Recent efforts have led to the design of new anolytes for nonaqueous flow batteries that exhibit reversible redox couples at low potentials. However, these molecules generally cycle through just a single electron-transfer event, which limits the overall energy density of resulting batteries on account of the undesirably high equivalent weight (i.e., ratio of anolyte/supporting electrolyte molecular weight to electrons transferred). In addition, these anolytes generally require expensive alkylammonium salts as supporting electrolytes for stable cycling, which further increases the equivalent weight of the system. The current work describes the multielectron redox cycling of a low-potential anolyte using alkali metal salts as supporting electrolytes. These studies reveal that potassium hexafluorophosphate (KPF6) dramatically lowers the equivalent weight of the anolyte system while supporting flow cell cycling through two redox events at low potentials for 150 cycles with no detectable degradation.

Aqueous redox flow batteries (RFBs) can serve as inexpensive grid-scale energy storage devices. A key challenge for developing these systems is identifying storage materials that undergo reversible redox events at potentials near the voltaic limits of aqueous media. This work details the development of a benzoylpyridinium-based anolyte for this application. A combination of electrochemical and spectroscopic studies guided the selection of a supporting electrolyte to mitigate anolyte-catalyzed proton reduction at the low potentials. These insights were used to achieve stable one-electron cycling with KOH as the support in both a H cell and in a laboratory-scale flow cell. In the latter experiment, cycling versus iron ferrocyanide afforded an aqueous RFB with an open-circuit voltage exceeding 1 V.

This article describes a detailed comparison of the organometallic chemistry of high-valent nickel and palladium model complexes supported by tris(pyrazolyl)borate and cycloneophyl ligands. The accessibility of the MIII and MIV oxidation states with each metal is investigated through electrochemical and chemical oxidation of the MII precursors. These studies show that the NiII precursor readily undergoes both one- and two-electron oxidations to generate stable NiIII and NiIV products. In contrast, under the conditions examined, the PdII analogue undergoes exclusively two-electron-oxidation reactions to form PdIV. Reactivity studies of isolated NiIV and PdIV complexes show that both participate in C(sp3)–heteroatom coupling reactions and that the reactions at NiIV are approximately 2 orders of magnitude faster than those at PdIV. Experimental and computational mechanistic studies implicate outer-sphere SN2-type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C(sp3)–heteroatom coupling reaction yields a TpMII(σ-aryl) product. However, with azide as the nucleophile, the NiII product of initial C(sp3)–N3 coupling undergoes a subsequent C(sp2)–N insertion reaction. Computations implicate an anionic NiIII–nitrene intermediate in this process and show that the Pd analogue of this species is a much higher energy species. Overall, the combined experimental and computational studies demonstrate remarkable similarities in the chemistry of NiIV and PdIV but an enhanced role for NiIII in enabling reactivity which is distinct from that of palladium.

This communication describes a series of oxidatively induced intramolecular arene C–H activation reactions of NiII model complexes to yield NiIV σ-aryl products. These reactions proceed within 10 min at room temperature, which represents among the mildest conditions reported for C–H cleavage at a Ni center. A combination of density functional theory and preliminary experimental mechanistic studies implicate a pathway involving initial 2e– oxidation of the NiII starting materials by the F+ transfer reagent N-fluoro-2,4,6-trimethylpyridinium triflate followed by triflate-assisted C–H cleavage at NiIV to yield the products.

This article focuses on the development of practical approaches to the in situ generation of anhydrous fluoride salts for applications in nucleophilic aromatic substitution (SNAr) reactions. We report herein that a variety of combinations of inexpensive nucleophiles (e.g., tetraalkylammonium cyanide and phenoxide salts) and fluorine-containing electrophiles (e.g., acid fluoride, fluoroformate, benzenesulfonyl fluoride, and aryl fluorosulfonate derivatives) are effective for this transformation. Ultimately, we demonstrate that the combination of tetramethylammonium 2,6-dimethylphenoxide and sulfuryl fluoride (SO2F2) serves as a particularly practical route to anhydrous tetramethylammonium fluoride. This procedure is applied to the SNAr fluorination of a range of electron-deficient aryl and heteroaryl chlorides as well as nitroarenes.

This communication describes a method for the nucleophilic radiofluorination of electron-rich arenes. The reaction involves the initial C(sp2)–H functionalization of an electron-rich arene with MesI(OH)OTs to form a (mesityl)(aryl)iodonium salt. This salt is then used in situ in a Cu-mediated radiofluorination with [18F]KF. This approach leverages the stability and availability of electron-rich arene starting materials to enable mild late-stage radiofluorination of toluene, anisole, aniline, pyrrole, and thiophene derivatives. The radiofluorination has been automated to access a 41 mCi dose of an 18F-labeled nimesulide derivative in high (2800 ± 700 Ci/mmol) specific activity.

This report describes a method for Pd-catalyzed decarbonylative cross-coupling that enables the conversion of carboxylic acid derivatives to biaryls, aryl amines, aryl ethers, aryl sulfides, aryl boronate esters, and trifluoromethylated arenes. The success of this transformation leverages the Pd0/Brettphos-catalyzed decarbonylative chlorination of aroyl chlorides, which can then participate in diverse cross-coupling reactions in situ using the same Pd catalyst.

This Communication describes studies of Ph–RF (RF = CF3 or CF2CF3) coupling at Pd complexes of general structure (PtBu3)PdII(Ph)(RF). The CF3 analogue participates in fast Ph-CF3 coupling (<5 min at 80 °C). However, the formation of side products limits the yield of this transformation as well as its translation to catalysis. DFT and experimental studies suggest that the side products derive from facile α-fluoride elimination at the 3-coordinate PdII complex. Furthermore, they show that this undesired pathway can be circumvented by changing from a CF3 to a CF2CF3 ligand. Ultimately, the insights gained from stoichiometric studies enabled the identification of Pd(PtBu3)2 as a catalyst for the Pd-catalyzed cross-coupling of aryl bromides with TMSCF2CF3 to afford pentafluoroethylated arenes.

Difluoromethyl copper complexes have been proposed as key intermediates in a variety of Cu-catalyzed difluoromethylation reactions. However, studies of these putative intermediates have been impeded by the low stability of these [Cu(CHF2)] species. This report describes the synthesis of isolable N-heterocyclic carbene ligated copper(I) difluoromethyl complexes. The stoichiometric reactions of these complexes with aryl electrophiles (i.e., diaryliodonium salts, aryl iodides, and aryl bromides) are described. In addition, N-heterocyclic carbene copper(I) species are demonstrated to serve as catalysts for the cross-coupling of aryl iodides with (difluoromethyl)trimethylsilane to afford difluoromethyl arene products.

This report describes a method for the deoxyfluorination of phenols with sulfuryl fluoride (SO2F2) and tetramethylammonium fluoride (NMe4F) via aryl fluorosulfonate (ArOFs) intermediates. We first demonstrate that the reaction of ArOFs with NMe4F proceeds under mild conditions (often at room temperature) to afford a broad range of electronically diverse and functional group-rich aryl fluoride products. This transformation was then translated to a one-pot conversion of phenols to aryl fluorides using the combination of SO2F2 and NMe4F. Ab initio calculations suggest that carbon–fluorine bond formation proceeds via a concerted transition state rather than a discrete Meisenheimer intermediate.

This letter describes the development of a method for selective remote C(sp3)–H oxygenation of protonated aliphatic amines using aqueous potassium persulfate. Protonation serves to deactivate the proximal C(sp3)–H bonds of the amine substrates and also renders the amines soluble in the aqueous medium. These reactions proceed under relatively mild conditions (within 2 h at 80 °C with amine as limiting reagent) and do not require a transition metal catalyst. This method is applicable to a variety of types of C(sp3)–H bonds, including 3°, 2°, and benzylic C–H sites in primary, secondary, and tertiary amine substrates.

