Catalytic Reduction of Hydrazine to Ammonia with MoFe(3)S(4)-Polycarboxylate Clusters. Possible Relevance Regarding the Function of the Molybdenum-Coordinated Homocitrate in Nitrogenase

The catalytic function of the previously synthesized and characterized [(L)MoFe(3)S(4)Cl(3)](2)(-)(,3)(-) clusters (L = tetrachlorocatecholate, citrate, citramalate, methyliminodiacetate, nitrilotriacetate, thiodiglycolate) and of the [MoFe(3)S(4)Cl(3)(thiolactate)](2)(4)(-) and [(MoFe(3)S(4)Cl(4))(2)(&mgr;-oxalate)](4)(-) clusters in the reduction of N(2)H(4) to NH(3) is reported. In the catalytic reduction, which is carried out at ambient temperature and pressure, cobaltocene and 2,6-lutidinium chloride are supplied externally as electron and proton sources, respectively. In experiments where the N(2)H(4) to the [(L)MoFe(3)S(4)Cl(3)](n)()(-) catalyst ratio is 100:1, and over a period of 30 min, the reduction proceeds to 92% completion for L = citrate, 66% completion for L = citramalate, and 34% completion for L = tetrachlorocatecholate. The [Fe(4)S(4)Cl(4)](2)(-) cluster is totally inactive and gives only background ammonia measurements. Inhibition studies with PEt(3) and CO as inhibitors show a dramatic decrease in the catalytic efficiency. These results are consistent with results obtained previously in our laboratory and strongly suggest that N(2)H(4) activation and reduction occur at the Mo site of the [(L)MoFe(3)S(4)Cl(3)](2)(-)(, 3)(-) clusters. A possible pathway for the N(2)H(4) reduction on a single metal site (Mo) and a possible role for the carboxylate ligand are proposed. The possibility that the Mo-bound polycarboxylate ligand acts as a proton delivery "shuttle" during hydrazine reduction is considered.     

The fully deprotonated anion of 1,3,5-benzene-triphosphonic acid: 1H, 31P,and 13C{1H} NMR and some comments on corresponding [AX]3 and AXX′2 spin systems and spectra

Abstract

The fully deprotonated anion of 1,3,5-benzene-triphosphonic acid 1,3,5-C₆H₃[PO₃^2?]₃ gives rise to deceptively simple ¹H and ³¹P NMR spectra due to a corresponding [AX]₃ spin system. The ¹³C{¹H} NMR spectrum reveals two ¹³C isotopomers which are identified via AXX′₂ systems. Results from analysis and iteration are described.     

High proton conductivity in a flexible, cross-linked, ultramicroporous magnesium tetraphosphonate hybrid framework

Multifunctional materials, especially those combining two or more properties of interest, are attracting immense attention due to their potential applications. MOFs, metal organic frameworks, can be regarded as multifunctional materials if they show another useful property in addition to the adsorption behavior. Here, we report a new multifunctional light hybrid, MgH(6)ODTMP·2H(2)O(DMF)(0.5) (1), which has been synthesized using the tetraphosphonic acid H(8)ODTMP, octamethylenediamine-N,N,N',N'-tetrakis(methylenephosphonic acid), by high-throughput methodology. Its crystal structure, solved by Patterson-function direct methods from synchrotron powder X-ray diffraction, was characterized by a 3D pillared open framework containing cross-linked 1D channels filled with water and DMF. Upon H(2)O and DMF removal and subsequent rehydration, MgH(6)ODTMP·2H(2)O (2) and MgH(6)ODTMP·6H(2)O (3) can be formed. These processes take place through crystalline-quasi-amorphous-crystalline transformations, during which the integrity of the framework is maintained. A water adsorption study, at constant temperature, showed that this magnesium tetraphosphonate hybrid reversibly equilibrates its lattice water content as a function of the water partial pressure. Combination of the structural study and gas adsorption characterization (N(2), CO(2), and CH(4)) indicates an ultramicroporous framework. High-pressure CO(2) adsorption data are also reported. Finally, impedance data indicates that 3 has high proton conductivity σ = 1.6 × 10(-3) S cm(-1) at T = 292 K at ~100% relative humidity with an activation energy of 0.31 eV.     

Spherically symmetric inhomogeneous dust collapse in higher dimensional space-time and cosmic censorship hypothesis

We consider a collapsing spherically symmetric inhomogeneous dust cloud in higher dimensional space-time. We show that the central singularity of collapse can be a strong curvature or a weak curvature naked singularity depending on the initial density distribution.     

Semi-wet growth and characterization of multi-functional nano-engineered mixed metal oxides for industrial application

This review paper covers the low temperature wet growth of nano-engineered particles of ZnO-based mixed metal oxides, their growth mechanism, and characterization using X-ray diffraction, SEM, TEM and IR, UV–visible, and XPS spectral techniques. Main focus of this article is centered on low temperature semi-wet methods of synthesis that are suitable for large scale production of zinc oxide-based systems mixed with iron oxide, copper oxide, nickel oxide and cobalt oxide. These mixed metal oxides have broad industrial applications as catalyst, semiconductors, adsorbents, superconductors, electro-ceramics, and antifungal agents in addition to extensive applications in medicines. This paper discusses the low-cost and environment friendly synthesis of these mixed metal oxides, measurement of properties and applicability of these materials systems.     

