Transition Metal Polysulfides: Coordination Compounds with Purely Inorganic Chelate Ligands

Metal-cation-induced shifts in thiolate redox and reduced sulfur speciation

Sulfur-containing anions (e.g. thiolates, polysulfides) readily exchange in solution, making control over their solution speciation and distribution challenging. Here, we demonstrate that different redox-inactive alkali, alkaline earth, and transition metals (Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Cd2+) shift the equilibria of sulfur catenation or sulfur reduction/oxidation between thiolate, polysulfanide, and polysulfide anions in acetonitrile solution. The thermodynamic factors that govern these equilibria are examined by identification of intermediate metal thiolate and metal polysulfide species using a combination of NMR spectroscopy, electronic absorption spectroscopy, and mass spectrometry. Electrochemical measurements demonstrate that the metal cation of the electrolyte modulates both sulfur reduction and thiolate oxidation potentials. DFT calculations suggest that the changes in equilibria are driven by stronger covalent interactions between polysulfide anions and more highly charged cations.

Polysulfido Chain in Binuclear Zinc(II) Complexes

The synthesis and a detailed reactivity study of a binuclear zinc(II) bis(benzenethiolate) complex, [Zn₂(BPMP)(SPh)₂]⁺ (4), and an unprecedented binuclear zinc(II) pentasulfido complex, [Zn₂(BPMP)(μ₂-S₅)]⁺ (6), are presented. While one-electron oxidation of the coordinated benzenethiolate ligands in 4 by Cp₂Fe⁺ produces diphenyl disulfide and [Zn₂(BPMP)(μ₂-OH)]²⁺ (5), a two-electron redox reaction between coordinated benzenethiolate ligands in 4 and elemental S (S₈) generated diphenyl disulfide and the binuclear zinc(II) pentasulfido complex 6. Complex 6 features a chelating, dianionic, pentasulfido (S₅^2-) chain and can consume up to a maximum of 3 equiv of PPh₃ to generate Ph₃PS and 5, while the reaction of 6 with 1 equiv of diphenylphosphinoethane allowed the isolation of [Zn₂(BPMP)(μ₂-S₄)]⁺ (7). A proteolysis reaction of the coordinated S₅^2- chain in 6 with fluoroboric acid (HBF₄), benzoic acid (PhCOOH), and thioacetic acid (MeCOSH) generates the complexes [Zn₂(BPMP)(MeCN)₂]³⁺ (1), [Zn₂(BPMP)(μ₂-PhCOO)₂]⁺ (8), and [Zn₂(BPMP)(μ₂-SCOMe)₂]⁺ (9), respectively, while the protonated S₅^2- chain liberates S₈ and hydrogen sulfide (H₂S). Finally, the transfer of the coordinated benzenethiolate ligands in 4 and the S₅^2- chain in 6 to selected organic compounds, namely, PhCH₂Br and PhC(O)Cl, for the generation of various organosulfur compounds is demonstrated.

Flux Crystal Growth, Crystal Structure, and Magnetic Properties of a Ternary Chromium Disulfide Ba9Cr4S19 with Unusual Cr4S15 Tetramer Units

A new ternary chromium disulfide, Ba₉Cr₄S₁₉, has been grown out of BaCl₂ molten salt. Single-crystal structure analysis revealed that it crystallizes in the centrosymmetric space group C 2/c with lattice parameters: a = 12.795(3) Å, b = 11.3269(2) Å, c = 23.2057(6) Å, β = 104.041(3)°, and Z = 4. Ba₉Cr₄S₁₉ comprises four face-sharing Cr-centered octahedra with disulfide ions occupying sites on each terminal face. The resulting Cr₄S₁₅ tetramer units are isolated by nonmagnetic Ba-centered polyhedra in the ab plane and barium disulfide (=Ba₄(S₂)₂) layers along the c-axis. Following the structure analysis, the title compound should be expressed as [Ba²⁺]₉[Cr³⁺]₄[(S₂)^2-]₄[S^2-]₁₁, which is also consistent with Cr2p X-ray photoemission spectra showing trivalent states of the Cr atoms. The unique Cr-based zero-dimensional structure with the formation of these disulfide ions can be achieved for the first time in ternary chromium sulfides, which adopt 1-3 dimensional frameworks of Cr-centered polyhedra.

