Friday, August 16, 2013

Catching Copper's Ghosts

Copper, copper, everywhere (and much more than you'd think). It's found in coins, wiring, statues, paints, and even as part of a balanced diet. Chemists, in particular, have long loved copper for its ready availability, well-defined redox states, and its wealth of reactions; just last week, Prof. Sherry Chemler (SUNY-Buffalo) recounted nearly 100 years of copper's catalytic successes* in a Science perspective.
Source: Ogle / Bertz group | Angew. Chem.

Though scientists have long studied copper-catalyzed reaction, several short-lived, unstable intermediates
have defied characterization. Now, Profs. Craig Ogle and Steven Bertz (UNC-Charlotte) may have caught one of these ghosts: an elusive C=O copper pi complex. Using rapid-injection techniques at -100 degrees C, the team "freezes out" the complex, which they study by 2D NMR (which shows relative positions of various atoms) and cryoloop X-ray crystallography (shows absolute position in a fixed crystal lattice).

When the team warms the compound much above -10 degrees C, it immediately falls apart.

Isolating otherwise reactive intermediates lets us peer inside** the "black box" of catalysis. In this structure, the lithium atom tugs at the oxygen's lone pair, allowing the copper to slip into pi-coordination in a "side-on" fashion. Though it's tough to see from this picture (left), the authors point out that five atoms (O, C, Cu, Me-a, Me-b) all sit together in one plane, which validates earlier NMR models. Finally, there's some hints of reactive fate here, as the "bottom" methyl group shortens up, preparing to jump off the copper atom and onto the central carbon, while at the same time, the copper atom cozies up to the oxygen. Remarkable stuff.

* And that was just on one class of reactions!
**The deeper we look, the more crazy, head-scratching stuff we find. Ask your local organometallic enthusiast for more info...

2 comments:

  1. This is a very cool structure. It makes me sad to think of the times I have trouble getting crystals of well behaved species when they get this tricky guy.

    I am a little confused as to why the Cu-C bond would shorten before reacting. Typically a bond would elongate on the reaction pathway to prepare to "jump off". These pi complexes are a little tricky sometimes and can be red herrings mechanistically.

    There's a famous paper (well, famous in my circles anyway) that surprisingly shows that aryl C-H activation by certain metal complexes can be hampered by pi complexation of the metal to the ring (i.e. the pi complex is an energetically favorable complex but not on the reaction pathway to C-H activation). This was surprising to many who assumed that such a pi complex accounted for the increased reactivity of the metal toward aryl C-H bonds.

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    1. Andre, I work in catalysis and am currently characterizing types of CH activation that my complex can do. Would you mind linking me to that famous paper you spoke of? It sounds interesting and would give me ammunition when arguing with my PI about which substrates to try.

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