This communication describes the synthesis of an organometallic NiIV complex bearing a labile trifluoroacetate (OTFA) ligand via the oxidation of a NiII precursor with PhI(OTFA)2. Intramolecular C(sp2)–O bond-forming reductive elimination from this NiIV complex is relatively slow, requiring 6 h at 70 °C to reach completion. In contrast, transmetalation with TMSCF3 occurs within just 1 h at room temperature to generate a NiIV–CF3 complex. These studies show that intermolecular reactions such as transmetalation can be competitive with intramolecular reductive elimination processes at NiIV centers.

Silver carboxylate salts are widely used as additives in palladium-catalyzed C–H functionalization reactions. However, the role of these silver additives is often not fully understood. This paper describes an investigation of the role of AgOPiv in the stoichiometric activation of C6F5H at a well-defined PdII complex as well as in the PdII-catalyzed oxidative dimerization of 2-alkylthiophenes. Both in situ NMR spectroscopy and H/D exchange studies of the reactions of C6F5H implicate a role for AgOPiv in the C–H cleavage event, generating Ag–C6F5 as an intermediate. The catalytic studies show similar trends despite the different conditions and substrates, suggesting that AgOPiv promotes a similar metalation of the thiophene in the catalytic transformations. This proposal is supported by DFT calculations, which show energetically feasible pathways for concerted metalation–deprotonation of both 2-methylthiophene and pentafluorobenzene at [Ag(OPiv)]2. These studies suggest that initial metalation of C–H substrates at AgI carboxylates should be considered as a plausible pathway in C–H functionalization reactions involving mixtures of Ag and Pd salts.

This report describes a computational study of C(sp3)–OR bond formation from PdIV complexes of general structure PdIV(CH2CMe2-o-C6H4-C,C′)(F)(OR)(bpy-N,N′) (bpy = 2,2′-bipyridine). Dissociation of −OR from the different octahedral PdIV starting materials results in a common square-pyramidal PdIV cation. An SN2-type attack by −OR (−OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp3)–OR bond formation. In contrast, when −OR = triflate, concerted C(sp3)–C(sp2) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and SN2 steps with different −OR follow opposing trends. The SN2 transition states exhibit “Pd⋯C⋯O” angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally.

This report describes a combined experimental and computational investigation of the mechanism of C(sp3)–N bond-forming reductive elimination from sulfonamide-ligated PdIV complexes. After an initial experimental assessment of reactivity, we used ZStruct, a computational combinatorial reaction finding method, to analyze a large number of multistep mechanisms for this process. This study reveals two facile isomerization pathways connecting the experimentally observed PdIV isomers, along with two competing SN2 pathways for C(sp3)–N coupling. One of these pathways involves an unanticipated oxygen–nitrogen exchange of the sulfonamide ligand prior to an inner-sphere SN2-type reductive elimination. The calculated ΔG⧧ values for isomerization and reductive elimination with a series of sulfonamide derivatives are in good agreement with experimental data. Furthermore, the simulations predict relative reaction rates with different sulfonamides, which is successful only after considering competition between the proposed operating mechanisms. Overall, this work shows that the combination of experimental studies and new computational tools can provide fundamental mechanistic insights into complex organometallic reaction pathways.

The development of nonaqueous redox flow batteries (NRFBs) has been impeded by a lack of electroactive compounds (anolytes and catholytes) with the necessary combination of (1) redox potentials that exceed the potential limits of water, (2) high solubility in nonaqueous media, and (3) high stability toward electrochemical cycling. In addition, ideal materials would maintain all three of these properties over multiple electron transfer events, thereby providing a proportional increase in storage capacity. This paper describes the mechanism-based design of a new class of metal-coordination complexes (MCCs) as anolytes for NRFBs. The tridentate bipyridylimino isoindoline (BPI) ligands of these complexes were designed to enable multielectron redox events. These molecules were optimized using a combination of systematic variation of the BPI ligand and the metal center along with mechanistic investigations of the decomposition pathways that occur during electrochemical cycling. Ultimately, these studies led to the identification of nickel BPI complexes that could undergo stable charge-discharge cycling (<5% capacity loss over 200 cycles) as well as a derivative that possesses the previously unprecedented combination of high solubility (>700 mM in CH3CN), multiple electron transfers at low redox potentials (–1.7 and –1.9 V versus Ag/Ag+), and high stability in the charged state for days at high concentration. Overall, the studies described herein have enabled the identification of a promising anolyte candidate for NRFBs and have also provided key insights into chemical design principles for future classes of MCC-based anolytes.

This manuscript describes the design, synthesis, characterization, and reactivity studies of organometallic NiIII complexes of general structure TpNiIII(R)(R1) (Tp = tris(pyrazolyl)borate). With appropriate selection of the R and R1 ligands, the complexes are stable at room temperature and can be characterized by cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. Upon heating, many of these NiIII compounds undergo C(sp2)–C(sp2) or C(sp3)–C(sp2) bond-forming reactions that are challenging at lower oxidation states of nickel.

This article describes the iron-catalyzed hydrogenation of unactivated amides. Under the optimal conditions, a PNP-ligated Fe catalyst affords 25–300 turnovers of products derived from C–N bond cleavage. This reaction displays a broad substrate scope, including a variety of 2° and 3° amide substrates. The reaction progress of N,N-dimethylformamide hydrogenation has been monitored in situ using Raman spectroscopy. This technique enables direct comparison of the relative activity of the Fe-PNP catalyst to that of its Ru analogue. Under otherwise identical conditions, the Fe and Ru catalysts exhibit rates within a factor of 2.Keywords: amide hydrogenation; bifunctional catalysis; homogeneous catalysis; iron catalysis; kinetics; pincer complexes

The cationic rhodium complexes (dppe)Rh(COD)BF4 and (MeCN)2Rh(COD)BF4 have been supported in metal–organic frameworks bearing anionic nodes (ZJU-28) and anionic linkers (MIL-101-SO3) via ion exchange. These MOF-supported Rh species serve as recyclable catalysts for the hydrogenation of both the terminal alkene substrate 1-octene and the internal alkene substrate 2,3-dimethylbutene. The nature of the MOF support impacts various aspects of catalysis, including: (i) the rate of 1-octene hydrogenation, (ii) the activity and recyclability of the catalyst in 2,3-dimethylbutene hydrogenation, and (iii) the size selectivity of hydrogenation with alkene substrates appended to calixarenes.Keywords: catalysis; hydrogenation; metal organic frameworks; rhodium; size exclusion

Half-sandwich iridium bipyridine complexes catalyze the hydrogenation of esters and lactones under base-free conditions. The reactions proceed with a variety of ester and lactone substrates. Mechanistic studies implicate a pathway involving rate-limiting hydride transfer to the substrate at high pressures of H2 (≥50 bar).Keywords: acid; ester; homogeneous catalysis; hydrogenation; iridium

We report the development of an iron-catalyzed method for the selective oxyfunctionalization of benzylic C(sp3)–H bonds in aliphatic amine substrates. This transformation is selective for benzylic C–H bonds that are remote (i.e., at least three carbons) from the amine functional group. High site selectivity is achieved by in situ protonation of the amine with trifluoroacetic acid, which deactivates more traditionally reactive C–H sites that are α to nitrogen. The scope and synthetic utility of this method are demonstrated via the synthesis and derivatization of a variety of amine-containing, biologically active molecules.

A copper-mediated nucleophilic radiofluorination of aryl- and vinylstannanes with [18F]KF is described. This method is fast, uses commercially available reagents, and is compatible with both electron-rich and electron-deficient arene substrates. This method has been applied to the manual synthesis of a variety of clinically relevant radiotracers including protected [18F]F-phenylalanine and [18F]F-DOPA. In addition, an automated synthesis of [18F]MPPF is demonstrated that delivers a clinically validated dose of 200 ± 20 mCi with a high specific activity of 2400 ± 900 Ci/mmol.

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.

