AN INITIAL APPROACH TO BIOLOGICALLY RELATED BRIDGED ASSEMBLIES - PYRIDINETHIOLATE-LINKED FE4S4-FE COMPLEX-SYSTEMS

The subsite-differentiated cubane-type clusters [Fe4S4(LS3)L']z- (LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolythio)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(acetylacetoneiminate(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(trifluoroacetylacetoneiminate(2-)) forms 9-Fe(acen) and 9-Fe(tfacen). Bridge formation was readily detected from the isotropically shifted H-1 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), K(f) = 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.