Alkali Metal Control over N–N Cleavage in Iron Complexes

Though N2 cleavage on
K-promoted Fe surfaces is important
in the large-scale Haber–Bosch process, there is still ambiguity
about the number of Fe atoms involved during the N–N cleaving
step and the interactions responsible for the promoting ability of
K. This work explores a molecular Fe system for N2 reduction,
particularly focusing on the differences in the results obtained using
different alkali metals as reductants (Na, K, Rb, Cs). The products
of these reactions feature new types of Fe–N2 and
Fe-nitride cores. Surprisingly, adding more equivalents of reductant
to the system gives a product in which the N–N bond is not
cleaved, indicating that the reducing power is not the most important
factor that determines the extent of N2 activation. On
the other hand, the results suggest that the size of the alkali metal
cation can control the number of Fe atoms that can approach N2, which in turn controls the ability to achieve N2 cleavage. The accumulated results indicate that cleaving the triple
N–N bond to nitrides is facilitated by simultaneous approach
of least three low-valent Fe atoms to a single molecule of N2.