Evolutionary Design of Low Molecular Weight Organic Anolyte Materials for Applications in Nonaqueous Redox Flow Batteries.

The integration of renewable energy sources into the electric grid requires low-cost energy storage systems that mediate the variable and intermittent flux of energy associated with most renewables. Nonaqueous redox-flow batteries have emerged as a promising technology for grid-scale energy storage applications. Because the cost of the system scales with mass, the electroactive materials must have a low equivalent weight (ideally 150 g/(mol·e(-)) or less), and must function with low molecular weight supporting electrolytes such as LiBF4. However, soluble anolyte materials that undergo reversible redox processes in the presence of Li-ion supports are rare. We report the evolutionary design of a series of pyridine-based anolyte materials that exhibit up to two reversible redox couples at low potentials in the presence of Li-ion supporting electrolytes. A combination of cyclic voltammetry of anolyte candidates and independent synthesis of their corresponding charged-states was performed to rapidly screen for the most promising candidates. Results of this workflow provided evidence for possible decomposition pathways of first-generation materials and guided synthetic modifications to improve the stability of anolyte materials under the targeted conditions. This iterative process led to the identification of a promising anolyte material, N-methyl 4-acetylpyridinium tetrafluoroborate. This compound is soluble in nonaqueous solvents, is prepared in a single synthetic step, has a low equivalent weight of 111 g/(mol·e(-)), and undergoes two reversible 1e(-) reductions in the presence of LiBF4 to form reduced products that are stable over days in solution.

Tandem gold/silver-catalyzed cycloaddition/hydroarylation of 7-aryl-1,6-enynes to form 6,6-diarylbicyclo[3.2.0]heptanes.

Mixtures of [{PCy2(o-biphenyl)}AuCl] and AgSbF6 catalyze the tandem cycloaddition/hydroarylation of 7-aryl-1,6-enynes with electron-rich arenes to form 6,6-diarylbicyclo[3.2.0]heptanes in good yield under mild conditions. Experimental observations point to a mechanism involving gold-catalyzed cycloaddition followed by silver-catalyzed hydroarylation of a bicyclo[3.2.0]hept-1(7)-ene intermediate.

Evaluation of Tris-Bipyridine Chromium Complexes for Flow Battery Applications: Impact of Bipyridine Ligand Structure on Solubility and Electrochemistry.

This report describes the design, synthesis, solubility, and electrochemistry of a series of tris-bipyridine chromium complexes that exhibit up to six reversible redox couples as well as solubilities approaching 1 M in acetonitrile. We have systematically modified both the ligand structure and the oxidation state of these complexes to gain insights into the factors that impact solubility and electrochemistry. The results provide a set of structure-solubility-electrochemistry relationships to guide the future development of electrolytes for nonaqueous flow batteries. In addition, we have identified a promising candidate from the series of chromium complexes for further electrochemical and battery assessment.

Synthesis and study of cationic, two-coordinate triphenylphosphine-gold-π complexes.

Cationic, two-coordinate triphenylphosphine-gold(I)-π complexes of the form [(PPh3)Au(π ligand)]⁺SbF6⁻ (π ligand=4-methylstyrene, 1∙SbF6), 2-methyl-2-butene (3∙SbF6), 3-hexyne (6∙SbF6), 1,3-cyclohexadiene (7∙SbF6), 3-methyl-1,2-butadiene (8∙SbF6), and 1,7-diphenyl-3,4-heptadiene (10∙SbF6) were generated in situ from reaction of [(PPh3)AuCl], AgSbF6, and π ligand at -78 °C and were characterized by low-temperature, multinuclear NMR spectroscopy without isolation. The π ligands of these complexes were both weakly bound and kinetically labile and underwent facile intermolecular exchange with free ligand (ΔG(≠) ≈9 kcal mol(-1) in the case of 6∙SbF6) and competitive displacement by weak σ donors, such as trifluoromethane sulfonate. Triphenylphosphine-gold(I)-π complexes were thermally unstable and decomposed above -20 °C to form the bis(triphenylphosphine) gold cation [(PPh3)2Au]⁺SbF6⁻ (2∙SbF6).

Cationic, two-coordinate gold π complexes.

Cationic, two-coordinate gold π complexes that contain a phosphine or N-heterocyclic supporting ligand have attracted considerable attention recently owing to the potential relevance of these species as intermediates in the gold-catalyzed functionalization of C-C multiple bonds. Although neutral two-coordinate gold π complexes have been known for over 40 years, examples of the cationic two-coordinate gold(I) π complexes germane to catalysis remained undocumented prior to 2006. This situation has changed dramatically in recent years and well-defined examples of two-coordinate, cationic gold π complexes containing alkene, alkyne, diene, allene, and enol ether ligands have been documented. This Minireview highlights this recent work with a focus on the structure, bonding, and ligand exchange behavior of these complexes.

Experimental evaluation of the electron donor ability of a gold phosphine fragment in a gold carbene complex.

X-ray analysis of the gold cyclopropyl(methoxy)carbene complex [(P)AuC(OMe)(c-Pr)](+) SbF6(-) [P = P(t-Bu)2o-biphenyl] and comparison to extant protonated cyclopropyl ketones indicates that electron donation from the (P)Au fragment to the electron-deficient C1 atom is similar to that provided by a cyclopropyl group.