction sheet for this available on the Departmental NMR page. Do not UNDER ANY CIRCUMSTANCES attempt to tune the probe without the direct supervision of an instructor or John Layton.Obtain from your instructor an NMR tube that contains a sealed capillary of 85% phosphoric acid suspended in C6D6. Phosphoric acid is the standard reference material for 31P just as tetramethylsilane is for 1H NMR. We could place the capillary in your tube (as an "internal external" standard), but we might not be certain which peak comes from your standard and which from your sample. To get around this we'll run a (truly) external standard.
Eject your sample and place the tube containing the phosphoric acid standard in the probe. There is no need to reshim the instrument, but make sure the sample is locked. Obtain a 31P NMR spectrum of the standard following the directions on the instruction sheet but type sw=14000 tof=-3000 wshim='n' before acquiring your data (this will make sure the spectral window is wide enough to include the peak(s) we'll be looking for). Also set nt=4 since your signal will be easily observed. Place a cursor on the phosphoric acid resonance and type nl rl(0p) to set the reference to zero. There is no need to print this spectrum. Eject the tube, replace it with your sample and obtain your spectrum after setting nt=32. Print it out, making sure you get a line list.
Finally, run a proton-coupled 31P spectrum by typing dm='nnn' su ga. Again, print this out and make sure you get a line list.
H,P-HETCOR spectrum
Obtain a proton-decoupled 31P spectrum (type dm='yyy' nt=4 su ga). Leave the probe tuned to phosphorous and then, following the instructions on the HETCOR instruction sheet (available at the Departmental NMR page), obtain a HETCOR spectrum. Print out the result.Optional: Homonuclear decoupling
This is rather fun and interesting. It will provide you with somewhat redundant information, but is so easy it is worth trying if you have the time. Type jexp1 to switch to the first experiment workspace (where you originally obtained your proton spectrum). Follow the (short and easy) instructions on the Homonuclear decoupling instruction sheet at Departmental NMR page to selectively decouple your peaks one at a time. Print out each spectrum, obtaining a line list for each.WHEN YOU ARE FINISHED
Be sure that the probe is tuned back to 13C when you are finished (again, ask your instructor...DO NOT try this yourself).
Synthesis Procedure, Day 1
In the metal glovebox place a 1/2 inch stirbar, 4.0 g of PCl5 and 40 mL of CCl4 in a 100 mL Schlenk flask. Place a greased stopper 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. Check that the sidearm is closed and bring the flask out of the box. HINT: if you stand the flask up in a beaker it won't tip over while you are doing all this.
Outside the box, place a nitrogen inlet adapter on the top of a 60-125 mL pressure-equalizing addition funnel and attach a rubber hose from the Schlenk line to the nitrogen inlet adapter. Open the inlet adapter to the bubbler and purge the funnel with nitrogen for a few minutes. Grease the bottom joint of the funnel and then, under counterflow, place the funnel on the flask containing the PCl5 solution. As all the components are already purged with nitrogen, it is not necessary to purge the system after assembly (otherwise you would simply evaporate most of your solvent).
Remove the nitrogen inlet adapter from the funnel under counterflow and replace it with a rubber septum. Double check that the stopcock on the addition funnel is closed and that the system is open to the bubbler. Transfer 25 mL of CCl4 into the addition funnel using a cannula or syringe and then add the required amount of isobutylamine (you must calculate this!) using a syringe. Add the isobutyl amine/CCl4 solution to the stirring phosphorous pentachloride solution dropwise over a period of 10 minutes. Note any changes or observations (or lack thereof).
Once the addition is complete and the reaction has subsided, place a nitrogen inlet adapter on the top of a 19/22 condenser and purge it with nitrogen for a few minutes. Under counterflow, remove the now-empty addition funnel and replace it with the condenser. Turn the nitrogen flow down to a slow bubble and close the sidearm on the Schlenk flask (why?). Be sure the system is open to the bubbler through the condenser.
Connect Tygon tubing to the condenser inlet (bottom...always have cooling water flow into the condenser through the nipple closest to the heat source) and outlet. Connect the inlet hose to a cold water tap and make sure the outlet hose is firmly positioned in a sink. Wire the three hose connections using heavy gauge copper wire or special tubing clips (ask your instructor for a demonstration of good wiring). This ensures that we won't pop a hose during the night and flood the laboratory (and have our reaction evaporate and/or catch fire).
Place a 100 mL heating mantle under the flask and plug the mantle into a Variac (Caution: NEVER plug a heating mantle directly into an outlet!). Plug the Variac into an electrical outlet, set the initial heat at about 10-15% scale and turn it on. Adjust the heat as necessary to obtain a gentle reflux (approximately 65 degrees C). Place a clear and informative label on your reaction and let it reflux under nitrogen overnight. Your TA will turn it off the next day.
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.Recrystallize your material under nitrogen from a minimal volume of hot toluene. Let the hot solution cool and collect the crystals on a frit. While you can collect the crystals in air if you work quickly, it is preferable to do this in the glovebox (use the metal glovebox since it is the only one equipped with a vacum connection). Check with your instructor to make sure that the stopper is properly fastened before attempting to pump your flask into the box.
Dry your crystals under vacuum by simply placing a large rubber stopper over the top of your frit (don't jam it into the frit) and placing the vacuum hose on the bottom of the frit (don't jam it on too hard). Place your product in a properly-labeled vial and store it in the drybox.
As your compound is air-sensitive, you will need to obtain the melting point using a sealed glass capillary. Load your melting point capillary with sample in the glovebox. Squish some silicone grease into the open end of the capillary to temporarily seal it and then bring it out of the box. Seal the end of the capillary with a glassblowing torch at a point below the grease plug and then obtain your melting point measurement.
Make several capillaries so you can take one or two of them to the Mass Spectrometry Facility (located in the basement of the ASTeCC building) to get an EI-MS. You need about 1 mg of sample or less in the capillary for this purpose.
Spectral assignments and analysis, post-laboratory
In addition to reporting on the synthesis and characterization of your product, be sure to fully explain the NMR spectra of your compound. This means assigning all connectivities and coupling constants and discussing whether these assignments are consistent with the proposed structure(s).