High-Tc Molecular-Based Magnets: Ferrimagnetic Mixed-Valence Chromium(III)-Chromium(II) Cyanides with Tc at 240 and 190 Kelvin

Molecular-based magnets with high magnetic-ordering temperatures, T(c), can be obtained by mild chemistry methods by focusing on the bimetallic and mixed-valence transition metal micro-cyanide of the Prussian blue family. A simple orbital model was used to predict the electronic structure of the metal ions required to achieve a high ordering temperature. The synthesis and magnetic properties of two compounds, [Cr(5)(CN)(12)].10H(2)O and Cs(0.75) [Cr(2.125)(CN)(6)].5H(2)O, which exhibited magnetic-ordering temperatures of 240 and 190 kelvin, respectively, are reported, together with the strategy for further work.     

Pentaborate Polyanion in the Crystal Structure of Ulexite, NaCaB5O6(OH)6{middle dot}5H2O

Triclinic ulexite crystals contain isolated borate polyanions [B(5)O(6)(OH)(6)](3-) related to the well known pentaborate polyanion [B(5)O(6)(OH)(4)](-) by addition of two hydroxyl groups to two opposite B-O triangles. The isolated ulexite polyanions form the [B(5)O(7)(OH)(4)](n)(3n-) chains previously found in crystals of the related mineral probertite, NaCaB(5)O(7)(OH)(4).3H(2)O.     

Kinetic evidence for five-coordination in AlOH(aq)2+ ion

Trivalent aluminum ions are important in natural bodies of water, but the structure of their coordination shell is a complex unsolved problem. In strong acid (pH < 3.0), Al(III) exists almost entirely as the octahedral Al(H2O)6(3+) ion, whereas in basic conditions (pH > 7), a tetrahedral Al(OH)(4- structure prevails. In the biochemically and geochemically critical pH range of 4.3 to 7.0, the ion structures are less clear. Other hydrolytic species, such as AlOH(aq)2+, exist and are traditionally assumed to be hexacoordinate. We show, however, that the kinetics of proton and water exchange on aqueous Al(III), coupled with Car-Parrinello simulations, support a five-coordinate Al(H2O)4OH2+ ion as the predominant form of AlOH(aq)2+ under ambient conditions. This result contrasts Al(III) with other trivalent metal aqua ions, for which there is no evidence for stable pentacoordinate hydrolysis products.     

Structure of a three-dimensional, microporous molybdenum phosphate with large cavities

The synthesis, single-crystal x-ray structural characterization, and sorption properties of a microporous molybdenum phosphate, (Me(4)N)(1.3)(H(3)O)(0.7)[Mo(4)O(8)(PO(4))(2)] . 2H(2)O (Me, methyl), are presented. The three-dimensional framework is built up from Mo(4)O(8)(4+) cubes and PO(4)(3-) tetrahedra that are connected in such a way that large, cation-filled voids are generated; these voids constitute 25% of the volume of the solid. Absorption isotherms for water show the completely reversible uptake of 4 to 5 percent by weight water into the micropores of this compound, which corresponds to 10 to 12 percent by volume.     

Eight-membered cyclosilicate rings in muirite

The determination of the crystal structure of muirite, Ba(10)(Ca,Mn,Ti)(4)Si(8)O(24)(Cl,OH,O)(12) . 4H(2)O, revealed the presence of discrete cyclic silicate anions, (Si(8)O(24))(16-), formed by the condensation of eight silicate tetrahedra. This first reported occurrence of eight-membered rings is of particular interest, because rings with eight tetrahedra are reported to be energetically less stable than rings with six tetrahedra, which have been found in many minerals.     

Boron-Oxygen Polyanion in the Crystal Structure of Tunellite

The crystal structure of tunellite, SrO.3B(2)O(3).4H(2)O, with infinite sheets of composition n[B(6)O(9)(OH)(2)](2-), has cations and water molecules in the spaces within the sheets. Adjacent sheets are held together by hydrogen bonding through the water molecules. The boron-oxygen polyanions provide the first example in hydrated borate crystals of one oxygen linked to three borons.     