Sulfur transfer reactions of a zinc tetrasulfanido complex

A zinc tetrasulfanido complex supported by a bis(carboxamide)pyridine ligand framework has been synthesized by the insertion of elemental sulfur into the zinc-S(thiolate) bond of a zinc dithiolate complex ([LZn]2-). This paper reports on sulfur transfer reactions of this polysulfanido complex ([1]2-) and compares this behavior to known reactions of metal polysulfido complexes. Complex [1]2- was demonstrated to be in exchange with [LZn]2- and free elemental sulfur in solution. Although triphenylphosphine abstracts sulfur from [1]2- to form [LZn]2-, complex [LZn]2- can abstract sulfur from the zinc polysulfido complex (TMEDA)ZnS6 (TMEDA = N,N,N',N'-tetramethylethylenediamine). The tetrasulfanido complex [1]2- can also transfer sulfur to dimethyl acetylenedicarboxylate to form a zinc dithiolene complex. These studies demonstrate that the zinc complex with a tetrasulfanido moiety can undergo similar reactions as metal complexes with purely inorganic polysulfido groups, although the final metal-containing products are different.

Reactivity of Binary and Ternary Sulfur Halides towards Transition-Metal Compounds

Binary sulfur fluorides exhibit an interesting reactivity towards transition metal complexes. They open up routes for the generation of sulfur‐containing building blocks. Often ligands with particular properties can be constructed. This includes their ability to transfer sulfur atoms or polysulfide units as well as fluorination reactions. This Minireview provides an insight into the reactivity of the binary and ternary sulfur halides S₂Cl₂, SCl₂, SF₄, SF₆ and SF₅Cl towards transition‐metal compounds.

Elastomer Reinforced with Innate Sulfur-Based Cross-Links as Ligands

Although the incorporation of sacrificial bonds into an elastomer is an effective way to provide a combination of high strength and high fracture toughness, this method normally involves complicated chemical processes. The coordination between metal ions and polysulfides has been documented. However, the potential of polysulfide structures in vulcanizates as ligands has long been neglected. Using innate sulfur-based cross-links, we show how weak and nonpolar elastomers achieve significant reinforcement without modification of the backbone. By simply soaking vulcanizates into solutions containing metal ions, dual ions are simultaneously introduced into the vulcanizate to generate coordinations with different bond strengths, resulting in an unprecedented high modulus. Overall, this work presents a universal yet high-efficiency reinforcing strategy to prepare high-performance elastomers without additional chemical modifications, which should promote comprehensive research and industrial application of sacrificial bond strategies for elastomers.

Multi-electron reduction of sulfur and carbon disulfide using binuclear uranium(iii) borohydride complexes

