Redox Champ: NHC Ipso, Facto

Bond density difference between electron-rich and -poor NHCs
Source: Cavallo, JACS  ASAP

On the surface, N-heterocyclic carbenes (NHCs) seem like dream ligands. They have strong, neutral dative electron pairs, steric bulk, and enough variants now exist that chemists can "dial in" chirality, control R-group rotation, or add them as salts.
Could we possibly improve them any further?

A new JACS ASAP answers: of course! Professor Luigi Cavallo, and KAUST / U. de Salerno coworkers, like to examine NHC ligands computationally. Using Gaussian '09 molecular modeling software, the authors toss in a variety of electron-donating (ex: NMe2, Me) or withdrawing (ex: Br, NO2) substituents on the aryl rings of mock NHC ligands. They then virtually "staple" them onto Grubbs, Grela, and Ir-COD catalysts, and calculate the redox potentials of the Ru and Ir metal centers.

These data trend like you might expect: electron-withdrawing groups on the NHC increase the redox potential, while donating groups reduce it. Well, what's driving these effects? The authors initially suspect σ/π donicity (that's a $5.00 word, right there), the ability of the NHC-M bond to directly influence metal properties. However, they note something odd: there's precious little change in the bond lengths between complexes, only 0.005-0.05Å; by contrast, in carbon-carbon bonding, moving from alkane to alkene shaves off 0.2Å! Thus, they suspect that the NHCs' influence isn't coming through their main bond.

Well, if it's not electronics, then what's changing these redox properties? Enter the 'ipso effect,' a long-observed, yet little-invoked, mechanism for ligand-to-metal charge transfer. The arene carbon directly connected to the heteroatom rubs up against the metal d orbitals, and charge can move through space. Perhaps the best known non-NHC systems to exhibit this effect are the highly-active Buchwald biaryl phosphines, which show ipso interactions with palladium and gold, among other metals. Cavallo and coworkers compare electron density maps (the trippy blue-and-red models, above R) between cationic and neutral metal states, and note a huge red splotch of π-to-d donation (see d) from the NHC orbitals to the metal.

NHC electronics lower intermediate energies
Source: Cavallo, JACS ASAP

But wait, there's more! The authors apply this model to previous observations in Ru metathesis chemistry, then extend the metaphor and tackle Pd biaryl coupling. As shown in the graphic (left), they calculate a ~3 kcal / mol difference for the first catalytic intermediate, depending on the electronic environment of the NHC used. For a throwaway closing line, they let us in on a final trade secret: this effect could be used to stabilize lots of high-valent metal species...do I hear any calls for nickel (IV) chemistry???

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