Non-coordinating Research Articles

Palladium-Catalyzed Enantioselective β-C(sp3)-H Activation Reactions of Aliphatic Acids: A Retrosynthetic Surrogate for Enolate Alkylation and Conjugate Addition.

Enolate alkylation and conjugate addition into an α,β-unsaturated system have served as long-standing strategic disconnections for the installation of α- or β-substituents on carbonyl-containing compounds. At the onset of our efforts to develop C-H activation reactions for organic synthesis, we set our eye toward developing asymmetric β-C-H activation reactions of aliphatic acids with the perspective that this bond-forming event could serve as a more flexible retrosynthetic surrogate for both canonical carbonyl-related asymmetric transformations.In this Account, we describe our early efforts using strongly coordinating chiral oxazolines to probe reaction mechanism and the stereochemical nature of the C-H cleavage transition state. The characterization of key reactive intermediates through X-ray crystallography and computational studies suggested a transition state with C-H and Pd-OAc bonds being approximately coplanar for optimum interaction. We then moved forward to develop more practical, weakly coordinating monodentate amide directing groups, a necessary advance toward achieving the β-C-H activation of weakly coordinating native carboxylic acids. Throughout this journey, gradual deconvolution between a substrate's directing effect and its intimate interplay with ligand properties has culminated in the design of new ligand classes that ultimately allowed the competency of native carboxylic acids in β-C-H activation. These efforts established the importance of ligand acceleration in Pd-catalyzed C-H activation, where the substrate's weak coordination is responsible for positioning the catalyst for C-H cleavage, while the direct participation from the bifunctional ligand is responsible for enthalpically stabilizing the C-H cleavage transition state.Building upon these principles, we developed five classes of chiral ligands (MPAA, MPAQ, MPAO, MPAThio, MPAAM) to enable enantioselective β-C-H activation reactions, including carbon-carbon and carbon-heteroatom bond formation. The accumulated data from our developed enantioselective C-H activation reactions indicate that ligands possessing point chirality are most effective for imparting stereoinduction in the C-H activation step, the application of which enabled the desymmetrization and subsequent C-H functionalization of enantiotopic carbon and protons across a range of weakly coordinating arylamides and, more recently, free carboxylic acids. Progress in ligand design, in conjunction with the enabling nature of alkali metal countercations, led to the realization of a suite of β-methyl and now methylene C(sp3)-H activation reactions. These advancements also enabled the use of economical oxidants, such as peroxides and molecular oxygen, to facilitate catalyst turnover. In the future, continued progress in designing more efficient bifunctional chiral ligands is likely to provide a myriad of enantioselective β-C-H activation reactions of readily available native substrates.

Different types of interactions between fullerene C60 and C70 anions and metal tetraphenylporphyrins in the (PMDAE+)(MIITPP)(Fullerene−)⋅Solvent complexes (M = Co, Mn, Zn) containing coordinating N, N, N, N‘, N‘-pentamethyldiaminoethane cations

