Natural Assembly of Electroactive Metallopolymers on the Electrode Surface: Enhanced Electrocatalytic Production of Hydrogen by [2Fe-2S] Metallopolymers in Neutral Water.

A molecular catalyst attached to an electrode surface can offer the advantages of both homogeneous and heterogeneous catalysis. Unfortunately, some molecular catalysts constrained to a surface lose much or all of their solution performance. In contrast, we found that when a small molecule [2Fe-2S] catalyst is incorporated into metallopolymers of the form PDMAEMA-g-[2Fe-2S] (PDMAEMA = poly(2-dimethylamino)ethyl methacrylate) and adsorbed to the surface, the observed rate of hydrogen production increases to kobs > 105 s-1 per active site with lower overpotential, increased lifetime, and tolerance to oxygen. Herein, the electrocatalytic performances of these metallopolymers with different length polymer chains are compared to reveal the factors that lead to this high performance. It was anticipated that smaller metallopolymers would have faster rates due to faster electron and proton transfers to more accessible active sites, but the experiments show that the rates of catalysis per active site are independent of the polymer size. Molecular dynamics modeling reveals that the high performance is a consequence of adsorption of these metallopolymers on the surface with natural assembly that brings the [2Fe-2S] catalytic sites into close contact with the electrode surface while maintaining exposure of the sites to protons in solution. The assembly is conducive to fast electron transfer, fast proton transfer, and a high rate of catalysis regardless of the polymer size. These results offer a guide to enhancing the performance of other electrocatalysts with incorporation into a polymer that provides an optimal interaction of the catalyst with the electrode and solution.

On the Mechanism of the Inverse Vulcanization of Elemental Sulfur: Structural Characterization of Poly(sulfur-random-(1,3-diisopropenylbenzene)).

Organosulfur polymers, such as those derived from elemental sulfur, are an important new class of macromolecules that have recently emerged via the inverse vulcanization process. Since the launching of this new field in 2013, the development of new monomers and organopolysulfide materials based on the inverse vulcanization process is now an active area in polymer chemistry. While numerous advances have been made over the last decade concerning this polymerization process, insights into the mechanism of inverse vulcanization and structural characterization of the high-sulfur-content copolymers that are produced remain challenging due to the increasing insolubility of the materials with a higher sulfur content. Furthermore, the high temperatures used in this process can result in side reactions and complex microstructures of the copolymer backbone, complicating detailed characterization. The most widely studied case of inverse vulcanization to date remains the reaction between S8 and 1,3-diisopropenylbenzene (DIB) to form poly(sulfur-random-1,3-diisopropenylbenzene)(poly(S-r-DIB)). Here, to determine the correct microstructure of poly(S-r-DIB), we performed comprehensive structural characterizations of poly(S-r-DIB) using nuclear magnetic resonance spectroscopy (solid state and solution) and analysis of sulfurated DIB units using designer S-S cleavage polymer degradation approaches, along with complementary de novo synthesis of the sulfurated DIB fragments. These studies reveal that the previously proposed repeating units for poly(S-r-DIB) were incorrect and that the polymerization mechanism of this process is significantly more complex than initially proposed. Density functional theory calculations were also conducted to provide mechanistic insights into the formation of the derived nonintuitive microstructure of poly(S-r-DIB).

Increasing the rate of the hydrogen evolution reaction in neutral water with protic buffer electrolytes.

Electrocatalytic generation of H2 is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H2 generation by 50 mA/cm2 at current densities over 100 mA/cm2 compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]+) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure-activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.

Polymerizations with Elemental Sulfur: From Petroleum Refining to Polymeric Materials.

The production of elemental sulfur from petroleum refining has created a technological opportunity to increase the valorization of elemental sulfur by the synthesis of high-performance sulfur-based plastics with improved optical, electrochemical, and mechanical properties aimed at applications in thermal imaging, energy storage, self-healable materials, and separation science. In this Perspective, we discuss efforts in the past decade that have revived this area of organosulfur and polymer chemistry to afford a new class of high-sulfur-content polymers prepared from the polymerization of liquid sulfur with unsaturated monomers, termed inverse vulcanization.

Sulfenyl Chlorides: An Alternative Monomer Feedstock from Elemental Sulfur for Polymer Synthesis.

