Synthesis and Study of Olefin Metathesis Catalysts Supported by Redox-Switchable Diaminocarbene 3 Ferrocenophanes

A redox-switchable ligand, N,N'-dimethyldiaminocarbene[3]ferrocenophane (5), was synthesized and incorporated into a series of Ir- and Ru-based complexes. Electrochemical and spectroscopic analyses of (5) Ir(CO)(2)Cl (15) revealed that 5 displayed a Tolman electronic parameter value of 2050 cm(-1) in the neutral state and 2061 cm(-1) upon oxidation. Moreover, inspection of X-ray crystallography data recorded for (5) Ir(cis,cis-1,5-cyclooctadiene)Cl (13) revealed that 5 was sterically less bulky (%V-Bur = 28.4) than other known diaminocarbene[3]ferrocenophanes, which facilitated the synthesis of (5)(PPh3)Cl2Ru-(3-phenylindenylid-1-ene) (18). Complex 18 exhibited quasi-reversible electrochemical processes at 0.79 and 0.98 V relative to SCE, which were assigned to the Fe and Ru centers in the complex, respectively, based on UV-vis and electron pair resonance spectroscopic measurements. Adding 2,3-dichloro-5,6-dicyanoquinone over the course of a ring-opening metathesis polymerization of cis, cis-1,5-cyclooctadiene catalyzed by 18 ([monomer](0)/[18](0) = 2500) reduced the corresponding rate constant of the reaction by over an order of magnitude (pre-oxidation: k(obs) = 0.045 s(-1); post-oxidation: k(obs) = 0.0012 s(-1)). Subsequent reduction of the oxidized species using decamethylferrocene restored catalytic activity (post-reduction: k(obs) = up to 0.016 s(-1), depending on when the reductant was added). The difference in the polymerization rates was attributed to the relative donating ability of the redox-active ligand (i.e., strongly donating 5 versus weakly donating 5(+)) which ultimately governed the activity displayed by the corresponding catalyst.U. S. Army Research Office W911NF-09-1-0446Chemistr

Distribution of Culex coronator in the USA

In 1920, Culex coronator was reported from San Benito, Texas, and later in Arizona, New Mexico, and Oklahoma. In 2005, this species was reported to be spreading across the southeastern USA. Now reported in 14 states, it has been found as far north as northern Oklahoma; Memphis, TN; and Suffolk, VA. The public health significance of Cx. coronator is not firmly established, even though it has been implicated as a potential vector of several arboviral diseases. This study aims to document additional Cx. coronator county-level records, to provide information about its continued expansion across the southern USA, and to provide a short research update into its vector potential. Data acquired through multistate collaborations and author collections resulted in 146 new county records from Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, and Texas. No new county records were presented for Arizona, New Mexico, Tennessee, or Virginia, which had previously reported this species. With these new data, this species has been documented in 386 counties in 14 states of the continental USA.Peer reviewedEntomology and Plant Patholog

1,1 '-Bis(N-benzimidazolylidene)ferrocene: synthesis and study of a novel ditopic ligand and its transition metal complexes

Diiridum complexes containing 1,1'-bis(N-benzimidazolylidene)ferrocene, a novel ditopic ligand comprised of two N-heterocyclic carbenes (NHCs) linked directly via their N-substituents to each cyclopentadienyl ring of a ferrocene moiety, were synthesized. Crystallographic analyses of these C-2-symmetric bimetallic complexes revealed the benzimidazolylidene moieties were intramolecularly stacked in nearly opposing orientations, effectively forming Janus-type bis(NHC) structures in the solid state. Using a variety of electrochemical techniques, the oxidation potentials of the ferrocenyl groups in these complexes were found to depend on the auxillary ligands coordinated to the Ir centers (i.e., 1,5-cyclooctadiene vs. carbonyl). Similarly, the nu(CO) of carbonyls ligated to the Ir centers varied in accord with the oxidation state of the ferrocene contained with the bis(NHC) ligand. These results suggest that the Ir and Fe centers in these complexes are electronically coupled and that the electron donating ability of the bis(NHC) ligand reported herein can be tuned electrochemically

Metal-centered oxidations facilitate the removal of ruthenium-based olefin metathesis catalysts

