An Iron–Sulfur Cluster with a Highly Pyramidalized Three-Coordinate Iron Center and a Negligible Affinity for Dinitrogen

Attempts
to generate open coordination sites for N2 binding
at synthetic Fe–S clusters often instead result in cluster
oligomerization. Recently, it was shown for Mo–Fe–S
clusters that such oligomerization reactions can be prevented through
the use of sterically protective supporting ligands, thereby enabling
N2 complex formation. Here, this strategy is extended to
Fe-only Fe–S clusters. One-electron reduction of (IMes)3Fe4S4Cl (IMes = 1,3-dimesitylimidazol-2-ylidene)
forms the transiently stable edge-bridged double cubane (IMes)6Fe8S8, which loses two IMes ligands
to form the face-bridged double-cubane, (IMes)4Fe8S8. The finding that the three supporting IMes ligands
do not confer sufficient protection to curtail cluster oligomerization
prompted the design of a new N-heterocyclic carbene,
SIArMe,iPr (1,3-bis(3,5-diisopropyl-2,6-dimethylphenyl)-2-imidazolidinylidene;
abbreviated as SIAr), that features bulky groups strategically placed
in remote positions. When the reduction of (SIAr)3Fe4S4Cl or [(SIAr)3Fe4S4(THF)]+ is conducted in the presence of SIAr, the
formation of (SIAr)4Fe8S8 is indeed
suppressed, permitting characterization of the reduced [Fe4S4]0 product. Surprisingly, rather than being
an N2 complex, the product is simply (SIAr)3Fe4S4: a cluster with a three-coordinate Fe
site that adopts an unusually pyramidalized geometry. Although (SIAr)3Fe4S4 does not coordinate N2 to any appreciable extent under the surveyed conditions, it does
bind CO to form (SIAr)3Fe4S4(CO).
This finding demonstates that the binding pocket at the unique Fe
is not too small for N2; instead, the exceptionally weak
affinity for N2 can be attributed to weak Fe–N2 bonding. The differences in the N2 coordination
chemistry between sterically protected Mo–Fe–S clusters
and Fe-only Fe–S clusters are discussed.