Tetramethylcyclobutadienecobalt(I) complexes containing pyrazolate or tetrazolate ligands with various coordination modes

Treatment of Cb^∗Co(CO)₂I (Cb^∗ = η⁴-C₄Me₄) with one equivalent of potassium 3,5-dimethylpyrazolate (Me₂pzK) or potassium 3,5-bis(trifluoromethyl)pyrazolate ((CF₃)₂pzK) in tetrahydrofuran at 0 °C afforded (η⁴-C₄Me₄(Me₂pz))Co(CO)₂ and Cb^∗Co((CF₃)₂pz)(CO)₂ in 90 and 71% yields, respectively. Treatment of Cb^∗Co(CO)₂I with one equivalent of Me₂pzH followed by the addition of one equivalent Me₂pzK in tetrahydrofuran afforded Cb^∗Co(Me₂pzH)(Me₂pz)(CO) in 74% yield. The reaction of Cb^∗Co(CO)₂I with one equivalent of potassium phenyl tetrazolate (PhtetzK) in tetrahydrofuran at 0 °C afforded [Cb^∗Co(Phtetz)(CO)]₂ in 44% yield. The solid state structure of (η⁴-C₄Me₄(Me₂pz))Co(CO)₂ revealed nucleophilic addition of a pyrazolate nitrogen atom to a Cb^∗ ligand carbon atom to afford a novel tetradentate ligand that bonds to the cobalt ion through an η³-π-allyl interaction and one pyrazolyl nitrogen atom. Two carbonyl ligands are also present. An X-ray crystal structure determination of Cb^∗Co((CF₃)₂pz)(CO)₂ showed η¹-coordination of the (CF₃)₂pz ligand and η⁴-coordination of the Cb^∗ ligand. The solid state structure of Cb^∗Co(Me₂pz)(Me₂pzH)(CO) is monomeric with one η¹-Me₂pz, one η¹-Me₂pzH, two carbonyl, and one η⁴-Cb^∗ ligands. The η¹-Me₂pz and η¹-Me₂pzH ligands are linked by a hydrogen bond involving the uncoordinated nitrogen atoms. The X-ray crystal structure determination of [Cb^∗Co(Phtetz)(CO)]₂ showed a dimeric molecular structure with two μ:η¹,η¹-Phtetz ligands connected to the cobalt ions through the 2- and 3-nitrogen atoms, one η⁴-Cb^∗ ligand, and one carbonyl ligand per cobalt center. (η⁴-C₄Me₄(Me₂pz))Co(CO)₂ is highly volatile and sublimes at 60 °C/0.03 Torr.     

Nickel-catalyzed synthesis of nanoporous organic frameworks and their potential use in gas storage applications

The synthesis of highly nanoporous organic frameworks (NPOFs) has been established using nickel(0)-catalyzed Yamamoto coupling reactions, which has afforded highly porous polymers featuring remarkable chemical and thermal stability. Treatment of 1,3,5-tris(4-bromophenyl)benzene, 1,2,4,5-tetrakis(4-bromophenyl)benzene, or 1,3,5,7-tetrakis(4-iodophenyl)adamantane with Ni(cod)₂ in DMF at 80°C for 48 h afforded the nanoporous organic frameworks, NPOF-1, NPOF-2, and NPOF-3, respectively, as white powders in quantitative yields. All NPOFs are insoluble in common organic solvents such as dimethylformamide, tetrahydrofuran, toluene, dichloromethane, and methanol. The chemical composition and structural aspects of NPOFs were investigated by spectral and analytical methods while porosity was examined by nitrogen porosity measurements. In spite of their amorphous nature, NPOFs exhibit permanent porosity and high Langmuir surface areas (NPOF-1: 2,635 m² g^?1; NPOF-2: 4,227 m² g^?1; NPOF-3: 2,423 m² g^?1), which make them attractive for subsequent use in gas storage and separation applications, among others. The performance of NPOFs in hydrogen storage was evaluated at 1 bar and 77 K and revealed that these highly porous architectures can store up to 1.45 wt% of hydrogen.     