This communication describes the synthesis and reactivity of NiIV(aryl)(CF3)2 complexes supported by trispyrazolylborate and 4,4′-di-tert-butylbipyridine ligands. We demonstrate that isolable NiIV complexes can be accessed under mild conditions via the oxidation of NiII precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The NiIV intermediates undergo high yielding aryl–CF3 bond-forming reductive elimination. These studies support the potential viability of NiIV intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.

This Communication describes the terminal-selective, Pt-catalyzed C(sp3)–H oxidation of aliphatic amines without the requirement for directing groups. CuCl2 is employed as a stoichiometric oxidant, and the reactions proceed in high yield at Pt loadings as low as 1 mol%. These transformations are conducted in the presence of sulfuric acid, which reacts with the amine substrates in situ to form ammonium salts. We propose that protonation of the amine serves at least three important roles: (i) it renders the substrates soluble in the aqueous reaction medium; (ii) it limits binding of the amine nitrogen to Pt or Cu; and (iii) it electronically deactivates the C–H bonds proximal to the nitrogen center. We demonstrate that this strategy is effective for the terminal-selective C(sp3)–H oxidation of a variety of primary, secondary, and tertiary amines.

This article describes detailed mechanistic studies focused on elucidating the impact of pyridine ligands on the Pd-catalyzed C–H acetoxylation of benzene. Three different catalysts, Pd(OAc)2, Pd(OAc)2/pyridine (1:1), and Pd(OAc)2/pyridine (1:2), are compared using a combination of mechanistic tools, including rate and order studies, Hammett analysis, detailed characterization of catalyst resting states, and isotope effects. The data from these experiments implicate C–H activation as the rate-limiting step in all cases. The major difference between the three catalysts is proposed to be the resting state of Pd. Under the reaction conditions, Pd(OAc)2 rests as an acetate bridged dimer, while the Pd(OAc)2/pyridine (1:2) catalyst rests as the monomer (pyridine)2Pd(OAc)2. In contrast, a variety of experiments suggest that the highly active catalyst generated from the 1:1 combination of Pd(OAc)2 and pyridine rests as the dimeric structure [(pyridine)Pd(OAc)2]2.

This Communication describes the hydrogenation of carbon dioxide to methanol via tandem catalysis with dimethylamine and a homogeneous ruthenium complex. Unlike previous examples with homogeneous catalysts, this CO2-to-CH3OH process proceeds under basic reaction conditions. The dimethylamine is proposed to play a dual role in this system. It reacts directly with CO2 to produce dimethylammonium dimethylcarbamate, and it also intercepts the intermediate formic acid to generate dimethylformamide. With the appropriate selection of catalyst and reaction conditions, >95% conversion of CO2 was achieved to form a mixture of CH3OH and dimethylformamide.

The direct functionalization of carbon–hydrogen (C–H) bonds has emerged as a versatile strategy for the synthesis and derivatization of organic molecules. Among the methods for C–H bond activation, catalytic processes that utilize a PdII/PdIV redox cycle are increasingly common. The C–H activation step in most of these catalytic cycles is thought to occur at a PdII centre. However, a number of recent reports have suggested the feasibility of C–H cleavage occurring at PdIV complexes. Importantly, these latter processes often result in complementary reactivity and selectivity relative to analogous transformations at PdII. This mini review highlights proposed examples of C–H activation at PdIV centres. Applications of this transformation in catalysis as well as mechanistic details obtained from stoichiometric model studies are discussed. Furthermore, challenges and future perspectives for the field are reviewed.

This paper describes the design, synthesis, and fundamental characterization of a series of Cr and V acetylacetonate (acac) complexes for use in redox flow batteries (RFBs). These materials offer a significant improvement in theoretical energy density relative to state-of-the-art aqueous chemistries. A detailed assessment of the solubility, cyclic voltammetry, and charge–discharge behavior of the complexes is presented. Their solubilities in acetonitrile vary by more than four orders of magnitude based on the structure/substituents on the acac ligand. Complexes bearing acac ligands with ester substituents have solubilities of up to 1.8 M, a significant improvement over most other metal complexes that have been considered for non-aqueous RFB applications. While the acac ligand substituents have a dramatic impact on solubility, they do not, in most cases, impact the electrochemical properties of the complexes. For instance, voltammetry for all of the V(acac)3 derivatives examined exhibit two quasi-reversible redox events separated by approximately 2.1 V. Charge–discharge testing in static H-cell and laboratory-scale flow batteries yielded energy densities that were consistent with the voltammetry and coulombic and energy efficiencies of up to 92% and 87%, respectively. Overall, these studies provide the basis for the development of structure–function relationships that could lead to new and even better performing energy storage chemistries in the future.

A copper-mediated radiofluorination of aryl- and vinylboronic acids with K18F is described. This method exhibits high functional group tolerance and is effective for the radiofluorination of a range of electron-deficient, -neutral, and -rich aryl-, heteroaryl-, and vinylboronic acids. This method has been applied to the synthesis of [18F]FPEB, a PET radiotracer for quantifying metabotropic glutamate 5 receptors.

The reaction of acid fluorides with N-heterocyclic carbenes (NHCs) produces anhydrous acyl azolium fluorides. With appropriate selection of acid fluoride and NHC, these salts can be used for the room temperature SNAr fluorination of a variety of aryl chlorides and nitroarenes.

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.

A computational study of the palladation of a pendant phenyl group in the PdIV complex PdCl2(CF3)(biH)(But2bpy) to form PdCl(CF3)(bi)(But2bpy) (bi = biphenylenediyl) indicates that this reaction occurs via an SEAr mechanism after dissociation of a chloride ion from the Pd center. Chloride dissociation occurs trans to the Pd–C(biH) bond. This dissociation is followed by rate-limiting isomerization of the square-pyramidal five-coordinate cation to place the remaining chloride ligand trans to the pendant phenyl group, with concomitant formation of a Pd-arenonium species. The free chloride ion then assists the departure of the Cipso proton from the arenonium complex as HCl. Key findings of this study include the occurrence of facile ligand dissociation and isomerization at this PdIVcenter as well as the participation of a classical SEAr mechanistic pathway in this transformation.

This paper describes the room-temperature SNAr fluorination of aryl halides and nitroarenes using anhydrous tetramethylammonium fluoride (NMe4F). This reagent effectively converts aryl-X (X = Cl, Br, I, NO2, OTf) to aryl-F under mild conditions (often room temperature). Substrates for this reaction include electron-deficient heteroaromatics (22 examples) and arenes (5 examples). The relative rates of the reactions vary with X as well as with the structure of the substrate. However, in general, substrates bearing X = NO2 or Br react fastest. In all cases examined, the yields of these reactions are comparable to or better than those obtained with CsF at elevated temperatures (i.e., more traditional halex fluorination conditions). The reactions also afford comparable yields on scales ranging from 100 mg to 10 g. A cost analysis is presented, which shows that fluorination with NMe4F is generally more cost-effective than fluorination with CsF.

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. Nonaqueous 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. In this paper we describe the design, synthesis, and characterization of metal coordination complexes containing noninnocent 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 H-cell 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.

This communication describes the design of a model system that allows direct investigation of competing sp3-C–N and sp3-C–F bond-forming reductive elimination from a PdIV fluoro sulfonamide complex. The reductive elimination selectivity varies dramatically as a function of reaction additives. A mechanism is proposed that provides a rationale for these effects.

This paper reports a room temperature visible light photocatalyzed method for the C–H amination of arenes and heteroarenes. A key enabling advance in this work is the design of N-acyloxyphthalimides as precursors to nitrogen-based radical intermediates for these transformations. A broad substrate scope is presented, including the selective meta-amination of pyridine derivatives. A radical aromatic substitution mechanism is proposed.

This article describes the high-yielding and selective oxidatively induced formation of ethane from mono-methyl palladium complexes. Mechanistic details of this reaction have been explored via both experiment and computation. On the basis of these studies, a mechanism involving methyl group transmetalation between PdII and PdIV interediates is proposed.

