Central Heteroatom Research Articles

Electrode kinetics for the reduction of central hetero VinV and framework addenda VoutV and WVI atoms in the [VinVoutW11O40]4− polyoxometalate: Comparisons with [SVoutW11O40]3− and [VinW12O40]3−

The electrode kinetics associated with the polyoxometalate (POM) [VinVoutW11O40]4-/5-/6−, [VinW12O40]3-/4-/5− and [SVoutW11O40]3-/4-/5− (Vin – central hetero vanadium atom, Vout – framework addenda vanadium atom) reduction processes in CH3CN (0.10 M [Bu4N][PF6]) have been determined by Fourier Transformed large amplitude alternating current voltammetry at a glassy carbon electrode. [VinVoutW11O40]4− provides an interesting situation where both the VinV and VoutV vanadium atoms can be reduced to the VIV state. The heterogeneous charge transfer rate constant kVout0′ value for the framework [VinVVoutVW11O40]4−/[VinVVoutIVW11O40]5− process of 0.15 cm s−1 (reversible formal potential E0ʹ = −223 mV vs Fc0/+) is almost ten times that of 0.020 for the kVin0′ (E0ʹ = −1027 mV vs Fc0/+) one associated with the central [VinVVoutIVW11O40]5−/[VinIVVoutIVW11O40]6− reaction. The significant decrease in the electrode kinetics for the process involving reduction of the central vanadium could in principle be attributed to differences in the Vin and Vout coordination geometry or a longer distance required for electron transfer between the electrode surface and Vout. However, neither of these factors is considered to be dominant as kVin0′ associated with the [VinVW12O40]3−/[VinIVW11O40]4- reaction is 0.19 cm s−1 (E0ʹ is −114 mV vs Fc0/+). With this POM, the more negative second process (E0ʹ = −918 mV vs Fc0/+) is associated with a WVI/WV framework reduction with a k0ʹ value of 0.080 cm s−1. With the second control POM, [SVoutW11O40]3−, the initial very fast framework VoutV/VoutIV reduction step (E0ʹ = 145 mV vs Fc0/+) has a k0ʹ value of 0.19 cm s−1 and is followed at considerably more negative potentials (E0ʹ = −1343 mV vs Fc0/+) by the slightly slower framework WVI/WV process with k0ʹ = 0.13 cm s−1. Thus, the k0ʹ values are most strongly influenced by the negative charge (n value) associated with the POMn-(/n+1)- reaction, being close to reversible when n = 3 as in the [VinW12O40]3−4− and [SVoutW12O40]3−/4− reactions, but much slower when n = 5 as in the case of the [VinVoutW11O40]5-/6- reaction. Accordingly, the initial reduction step is always faster than the second one that occurs at more negative potentials, irrespective of whether Vin, Vout or W are reduced. In the case of thermodynamically relevant E0ʹ values, it has been established that a strong correlation exists with the n value in POMn-/(n+1)− process. It is now shown that a similar correlation of k0ʹ with n applies.

Recyclization reaction of <i>S</i>-, <i>N</i>-containing cyclic peroxides with aromatic amines

The recyclization reaction of tetraoxathiaspiroalkanes and tetraoxathiocanes with primary amines ( o , p -fluoroanilines, m , p -chloroanilines, o -toluidine, o , p -anisidines, ammonium chloride) to obtain cycloazadiperoxides under the action of samarium nitrate crystal hydrate as a catalyst was studied. The possibility of recycling N -( tert -butyl)hexaoxaazadispirocycloalkanes with arylamines to the corresponding azatriperoxides was shown. It was found that the rate of the recyclization reaction depends on the nature of the central heteroatom in the heterocycle (S > N > O).

