Counterion Conundrum

(For Popkin's great Science News writeup, click here)
(Update - Also, Chemistry World!)

Update (11/13/13) - Blog buddy Lila Guterman of Science News has the inside scoop:
"An answer! Authors via @gabrielpopkin: cerium(III)chloride heptahydrate (99.9% pure) from Sigma (cat. nr. 228931-25G)"

~~

Fascinating news for the inorganic biochemistry fans out there: Scientists have ID'd a bacterium (Methylacidiphilum fumariolicum) living in highly acidic volcanic pools that seems to use rare earth metals in one of its enzymes. A multinational team modeled the enzyme with a variety of rare earth cores, and the bacterium appeared to selectively take them up in cell culture. Cool!

Just one small problem: What's the counterion?!?

My crack online reporting team has scoured the manuscript, finding only mentions of a mysterious Ce(III), along with triply-oxidized La* and Pr. Nowhere in the Supporting Information do they mention reagents used, and the reporter has confirmed that this subject wasn't broached.

Given the other salts the researchers added to the media, it's likely that they used either cerium (III) sulfate or cerium (III) chloride. However, Sigma-Aldrich sells no less than 10 different cerium (III) salts (Strem has over 20!), and I'm willing to bet they have markedly different bioavailability, oxidation potential, aqueous solubility...the works.

Readers, does anyone know what the cerium source is in this paper?** I certainly don't wish to draw unwarranted conclusions, but we're all still touchy over another recent dust-up having to do with a miraculous trace element.

Please let me know in the comments.

*IUPAC police: Throughout the paper, the authors refer to "Ln ions." Do you suppose they mean La (lanthanum)? Are elements abbreviated differently in other places?

**Interestingly, the authors note that their acidic growth media leached trace rare earths out of the glassware. I've never looked at that as a reaction contaminant, but I guess I'll have to start!

Rare Earths, Common Problem

Rare earth elements have made quite a stir lately: just last month, both Discover and National Geographic have written full articles about these 17 unique metals, which comprise the top part of the periodic table “f-block” (plus scandium and yttrium). Pundits and scientists alike are anxious that the US won’t be able to compete in the high-tech sector with scarce domestic rare earth supply.

Discover’s Hugh Aldersey-Williams (July / Aug 2011, p. 62) takes the historic view, starting from the elements’ first discovery in Ytterby, Sweden (1787, yttrium) and wending through the myriad of uses for the rare earths in modern-day electronics, hybrid cars, lighting, and materials.  The NatGeo article (June 2011, p. 136) takes a decidedly more polemically charged stance, peeking over the fence at China’s 97% share of the world rare earths market. Reporter Tim Folger argues that China’s unmatched mining infrastructure, coupled with lax environmental restrictions and cheap labor, make it tough for US miners to compete, despite the importance of a regular supply – the world demand for technology items such as iPods, wind turbines, flatscreens, and military equipment may drive lanthanide demand to a projected 185,000 tons by 2015, of which the US can only account for 5,000 tons of production. Worse, Folger bases this estimate on the production of a single mine (Molycorp) in California.

So what’s the impact for synthetic chemists?

Many of our favorite reactions use these metals. Lanthanum and scandium triflate promote aldols, acetylation, imine addition, cyclopropanation, and guest-star in new reagents like Leighton’s “EZ-crotyl” (JACS 2011, 6517). Samarium diiodide, a 1-electron reducing agent with a penchant for carbonyls and halides, underlies the Evans-Tischenko and Barbier couplings.  Cerium ammonium nitrate (CAN), a stable, off-the-shelf oxidizer, plucks off TBS and PMB groups, and promotes oxidative fragmentations. Perhaps even more worrisome is that the cerium and samarium reactions usually use the metal-containing reagent in large excess.

How can we fix the problem? New labs might find themselves conducting cost-benefit analyses simply to see if the improved reactivity or selectivity offered by these metals is worth their increased price (the Hoveyda-Grubbs 2^nd-gen catalyst, a highly active precious metal catalyst based on still-rarer ruthenium, runs $671 USD / 2g).  Perhaps the NSF will step in to issue challenge grants to develop catalytic processes intended to wean us from rare earth excesses. Either way, we’ve got to figure it out soon; as the US has shifted to a service-based economy, we’ve lost many skilled laborers (steel workers, miners, heavy industry) and may not be able to increase our rare earth capacity quickly enough.

Updates (July 28, 8:15PM) - Ever-helpful editor @carmendrahl informs me of a fantastic rare earth cover story from C&EN.  Others showed me the WIRED post about the US stealth fleet.

(July 30, 7:40AM) - Here's a July 2011 story in Scientific American debating the potential for harvesting rare earths from ocean floor sediment. Says Duke researcher Cindy Van Dover: "Four thousand meters in the deep ocean is a long way down"

(August 27, 10:36PM) - Commenter gippgig refers to Science News cover story, see here