137003-15-7Relevant articles and documents
Synthesis and electron delocalization of [Fe4S4]-S-Fe(III) bridged assemblies related to the exchange-coupled catalytic site of sulfite reductases
Cai, Lisheng,Holm
, p. 7177 - 7188 (1994)
Because of the pervasive occurrence of magnetically coupled siroheme and Fe4S4 units in assimilatory and dissimilatory sulfite and nitrite reductases, we have undertaken the synthesis of the sulfide-bridged assembly Fe4S4-S-heme as a possible analogue to the active sites of certain assimilatory enzymes. The approach has utilized iron subsite-differentiated clusters of the type [Fe4S4(LS3)L′]2-, which undergo regiospecific substitution at the unique subsite. Reaction of [Fe4S4(LS3)(SEt)]2- with limited H2S in acetonitrile affords the functionalized cluster [Fe4S4(LS3)(SH)]2- (4), which exists in equilibrium with the μ-S double cubane {[Fe4S4(LS3)]2S}4- (6) and H2S. Reaction of 4 and [Fe(salen)]2O gave the bridged assembly [Fe4S4(LS3)-S-FeIII(salen)] 2- (8), detectable by its characteristic isotropically shifted 1H NMR spectrum. Six routes were devised to a related heme-based assembly: directed acid-base coupling of 4 with [Fe(OEP)]2O, [Fe(OEP)(OMe)], [Fe(OEP)(OC(Me)=CH2)], and [Fe(OEP)-(OClO3)]/Et3N; Si-S bond cleavage in the reaction of [Fe4S4(LS3)(SSiEt3)]2- with [Fe(OEP)F]; oxidative addition of [FeII(OEP)] to the disulfide bond of the μ-S2 double cubane {[Fe4S4(LS3)]2S2} 4- (7). In each case, the product was [Fe4S4(LS3)-S-FeIII(OEP)] 2- (9), recognizable by UV-visible absorption and 1H NMR spectra. Both 8 and 9 contain [Fe4S4]2+ and high-spin Fe(III) fragments. Isotropic shifts mainly contact in origin that are enhanced by factors of 7-12 compared to those of precursor cluster 4, and the Curie-type temperature dependence of the shifts of 9, originate from extensive spin localization from the Fe(III) fragment to the cluster. This effect requires the existence of a covalent bridge between the fragments and, together with the methods of synthesis and other spectroscopic observations, provides structure proof of the bridged assemblies. These species sustain two one-electron reduction reactions; other reactions of 9, which alter or cleave the bridge, are summarized. The electronic features of bridged assemblies such as 8 and 9 approach the intrinsic magnetic and spectroscopic properties of a structurally similar unit in the oxidized enzymes and potentially provide a means of identification of such units. (LS3 = 1,3,5-tris[(4,6-dimethyl-3-mercaptophenyl)-thio]-2,4,6-tris(p-tolylthio) benzene(3-); OEP = octaethylporphyrinate(2-); salen = 1,2-bis(salicylideneamino)ethane-(2-).).
An initial approach to biologically related bridged assemblies: PYridinethiolate-Linked Fe4S4-Fe complex systems
Liu, Hong Ye,Scharbert, Bernd,Holm
, p. 9529 - 9539 (2007/10/02)
The subsite-differentiated cubane-type clusters [Fe4S4(LS3)L′]2- (LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio) benzene(3-)) undergo substitution reactions at the unique subsite. This property has been exploited in the formation of bridged assemblies in which an Fe4S4 cluster and a high-spin Fe(II) complex are covalently linked. Reaction of [Fe4S4(LS3)(SR)]2- (R = Me, Et) with isomeric pyridinethiols (HSpy) affords [Fe4S4(LS3)(S-2-py)]2- (7), [Fe4S4(LS3)(S-3-py)]2- (8), and [Fe4S4(LS3)(S-4-py)]2- (9). Cluster 7 did not react with Fe(acen) (acen = N,N′-ethylene-bis(acctylacetoneiminate(2-)) because of the chelate structure at the unique subsite. Reaction of 8 with Fe(acen) gives 8-Fe(acen), and reaction of 9 with Fe(acen) and Fe(tfacen) (tfacen = N,N′-bis(trinuoroacetylacetoneiminate(2-)) forms 9-Fe(acen) and 9-Fe(tfacen). Bridge formation was readily detected from the isotropically shifted 1H NMR spectra, which are fast-exchange averages over the bridged species and their separate components. An NMR method was developed for determination of formation constants of pyridine adducts in the systems Fe(acen)/py and Fe(tfacen)/py and in those containing the bridged species. For 9-Fe(acen) and 9-Fe(tfacen), Kf = 790 and 920 M-1, respectively, in acetonitrile solution. Bridge-Fe(II) binding is expected to resemble that in Fe(tfacen)(py)2, whose structure (trans octahedral) is reported. Bridge formation was also detected electrochemically; in the case of Fe(acen) a reversible oxidation reaction occurs only when the complex is axially ligated by a pyridyl ligand. The effects of ligand substituents on equilibrium constants and redox potentials are described. The preparative and redox reactions generate the bridged assembly oxidation levels [Fe4S4]1+/[Fe(II), [Fe4S4]2+/Fe(II), and [Fe4S4]2+/Fe(III), which are those known for the bridged Fe4S4-siroheme active site assembly of E. coli sulfite reductase. This work provides the initial experimental protocol for the construction of biologically related, bridged Fe4S4-Fe complex assemblies that should allow examination of such matters as electron transfer and magnetic coupling between cluster and Fe(II,III) sites. Potential means for forming nonlabile bridged assemblies are outlined, including the covalent attachment of an iron complex to the bridging group with use of a five-coordinate ligand. The preparation and structure of one such complex, N,N′-2,6-diethylpyridinebis(trifluoroacetyliminato)iron(II), are described.