This article describes an investigation of C(sp3)–O bond-forming reductive elimination reactions from PdIV complexes. Phenoxide, acetate, difluoroacetate, dimethylphosphate, tosylate, and nitrate nucleophiles are shown to participate in this reaction. In all cases, C(sp3)–O bond formation occurs with high selectivity over potentially competing C(sp2)–O coupling. Additives have a profound impact on the chemoselectivity of these reductive elimination reactions. An excess of RO– was found to limit competing C(sp3)–C(sp2) bond-forming reductive elimination, while the presence of Lewis acidic cations was found to suppress competing C(sp3)–F coupling. Mechanistic investigations were conducted, and the available data are consistent with a sequence involving pre-equilibrium dissociation of the oxyanion ligand (RO–) followed by nucleophilic attack of RO– on a cationic PdIV–alkyl intermediate.

The last 2–3 years have seen numerous relationships develop between organometallic chemists, fluorine chemists and PET Centers around the world. These collaborations have led to the development of many new strategies for the late-stage introduction of fluorine-18 into complex bioactive molecules. In this perspective we highlight recent developments and key milestones since 2011.

A practical, rapid, and highly regioselective Cu-catalyzed radiofluorination of (mesityl)(aryl)iodonium salts is described. This protocol utilizes [18F]KF to access 18F-labeled electron-rich, -neutral, and -deficient aryl fluorides under a single set of mild conditions. This methodology is applied to the synthesis of protected versions of two important radiotracers: 4-[18F]fluorophenylalanine and 6-[18F]fluoroDOPA.

A combination of experimental and density functional theory (DFT) investigations suggests that the Cu-catalyzed fluorination of unsymmetrical diaryliodonium salts with general structure [Mes(Ar)I]+ in N,N′-dimethylformamide proceeds through a CuI/CuIII catalytic cycle. A low concentration of fluoride relative to combined iodonium reagent plus copper ensures that [Mes(Ar)I]+ is available as the reactive species for oxidative “Ar+” transfer to a CuI center containing one or two fluoride ligands. A series of different possible CuI active catalysts (containing fluoride, triflate, and DMF ligands) have been evaluated computationally, and all show low-energy pathways to fluorinated products. The oxidation of these CuI species by [Mes(Ar)I]+ to form cis-Ar(F)CuIII intermediates is proposed to be rate-limiting in all cases. Ar–F bond-forming reductive elimination from CuIII is computed to be very facile in all of the systems examined. The conclusions of the DFT experiments are supported by several experimental studies, including tests showing that CuI is formed rapidly under the reaction conditions and that the fluoride concentration strongly impacts the reaction yields/selectivities.

The synthesis and characterization of a series of fluoroalkyl palladium(II) and nickel(II) complexes via decarbonylation of the corresponding acylmetal species is reported. At palladium(II), labile supporting ligands such as tri-o-tolylphosphine are required to achieve decarbonylation within 30 min at 85 °C. In contrast, decarbonylation at (PPh3)2Ni(C(O)RF)(OCORF) (RF = fluoroalkyl) complexes proceeds rapidly at or below room temperature.

This paper demonstrates a catalytic cycle for Pd-catalyzed decarbonylative trifluoromethylation using trifluoroacetic esters as CF3 sources. The proposed cycle consists of four elementary steps: (1) oxidative addition of a trifluoroacetic ester to Pd0, (2) CO deinsertion from the resulting trifluoroacyl PdII complex, (3) transmetalation of a zinc aryl to PdII, and (4) aryl–CF3 bond-forming reductive elimination. The use of RuPhos as the supporting ligand enables each of these steps to proceed under mild conditions (<100 °C). These studies set the stage for the development of catalytic arene trifluoromethylation and perfluoroalkylation reactions using inexpensive trifluoroacetic acid derived CF3 sources.

This paper describes the fluorination of nitrogen heterocycles using anhydrous NBu4F. Quinoline derivatives as well as a number of 3- and 5-substituted pyridines undergo high-yielding fluorination at room temperature using this reagent. These results with anhydrous NBu4F compare favorably to traditional halex fluorinations using alkali metal fluorides, which generally require temperatures of ≥100 °C.

The preparation of diaryl and alkyl aryl sulfides via acid-mediated coupling of thiols and thioethers with diaryliodonium salts is reported. The scope, limitations, and mechanism of the transformation are discussed.

This report describes a detailed investigation of acetate-assisted C–H activation at PdIV centers supported by the tris(2-pyridyl)methane (Py3CH) ligand. Mechanistic information about this transformation has been obtained through the following: (i) extensive one- and two-dimensional NMR analysis, (ii) reactivity studies of a series of substituted analogues, and (iii) isotope effect studies. These experiments all suggest that C–H activation at [(Py3CH)PdIV(biphenyl)Cl2]+ occurs via a multistep process involving chloride-to-acetate ligand exchange followed by conformational and configurational isomerization and then C–H cleavage. The data also suggest that C–H cleavage proceeds via an acetate-assisted mechanism with the carboxylate likely serving as an intramolecular base. The viability of acetate-assisted C–H activation at high valent palladium has important implications for the design and optimization of catalytic processes involving this transformation as a key step.

This communication describes a mild copper-mediated fluorination of aryl stannanes and aryl trifluoroborates with N-fluoro-2,4,6-trimethylpyridinium triflate. This protocol demonstrates broad substrate scope and functional group tolerance, and does not require the use of any noble metal additives. The reaction is proposed to proceed via an arylcopper(III) fluoride intermediate.

This paper describes the heterogenization of single-site transition-metal catalysts in metal–organic frameworks (MOFs) via cation exchange. A variety of cationic complexes of Pd, Fe, Ir, Rh, and Ru have been incorporated into ZJU-28, and the new materials have been characterized by optical microscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction. MOF-supported [Rh(dppe)(COD)]BF4 catalyzes the hydrogenation of 1-octene to n-octane. The activity of this supported catalyst compares favorably to its homogeneous counterpart, and it can be recycled at least four times. Overall, this work provides a new and general approach for supporting transition-metal catalysts in MOFs.

This report describes the Na2PtCl4 catalyzed C–H arylation of arene substrates with diaryliodonium salts. The site selectivity of these reactions is predominantly controlled by steric factors. Remarkably, Na2PtCl4-catalyzed naphthalene arylation proceeds with opposite site selectivity compared to that obtained with Na2PdCl4 as the catalyst. Preliminary mechanistic studies provide evidence for a PtII/PtIV catalytic cycle involving rate-limiting C–C bond-forming reductive elimination.

This Communication describes the Cu(OTf)2-mediated fluorination of aryltrifluoroborates with KF. The reaction proceeds under mild conditions (at 60 °C over 20 h) and shows a broad substrate scope and functional group tolerance. The Cu is proposed to play two separate roles in this transformation: (1) as a mediator for the aryl–F coupling and (2) as an oxidant for accessing a proposed CuIII(aryl)(F) intermediate.

The replacement of an acetate ligand for carbonate leads to a reversal in site-selectivity in the Pd-mediated C–H oxidative coupling of benzo[h]quinoline with 1,3-dimethoxybenzene. This report describes Density Functional Theory studies designed to elucidate the origin of this selectivity change. These studies focused on two key mechanistic steps: C–H activation and C–C bond-forming reductive elimination. We considered monometallic and bimetallic reaction pathways for acetate and carbonate conditions. The favored C–H activation pathway proceeds via a concerted metalation deprotonation (CMD) mechanism, independent of the nature of anionic ligand (acetate versus carbonate). The predicted selectivity is ortho/para for the C–H activation for both the acetate and carbonate-ligated Pd complexes. Further, we determined that the reductive elimination step is greatly facilitated by the coordination of benzoquinone (by ΔΔG‡ ∼ 20 kcal mol−1) and is predicted to be meta–meta selective with both anionic ligands. Overall, the DFT studies indicate that the anionic ligand does not induce a mechanism change at the elementary steps, and the predicted selectivity at all steps is equivalent for carbonate and acetate, no matter whether a dinuclear or mononuclear pathway is considered. These studies lead us to propose that the role of the anionic ligand is to control which step of the mechanism is overall selectivity-determining. This proposal has been tested experimentally using appropriately designed experiments. Notably, the insoluble base MgO as an acid trap under acetate conditions (with the aim of making the C–H insertion step less reversible), gave rise to predominant ortho/para selectivity in the presence of acetate, in analogy to the results previously seen under carbonate conditions.

