Long-range magnetic ordering in iron jarosites prepared by redox-based hydrothermal methods

The iron jarosites, plumbojarosite, Pb0.5Fe3(OH)6(SO4)2, argentojarosite, AgFe3(OH)6(SO4)2, and thallium jarosite, TlFe3(OH)6(SO4)2, along with the selenate-capped jarosite analogues of potassium, KFe3(OH)6(SeO4)2, and rubidium, RbFe3(OH)6(SeO4)2, have been prepared in their analytically pure forms by employing redox-based hydrothermal methods. The crystal structures of these materials have been determined, and all are found to be essentially isostructrual including Pb0.5Fe3(OH)6(SO4)2, which is distinct from the structure reported for naturally mined samples. All iron jarosites show long-range order (LRO), signified by a sharp transition temperature, T(N), which falls in the narrow temperature range of 61.4 +/- 5 K. The mechanism responsible for this ordering has been established by examining magnetostructural correlations for the jarosites possessing various interlayer cation and capping groups. We show that all magnetic properties of jarosites, including LRO, find their origin in the basic magnetic unit, the intralayer Fe3(mu-OH)3 triangle. Field-dependent magnetization experiments are consistent with the antiferromagnetic stacking of an out of plane moment developed from spin canting within Fe3(mu-OH)3 triangles. Together with the previously reported AFe3(OH)6(SO4)2 (A = Na+, K+, Rb+ and NH4+) jarosites, these compounds provide a framework for probing magnetic ordering in a spin frustrated lattice of the largest series of isoelectronic and isostructural kagomé systems yet discovered.     

Aerobic catalytic photooxidation of olefins by an electron-deficient Pacman bisiron(III) mu-oxo porphyrin

The synthesis and oxygen atom transfer (OAT) photoreactivity of a diiron(III) mu-oxo meso-tripentafluorophenyl bisporphyrin appended to a dibenzofuran spacer are presented. Reaction of 4,6-diformyldibenzofuran under standard Lindsey conditions furnishes the parent cofacial porphyrin architecture in a single step. These cofacial porphyrins photocatalyze the oxidation of sulfides and olefins using visible light and molecular oxygen as the terminal oxidant. High turnover numbers reflect the enhanced stability of the electron-deficient diiron(III) mu-oxo bisporphyrin core appended to a dibenzofuran spacer under aerobic conditions.     

The preparation of dimolybdenum(V,V) complexes from molybdenum quadruply bonded metal-metal dimers

Oxidation of quadruply bonded metal-metal dimers in the presence of good π-accepting ligands results in the formation of Mo^V---Mo^V compounds of the type [MO₂(μ-X)₂(Y)(Y′)]²⁺ (X = O or S; Y,Y′ = O,O; S,S; O,S). Reaction of MO₂(O₂CCH₃)₄ with oxygen in the presence of Na₂mnt (mnt = 1,2-dicyanoethylene-2,2-dithiolate) gives [MO₂(μ-S)₂(O)(S)(mnt)₂]²⁻ (1). The compound crystallizes in the monoclinic space group P2₁/c, with cell dimensions a = 19.547(4), b = 15.210(4), c = 18.754(6) Å, β = 101.69(2)°, V= 5460(2) ų, and Z = 4. Similarly, oxidation of o-dichlorobenzene solutions of Mo₂Cl₄(CH₃CN)₄ and 4,4′-dimethyl-2,2′-dipyridyl (dmpby) or, more directly, the reaction of Mo₂Cl₄(dmbpy)₂ with oxygen leads to the formation of a red solid, which was characterized by X-ray crystallography to be Mo₂(μ-O)₂(O)₂(Cl)₂(dmbpy)₂ (2). Red diamond crystals, prepared by slow evaporation of CH₃CN solutions of 2, are trigonal and in the space group P3₁21 with cell dimensions a = 16.135(4), b = 16.135(4), c = 10.709(3) Å, V = 2414.4(13) ų and Z = 3. In both structures, the geometry about each of the molybdenum atoms is a distorted square pyramid with terminal oxygen or sulphur atoms at the apices and in a syn conformation. The molybdenum-molybdenum bond distances of 2.858(1) Å and 2.562(2) Å in structures of 1 and 2, respectively, are typical of other Mo^V---Mo^V dimers and indicative of a single Mo---Mo bond.     