Series of anionic salts of fullerenes C60 and C70 with different metal tetraphenylporphyrins, MIITPP, where M = Co, Mn and Zn, and N, N, N, N‘, N‘-pentamethyldiaminoethane cations (PMDAE+) has been obtained in a crystalline form. Composition of the complexes is (PMDAE+)⋅(MIITPP)(C60−)⋅XC6H4Cl2 (M = Co, X = 1.25 (1), M = Mn, X = 0.75 (2) and M = Zn, X = 0.75 (3)) and {(PMDAE+)⋅ZnIITPP}2⋅(C70−)2⋅4C6H4Cl2 (4). Both fullerene anion and nitrogen-containing PMDAE+ cation can coordinate to metal porphyrin competing with each other. Weak Co-C(C60−) coordination bond of 2.426(7) Å length (120 K) is formed in 1 whereas PMDAE+ cation only weakly coordinates to CoIITPP. Dissociation of Co-C(C60−) bond observed at 90–310 K is accompanied by a transition of 1 from singlet to triplet state with an energy gap of 574 ± 3 K. According to the calculations negative charge on C60− is only −0.548 due to electron transfer to the macrocyclic ligand through the cobalt atom. Charge transfer bands are observed in {CoIITPP(C60)}− at 700 and 1240 nm. On the contrary, PMDAE+ cations coordinate to MnIITPP and ZnIITPP in 2 and 3, respectively, with short M−N(PMDAE+) bonds of 2.27–2.33 Å length. In these cases fullerenes are preserved in the non-coordinated radical anion state. Fullerenes are arranged in closely packed π-stacking {C60•−}2 dimers with short interfullerene center-to-center distances of 9.92–9.95 Å. Nevertheless, the C60•− radical anions do not form intercage CC bonds down to 1.9 K. Strong antiferromagnetic coupling of spins is observed in the {C60•−}2 dimers of 3 with exchange interaction of J/kB = -21 K. Both {(PMDAE+)⋅MnIITPP} and C60•− species are paramagnetic in 2 having S = 5/2 and S = 1/2 spin states, respectively. These spins are coupled antiferromagnetically with Weiss temperature of −16 K. It is obvious that magnetic coupling occurs not only in the {C60•−}2 dimers but MnIITPP is also involved in this coupling. The C70•− radical anions are dimerized in 4 forming diamagnetic singly-bonded (C70−)2 dimers which manifest intense NIR absorption. As a result, complex 4 is EPR silent.

Increasing the oxidation power of TCNQ by coordination of B(C6F5)3.

The oxidation power of the cyanocarbon TCNQ (tetracyano-quinodimethane) can be significantly increased to approximately E = +0.9 V vs. Cp2Fe by coordination of up to four equivalents of the strong fluorinated Lewis acid B(C6F5)3, resulting in a highly reactive but easy-to-use oxidation system. Thianthrene and tris(4-bromophenyl)amine were oxidized to the corresponding radical cations. Dianionic [TCNQ·4 B(C6F5)3]2- was formed upon reduction with two equivalents of ferrocene or decamethylcobaltocene. [TCNQ·4 B(C6F5)3]- and [TCNQ·4 B(C6F5)3]2- are rare cases of redox-active weakly-coordinating anions.

Room temperature light induced spin state switching in a FeII coordination polymer featuring a photo responsive anion

The FeII coordination polymer, [Fe(NH2trz)3](psca)2·2H2O, incorporating a photo-responsive non-coordinated anion, p-sulfocinnamic acid (psca), offers the first example of AD-LISC at room temperature.

Crystal structure of a TbIII-CuII glycine-hydroxamate 15-metallacrown-5 sulfate complex.

The core of the title complex, bis-[hexa-aqua-hemi-aqua-penta-kis-(μ3-glycine-hydroxamato)sulfato-penta-copper(II)terbium(III)] sulfate hexa-hydrate, [TbCu5(SO4)(GlyHA)5(H2O)6.5]2(SO4)·6H2O (1), which belongs to the 15-metalla-crown-5 family, consists of five glycine-hydroxamate dianions (GlyHA2-; C2H4N2O2) and five copper(II) ions linked together forming a metallamacrocyclic moiety. The terbium(III) ion is connected to the centre of the metallamacrocycle through five hydroxamate oxygen atoms. The coordination environment of the Tb3+ ion is completed to an octa-coordination level by oxygen atoms of a bidentate sulfate and an apically coordinated water mol-ecule, while the copper(II) atoms are square-planar, penta- or hexa-coordinate due to the apical coordination of water mol-ecules. Continuous shape calculations indicate that the coordination polyhedron of the Tb3+ ion in 1 is best described as square anti-prismatic. The positive charge of each pair of [TbCu5(GlyHA)5(H2O)6.5(SO4)]2 2+ fragments is compensated by a non-coordinated sulfate anion, which is located on an inversion center with 1:1 disordered oxygen atoms. Complex 1 is isomorphous with the previously reported compounds [LnCu5(GlyHA)5(SO4)(H2O)6.5]2(SO4), where Ln III = Pr, Nd, Sm, Eu, Gd, Dy and Ho.

Investigation of Bis(Perfluoro-tert-Butoxy) Halogenates(I/III).