Synaptic depression and short-term habituation are located in the sensory part of the mammalian startle pathway

Background  

Short-term habituation of the startle response represents an elementary form of learning in mammals. The underlying mechanism is located within the primary startle pathway, presumably at sensory synapses on giant neurons in the caudal pontine reticular nucleus (PnC). Short trains of action potentials in sensory afferent fibers induce depression of synaptic responses in PnC giant neurons, a phenomenon that has been proposed to be the cellular correlate for short-term habituation. We address here the question whether both this synaptic depression and the short-term habituation of the startle response are localized at the presynaptic terminals of sensory afferents. If this is confirmed, it would imply that these processes take place prior to multimodal signal integration, rather than occurring at postsynaptic sites on PnC giant neurons that directly drive motor neurons.     

Chemistry of organophosphonate scale growth inhibitors: two-dimensional, layered polymeric networks in the structure of tetrasodium 2-hydroxyethyl-amino-bis(methylenephosphonate)

Aminomethylene phosphonates are important scale inhibitors applied in diverse areas of technology. This study adds to the existing body of information on this subject and reports the crystal and molecular structures of tetrasodium 2-hydroxyethylamino-bis(methylene-phosphonate) decahydrate ([Na₄(HOCH₂CH₂N(CH₂PO₃)₂)]·10H₂O, 1). The crystal structure of 1 could be described as two-dimensional polymeric layered structure hydrogen bonded into a 3D supramolecular polymeric network. The structure of the tetraanion consists of a “three-arm” backbone stemming from the N atom. Two “arms” are deprotonated methylene phosphonate (-CH₂PO₃²⁻) moieties and the third is a hydroxyethyl (-CH₂CH₂OH) moiety. One Na cation forms an intramolecular complex with two oxygens from separate phosphonate groups, a hydroxyl oxygen, the nitrogen and two lattice water molecules. The position of this Na cation points to a possible coordination site for Ca in a proposed Ca-HEABMP complex (HEABMP=2-hydroxyethylamino-bis(methylene-phosphonate).     

Oxidation of ammonia in osmium polypyridyl complexes

The oxidations of cis- and trans-[OsIII(tpy)(Cl)2(NH3)](PF6), cis-[OsII(bpy)2(Cl)(NH3)](PF6), and [OsII(typ)(bpy)(NH3)](PF6)2 have been studied by cyclic voltammetry and by controlled-potential electrolysis. In acetonitrile or in acidic, aqueous solution, oxidation is metal-based and reversible, but as the pH is increased, oxidation and proton loss from coordinated ammonia occurs. cis- and trans-[OsIII(tpy)(Cl)2(NH3)](PF6) are oxidized by four electrons to give the corresponding OsVI nitrido complexes, [OSVI(typ)(Cl)2(N)]+. Oxidation of [Os(typ)(bpy)(NH3)](PF6)2 occurs by six electrons to give [Os(tpy)(bpy)(NO)](PF6)3. Oxidation of cis-[OsII(bpy)2(Cl)(NH3)](PF6) at pH 9.0 gives cis-[OsII(bpy)2(Cl)(NO)](PF6)2 and the mixed-valence form of the mu-N2 dimer [cis-[Os(bpy)2(Cl)2[mu-N2)](PF6)3. With NH4+ added to the electrolyte, cis-[OsII(bpy)2(Cl)(N2)](PF6) is a coproduct. The results of pH-dependent cyclic voltammetry measurements suggest OsIV as a common intermediate in the oxidation of coordinated ammonia. For cis- and trans-[OsIII(tpy)(Cl)2(NH3)]+, OsIV is a discernible intermediate. It undergoes further pH-dependent oxidation to [OsVI(tpy)(Cl)2(N)]+. For [OsII(tpy)(bpy)(NH3)]2+, oxidation to OsIV is followed by hydration at the nitrogen atom and further oxidation to nitrosyl. For cis-[OsII(bpy)2(Cl)-(NH3)]+, oxidation to OsIV is followed by N-N coupling and further oxidation to [cis-[Os(bpy)2(Cl)2(mu-N2)]3+. At pH 9, N-N coupling is competitive with capture of OsIV by OH- and further oxidation, yielding cis-[OsII(bpy)2(Cl)(NO)]2+.     

Metal-organotetraphosphonate inorganic-organic hybrids: crystal structure and anticorrosion effects of zinc hexamethylenediaminetetrakis(methylenephosphonate) on carbon steels

The synthesis and structural characterization of the first polymeric M-HDTMP organic-inorganic hybrids are described [M = Zn2+, Ca2+; HDTMP = hexamethylenediaminetetrakis(methylenephosphonate)]. The 3D crystal structure of the Zn2+ analogue [Zn(HDTMP) x H2O] is described. The Zn center is found in a distorted octahedral environment of phosphonate oxygens. There is a long Zn...O interaction (2.622 A) originating from a protonated -P-OH group. Synergistic combinations of Zn2+ and the tetraphosphonate are found to form films that protect against the corrosion of carbon steels.