Reduced Mono-, Di-, and Tetracubane-Type Clusters Containing the [MoFe₃S₄]²⁺ Core Stabilized by Tertiary Phosphine Ligation

Abstract

Treatment of oxidized clusters [(Cl4cat)(MeCN)MoFe3S4Cl3]2- (1) and [(Meida)MoFe3S4Cl3]2- (2) with tertiary phosphines in the presence of NaBPh4 in acetonitrile results in chloride substitution at the iron sites and the formation of clusters with the reduced [MoFe3S4]2+ core. Thus, 1 is a precursor to [(Cl4cat)(MeCN)MoFe3S4(PR3)3] (R = But (3), Pri (4)) and [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] (5). Cluster 2 affords [[(Meida)MoFe3S4(PCy3)3]4Fe2(mu-Cl)L2]3+ (L = THF (6), MeCN (7)). The structures of 3-7 were established by X-ray analysis. Clusters 3 and 4 are single cubanes, centrosymmetric 5 (previously reported in a different space group: Demadis, K. D.; Campana, C. F.; Coucouvanis, D. J. Am. Chem. Soc. 1995, 117, 7832) is a double cubane with a rhomboidal Fe2S2 bridge, and 6 and 7 are tetracubanes. In the latter, four Meida oxygen atoms from different cubanes bind each of two central high-spin Fe(II) atoms in trans-Fe(mu-Cl)LO4 coordination. The topology of these clusters is not precedented. Zero-field Mössbauer parameters for all clusters are reported. Isomer shift considerations suggest the formulation [Mo3+Fe2+2Fe3+S4] for reduced clusters. Voltammetry of 3 and 4 reveals four-member electron transfer series encompassing the oxidation levels [MoFe3S4]4+,3+,2+,+ in the potential interval + 1.0 to -1.3 V vs SCE in dichloromethane. Compared to the clusters with monoanionic ligands at the iron sites, phosphine ligation shifts redox potentials to more positive values. This effect arises from reduction of cluster negative charge and the tendency of phosphines to stabilize lower oxidation states. The synthesis of reduced clusters 4 from 1 and of [Fe4S4(PPri3)4]+ from [Fe4S4Cl4]2- is accompanied by the formation of Pri3PS, detected by 31P NMR, indicating that the phosphine is the reductant. This result implies a similar function of tertiary phosphines in the synthesis of 3 and 5-7. (Cl4cat = tetrachlorocatecholate(2-); Meida = N-methyliminodiacetate(2-).)

The [MoFe3S4]2+ Oxidation State: Synthesis, Substitution Reactions, and Structures of Phosphine-Ligated Cubane-Type Clusters with the S = 2 Ground State

“…Magnetic and Mössbauer spectroscopic properties for four representative polycrystalline clusters and two others reported earlier 24,25 are provided in Figure 5 and Table 3. Clusters 3 and 4 exhibit Curie-type behavior at 2-20 K with Curie constants close to C = 3 emu K/mol for an S = 2 ground state (g = 2).…”

“…5 In several instances, chemical reduction of [MoFe 3 S 4 ] 3+ clusters has afforded isolable but oxidatively sensitive [MoFe 3 S 4 ] 2+ species. However, the initial preparation of the edge-bridged double cubane [(Cl 4 cat) 2 (Et 3 P) 2 Mo 2 Fe 6 S 8 (PEt 3 ) 4 ] 29 followed by demonstration that tertiary phosphines are reductants of oxidized clusters 25 has led to more direct syntheses of reduced clusters. 6,7,10 These results show that stabililty of the [MoFe 3 S 4 ] 2+ oxidation state is clearly enhanced by phosphine coordination at the iron sites, irrespective of ligation at molybdenum.…”

Selenium as a Structural Surrogate of Sulfur: Template-Assisted Assembly of Five Types of Tungsten–Iron–Sulfur/Selenium Clusters and the Structural Fate of Chalcogenide Reactants

“…One-electron cluster reduction by phosphine to the [MoFe 3 S 4 ] 2+ oxidation level is precedented. 15,40 Mössbauer spectra consist of two overlapping quadrupole doublets. Mean isomer shifts are suggestive of the description W 3+ Fe 2+ 2 Fe 3+ .…”

Stabilization of Fully Reduced Iron−Sulfur Clusters by Carbene Ligation: The [FenSn]0 Oxidation Levels (n = 4, 8)

“…Ligands at the two sites give rise to isopropyl and olefinic methyl resonances in 2:1 intensity ratio, with the signals from the m ′ sites twice as intense as from the m sites. However, the m ′ isopropyl methyl groups are diastereotopic (as in 6 23 and other edge-bridged double cubanes41). Consequently, the m (1, 2) and m ′ (2, 2, 2) ligands should generate five methyl signals with the indicated relative intensities, as observed, with that at 2.08 ppm due to the olefinic methyl groups at m. The spectra in toluene at 193–313 K showed increasing downfield shifts with decreasing temperature but no features indicative Fe–C hindered ligand rotation or other dynamical process.…”