Cyclopentadienyl substituent effects on reductive elimination reactions in group 4 metallocenes: kinetics,mechanism, and application to dinitrogen activation

The rate of reductive elimination for a family of zirconocene isobutyl hydride complexes, Cp(CpR(n)())Zr(CH(2)CHMe(2))H (Cp = eta(5)-C(5)Me(5), CpR(n)() = substituted cyclopentadienyl), has been measured as a function of cyclopentadienyl substituent. In general, the rate of reductive elimination increases modestly with the incorporation of sterically demanding substituents such as [CMe(3)] or [SiMe(3)]. A series of isotopic labeling experiments was used to elucidate the mechanism and rate-determining step for the reductive elimination process. From these studies, a new zirconocene isobutyl hydride complex, Cp' '(2)Zr(CH(2)CHMe(2))(H) (Cp' ' = eta(5)-C(5)H(3)-1,3-(SiMe(3))(2)), was designed and synthesized such that facile reductive elimination of isobutane and activation of dinitrogen was observed. The resulting dinitrogen complex, [Cp' '(2)Zr](2)(mu(2), eta(2),eta(2)-N(2)), has been characterized by X-ray diffraction and displays a bond length of 1.47 A for the N(2) ligand, the longest observed in any metallocene dinitrogen complex. Solution magnetic susceptibility demonstrates that [Cp' '(2)Zr](2)(mu(2), eta(2), eta(2)-N(2)) is a ground-state triplet, consistent with two Zr(III), d(1) centers. Mechanistic studies reveal that the dinitrogen complex is derived from the reaction of N(2) with the resulting cyclometalated zirconocene hydride rather than directly from reductive elimination of alkane.     

Kinetic study of the oxidative addition of methyl iodide to Vaska’s complex in ionic liquids

Oxidative addition of methyl iodide to Vaska’s complex in the ionic liquids 1-butyl-3-methylimidazolium triflate [C₄mim][OTf], [C₄mim] bis(trifluormethylsulfonyl)imide [Tf₂N], and N-hexylpyridinium [C₆pyr][Tf₂N] occurred cleanly to give the expected Ir(III) oxidative addition product. Pseudo-first order rate constants were determined for the oxidative addition reaction in each solvent ([Vaska’s] = 0.25 mM, [CH₃I] = 37.5 mM). The observed rate constants under these conditions were 5-10 times slower than the rate seen in DMF. At high methyl iodide concentrations (>23 mM), the expected first order dependence on methyl iodide was not observed. In each ionic liquid, there was no change in the reaction rates within experimental error over the methyl iodide concentration range of 23-75 mM. At lower methyl iodide concentration, a decrease in rate was observed in [C₄mim][Tf₂N] with decreasing concentration of methyl iodide.     

Accurate free energies of micelle formation

We determine the free energy of micelle formation for model surfactants in a Lennard-Jones solvent by employing a hybrid semi-grand Monte Carlo simulation scheme in combination with umbrella sampling and configurational bias techniques. Comparing the results to theoretical prediction, we obtain good agreement for large micellar sizes. We also study the effect of changing the surfactant headgroup size and tail length on the critical micelle concentration. The values of and the trends in the calculated critical micelle concentrations do agree with experimental observation for nonionic surfactants. The results open up the way for the calculation of critical micelle concentrations using realistic atomic force fields.     

The influence of micelle formation on the stability of colloid surfactant mixtures

The stability of colloidal dispersions can be severely affected by the presence of surfactants. Because surfactants can adsorb at colloidal surfaces as well as form micelles, one can expect an interplay between both phenomena. Using grand-canonical coarse-grained Monte Carlo simulations on surfactant solutions confined between two surfaces, we investigate how adsorption and micelle formation affects the effective interaction between two colloidal particles, and hence, the stability of the colloidal dispersion. For solvophilic colloidal surfaces, we observe a short-ranged oscillatory solvation pressure that is hardly affected by the presence of surfactants in the system. The effective surface-surface interaction, however, reveals a decrease in solvophilic stabilization as a function of surfactant chemical potential. For solvophobic surfaces, we find that the capillary evaporation observed in a confined pure solvent, is counteracted by the addition of surfactants. Around the critical micelle concentration (CMC), the surface-surface interaction even becomes repulsive, enhancing stabilization of the colloidal dispersion. In contrast, the formation of micelles at concentrations above the CMC causes an additional depletion effect, resulting in an effective attraction, which in turn can destabilize a colloidal dispersion.     