A systematic study of halogenate(I/III) anions with polyatomic ligands is presented. The bis(perfluoro‐tert‐butoxy) halogenates(I) [X(OC4F9)2]−, X=Cl, Br, I, of chlorine, bromine, and iodine are prepared as their tetraethylammonium salts and characterized with IR, Raman, and NMR spectroscopic methods, as well as single‐crystal X‐ray diffraction analyses. Spectroscopical data are supported by quantum‐chemical calculations. Additionally, the bonding situation of the species in question are analyzed and discussed. Furthermore, the oxidation to the corresponding halogenate(III) derivatives was studied. For [Br(OC4F9)2]−, oxidation with elemental fluorine gave [BrF2(OC4F9)2]−. Iodide was directly oxidized by ClOC4F9 to the IIII species [I(OC4F9)4]−, which is a surprisingly inert anion that might be used as a weakly coordinating anion (WCA) in the future. For [Cl(OC4F9)2]−, the decomposition products of the synthetic approaches towards a chlorine(III) system were analyzed.

Crystal structures of Zn(cyclam)I2 (second monoclinic polymorph) and Zn(cyclam)I(I3).

The asymmetric unit of the first title compound iodido-(1,4,8,11-tetra-aza-cyclo-tetra-decane-κ4 N 1,N 4,N 8,N 11)zinc(II) iodide, [ZnI(C10H24N4)]I, I, consists of the zinc-cyclam macrocyclic cation with one iodide anion coordinated to the metal ion [Zn-I = 2.6619 (5) Å] and the second present as a counter-ion. The asymmetric unit of the second title compound iodido-(1,4,8,11-tetra-aza-cyclo-tetra-decane-κ4 N 1,N 4,N 8,N 11)zinc(II) triiodide, [ZnI(C10H24N4)]I3, II, consists of half of the centrosymmetric macrocyclic cation, in which the ZnII ion coordinated to an iodide anion [Zn-I = 2.766 (2) Å] is disordered over two positions [Zn⋯Zn = 0.810 (3) Å], and of the two halves of the crystallographically non-equivalent, non-coordinated, centrosymmetric triiodide anions. In both compounds, the N,N,N,N-tetra-dentate macrocyclic ligand is present in the most energetically favored trans-III conformation. In the crystals of I, the [Zn(C10H24N4)I]+ cations and the non-coordinated iodide anions are linked by N-H⋯I and bifurcated N-H⋯(I,I) hydrogen bonds, resulting in the formation of two-dimensional networks lying parallel to the (001) and (101) planes. In contrast, the crystals of II are built up from infinite chains of the five-coordinate macrocyclic units arranged along the b-axis direction and perpendicular sheets formed of the triiodide counter-ions without significant hydrogen bonding between them.

Ultra‐Stable Cycling of High Capacity Room Temperature Sodium‐Sulfur Batteries Based on Sulfurated Poly(acrylonitrile)

AbstractWe report on a room temperature (RT) sodium‐sulfur (Na−S) battery based on a sodium anode, a sulfurated poly(acrylonitrile) (SPAN) cathode and an electrolyte containing sodium tetrakis(hexafluoroisopropyloxy) borate (Na[B(hfip)4]; hfip=hexafluoroisopropoxide) in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC). The hfip anion as a weakly coordinating anion (WCA) provides high anodic stability, high ionic conductivity, and superior electrochemical performance in carbonate‐based solvents. The Na‐SPAN cell exhibits an initial discharge capacity of 1360 and a remarkable reversible capacity of 1072 after 1000 cycles at 3 C (C=C‐rate, 5.025 ) with an insignificant average capacity decay of less than 0.021 % per cycle. A careful choice of the discharge cut‐off potential (DCP) reveals that a DCP of 0.2 V allows for stable cycling for more than 500 cycles while a DCP of 0.5 V results in a constant capacity decay. The excellent cycle stability at a DCP of 0.2 V is likely to be caused by the high conversion of the SPAN‐bound sulfur into Na2S.

Versatile Coordination of AgI and CuI Ions towards cyclo-As5 Ligands.