A polymerization methodology is reported using sulfur monochloride (S2Cl2) as an alternative feedstock for polymeric materials. S2Cl2 is an inexpensive petrochemical derived from elemental sulfur (S8) but has numerous advantages as a reactive monomer for polymerization vs S8. This new process, termed sulfenyl chloride inverse vulcanization, exploits the high reactivity and miscibility of S2Cl2 with a broad range of allylic monomers to prepare soluble, high molar-mass linear polymers, segmented block copolymers, and crosslinked thermosets with greater synthetic precision than achieved using classical inverse vulcanization. This step-growth addition polymerization also allows for preparation of a new class of thiol-free, inexpensive, highly optically transparent thermosets (α = 0.045 cm-1 at 1310 nm), which exhibit among the best optical transparency and low birefringence relative to commodity optical polymers, while possessing a higher refractive index (n > 1.6) in the visible and near-infrared spectra. The fabrication of large-sized optical components is also demonstrated.

Synthesis of Metallopolymers via Atom Transfer Radical Polymerization from a [2Fe-2S] Metalloinitiator: Molecular Weight Effects on Electrocatalytic Hydrogen Production.

Small molecule biomimetics inspired by the active site of the [FeFe]-hydrogenase enzymes have shown promising electrocatalytic activity for hydrogen (H2 ) generation. However, most of the active-site mimics based on [2Fe-2S] clusters are not water-soluble which limits the use of these electrocatalysts to organic media. Polymer-supported [2Fe-2S] systems, in particular, single-site metallopolymer catalysts, have shown drastic improvements for electrocatalytic H2 generation in aqueous milieu. [2Fe-2S] complexes functionalized within well-defined macromolecular supports via covalent bonding have demonstrated water solubility, enhanced site-isolation, and improved chemical stability during catalysis. In this report, the synthesis of a new propanedithiolate (pdt)-[2Fe-2S] complex bearing a single α-bromoester moiety for use in atom transfer radical polymerization (ATRP) is demonstrated as a novel metalloinitiator to prepare water-soluble poly(2-dimethylaminoethyl methacrylate) grafted (PDMAEMA-g-[2Fe-2S]) metallopolymers. Using this approach, metallopolymers with controllable molecular weights (Mn = 5-40 kg mol-1 ) and low dispersity (D, Mw /Mn = 1.09-1.36) are prepared, which allows for the first time observation of the effect of the metallopolymers' chain length on the electrocatalytic activity. The ability to control the composition and molecular weight of these metallopolymers enables macromolecular engineering via ATRP of these materials to determine optimal structural features of metallopolymer catalysts for H2 production.

Catalytic Metallopolymers from [2Fe-2S] Clusters: Artificial Metalloenzymes for Hydrogen Production.

Reviewed herein is the development of novel polymer-supported [2Fe-2S] catalyst systems for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe] hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small-molecule, [2Fe-2S]-containing mimetics of the active site (H-cluster) of these metalloenzymes have been synthesized for years. These small [2Fe-2S] complexes have not yet reached the same capacity as that of enzymes for hydrogen production. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe-2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this review, the various macromolecular motifs and the catalytic properties of these polymer-supported [2Fe-2S] materials are surveyed. The most recent catalysts that incorporate a single [2Fe-2S] complex, termed single-site [2Fe-2S] metallopolymers, exhibit superior activity for H2 production.

Infrared Fingerprint Engineering: A Molecular-Design Approach to Long-Wave Infrared Transparency with Polymeric Materials.

Optical technologies in the long-wave infrared (LWIR) spectrum (7-14 µm) offer important advantages for high-resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR-transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so-called IR-fingerprint region. We report on a new molecular-design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co-monomers.

[FeFe]-Hydrogenase Mimetic Metallopolymers with Enhanced Catalytic Activity for Hydrogen Production in Water.

Electrocatalytic [FeFe]-hydrogenase mimics for the hydrogen evolution reaction (HER) generally suffer from low activity, high overpotential, aggregation, oxygen sensitivity, and low solubility in water. By using atom-transfer radical polymerization (ATRP), a new class of [FeFe]-metallopolymers with precise molar mass, defined composition, and low polydispersity, has been prepared. The synthetic methodology introduced here allows facile variation of polymer composition to optimize the [FeFe] solubility, activity, and long-term chemical and aerobic stability. Water soluble functional metallopolymers facilitate electrocatalytic hydrogen production in neutral water with loadings as low as 2 ppm and operate at rates an order of magnitude faster than hydrogenases (2.5×105  s-1 ), and with low overpotential requirement. Furthermore, unlike the hydrogenases, these systems are insensitive to oxygen during catalysis, with turnover numbers on the order of 40 000 under both anaerobic and aerobic conditions.

[FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar µ-(SCH2 )2 ER2 Linkers (E=Ge and Sn).

Analogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-membered FeSCECS rings in these complexes (E=Ge or Sn) adopt an unusual conformation with nearly co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe2 (CO)6 cluster and low torsional strain for GeMe2 and SnMe2 moieties owing to the long C-Ge and C-Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal-metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe2 S2 (CO)6 cores bridged by CH2 SnR2 CH2 , R=Me, Ph, linkers were also obtained and characterized.

