Electrochemical Evidence for Metal Polysulfide Complexes: Tetrasulfide (S2-4) Reactions with Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+

Generation and Reactivity of Polychalcogenide Chains in Binuclear Cobalt(II) Complexes

A series of six binuclear Co(II)-thiolate complexes, [Co2(BPMP)(S-C6H4-o-X)2]1+ (X = OMe, 2; NH2, 3), [Co2(BPMP)(μ-S-C6H4-o-O)]1+ (4), and [Co2(BPMP)(μ-Y)]1+ (Y = bdt, 5; tdt, 6; mnt, 7), has been synthesized from [Co2(BPMP)(MeOH)2(Cl)2]1+ (1a) and [Co2(BPMP)(Cl)2]1+ (1b), where BPMP1- is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol. While 2 and 3 could allow the two-electron redox reaction of the two coordinated thiolates with elemental sulfur (S8) to generate [Co2(BPMP)(μ-S5)]1+ (8), the complexes, 4-7, could not undergo a similar reaction. An analogous redox reaction of 2 with elemental selenium ([Se]) produced [{Co2(BPMP)(μ-Se4)}{Co2(BPMP)(μ-Se3)}]2+ (9a) and [Co2(BPMP)(μ-Se4)]1+ (9b). Further reaction of these polychalcogenido complexes, 8 and 9a/9b, with PPh3 allowed the isolation of [Co2(BPMP)(μ-S)]1+ (10) and [Co2(BPMP)(μ-Se2)]1+ (11), which, in turn, could be converted back to 8 and 9a upon treatment with S8 and [Se], respectively. Interestingly, while the redox reaction of the polyselenide chains in 9a and 11 with S8 produced 8 and [Se], the treatment of 8 with [Se] gave back only the starting material (8), thus demonstrating the different redox behavior of sulfur and selenium. Furthermore, the reaction of 8 and 9a/9b with activated alkynes and cyanide (CN-) allowed the isolation of the complexes, [Co2(BPMP)(μ-E2C2(CO2R)2)]1+ (E = S: 12a, R = Me; 12b, R = Et; E = Se: 13a, R = Me; 13b, R = Et) and [Co2(BPMP)(μ-SH)(NCS)2] (14), respectively. The present work, thus, provides an interesting synthetic strategy, interconversions, and detailed comparative reactivity of binuclear Co(II)-polychalcogenido complexes.

Switching the memory behaviour from binary to ternary by triggering S62- relaxation in polysulfide-bearing zinc-organic complex molecular memories

The use of crystalline metal-organic complexes with definite structures as multilevel memories can enable explicit structure-property correlations, which is significant for designing the next generation of memories. Here, four Zn-polysulfide complexes with different degrees of conjugation have been fabricated as memory devices. ZnS₆(L)₂-based memories (L = pyridine and 3-methylpyridine) can exhibit only bipolar binary memory performances, but ZnS₆(L)-based memories (L = 2,2'-bipyridine and 1,10-phenanthroline) illustrate non-volatile ternary memory performances with high ON2/ON1/OFF ratios (10^4.22/10^2.27/1 and 10^4.85/10^2.58/1) and ternary yields (74% and 78%). Their ON1 states stem from the packing adjustments of organic ligands upon the injection of carriers, and the ON2 states are a result of the ring-to-chain relaxation of S₆^2- anions. The lower conjugated degrees in ZnS₆(L)₂ result in less compact packing; consequently, the adjacent S₆^2- rings are too long to trigger the S₆^2- relaxation. The deep structure-property correlation in this work provides a new strategy for implementing multilevel memory by triggering polysulfide relaxation based on the conjugated degree regulation of organic ligands.

Effect of speciation and composition on the kinetics and precipitation of arsenic sulfide from industrial metallurgical wastewater

Precipitation of arsenic as As₂S₃ produces little waste sludge, has the potential for low chemical consumption and for selective metal(loid) removal. In this study, arsenic removal from acidic (pH 2), metallurgical wastewater was tested in industrially relevant conditions. Sulfides added at a S:As molar ratio of 2.5 and 5 resulted in removal of 99% and 84% of As(III) and As(V). Precipitation of As₂S₃ from the As(III) and industrial wastewater containing 17% As(V) was nearly instantaneous. For the synthetic As(V) solution, reduction to As(III) was the rate limiting step. At a S:As ratio of 20 and an observed removal rate (k₂ = 4.8 (mol L^-1) h^-1), two hours were required to remove of 93% of arsenic from a 1 g As L^-1 solution. In the case of As(V) in industrial samples this time lag was not observed, showing that components in the industrial wastewater affected the removal and reduction of arsenate. Speciation also affected flocculation and coagulation characteristics of As₂S₃ particles: As(V) reduction resulted in poor coagulation and flocculation. Selective precipitation of arsenic was possible, but depended on speciation, S:As ratio and other metals present.