Parton distributions and new physics searches: the Drell–Yan forward–backward asymmetry as a case study

We present the first unquenched lattice-QCD calculation of the form factors for the decay \(B\rightarrow D^*\ell \nu \) at nonzero recoil. Our analysis includes 15 MILC ensembles with \(N_f=2+1\) flavors of asqtad sea quarks, with a strange quark mass close to its physical mass. The lattice spacings range from \(a\approx 0.15\) fm down to 0.045 fm, while the ratio between the light- and the strange-quark masses ranges from 0.05 to 0.4. The valence b and c quarks are treated using the Wilson-clover action with the Fermilab interpretation, whereas the light sector employs asqtad staggered fermions. We extrapolate our results to the physical point in the continuum limit using rooted staggered heavy-light meson chiral perturbation theory. Then we apply a model-independent parametrization to extend the form factors to the full kinematic range. With this parametrization we perform a joint lattice-QCD/experiment fit using several experimental datasets to determine the CKM matrix element \(|V_{cb}|\). We obtain \(\left| V_{cb}\right| = (38.40 \pm 0.68_{\text {th}} \pm 0.34_{\text {exp}} \pm 0.18_{\text {EM}})\times 10^{-3}\). The first error is theoretical, the second comes from experiment and the last one includes electromagnetic and electroweak uncertainties, with an overall \(\chi ^2\text {/dof} = 126/84\), which illustrates the tensions between the experimental data sets, and between theory and experiment. This result is in agreement with previous exclusive determinations, but the tension with the inclusive determination remains. Finally, we integrate the differential decay rate obtained solely from lattice data to predict \(R(D^*) = 0.265 \pm 0.013\), which confirms the current tension between theory and experiment.

     

Eclipsed M2X6 compounds exhibiting very short metal-metal triple bonds

The preparation, characterization, and electronic structure of homoleptic complexes of molybdenum and tungsten bridged by bis(alkylamido)phenylboranes, M(2)[RN-B(Ph)-NR](3) (M = Mo, R = Et (1), (i)Pr (2); M = W, R = Et (3), (i)Pr (4)), are described. These triple metal-metal bond species (i) exhibit a nearly eclipsed ligand geometry and (ii) possess the shortest metal-metal bonds of neutral dimolybdenum and ditungsten M(2)X(6) complexes observed to date (d(Mo-Mo) = 2.1612(6) A (1); d(W-W) = 2.2351(7) A (4)).     

Electron transfer reactions of fluorotyrosyl radicals

The complex Re(bpy)(CO)3CN is an excited state oxidant of tyrosine upon deprotonation of the tyrosyl phenol. A series of Re(bpy-FnY)(CO)3CN complexes ([Re]-FnY: [Re]-Y, [Re]-3-FY, [Re]-3,5-F2Y, [Re]-2,3-F2Y, [Re]-2,3,5-F3Y, [Re]-2,3,6-F3Y, and [Re]-F4Y) have been prepared so as to vary the FnY*/FnY- reduction potential and thus the driving force for electron transfer in this system. Time-resolved emission and nanosecond absorption spectroscopies have been used to measure the rates for charge separation, CS, and charge recombination, CR, for each complex. A driving force analysis reveals that CS is well described by Marcus' theory for ET, is strongly driving force dependent (activated), and occurs in the normal region for ET. CR, on the other hand, is weakly driving force dependent (near activationless) and occurs in the inverted region for ET. These data demonstrate that fluorotyrosines will be powerful probes for unraveling charge transport mechanisms in enzymes that utilize tyrosyl radicals.     

A supramolecular microfluidic optical chemosensor

A supramolecular microfluidic optical chemosensor (muFOC) has been fabricated. A serpentine channel has been patterned with a sol-gel film that incorporates a cyclodextrin supramolecule modified with a Tb(3+) macrocycle. Bright emission from the Tb(3+) ion is observed upon exposure of the (mu)FOC to biphenyl in aqueous solution. The signal transduction mechanism was elucidated by undertaking steady-state and time-resolved spectroscopic measurements directly on the optical chemosensor patterned within the microfluidic network. The presence of biphenyl in the cyclodextrin receptor site triggers Tb(3+) emission by an absorption-energy transfer-emission process. These results demonstrate that the intricate signal transduction mechanisms of supramolecular optical chemosensors are successfully preserved in microfluidic environments.     

Bidirectional and unidirectional PCET in a molecular model of a cobalt-based oxygen-evolving catalyst

The oxidation of water to molecular oxygen is a kinetically demanding reaction that requires efficient coupling of proton and electron transfer. The key proton-coupled electron transfer (PCET) event in water oxidation mediated by a cobalt-phosphate-based heterogeneous catalyst is the one-electron, one-proton conversion of Co(III)-OH to Co(IV)-O. We now isolate the kinetics of this PCET step in a molecular Co(4)O(4) cubane model compound. Detailed electrochemical, stopped-flow, and NMR studies of the Co(III)-OH to Co(IV)-O reaction reveal distinct mechanisms for the unidirectional PCET self-exchange reaction and the corresponding bidirectional PCET. A stepwise mechanism, with rate-limiting electron transfer is observed for the bidirectional PCET at an electrode surface and in solution, whereas a concerted proton-electron transfer displaying a moderate KIE (4.3 ± 0.2), is observed for the unidirectional self-exchange reaction.