Spectroscopic Evidence for Through-Space Arene-Sulfur-Arene Bonding Interaction in m-Terphenyl Thioether Radical Cations.

Electronic absorption spectra and quantum chemical calculations of the radical cations of m-terphenyl tert-butyl thioethers, where the S-t-Bu bond is forced to be perpendicular to the central phenyl ring, show the occurrence of through-space [π···S···π](+) bonding interactions which lead to a stabilization of the thioether radical cations. In the corresponding methyl derivatives there is a competition between delocalization of the hole that is centered on a p-AO of the S atom into the π-system of the central phenyl ring or through space into the flanking phenyl groups, which leads to a mixture of planar and perpendicular conformations in the radical cation. Adding a second m-terphenyl tert-butyl thioether moiety does not lead to further delocalization; the spin and charge remain in one of the two halves of the radical cation. These findings have interesting implications with regard to the role of methionines as hopping stations in electron transfer through proteins.

Intramolecular electron transfer in bipyridinium disulfides.

Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4'-bipyridyl-3,3'-disulfide (1) and the di-N-methylated derivative (2(2+)) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S-S bond to form the dithiolate. In contrast, compound 2(2+) is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S-S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S-S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

Biosynthesis of selenocysteine on its tRNA in eukaryotes.

Selenocysteine (Sec) is cotranslationally inserted into protein in response to UGA codons and is the 21st amino acid in the genetic code. However, the means by which Sec is synthesized in eukaryotes is not known. Herein, comparative genomics and experimental analyses revealed that the mammalian Sec synthase (SecS) is the previously identified pyridoxal phosphate-containing protein known as the soluble liver antigen. SecS required selenophosphate and O-phosphoseryl-tRNA([Ser]Sec) as substrates to generate selenocysteyl-tRNA([Ser]Sec). Moreover, it was found that Sec was synthesized on the tRNA scaffold from selenide, ATP, and serine using tRNA([Ser]Sec), seryl-tRNA synthetase, O-phosphoseryl-tRNA([Ser]Sec) kinase, selenophosphate synthetase, and SecS. By identifying the pathway of Sec biosynthesis in mammals, this study not only functionally characterized SecS but also assigned the function of the O-phosphoseryl-tRNA([Ser]Sec) kinase. In addition, we found that selenophosphate synthetase 2 could synthesize monoselenophosphate in vitro but selenophosphate synthetase 1 could not. Conservation of the overall pathway of Sec biosynthesis suggests that this pathway is also active in other eukaryotes and archaea that synthesize selenoproteins.

Synthesis and structure of m-terphenyl thio-, seleno-, and telluroethers.

Several routes for the synthesis of m-terphenyl thio-, seleno-, and telluroethers were investigated. m-Terphenyl iodides react with diphenyl diselenides or ditellurides (CsOH·H(2)O, DMSO, 110 °C) to give the desired compounds in 19-84% yield which significantly extends the previously reported such reactions because o-benzyne cannot be an intermediate as previously suggested. However, the most general synthetic route was that involving reaction of 2,6-diaryl Grignard reagents with sulfur, selenium, or tellurium electrophiles. The m-terphenyl thio-, seleno-, and telluroethers were characterized spectroscopically and, in one case, by single-crystal X-ray analysis. Certain of these compounds showed atropisomerism and barriers for interconversion of isomers were determined by variable-temperature NMR spectroscopy. The barriers for interconverting the syn and anti atropisomers increase on going from the analogous S to Se to Te compounds. Calculations on this isomerization revealed that the barriers are due to rotation about the aryl-aryl bond and that the barriers for rotation about the aryl-chalcogen bond are much lower.

Electrochemical and chemical oxidation of dithia-, diselena-, ditellura-, selenathia-, and tellurathiamesocycles and stability of the oxidized species.

The diverse electrochemical and chemical oxidations of dichalcogena-mesocycles are analyzed, broadening our understanding of the chemistry of the corresponding radical cations and dications. 1,5-Diselenocane and 1,5-ditellurocane undergo reversible two-electron oxidation with inverted potentials analogous to 1,5-dithiocane. On the other hand, 1,5-selenathiocane and 1,5-tellurathiocane undergo one-electron oxidative dimerization. The X-ray crystal structures of the Se-Se dimer of the 1,5-selenathiocane one-electron oxidized product and the monomeric two-electron oxidized product (dication) of 1,5-tellurathiocane are reported. 1,5-Dithiocanes and 1,5-diselenocanes with group 14 atoms as ring members undergo irreversible oxidation, unlike the reversible two-electron oxidation of the corresponding silicon-containing 1,5-ditellurocanes. These results demonstrate the chemical consequences of the dication stabilities Te(+)-Te(+) > Se(+)-Se(+) > S(+)-S(+), as well as Se(+)-Se(+) > Se(+)-S(+) and Te(+)-Te(+) > Te(+)-S(+).

