Nitride-Bridged Triiron Complex and Its Relevance to Dinitrogen Activation

Using a simple metathesis approach,
the triiron­(II) tribromide complex Fe3Br3L (1) reacts with tetrabutylammonium azide to afford the monoazide
dibromide analogue Fe3(Br)2(N3)­L
(2) in high yield. The inclusion of azide was confirmed
by IR spectroscopy with a ν­(N3) = 2082 cm–1 as well as combustion analysis and X-ray crystallography. Heating 2 in the solid state results in the complete loss of the azide
vibration in the IR spectra and the isolation of the olive-green mononitride
complex Fe3(Br)2(N)­L (3). Solution
magnetic susceptibility measurements support that the trimetallic
core within 2 is oxidized upon generation of 3 (5.07 vs 3.09 μB). Absorption maxima in the UV–visible–near-IR
(NIR) spectra of 2 and 3 support the azide-to-nitride
conversion, and a broad NIR absorption centered at 1117 nm is similar
to that previously reported for the intervalence charge-transfer band
for a mixed-valent nitridodiiron cluster. The cyclic voltammograms
recorded for 3 are comparable to those of 1 with no reductive waves observed between ∼0 and −2.5
V (vs Fc/Fc+), whereas a reversible one-electron redox
process is observed for Fe3(NH2)3L (4). These results suggest that intercluster cooperativity
is unlikely to predominate the dinitrogen reduction mechanism when 1 is treated with KC8 under N2.