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

Abstract

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.

Topics & Concepts