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 Nheterocyclic carbene, SIAr Me,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) 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.

Three-Coordinate Nickel and Metal–Metal Interactions in a Heterometallic Iron–Sulfur Cluster

“…We have previously reported several dinuclear complexes with FeSFe cores , as well as a Fe 3 S 4 cluster with a planar three-coordinate iron having only S donors . Very recently, Suess has described a Fe 4 S 4 cluster in which one of the Fe vertices is pyramidal and lacks a terminal ligand, rendering it three-coordinate . Since WFe 2 Ni-red provides the first example of a three-coordinate nickel site in a synthetic iron–sulfur cluster, it gives us an exciting opportunity to understand the properties of nickel in this environment as well as its magnetic interactions with the rest of the cluster.…”

Topological structures and adsorption properties of the [Fe4S4] clusters

“…In controlling the AIMD temperatures of the [Fe 4 S 4 ] 0 , the Nose-Hoover method is employed [49]. ] are widely present in biological systems, serving as active centers in electron transfer proteins or integral components of complex catalytic mechanisms, and the cube cage of them is adopted as the basic structural model [3]. It is well-known that charge states can greatly affect the structure and properties of clusters.…”

“…Iron-sulfur (Fe-S) clusters constitute the active sites of metalloproteins and are evolutionarily ancient cofactors necessary to maintain basic life evolutionary processes [1][2][3][4]. They play essential roles in the reactions such as electron transfer chain, photosynthesis, respiratory chain and nitrogen fixation and are also considered to be among the first catalysts in live organisms [5,6].…”

Highly Activated Terminal Carbon Monoxide Ligand in an Iron–Sulfur Cluster Model of FeMco with Intermediate Local Spin State at Fe

“…Synthetic models promise to facilitate a better understanding of the impact of cluster structure on substrate binding and level of activation. − However, few examples of synthetic iron–sulfur clusters with terminal or bridging N 2 or CO ligands have been reported, many of which possess multiple CO ligands that drastically alter the electronic structure of the cluster and complicate comparisons to FeMoco (Figure ). − Only one type of FeS cluster with a single terminal CO ligand has been characterized, ligated by three carbene ligands. , …”

“… 23 − 29 Only one type of FeS cluster with a single terminal CO ligand has been characterized, ligated by three carbene ligands. 30 , 31 …”