Molecular Engineering of Enamine‐Based Hole‐Transporting Materials for High‐Performing Perovskite Solar Cells: Influence of the Central Heteroatom

Stabilizing the high‐performing perovskite solar cells (PSCs) with low‐cost and simply affordable hole‐transporting materials (HTMs) has been identified as an ongoing ambitious challenge. Herein, a series of enamine‐based HTMs having different central heteroatoms (C, N, O, and S) and a number of enamine branches is designed and synthesized. The impact of varied central heteroatom cores is investigated in‐depth including thermal, photophysical, and photovoltaic properties. Importantly, molecularly engineered HTMs are obtained by a single condensation reaction without the need for expensive catalysts, inert reaction conditions, or tedious product purification. PSCs with a power conversion efficiency (PCE) of over 20% can be realized with the triphenylamine core HTM (V1435), a result comparable with spiro‐OMeTAD. HTMs based on tetraphenylmethane (V1431) and diphenyl sulfide (V1434) cores give a slightly lower performance under similar device fabrication conditions. This work demonstrates how rational molecular engineering of a simple condensation approach can produce HTMs for high‐performing PSCs without sacrificing the PCE.

The role of central heteroatom in electrochemical nitrogen reduction catalyzed by polyoxometalate-supported single-atom catalyst

The role of central heteroatom in electrochemical nitrogen reduction catalyzed by polyoxometalate-supported single-atom catalyst

Unusual redox activity of the central heteroatom manganese in Anderson anion: Modulating its oxidation state in a gas solid reaction

The orange crystalline [N(C4H9)4]3[MnIIIMo6O18{(OCH2)3CNH2}2] (1) with central Mn(III), on exposure to vapors of conc. HCl is converted to green colour solid of tentative formula, [N(C4H9)4]2[MnIIMo6O18{(OCH2)3CNH3}2] ∙ [N(C4H9)4]Cl (1red). Compound 1, on exposure to conc. HNO3 vapours is converted to brown solid, which has tentatively been formulated as [MnIVMo6O18{(OCH2)3CNH3}2] ∙ 3[N(C4H9)4]NO3 (1ox). The color changes are associated with oxidation of central Mn(III) center of compound 1 into Mn(IV) in compound 1ox and reduction of central Mn(III) center of compound 1 into Mn(II) in compound 1red. Both solid state reduction and oxidation of Mn(III) center are accompanied by the protonation of amino group of tris(hydroxymethyl) aminomethane during the gas–solid interface reactions. The conversion of compound 1 into compounds 1red and 1ox in gas solid redox reactions has been characterized by various spectroscopic techniques, e.g., FT-IR, PXRD, EPR, DRS, 1H NMR, 13C NMR and XPS. Also, electrochemical analyses were carried out for all the compounds.

Polarizability is a key parameter for molecular electronics.

Identifying descriptors that govern charge transport in molecular electronics is of prime importance for the elaboration of devices. The effects of molecule characteristics, such as size, bulkiness or charge, have been widely reported. Herein, we show that the molecule polarizability can be a crucial parameter to consider. To this end, platinum nanoparticle self-assemblies (PtNP SAs) are synthesized in solution, including a series of polyoxometalates (POMs). The charge of the POM unit can be modified according to the nature of the central heteroatom while keeping its size constant. POM hybrids that display remote terminal thiol functions strongly anchor the PtNP surface to form robust SAs. IV curves, recorded by conductive AFM, show a decrease in Coulomb blockade as the dielectric constant of the POMs increases. In this system, charge transport across molecular junctions can be interpreted as variations in polarizability, which is directly related to the dielectric constant.

Deep aerobic oxidative desulfurization of model fuel by Anderson-type polyoxometalate catalysts

Anderson-type polyoxometalate (POM) catalysts were synthesized and applied for deep oxidative desulfurization of model fuel containing dibenzothiophene. O2 contained in air was used as oxidant under mild conditions. The influence of the nature of central heteroatom and that of the quaternary ammonium cation used on the conversion of DBT was investigated. For the first time to the best of our knowledge, the role of the solvent in aerobic oxidative desulfurization was investigated and the possible mechanism of DBT oxidation was suggested. It was shown that the key stage in the oxidation of the sulfur-containing compound is the generation of alkyl peroxides from the solvent.

MW12O44] clusters: unprecedented central heteroatoms atomically dispersed in the eight coordination state bridging the 1 : 12 polyoxometalate family of Keggin and Silverton.

