Sonny C. Lee

Ligand K-edge X-ray absorption spectroscopy was used to study dimeric and tetrameric Cl-terminated Fe–S clusters with variable numbers of S^2– substituted by N^tBu^2– (N^tBu^2– = tertbutylimido) ligands to gain insights into the functional role of the interstitial light atom in the iron–molybdenum cofactor (FeMoco) of nitrogenase. These studies are complemented by time-dependent density functional theory analysis to quantify the relative effects on Fe–S and Fe–Cl bonding. The results show that N^tBu^2– substitution dramatically affects the electronic structure of dimeric clusters, while the impact on tetrameric clusters is small. Strong agreement between experiment and theory merited extension of this analysis to hypothetical clusters with S^2– substituted by N and C-atom donor ligands as well as FeMoco itself. These results show that very strong electron donors are required to appreciably modulate the electronic structure of tetrameric (or larger) iron sulfur clusters, pointing to a possible role of the central C^4– in FeMoco.

The dinuclear precursors Fe₂(N^tBu)₂Cl₂(NH₂^tBu)₂, [Fe₂(N^tBu)(S)Cl₄]²⁻, and Fe₂(NH^tBu)₂(S)(N{SiMe₃}₂)₂ allowed the selective syntheses of the cubane clusters [Fe₄(N^tBu)_n(S)_4−nCl₄]^z with [n, z] = [3, 1−], [2, 2−], [1, 2−]. Weak-field iron−sulfur clusters with heteroleptic, nitrogen-containing cores are of interest with respect to observed or conjectured environments in the iron−molybdenum cofactor of nitrogenase. In this context, the present iron−imide−sulfide clusters constitute a new class of compounds for study, with the Fe₄NS₃ core of the [1, 2−] cluster affording the first synthetic representation of the corresponding heteroligated Fe₄S₃X subunit in the cofactor.