The reactions of the cyclo‐As5 complex [Cp*Fe(η5‐As5)] (B) with the AgI and CuI salts of the weakly coordinating anion (WCA) [FAl{OC6F10(C6F5)}3]− ([FAl]−) are studied. These reactions allow the synthesis of the mononuclear complexes [M(η5 : η2‐B)2][FAl] (M=Ag (1), Cu (2)) when a ratio of B/M(FAl) 2 : 1 is used. Compound 1 shows an unusual disorder of the central AgI cation between two π‐coordinating cyclo‐As5 ligands, which is absent in 2 pointing to a weak interaction of the Ag center towards the cyclo‐As5 ligands in B. When the ratio of B/Ag(FAl) is changed to 3 : 1 or 1 : 1, the respective coordination compounds [Ag(η2‐B)3][FAl] (3) and [Ag2(η2 : η2‐B)2][FAl]2 (4) are accessible. The coordination modes of the cyclo‐As5 units in 1, 3 and 4 are all different, reflecting the adaptive coordination behavior of B towards AgI ions. The optimized geometries in the gas phase of 1–4 are determined by DFT calculations to support the bonding situation observed in their solid‐state structures.

Design & Characterization of Pb Electrode for Sensing Trichloroethylene in Ionic Liquid/Acetonitrile Co-Solvents

Design & Characterization of Pb Electrode for Sensing Trichloroethylene in Ionic Liquid/Acetonitrile Co-Solvents

(Invited) Developing Common Descriptors for Plating/Stripping of Divalent Metals in Organic Electrolytes

A significant challenge facing the development of new chemistries for multivalent batteries is the fact that many promising electrolytes are incompatible with either metallic anodes or high voltage cathodes, both of which are required in order for such systems to compete with existing Li-ion batteries. Furthermore, the specific species responsible for delivering ions to the active electrode are generally not known, as solvation structure may change significantly relative to the bulk as solution complexes approach the electrified interface. Many Mg electrolytes in particular rely on the presence of significant concentrations of Cl- to enable reversible metal plating/stripping at the anode, which preclude their use with oxide cathodes due to parasitic reactions that take place at high voltage. Beyond this limitation, however, it is also unclear what the specific role of Cl- is in promoting reversibility in Mg and other multivalent systems, making it challenging to develop general design rules for synthesizing new electrolytes that can avoid the use of Cl- altogether. We have recently revealed that a cooperative effect between Cl- and bis-(trifluoromethane sulfonyl) imide (TFSI-) anions yields the overall more reversible behavior of mixed TFSI-/Cl- electrolytes relative to electrolytes containing only TFSI- – an effect that is general across multiple multivalent systems (i.e., Mg, Zn and Cu). Mechanistic understanding of the role of Cl- in improving reversibility of TFSI-based electrolytes is provided through the combination of electrochemical measurments with computational evaluation of the activity and stability of various TFSI- and Cl-based solution complexes. This cooperative anion effect is generalizable to dual anion electrolytes without Cl- as well, with the strength of anion association to the working cation systematically modifying the electrochemical response in direct agreement with trends predicted by theory. These results further indicate that interfacial speciation indeed deviates from that expected in the bulk, as even nominally weakly-coordinating anions appear to participate strongly in the plating/stripping reactions. Overall, this work points to the existence of a common descriptor for divalent metal plating/stripping – anion association strength – and suggests a set of general design principles for developing new multivalent electrolytes.

Multistimuli-Responsive Fluorescent Organometallic Assemblies Based on Mesoionic Carbene-Decorated Tetraphenylethene Ligands and Their Applications in Cell Imaging

Multistimuli-Responsive Fluorescent Organometallic Assemblies Based on Mesoionic Carbene-Decorated Tetraphenylethene Ligands and Their Applications in Cell Imaging

Ni-catalyzed enantioselective [2 + 2 + 2] cycloaddition of malononitriles with alkynes

Summary Efficient strategies to assemble enantioenriched pyridine derivatives are highly important, given their significance in both synthetic and medicinal chemistry. Here, we report an enantioselective nickel-catalyzed intermolecular [2 + 2 + 2] cycloaddition of alkyne-tethered malononitriles with alkynes for the synthesis of densely substituted pyridines. The α-all-carbon quaternary center adjacent to pyridine is introduced by desymmetrizing the two cyano groups of disubstituted malononitriles. Notably, terminal alkynes are also tolerated with good regioselectivity to afford tetrasubstituted pyridines. Zinc halide is essential to enable the occurrence of the transformation by promoting the hetero-cyclometallation step. The reaction uses bio-renewable 2-MeTHF as the solvent and features mild reaction conditions, good functional group compatibilities, and enantioselectivities. This study offers a straightforward approach to the valuable enantioenriched heteroarenes from feedstock chemicals by forming three bonds and one quaternary stereocenter simultaneously in a single reaction step.

