OK, this one’s a fairly easy procedure, which you will find way back in Volume 6 of the ever-entertaining Inorganic Syntheses. You’ll need a tube furnace, though, and you may or may not have one of those sitting around. You’ll also need to soften up the glass tube that goes into it, enough to make a couple of fairly deep indentations on the sides. Maybe you can buy tubes like that, but I believe that I had to make my own back in 1982 as an undergraduate, when I last did this preparation myself. I haven’t checked the catalogs, because I have absolutely no intention of ever doing this again.
At any rate, those shoulders in the tube are for holding a pile of magnesium turnings, because this is a prep of the magnesium analog of ferrocene, magnecene. Ferrocene is, of course, a perfectly well-behaved compound (and an interesting story in itself, which I included in The Chemistry Book), but its magnesium cousin is not so much at peace with itself. It reacts with oxygen, it reacts with moisture, it reacts with carbon dioxide – all kinds of stuff. I believe it’s a good starting material for preparing all sorts of other metal cyclopentadienyl sandwich compounds, because almost anything would rather be in there than the magnesium atom. In this decadent era you can buy it from Aldrich, but where’s the fun in that?
Now, turn your tube furnace on the side so it’s vertical. I would recommend using something non-combustible for the needed support rather than, say, old lab-supply catalogs or cans of acetone. You’ll need to rig up some sort of connection to a flask or receiver on the bottom end, with a venting stopcock or something, because there will be a steady flow of nitrogen through the whole shebang. That flow is coming in from the top, where you’ll need to rig up another connection to the head of a dicyclopentadiene cracking still. “Ah-hah!” say the chemists in the crowd, who have now grasped the Spartan simplicity of this reaction. Two cyclopentadienes, one magnesium – what could be more atom-efficient? Of course, cyclopentadiene doesn’t exist as such in the bottle – it Diels-Alders with itself to the dimer, but as any synthetic organic chemist knows, it reverts on heating and distillation, and the vapors at the top of the still are cyclopentadiene monomer. So’s the distillate, for that matter, but that’s a limited-time offer and you’d better work fast. In this case, though, a flow of nitrogen gas will sweep the pure vapor over the magnesium turnings, and there you have it.
Oh, there’s one other little detail. This doesn’t exactly happen at room temperature, thus all that fiddling with the tube furnace. The bolus of Mg metal needs to be up around 500 to 600 degrees. Make sure that your thermostat is working, because Pyrex will start to goop up on you if you go over 800, and the next person who wants to use the tube furnace, assuming there’s a next person, assuming that there’s still a tube furnace, will be very unhappy with you. Assuming that you’re still around as well. And make sure that your apparatus is reasonably gas-tight, because if oxygen gets in there with those five-hundred-degrees magnesium turnings you’re going to kick off a very bright fireworks display. If at any point a blinding white light starts shining out the top and bottom of the tube, probably to the accompaniment of interesting noises, something has gone wrong and it will indeed be one of those days.
But if you don’t set off a magnesium flare, then you should produce a pile of thin white flaky crystals in the receiver flask. The magnecene is still in the vapor phase in the hot zone of the tube, but sublimes back out once things return to what passes for normal. For this reason, you probably also want to have a short path out the bottom of the apparatus, for fear of clogging the tube with product. I remember thinking a bit about what to do in that case – I mean, it’s not like I was going to be able to ram a bent coathanger through the thing – and I decided that I should have a heat gun handy to resublime the product along if needed.
Not being stocked with fancy glassware, I held everything together in my setup with big honking pieces of Tygon tubing. The result did not exactly look like a glassware advertisement, although I suppose it could have been a Tygon advertisement, but it did the job. Actually, it did even more of a job than I was planning on. The first time I ran this reaction – I was only planning on one time, but you know how it goes – something happened to the gas flow, and while frantically messing around with the line, I seem to have caused things to go in reverse. My tip-off to that was looking up at the big Tygon elbow at the top of the apparatus, where the tube furnace part was connected over to the still. With some consternation, I noticed that this was no longer a big clear piece of plastic tubing, but was now, for some reason, black. And bubbling. It occurred to me, after a few seconds of intense puzzlement that I would enjoy having seen captured on video, that the wad of red-hot magnesium turnings had been propelled back up the tube and was now stuck inside the Tygon elbow part. I’m pretty sure that I said something vulgar and shameful at this point, as who the hell wouldn’t, because I had visions of the magnesium chewing its way through the obviously hard-pressed plastic tubing and giving me just the sort of Fourth of July experience I’d been trying to avoid.
The tubing held, though, and should the makers of Tygon wish me to provide a testimonial, I’m more than happy to deliver. After waiting for a bit to make sure of things, I decided that this was a good time to let the whole rig come back to room temperature under a healing stream of nitrogen, at which point I would remount and reload. I adjourned for a few hours, but I still remember the blackened, crackling lump of fused magnesio-gunk that I cleaned out in preparation for another (successful) run.