Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer

The control of the solution electrochemical potential as well as pH impacts products in redox reactions, but the former gets far less attention. Redox buffers facilitate the maintenance of potentials and have been noted in diverse cases, but they have not been a component of catalytic systems. We report a catalytic system that contains its own built-in redox buffer. Two highly synergistic components (a) the tetrabutylammonium salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) and (b) Cu­(ClO4)2 in acetonitrile catalyze the aerobic oxidative deodorization of thiols by conversion to the corresponding nonodorous disulfides at 23 °C (each catalyst alone is far less active). For example, the reaction of 2-mercaptoethanol with ambient air gives a turnover number (TON) = 3 × 102 in less than one hour with a turnover frequency (TOF) of 6 × 10–2 s–1 with respect to PV6Mo6. Multiple electrochemical, spectroscopic, and other methods establish that (1) PV6Mo6, a multistep and multielectron redox buffering catalyst, controls the speciation and the ratio of Cu­(II)/Cu­(I) complexes and thus keeps the solution potential in different narrow ranges by involving multiple POM redox couples and simultaneously functions as an oxidation catalyst that receives electrons from the substrate; (2) Cu catalyzes two processes simultaneously, oxidation of the RSH by PV6Mo6 and reoxidation of reduced PV6Mo6 by O2; and (3) the analogous polytungstate-based system, TBA4H5[PW6V6O40] (PV6W6), has nearly identical cyclic voltammograms (CV) as PV6Mo6 but has almost no catalytic activity: it does not exhibit self-redox buffering

Role of Multiple Vanadium Centers on Redox Buffering and Rates of Polyvanadomolybdate-Cu(II)-Catalyzed Aerobic Oxidations

A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number (x = 0–4 and 6) in TBA salts of PVxMo12–xO40(3+x)– (PVMo) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to −2000 mV vs Fc/Fc+ under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo/Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV3Mo9 and PV4Mo8 are 8.9 and 48 s–1, respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo12 is more positive than that of PVMo11 (−236 and −405 mV vs Fc/Fc+); however, the initial reduction rates are 1.06 × 10−4 s−1 and 0.036 s–1 for PMo12 and PVMo11, respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo11 and PV2Mo10, where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity

Table_1_Associations of follicle-stimulating hormone and luteinizing hormone with metabolic syndrome during the menopausal transition from the National Health and Nutrition Examination Survey.docx

BackgroundThe increased risk of metabolic syndrome (MetS) during the menopausal transition might partly attribute to the changes in follicle-stimulating hormone (FSH) and luteinizing hormone (LH). However, few studies were conducted to examine the associations of FSH and LH concentrations with MetS at the full range of reproductive aging, especially in the US population. The aim of this study is to examine the associations of FSH, LH, and LH/FSH ratio with the risk of MetS and severity score in the US women.MethodsData were derived from the National Health and Nutrition Examination Survey. Women aged from 35 to 60 years were eligible. MetS was defined as having at least 3 of the following: a waist circumference ≥ 88 cm, a triglycerides level ≥ 150 mg/dL, a high density lipoprotein ResultsThere were 3,831 women included in this study. Increases in serum FSH and LH levels per 1 SD were separately linked to a 22.6% (OR: 0.774; 95% CI: 0.646, 0.929; and P= 0.006) and 18.5% (OR: 0.815; 95% CI: 0.690, 0.962; and P= 0.006) lower risk of MetS only in postmenopausal women. Meanwhile, increases in serum FSH and LH levels per 1SD were associated with a decrease of -0.157 (95% CI :-2.967, -2.034) and -0.078 (95% CI: -2.688, -1.806) MetS severity score in perimenopausal women and -0.195 (95% CI: -2.192, -1.023) and -0.098 (95% CI:-1.884, -0.733) in postmenopausal women. However, LH/FSH ratio had no connections with the risk of MetS and MetS severity score across the menopausal transition.ConclusionsElevated serum FSH and LH levels, but not LH/FSH ratio, were associated with a lower risk of MetS and MetS severity score, especially in postmenopausal women. Therefore, serum FSH and LH levels might be efficient predictors for screening and identifying women at risk of MetS across the menopausal transition.</p

Multilevel Manipulation of Supramolecular Structures of Giant Molecules via Macromolecular Composition and Sequence