Interwoven metal-organic framework on a periodic minimal surface with extra-large pores

Interpenetration (catenation) has long been considered a major impediment in the achievement of stable and porous crystalline structures. A strategy for the design of highly porous and structurally stable networks makes use of metal-organic building blocks that can be assembled on a triply periodic P-minimal geometric surface to produce structures that are interpenetrating-more accurately considered as interwoven. We used 4,4',4"-benzene-1,3,5-triyl-tribenzoic acid (H(3)BTB), copper(II) nitrate, and N,N'-dimethylformamide (DMF) to prepare Cu(3)(BTB)(2)(H(2)O)(3).(DMF)(9)(H(2)O)(2) (MOF-14), whose structure reveals a pair of interwoven metal-organic frameworks that are mutually reinforced. The structure contains remarkably large pores, 16.4 angstroms in diameter, in which voluminous amounts of gases and organic solvents can be reversibly sorbed.     

High-temperature molecular magnets based on cyanovanadate building blocks: spontaneous magnetization at 230 k

The molecular-based magnetic materials Cs(2)Mn(||)[V(||)(CN)(6)] (1) and (Et(4)N)(0.5)Mn(l.25)- [V(CN)(5)].2H(2)O (2) (where Et is ethyl) were prepared by the addition of manganese(II) triflate to aqueous solutions of the hexacyanovanadate(II) ion at 0 degrees C. Whereas 1 crystallizes in a face-centered cubic lattice, 2 crystallizes in a noncubic space group. The cesium salt (1) has features characteristic of a three-dimensional ferrimagnet with a Néel transition at 125 kelvin. The tetraethylammonium salt (2) also behaves as a three-dimensional ferrimagnet with a Néel temperature of 230 kelvin; only two other molecular magnets have higher magnetic ordering temperatures. Saturation magnetization measurements indicate that in both compounds the V(II) and high-spin Mn(II) centers are antiferromagnetically coupled. Both 1 and 2 exhibit hysteresis loops characteristic of soft magnets below their magnetic phase-transition temperatures. The high magnetic ordering temperatures of these cyano-bridged solids confirm that the incorporation of early transition elements into the lattice promotes stronger magnetic coupling by enhancing the backbonding into the cyanide pi* orbitals.     

Synthesis and Characterization of Molybdenum Carbide Clusters MonC4n, (n = 1 to 4)

Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO)(6) vapor produced ultrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry MonC4n (n = 1 to 4). The MonC4n(-) (n = 2 to 4) ions are completely unreactive with NH(3), H(2)O, and O(2), suggesting structures in which the molybdenum atoms are unavailable for coordination to additional ligands. Collision-induced dissociation studies of these anions show the loss of MoC(4) units as the main fragmentation pathway. This observation, together with the lack of addition reactions, provides a basis for structures in which a planar cluster of two, three, or four molybdenum atoms is surrounded by, and bonded to, carbon dimers.     

Crystal Structure of Kernite, Na2B4O6(OH)2 {middle dot} 3H2

Kernite, Na(2)B(4)O(6)(OH)(2). .3H(2)0, contains parallel infinite chains of the borate polyanion[B(4)O(6)(OH)(2)](n)(2n-). The chains are composed of six-membered rings containing one boron-oxygen triangle and two boron-oxygen tetrahedra. The rings are linked through commonly shared boron-oxygen tetrahedra.     

Ammonioborite: new borate polyion and its structure

The mineral ammonioborite is monoclinic with unit cell dimensions (in angstroms): a = 25.27, b = 9.65, and c = 11.56; beta = 94 degrees 17', and the space group is C 2/c. Analysis of the crystal structure revealed the crystallo-chemical formula (NH(4))(3)B(15)O(20)(OH)(8).4H(2)O, with four such formula units in the unit cell. The basic structural unit is the double ring consisting of one BO(4) tetrahedron and four BO(3) triangles: in ammonioborite three of these units are connected to give trimeric ions [B(15)O(20)(OH)(8)](3-).     