The first use of a dinuclear UIII/UIII complex in the activation of small molecules is reported. The octadentate Schiff-base pyrrole, anthracene-hinged 'Pacman' ligand LA combines two strongly reducing UIII centres and three borohydride ligands in [M(THF)4][{U(BH4)}2(μ-BH4)(LA)(THF)2] 1-M, (M = Li, Na, K). The two borohydride ligands bound to uranium outside the macrocyclic cleft are readily substituted by aryloxide ligands, resulting in a single, weakly-bound, encapsulated endo group 1 metal borohydride bridging the two UIII centres in [{U(OAr)}2(μ-MBH4)(LA)(THF)2] 2-M (OAr = OC6H2t Bu3-2,4,6, M = Na, K). X-ray crystallographic analysis shows that, for 2-K, in addition to the endo-BH4 ligand the potassium counter-cation is also incorporated into the cleft through η5-interactions with the pyrrolides instead of extraneous donor solvent. As such, 2-K has a significantly higher solubility in non-polar solvents and a wider U-U separation compared to the 'ate' complex 1. The cooperative reducing capability of the two UIII centres now enforced by the large and relatively flexible macrocycle is compared for the two complexes, recognising that the borohydrides can provide additional reducing capability, and that the aryloxide-capped 2-K is constrained to reactions within the cleft. The reaction between 1-Na and S8 affords an insoluble, presumably polymeric paramagnetic complex with bridging uranium sulfides, while that with CS2 results in oxidation of each UIII to the notably high UV oxidation state, forming the unusual trithiocarbonate (CS3)2- as a ligand in [{U(CS3)}2(μ-κ2:κ2-CS3)(LA)] (4). The reaction between 2-K and S8 results in quantitative substitution of the endo-KBH4 by a bridging persulfido (S2)2- group and oxidation of each UIII to UIV, yielding [{U(OAr)}2(μ-κ2:κ2-S2)(LA)] (5). The reaction of 2-K with CS2 affords a thermally unstable adduct which is tentatively assigned as containing a carbon disulfido (CS2)2- ligand bridging the two U centres (6a), but only the mono-bridged sulfido (S)2- complex [{U(OAr)}2(μ-S)(LA)] (6) is isolated. The persulfido complex (5) can also be synthesised from the mono-bridged sulfido complex (6) by the addition of another equivalent of sulfur.

Multi-electron reactivity of a cofacial di-tin(ii) cryptand: partial reduction of sulfur and selenium and reversible generation of S3˙. Chem Sci 2016;7:6928-6933. [PMID: 28567264 PMCID: PMC5450590 DOI: 10.1039/c6sc01754a]

A cofacial di-tin(ii) hexacarboxamide cryptand that binds sulfur to form a complex containing μ-S and bridging μ-S₅ ligands and acts reversibly as a source of S₃˙^– in DMF solution is described.

Cofacial bimetallic tin(ii) ([Sn₂(mBDCA-5t)]^2–, 1) and lead(ii) ([Pb₂(mBDCA-5t)]^2–, 2) complexes have been prepared by hexadeprotonation of hexacarboxamide cryptand mBDCA-5t-H₆ together with double Sn(ii) or Pb(ii) insertion. Reaction of 1 with elemental sulfur or selenium generates di-tin polychalcogenide complexes containing μ-E and bridging μ-E₅ ligands where E = S or Se, and the Sn(ii) centers have both been oxidized to Sn(iv). Solution and solid-state UV-Vis spectra of [(μ-S₅)Sn₂(μ-S)(mBDCA-5t)]^2– (4) indicate that the complex acts reversibly as a source of S₃˙^– in DMF solution with a K_eq = 0.012 ± 0.002. Reductive removal of all six chalcogen atoms is achieved through treatment of [(μ-E₅)Sn₂(μ-E)(mBDCA-5t)]^2– with PR₃ (R = ^tBu, Ph, OⁱPr) to produce six equiv. of the corresponding EPR₃ compound with regeneration of di-tin(ii) cryptand complex 1.

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.

