Saturday, May 19, 2007

You want the Chemistry? ... You can't handle the Chemistry!

OK I guess I need to provide some detail. I was a graduate student in an inorganic fluorine chemistry research group. We were investigating Main Group compounds that had inorganic pi systems created by oxidizing electron rich molecules using Arsenic pentafluoride in liquid sulphur dioxide.

The graduate student ahead of me discovered reaction 1. S4N4 is a contact explosive prepared from one of those classic fire from God reactions that were popular in the late 19th century. So you prepare the explosive and then grind it to a fine powder (grind gently because I was in a lab in England where a small beaker of the stuff spontaneously exploded and split a lab bench from end to end). You put the fine powder and some sulphur in a vessel with lots of sulphur dioxide as solvent and then add lashings of AsF5. About a week of recrystallizations later gets you to SNSAsF6 which contains the dithianotronium cation [SNS]+ (think ONO+).

Now SNS+ is a lot of fun but I discovered rxn 2. It turns out that SNS+ does not react like ONO+. SNS+ is a propargyl-allenyl 1,3-dipole favouring reverse electron demand cycloaddition reactions with pi bonds. Thus the reaction with alkynes gives the derivatives of CAT in high yield.

The hero of this story is the 4,5-bis(trifluoromethyl)-1,3,2-dithiazolium cation. I discovered that the reduction of this cation by sodium dithionite in liquid sulphur dioxide gives the blue gas radical in reasonable yield but the reaction works the best because the products are the radical, insoluble NaAsF6 and the solvent SO2. Laborious fractional distillation of the reaction volatiles give the black liquid referred to in the previous post. And I always wondered why the preparation wasn't repeated more often.
Thanks for the interest in the chemistry.

Friday, May 18, 2007

My Small Contribution to Compounds with Pretty Colours

It was the summer of 1984. I was in the first year of my graduate work and I was cleaning up a project initiated by a previous graduate student. I had come along at the end of his run and picked up the SNS+ reactions with pi bonds and had started with alkynes. I had prepared a variety of compounds using the SNS+ / alkyne reaction and they had worked well. My supervisor decided that I should go to England to learn how to do cyclic voltametry in liquid sulphur dioxide. That brought me into the sphere of a fragile, dedicated chemist of the collegial old school. It was in Durham that I was trying to do anion exchange reactions to get rid of the fluorinated anions so that the cyclic voltametry would work better (in fact the CV never did "work"). One day, late in the afternoon, I would wash a solution of the cation into a solution of potassium iodide and to my wondering eyes the whole flask filled with a pale blue gas. In my small area I knew all that had come before me and I knew that I had discovered something new. I was seeing something important for the first time and it was important and new not only for me but for my whole discipline. My real research career had begun.

It is still unique in my area of chemistry.

The product of the reaction was a radical, traditionally an unstable, reactive family of compounds with only a fleeting existence. In this case the heterocycle was stabilized by extensive pi delocalization and the carbon atoms were shielded and electronically changed by attachment to electronegative, kinetically stable CF3 groups. This also meant that the melting point of 12 degrees C resulted in a liquid with a significant vapour pressure at room temperature. This resulted in the blue gas.

I remember when I did the melting point. I had laboriously prepared the cation salt and reduced with huge excess of sodium dithionite and then fractionally distilled the volatile products three times to remove solvent and bi-products (which I would discover later to be the result of facile photolysis reactions). Then when I had grams of pure dark green black liquid I froze it and slowly warmed the solid in a Dewar until I reached the melting point. What
I was not ready for was the high volatility of the solid near its melting point and soon I had huge shiny blue-black crystals growing perpendicular from the side of the flask. They were altogether some of the most beautiful things that I had ever seen. I would spend a whole year of my life preparing and characterizing this compound. The last problem to fall was the precipitation of the photolytic impurities using a method I had stumbled across in an early JACS paper written by Speed Marvel in which he noted that he had precipitated polysulphide impurities with a catalytic addition of amine. I found a whisper of triethyl amine did the trick just fine.

And that is my coloured compound story.

About Me

My Photo
For a while it was all about research and then it was all about teaching and now it's all about trying to find a balance while teaching at a small liberal arts and science university.