Synthesis and electron delocalization of [Fe4S4]-S-Fe(III) bridged assemblies related to the exchange-coupled catalytic site of sulfite reductases

Article abstract of DOI:10.1021/ja00095a021

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-Fe^III(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 [Fe^II(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-Fe^III(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-).).