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

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).

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.

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.

The cycloisomerization of enynes catalyzed by electrophilic noble-metal complexes, [1] in particular Au and Pt, [2] has attracted considerable attention owing to the formation of skeletal rearrangement products, including vinylcyclopentenes (B and C), bicyclo[4.1.0]heptenes (D), and bicyclo-[3.2.0]hept-6-enes (E) from 1,6-enynes (A; Scheme 1). [3,4] Fürstner first posited that these outcomes were consistent with the intermediacy of metal-stabilized nonclassical cyclopropylmethyl-, cyclobutyl-, and homoallylic carbocations/ carbenes accessed through attack of the C=C moiety on a metal-complexed CC bond (Scheme 1), [5] and mechanistic thought in this area has evolved largely within this conceptual framework. The involvement of nonclassical carbocations/ carbenes is supported by a wealth of indirect experimental evidence, including trapping experiments, isotopic labeling studies, and stereochemical analyses, and through numerous computational studies. [1, 2] Absent, however, is direct experimental evidence regarding the structure and reactivity of these cationic complexes, as no organometallic intermediate has been observed spectroscopically in any of these transformations. [6][7][8][9][10] Platinum(II), [11] gold(I), [12,13] and rhodium(II) [14] complexes catalyze the cycloisomerization of 7-aryl-1,6-enynes (A, R = Ar) to form vinylcyclopentenes C and/or bicyclo-[3.2.0]hept-6-enes E (Scheme 1). Directly implicated in these and related [15,16] transformations is the strained bicyclo-[3.2.0]hept-1(7)-ene species I, presumably generated through 6-endo-cyclization followed by 1,2-alkyl migration from cyclopropyl carbene II and consumed either by ring opening to form B and/or 1,3-hydrogen migration to form E (Scheme 1). [11][12][13][14] Possible contributors to I include the metalated cyclobutyl carbenium ion I a, the p-alkene complex I b, and the metallacyclopropane complex I c. In this context, it appeared likely that contribution of the latter forms would engender stability to I not realized in the corresponding cyclopropyl carbene intermediates II or III, such that I might represent a local minimum on the reaction coordinate. Indeed, here we report the selective generation, spectroscopic characterization, and reactivity analysis of a gold-bicyclo-[3.2.0]hept-1(7)-ene complex formed in the gold-catalyzed cycloisomerization of a 7-phenyl-1,6-enyne.Toward detection of a reactive bicyclo[3.2.0]hept-1(7)-ene complex, we investigated the gold(I)-catalyzed conversion of the 7-phenyl-1,6-enyne 1 into the 6-phenylbicyclo[3.2.0]hept-6-ene 2 reported by Echavarren (Scheme 2). [12] In an initial experiment, a 1:1:1 mixture of 1, [LAuCl] (L = P(tBu) 2 (obiphenyl)), and AgSbF 6 in CD 2 Cl 2 was mixed thoroughly at À78 8C. 31 P and 1 H NMR analysis at À80 8C showed formation of an approximately 9:1 mixture of p-alkene and p-alkyne Scheme 1. Ligand-and substrate-dependent pathways for enyne cycloaddition catalyzed by electrophilic noble-metal complexes.Scheme 2. Gold-mediated conversion of 1 into 4.

Through employment of deuterium-labelled substrates, the triflic acid-catalyzed intramolecular exo-addition of the X–H(D) (X = N, O) bond of a sulfonamide, alcohol, or carboxylic acid across the C=C bond of a pendant cyclohexene moiety was found to occur, in each case, with exclusive formation (≥90%) of the anti-addition product without loss or scrambling of deuterium as determined by 1H and 2H NMR spectroscopy and MS analysis. Kinetic analysis of triflic acid-catalyzed intramolecular hydroamination of N-(2-cyclohex-2'-enyl-2,2-diphenylethyl)-p-toluenesulfonamide (1a) established the second-order rate law: rate = k2[HOTf][1a] and the activation parameters: ΔH‡ = 9.7 ± 0.5 kcal mol−1 and ΔS‡ = −35 ± 5 cal K−1 mol−1. An inverse α-secondary kinetic isotope effect of kD/kH = 1.15 ± 0.03 was observed upon deuteration of the C2' position of 1a, consistent with partial C–N bond formation in the highest energy transition state of catalytic hydroamination. The results of these studies were consistent with a mechanism for the intramolecular hydroamination of 1a involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C3' position of 1a coupled with intramolecular anti-addition of the pendant sulfonamide nitrogen atom to the alkenyl C2' position.