Bambusuril Macrocycles as Mediators of Supramolecular Interactions: Application to the Europium Cage Helicate.

Herein, it is shown how bambusurils can be used for tuning and/or characterizing supramolecular systems. Indeed, the addition of bambusurils as anion scavengers to metal-mediated self-assemblies allows manipulation of the subtle equilibria in the given system. This is demonstrated for the case of the tetranuclear europium helical cage, which is well suited to different applications. Among the reported results, experimental evidence is provided showing that perchlorate and triflate anions act as a molecular template for the cage assembly. The complexation of inorganic anions with neutral bambusurils resulted in bulky non-coordinating counterions that may trigger the self-assembly process or stimulate specific interactions between components. Moreover, bambusuril was able to selectively remove coordinating nitrates from the mixture with non-coordinating anions, enabling the regeneration of the helical cage.

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry.

Fluoride abstraction from different types of transition metal fluoride complexes [LnMF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C2F5)3PF2 to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2F5)3PF3]−) is reported. (C2F5)3PF2 reacted with trans‐[Ni(iPr2Im)2(ArF)F] (iPr2Im=1,3‐diisopropylimidazolin‐2‐ylidene; ArF=C6F5, 1 a; 4‐CF3‐C6F4, 1 b; 4‐C6F5‐C6F4, 1 c) through fluoride transfer to form the complex salts trans‐[Ni(iPr2Im)2(solv)(ArF)]FAP (2 a‐c[solv]; solv=Et2O, CH2Cl2, THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh3, solvent coordination was suppressed and the complexes trans‐[Ni(iPr2Im)2(PPh3)(C6F5)]FAP (trans ‐2 a[PPh3]) and cis‐[Ni(iPr2Im)2(Dipp2Im)(C6F5)]FAP (cis ‐2 a[Dipp2Im]) (Dipp2Im=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were isolated. Fluoride abstraction from [(Dipp2Im)CuF] (3) in CH2Cl2 or 1,2‐difluorobenzene led to the isolation of [{(Dipp2Im)Cu}2]2+2 FAP− (4). Subsequent reaction of 4 with PPh3 and different carbenes resulted in the complexes [(Dipp2Im)Cu(LB)]FAP (5 a–e, LB=Lewis base). In the presence of C6Me6, fluoride transfer afforded [(Dipp2Im)Cu(C6Me6)]FAP (5 f), which serves as a source of [(Dipp2Im)Cu)]+. Fluoride abstraction of [Cp2TiF2] (7) resulted in the formation of dinuclear [FCp2Ti(μ‐F)TiCp2F]FAP (8) (Cp=η5‐C5H5) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.

Super-electrophiles of tri- and tetra-anions stabilized by selected terminal groups and their role in binding noble gas atoms.

Stabilization of multiply-charged atomic clusters in the gas phase has been a topic of great interest not only because of their potential applications as weakly-coordinating anions, but also for their ability to promote unusual reactions and serve as building blocks of materials. Recent experiments have shown that, after removing one terminal ligand from the closo-dodecacyano-borate, B12(CN)122-, the cluster can strongly bind an argon atom at room temperature. Bearing this in mind, here, we have developed more than a dozen highly stable tri- and tetra-anions using density functional theory (DFT) calculations with hybrid functional (B3LYP) and semi-empirical dispersion corrections. The interactions between the clusters and noble gas atoms, including Ne, Ar and Kr, are studied. The resulting super-electrophilic sites embedded in these charged clusters can bind noble gas atoms with binding energies up to 0.7 eV. This study enriches the database of highly-charged clusters and provides a viable design rule for super-electrophiles that can strongly bind noble gas atoms.