A general synthetic protocol is developed to afford a series of [MW12O44] (M = Ni2+, Co2+ and Fe3+) clusters with diverse central heteroatoms, by employing [W12O44]16- as the structure-directing precursor. The structures of the [MW12O44] clusters are definitively confirmed by single crystal X-ray diffraction (XRD). The central heteroatoms are monodispersed and capture the "empty" cavity of [W12O44]16- with an 8 coordination number state, as demonstrated by the combination of single crystal structure extended X-ray absorption fine structure (EXAFS) fitting analysis and wavelet transform EXAFS (WTEXAFS). The chemical state of the heteroatoms is confirmed by analyses of high resolution synchrotron radiation photoelectron spectroscopy (HR-SRPES), high resolution X-ray photoelectron spectroscopy (HR-XPS) and EXAFS. The exact components of the [MW12O44] clusters and their thermal stability are also investigated by inductively coupled plasma-mass spectrometry (ICP-MS) and thermal gravimetric analyses (TGA). The powder XRD patterns indicate their phase purity. Moreover, the newly discovered [MW12O44] clusters bridge the Keggin and Silverton polyoxometalate family structures and so unify the 1 : 12 hetero-polyoxometalates.

Recent advances in controllable alkoxylation chemistry of Anderson-type polyoxometalates from synthetic strategies perspective

Polyoxometalates (POMs) are an exceptional family of inorganic oxide anions consisting of Mo, W, V, etc. early transition metal ions at their highest oxidization states with structural versatility. The Anderson-type hetreopolyanions are one of the most important subclass since their most flexibility benefited from the central heteroatom. Among the frontier of current POMs investigations, organic modification and functionalization of POMs stands for one of most hot topics. The organic modification of POMs can incorporate POMs with advanced functional organic moieties on their surfaces to make the structure versatility more accessible and the design of advanced functional materials more rational. In this review, recent advances in controllable alkoxylation chemistry of Anderson-type polyoxometalates are briefly summarized into 9 parts as following: (1) the synthesis strategy development of direct modification protocol for the synthesis of single-side triol-functionalized Anderson δ isomer organic hybrids. (2) Based on direct modification protocol, the synthesis strategy development of μ 2-O atom pronation and activation chemistry in Anderson cluster to afford single-side triol-functionalized Anderson χ and ψ isomer organic hybrids. The proton-controlled isomer transformation between the δ , χ and ψ isomer was discovered. This will open up a new era for developing chemical reactivity of μ 2-O atoms in the Anderson cluster. (3) By precise proton control, the synthesis strategy development to obtain diol functionalized Anderson organic hybrids. Such diol functionalization mode not only works for some specific triol ligands but also can readily be extended to the diol ligands, which will greatly enrich the species of alkoxo-derivatized Anderson POM clusters. (4) Based on direct modification protocol, extending special tripodal ligands with carboxyl group to symmetrically functionalized the Anderson cluster. Such Anderson organic hybrids cannot be obtained for the traditional reconstruction protocol. This has extended the versatility of tripodal ligands applied for Anderson-type POMs modification. (5) Through stepwise synthesis strategy, single-side δ isomer was utilized as a precursor to efficiently synthesized asymmetrically triol-functionalized Anderson organic hybrids. (6) The synthesis strategy development of direct parent Anderson alkoxylation protocol for triol-functionalized butterfly-shaped β isomers of Anderson POMs hybrids. (7) The synthesis strategy development to extend the triol-functionalized Anderson from polymolybdates(Mo6O24) to polyoxotungstates(W6O24). (8) The newly development and revised synthesis strategy of Mo8 reconstruction protocol. (9) The original concept proposed by our group, organic ligand protected and inorganic-ligand supported atomically precise Anderson nanocluster based molecular armor hybrid material design as effective catalysts for green aerobic oxidation catalysis applications. In summary, the synthetic strategies and structures of novel alkoxyl-functionalized Anderson cluster have been discussed in this review. These newly developed synthesis strategies have greatly enriched the category of organoalkoxylation derivatives of Anderson cluster, providing a reliable and convenient synthetic methodology for the controllable design of triol-functionalized Anderson organic hybrids with specific and desirable functionalization modes. The extension of triol ligands with functional groups to other POMs units are still need to be developed. One of the important application fields for POMs is catalysis, however, the exploration of catalysis of alkoxyl-functionalized Anderson nanoclusters is quite limited to the recent cases we mentioned in this review. It is worth exploring to develop more alkoxyl-functionalized Anderson nanoclusters as catalysts for useful reactions, because alkoxyl-functionalized Anderson nanoclusters have multiple active sites and special properties derived from functional groups of alkoxyl ligands as well as cooperativities. The charge transfer properties between POMs and organic moieties in alkoxyl-functionalized Anderson nanoclusters have also attracted widely attentions. Using alkoxyl ligands with conjugated groups to functionalize Anderson nanoclusters may enhance the ability of electron transfer to improve the intrinsic redox properties of Anderson nanoclusters. It can be concluded with confidence that the persistent and dedicated efforts of chemists have revealed a vast advance in alkoxylation of Anderson nanoclusters, which may in fact just be the reason why the alkoxylation chemistry of Anderson nanoclusters research field has flourished in recent years. Of course, more Anderson nanoclusters-based materials with novel structures, interesting and functional properties remain to be explored and the alkoxylation chemistry of Anderson nanoclusters still has great growing opportunities in the future.

