An octanuclear molybdenum(VI) complex containing coordinatively bound 4,4'-di-tert-butyl-2,2'-bipyridine, [Mo8O22(OH)4(di-tBu-bipy)4]: synthesis, structure, and catalytic epoxidation of bio-derived olefins

The reaction of [MoO(2)Cl(2)(di-tBu-bipy)] (1) (di-tBu-bipy = 4,4'-di-tert-butyl-2,2'-bipyridine) with water at 100-120 °C in a Teflon-lined stainless steel autoclave, in an open reflux system, or in a microwave synthesis system gave the octanuclear complex [Mo(8)O(22)(OH)(4)(di-tBu-bipy)(4)] (2) as a microcrystalline powder in good yields. Single crystals of 2 suitable for X-ray diffraction were obtained by the reaction of MoO(3) and di-tBu-bipy in water at 160 °C for 3 days. The molecular structure of 2 comprises a purely inorganic core, Mo(4)O(8)(μ(3)-OH)(2)(μ(2)-O)(2), attached to two peripheral oxo-bridged binuclear units, Mo(2)O(4)(μ(2)-O)(2)(OH)(di-tBu-bipy)(2). The inorganic core is composed of a unique assembly of four {MoO(5)} distorted square pyramids connected to each other via edge-sharing. Overall, the octanuclear complex adopts a highly distorted form strongly resembling an "S"-shaped molecular unit. Complex 2 was applied in the catalytic epoxidation of the biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using tert-butylhydroperoxide (TBHP) as an oxygen donor, under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high selectivities at high conversions (89% 1,2-epoxy-p-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable "non-productive" decomposition of TBHP.     

Multiphasic Individual Growth Models in Random Environments

The evolution of the growth of an individual in a random environment can be described through stochastic differential equations of the form dY _t  = β(α − Y _t )dt + σdW _t , where Y _t  = h(X _t ), X _t is the size of the individual at age t, h is a strictly increasing continuously differentiable function, α = h(A), where A is the average asymptotic size, and β represents the rate of approach to maturity. The parameter σ measures the intensity of the effect of random fluctuations on growth and W _t is the standard Wiener process. We have previously applied this monophasic model, in which there is only one functional form describing the average dynamics of the complete growth curve, and studied the estimation issues. Here, we present the generalization of the above stochastic model to the multiphasic case, in which we consider that the growth coefficient β assumes different values for different phases of the animal’s life. For simplicity, we consider two phases with growth coefficients β ₁ and β ₂. Results and methods are illustrated using bovine growth data.     

Crossover to the KPZ Equation

We characterize the crossover regime to the KPZ equation for a class of one-dimensional weakly asymmetric exclusion processes. The crossover depends on the strength asymmetry an ^2-γ (a, γ >  0) and it occurs at γ = 1/2. We show that the density field is a solution of an Ornstein-Uhlenbeck equation if \({\gamma\in(1/2,1]}\) , while for γ = 1/2 it is an energy solution of the KPZ equation. The corresponding crossover for the current of particles is readily obtained.     

Self-degassing SARA ATRP mediated by Na2S2O4 with no external additives

The supplemental activator and reducing agent (SARA) atom transfer radical polymerization (ATRP) mediated by Na₂S₂O₄ in the presence of air, without external deoxygenation or additional oxygen scavengers, is reported for several vinyl monomers: methyl acrylate (MA), n-butyl acrylate (n-BA), methyl methacrylate (MMA), 2-(dimethylamino)ethyl methacrylate (DMAEMA), poly(ethylene glycol) methyl ether acrylate (OEOA), and styrene (Sty). The polymerizations can be conducted in aqueous medium or using organic/water mixtures as solvent, with low concentration of copper, near room temperature. In the absence of any external deoxygenation, several well-defined homopolymers and block copolymers were obtained (Ð < 1.3). The evolution of the oxygen concentration during the polymerizations was monitored with an optical oxygen sensor. The consumption of oxygen prior polymerization in ethanol/water mixtures was attributed to the combined presence of Na₂S₂O₄ and alkyl halide initiator, which led to a lower initiation efficiency (I_eff). This could be overcome by decreasing the headspace volume of the reaction. The system reported exhibited the potential to be scalable, which is very relevant from an industrial standpoint. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 145–153     

Ab initio and DFT benchmark study for nucleophilic substitution at carbon (SN2@C) and silicon (SN2@Si)

