Time to get down and get chemical today, with a look at “Rieke metals“. These are named after their discoverer, Rueben Rieke, who founded a company dedicated to their production and use in chemical building-block synthesis. In general, these are extremely fine particles of your desired metal, produced by chemical synthesis in a way that gives you both huge surface areas and “naked” metal surfaces that are largely free of oxide layers. That’s a recipe for very high reactivity, and indeed these reagents can do some really interesting reactions that are just not going to happen in any useful way using your handy jar of magnesium turnings or what have you.
Metal-surface heterogeneous reactions always have the aroma of witchcraft around them, anyway. There are a lot of variables that can turn out to have huge effects – particle size and surface area, of course, as mentioned, cleanness of the metal surface, definitely, but also things like particle shape: corners and ridges tend to contain more reactive metal atoms than flat planes do (they’re more naked out there). Then there are effects from tiny amounts of impurities in the metal sources themselves – several times it’s been discovered that trace amounts of nickel or other metals are crucial additives, and if you use extremely high-purity metals or salts things actually go awry. The choice of solvent can be critical, too, because some of them will complex with your desired metal-organic species and stabilize them, perhaps even too much in some cases.
The general chemical recipe for Rieke metals is to start with a chloride salt of your desired metal and reduce that with a more reactive metal like lithium, sodium, or potassium. You can do that at higher temperatures where the reactive metals themselves have melted under hot solvent, or at room temperature in the presence of an “electron carrier” like napthalene. If you want to go all the way, you can prepare something like lithium napthalide first and use that (in goodly amounts) to reduce the metal chloride, and that’s a reactive enough system to run at much lower temperatures. Roughly speaking, the lower the temperature of the method, the more reactive the resulting divided metal powders will be. They land in the bottom of the reaction vessel as dark powders, generally with the lithium/sodium/potassium chloride along with them in greater or lesser amounts, depending on temperature and solvent conditions.
And they are pretty lively! Rieke metals are not for weighing out on the balance pan on the bench, not unless you have been longing to start a fire over there. Even if they don’t ignite for some reason, you will surely be oxidizing their carefully prepared metal surfaces by exposing them to air, and what’s the point in that? No, you handle these guys under solvent as a slurry, or in a well-maintained glove box. Rieke zinc and magnesium are perhaps the most famous – you can use them to form organometallic reagents (such as Grignards) that are basically impossible to get with other types of Zn or Mg. But there are Rieke versions of copper, palladium, platinum, tungsten, nickel, calcium, barium, aluminum, indium, all kinds of stuff depending on your ever-changing moods.
You’d think that after forty or fifty years of work on these things that we’d know all about them, but here’s a new paper from a team at UC-Irvine that adds to the story. It’s been known for a long time that Rieke zinc prepared via lithium metal routes seems to be more reactive than that prepared via sodium metal routes. This has long been believed to be down to the lithium chloride and sodium chloride that end up in the solid reagents, but this new work – with lots of painstaking analytical chemistry behind it – does a thorough job of showing that that’s not the case. Weirdly and unexpectedly, it turns out that it’s the metal salts in the supernatant liquid that are behind these effects, specifically the more soluble lithium chloride. The paper shows that with sodium-formed Rieke particles that the organometallic species tend to sit on the surface and that these cannot be removed even with further solvent washing with something like THF (tetrahydrofuran, which is generally a powerful solvent for such species). The lithium-formed Rieke particles undergo the same general chemistry when making organometallics, but the LiCl in the supernatant changes the equilibria completely and solubilizes the organometallic reagents right off the zinc surface.
These salt effects not only change the kinetics of the further reactions with the metal particles, but they can change the ratios of the organometallic reagents that are formed (dialkyl zincs versus monoaklyl zinc halides, for example) There are experimental procedures that involve swapping the solvent after preparation of the reagent, which would wash out any soluble salts, and as the authors say, “Users of this methodology have presumably not known they were changing the structure of the resulting reagent. . .” This also means – now that we know what the heck is going on – that you can use these supernatant salts to your own advantage, adding extra lithium chloride, for example, to crank up the reactivity. This is a lot easier than trying to make subtle variations of the metal particles themselves, and anyone who’s using finely divided metal reagents is going to want to read this one immediately.