Heteropoly Acid Functionalized Membranes for High Performance and Chemically Stable Fuel Cell Operation

There is still a need for membranes that operate in proton exchange membrane (PEM) fuel cells at hotter and drier conditions than can be achieved with current materials, >100°C and <50%RH. One approach pursued by us and independently by Kunz and co-workers was the use of inorganic super acids, such as the heteropoly acids (HPAs). HPAs are a sub group of the large class of metal oxygen clusters known as polyoxometalates in which a central heteroatom is surrounded by a number of W or Mo oxygen octahedra. For proton conductivity it is desirable that a strong negative charge be delocalized across the whole anion so that the proton will be as dissociated as possible. This limits the choice of HPA to the spherical tungsten based Keggin anion with as light as possible a heteroatom. This limit is reached with Si as the P based HPA is known to decompose in the presence of peroxide and the electron deficient nature of B renders the spherical Keggin anion unstable. Many fundamental studies have been undertaken on solid state HPA systems. In addition the Si based HPA are known to decompose oxygenated radicals and so offer a path to the development of a PEM that is oxidatively stable under fuel cell operating conditions. These studies indicated that despite the original report in the early 80’s that HPA had the highest proton conductivity reported at that time, that when dry there proton conductivity was disappointingly low at moderate temperatures. The key is to distribute the HPA on a perfluorinated polymer backbone in such a way that the natural tendency of the HPA to cluster and crystalize is suppressed. Using dehydroflourination chemistry we have learnt to functionalism PVDF type polymers with molecules that can act as anchor points for lacunary HPA, where a lacunary HPA has 1,2 or 3 tungsten oxygen clusters removed to give bonding points. After much research we now can produce these materials in large area thin film membranes. The base material outperforms the state of the art PEM under standard conditions and has the highest chemical stability of any PEM proposed to date. We are now varying the HPA structure to try and avoid the HPA clustering issue, these membranes are being sprayed on gas diffusion electrodes which act as the support to the tin films. This is allowing us to run these as fuel cells at higher temperatures to asses the potential of HPA functionalized membranes in these applications.