Formation and Reactivity of Organo-Functionalized Tin Selenide Clusters

Reactions of R(1) SnCl3 (R(1) =CMe2 CH2 C(O)Me) with (SiMe3 )2 Se yield a series of organo-functionalized tin selenide clusters, [(SnR(1) )2 SeCl4 ] (1), [(SnR(1) )2 Se2 Cl2 ] (2), [(SnR(1) )3 Se4 Cl] (3), and [(SnR(1) )4 Se6 ] (4), depending on the solvent and ratio of the reactants used. NMR experiments clearly suggest a stepwise formation of 1 through 4 by subsequent condensation steps with the concomitant release of Me3 SiCl. Furthermore, addition of hydrazines to the keto-functionalized clusters leads to the formation of hydrazone derivatives, [(Sn2 (μ-R(3) )(μ-Se)Cl4 ] (5, R(3) =[CMe2 CH2 CMe(NH)]2 ), [(SnR(2) )3 Se4 Cl] (6, R(2) =CMe2 CH2 C(NNH2 )Me), [(SnR(4) )3 Se4 ][SnCl3 ] (7, R(4) =CMe2 CH2 C(NNHPh)Me), [(SnR(2) )4 Se6 ] (8), and [(SnR(4) )4 Se6 ] (9). Upon treatment of 4 with [Cu(PPh3 )3 Cl] and excess (SiMe3 )2 Se, the cluster fragments to form [(R(1) Sn)2 Se2 (CuPPh3 )2 Se2 ] (10), the first discrete Sn/Se/Cu cluster compound reported in the literature. The derivatization reactions indicate fundamental differences between organotin sulfide and organotin selenide chemistry.

Methodology for assessing thioarsenic formation potential in sulfidic landfill environments

Arsenic leaching and speciation in landfills, especially those with arsenic bearing waste and drywall disposal (such as construction and demolition (C&D) debris landfills), may be affected by high levels of sulfide through the formation of thioarsenic anions. A methodology using ion chromatography (IC) with a conductivity detector was developed for the assessment of thioarsenic formation potential in sulfidic landfill environments. Monothioarsenate (H2AsSO3(-)) and dithioarsenate (H2AsS2O2(-)) were confirmed in the IC fractions of thioarsenate synthesis mixture, consistent with previous literature results. However, the observation of AsSx(-) (x=5-8) in the supposed trithioarsenate (H2AsS3O(-)) and tetrathioarsenate (H2AsS4(-)) IC fractions suggested the presence of new arsenic polysulfide complexes. All thioarsenate anions, particularly trithioarsenate and tetrathioarsenate, were unstable upon air exposure. The method developed for thioarsenate analysis was validated and successfully used to analyze several landfill leachate samples. Thioarsenate anions were detected in the leachate of all of the C&D debris landfills tested, which accounted for approximately 8.5% of the total aqueous As in the leachate. Compared to arsenite or arsenate, thioarsenates have been reported in literature to have lower adsorption on iron oxide minerals. The presence of thioarsenates in C&D debris landfill leachate poses new concerns when evaluating the impact of arsenic mobilization in such environments.

Polychalcogenide Complexes as Low Temperature Precursors for Quantum Size and Bulk Binary and Ternary Semiconductors

We report the utility of \Cd(Se4)2]2-, \Cu4Se12]2- and \In3Se15]3- as convenient low temperature precursors to semiconducting CdSe and CuInSe2. DMF and DMSO solutions of these complexes react with Se-abstracting reagents such as CN- and n-(Bu3)P to yield the corresponding binary solids at 155 °C or less. Appropriate stoichiometric mixtures of \Cu4Se12]2- and \In3Se15]3- react to give CuInSe2. The semiconducting solids were characterized with UV/vis spectroscopic, X-ray crystallographic and electron (scanning and transmission) microscopic techniques. The particle size of these materials can range from the quantum-size regime to the bulk regime, depending on the reaction conditions, metal precursor complex and Se-abstracting reagent used.

Facets of early transition metal–sulfur chemistry: Metal–sulfur ligand redox, induced internal electron transfer, and the reactions of metal–sulfur complexes with alkynes

Metal-sulfur ligand redox interplay, induced internal electron transfer reactions, and the generation of dithiolene and organosulfur ligands in the reactions of metal-sulfur compounds with alkynes are important and useful facets of early transition metal-sulfur chemistry. This review focuses on developments in these areas over the past 30 years.

