ic301868m_si_002.cif (225.74 kB)
Iron–Amide–Sulfide and Iron–Imide–Sulfide Clusters: Heteroligated Core Environments Relevant to the Nitrogenase FeMo Cofactor
dataset
posted on 2016-02-20, 04:56 authored by Xu-Dong Chen, Wei Zhang, Jeremiah S. Duncan, Sonny C. LeeHeteroligated 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 Fe2(μ-NHtBu)2[N(SiMe3)2]2 (6), Fe2(μ-NHtBu)2(μ-S)[N(SiMe3)2]2 (7), which has a rare
confacial bitetrahedral geometry previously unknown in iron chemistry,
[Fe2(μ-NtBu)(μ-S)Cl4]2– (2), and cuboidal [Fe4(μ3-NtBu)3(μ3-S)Cl4]− (8), [Fe4(μ3-NtBu)2(μ3-S)2Cl4]2– (9), and [Fe4(μ3-NtBu)(μ3-S)3Cl4]2– (10), as
well as selenide-substituted derivatives Fe2(μ-NHtBu)2(μ-Se)[N(SiMe3)2]2 (7-Se) and [Fe4(μ3-NtBu)(μ3-Se)3Cl4]2– (10-Se). The imide–sulfide clusters complete the compositional
sets [Fe2(μ-NtBu)n(μ-S)2–nCl4]2– (n = 0–2)
and [Fe4(μ3-NtBu)n(μ3-S)4–nCl4]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 [Fe4NS3] core framework
virtually isometric with the isostructural [Fe4S3X] subunit of the FeMo cofactor, thus providing a synthetic structural
representation for this portion of the cofactor core.