The Role of Central Metal Atom and Ligand in Transition Metal Based Metal Organic Frameworks for Selective Electrochemical Reduction of CO2 to Value-Added Chemicals

Considering the rapid decline of the cost of renewable electrons (wind and solar), electrochemical conversion of carbon dioxide (CO2) to energy-dense or value-added chemicals with high selectivity (Faradaic efficiency >80%) is an increasing economic imperative. Despite the recent optimization efforts on noble, alkali, alkaline-earth and transition metal (TM) based electrocatalysts for efficient CO2 reduction, the rising cost of these metals (esp. noble metals) hinders large-scale commercialization. Recently, research into the electrocatalytic reduction of CO2 has focused on efficient and cost-effective catalysts such as metal-free, organometallic, hetero-atom doped or conducting polymers (1). Alternatively, electrocatalysts containing pyridinium derivatives have been proposed to avoid the overpotentials required for activating of carbon dioxide (via a high energy intermediate radical of CO2 -) and to promote CO2 reduction through successive proton-hydrate transfers (PT-HT) (2-4). To this end, Metal-Organic Framework (MOF) catalysts with central (TM based) heteroatom and surrounding pyridinic or pyrrolic ligands have been prepared and electrochemically characterized for selective CO2 reduction to methanol or carboxylic acids. In this study, the effect of electrolyte pH on reaction selectivity and reaction kinetics via Tafel analysis of the electrocatalysts were determined. Additional variables such as reactant’s partial pressure and phase, and electrolyzer temperature for optimum electrochemical reduction of CO2 will be also be discussed. This presentation will showcase our efforts to tailor electrocatalyst for value added chemicals such as methanol and formic acid. We will present electrochemical data such as Tafel kinetics and Faradaic efficiency in close concert with spectroscopy such as in situ Raman and Synchrotron X-ray Absorption. Acknowledgements: The authors gratefully acknowledge the financial support from arpa.e under the auspices of the REFUELS initiative. This grant lead by Sustainable Innovation (grant # DE-AR0000810) involves the participation of Northeastern University Center for Renewable Energy Technology as a catalyst partner. The authors also acknowledge instrumental support from Thermo Fisher Corp., and access to synchrotron based facility for x-ray absorption spectroscopy at National Synchrotron Light Source-II (NSLS-II) situated in Brookhaven National Laboratory (BNL), Upton, NY, under grant # DE-SC0012704.

Effect of protonation, composition and isomerism on the redox properties and electron (de)localization of classical polyoxometalates

Abstract This publication reviews some relevant features related with the redox activity of two inorganic compounds: [XM12O40]q- (Keggin structure) and [X2M18O62]q- (Wells-Dawson structure). These are two well-known specimens of the vast Polyoxometalate (POM) family, which has been the subject of extensive experimental and theoretical research owing to their unmatched properties. In particular, their redox activity focus a great deal of attention from scientists due to their prospective related applications. POMs are habitually seen as ‘electron sponges’ since many of them accept several electrons without losing their chemical identity. This makes them excellent models to study mechanisms of electrochemical nature. Their redox properties depend on: (i) the type and number of transition metal atoms in the structure, (ii) the basicity of the first reduced species and, occasionally, of the fully oxidized species; (iii) the size of the molecule, (iv) the overall negative charge of the POM, and (v) the size of the central heteroatom. In the last years, important collaboration between the experimental and theoretical areas has been usual on the development of POM science. In the present chapter three of these synergies are highlighted: the influence of the internal heteroatom upon the redox potentials of Keggin anions; the dependence of the redox waves of Fe-substituted Wells-Dawson compounds with pH; and the role of electron delocalization and pairing in mixed-metal Mo/W Wells-Dawson compounds in their ability to accept electrons. In these three cases, a complete understanding of the problem would not have been possible without the mutual benefit of experimental and computational data.

(Invited) Highly Proton Conducting Membranes for Electrochemical Energy Conversion Applications

