In the latest effort to activate and functionalize nitrogen to produce ammonia, assistant chemistry professor Paul J. Chirik and coworkers at Cornell University have found that tweaking the ligand in a soluble zirconium complex provides a valuable clue that may help chemists develop a homogeneous catalytic route for nitrogen fixation [Nature, 427, 527 (2004)].
Dozens of transition-metal N2 complexes have been reported over the years as chemists have tried--with little success--to develop an alternative route to the Haber-Bosch industrial process, which makes NH3 using high temperature and pressure.
The Cornell chemists began by reacting a zirconocene dichloride with N2 in an organic solvent using sodium amalgam as the reducing agent. This reaction formed a bridging bimetallic complex where the N2 is coordinated side-on between the metal atoms. When H2 is introduced, hydrogen atoms add to the N2 bridge to form a new complex. Upon heating to 85 °C under H2, this complex dissociates to form zirconocene dihydride and a small amount of NH3.
One twist to the chemistry is that NH3 forms when tetramethylated cyclopentadienyl ligands are used. When pentamethylated ligands are used, as shown by other researchers, N2 binds end-on to the zirconium metal centers (Zr–N=N–Zr). Upon further addition of H2, the end-on complex dissociates to zirconocene dihydride and liberates N2--a typical observation for metal-N2 complexes. This finding indicates that the tetramethyl ligands alter the coordination geometry of N2 in such a way to allow NH3 formation.
"While our reaction sequence isn't catalytic at this time, it may shed light on how to assemble N–H bonds under mild conditions," Chirik says.
Caltech assistant chemistry professor Jonas C. Peters calls the work "extremely fascinating," adding that it "reminds us all how seemingly subtle changes in ligand architecture can dramatically alter the course of a chemical reaction profile."