Chemistry 407

Inorganic Synthesis and Properties

dichloro bis(ethylenediamine) Cobalt(III) chloride

 revised: August 5, 2002, created October 22, 2000
file called ../ch407/cobalt_remarks.htm<>
During the first two weeks of this experiment we ran into a number of difficulties. Some parts were successful, but overall we didn't get as far as we wanted.

Phase 1-- preparation of the green salt

trans- dichloro bis(ethylenediamine) Cobalt(III) chloride hydrochloride

The initial stages of this experiment work reasonably well. We see a strong color reaction as the Co+2 reacts with two equivalents of ethylenediamine, probably forming the Co(en)2(H2O)22+ complex ion, oxidation state +2. This then is slowly oxidized by air to form the Co(III) complex. These complexes are both characterized as dark reddish brown color.

The next stage involves displacing the water with two chloride ions. The source is an excess of HCl . The reaction is encouraged by heating and by evaporation to concentrate the solution. I assume that the reaction mixture contains both cis and trans forms of the complex, but the trans form has lower solubility so it becomes the solid product. This also means that the green color of the new product is largely masked by the brownish cis species.

This evaporation stage was not very successful.
Probably, we scaled up the preparation without providing for a comparable scaling up of the evaporation process. I'm guessing that this stage would be better accomplished in an evaporating dish or shallow glass container like a petri dish. A beaker is less effective since the interior fills with moist air and this decreases the evaporation rate. This provides more surface for evaporation. A gentle stream of air blowing across the liquid would also help but removing moist air. Placing the system near the front of a hood would guarantee a reasonable air flow. (Unless we could carefully filter the compressed air it's not a good idea to use compressed air from the building lines. That air supply always contains traces of oil.) If you use an evaporating dish it might be good to set it into a shallow dish of sand on the hot plate so that most of the bottom is in good thermal contact. The temperature controller unit may also be desirable with the probe inserted into the sand. We want to avoid boiling this mixture. If available, rigging a steam bath would provide good heating and guarantee no overheating.

Still, some of you collected modest samples of a brilliant green compound. This is the hydrochloride salt (containing one HCl of crystallization.) The direction say you can remove this by heating the sample in a drying oven for about an hour. For one group at least, this led to an unpleasant surprise. It appears that the sample melts and/or decomposes at moderately low temperatures (say 150oC.) The drying oven in the lab is normally used for glassware, so we are not very careful about the internal temperature. You need to monitor the oven temperature and turn back the temperature control as necessary. The directions in Inorganic Syntheses, vol 2 say that 110oC is satisfactory.

forming the cis-

The next stage is the isomerization of trans to cis by heating in aqueous solution. This was generally successful. The green starting material forms a green solution. As the solution evaporates, it's color lightens and shifts to a pale purple. A crust of purple crystals forms and this is, according to the directions, the cis compound. Harvesting is a little awkward since you need to scrape and chip the product from the glass and the yield is modest. Before I was taken from the scene, I don't know that anyone collected enough material at this stage to go on to the final stage.

Isolating optical isomers

The final stage was the separation of the two optical isomers of the cis- complex using crystallization with an optically active tartrate salt. Did anyone get this far? Does anyone have any material or results?

Follow Up Stage for Optical Isomers

I'm trying to locate a commercial source for the cis complex or to do a large scale synthesis. Perhaps after mid-term break we can spend an afternoon focusing on this final stage of the synthesis at a scale large enough to isolate product. (see calculation below.) We can then check optical rotation, borrowing the polarimeter in the organic lab or by rigging one of our own using crossed polarizers..

Characterizing Optical Isomers

The observed Optical Rotation generally is proportional to concentration and proportional to the length of the polarimeter tube. You often examine optical rotation with concentrated sugar solutions in an organic lab. (Perhaps 1-2 Molar solutions, but the sugar is cheap.) However, the conventional polarimeter needs 50-100 ml of solution to fill the tube and probably needs at least 0.5M to measure any significant optical rotation. This suggests we need to collect about 50 millimoles of the species (and start with 100 millimoles of the racemic mixture.) The formula weight of our Co complex is 321 so we need to start with at least 3 grams if we use conventional polarimeter tubes.

