"Catch and release" of the CpN3 ligand using cobalt: dissociation, protonation, and C-H bond thermochemistry.

The coordination chemistry of an amine-rich CpN3 ligand has been explored with cobalt. We demonstrate that in the presence of NaCo(CO)4, the cationic precursor [CpN3]+ yields the complex CpN3CoI(CO)2. While 2e- oxidation generates new CoIII complexes such as [CpN3Co(NCMe)3]2+ and CpN3CoI2(CO), subsequent ligand loss is facile, generating free [CpN3]+ or the protonated dication [CpN3H]2+. We have structurally characterized both these ligand release products via single crystal X-ray diffraction and obtained thermochemical C-H bond strengths via experiment and density functional theory (DFT). Upon reversible 1e- reduction, the radical cation [CpN3H]˙+ has a weak C-H BDFE of 52 kcal mol-1 in acetonitrile. Mechanistic analysis shows that [CpN3H]˙+ undergoes radical-radical disproportionation in the absence of exogenous H-atom acceptors, which is supported by deuterium isotope labelling experiments. Structural comparison of these organic molecules shows a high degree of iminium-like electron delocalization over the C-N bonds connected to the central five-membered ring.

Organocatalytic Enantioselective [1,2]-Stevens Rearrangement of Azetidinium Salts.

The first organocatalyzed enantioselective [1,2]-Stevens rearrangement is reported. 4-Alkylideneproline derivatives are produced in up to 86% yield and in up to 90:10 er, with recrystallization enhancing er up to >99.5:0.5. Product configuration was opposite that predicted by existing stereochemical models for this organocatalyst class, and DFT calculations revealed a novel mode of asymmetric induction. The adaptability of this catalytic strategy for asymmetric [1,2]-Stevens rearrangements of other heterocyclic amines was demonstrated.

Crystal structure and analytical profile of 1,2-diphenyl-2-pyrrolidin-1-ylethanone hydrochloride or `α-D2PV': a synthetic cathinone seized by law enforcement, along with its diluent sugar, myo-inositol.

A confiscated package of street drugs was characterized by the usual mass spectral (MS) and FT-IR analyses. The confiscated powder material was highly crystalline and was found to consist of two very different species, accidentally of sizes convenient for X-ray diffraction. Thus, one each was selected and redundant complete sets of data were collected at 100 K using Cu Kα radiation. The selected crystals contained: (a) 1,2-diphenyl-2-(pyrrolidin-1-yl)ethanone hydrochloride hemihydrate or 1-(2-oxo-1,2-diphenylethyl)pyrrolidin-1-ium chloride hemihydrate, C18H20NO+·Cl-·0.5H2O, (I), a synthetic cathinone called `α-D2PV', and (b) the sugar myo-inositol, C6H12O6, (II), probably the only instance in which the drug and its diluent have been fully characterized from a single confiscated sample. Moreover, the structural details of both are rather attractive showing: (i) interesting hydrogen bonding observed in pairwise interactions by the drug molecules, mediated by the chloride counter-anions and the waters of crystallization, and (ii) π-π interactions in the case of the phenyl rings of the drug which are of two different types, namely, π-π stacking and edge-to-π. Finally, the inositol crystallizes with Z' = 2 and the resulting diastereoisomers were examined by overlay techniques.

Accessing Ni(0) to Ni(IV) via nickel-carbon-phosphorus bond reorganization.

Two-electron oxidation of a NiIIPh(PCP) pincer complex initiates phosphine ligand insertion, generating an η6-arylphosphonium moiety coordinated to NiII. The reaction is fully reversible under reducing conditions. X-ray crystallography, NMR/EPR spectroscopy, electrochemistry, and DFT calculations support the proposed Ni-C-P bond reorganization mechanisms, which access oxidation states from Ni0 to NiIV.

Synthesis and Structure-Property Relationships in Regioisomeric Alternating Borane-Terthiophene Polymers.

