Mössbauer 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 Mössbauer
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 N2 per two Fe atoms, has THF associated with it, and
has NRVS features indicative of bridging N2. Computational
models with a formally diiron(0)–N2 core are most
consistent with the accumulated data, and on this basis, a mechanism
for N2 splitting is suggested. This work shows the power
of combining NRVS, Mössbauer, NMR, and vibrational spectroscopies
with computations for revealing the nature of transient iron species
during N2 cleavage.