Characteristic Spectra

One other aspect of this experiment was characterization of the several synthesized forms. For strongly colored compounds, visible spectra is an obvious way to characterize the material. The green trans- species has a distinctive color and spectrum. The pink-purple cis form is clearly different. Last year we observed that trans- solutions, when allowed to stand for 24 hr., almost complete isomerized to the cis. It would be desirable for one group to try to follow this process by recording a spectrum at the start of a period and at the end of the period and making arrangements to record another spectrum the following morning and perhaps afternoon.. An alternative might be to try to observe spectral changes over 3 hr., keeping the sample at an elevated temperature (I'd guess 50oC would be a start.) Dr. DenBesten can show you a constant temperature bath-- there's one in the lab and there's one by the HP8452a spectrometer.

IR SPectra

(See the first reference at bottom of this document.)

We would also like to characterize the compounds by IR spectrophometry. Clearly we don't do IR with aqueous solutions and these ionic compounds are not likely to dissolve in a typical IR solvent. The simple approach is to make a KBr pellet. A few milligrams of sample is ground with about 1 gram of KBr solid. This sample is then places in a metal die and is pressed to form a pellet. The IR of the pellet is recorded.

Sidebar-- remarks on KBr Pellets and IR Spectra

It's worth taking a brief detour to discuss why a KBr pellet works. Most salts appear white because they reflect and scatter light. (If certain wavelength are actually absorbed, the reflected light will appear to have the complementary color.) Even a thin layer of powder stops all light and would appear opaque.

Each tiny crystal is probably completely transparent. However, as light leaves air and enters a denser material, about 5-10% of the light is reflect and the rest is transmitted. After light has encountered a few time crystals, most has been multiply reflected and very little is transmitted directly through the sample.

One way to greatly reduce the %-reflection is to reduce the differences in the index of refraction between the crystal and the surroundings. When we form a KBr pellet, we replace the air with crystalline KBr which has a refractive index quite close to that of our samples. This, only a small fraction of light is reflected and the rest is transmitted. It also helps if the particle size is comparable to the wavelength of light (in this case IR light at perhaps 5 micrometers.) The choice of KBr is partly because the crystal is easily deformed, so a high pressure can force it to uniformly fill the disk with a continuous, crystal-like piece of KBr.

You can grind the KBr and sample with a mortar and pestle. We also have a mechanical grinder that uses a ball bearing in a metal capsule. The organic labs use a simple screw type die which you tighten with a large wrench. Better quality disks are possible using a multi-part die and a hydraulic press. These specialized facilities are available on the fourth floor of PSLB is the IR room. Either IR (organic lab or 4th floor) will produce adequate spectra for these compounds. I'll make arrangements to get IR samples when we return from mid term break.

An alternative is to record te spectum of a Nujol mull, sandwiched between two NaCl plates. You grind a few mg. of the sample in a mortar, with a few drops of mineral oil (nujol.) Again, this places the crystals in a medium of comparable refractive index, so we greatly reduce the reflections. Of course, you must avoid interpreting any spectral lines associated with the Nujol itself.

Added August 2002-- ATR Spectra In late spring 2002 we purchased a new IR for the organic labs and it included an ATR sampling device. ATR= Attenuated Total Reflection. This allows much simpler ways of getting spectra of solids. Details later, but will use this technique and compare with KBr spectra.

What's the Status of this Experiment?

If you have completed the experiment, prepare a basic report. Tell what you did, what results you had, what you observed and what was unsuccessful. If you have spectra, include them in your report. (If you didn't get the spectra, do you have a chance to run them yet?) Keep an electronic copy.

Where the report is incomplete, we'll try to get the missing elements completed after the break. I'd then like you to go back and incorporate the new material into the ultimate report.

References (copies available in the lab) Comment: The third reference generally supplied air for 10-12 hours at room temperature. It noted that reaction can be accelerated with charcoal (as a catalyst.) Other references use H2O2 for the oxidation.

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