To obtain a set of consistent benchmark potential energy surfaces (PES) for the two archetypal nucleophilic substitution reactions of the chloride anion at carbon in chloromethane (S(N)2@C) and at silicon in chlorosilane (S(N)2@Si), we have explored these PESes using a hierarchical series of ab initio methods [HF, MP2, MP4SDQ, CCSD, CCSD(T)] in combination with a hierarchical series of six Gaussian-type basis sets, up to g polarization. Relative energies of stationary points are converged to within 0.01 to 0.56 kcal/mol as a function of the basis-set size. Our best estimate, at CCSD(T)/aug-cc-pVQZ, for the relative energies of the [Cl(-), CH(3)Cl] reactant complex, the [Cl-CH(3)-Cl](-) transition state and the stable [Cl-SiH(3)-Cl](-) transition complex is -10.42, +2.52, and -27.10 kcal/mol, respectively. Furthermore, we have investigated the performance for these reactions of four popular density functionals, namely, BP86, BLYP, B3LYP, and OLYP, in combination with a large doubly polarized Slater-type basis set of triple-zeta quality (TZ2P). Best overall agreement with our CCSD(T)/aug-cc-pVQZ benchmark is obtained with OLYP and B3LYP. However, OLYP performs better for the S(N)2@C overall and central barriers, which it underestimates by 2.65 and 4.05 kcal/mol, respectively. The other DFT approaches underestimate these barriers by some 4.8 (B3LYP) to 9.0 kcal/mol (BLYP).     

Keto-Enolic Equilibria of an Isatin-Schiff Base Copper(II) Complex and its Reactivity toward Carbohydrate Oxidation

An interesting isatin-Schiff base copper(II) complex, [Cu(isapn)](ClO₄)₂ where isapn= N,N-[bis-(3,3-indolin-2-one)]-1,3-diiminepropane, was prepared and characterized by different techniques, both in the solid state and in solution, and its reactivity toward carbohydrate oxidation was verified. The positive ion electrospray mass spectrum detects the complex as an isotopologue cluster of singly charged intact isatin-copper(I) ions of m/z 395 (for ⁶⁵Cu) with an isotopic pattern identical to that calculated for C₁₉H₁₆CuN₄O₂ ⁺. Tandem mass spectrometry reveals an interesting and structurally diagnostic collision-induced dissociation behavior for this ionized complex, which is dominated by the cleavage of the N—(CH₂)₃—N propylene bridge. In aqueous solution, this complex undergoes a peculiar keto-enolic equilibrium, verified at different pH's by spectroscopic methods (u.v.–vis. and e.p.r.), with a corresponding pK _a determined as 9.5. The e.p.r. parameter ratio g /A for this complex, in frozen MeOH/H₂O (4:1, v/v) solution at 77 K, changes from 188 cm in acidic medium (pH 2.5–3.0) to 118 cm in basic medium (pH 11), indicating a significant structural change from a distorted tetrahedral to a more tetragonal geometry around the copper ion. This compound was shown to catalyze the oxidation of hexoses (glucose, fructose and galactose), in alkaline media, via reactive oxygen species, which were detected by using specific enzymes, and by e.p.r. spin trapping. The reaction was monitored at (25.0 ± 0.1)°C by the consumption of oxygen, and showed first-order dependence on catalyst, followed by a saturation effect. First-order kinetics with respect to [OH^–] concentration was also observed, indicating that enolization of the substrate as well as the metal-catalyzed enediol oxidation are the rate-determining steps.     

Kinetics of pyrolysis of some biomasses widely available in Brazil

Journal of Thermal Analysis and Calorimetry - Biomass conversion via thermal processes to generate energy will be an important part of the future energy landscape. The objective of this study was...     

Thermal,antioxidant, morphological and bioactive properties of starchy material extracted from the bacupari (Garcinia brasiliensis (Mart.)) seed using aqueous and alkaline maceration

Journal of Thermal Analysis and Calorimetry - Despite their being rich in bioactive compounds, by-products produced during fruit pulping are usually discarded. The present paper aimed to...     

The (2,2,0) Drazin inverse problem

We consider the additive Drazin problem and we study the existence of the Drazin inverse of a two by two matrix with zero (2,2) entry.     

Orientability of vector bundles over real flag manifolds

We investigate the orientability of a class of vector bundles over flag manifolds of real semi-simple Lie groups, which include the tangent bundle and also stable bundles of certain gradient flows. Closed formulas, in terms of roots, are provided.