Thermodynamic,kinetic, and structural factors in the synthesis of imine-linked dynamic covalent frameworks

The formation of imine-linked dynamic covalent frameworks (DCFs) has been systematically studied to determine how different factors such as reaction solvent and composition, reaction kinetics and thermodynamics, and structure of the building units influence the yield, degree of condensation, and homogeneity of the resulting products. Using molecular tetraimine analogs, we first investigated the kinetics and thermodynamics of imine condensation reactions under a variety of conditions, as well as the structures of the basic tetraimine units by X-ray crystallography. These model systems allowed us to identify conditions that favored both high reversibility and fast reaction kinetics for the imine condensation, leading in the end to the development of effective synthetic routes toward imine-linked DCFs with high degrees of cross-linking and homogeneity.     

Comparison of amorphous iridium water-oxidation electrocatalysts prepared from soluble precursors

Electrodeposition of iridium oxide layers from soluble precursors provides a route to active thin-layer electrocatalysts for use on water-oxidizing anodes. Certain organometallic half-sandwich aqua complexes of iridium form stable and highly active oxide films upon electrochemical oxidation in aqueous solution. The catalyst films appear as blue layers on the anode when sufficiently thick, and most closely resemble hydrous iridium(III,IV) oxide by voltammetry. The deposition rate and cyclic voltammetric response of the electrodeposited material depend on whether the precursor complex contains a pentamethylcyclopentadieneyl (Cp*) or cyclopentadienyl ligand (Cp), and do not match, in either case, iridium oxide anodes prepared from non-organometallic precursors. Here, we survey our organometallic precursors, iridium hydroxide, and pre-formed iridium oxide nanoparticles. From electrochemical quartz crystal nanobalance (EQCN) studies, we find differences in the rate of electrodeposition of catalyst layers from the two half-sandwich precursors; however, the resulting layers operate as water-oxidizing anodes with indistinguishable overpotentials and H/D isotope effects. Furthermore, using the mass data collected by EQCN and not otherwise available, we show that the electrodeposited materials are excellent catalysts for the water-oxidation reaction, showing maximum turnover frequencies greater than 0.5 mol O(2) (mol iridium)(-1) s(-1) and quantitative conversion of current to product dioxygen. Importantly, these anodes maintain their high activity and robustness at very low iridium loadings. Our organometallic precursors contrast with pre-formed iridium oxide nanoparticles, which form an unstable electrodeposited material that is not stably adherent to the anode surface at even moderately oxidizing potentials.     

An N-heterocyclic carbene containing a bithiophene backbone: synthesis and coordination chemistry

A new N-heterocyclic carbene (NHC) containing a fused bithiophene backbone has been synthesized along with its silver(I) and BPh(3) complexes. The donor strength of this unique NHC has been determined from the IR stretching frequencies of the isolated NHC-Rh(CO)(2)Cl complex. The photophysical properties of all of the novel compounds have been investigated and are presented.     

Photocatalyzed sulfide oxygenation with water as the unique oxygen atom source

In our research program aiming to develop new ruthenium-based polypyridine catalysts for oxidation we were interested in combining a photosensitizer and a catalytic fragment within the same complex to achieve catalytic light-driven oxidation. To respond to the lack of such conjugates, we report here a new catalytic system capable of using light to activate water molecules in order to perform selective sulfide oxygenation into sulfoxide via an oxygen atom transfer from H(2)O to the substrate with a TON of up to 197 ± 6. On the basis of electrochemical and photophysical studies, a proton-coupled electron-transfer process yielding to an oxidant Ru(IV)-oxo species was proposed. In particular, the synergistic effect between both partners in the dyad yielding a more efficient catalyst compared to the bimolecular system is highlighted.     

Scandium and yttrium metallocene borohydride complexes: comparisons of (BH4)1- vs. (BPh4)1- coordination and reactivity