Eigenvalue Boundary Problems for the Dirac Operator

 On a compact Riemannian spin manifold with mean-convex boundary, we analyse the ellipticity and the symmetry of four boundary conditions for the fundamental Dirac operator including the (global) APS condition and a Riemannian version of the (local) MIT bag condition. We show that Friedrich's inequality for the eigenvalues of the Dirac operator on closed spin manifolds holds for the corresponding four eigenvalue boundary problems. More precisely, we prove that, for both the APS and the MIT conditions, the equality cannot be achieved, and for the other two conditions, the equality characterizes respectively half-spheres and domains bounded by minimal hypersurfaces in manifolds carrying non-trivial real Killing spinors. Received: 12 November 2001 / Accepted: 25 June 2002 Published online: 21 October 2002 RID="*" ID="*" Research of S. Montiel is partially supported by a Spanish MCyT grant No. BFM2001-2967 and by European Union FEDER funds     

Anomalous structure-luminescence relationship in phosphorescent gold(I) isonitrile neutral complexes

A new compound that exhibits the shortest intermolecular Au...Au distance ever reported for neutral RNCAuX complexes is found to exhibit a counterintuitive higher-energy Au-centered phosphorescence than that in an analogous compound with a much longer Au...Au distance, presumably due to a different extent of excited-state distortion in dimers vs. extended chains.     

Photochemistry of neutral isonitrile gold(I) complexes: modulation of photoreactivity by aurophilicity and pi-acceptance ability

This work demonstrates for the first time that aurophilicity and ligand pi-acceptance ability sensitize the photoreactivity of Au(I) complexes. Photolysis of LAu(I)Cl (L = RNC or CO) complexes leads to free L, Au(III), and Au(0) photoproducts. Solutions of (p-tosyl)CH(2)NCAuCl in dichloromethane undergo significant oligomerization leading to dimers and trimers with formation constants of 1.61 x 10(3) and 6.61 x 10(3) M(-1), respectively, representing the highest values reported to date for complexes that exhibit aurophilic association in solution. The photoproduct quantum yield (Phi) varies with the LAu(I)Cl concentration in solution. For (p-tosyl)CH(2)NCAuCl, metallic gold forms with Phi = 0.0065 and 0.032 in 4.0 x 10(-5) and 4.0 x 10(-3) M dichloromethane solutions, respectively. Meanwhile, irradiation of t-BuNCAuCl primarily produces t-BuNCAuCl(3) with Phi = 0.0045 and 0.013 for 5.0 x 10(-5) and 5.0 x 10(-3) M dichloromethane solutions, respectively. For Au(CO)Cl, metallic gold forms with Phi = 0.013 and 0.065 upon irradiation of 8.0 x 10(-5) and 8.0 x 10(-3) M dichloromethane solutions, respectively. Hence, *[LAuX](n) oligomeric species are more photoreactive than monomeric species. The results also demonstrate intuitive control of Phi via modulation of the pi-acceptance ability of L, as both follow CO > (p-tosyl)CH(2)NC > (alkyl)NC in LAuCl, a trend that is also commensurate with the relative long-term photosensitivity of the corresponding solids and solutions. A new method for preparing stable small gold nanoparticles is described based on the fundamental findings above. Thus, photolysis of different concentrations of LAuX in solutions containing a primary amine-terminated dendrimer leads to clear solutions exhibiting tunable visible plasmon absorptions of gold nanoparticles; these solutions maintain their colors and stability indefinitely. TEM measurements for representative samples prepared by photolysis of (p-tosyl)CH(2)NCAuCl solutions give rise to spherical nanoparticles as small as 5 nm.     