Synthesis and characterization of well-defined aluminum-containing heterobimetallic selenides

A series of novel aluminum heterobimetallic selenides were reported. The reaction of LAl(SeH)2 (1) with LiN(SiMe3)2 resulted in the formation of [LAl(SeLi)2(THF)2] (2) (L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3). Compound 2 reacted with Me2GeCl2, Ph2GeCl2, Cp2TiCl2, and Cp2ZrCl2, respectively, to produce LAl(mu-Se)2GeMe2 (3), LAl(mu-Se)2GePh2 (4), LAl(mu-Se)2TiCp2 (5), and LAl(mu-Se)2ZrCp2 (6) in moderate yields. Compounds 2-6 were characterized by elemental analysis, NMR, and electron impact-MS. The X-ray single-crystal structure of 3 is reported and confirms the spirocyclic arrangement of the aluminum atom within the six-membered AlN2C3 and four-membered AlSe2Ge rings.

Synthesis, Characterization, and Biomimetic Chemistry of cis-Oxosulfidomolybdenum(VI) Complexes Stabilized by an Intramolecular Mo(O)S···S Interaction

The reactions of jade-green Tp*MoIVO(S2PR2) [Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Et, Pri, Ph] with propylene sulfide produce ochre-red Tp*MoVIOS{SP(S)R2}. The complexes have been characterized by microanalysis, mass spectrometry, cyclic voltammetry, spectroscopy (IR, NMR, UV-vis, and X-ray absorption), and X-ray crystallography. The distorted-octahedral isopropyl and phenyl derivatives feature a tridentate fac-Tp* ligand, a terminal oxo ligand, and a unique five-membered Mo(=S){SP(=S)R2 ring moiety formed by a weak, intramolecular, bonding interaction between the Mo=S1 and (uncoordinated) S3=P moieties. The Mo=S1 [2.227(2) A (R = Pri) and 2.200(2) A (R = Ph)] and S1...S3 distances [2.396(3) A (R = Pri) and 2.383(2) A (R = Ph)] are indicative of a pi-bonded Mo=S1 unit and a weak (bond order ca. 1/3) S1...S3 interaction; the solid-state structures are maintained in solution according to S K-edge X-ray absorption data. The complexes react with excess cyanide to form thiocyanate and Tp*MoO(S2PR2), under anaerobic conditions, or Tp*MoO2(S2PR2), under aerobic conditions; the latter models the production of thiocyanate and desulfo molybdenum hydroxylases upon cyanolysis of molybdenum hydroxylases. The complexes react with triphenylphosphine to give Tp*MoO(S2PR2) and SPPh3, with cobaltocene or hydrosulfide ion to produce [Tp*MoVOS(S2PR2)]-, and with ferrocenium salts to yield [Tp*MoVO(S3PR2)]+; in the last two reactions, Mo(V) is produced by direct or induced internal redox reactions, respectively. The presence of the Mo(O)=S...S interaction does not radically lengthen the Mo=S bond in the complexes or preclude them from reactions typical of unperturbed oxosulfidomolybdenum(VI) complexes.

Synthesis and Structural Characterization of Some Selenoruthenates and Telluroruthenates

The reaction of solid [RuClCp(PPh(3))(2)] with TeSe(3)(2-) or Se(n)(2-) in DMF leads to the formation of [RuCp(PPh(3))(mu(2)-Se(2))](2) (1). In the structure of this compound the two bridging Se(2) groups lead to a six-membered Ru(2)Se(4) ring in a chair conformation. Attached to each Ru center is a PPh(3) ligand in an equatorial position and a Cp ring in an axial position. The compound is diamagnetic. The compound [Ru(2)Cp(2)(mu(3)-Se(2))(mu(3)-Se)](2) (2) is obtained under similar conditions in the presence of air. This structure comprises a centrosymmetric Ru(4)Se(6) dimer formed from the two bridging Se groups and the two bridging Se(2) groups. Each Ru center is pi-bonded to a Cp ring. The reaction of solid [RuClCp(PPh(3))(2)] with a Te(n)(2-) polytelluride solution in DMF leads to the diamagnetic compound [(RuCp(PPh(3)))(2)(mu(2)-(1,4-eta:3,6-eta)Te(6))] (3). Here the Ru centers are bound to a bridging Te(6) chain at the 1, 4, 3, and 6 positions, leading to a bicyclic Ru(2)Te(6) ring. Each Ru atom is bound to a Cp ring and a PPh(3) group. This dimer possesses a center of symmetry. The structure of 3 is the first example of a bicyclic complex where fusion occurs along a Te-Te bond. If the same reaction is carried out in DMF/CH(2)Cl(2), rather than DMF, then [(RuCp(PPh(3)))(2)(mu(2)-(1,4-eta:3,6-eta)Te(6))].CH(2)Cl(2) (4) is obtained. In the solid state it possesses the same Ru(2)Te(6) structural unit as does 3, but the unit lacks a crystallographically imposed center of symmetry. The electronic structures of 3 and 4 have been analyzed with the use of first principles density functional theory. Bond order analysis indicates that the Te-Te bond where fusion occurs has a shared bonding charge of about (2)/(3) of that found for Te-Te single bonds.

