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 N 2 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 N 2 complex formation. Here, this strategy is extended to Fe-only Fe-S clusters. One-electron reduction of (IMes) 3 Fe 4 S 4 Cl (IMes = 1,3-dimesitylimidazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes) 6 Fe 8 S 8 , which loses two IMes ligands to form the face-bridged double-cubane, (IMes) 4 Fe 8 S 8 . 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, SIAr Me, i Pr (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) 3 Fe 4 S 4 Cl or [(SIAr) 3 Fe 4 S 4 (THF)] + is conducted in the presence of SIAr, the formation of (SIAr) 4 Fe 8 S 8 is indeed suppressed, permitting characterization of the reduced [Fe 4 S 4 ] 0 product. Surprisingly, rather than being an N 2 complex, the product is simply (SIAr) 3 Fe 4 S 4 : a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr) 3 Fe 4 S 4 does not coordinate N 2 to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr) 3 Fe 4 S 4 (CO). This finding demonstates that the binding pocket at the unique Fe is not too small for N 2 ; instead, the exceptionally weak affinity for N 2 can be attributed to weak Fe-N 2 bonding. The differences in the N 2 coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.