There is still a need for membranes that operate in proton exchange membrane (PEM) fuel cells at hotter and drier conditions than can be achieved with current materials, >100°C and <50%RH. One approach pursued by us is the use of inorganic super acids as the heteropoly acids (HPAs) r zirconium phosphonates. HPAs are a sub group of the large class of metal oxygen clusters known as polyoxometalates in which a central heteroatom is surrounded by a number of W or Mo oxygen octahedra. For proton conductivity it is desirable that a strong negative charge be delocalized across the whole anion so that the proton will be as dissociated as possible. This limits the choice of HPA to the spherical tungsten based Keggin anion with as light as possible a heteroatom. This limit is reached with Si as the P based HPA is known to decompose in the presence of peroxide and the electron deficient nature of B renders the spherical Keggin anion unstable. Many fundamental studies have been undertaken on solid state HPA systems. These studies indicated that despite the original report in the early 80’s that HPA had the highest proton conductivity reported at that time, that when dry there proton conductivity was disappointingly low at moderate temperatures. We have doped both PFSA and PBI polymers with HPA and shown that both approaches lead to membrnes with much improved properties for proton cnduction under higher an drier conditions. The HPA/PBI/phosphorics acid system that we have developed may be used to investigate direct fuel cells with a number of fuels beyond hydrogen as the membranes can opearte at temperatues >160°C. Our most ambitious approach is to make monomers from HPA and immobilize the HPA by polymerization into hybrid systems. In order to functionalize the Keggin anion one W oxygen octahedra is removed and a Si or P based organic functionality introduced that may be a monomer or a tether to a functionalized polymer backbone. Our first generation materials based on divinyl functionalized HPA and acrylate chemistry produced films with impressive conductivities, >100 mS cm-1 T >80°C and 50% RH. This model system contained ester linkages that we think may be hydrolysed under the harsh conditions of fuel cell operation and so we attached HPA via phosphonate linkages to perfluorinated polymers. Very recently we have fully perfected this chemistry and can now produce large area thin high loaded HPA films that have demonstrated high performance fuel cell operation. We will show performance data in both hot and dry fuel cell operation and also durability data that shows the materials pass the DOE tests. These membranes are currently being investigated as seperators in redox flow batteries using a large number of redox couples.

Triol-Ligand Modification and Structural Transformation of Anderson-Evans Oxomolybdates via Modulating Oxidation State of Co-Heteroatom.

Various covalent decorations and structural transformations of a series of R-C(CH2OH)3 triol ligands on CoII/CoIII-centered Anderson-Evans polyoxomolybdates were studied in aqueous and/or organic solution. Single-crystal X-ray structural analysis demonstrated that four manners, including (1) double-sided χ/χ-isomer and (2) double-sided δ/χ-isomer with CoII central heteroatom, and (3) single-sided δ-isomer and (4) double-sided δ/δ-isomer with CoIII central heteroatom, appeared on the decoration of the mother polyanion [CoII/III(OH)6Mo6O18]4-/3-. Through careful manipulation of the reaction conditions, a gradual oxidation process from CoII to CoIII was disclosed to happen in air, and accompanying that process, the CoII-centered double-sided δ/χ-isomer that formed in the initial stage transformed into a CoIII-centered double-sided δ/δ-isomer. When the electron-withdrawing group -NO2 substituted triol ligand was used to replace CH3C(CH2OH)3, the oxidation process accelerated spontaneously. A further investigation showed that this oxidation process can be blocked by acid. While the triol ligands' decoration on Anderson-Evans polyoxometalates (POMs) took place along with the oxidation of the central heteroatom, that codecoration of other components was also accomplished. In the presence of excess acetic acid, an unprecedented single-sided δ-isomer with an acetate ligand covalently attaching on the other side by replacing one μ3-O atom was obtained in aqueous solution. The reaction of the mother cluster [(n-C4H9)4N]3[Co(OH)6Mo6O18] and triol ligands in methanol resulted in a codecoration of methanol with the double-sided δ/δ-isomer, which has never been reported in B-type Anderson-Evans POMs. The obtained results revealed that the oxidation state of the heteroatom has a significant impact on the triol ligand decoration styles, such as the χ/χ- or δ/χ-isomer for the divalent heteroatom and the δ- or δ/δ-isomer for the trivalent heteroatom.

