Stabilization of fully reduced iron-sulfur clusters by carbene ligation: the [FenSn]0 oxidation levels (n = 4, 8)

Abstract

The all-ferrous [Fe4S4](0) state has been demonstrated in the fully reduced Fe protein of the Azotobacter vinelandii nitrogenase complex. We seek synthetic analogues of this state more tractable than the recently prepared but highly unstable cluster [Fe4S4(CN)4](4-) (Scott, Berlinguette, Holm, and Zhou, Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Pr(i)2NHCMe2) has been found to stabilize the fully reduced clusters [Fe8S8(Pr(i)2NHCMe2)6] (4) and [Fe4S4(Pr(i)2NHCMe2)4] (5), which are prepared by cluster assembly or phosphine substitution of FenSn (n = 8, 16) clusters. Cluster 4 is also obtained by reaction of the carbene with all-ferrous [Fe7S6(PEt3)5Cl2] (3) and cluster 5 by carbene cleavage of 4. Detailed structures of 3 (monocapped prismatic), 4, and 5 are described; the latter two are the first iron-sulfur clusters with Fe-C sigma bonds. Cluster 4 possesses the [Fe8(mu3-S) 6(mu4-S)2] edge-bridged double cubane structure and 5 the cubane-type [Fe4(mu3-S)4] stereochemistry. The all-ferrous formulations of the clusters are confirmed by X-ray structure parameters and (57)Fe isomer shifts. Both clusters are stable under conventional aprotic anaerobic conditions, enabling further study of reactivity. The collective properties of 5 indicate that it is a meaningful synthetic analogue of the core of the fully reduced protein-bound cluster.

Figure 1

Synthesis and conversion reactions of all-ferrous iron–sulfur clusters 3–7 derived from Fe^II precursors 1 and 2 and (Me₃Si)₂S as the sulfur source.

Figure 2

Structure of [Fe₇S₆(PEt₃)₅Cl₂] showing 30% probability ellipsoids. For clarity, the ethyl groups are omitted.

Figure 3

Structure of [Fe₈S₈(Prⁱ₂NHCMe₂)₆] showing 30% probability ellipsoids and a partial atom labeling scheme. Atoms n and nA are related by imposed centrosymmetry.

Figure 4

Mössbauer spectra of polycrystalline [Fe₄S₄(Prⁱ₂NHCMe₂)₄] (A, at 77 K) and [Fe₈S₈(Prⁱ₂NHCMe₂)₆] (B, at 4.2 K). The spectra were fitted using the parameters of Table 4.

Figure 5

¹H NMR spectra of [Fe₄S₄(Prⁱ₂NHCMe₂)₄] (upper), [Fe₈S₈(Prⁱ₂NHCMe₂)₆] (lower), in C₆D₆ at room temperature. The signals of one set of ligand methyl groups of [Fe₈S₈(Prⁱ₂NHCMe₂)₆] and solvent overlap at 7.11 ppm.

Figure 6

Structure of [Fe₄S₄(Prⁱ₂NHCMe₂)₄] showing 30% probability ellipsoids and a partial atom labeling scheme.

Figure 7

UV– visible absorption spectra in benzene of [Fe₄S₄(Prⁱ₂NHCMe₂)₄] (red), [Fe₈S₈(Prⁱ₂NHCMe₂)₆] (black), and free ligand Prⁱ₂NHCMe₂ (blue). (Inset) Enlarged absorption spectra between 400 and 800 nm.

Figure 8

Cyclic voltammograms (50 mV/s) of 3.7 mM [Fe₄S₄(Prⁱ₂NHCMe₂)₄] in THF with 0.10 M (Buⁿ ₄N)(PF₆) supporting electrolyte. (Inset) Voltammogram at −1.0 to −1.6 V. Number peak potentials vs SCE are indicated.

Chart 1

Designation of Compounds