Valence Localization in Alkyne and Alkene Adducts of Synthetic [Fe 4 S 4 ] + Clusters.

Reported herein are alkyne and alkene adducts of synthetic [Fe 4 S 4 ] + clusters that model intermediates and inhibitor-bound states in enzymes involved in isoprenoid biosynthesis. Treatment of the N -heterocyclic carbene-ligated cluster [(IMes) 3 Fe 4 S 4 (OEt 2 )][BAr F 4 ] (IMes = 1,3-dimesitylimidazol-2-ylidene; [BAr F 4 ] - = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) with phenylacetylene (PhCCH) or cis -cyclooctene (COE) results in displacement of the Et 2 O ligand to yield the corresponding π complexes, [(IMes) 3 Fe 4 S 4 (PhCCH)][BAr F 4 ] and [(IMes) 3 Fe 4 S 4 (COE)][BAr F 4 ]. EPR spectroscopic analysis demonstrates that both clusters are doublets with g iso > 2 and thus are spectroscopically faithful models of the analogous species characterized in the isoprenoid biosynthetic enzymes IspG and IspH. Structural and Mössbauer spectroscopic analysis reveals that both complexes are best described as [Fe 4 S 4 ] + clusters in which the unique Fe site engages in modest back-bonding to the π-acidic ligand. Paramagnetic NMR studies show that, even at room temperature, the alkyne/alkene-bound Fe centers harbor minority spin and therefore adopt an Fe 2+ valence. We propose that such valence localization could likewise occur in Fe-S enzymes that interact with π-acidic molecules.