Electrophilic Attack on Sulfur-Sulfur Bonds: Coordination of Lithium Cations to Sulfur-Rich Molecules Studied by Ab Initio MO Methods

Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1-5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiS(n) heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from -88 kJ mol(-1) for [H2SLi]+ to -189 kJ mol(-1) for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5==S ligand have the highest binding energies (-163+/-1 kJ mol(-1)). However, the global minimum structure of [LiS6]+ is of C(3v) symmetry with the six-membered S(6) homocycle in the well-known chair conformation and three Li--S bonds with a length of 256 pm (binding energy: -134 kJ mol(-1)). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion-dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S4)]+. In the two most stable isomers of the [LiS6]+ complexes, the number of S--S bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the Li--S bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction.

Photochemistry of the Dinitrosyl Iron Complex [S5Fe(NO)2]- Leading to Reversible Formation of [S5Fe(μ-S)2FeS5]2-: Spectroscopic Characterization of Species Relevant to the Nitric Oxide Modification and Repair of [2Fe−2S] Ferredoxins

The reaction of [PPN][Fe(CO)(3)(NO)] and S(8) in a 1:1 molar ratio in THF proceeded to give the dinitrosyl iron complex [PPN][S(5)Fe(NO)(2)] (1) and the known [PPN](2)[S(5)Fe(mu-S)(2)FeS(5)] (2). EPR signals of g values g(z) = 2.0148, g(x) = 2.0270, and g(y) = 2.0485 at 77 K confirmed the existence of the unpaired electron in compound 1. The temperature-dependent magnetic moment of complex 1 indicates that the ground state is one unpaired electron with (S(t), S(L)) = ((1)/(2), 1) at very low temperature (S(t) is the total spin quantum number of the system; S(L) is the sum of the spin quantum numbers of two NO ligands). The O K-edge absorptions of complex 1 and [(NO)Fe(S(2)CNEt(2))(2)] at 532.1 and 532.5 eV are assigned to the transition of 1s --> pi(NO) and 1s --> pi(NO(+)), respectively. For the electronic structure of the [Fe(NO)(2)] core, DFT calculations, magnetic susceptibility measurement, EPR, and Fe K-/L-edge XAS spectroscopy of complex 1 lead to a description of [Fe(1+)(.NO)(2)](9). [2Fe-2S] cluster 2 treated with nitric oxide in THF shows that cluster 2 is transformed into the dinitrosyl iron complex 1 identified by IR, UV-vis, and X-ray diffraction analysis. The reaction may be reversed by the photolysis of the THF solution of 1 in the presence of the NO-accepting reagent [(C(4)H(8)O)Fe(S,S-C(6)H(4))(2)](-) to reform 2. This result demonstrates a successful biomimetic reaction cycle of the degradation and reassembly of [2Fe-2S] cluster [S(5)Fe(mu-S)(2)FeS(5)](2)(-) relevant to the repair of nitric oxide-modified [2Fe-2S] ferredoxin by cysteine desulfurase and l-cysteine in vitro.