Mossbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N2 to give a tetra-iron complex with two nitrides (Rodriguez et al., Science, 2011, 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate. Here, we describe studies aiming to identify the characteristics of this intermediate, using a range of spectroscopic techniques, including Mo''ssbauer spectroscopy, electronic absorption spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) complemented by density functional theory (DFT) calculations. We successfully elucidated the nature of the starting iron(II) species and the bis(nitride) species in THF solution, and in each case, THF breaks up the multiiron species. Various observations on the green intermediate species indicate that it has one N-2 per two Fe atoms, has THF associated with it, and has NRVS features indicative of bridging N-2. Computational models with a formally diiron(0)-N-2 core are most consistent with the accumulated data, and on this basis, a mechanism for N-2 splitting is suggested. This work shows the power of combining NRVS, Mo''ssbauer, NMR, and vibrational spectroscopies with computations for revealing the nature of transient iron species during N2 cleavage.

Automated summary

This study successfully elucidated the characteristics and mechanism of the intermediate species during the cleavage of the strong triple bond of N2 by Diketiminate-supported iron complexes, using a range of spectroscopic techniques and computational models.