We have successfully synthesized a series of monodispersed chain-like giant molecules with precisely controlled macromolecular composition and sequence based on polyhedral oligomeric silsesquioxane (POSS) nanoparticles using an orthogonal “click” strategy. Their nonspherical supramolecular structures, such as lamellae, double gyroids, and hexagonal packed cylinders, are mainly determined by the composition (namely, the number of incorporated amphiphilic nanoparticles). In addition, by precisely alternating the sequence of arranged nanoparticles in the giant molecules with identical chemical compositions, the domain sizes of their supramolecular structures could be fine-tuned. This is attributed to the macromolecular conformational differences caused by collective hydrogen bonding interactions in each set of sequence isomeric giant molecules. This work has demonstrated multilevel manipulation of supramolecular structures of giant molecules: coarse tuning by composition and fine-tuning by sequence

Speciation and Dynamics in the [Co₄V₂W₁₈O₆₈]^10–/Co(II)_aq/CoO_<i>x</i> Catalytic Water Oxidation System

Our group reported that the polyoxometalate Na10[Co4V2W18O68]·26H2O (Co4V2) is an active water oxidation catalyst and provided characterization of this system (J. Am. Chem. Soc. 2014, 136 (26), 9268). Two recent publications called into question the stability of Co4V2, one noting the miss-assignment of a 51V NMR peak (Inorg. Chem. 2016, 55 (11), 5343) and another providing additional stability studies (ACS Catal., 2017, 7 (1), 7). We report here solution studies that further clarify stability limitations in this system by locating the correct 51V NMR resonance of Co4V2 and the other V-containing species present. Furthermore, we demonstrate that the observed catalytic activity cannot be explained simply by Co­(II)aq, but arises from multiple active WOC species in solution. Key points about investigating such complex equilibrating aqueous catalyst systems are addressed

Speciation and Dynamics in the [Co₄V₂W₁₈O₆₈]^10–/Co(II)_aq/CoO_<i>x</i> Catalytic Water Oxidation System

Our group reported that the polyoxometalate Na10[Co4V2W18O68]·26H2O (Co4V2) is an active water oxidation catalyst and provided characterization of this system (J. Am. Chem. Soc. 2014, 136 (26), 9268). Two recent publications called into question the stability of Co4V2, one noting the miss-assignment of a 51V NMR peak (Inorg. Chem. 2016, 55 (11), 5343) and another providing additional stability studies (ACS Catal., 2017, 7 (1), 7). We report here solution studies that further clarify stability limitations in this system by locating the correct 51V NMR resonance of Co4V2 and the other V-containing species present. Furthermore, we demonstrate that the observed catalytic activity cannot be explained simply by Co­(II)aq, but arises from multiple active WOC species in solution. Key points about investigating such complex equilibrating aqueous catalyst systems are addressed

Structurally Precise Two-Transition-Metal Water Oxidation Catalysts: Quantifying Adjacent 3d Metals by Synchrotron X‑Radiation Anomalous Dispersion Scattering

Mixed 3d metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know the locations and percent occupancies of different 3d metals in these heterogeneous catalysts. Without such information, it is hard to quantify catalysis, stability, and other properties of the WOC as a function of the catalyst active site structure. This study combines the site selective synthesis of a homogeneous WOC with two adjacent 3d metals, [Co2Ni2(PW9O34)2]10– (Co2Ni2P2) as a tractable molecular model for CoNi oxide, with the use of multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS) that quantifies both the location and percent occupancy of Co (∼97% outer-central-belt positions only) and Ni (∼97% inner-central-belt positions only) in Co2Ni2P2. This mixed-3d-metal complex catalyzes water oxidation an order of magnitude faster than its isostructural analogue, [Co4(PW9O34)2]10– (Co4P2). Four independent and complementary lines of evidence confirm that Co2Ni2P2 and Co4P2 are the principal WOCs and that Co2+(aq) is not. Density functional theory (DFT) studies revealed that Co4P2 and Co2Ni2P2 have similar frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexes follows analogous multistep mechanisms, including likely Co–OOH formation, with the energetics of most steps being lower for Co2Ni2P2 than for Co4P2. Synchrotron XRAS should be generally applicable to active-site-structure-reactivity studies of multi-metal heterogeneous and homogeneous catalysts