Commercially available catalysts (SIMes)(PCy3)Cl2Ru(=CHPh) (2) and (SIMes)Cl2Ru(=CH-o-O-i-PrC6H4) (3) (SIMes = 1,3-dimesitylimidazolin-2-ylidene) were found to display reversible Ru oxidations via a series of electrochemical measurements. The redox processes enabled the catalysts to be switched between two different states of activity in ring opening metathesis polymerizations and ring closing metathesis reactions, primarily through changes in catalyst solubility. Moreover, treating a solution of 2 dissolved in C6H6/CH2Cl2/[1-butyl-3-methylimidazolium][PF6] (6:1:1.1 v/v/v) with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone was found to remove >99.9% of the catalyst, as determined by UV/vis spectroscopy. The methodology described herein establishes a new approach for controlling the activities displayed by commercially available olefin metathesis catalysts and for removing residual Ru species using redox-driven processes

Pyridine- and pyrimidine-functionalized poly(sulfone)s: performance-enhancing crosslinkers for acid/base blend proton exchange membranes used in direct methanol fuel cells

The direct methanol fuel cell (DMFC) is a promising energy conversion device that holds the potential to replace lithium ion batteries used in portable electronic devices. At present, widespread commercialization is impeded in part by the absence of a suitable proton exchange membrane (PEM). To address this need, pyridine- and pyrimidine-functionalized poly(sulfone) s (PPS and PMPS, respectively) were prepared in two high yielding post-polymerization C-H borylation/Suzuki coupling steps from commercially available UDEL (R) poly(sulfone). Membranes comprised of the aforementioned polymers blended with sulfonated poly(ether ether ketone) (SPEEK) were found to exhibit improved single cell performance and decreased methanol crossover in comparison to plain SPEEK and showed higher power densities than Nafion (R) 112, an exchange membrane commonly used in DMFCs. Blend properties and single cell performances were found to be dependent on the basicity and steric parameters of the N-heterocycle incorporated into the polymeric material used in the aforementioned membranes

Molecular Recognition of Methionine-Terminated Peptides by Cucurbit[8]uril

This Article describes the molecular recognition of peptides containing an N-terminal methionine (Met) by the synthetic receptor cucurbit[8]uril (Q8) in aqueous solution and with submicromolar affinity. Prior work established that Q8 binds with high affinity to peptides containing aromatic amino acids, either by simultaneous binding of two aromatic residues, one from each of two different peptides, or by simultaneous binding of an aromatic residue and its immediate neighbor on the same peptide. The additional binding interface of two neighboring residues suggested the possibility of targeting nonaromatic peptides, which have thus far bound only weakly to synthetic receptors. A peptide library designed to test this hypothesis was synthesized and screened qualitatively for Q8 binding using a fluorescent indicator displacement assay. The large fluorescence response observed for several Met-terminated peptides suggested strong binding, which was confirmed quantitatively by the determination of submicromolar equilibrium dissociation constant values for Q8 binding to MLA, MYA, and MFA using isothermal titration calorimetry (ITC). This discovery of high affinity binding to Met-terminated peptides and, more generally, to nonaromatic peptides prompted a detailed investigation of the determinants of binding in this system using ITC, electrospray ionization mass spectrometry, and H-1 NMR spectroscopy for 25 purified peptides. The studies establish the sequence determinants required for high-affinity binding of Met-terminated peptides and demonstrate that cucurbit[n]uril-mediated peptide recognition does not require an aromatic residue for high affinity. These results, combined with the known ability of cucurbit[n]urils to target N-termini and disordered loops in folded proteins, suggest that Q8 could be used to target unmodified, Met-terminated proteins

Chemical and Electrochemical Lithiation of LiVOPO₄ Cathodes for Lithium-Ion Batteries

The theoretical capacity of LiVOPO₄ could be increased from 159 to 318 mAh/g with the insertion of a second Li⁺ ion into the lattice to form Li₂VOPO₄, significantly enhancing the energy density of lithium-ion batteries. The phase changes accompanying the second Li⁺ insertion into α-LiVOPO₄ and β-LiVOPO₄ are presented here at various degrees of lithiation, employing both electrochemical and chemical lithiation. Inductively coupled plasma, X-ray absorption spectroscopy, and Fourier transform infrared spectroscopy measurements indicate that a composition of Li₂VOPO₄ can be realized with an oxidation state of V³⁺ by the chemical lithiation process. The accompanying structural changes are evidenced by X-ray and neutron powder diffraction. Spectroscopic and diffraction data collected with the chemically lithiated samples as well as diffraction data on the electrochemically lithiated samples reveal that a significant amount of lithium can be inserted into α-LiVOPO₄ before a phase change occurs. In contrast, lithiation of β-LiVOPO₄ is more consistent with the formation of a two-phase mixture throughout most of the lithiation range. The phases observed with the ambient-temperature lithiation processes presented here are significantly different from those reported in the literature