This paper reports the hydrogenation of carbon dioxide to formate catalyzed by the Ru pincer complex Ru(PNN)CO(H) (PNN = 6-(di-tert-butylphosphinomethylene)-2-(N,N-diethylaminomethyl)-1,6-dihydropyridine). Stoichiometric studies are presented that support the feasibility of the individual steps in a proposed catalytic cycle for this transformation. The influence of base and solvent on catalyst performance is explored. Overall, under optimized conditions (using diglyme as the solvent and potassium carbonate as the base) up to 23,000 turnovers of formate and a turnover frequency of up to 2,200 h–1 can be achieved.Keywords: formic acid; homogeneous catalysis; hydrogenation; pincer ligands; ruthenium

This manuscript describes the development of Pd catalysts with pyridinium-substituted pyridine ligands for the C–H oxygenation of benzene with potassium persulfate. These new catalysts provide dramatically improved activity compared to simple Pd(OAc)2 in this transformation. Furthermore, the reaction proceeds with high selectivity for phenyl acetate over biphenyl. Preliminary investigations suggest that a key role for the cationic pyridinium ligand is to serve as a phase transfer catalyst for the K2S2O8 oxidant.Keywords: benzene; catalysis; cationic; ligand; oxidation; palladium; phase-transfer

This report describes the use of an oxidant and a ligand to control site selectivity in the Pd(OAc)2-catalyzed C–H acetoxylation of simple arenes. The use of MesI(OAc)2 as the terminal oxidant in combination with acridine as the ligand results in primarily sterically controlled selectivity. In contrast, with Pd(OAc)2 as the catalyst and PhI(OAc)2 as the oxidant, electronic effects dominate the selectivity of arene C–H acetoxylation.

A mild Cu-catalyzed nucleophilic fluorination of unsymmetrical diaryliodonium salts with KF is described. This protocol preferentially fluorinates the smaller aromatic ligand on iodine(III). The reaction exhibits a broad substrate scope and proceeds with high chemoselectivity and functional group tolerance. DFT calculations implicate a CuI/CuIII catalytic cycle.

A Pd-catalyzed asymmetric alkene 1,2-dioxygenation reaction is described. The diastereoselectivity of the reaction is controlled by tethering a chiral oxime ether directing group to the alkene substrate. The best selectivities are obtained with 8-substituted menthone-derived oxime ether auxiliaries.

This communication describes the reversible reaction of a ruthenium pincer complex with a variety of carbonyl compounds. Both NMR spectroscopic and X-ray crystallographic characterization of isomeric carbonyl adducts are reported, and the equilibrium constants for carbonyl binding have been determined.

A Pd-catalyzed method for ligand-directed C–H alkylation with organoboron reagents is described. The combination of potassium organotrifluoroborates, MnF3, and a PdII catalyst effects pyridine and amide-directed C–H alkylation. These reactions proceed under mild conditions (25–40 °C in weakly acidic media), are effective for installing methyl and 1° alkyl groups, and do not require promoters such as benzoquinone.

A computational analysis of the Pd-catalyzed coupling of 3-methyl-2-phenylpyridine (mppH) with [Ph2I]BF4 to form mppPh is supportive of a prior synthetic and kinetic study implicating binuclear palladium species in a rate-limiting oxidation step. The Pd(OAc)2 precatalyst forms the “clamshell” orthopalladated complex [Pd(mpp)(μ-OAc)]2 (8) as the active catalyst, which is oxidized by [Ph2I]+ in a reaction having the highest energy requirement of all steps in the catalytic cycle. In the oxidation reaction, involving formal transfer of Ph+, the electrophilic iodine center interacts initially with a bridging acetate oxygen atom of [Pd(mpp)(μ-OAc)]2 (8), “Pd–O···IPh2”, which transforms to a transition structure with retention of the O···I interaction and formation of a “Pd(μ-Ph-η1)I” bridge in a four-membered ring, “Pd···Ph···I(Ph)···O–Pd”, followed by elimination of PhI with formation of a binuclear Pd(III) cation containing a Pd–Pd bond, [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14). Cation 14 undergoes mpp···Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh-N)Pd(μ-OAc)2Pd(mpp)]+ (Da). Cation Da may fragment to release mppPh and mononuclear palladium species, followed by orthopalladation at a mononuclear center. However, in an environment of very low acetate concentration and high nitrogen-donor concentration, it is considered far more likely that Da undergoes ligand exchange with release of mppPh and formation of [(mppH-N)Pd(μ-OAc)2Pd(mpp)]+ (I). Computation shows a low-energy pathway for orthopalladation at cation I that involves nitrogen-donor reagents mppH and mppPh acting as bases to remove a proton as [HN-donor]+. This orthopalladation would complete the cycle and regenerate the catalyst, [Pd(mpp)(μ-OAc)]2 (8). A Hammett plot obtained from a computational analysis of the reaction of [(p-X-C6H4)(Mes)I]BF4 (X = H, Me, OMe, F, Cl, COMe, CF3) has a reaction constant (ρ) of 1.8, which compares well with the experimental result (ρ = 1.7 ± 0.2). Consistent with this, the analysis reveals the dominant role of the interaction energy for palladium- and iodine-containing fragments in the transition structure.

Effective methodology to functionalize C–H bonds requires overcoming the key challenge of differentiating among the multitude of C–H bonds that are present in complex organic molecules. This Account focuses on our work over the past decade toward the development of site-selective Pd-catalyzed C–H functionalization reactions using the following approaches: substrate-based control over selectivity through the use of directing groups (approach 1), substrate control through the use of electronically activated substrates (approach 2), or catalyst-based control (approach 3). In our extensive exploration of the first approach, a number of selectivity trends have emerged for both sp2 and sp3 C–H functionalization reactions that hold true for a variety of transformations involving diverse directing groups. Functionalizations tend to occur at the less-hindered sp2 C–H bond ortho to a directing group, at primary sp3 C–H bonds that are β to a directing group, and, when multiple directing groups are present, at C–H sites proximal to the most basic directing group. Using approach 2, which exploits electronic biases within a substrate, our group has achieved C-2-selective arylation of indoles and pyrroles using diaryliodonium oxidants. The selectivity of these transformations is altered when the C-2 site of the heterocycle is blocked, leading to C–C bond formation at the C-3 position. While approach 3 (catalyst-based control) is still in its early stages of exploration, we have obtained exciting results demonstrating that site selectivity can be tuned by modifying the structure of the supporting ligands on the Pd catalyst. For example, by modulating the structure of N–N bidentate ligands, we have achieved exquisite levels of selectivity for arylation at the α site of naphthalene. Similarly, we have demonstrated that both the rate and site selectivity of arene acetoxylation depend on the ratio of pyridine (ligand) to Pd. Lastly, by switching the ligand on Pd from an acetate to a carbonate, we have reversed the site selectivity of a 1,3-dimethoxybenzene/benzo[h]quinoline coupling. In combination with a growing number of reports in the literature, these studies highlight a frontier of catalyst-based control of site-selectivity in the development of new C–H bond functionalization methodology.

This communication describes the development of a mild method for the cross-coupling of arylboronic acids with CF3I via the merger of photoredox and Cu catalysis. This method has been applied to the trifluoromethylation of electronically diverse aromatic and heteroaromatic substrates and tolerates many common functional groups.