100th Anniversary of Macromolecular Science Viewpoint: High Refractive Index Polymers from Elemental Sulfur for Infrared Thermal Imaging and Optics.

Optical technologies in the midwave and long wave infrared spectrum (MWIR, LWIR) are important systems for high resolution thermal imaging in near, or complete darkness. While IR thermal imaging has been extensively utilized in the defense sector, application of this technology is being driven toward emerging consumer markets and transportation. In this viewpoint, we review the field of IR thermal imaging and discuss the emerging use of synthetic organic and hybrid polymers as novel IR transmissive materials for this application. In particular, we review the critical role of elemental sulfur as a novel feedstock to prepare high refractive index polymers via inverse vulcanization and discuss the fundamental chemical insights required to impart improved IR transparency into these polymeric materials.

One- to two-electron reduction of an [FeFe]-hydrogenase active site mimic: the critical role of fluxionality of the [2Fe2S] core.

The one- to two-electron reduction of mu-(1,2-ethanedithiolato)diironhexacarbonyl that has been observed under electrochemical conditions is dependent on scan rate and temperature, suggesting activation of a structural rearrangement. This structural rearrangement is attributed to fluxionality of the [2Fe2S] core in the initially formed anion. Computations support this assessment. Upon an initial one-electron reduction, the inherent fluxionality of the [2Fe2S] complex anion allows for a second one-electron reduction at a less negative potential to form a dianionic species. The structure of this dianion is characterized by a rotated iron center, a bridging carbonyl ligand, and, most significantly, a dissociated Fe-S bond. This fluxionality of the [2Fe2S] core upon reduction has direct implications for the chemistry of [FeFe]-hydrogenase mimics and for iron-sulfur cluster chemistry in general.

Neighboring amide participation in thioether oxidation: relevance to biological oxidation.

To investigate neighboring amide participation in thioether oxidation, which may be relevant to brain oxidative stress accompanying beta-amyloid peptide aggregation, conformationally constrained methylthionorbornyl derivatives with amido moieties were synthesized and characterized, including an X-ray crystallographic study of one of them. Electrochemical oxidation of these compounds, studied by cyclic voltammetry, revealed that their oxidation peak potentials were less positive for those compounds in which neighboring group participation was geometrically possible. Pulse radiolysis studies provided evidence for bond formation between the amide moiety and sulfur on one-electron oxidation in cases where the moieties are juxtaposed. Furthermore, molecular constraints in spiro analogues revealed that S-O bonds are formed on one-electron oxidation. DFT calculations suggest that isomeric sigma*(SO) radicals are formed in these systems.

Observation and characterization of the CH3S(O)CH- and CH3S(O)CH- x H2O carbene anions by photoelectron imaging and photofragment spectroscopy.

We report the observation of the CH(3)S(O)CH(-) and CH(3)S(O)CH(-) x H(2)O carbene anions formed upon overall H(2)(+) abstraction from dimethyl sulfoxide by O(-). Photoelectron spectroscopy reveals singlet and triplet carbenes for the remaining neutral, with the singlet state assigned as the ground state. Although some formation of the distonic CH(2)S(O)CH(2)(-) radical anion is also expected, no conclusive evidence of the presence of this isomer is found. The photoelectron spectrum of HCSO(-) is also reported for the first time. Photofragmentation of CH(3)S(O)CH(-) with 532 nm light reveals two main types of anionic products: a dominant HCSO(-) fragment, resulting from methyl elimination, and a less intense SO(-) product. For the monohydrated anion, an additional SO(-) x H(2)O fragment is observed. Intriguingly, both the SO(-) x H(2)O and SO(-) products are produced with much higher yields in the fragmentation of CH(3)S(O)CH(-) x H(2)O, compared to the SO(-) yield from the dissociation of the bare CH(3)S(O)CH(-) anion. Two possible pathways are proposed as likely mechanisms for the SO(-)-based photoproducts, both involving a photoinduced intramolecular rearrangement and the formation of a C-C bond.

Sulfur Radicals and Their Application.

The present review highlights recent developments in the chemistry of sulfur radicals. Background material essential to the understanding of these developments is briefly reviewed and references to previous in depth reviews are cited. The exciting applications of current research involving sulfur radicals in bonding theory, organic synthesis, polymer chemistry, materials science, and biochemistry are outlined.