Size-Specific Infrared Spectra of Benzene-(H2O)n Clusters (n = 1 through 7): Evidence for Noncyclic (H2O)n Structures

Resonant ion-dip infrared spectroscopy has been used to record size-specific infrared spectra of C(6)H(6)-(H(2)O)n clusters with n = 1 through 7 in the O-H stretch region. The O-H stretch spectra show a dramatic dependence on cluster size. For the n = 3 to 5 clusters, the transitions can be divided into three types-attributable to free, pi hydrogen-bonded, and single donor water-water O-H stretches-consistent with a C(6)H(6)-(H(2)O)n structure in which benzene is on the surface of a cyclic (H(2)O)n cluster. In n = 6 and 7 clusters, the spectra show distinct new transitions in the 3500 to 3600 wave number region. After comparison of these results with the predictions of ab initio calculations on (H(2)O)n clusters, these new transitions have been assigned to double donor O-H stretches associated with the formation of a more compact, noncyclic structure beginning with (H(2)O)(6). This is the same size cluster for which ab initio calculations predict that a changeover to noncyclic (H(2)O)n structures will occur.     

Large first hyperpolarizabilities in push-pull polyenes by tuning of the bond length alternation and aromaticity

Conjugated organic compounds with 3-phenyl-5-isoxazolone or N,N'-diethylthiobarbituric acid acceptors have large first molecular hyperpolarizabilities (beta) in comparison with compounds with 4-nitrophenyl acceptors. For example, julolidinyl-(CH=CH)(3)-CH=N,N'- diethylthiobarbituric acid, which has 12 atoms between the donor and acceptor, has a beta(0) of 911 x 10(-30) electrostatic units, whereas (CH(3))(2)NC(6)H(4),-(CH=CH)(4)-C(6)H(4)NO(2), with 16 atoms between its donor and acceptor, has a beta(0) of 133 x 10(-30) electrostatic units. The design strategies demonstrated here have resulted in chromophores that when incorporated into poled-polymer electrooptic modulators exhibited significant enhancements in electrooptic coefficients relative to polymers containing the commonly used dye Disperse Red-1. Poled polymer devices based on these or related chromophores may ultimately lead to high-speed electrooptic switching elements with low drive-power requirements, suitable for telecommunications applications.     

Shattuckite and plancheite: a crystal chemical study

The orthorhombic crystal structures of shattuckite, Cu(5)( SiO(3))(4)(OH)(2) and planchétite, Cu(8)(Si(4)0(11))(2)(OH)(4) H(2)O, have been solved. Shattuckite contains silicate chains similar to pyroxene in a complex association with copper atoms, while the closely related planchéite contains silicate chains similar to amphibole.     

Synthesis, structure, and reactivity of an iron(V) nitride

Despite being implicated as important intermediates, iron(V) compounds have proven very challenging to isolate and characterize. Here, we report the preparation of the iron(V) nitrido complex, [PhB((t)BuIm)(3)Fe(V)≡N]BAr(F24) (PhB((t)BuIm)(3)(-) = phenyltris(3-tert-butylimidazol-2-ylidene)borato, BAr(F24) = B(3,5-(CF(3))(2)C(6)H(3))(4)(-)), by one electron oxidation of the iron(IV) nitrido precursor. Single-crystal x-ray diffraction of the iron(V) complex reveals a four-coordinate metal ion with a terminal nitrido ligand. M??bauer and electron paramagnetic resonance spectroscopic characterization, supported by electronic structure calculations, provide evidence for a d(3) iron(V) metal center in a low spin (S = 1/2) electron configuration. Low-temperature reaction of the iron(V) nitrido complex with water under reducing conditions leads to high yields of ammonia with concomitant formation of an iron(II) species.     

