SYNTHESIS AND ELECTRON DELOCALIZATION OF [FE4S4]-S-FE(III) BRIDGED ASSEMBLIES RELATED TO THE EXCHANGE-COUPLED CATALYTIC SITE OF SULFITE REDUCTASES

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(LS(3))L'](2-), which undergo regiospecific substitution at the unique subsite. Reaction of [Fe4S4(LS(3))(SEt)](2-) with limited H2S in acetonitrile affords the functionalized cluster [Fe4S4(LS(3))(SH)](2-) (4), which exists in equilibrium with the mu-S double cubane {[Fe4S4(LS(3))]S-2}(4-) (6) and H2S. Reaction of 4 and [Fe(salen)]O-2 gave the bridged assembly [Fe4S4(LS(3))-S-Fe-III(salen)](2-) (8), detectable by its characteristic isotropically shifted H-1 NMR spectrum. Six routes were devised to a related heme-based assembly: directed acid-base coupling of 4 with [Fe(OEP)]O-2, [Fe(OEP)(OMe)], [Fe(OEP)(OC(Me)=CH2)], and [Fe(OEP)(OClO3)]/Et(3)N; Si-S bond cleavage in the reaction of [Fe4S4(LS(3))(SSiEt(3))](2-) with [Fe(OEP)F]; oxidative addition of [Fe-II(OEP)] to the disulfide bond of the mu-S-2 double cubane {[Fe4S4(LS(3))]S-2(2)}(4-) (7). In each case, the product was [Fe4S4(LS(3))-S-Fe-III(OEP)](2-) (9), recognizable by UV-visible absorption and H-1 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. (LS(3) = 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(salicylidenearnino)ethane-(2-).)