Iron–Amide–Sulfide and Iron–Imide–Sulfide Clusters: Heteroligated Core Environments Relevant to the Nitrogenase FeMo Cofactor
Bibliographic record
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.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; both teacher heads agree on what is shown here.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".