Iron–Amide–Sulfide and Iron–Imide–Sulfide Clusters: Heteroligated Core Environments Relevant to the Nitrogenase FeMo Cofactor

Abstract

Heteroligated cluster cores consisting of weak-field iron, strongly basic nitrogen anions, and sulfide are of interest with respect to observed and conjectured environments in the FeMo cofactor of nitrogenase. Selective syntheses have been developed to achieve such environments with tert-butyl-substituted amide and imide core ligands. A number of different routes were employed, including nominal ligand substitution and oxidative addition reactions, as well as novel transformations involving the combination of different cluster precursors. New cluster products include precursor Fe(2)(μ-NH(t)Bu)(2)[N(SiMe(3))(2)](2) (6), Fe(2)(μ-NH(t)Bu)(2)(μ-S)[N(SiMe(3))(2)](2) (7), which has a rare confacial bitetrahedral geometry previously unknown in iron chemistry, [Fe(2)(μ-N(t)Bu)(μ-S)Cl(4)](2-) (2), and cuboidal [Fe(4)(μ(3)-N(t)Bu)(3)(μ(3)-S)Cl(4)](-) (8), [Fe(4)(μ(3)-N(t)Bu)(2)(μ(3)-S)(2)Cl(4)](2-) (9), and [Fe(4)(μ(3)-N(t)Bu)(μ(3)-S)(3)Cl(4)](2-) (10), as well as selenide-substituted derivatives Fe(2)(μ-NH(t)Bu)(2)(μ-Se)[N(SiMe(3))(2)](2) (7-Se) and [Fe(4)(μ(3)-N(t)Bu)(μ(3)-Se)(3)Cl(4)](2-) (10-Se). The imide-sulfide clusters complete the compositional sets [Fe(2)(μ-N(t)Bu)(n)(μ-S)(2-n)Cl(4)](2-) (n = 0-2) and [Fe(4)(μ(3)-N(t)Bu)(n)(μ(3)-S)(4-n)Cl(4)](z) (n = 0-4), represented previously only by the all-imide and all-sulfide core congeners, and they share chemical and physical properties with the parent homoleptic core species. All imide-sulfide cores are compositionally stable and show no evidence of core ligand exchange over days in solution. Beyond structural differences, the impact of mixed core ligation is most evident in redox potentials, which show progressive decreases of -435 (for z = 1-/2-) or -385 mV (for z = 2-/3-) for each replacement of sulfide by the more potent imide donor, and a corresponding effect may be expected for the interstitial heteroligand in the FeMo cofactor. Cluster 10 presents an [Fe(4)NS(3)] core framework virtually isometric with the isostructural [Fe(4)S(3)X] subunit of the FeMo cofactor, thus providing a synthetic structural representation for this portion of the cofactor core.

Developments in the Biomimetic Chemistry of Cubane-Type and Higher Nuclearity Iron–Sulfur Clusters

“…121,122a Initial reactivity studies summarized in Figure 14 reveal both similarities and differences between this heteroligated core and the well-known homoleptic all-sulfide species. 122b The terminal chloride ligands in both clusters are readily replaced by thiolates, lending enhanced redox stability to the cores and allowing electrochemical observation of reversible oxidative (z = 1−/2−) and reductive (z = 2−/3−) processes, albeit with more reducing potentials ( ca . −300 mV shifts) for the imide-containing core.…”

“…48,122 Metal sites are well-described as independent, weakly-coupled high-spin tetrahedral centers. The cluster structures combine the specific geometric characteristics associated with imide and sulfide core ligation; e.g., Fe-Q distances are shorter, Q-Fe-Q angles are sharper, and Fe-Q-Fe angles are wider for Q = N vs Q = S. 48 Multiple redox states are accessible, with oxidative redox stabilization due to strongly basic nitrogen anion donors reflected in progressive linear decreases in redox potentials (−435 mV for z = 1−/2−, −385 mV for z = 2−/3−) for each replacement of sulfide by imide in the cubane core.…”

“…To explore this possibility, selenide analogues of 53 and [Fe 4 S 4 Cl 4 ] 2− were prepared and deployed in an attempt to track the fate of the chalcogenide according to reactions [17]–[19]. 122 The results of these labelling experiments were mechanistically uninterpretable: congeners spanning all possible sulfide-selenide compositions were produced in reactions [17] and [18] whereas an all-selenide product dominated in reaction [19]. The cluster assembly pathways for these systems remain unknown at present.…”

“…In 47 , 58 , and 61 , for example, mean Fe-Q bond lengths (Å) are approximately 1.95 (Q = N), 1.91-2-19 (Q = O, four-coordinate), 2.30 (Q = S), and 2.40 (Q = Se). 122,126,127 Extensive metric data have been compiled for Fe 4 S 4- n (NR) n clusters. 122 …”

A Low‐Valent Iron Imido Heterocubane Cluster: Reversible Electron Transfer and Catalysis of Selective C–C Couplings

“…2.3-2.6 3 . Theaccessible potential range and the number of chemically reversible redox events can be modified to ag reater extent in these [Fe 4 Q 4 L 4 ]c lusters with Q = O, [8][9][10] S, [11][12][13][14][15][16][17][18][19] Se, [20,21] NR, [22][23][24][25][26][27] S/NR, [28][29][30][31] CO; [16,[32][33][34] L = neutral or anionic ligand (see the Supporting Information for details). À0.1 Vto À1.4 Vv s. ferrocene/ferrocenium (Fc/Fc + )i nb iologically relevant Fe 4 S 4 based systems.…”