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)(3)Fe4S4Cl (IMes = 1,3-dimesityli-midazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes)(6)Fe8S8, 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-dime-thylphenyl)-2-imidazolidinylidene; abbreviated as SIAr), that features bulky groups strategically placed in remote positions. When the reduction of (SIAr)(3)Fe4S4Cl or [(SIAr)3Fe4S4(THF)]+ is conducted in the presence of SIAr, the formation of (SIAr)(4)Fe8S8 is indeed suppressed, permitting characterization of the reduced [Fe4S4]0 product. Surprisingly, rather than being an N2 complex, the product is simply (SIAr)(3)Fe4S4: a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr)(3)Fe4S4 does not coordinate N-2 to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr)(3)Fe4S4(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 N2 can be attributed to weak Fe-N-2 bonding. The differences in the N2 coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.

Automated summary

Attempts to generate open coordination sites for N2 binding at synthetic Fe-S clusters often result in cluster oligomerization. However, recent studies have shown that oligomerization reactions can be prevented by using sterically protective supporting ligands in Mo-Fe-S clusters, enabling N2 complex formation. This strategy has now been extended to Fe-only Fe-S clusters. Evaluation: 7 out of 10.