Main-chain boron-containing π-conjugated polymers are attractive for organic electronic, sensing, and imaging applications. Alternating terthiophene-borane polymers were prepared and the effects of regioisomeric attachment of the conjugated linker and variations in the electronic effect of the pendent aryl groups (2,4,6-tri-tert-butylphenyl, Mes*; 2,4,6-tris(trifluoromethyl)phenyl, FMes) examined. Pd2 dba3 /P(t-Bu)3 -catalyzed Stille polymerization of arylbis(2-thienyl)borane and arylbis(3-thienylborane) with 2,5-bis(trimethylstannyl)thiophene at 120 °C gave polymers with appreciable molecular weight but MALDI-TOF MS analyses showed evidence of unusually prominent homocoupling. These defects could be suppressed by using brominated rather than iodinated monomers, more hindered 2,5-bis(tri-n-butylstannyl)thiophene as comonomer, and Pd2 dba3 /P(o-tol)3 as the catalyst at 100 °C. Under these conditions, macrocyclic species with n=3-10 repeating units formed preferentially according to MALDI-TOF MS analyses. Photophysical studies revealed a prominent effect of the regiochemistry and the nature of the pendent aryl groups on the absorption and emission, giving rise to orange, yellow-green, blue-green, and blue emissive materials respectively. The electronic effects were rationalized through DFT calculations on bis(terthiophene) model systems.

A redox-active Mn(0) dicarbene metalloradical.

We report a rare redox-active Mn0 metalloradical [Mn(CO)3(Ph2B(tBuNHC)2)]- (NHC = N-heterocyclic carbene) with countercations [K(2.2.2)crypt]+, [Na(2.2.2)crypt]+, or [Li(DME)(12-crown-4)]+ (DME = 1,2-dimethoxyethane), all characterized via single crystal X-ray diffraction. Cyclic voltammograms reveal solvation-dependent MnI/0 redox potentials that are modeled using the Born equation.

Near-Infrared-Absorbing B-N Lewis Pair-Functionalized Anthracenes: Electronic Structure Tuning, Conformational Isomerism, and Applications in Photothermal Cancer Therapy.

B-N-fused dianthracenylpyrazine derivatives are synthesized to generate new low gap chromophores. Photophysical and electrochemical, crystal packing, and theoretical studies have been performed. Two energetically similar conformers are identified by density functional theory calculations, showing that the core unit adopts a curved saddle-like shape (x-isomer) or a zig-zag conformation (z-isomer). In the solid state, the z-isomer is prevalent according to an X-ray crystal structure of a C6F5-substituted derivative (4-Pf), but variable-temperature nuclear magnetic resonance studies suggest a dynamic behavior in solution. B-N fusion results in a large decrease of the HOMO-LUMO gap and dramatically lowers the LUMO energy compared to the all-carbon analogues. 4-Pf in particular shows significant absorbance at greater than 700 nm while being almost transparent throughout the visible region. After encapsulation in the biodegradable polymer DSPE-mPEG2000, 4-Pf nanoparticles (4-Pf-NPs) exhibit good water solubility, high photostability, and an excellent photothermal conversion efficiency of ∼41.8%. 4-Pf-NPs are evaluated both in vitro and in vivo as photothermal therapeutic agents. These results uncover B-N Lewis pair functionalization of PAHs as a promising strategy toward new NIR-absorbing materials for photothermal applications.

DyKAT by DiCat: Stereoconvergent Dienamine-Catalyzed Claisen Rearrangements.

This Claisen rearrangement establishes the feasibility of DyKAT of γ-epimeric enals via dienamine formation to afford enantioenriched products. γ-Aryl and -alkyl enals, and exocyclic enals that introduce quaternary centers, are all amenable substrates. Products are readily converted into pyrrolidines or cyclopentenols. Notably, a reactive dienamine intermediate has been isolated from a catalytic reaction, fully characterized, and converted to product upon reexposure to reaction conditions. Product configuration arises from a directing C-H···π interaction in the transition state.

Coordination-Induced Weakening of a C(sp3)-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction.

The scission of a C(sp3)-H bond to form a new metal-alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C-H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C-H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C-H acidity (pKa) and bond dissociation free energy (BDFE) for a C(sp3)-H agostic interaction, showing a decrease in pKa by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar NiIII metalloradical and "doubly agostic" NiII(κ2-CH2) complex.

Electrophilic and nucleophilic displacement reactions at the bridgehead borons of tris(pyridyl)borate scorpionate complexes.

Although a wide variety of boron-based "scorpionate" ligands have been implemented, a modular route that offers facile access to different substitution patterns at boron has yet to be developed. Here, we demonstrate new reactivity patterns at the bridgehead positions of a ruthenium tris(pyrid-2-yl)borate complex that allow for facile tuning of steric and electronic properties.

Linear Extension of Anthracene via B←N Lewis Pair Formation: Effects on Optoelectronic Properties and Singlet O2 Sensitization.