The synthetically accessible borohydride complexes (C(5)Me(4)H)(2)Ln(THF)(BH(4)) and (C(5)Me(5))(2)Ln(THF)(BH(4)) (Ln = Sc, Y) were examined as precursors alternative to the heavily-used tetraphenylborate analogs, [(C(5)Me(4)H)(2)Ln][BPh(4)] and [(C(5)Me(5))(2)Ln][BPh(4)], employed in LnA(2)A'/M reduction reactions (A = anion; M = alkali metal) that generate "LnA(2)" reactivity and form reduced dinitrogen complexes [(C(5)R(5))(2)(THF)(x)Ln](2)(μ-η(2):η(2)-N(2)) (x = 0, 1). The crystal structures of the yttrium borohydrides, (C(5)Me(4)H)(2)Y(THF)(μ-H)(3)BH, 1, and (C(5)Me(5))(2)Y(THF)(μ-H)(2)BH(2), 2, were determined for comparison with those of the yttrium tetraphenylborates, [(C(5)Me(4)H)(2)Y][(μ-Ph)(2)BPh(2)], 3, and [(C(5)Me(5))(2)Y][(μ-Ph)(2)BPh(2)], 4. The complex (C(5)Me(4)H)(2)Sc(μ-H)(2)BH(2), 5, was synthesized and structurally characterized for comparison with (C(5)Me(5))(2)Sc(μ-H)(2)BH(2), 6, [(C(5)Me(4)H)(2)Sc][(μ-Ph)BPh(3)], 7, and [(C(5)Me(5))(2)Sc][(μ-Ph)BPh(3)], 8. Structural information was also obtained on the borohydride derivatives, (C(5)Me(4)H)(2)Sc(μ-H)(2)BC(8)H(14), 9, and (C(5)Me(5))(2)Sc(μ-H)(2)BC(8)H(14), 10, obtained from 9-borabicyclo(3.3.1)nonane (9-BBN) and (C(5)Me(4)R)(2)Sc(η(3)-C(3)H(5)), where R = H, 11; Me, 12. The preference of the metals for borohydride over tetraphenylborate binding was shown by the facile displacement of (BPh(4))(1-) in 3, 4, 7, and 8 by (BH(4))(1-) to make the respective borohydride complexes 1, 2, 5, and 6. These results are consistent with the fact that the borohydrides are not as useful as precursors in A(2)LnA'/M reductions of N(2). An unusual structural isomer of [(C(5)Me(4)H)(2)Sc](2)(μ-η(2):η(2)-N(2)), 13', was isolated from this study that shows the variations in ligand orientation that can occur in the solid state.     

Heartbeat sensors under pressure: a new method for assessing hyperbaric physiology

Non-invasive heartbeat sensors to measure the cardiac activity of crustaceans have been adapted for use under hyperbaric conditions. Able to record data continuously over long timescales, these sensors can collect high-resolution data on the physiological state of an organism up to a tested limit of 300 atm. Using this technique, heart rate was recorded in a juvenile of the sublittoral spider crab, Maja brachydactyla (Decapoda: Majidae), when subjected to hydrostatic pressures of 1, 50, 100, and 150 atm for periods of 30 min. Heart rate increases with pressure until 100 atm (one-way repeated measures ANOVA: F _{(4, 25)}=154.76, p<0.001). However, the significant decrease in the mean heart rate from 137.07 bpm at 100 atm to 118.40 bpm at 150 atm (t-test: t=4.581, d.f.=10, p<0.001) indicates a mechanistic limit in the cardiac response of this species to pressures beyond 100 atm. This method could be potentially applied to any marine invertebrate with a neurogenic heart.     

Global Weyl modules for the twisted loop algebra

We define global Weyl modules for twisted loop algebras and analyze their highest weight spaces, which are in fact isomorphic to Laurent polynomial rings in finitely many variables. We are able to show that the global Weyl module is a free module of finite rank over these rings. Furthermore we prove, that there exist injective maps from the global Weyl modules for twisted loop algebras into a direct sum of global Weyl modules for untwisted loop algebras. Relations between local Weyl modules for twisted and untwisted generalized current algebras are known; we provide for the first time a relation on global Weyl modules.     

Second-chance forward isomerization dynamics of the red/green cyanobacteriochrome NpR6012g4 from Nostoc punctiforme

The primary ultrafast Z-to-E isomerization photodynamics of the phytochrome-related cyanobacteriochrome NpR6012g4 from Nostoc punctiforme was studied by transient absorption pump-dump-probe spectroscopy. A 2 ps dump pulse resonant with the stimulated emission band depleted 21% of the excited-state population, while the initial photoproduct Lumi-R was depleted by only 11%. We observed a red-shifted ground-state intermediate (GSI) that we assign to a metastable state that failed to isomerize fully. Multicomponent global analysis implicates the generation of additional Lumi-R from the GSI via crossing over the ground-state thermal barrier for full isomerization, explaining the discrepancy between excited-state and Lumi-R depletion by the dump pulse. This second-chance ground-state dynamics provides a plausible explanation for the unusually high quantum yield of 40% for the primary isomerization step in the forward reaction of NpR6012g4.     

Topographic effects on infrasound propagation

Infrasound data were collected using portable arrays in a region of variable terrain elevation to quantify the effects of topography on observed signal amplitude and waveform features at distances less than 25 km from partially contained explosive sources during the Frozen Rock Experiment (FRE) in 2006. Observed infrasound signals varied in amplitude and waveform complexity, indicating propagation effects that are due in part to repeated local maxima and minima in the topography on the scale of the dominant wavelengths of the observed data. Numerical simulations using an empirically derived pressure source function combining published FRE accelerometer data and historical data from Project ESSEX, a time-domain parabolic equation model that accounted for local terrain elevation through terrain-masking, and local meteorological atmospheric profiles were able to explain some but not all of the observed signal features. Specifically, the simulations matched the timing of the observed infrasound signals but underestimated the waveform amplitude observed behind terrain features, suggesting complex scattering and absorption of energy associated with variable topography influences infrasonic energy more than previously observed.