This paper describes a new method for the catalytic aerobic oxygenation of unactivated sp3-C–H bonds. This transformation utilizes Pd(OAc)2 as a catalyst in conjunction with NaNO3 as a redox co-catalyst. Both oxime ether and pyridine derivatives are effective directing groups for these reactions. The oxygen incorporated into the product derives from the solvent (acetic acid). Preliminary results show that the addition of simple NaCl to the reaction mixture results in aerobic chlorination under analogous conditions.

The palladium-catalyzed C–H fluorination of 8-methylquinoline derivatives with nucleophilic fluoride is reported. This transformation involves the use of AgF as the fluoride source in combination with a hypervalent iodine oxidant. Both the scope and mechanism of the reaction are discussed.

Commercially available pyridine ligands can significantly enhance the rate, yield, substrate scope, and site selectivity of arene C–H olefination (Fujiwara–Moritani) reactions. The use of a 1:1 ratio of Pd/pyridine proved critical to maximize reaction rates and yields.

A mild and practical protocol for the copper-mediated trifluoromethylation of aryl and heteroaryl boronic acids using NaSO2CF3 (Langlois’ reagent) and TBHP is described. The reaction proceeds at room temperature under ambient conditions, and the products can be readily purified by extraction or column chromatography.

This communication describes the activation of CO2 at the Ru pincer complex (PNN)Ru(H)(CO) (PNN = 6-(di-tert-butylphosphinomethylene)-2-(N,N-diethylaminomethyl)-1,6-dihydropyridine). The reaction proceeds to completion within minutes at room temperature to form a C–C bond between the pincer ligand and the electrophilic carbon atom of CO2. The characterization of both the kinetic and thermodynamic products of CO2 activation and the reversibility of this C–C bond formation are discussed.

This report describes a comparison of the reactivity of a series of diimine PtII complexes as catalysts for H/D exchange between benzene and RCO2D. Substitution at the 2- and 6-positions of the N-aryl group and especially incorporation of halogens at these sites lead to significant increases in catalyst reactivity. Interestingly, marked differences in the structure activity relationship were observed in this catalytic study compared to previously reported stoichiometric investigations of C–H activation at PtII diimine complexes.

This article describes the rational design of first generation systems for oxidatively induced Aryl−CF3 bond-forming reductive elimination from PdII. Treatment of (dtbpy)PdII(Aryl)(CF3) (dtbpy = di-tert-butylbipyridine) with NFTPT (N-fluoro-1,3,5-trimethylpyridinium triflate) afforded the isolable PdIV intermediate (dtbpy)PdIV(Aryl)(CF3)(F)(OTf). Thermolysis of this complex at 80 °C resulted in Aryl−CF3 bond-formation. Detailed experimental and computational mechanistic studies have been conducted to gain insights into the key reductive elimination step. Reductive elimination from this PdIV species proceeds via pre-equilibrium dissociation of TfO− followed by Aryl−CF3 coupling. DFT calculations reveal that the transition state for Aryl−CF3 bond formation involves the CF3 acting as an electrophile with the Aryl ligand serving as a nucleophilic coupling partner. These mechanistic considerations along with DFT calculations have facilitated the design of a second generation system utilizing the tmeda (N,N,N′,N′-tetramethylethylenediamine) ligand in place of dtbpy. The tmeda complexes undergo oxidative trifluoromethylation at room temperature.

High reversibility during crystallization leads to relatively defect-free crystals through repair of nonperiodic inclusions, including those derived from impurities. Microporous coordination polymers (MCPs) can achieve a high level of crystallinity through a related mechanism whereby coordination defects are repaired, leading to single crystals. In this work, we discovered and exploited the fact that this process is far from perfect for MCPs and that a minority ligand that is coordinatively identical to but distinct in shape from the majority linker can be inserted into the framework, resulting in defects. The reaction of Zn(II) with 1,4-benzenedicarboxylic acid (H2BDC) in the presence of small amounts of 1,3,5-tris(4-carboxyphenyl)benzene (H3BTB) leads to a new crystalline material, MOF-5(Oh), that is nearly identical to MOF-5 but has an octahedral morphology and a number of defect sites that are uniquely functionalized with dangling carboxylates. The reaction with Pd(OAc)2 impregnates the metal ions, creating a heterogeneous catalyst with ultrahigh surface area. The Pd(II)-catalyzed phenylation of naphthalene within Pd-impregnated MOF-5(Oh) demonstrates the potential utility of an MCP framework for modulating the reactivity and selectivity of such transformations. Furthermore, this novel synthetic approach can be applied to different MCPs and will provide scaffolds functionalized with catalytically active metal species.

This communication describes the development of a room-temperature ligand-directed C–H arylation reaction using aryldiazonium salts. This was achieved by the successful merger of palladium-catalyzed C–H functionalization and visible-light photoredox catalysis. The new method is general for a variety of directing groups and tolerates many common functional groups.

This communication describes the first observation and study of C–H activation at a PdIV center. This transformation was achieved by designing model complexes in which the rate of reductive elimination is slowed relative to that of the desired C–H activation process. Remarkably, the C–H activation reaction can proceed under mild conditions and with complementary site selectivity to analogous transformations at PdII. These results provide a platform for incorporating this new reaction as a step in catalytic processes.

This communication demonstrates the homogeneous hydrogenation of CO2 to CH3OH via cascade catalysis. Three different homogeneous catalysts, (PMe3)4Ru(Cl)(OAc), Sc(OTf)3, and (PNN)Ru(CO)(H), operate in sequence to promote this transformation.

This letter describes a new method for the highly site- and chemoselective Pd-catalyzed direct arylation of naphthalene. Tuning the structure of the diimine-ligated Pd catalyst results in formation of the α-arylated product in high yield and >50:1 selectivity. This is, to our knowledge, the first systematic evaluation of catalyst control in the C−H arylation of an unactivated aromatic substrate. Preliminary studies implicate an unusual mechanism involving sequential naphthalene π-coordination/metalation at PdIV.Keywords (keywords): catalyst control; C−H functionalization; naphthalene; palladium; regioselective functionalization

The silver-mediated C–H trifluoromethylation of aromatic substrates using TMSCF3 is described. The development, optimization, and scope of these transformations are reported. AgCF3 intermediates are proposed.

This paper describes the 1,1-arylacetoxylation of diverse α-olefins using organostannanes and hypervalent iodine oxidants. The reaction provides a convergent approach for generating a C−C and a C−O bond as well as a new stereocenter in a single catalytic transformation.

This paper describes the synthesis of a series of PdIV complexes containing modular monoanionic tridentate facially coordinated NNN and NNC donor ligands. In all cases, these complexes are stable to reductive elimination for a minimum of several days in solution at room temperature. With appropriately designed tridentate ligands, the PdIV adducts participate in both ligand substitution and C–H activation reactions. Overall, this work shows that unsymmetrical fac-L2X type ligands can serve as versatile and tunable scaffolds for modulating the reactivity of octahedral PdIV complexes.

This paper describes density functional theory (DFT) calculations of the mechanism of carbon-acetate bond-forming reductive elimination from Pd centers. This C–O coupling reaction has been studied at a series of different PdIV, PdIII, and PdII complexes. In all cases, three-membered transition states involving direct coupling of the Pd-bound O and C atoms have been compared to five-membered transition states where the pendant carbonyl oxygen participates in the C–O coupling event. These calculations show that the five-membered transition state is kinetically favored by between 4.7 and 13.2 kcal/mol.

Tuning the aryl substituents of N,N′-diaryl-2,3-dimethyl-1,4-diaza-1,3-butadiene (DAB) ligands promotes the challenging C–C bond-forming reductive elimination from PtIV diimine complexes [(DAB)Pt(CH3)3(solvent)]+ (2) under mild conditions. Experimental results and density functional calculations indicate that 2,6-aryl substitution promotes reductive elimination by facilitating dissociation of the coordinating solvent by close to 10 kcal mol–1, but too much steric bulk inhibits the formation of 2 in the one-electron outersphere oxidation of (DAB)Pt(CH3)2 (1).

