Martha S. Reynolds

Binding of oxysulfur anions to macrocyclic iron(II,III): [(Fe(TPP))2SO4] and [Fe(Me6[14]-4,11-dieneN4)(S2O5)]

Abstract In an attempt to produce substrate-Fe(II) complexes potentially relevant to the mechanism of action of sulfite reductase, several anaerobic reaction systems were investigated. The system Fe(TPP)+SO2 in benzene afforded [(FE(TPP))2SO4] (1), which was obtained in single crystal form as the solvate [(Fe- (TPP))2SO4]·C6H6·2SO2. This compound crystallizes in orthorhombic space group P21221 with a= 12.647(2), b=13.483(3), c=23.409(6) A and Z=2. Complex 1 contains five-coordinate Fe(III) units bridged by sulfate ion binding as a monodentate ligand to each metal. The structure of 1 is closely similar to that in a differently solvated, rhombohedral lattice reported earlier. The oxidized product results from traces of dioxygen and is probably formed from a bridged peroxide intermediate in a reaction demonstrated earlier. The system [Fe(Me6- [14]-4,11-dieneN4)(MeCN)2]2++(n-Bu4N)HSO3 in acetonitrile yielded yellow, high-spin [Fe(Me6[14]- 4,11-dieneN4)(S2O5)] (2). Compound 2 crystallizes as the acetonitrile monosolvate in monoclinic space group C2/c with a=9.575(3), b=16.557(4), c= 15.409(4) A, β=96.85(2)° and Z=4. Complex 2 contains a chelating disulfite, generated in the equilibrium 2HSO3−⇌S2O52−+H2O. Dimensions of coordinated and free disulfite are not significantly different; however, the ligand is disordered around a C2 axis passing through the Fe atom and bisecting the SS bond of the chelate ring. This is the first structure of a transition metal disulfite complex. The results suggest the conditions necessary to the formation of an authentic sulfite or bisulfite complex of a Fe(II) tetraaza macrocycle, species potentially suitable as models of the enzyme-substrate complex of sulfite reductase.

Electronic properties of monocapped prismane and basket iron-sulphur clusters

Abstract The ground state electronic properties of the mixed-valence clusters Fe7S6 (PEt3)4Cl3 (1), Fe6S6(PEt3)4L2 [L = Cl− (2), Br− (4), I− (5) and PhS− (6)], Fe6Se6 (PEt3)4Cl2 (3) and [Fe6S6(PEt)6]1− (9), have been investigated by magnetic susceptibility, magnetization and Mossbauer spectroscopic measurements. Cluster 1 has a (idealized C3v) monocapped prismane structure and clusters 2–6 and 9 adopt the C2v “basket” configuration based on the [Fe6(μ2-S)(μ3-S)4(μ4-S)]2+,1+ core unit. From magnetic properties, the ground states S = 1/2 (1, 9) and S = 1 (2–5) were established. Unlike the other clusters, 6 did not show a Curie region of susceptibility; its ground state was not directly determined but is probably S = 1. Mossbauer spectra were successfully analysed in terms of a 1 : 1 : 1 iron site population. Isomer shifts and quadrupole splittings were assigned to each site. Magnetically perturbed spectra obtained in applied fields of 60–80 kOe were analysed to give the magnetic hyperfine parameters and magnetic hyperfine fields in clusters 1–5 and 9. The magnetic spectra demonstrate antiferromagnetic spin coupling which affords the indicated ground states.