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.

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.

Redox flow batteries (RFBs) hold promise for use in large-scale energy storage applications, but new electrolyte chemistries are needed to significantly enhance their energy densities and lower their cost. The energy density is governed by the cell voltage, active species concentration and number of electrons transferred at each electrode. Non-aqueous solvents offer wider voltage windows than water; however, most if not all of the previously reported active species have low solubilities and/or are limited to single electron transfer at each electrode. This paper describes the design, synthesis, and characterization of metal coordination complexes containing non-innocent ligands that demonstrate enhanced solubilities at different oxidation states along with multiple electron transfers. In particular, a series of ester functionalized chromium bipyridine complexes are demonstrated that afford six reversible redox couples over ~2 V and solubilities approaching 1 M. These characteristics allow the same complex to be used at the negative and positive electrodes. Using an electrolyte consisting of the tris(4,4'(bis(2-(2-methoxyethoxy)ethyl)ester-2,2'-bipyridine)chromium complex ([Cr(L3) 3 ]) in acetonitrile, we demonstrate two reversible electron transfers at each electrode in an unoptimized, symmetric Hcell with efficiencies of ~70%. With further enhancements in the electrolyte chemistry and cell design, this approach could lead to the demonstration of highly energy dense RFB chemistries for grid-scale storage applications. INTRODUCTION With increasing efforts to incorporate renewable energy sources such as