This article describes the development of a Pd-catalyzed reaction for the arylhalogenation (halogen = Cl or Br) of diverse α-olefins by oxidatively intercepting Mizoroki−Heck intermediates. These transformations afford synthetically useful 1,2- and 1,1-arylhalogenated products in good yields with good to excellent selectivities that can be modulated by changing the nature of the halogenating reagent and/or the reaction conditions. The selectivity of these reactions can be rationally tuned by (i) controlling the relative rates of oxidative functionalization versus β-hydride elimination from equilibrating PdII-alkyl species and (ii) stabilization of organometallic PdII intermediates through the formation of π-benzyl adducts. These arylhalogenations exhibit modest to excellent levels of stereoselectivity, and the key carbon−halogen bond-forming step proceeds with predominant retention of stereochemistry at carbon.

This communication describes oxidatively induced Ar−CF3 bond-forming reductive elimination from new PdII complexes of general structure (L∼L)PdII(Ar)(CF3). The electrophilic fluorinating reagent N-fluoro-2,4,6-trimethylpyridinium triflate promotes these reactions in good to excellent yields. The palladium(IV) intermediate (tBu-bpy)PdIV(CF3)(F)(OTf)(C6H4F) has been isolated, characterized, and demonstrated to undergo high yielding Ar−CF3 coupling upon thermolysis. This work provides an attractive conceptual framework for the development of PdII/IV-catalyzed arene trifluoromethylation reactions.

The reaction of [(bzq)Pd(OAc)]2 (bzq = benzo[h]quinoline) with “CF3+” reagents to afford the monomeric PdIV aquo complex (bzq)Pd(CF3)(OAc)2(OH2) is demonstrated. Heating this PdIV adduct at 60 °C for 12 h leads to highly chemoselective aryl−CF3 bond-forming reductive elimination. The rate and yield of this transformation are both significantly enhanced by the addition of Brønsted and Lewis acidic additives. The mechanism of C−CF3 bond formation from (bzq)Pd(CF3)(OAc)2(OH2) has been studied, and the major pathway is proposed to involve pre-equilibrium acetate dissociation followed by C−CF3 coupling. Finally, this monomeric PdIV complex is shown to be a kinetically competent intermediate for C−H trifluoromethylation with “CF3+” reagents. Collectively, these studies provide valuable insights about the speciation, nuclearity, and reactivity of Pd intermediates in catalytic C−H trifluoromethylation reactions.

O-Acetyl oximes serve as effective directing groups for Pd-catalyzed sp2 and sp3 C−H functionalization reactions. The C−H functionalization products can be subsequently transformed into ortho- or β-functionalized ketones, alcohols, amines, and heterocycles.

This article describes the synthesis, characterization, and reactivity of palladium(II) fluoride complexes containing sp2 and sp3 nitrogen-containing supporting ligands. Both cis and trans complexes of general structure (N)(N′)PdII(R)(F) (R = Ar or CH3) as well as cis-(N)2PdII(F)2 are reported. Crystallographic characterization of these molecules has allowed structural comparisons to related phosphine-ligated species. Furthermore, these studies have revealed that nitrogen-donor ligands support some of the longest and the shortest Pd–F bonds reported to date. The thermal decomposition of (N)(N′)PdII(R)(F) has also been examined, and no products of C–F bond-forming reductive elimination were obtained in any case.

The disproportionation of 2 equiv of (L)2Pd(CH3)(X) to form 1 equiv of (L)2Pd(CH3)2 and 1 equiv of (L)2Pd(X)2 could serve as a key step in the catalytic oxidative oligomerization of methane. The thermodynamics associated with this transformation have been evaluated as a function of the supporting ligands L and X using DFT calculations. With these calculations as a guide, we demonstrate the first experimental example of disproportionation of a PdII monomethyl complex to generate a PdII dimethyl species using (tBu-bpy)Pd(CH3)(CH2COCH3) (tBu-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine) as the starting material.

This paper describes an efficient synthesis of the cationic platinum complex [(N-CH3-bpym)PtCl2]+ (N-CH3-bpym = N-methylbipyrimidinium) and evaluation of its catalytic activity in H/D exchange reactions of CH4 (with D2SO4) and benzene (with CF3CO2D). With both substrates [(N-CH3-bpym)PtCl2]+ shows C−H activation reactivity comparable to that of its neutral analogue (bpym)PtCl2 (bpym = bipyrimidine). The origin of this similar reactivity is proposed to be the in situ formation of the same active catalyst in both cases.

This article describes the synthesis and reactivity of NiII(Phpy)2 (Phpy = 2-phenylpyridine) with a variety of oxidants, including O2, Br2, PhICl2, N-fluoropyridinium salts, CuII salts, and N-halosuccinimides. High-oxidation-state Ni intermediates were not detected in any of these transformations. In all cases, the major organic product resulted from oxidatively induced C−C bond formation to generate the Phpy−Phpy dimer. Traces (2−16%) of organic products resulting from C−O, C−Br, C−Cl, and C−N bond-forming reductive elimination were also observed.

This contribution describes the substrate scope and mechanism of Pd-catalyzed ligand-directed C−H arylation with diaryliodonium salts. This transformation was applied to the synthesis of a variety of different biaryl products, using directing groups including pyridines, quinolines, pyrrolidinones, and oxazolidinones. Electronically and sterically diverse aryl groups (Ar) were transferred in high yield using iodine(III) reagents of general structure [Mes−I−Ar]BF4. Mechanistic investigations have been conducted that establish the kinetic order of the catalytic reaction in each component, determine the resting state of the catalyst and the iodine(III) reagent, quantify the electronic influence of the arylating reagent on the reaction rate, and establish the intra- and intermolecular 1° H/D kinetic isotope effect. On the basis of these studies, this transformation is proposed to proceed via turnover-limiting oxidation of the Pd dimer [Pd(N∼C)(OAc)]2 (N∼C = 3-methyl-2-phenylpyridine) by [Mes−I−Ph]BF4. This mechanism implicates a bimetallic high oxidation state Pd species as a key catalytic intermediate. The significance of this and other aspects of the proposed mechanism are discussed in detail.

This communication describes studies of oxidatively induced C−C bond-forming reductive elimination from (tBu2bpy)PdII(Me)2. With the outer-sphere oxidant ferrocenium, the data are consistent with a mechanism involving PdIII and PdIV intermediates, with C−C bond formation occurring from the latter. The reaction with Ag+ appears to proceed via a Pd−Ag+ adduct, which then undergoes inner sphere electron transfer to generate PdIII. In contrast, the slower benzoquinone reaction forms ethane by a different pathway that does not involve methyl group scrambling and generates Pd0 products.

A PdIV complex that represents a viable catalytic intermediate in Pd-catalyzed C−H bond halogenation reactions has been isolated and structurally characterized. It contains the first examples of both a PdIV NHC bond and a PdIV alkoxide bond and serves as a precatalyst for C−H bond halogenation. As such, this represents a new class of tunable supporting ligand systems in PdIV catalysis.

This communication describes detailed investigations of the mechanism of the Pd-catalyzed C−H chlorination and acetoxylation of 2-o-tolylpyridine. Under the conditions examined, both reactions proceed via rate-limiting cyclopalladation. However, substrate and catalyst order as well as Hammett data indicate that the intimate mechanism of cyclopalladation differs significantly between PdCl2-catalyzed chlorination and Pd(OAc)2-catalyzed acetoxylation.

This communication describes the first reported example of carbon−halogen bond formation from an isolable nickel aryl halide precursor. In addition, oxidatively induced Ar−Br bond-forming reactions from the nickel(II) complex NiII(phpy)(Br)(pic) (phpy = 2-phenylpyridine; pic = 2-picoline) are demonstrated using CuBr2, Br2, ceric ammonium nitrate, and ferrocenium as oxidants. On the basis of several pieces of evidence, the intermediacy of Ni(III) species is proposed in these transformations.