Polyoxometalate silver salts loaded with silver nanoparticles as new photocatalysts under visible light

A series of polyoxometalate silver salts (Ag n POM) with low solubility were prepared by mechanically grinding the dry solid mixture of Keggin-type polyoxotungstates with different central heteroatoms and the silver nitrate. The as-prepared Ag n POM solids were further UV-irradiated, resulting in the generation of metallic silver nanoparticles (NPs) on the surface of the Ag n POM salts. Thus, a series of composite Ag n POM materials loaded with different amount of Ag NPs (Ag0 x /AgI n POM) were obtained. The UV-Vis diffuse reflectance spectra show that the intensities of absorbing bands of the composite materials in visible area have been enhanced due to the plasmon resonance of Ag NPs. Such composite materials exhibit enhanced visible light-driven photocatalytic performances for dye degradation and the reduction of Cr(VI). Furthermore, the loading amount of Ag NPs on the photocatalysts can be tuned by changing the negative charge of POM anions. As a result, the relevant photocatalytic activity of such composite materials can be also adjusted.

Four Strandberg-type polyoxometalates with organophosphine centre decorated by transition metal-2,2'-bipy/H2O complexes

Four inorganic-organic hybrid compounds composed of Strandberg-type organophosphomolybdate anion [(C6H5PO3)2Mo5O15]4− (abbreviated as (PhP)2Mo5) and transition metal (TM)−2,2'-bipy/H2O complex units, namely [(TM(H2O)(bipy))2(C6H5PO3)2Mo5O15]n (TM = Co, (1); Ni, (2)), [(Cu(bipy)2)2(C6H5PO3)2Mo5O15]·2H2O (3) and [(Zn(bipy)(µ-OH))2(Zn(bipy)2(C6H5PO3)2Mo5O15)2]·3H2O (4) (bipy = 2,2'-bipyridine), were successfully constructed under hydrothermal conditions, and their structures were determined by single crystal X-ray diffraction analysis and spectroscopic methods. The central heteroatoms in these polyoxometalates (POMs) are all organophosphine (RP) groups. Compound 1 and compound 2 are isostructural (PhP)2Mo5-based TM-coordination polymers with the two-dimensional layer frameworks. In compound 3, the bi-supporting structure containing one (PhP)2Mo5 unit and two [Cu(bipy)2]2+ cations. For compound 4, it can be regarded as a dimer of two bi-supporting {Zn(bipy)2(PhP)2Mo5Zn(bipy)} clusters that were connected by two µ-OH groups. The acid-catalytic activities and fluorescence properties of the four hybrids have been investigated.

(Invited) Using Heteropoly Acids As a Proton Conductor in High Temperature Proton Exchange Membrane Fuel Cells

There is still a need for membranes that operate in proton exchange membrane (PEM) fuel cells at hotter and drier conditions than can be achieved with current materials, >100°C and <50%RH. One approach pursued by us and independently by Kunz and co-workers was the use of inorganic super acids as proton conducting entities, such as the heteropoly acids (HPAs). HPAs are a sub group of the large class of metal oxygen clusters known as polyoxometalates in which a central heteroatom is surrounded by a number of W or Mo oxygen octahedra. For proton conductivity it is desirable that a strong negative charge be delocalized across the whole anion so that the proton will be as dissociated as possible. This limits the choice of HPA to the spherical tungsten based Keggin anion with as light as possible a heteroatom. This limit is reached with Si as the P based HPA is known to decompose in the presence of peroxide and the electron deficient nature of B renders the spherical Keggin anion unstable. Many fundamental studies have been undertaken on solid state HPA systems. These studies indicated that despite the original report in the early 80’s that HPA had the highest proton conductivity reported at that time, that when dry there proton conductivity was disappointingly low at moderate temperatures. We and Kunz and coauthors independntly showed that HPAs could dramtically enhance the proton conductivity of perfluoro sulfonic acid (PFSA) PEM under hotter and drier operating conditions. Despite their desirable properties HPAs are highly water soluble. Kunz and co-workers took the approach of immobilizing the HPA as the insoluble Cs salt and very ingeniously dispersed the Cs HPA clusters as nano-sized moieties, thereby generating a composite Nafion® film with superior properties. HPAs are known to decompose peroxides and oxygen based radicals and so have been shown to enhance oxidative stability of PEMs. A furher advantage advantage of the Si centered HPA is that the parent H4SiW12O40 HPA showed enhanced oxidative stability and performance of perfluoro sulfonic acid polymers (PFSAs) when it was used as a dopant in fuel cell testing. Our approach is to make monomers from HPA and immobilize the HPA by polymerization into hybrid systems. In order to functionalize the Keggin anion one W oxygen octahedra is removed and a Si or P based organic functionality introduced that may be a monomer or a tether to a functionalized polymer backbone. Our first generation materials based on divinyl functionalized HPA and acrylate chemistry produced films with impressive conductivities, >100 mS cm-1 T >80°C and 50% RH. This model system contained ester linkages that we think may be hydrolysed under the harsh conditions of fuel cell operation and so we attached HPA via phosphonate linkages to perfluorinated polymers. Very recently we have fully perfected this chemistry and can now produce large area thin high loaded HPA films for both fuel cell and redox flow cell operation. We will show performance data in both hot and dry fuel cell operation and for redox flow batteries. This work is sponsored by DOE EERE and ARPA-E.

