Synthetic organic chemists spend a lot of time using organometallic coupling reactions, because they can be such great ways to make carbon-carbon (and carbon-heteroatom) bonds. And that’s the currency of the realm: do you want to build up larger molecules from smaller precursors in a controlled fashion? You’re going to have to make bonds when and where you want them to form.
But those organometallic reagents are tricky beasts, whether catalytic or stoichiometric. Just understanding what the actual structures of some of them are has occupied people for years on end, because a given system can adopt a whole range of structures – and thus form completely different reagents – depending on solvent, temperature, counterions, concentration, added salts, stoichiometry, order of addition, and so on. To make matters more interesting, some of the actual reactive species are formed only in small, transient amounts, which makes proving their existence even more fraught.
Here’s a recent example. One of the many things you can do with Grignard reagents, a classic organometallic species if ever there was, is to couple them onto vinyl bromides and haloalkenes in general, with iron catalysis. It turns out, though, that the yields for this are far better when the reaction is run in N-methylpyrrolidone (NMP) than in any other solvent (an effect discovered in 1998, over twenty years since the reaction was first reported). That one’s not exactly the first choice for this sort of thing, so it’s interesting to know that it works so well. But why?
It had already been figured out that if you take methylmagnesium bromide and an iron(III) salt in THF, that a new species forms: magnesium(penta-THF) chloride cation and a penta-iron-octa-methyl cluster anion. That was thought to be the active methyl-coupling reagent, but the yields are never that great in THF (which is probably what led to the solvent screening work that uncovered the NMP effect). Now we finally know what that works. In that new solvent, a completely different complex salt forms: magnesium hexa-NMP cation (+2 charge), with two trimethyliron anions.
Those irons are exceptionally reactive; you really don’t see alkyliron species without other stabilizing ligands on them very often, but for some reason you get them here. NMP is a pretty highly coordinating solvent, so in the absence of experimental evidence you’d figure that the solvent is in there on both the iron and magnesium centers. But it turns out that it’s just the latter (as suggested by magnetic circular dichroism spectra in solution, and Mössbauer spectroscopy along with X-ray diffraction on isolated crystals of the complex. Mössbauer is not the first technique you reach for (since you need an appropriate gamma-ray source) but it’s extremely sensitive to several different kinds of changes that affect the energy levels of sensitive nuclei, which are mostly metals.
So now we know, and another piece of knowledge gets added to the organometallic pile, and another idea for future reagents and reactions gets queued up. Eventually we’ll understand this stuff.