This paper describes a protocol for the direct comparison of diverse Pt catalysts in the H/D exchange between C6H6 and TFA-d1, CD3CO2D, and TFE-d3 using turnover number (TON) as a standard metric. An initial survey of Pt complexes, including commercial Pt salts (PtCl2, K2PtCl4) and Pt chloride complexes containing bidentate and tridentate nitrogen donor ligands, has been conducted. These studies have established that the addition of AgOAc (in TFA-d1) or AgBF4 (in CD3CO2D and TFE-d3) displaces the Cl ligands on the Pt precatalyst, which leads to dramatically increased turnover numbers. In general, PtCl2 and K2PtCl4 provided the fewest turnovers, and species containing bidentate ligands afforded higher turnover numbers than those with tridentate ligands. A diimine Pt complex was found to be a top performing catalyst for H/D exchange with all deuterium sources examined. Interestingly, the relative reactivity of many of the catalysts varied dramatically upon changing the deuterium source, highlighting the need to thoroughly assay potential catalysts under a variety of conditions.

This paper describes the oxidation of PtII(phpy)2 (phpy = 2-phenylpyridine) with two different electrophilic chlorinating reagents—PhICl2 and N-chlorosuccinimide (NCS). PtII(phpy)2 reacts with PhICl2 to provide the PtIV complex PtIVCl2(phpy)2 as a mixture of symmetrical and unsymmetrical isomers. In contrast, the reaction of PtII(phpy)2 with NCS affords a mixture of the PtIII dimer [(phpy)2(Cl)PtIII]2 and PtIVCl2(phpy)2, whose ratio varies as a function of reaction time, concentration, and the presence/absence of ambient light. The implications of these results for the mechanisms of the two reactions are discussed in detail.

The palladium-catalyzed direct arylation of 2,5-substituted pyrrole derivatives with diaryliodonium salts to generate tri-, tetra-, and penta-substituted pyrrole products is described. The scope and limitations of these transformations are also reported.

This report describes a computational study of C(sp3)–OR bond formation from PdIV complexes of general structure PdIV(CH2CMe2-o-C6H4-C,C′)(F)(OR)(bpy-N,N′) (bpy = 2,2′-bipyridine). Dissociation of −OR from the different octahedral PdIV starting materials results in a common square-pyramidal PdIV cation. An SN2-type attack by −OR (−OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp3)–OR bond formation. In contrast, when −OR = triflate, concerted C(sp3)–C(sp2) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and SN2 steps with different −OR follow opposing trends. The SN2 transition states exhibit “Pd⋯C⋯O” angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally.

This communication describes the reversible reaction of a ruthenium pincer complex with a variety of carbonyl compounds. Both NMR spectroscopic and X-ray crystallographic characterization of isomeric carbonyl adducts are reported, and the equilibrium constants for carbonyl binding have been determined.

The last 2–3 years have seen numerous relationships develop between organometallic chemists, fluorine chemists and PET Centers around the world. These collaborations have led to the development of many new strategies for the late-stage introduction of fluorine-18 into complex bioactive molecules. In this perspective we highlight recent developments and key milestones since 2011.

This paper describes a new method for the catalytic aerobic oxygenation of unactivated sp3-C–H bonds. This transformation utilizes Pd(OAc)2 as a catalyst in conjunction with NaNO3 as a redox co-catalyst. Both oxime ether and pyridine derivatives are effective directing groups for these reactions. The oxygen incorporated into the product derives from the solvent (acetic acid). Preliminary results show that the addition of simple NaCl to the reaction mixture results in aerobic chlorination under analogous conditions.

The replacement of an acetate ligand for carbonate leads to a reversal in site-selectivity in the Pd-mediated C–H oxidative coupling of benzo[h]quinoline with 1,3-dimethoxybenzene. This report describes Density Functional Theory studies designed to elucidate the origin of this selectivity change. These studies focused on two key mechanistic steps: C–H activation and C–C bond-forming reductive elimination. We considered monometallic and bimetallic reaction pathways for acetate and carbonate conditions. The favored C–H activation pathway proceeds via a concerted metalation deprotonation (CMD) mechanism, independent of the nature of anionic ligand (acetate versus carbonate). The predicted selectivity is ortho/para for the C–H activation for both the acetate and carbonate-ligated Pd complexes. Further, we determined that the reductive elimination step is greatly facilitated by the coordination of benzoquinone (by ΔΔG‡ ∼ 20 kcal mol−1) and is predicted to be meta–meta selective with both anionic ligands. Overall, the DFT studies indicate that the anionic ligand does not induce a mechanism change at the elementary steps, and the predicted selectivity at all steps is equivalent for carbonate and acetate, no matter whether a dinuclear or mononuclear pathway is considered. These studies lead us to propose that the role of the anionic ligand is to control which step of the mechanism is overall selectivity-determining. This proposal has been tested experimentally using appropriately designed experiments. Notably, the insoluble base MgO as an acid trap under acetate conditions (with the aim of making the C–H insertion step less reversible), gave rise to predominant ortho/para selectivity in the presence of acetate, in analogy to the results previously seen under carbonate conditions.

This article describes the synthesis, characterization, and reactivity of palladium(II) fluoride complexes containing sp2 and sp3 nitrogen-containing supporting ligands. Both cis and trans complexes of general structure (N)(N′)PdII(R)(F) (R = Ar or CH3) as well as cis-(N)2PdII(F)2 are reported. Crystallographic characterization of these molecules has allowed structural comparisons to related phosphine-ligated species. Furthermore, these studies have revealed that nitrogen-donor ligands support some of the longest and the shortest Pd–F bonds reported to date. The thermal decomposition of (N)(N′)PdII(R)(F) has also been examined, and no products of C–F bond-forming reductive elimination were obtained in any case.

This paper describes the design, synthesis, and fundamental characterization of a series of Cr and V acetylacetonate (acac) complexes for use in redox flow batteries (RFBs). These materials offer a significant improvement in theoretical energy density relative to state-of-the-art aqueous chemistries. A detailed assessment of the solubility, cyclic voltammetry, and charge–discharge behavior of the complexes is presented. Their solubilities in acetonitrile vary by more than four orders of magnitude based on the structure/substituents on the acac ligand. Complexes bearing acac ligands with ester substituents have solubilities of up to 1.8 M, a significant improvement over most other metal complexes that have been considered for non-aqueous RFB applications. While the acac ligand substituents have a dramatic impact on solubility, they do not, in most cases, impact the electrochemical properties of the complexes. For instance, voltammetry for all of the V(acac)3 derivatives examined exhibit two quasi-reversible redox events separated by approximately 2.1 V. Charge–discharge testing in static H-cell and laboratory-scale flow batteries yielded energy densities that were consistent with the voltammetry and coulombic and energy efficiencies of up to 92% and 87%, respectively. Overall, these studies provide the basis for the development of structure–function relationships that could lead to new and even better performing energy storage chemistries in the future.

The direct functionalization of carbon–hydrogen (C–H) bonds has emerged as a versatile strategy for the synthesis and derivatization of organic molecules. Among the methods for C–H bond activation, catalytic processes that utilize a PdII/PdIV redox cycle are increasingly common. The C–H activation step in most of these catalytic cycles is thought to occur at a PdII centre. However, a number of recent reports have suggested the feasibility of C–H cleavage occurring at PdIV complexes. Importantly, these latter processes often result in complementary reactivity and selectivity relative to analogous transformations at PdII. This mini review highlights proposed examples of C–H activation at PdIV centres. Applications of this transformation in catalysis as well as mechanistic details obtained from stoichiometric model studies are discussed. Furthermore, challenges and future perspectives for the field are reviewed.