The functionalization of atmospheric nitrogen (N2) to more value-added compounds, particularly those with nitrogen-carbon bonds, has been the area of longest standing interest in our laboratory. The project was initiated in 2001 motivated by a single concept – namely the synthesis of group 4 metallocene complexes with side-on dinitrogen ligands. Using a simple Lauher-Hoffmann molecular orbital rationale, we reasoned that these compounds, by virtue of four-electron N2 reduction, would impart “imido”-character into the metal-nitrogen bonds and open new pathways for the formation of new bonds to dinitrogen.
In 2004, we published the synthesis and initial reactivity of one the first realizations of our goal. The so-called “eight methyl” zirconium dinitrogen compound, [( h 5 -C5Me4)Zr]2( m 2 , h 2 , h 2 -N2), exhibits side-on N2 coordination and an elongated N-N bond of approximately 1.37 Å. Under mild conditions in solution, this compound undergoes rapid hydrogenation of the dinitrogen ligand to furnish the hydrido zirconocene hydrazide compound, [( h 5 -C5Me4)ZrH]2( m 2 , h 2 , h 2 -N2H2). This transformation can be viewed as the hydrogenation of coordinated N2 to coordinated hydrazine. Gentle heating to 85 ºC with continued hydrogenation resulted in liberation of ammonia.
Since this initial discovery, we have conducted both computational and experimental studies to establish both the mechanism and generality of this reaction. A host of zirconium and hafnium compounds have been discovered that promote the hydrogenation. More significantly, we have determined that the N2 hydrogenation proceeds through an ordered four-centered transition structure, reminiscent of the chemistry of group 4 imido compounds. Using indenyl-substituted metallocenes, we have also demonstrated that side-on coordination in the ground state is not a necessary condition for N2 hydrogenation.
Having discovered methods for N-H bond formation, including from terminal alkynes and intramolecular C-H processes, we have turned our attention to the more challenging problem of nitrogen-carbon bond forming reactions. What should be our carbon sources? Two attractive targets are carbon dioxide, CO2, and carbon monoxide, CO. The former was chosen because of its alleged overabundance in the atmosphere, the latter due to its isoelectronic relationship with N2. Carbon monoxide is also an attractive carbon source as this resource become particularly valuable in times when petroleum supplies are limited. Accordingly, we sought to discovery an “aza-variant” of CO homologation.
Our current efforts are primarily directed at so-called “ligand-induced” N2 bond cleavage reactions (Figure 2 - to be added). Addition of carbon monoxide to the ansa-hafnocene dinitrogen compound, [(Me2Si( h 5 -C5Me4)( h 5 -C5H3-3-tBu)Hf]2( m 2 , h 2 , h 2 -N2), with 1 atm of CO resulted in formation of an unprecedented oxamidide ligand, [N2C2O2]4-, arising from simultaneous N-N bond cleavage with formation of two N-C bonds and one C-C bond (Figure 2). Thus, the six electron cleavage of dinitrogen was accomplished by providing four electrons from the hafnocenes and the remaining electrons from the incoming ligand, CO, used to also form the new N-C bonds. Treatment of the oxamidide with proton sources yielded free oxamide, an important fertilizer.
With this discovery in hand, several questions motivate our continued efforts:
(1) What is the mechanism of this interesting transformation?
(2) What other organic molecules can be accessed from the oxamidide platform?
(3) What other ligands can be used to induce N2 cleavage and functionalization?
The pursuit of the answers to these queries are the subject of ongoing work in our laboratory. Mechanistic studies are combined with synthetic efforts to understand the observed reactivity. More long-term objectives are focused on extending these studies beyond the group 4 triad.
Chirik, P. J. “Dinitrogen functionalization with bis(cyclopentadienyl) complexes of zirconium and hafnium.” Dalton Trans. 2007, 16-26.
N-C Bond Forming Reactions:
Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. “Functionalization of hafnium oxamidide complexes prepared from CO-induced N2 cleavage.” J. Am. Chem. Soc. 2010, 132, 15340-15350.
Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. “Carbon monoxide-induced dinitrogen cleavage with group 4 metallocenes: Reaction scope and coupling to N-H bond formation and C-O deoxygenation.”J. Am. Chem. Soc. 2010, 132, 10553-10564.
Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. “Dinitrogen cleavage and functionalization by carbon monoxide promoted by a hafnium complex.” Nature Chemistry 2010, 2, 30-35.
Knobloch, D. J.; Benito-Garagorri, D.; Bernskoetter, W. H.; Keresztes, I.; Lobkovsky, E.; Toomey, H.; Chirik, P. J. “Addition of methyl triflate to a hafnocene dinitrogen complex: Stepwise N2 methylation and conversion to a hafnocene hydrazonato compound.” J. Am. Chem. Soc. 2009, 131, 14903-14912.
Knobloch, D. J.; Toomey, H. E.; Chirik, P. J. “Carboxylation of an ansa-zirconocene dinitrogen complex: Regiospecific hydrazine synthesis from N2 and CO2.” J. Am. Chem. Soc. 2008, 130, 4248-4249.
N-H Bond Forming Reactions:
Pun, D.; Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. “N2 hydrogenation from activated end-on bis(indenyl)zirconium dinitrogen complexes.” J. Am. Chem. Soc. 2008, 130, 14046-14047.
Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. “Kinetics and mechanism of N2 hydrogenation in bis(cyclopentadienyl)zirconium complexes and dinitrogen functionalization by 1,2-addition of a saturated C-H bond.” J. Am. Chem. Soc. 2005, 127, 14051-14061.
Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. “On the origin of dinitrogen hydrogenation promoted by [( h 5 -C5Me4H)2Zr]2( m 2 , h 2 , h 2 -N2).” J. Am. Chem. Soc. 2004, 126, 14326-14327.
Pool, J. A.; Lobkovsky, E.; Chirik, P. J. “Hydrogenation and cleavage of dinitrogen to ammonia with a well-defined zirconium complex.” Nature 2004, 427, 527-530.