Synthesis and study of olefin metathesis catalysts supported by redox-switchable diaminocarbene[3]ferrocenophanes

A redox-switchable ligand, N,N'-dimethyldiaminocarbene[3]ferrocenophane (5), was synthesized and incorporated into a series of Ir- and Ru-based complexes. Electrochemical and spectroscopic analyses of (5) Ir(CO)(2)Cl (15) revealed that 5 displayed a Tolman electronic parameter value of 2050 cm(-1) in the neutral state and 2061 cm(-1) upon oxidation. Moreover, inspection of X-ray crystallography data recorded for (5) Ir(cis,cis-1,5-cyclooctadiene)Cl (13) revealed that 5 was sterically less bulky (%V-Bur = 28.4) than other known diaminocarbene[3]ferrocenophanes, which facilitated the synthesis of (5)(PPh3)Cl2Ru-(3-phenylindenylid-1-ene) (18). Complex 18 exhibited quasi-reversible electrochemical processes at 0.79 and 0.98 V relative to SCE, which were assigned to the Fe and Ru centers in the complex, respectively, based on UV-vis and electron pair resonance spectroscopic measurements. Adding 2,3-dichloro-5,6-dicyanoquinone over the course of a ring-opening metathesis polymerization of cis, cis-1,5-cyclooctadiene catalyzed by 18 ([monomer](0)/[18](0) = 2500) reduced the corresponding rate constant of the reaction by over an order of magnitude (pre-oxidation: k(obs) = 0.045 s(-1); post-oxidation: k(obs) = 0.0012 s(-1)). Subsequent reduction of the oxidized species using decamethylferrocene restored catalytic activity (post-reduction: k(obs) = up to 0.016 s(-1), depending on when the reductant was added). The difference in the polymerization rates was attributed to the relative donating ability of the redox-active ligand (i.e., strongly donating 5 versus weakly donating 5(+)) which ultimately governed the activity displayed by the corresponding catalyst

Chemical and Electrochemical Lithiation of LiVOPO4 Cathodes for Lithium-Ion Batteries

The theoretical capacity of LiVOPO4 could be increased from 159 to 318 mAh/g with the insertion of a second Li+ ion into the lattice to form Li2VOPO4, significantly enhancing the energy density of lithium-ion batteries. The phase changes accompanying the second Li+ insertion into alpha-LiVOPO4 and beta-LiVOPO4 are presented here at various degrees of lithiation, employing both electrochemical and chemical lithiation. Inductively coupled plasma, X-ray absorption spectroscopy, and Fourier transform infrared spectroscopy measurements indicate that a composition of Li2VOPO4 can be realized with an oxidation state of V3+ by the chemical lithiation process. The accompanying structural changes are evidenced by X-ray and neutron powder diffraction. Spectroscopic and diffraction data collected with the chemically lithiated samples as well as diffraction data on the electrochemically lithiated samples reveal that a significant amount of lithium can be inserted into alpha-LiVOPO4 before a phase change occurs. In contrast, lithiation of beta-LiVOPO4 is more consistent with the formation of a two-phase mixture throughout most of the lithiation range. The phases observed with the ambient-temperature lithiation processes presented here are significantly different from those reported in the literature

Blind prediction of host-guest binding affinities: a new SAMPL3 challenge

The computational prediction of protein-ligand binding affinities is of central interest in early-stage drug-discovery, and there is a widely recognized need for improved methods. Low molecular weight receptors and their ligands-i.e., host-guest systems-represent valuable test-beds for such affinity prediction methods, because their small size makes for fast calculations and relatively facile numerical convergence. The SAMPL3 community exercise included the first ever blind prediction challenge for host-guest binding affinities, through the incorporation of 11 new host-guest complexes. Ten participating research groups addressed this challenge with a variety of approaches. Statistical assessment indicates that, although most methods performed well at predicting some general trends in binding affinity, overall accuracy was not high, as all the methods suffered from either poor correlation or high RMS errors or both. There was no clear advantage in using explicit versus implicit solvent models, any particular force field, or any particular approach to conformational sampling. In a few cases, predictions using very similar energy models but different sampling and/or free-energy methods resulted in significantly different results. The protonation states of one host and some guest molecules emerged as key uncertainties beyond the choice of computational approach. The present results have implications for methods development and future blind prediction exercises