A thiosulfate shunt in the sulfur cycle of marine sediments

The oxidation of sulfide, generated by bacterial sulfate reduction, is a key process in the biogeochemistry of marine sediments, yet the pathways and oxidants are poorly known. By the use of (35)S-tracer studies of the S cycle in marine and freshwater sediments, a novel shunt function of thiosulfate (S(2)O(3)(2-)) was identified. The S(2)O(3)(2-) constituted 68 to 78 percent of the immediate HS(-)-oxidation products and was concurrently (i) reduced back to HS(-), (ii) oxidized to SO(4)(2-), and (iii) disproportionated to HS(-) + SO(4)(2-). The small thiosulfate pool is thus involved in a dynamic HS(-) - S(2)O(3)(2-) cycle in anoxic sediments. The disproportionation of thiosulfate may help account for the large difference in isotopic composition ((34)S/(32)S) of sulfate and sulfides in sediments and sedimentary rocks.     

The "Ozone Deficit" Problem: O2(X, v ge 26) + O(3P) from 226-nm Ozone Photodissociation

Highly vibrationally excited O(2)(X(3)sigmag(-), v >/= 26) has been observed from the photodissociation of ozone (O(3)), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the "ozone deficit" problem, or why the previously predicted stratospheric O(3) concentration is less than that observed. Recent kinetic studies have suggested that O(2)(X(3)sigmag(-), v >/= 26) can react rapidly with O(2) to form O(3) + O and have led to speculation that, if produced in the photodissociation of O(3), this species might be involved in resolving the discrepancy. The sequence O(3) + hv --> O(2)(X(3)sigmag(-), v >/= 26) + O; O(2)(X(3)sigmag(-), v >/= 26) + O(2) --> O(3) + O (where hv is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O(3) deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O(3) in the stratosphere.     

Anionic constitution of 1-atmosphere silicate melts: implications for the structure of igneous melts

A structural model is proposed for the polymeric units in silicate melts quenched at 1 atmosphere. The anionic units that have been identified by the use of Raman spectroscopy are SiO(4)(4-) monomers, Si(2)O(7)(6-) dimers, SiO(3)(2-) chains or rings, Si(2)O(5)(2-) sheets, and SiO(2) three-dimensional units. The coexisting anionic species are related to specific ranges of the ratio of nonbridging oxygens to tetrahedrally coordinated cations (NBO/Si). In melts with 2.0 < NBO/Si < approximately 4.0, the equilibrium is of the type [See equation in the PDF file]. In melts with NBO/Si approximately 1.0 to 2.0, the equilibrium anionic species are given by [See equation in the PDF file]. In alkali-silicate melts with NBO/Si <~ 1.3 and in aluminosilicate melts with NBO/T < 1.0, where T is (Si + Al), the anionic species in equilibrium are given by [See equation in the PDF file]. In multicomponent melts with compositions corresponding to those of the major igneous rocks, the anionic species are TO(2), T(2)O(5), T(2)O(6), and TO(4), and the coexisting polymeric units are determined by the second and third of these disproportionation reactions.     

Hydrous sodium silicates from lake magadi, kenya: precursors of bedded chert

Two new hydrous sodium silicates, NaSi(7)O(13)(OH)(3).3H(2)O (magadiite) and NaSi(11)O(20.5)(OH)(4).3H(2)O (kenyaite), were found in lake beds at Lake Magadi, Kenya. Both are well-crystallized layered silicates with large basal spacings. Concretions within the magadiite bed consist of kenyaite or quartz (chert) in the center, surrounded by kenyaite. In dilute acids magadiite and kenyaite are converted to 6SiO(2).H(2)O (SH), the first known crystalline hydrate of silica. The magadiite bed probably represents a chemical precipitate from alkaline brines. Percolating waters convert magadiite to kenyaite and eventually to chert. Thus a mechanism has been outlined for the formation of bedded chert deposits through inorganic precipitation. Alternations between silica-rich and iron-rich bands of iron formations may be due to concentration cycles in alkaline lakes.     

Reversible, metal-free hydrogen activation

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100 degrees C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25 degrees C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.