Anion binding properties of a hollow PdL-cage.

The hollow [PdL][BArF]2 complex 1 of a tetra-pyridyl (py) ligand (L) has a [Pd(py)4]2+ coordination environment. Addition of coordinating anions resulted in the formation of a neutral species with Pd(py)2(anion)2 coordination environment (12A). These species bind further to the coordinating anions in the order Cl− > N3− > Br− > I− > AcO− with Ka1 : 1 ≤ 414 M−1. With relatively non-coordinating anions 1 remains intact and displays 1 : 2 binding behaviour dominated by the 1 : 1 stoichiometry in the order NO3− (∼105 M−1) » ClO4− and BF4− (∼103 M−1). As evidenced by crystal structure data, DFT calculations and {1H–19F}-HOESY NMR with BF4−, the anions are bound by charge assisted [C–H]+···anion interactions.

Structural Snapshots of π-Arene Bonding in a Gold Germylene Cation.

Heavier group 14 element cations exhibit a remarkable reactivity that has typically hampered their isolation. For the few available examples, the role of π‐arene interactions is crucial to provide kinetic stabilization, but dynamic and structural information on those contacts is yet limited. In this study we have accessed the metalogermylenium cation [(PMe2ArDipp2)AuGe(ArDipp2)Cl]+ (4+) (ArDipp2=C6H3‐2,6‐(C6H3‐2,6‐iPr2)2) that has been structurally characterized with three different non‐coordinating counter anions. These studies provide for the first time dynamic information about the conformational rearrangement that characterizes π‐arene bonding thorough a series of X‐ray diffraction structural snapshots. Computational studies reveal the weak character of the π‐arene bonding (ca. 2 kcal mol−1) that can be described as the donation from a πC=C bond toward the empty p valence orbital of germanium.

Quadruple C-H activation coupled to hydrofunctionalization and C-H silylation/borylation enabled by weakly coordinated palladium catalyst

Unlike the well-reported 1,2-difunctionalization of alkenes that is directed by classic pyridine and imine-containing directing groups, oxo-palladacycle intermediates featuring weak Pd-O coordination have been less demonstrated in C-H activated cascade transformations. Here we report a quadruple C-H activation cascade as well as hydro-functionalization, C-H silylation/borylation sequence based on weakly coordinated palladium catalyst. The hydroxyl group modulates the intrinsic direction of the Heck reaction, and then acts as an interrupter that biases the reaction away from the classic β-H elimination and toward C-H functionalization. Mechanistically, density functional theory calculation provides important insights into the key six-membered oxo-palladacycle intermediates, and indicates that the β-H elimination is unfavorable both thermodynamically and kinetically. In this article, we focus on the versatility of this approach, which is a strategic expansion of the Heck reaction.

Catena-Poly[[[bis-(aceto-nitrile-κN)(4,4'-dimeth-oxy-2,2'-bi-pyridine-κ2 N,N')copper(II)]-μ-tri-fluoro-methane-sulfonato-κ2 O:O'] tri-fluoro-methane-sulfonate

The central copper(II) atom of the title salt, {[Cu(CF3SO3)(CH3CN)2(C12H12N2O2)](CF3SO3)} n or [[Cu(CH3CN)2(diOMe-bpy)(CF3SO3)](CF3SO3)] n where diOMe-bpy is 4,4'-dimeth-oxy-2,2'-bi-pyridine, C12H12N2O2, is sixfold coordin-ated by the N atoms of the chelating bi-pyridine ligand, the N atoms of two aceto-nitrile mol-ecules, and two tri-fluoro-methane-sulfonate O atoms in a tetra-gonally distorted octa-hedral shape. The formation of polymeric chains [Cu(CH3CN)2(diOMe-bpy)(CF3SO3)]+ n leaves voids for the non-coordinating tri-fluoro-methane-sulfonate anions that inter-act with the complex through weak hydrogen bonds. The presence of weakly coordinating ligands like aceto-nitrile and tri-fluoro-methane-sulfonate makes the title compound a convenient starting material for the synthesis of novel metal-organic frameworks.