Prediction of an autocatalytic replication mechanism for micelle formation

We report molecular simulations suggesting that the kinetics of surfactant micelle formation can be sped up significantly by a replication mechanism, in which growing micelles become unstable and split into two similar sized micelles. We argue that for certain surfactants types around the critical micelle concentration, such a mechanism becomes more dominant than the commonly accepted nucleation pathway.     

Interactions between antimicrobial polynorbornenes and phospholipid vesicles monitored by light scattering and microcalorimetry

Antimicrobial polynorbornenes composed of facially amphiphilic monomers have been previously reported to accurately emulate the antimicrobial activity of natural host-defense peptides (HDPs). The lethal mechanism of most HDPs involves binding to the membrane surface of bacteria leading to compromised phospholipid bilayers. In this paper, the interactions between biomimetic vesicle membranes and these cationic antimicrobial polynorbornenes are reported. Vesicle dye-leakage experiments were consistent with previous biological assays and corroborated a mode of action involving membrane disruption. Dynamic light scattering (DLS) showed that these antimicrobial polymers cause extensive aggregation of vesicles without complete bilayer disintegration as observed with surfactants that efficiently solubilize the membrane. Fluorescence microscopy on vesicles and bacterial cells also showed polymer-induced aggregation of both synthetic vesicles and bacterial cells. Isothermal titration calorimetry (ITC) afforded free energy of binding values (Delta G) and polymer to lipid binding ratios, plus revealed that the interaction is entropically favorable (Delta S>0, Delta H>0). It was observed that the strength of vesicle binding was similar between the active polymers while the binding stoichiometries were dramatically different.     

Sampling the kinetic pathways of a micelle fusion and fission transition

The mechanism and kinetics of micellar breakup and fusion in a dilute solution of a model surfactant are investigated by path sampling techniques. Analysis of the path ensemble gives insight in the mechanism of the transition. For larger, less stable micelles the fission/fusion occurs via a clear neck formation, while for smaller micelles the mechanism is more direct. In addition, path analysis yields an appropriate order parameter to evaluate the fusion and fission rate constants using stochastic transition interface sampling. For the small, stable micelle (50 surfactants) the computed fission rate constant is a factor of 10 lower than the fusion rate constant. The procedure opens the way for accurate calculation of free energy and kinetics for, e.g., membrane fusion, and wormlike micelle endcap formation.     

Development of polymethacrylate nanospheres as targeted delivery systems for catechin within the gastrointestinal tract

In this study, pH-sensitive nanospheres were fabricated using a polymethacrylate-based copolymer to encapsulate, protect, and release catechin, and thereby overcome its poor water solubility and low oral bioaccessibility. The polymer used was a polymethacrylic acid-co-ethyl acrylate 1:1 copolymer that dissolves above pH 5.5, and so can be used to retain and protect bioactives within the stomach but releases them in the small intestine. Catechin-loaded nanospheres were fabricated using the solvent displacement method. Physicochemical characterization of the nanospheres indicated that they were relatively small (d = 160 nm) and had a high negative charge (ζ = ? 36 mV), which meant that they had good stability to aggregation under physiological conditions (pH 7.2). Catechin was trapped within the nanospheres at an encapsulation efficiency of about 51% in an amorphous state. A simulated gastrointestinal study showed that catechin was slowly released under gastric conditions (pH 2.5), but rapidly released under small intestine conditions (pH 7.2). The observed improvement in the antioxidant activity and bioaccessibility of catechin after encapsulation was attributed to the fact that it was in an amorphous state and had good water dispersibility. This study provides useful information for the formulation of novel delivery systems to improve the dispersibility, bioaccessibility, and bioactivity of catechin and potentially other active components. These delivery systems could be used to improve the efficacy of bioactive components in foods, supplements, and pharmaceutical products.