Hexanuclear iron-sulfur basket clusters: topological isomers of prismanes. Synthesis, structure, and reactions

Abstract

Thus neither e" nor ew for CN" can be even approximated from this treatment. It is also unrealistic to accept the best-fit f values, even when the propagated error turns out to be small, because changes in f induce smaller changes in the transition energies than would such factors as the inclusion of additional K+ (or [Cr-(CN)6]3~) ions in the model.The value of the Trees parameter, 211 cm™1 using the Pean structure, appears large by comparison with the 70-cm™1 value for the free Cr3+ ion.8 Smaller values of the Trees parameter place the 2T2g levels too high in energy when B and C are chosen to fit the quartets and the 2Eg and 2Tlg levels.The fit to the experimental energies is definitely better for the Pean geometry, but the model includes too many approximations to justify a prediction that this is the structure at 13 K, the temperature at which the excitation spectrum was measured. With either the Pean or P\ geometry, including a cation field of reasonable magnitude yields a better fit to experiment than no field at all. It is not obvious that this should be so, because at zero cation field the calculated 2T2g splitting is too large, while the 2Eg (20) Flint, C.

Structural Conversions of Synthetic and Protein-Bound Iron–Sulfur Clusters

“…In terms of just carbide coordination, no molecular entity comes closer than [Fe 6 C(CO) 16 ] 2−234 in which a carbon atom resides at the center of an Fe 6 octahedron at 1.88−1.91 Å, 235 but the formal (29) and supported by an idealized synthetic Cys-containing helical peptide (34). Discrete covalently bridged synthetic clusters (35,36) are perhaps plausible; however, 35 (X = HS − ) and 36 have not been prepared. iron oxidation state is completely unrealistic for a protein site.…”

“…[Fe 8 S 8 ] 0 (12) ↔ [Fe 4 S 4 ] 1+,0 (4) Double cubanes 12 are readily cleaved by excess carbene in reactions (32) (63%) and (33) (nearly quantitative) or with a variety of other ligands in the presence of a suitable oxidant to afford the cubane clusters 4 in the all-ferrous or one-electron oxidized state (20−50%). Reactions (32)−(35) are further examples of double to single cubane conversions leading to 4 in two different oxidation states.…”

“…Depictions of 2[Fe 4 S 4 ] cluster arrangements: in a protein environment without covalent attachment (29) and supported by an idealized synthetic Cys-containing helical peptide(34). Discrete covalently bridged synthetic clusters(35,36) are perhaps plausible; however, 35 (X = HS − ) and 36 have not been prepared.…”

Developments in the Biomimetic Chemistry of Cubane-Type and Higher Nuclearity Iron–Sulfur Clusters

“…There are of course some exceptions, including among others [Fe 4 S 4 (STip) 2 {SC(NMe 2 ) 2 } 2 ] prepared in toluene 140 and, more recently, [Fe 4 S 4 {N(SiMe 3 ) 2 } 4 ] ( 13 , Figure 3) also prepared in toluene. Octanuclear [Fe 8 S 12 (RNC) 12 ] clusters have been synthesized in benzene from the fragment coupling reaction of [Fe 4 S 4 (SEt) 2 (RNC) 6 ] and (PhCH 2 S) 2 S. 141 Certain neutral clusters such as [Fe 6 S 6 (PEt 3 ) 4 X 2 ] 116,142 (X = halide, PhS − ), [Fe 7 S 6 (PEt 3 ) 4 Cl 3 ], 143 and [Fe 8 S 8 (Pr i 2 NHCMe 2 ) 6 ( 12 , Figure 3) are pr143epared in THF whose dielectric constant (ε 7.52) is somewhat higher than that of toluene (ε 2.38).…”

Interconversion between [Fe4S4] and [Fe2S2] Clusters Bearing Amide Ligands

“…The biological significance of the oxidative disassembly of the [Fe 4 S 4 ] cluster draws attention to the molecular basis of this reaction. As fundamental knowledge for iron–sulfur clusters, the conversion of [Fe 4 S 4 ] cluster into [Fe 2 S 2 ] clusters is highly attractive but remains unprecedented, and notably, little has been known about transformations of synthetic [Fe 4 S 4 ] clusters. , Recently, we have demonstrated that the highly oxidized all-ferric [Fe 4 S 4 ] cluster Fe 4 S 4 {N­(SiMe 3 ) 2 } 4 ( 1 ) , can be transformed into [Fe 8 S 7 ] clusters representing the core of a nitrogenase P-cluster, through the reductive desulfurization of the cubic core induced by phosphines . This result prompted us to explore core transformations of all-ferric [Fe 4 S 4 ] cluster 1 , and herein we report our discovery of a novel transformation of an [Fe 4 S 4 ] core from cluster 1 into two [Fe 2 S 2 ] clusters in the presence of pyridines.…”