Stabilization of molecular lanthanide polysulfides by bulky scorpionate ligands

Well-defined lanthanide polysulfide complexes containing S4(2-) and S5(2-) ligands, the samarium(iii) pentasulfide complex Sm(Tp(iPr2))(κ(1)-3,5-(i)Pr2Hpz)(S5) and the tetrasulfide-bridged binuclear ytterbium(iii) complex (μ-S4)[Yb(Tp(iPr2))(κ(1)-3,5-(i)Pr2Hpz)(κ(2)-3,5-(i)Pr2pz)]2 (Tp(iPr2) = hydro-tris(3,5-diisopropylpyrazolyl)borate), have been synthesized and structurally characterized by single-crystal X-ray diffraction.

Catalysis of elemental sulfur nanoparticles on chromium(VI) reduction by sulfide under anaerobic conditions

Chromate (CrVI) reduction by sulfide was conducted in anaerobic batch experimental systems. The molar ratio of the reduced CrVI to the oxidized S(-II) was 1:1.5 during the reaction, suggesting that the product of sulfide oxidation was elemental sulfur. Under the anaerobic condition, the reaction was pseudo first order initially with respect to CrVI, but the rate was dramatically accelerated at the later stage of the reaction. The rate acceleration was due to catalysis by elemental sulfur nanoparticles; dissolved species such as monomeric elemental sulfur and polysulfides appeared to be ineffective catalysts. Elemental sulfur nanoparticles were capable of adsorbing sulfide and such adsorbed sulfide exhibited much higher reactivity toward CrVI reduction than the aqueous-phase sulfide, resulting in the observed rate acceleration. Kinetic data under various reactant concentrations can be represented by the following empirical kinetic equation: -d[CrVI]/dt = k1 [CrVI][H2S]0.63 + k3[CrVI][triple bond S--SH]0.57. The first term on the right-hand side corresponds to the noncatalytic pathway, with k1 = 1.0 x 10(-3) (microM)(-0.63) min(-1) at pH 7.60 and 8.2 x 10(-5) (microM)-0.63 min(-1) at pH 8.10. The second term, k3[CrVI][triple bond S--SH]b, is the catalytic term with [triple bond S--SH] representing the adsorbed concentration of sulfide on the elemental sulfur nanoparticles (microM). The catalytic term is more important at the later stage of the reaction, as indicated by the observed kinetics and the enhancement of the reaction rate by externally added elemental sulfur nanoparticles. At pH 8.10, k3 = 0.0057 (microM)(-0.57) min(-1).

Aqueous copper sulfide clusters as intermediates during copper sulfide formation

Using a combination of experimental techniques, we show that Cu(II) reduction by sulfide to Cu(I) occurs in solution prior to precipitation. EPR and 63Cu NMR data show that reduction to Cu(l) occurs during the reaction of equimolar amounts of Cu(II) with sulfide. 63Cu solution NMR data show that Cu(I) is soluble when bound to sulfide and is in a site of high symmetry. EPR data confirm that Cu(I) forms in solution and that the mineral covellite, CuS, contains only Cu(I). Mass spectrometry data from covellite as well as laboratory prepared solid and solution CuS materials indicate that Cu3S3 six-membered rings form in solution. These trinuclear Cu rings are the basic building blocks for aqueous CuS molecular clusters, which lead to CuS precipitation. In controlled titration experiments where sulfide is slowly added to Cu(II), Cu3S3 rings and tetranuclear Cu molecular clusters (Cu4S5, and Cu4S6) form; the rings are composed primarily of Cu(II). During cluster formation from Cu3S3 condensation, some Cu(II) is released back into solution, indicating that Cu(II) reduction does not occur until after Cu-S bond and higher order cluster formation. Analysis of the frontier molecular orbitals for Cu(II) and sulfide indicate that an outer-sphere electron transfer is symmetry forbidden. These results are consistent with the formation of CuS bonds prior to electron transfer, which occurs via an inner-sphere process.