Divergent Synthesis of Heteroatom‐Centered 4,8,12‐Triazatriangulenes

AbstractThe increasing attention devoted to triangulenes and their heteroatom derivatives inspired us to explore a divergent synthesis of heteroatom‐centered 4,8,12‐triazatriangulenes, which involved the preparation of a nitrogen‐containing macrocyclic precursor and subsequent central heteroatom introduction by electrophilic C−Li and C−H substitution. The boron‐centered triangulene has a planar structure unlike the bowl‐shaped phosphorus‐ and silicon‐centered triangulenes. The described synthetic procedure can be used to fabricate a broad range of attractive functional materials, for example, for organic light‐emitting diodes, based on heteroatom‐centered triangulenes.

Divergent Synthesis of Heteroatom-Centered 4,8,12-Triazatriangulenes.

The increasing attention devoted to triangulenes and their heteroatom derivatives inspired us to explore a divergent synthesis of heteroatom-centered 4,8,12-triazatriangulenes, which involved the preparation of a nitrogen-containing macrocyclic precursor and subsequent central heteroatom introduction by electrophilic C-Li and C-H substitution. The boron-centered triangulene has a planar structure unlike the bowl-shaped phosphorus- and silicon-centered triangulenes. The described synthetic procedure can be used to fabricate a broad range of attractive functional materials, for example, for organic light-emitting diodes, based on heteroatom-centered triangulenes.

Non-Aqueous Redox Flow Batteries Based on Polyoxometalates

Renewable energy generation around the world continues to see dramatic growth. According to a recent U.S. Energy Information Administration (EIA) report, 13% of US energy demand in 2013 was met by renewables and the amount is still increasing. Large scale energy storage systems promise to increase the effectiveness of renewable energy sources. Redox Flow Batteries (RFB) represent a highly scalable approach. Traditional RFBs are mostly aqueous which limits their operating voltage range due to water splitting. Non-aqueous systems offer a larger voltage window. Polyoxometalates (POMs), which exhibit versatile redox properties and multiple electron transfers were used as the active species in non-aqueous RFBs. POMs generally contain group 5 or group 6 transition metals linked by oxygen atoms and centered around a heteratom. Common POMs are of the Keggin type ([XM12O40]q−, where X = P, Si, Ge, etc., and M = Mo, W, V, etc. ) or Wells-Dawson type ([X2M18O62]q-). The redox properties of POMs can be altered by exchanging the framework metal atoms or the central heteroatom very easily. Li3PMo12O40, Li4PMo11VO40 and Li6P2W18O62 were tested in acetonitrile and the lithium was chosen as the counter-cation in order to avoid H2 generation. Owing to limitations on the transport rate of Li+ through conventional proton-exchange membranes such as Nafion, an aramid nano-fiber (ANF) membrane was utilized for charge-discharge experiments. The low permeability of ANF prevents the POM crossover, with less than 1% crossover observed after one week. Greater than 85% coulombic efficiency was observed for symmetric cells containing the POMs above, with good reversibility and stability. Both symmetric and asymmetric cells (different POMs utilized for the anolyte and catholyte) were tested. Asymmetric POM-based RFBs hold promise for increasing the voltage window and overall energy density.