Phase Transitions in Transfer Learning for High-Dimensional Perceptrons

Transfer learning seeks to improve the generalization performance of a target task by exploiting the knowledge learned from a related source task. Central questions include deciding what information one should transfer and when transfer can be beneficial. The latter question is related to the so-called negative transfer phenomenon, where the transferred source information actually reduces the generalization performance of the target task. This happens when the two tasks are sufficiently dissimilar. In this paper, we present a theoretical analysis of transfer learning by studying a pair of related perceptron learning tasks. Despite the simplicity of our model, it reproduces several key phenomena observed in practice. Specifically, our asymptotic analysis reveals a phase transition from negative transfer to positive transfer as the similarity of the two tasks moves past a well-defined threshold.     

Unique weak solutions of the d‐dimensional micropolar equation with fractional dissipation

This article examines the existence and uniqueness of weak solutions to the d‐dimensional micropolar equations (d=2 or d=3) with general fractional dissipation (?Δ)^αu and (?Δ)^βw. The micropolar equations with standard Laplacian dissipation model fluids with microstructure. The generalization to include fractional dissipation allows simultaneous study of a family of equations and is relevant in some physical circumstances. We establish that, when $\alpha \ge \frac{1}{2}$ and $\beta \ge \frac{1}{2}$, any initial data (u₀,w₀) in the critical Besov space $u_0\in B_{2,1}^{1+\frac{d}{2}?2\alpha }({?}^d)$ and $w_0\in B_{2,1}^{1+\frac{d}{2}?2\beta }({?}^d)$ yields a unique weak solution. For α ≥ 1 and β=0, any initial data $u_0\in B_{2,1}^{1+\frac{d}{2}?2\alpha }({?}^d)$ and $w_0\in B_{2,1}^{\frac{d}{2}}({?}^d)$ also leads to a unique weak solution as well. The regularity indices in these Besov spaces appear to be optimal and can not be lowered in order to achieve the uniqueness. Especially, the 2D micropolar equations with the standard Laplacian dissipation, namely, α=β=1, have a unique weak solution for $(u_0,w_0)\in B_{2,1}^0$. The proof involves the construction of successive approximation sequences and extensive a priori estimates in Besov space settings.     

Poly(ethylene glycol)-based shape-memory polymers

Poly(ethylene glycol) (PEG) blends photo-curable and thermal activated shape-memory polymers (SMPs), with different activation temperature (T_switch), have been synthesized and characterized. PEG blends with different molecular weights were chain-end functionalized with isocyanate ethyl methacrylate and photo-cured with UV lamp. Degree of cross-linking of the blend network, determined by gel content measurement, resulted as higher than 95%. The thermal and thermomechanical properties of these SMPs PEG blends were characterized by differential scanning calorimetry and dynamic mechanical analysis. The shape-memory properties of the networks were quantified using thermomechanical three-point bending experiments and showed strain fixity rates higher than 99% and a minimum strain recovery ratio of 82%.     

Grafted polymer membranes with extractive agents for the extraction process of VO2+ ions

To conduct experiments related to the facilitated extraction phenomenon and recovery of vanadium ions VO₂⁺, two grafted polymer membranes were prepared; containing respectively cholic acid and azithromycin as extractive agents and the obtained membranes were characterized. All experiments for the facilitated extraction phenomenon of these ions were carried out. Kinetic and thermodynamic models, based on the interaction of the substrate S (VO₂⁺) with the extractive agent T, and the diffusion of the formed entity (TS) through the membrane were developed, to determine the macroscopic parameters, permeability P and initial fluxes J₀, and the microscopic parameters, association constants K_ass and apparent diffusion coefficients D*, related to the formation of entities TS and their diffusion. A clear evolution of these parameters, depending on various factors (substrate concentration C₀, extractive agent concentration and feed and receiving phases temperature), and high permeabilities for this extraction phenomenon of VO₂⁺ ions were observed for this membrane type. Copyright © 2016 John Wiley & Sons, Ltd.