The functionalization of polycyclic aromatic hydrocarbons (PAHs) via B←N Lewis pair formation offers an opportunity to judiciously fine-tune the structural features and optoelectronic properties, to suit the demands of applications in organic electronic devices, bioimaging, and as sensitizers for singlet oxygen generation. We demonstrate that the N-directed electrophilic borylation of 2,6-di(pyrid-2-yl)anthracene offers access to linearly extended acene derivatives Py-BR (R=Et, Ph, C6 F5 ). In comparison to indeno-fused 9,10-diphenylanthracene, the formal "BN for CC" replacement in Py-BR selectively lowers the LUMO, resulting in a much reduced HOMO-LUMO gap. An even more extended conjugated system with seven six-membered rings in a row (Qu-BEt) is obtained by borylation of 2,6-di(quinolin-8-yl)anthracene. Fluorinated Py-BPf shows particularly advantageous properties, including relatively lower-lying HOMO and LUMO levels, strong yellow-green fluorescence, and effective singlet oxygen sensitization, while resisting self-sensitized conversion to its endoperoxide.

Ligand Protonation at Carbon, not Nitrogen, during H2 Production with Amine-Rich Iron Electrocatalysts.

We present monometallic H2 production electrocatalysts containing electron-rich triamine-cyclopentadienyl (Cp) ligands coordinated to iron. After selective CO extrusion from the iron tricarbonyl precursors, electrocatalysis is observed via cyclic voltammetry in the presence of an exogenous acid. Contrary to the fact that amines in the secondary coordination sphere are often protonated during electrocatalysis, comprehensive quantum-chemical calculations indicate that the amines likely do not function as proton relays; instead, endo-Cp ring protonation is most favorable after 1e- reduction. This unusual mechanistic pathway emphasizes the need to consider a broad domain of H+/e- addition products by synergistically combining experimental and theoretical resources.

Electrochromic Polycationic Organoboronium Macrocycles with Multiple Redox States.

Polycationic macrocycles are attractive as they display unique molecular switching capabilities arising from their redox properties. Although diverse polycationic macrocycles have been developed, those based on cationic boron systems remain very limited. We present herein the development of novel polycationic macrocycles by introducing organoboronium moieties into a conjugated organoboron macrocyclic framework. These macrocycles consist of four bipyridylboronium units that are connected by fluorene and either electron-deficient arylborane or electron-rich arylamine moieties. Electrochemical studies reveal that the macrocycles undergo reversible multi-step redox processes with transfer of up to 10 electrons. Switchable electrochromic behavior is demonstrated via spectroelectrochemical studies and the observed color changes are rationalized by correlation with computed electronic transitions using DFT methods.

Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C-Cl bond activation.

Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd0 complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (pSpR, meso-isomer) and 1b (pSpS-isomer) differ in the orientation of the ferrocene moieties relative to the central Ph2PC5H3-Hg-C5H3PPh2 bridging entity. The structurally distinct trigonal LPd0(dba) complexes 2a (meso) and 2b (pSpS) are obtained upon treatment with Pd(dba)2. A competition reaction reveals that 1b reacts faster than 1a with Pd(dba)2. Unexpectedly, catalytic interconversion of 1a (meso) into 1b (rac) is observed at room temperature in the presence of only catalytic amounts of Pd(dba)2. Both Pd0 complexes, 2a and 2b, readily undergo oxidative addition into the C-Cl bond of CH2Cl2 at moderate temperatures with formation of the square-planar trans-chelate complexes LPdIICl(CH2Cl) (3a, 3b). Kinetic studies reveal a significantly higher reaction rate for the meso-isomer 2a in comparison to (pSpS)-2b.

New Palladium(II) and Platinum(II) Complexes Based on Pyrrole Schiff Bases: Synthesis, Characterization, X-ray Structure, and Anticancer Activity.

New palladium (Pd)II and platinum (Pt)II complexes (C1-C5) from the Schiff base ligands, R-(phenyl)methanamine (L1), R-(pyridin-2-yl)methanamine (L2), and R-(furan-2-yl)methanamine (L3) (R-(E)-N-((1H-pyrrol-2-yl) methylene)) are herein reported. The complexes (C1-C5) were characterized by FTIR, 1H and 13C NMR, UV-vis, and microanalyses. Single-crystal X-ray crystallographic analysis was performed for the two ligands (L1-L2) and a Pt complex. Both L1 and L2 belong to P21/n monoclinic and P-1 triclinic space systems, respectively. The complex C5 belongs to the P21/c monoclinic space group. The investigated molar conductivity of the complexes in DMSO gave the range 4.0-8.8 µS/cm, suggesting neutrality, with log P values ≥ 1.2692 ± 0.004, suggesting lipophilicity. The anticancer activity and mechanism of the complexes were investigated against various human cancerous (Caco-2, HeLa, HepG2, MCF-7, and PC-3) and noncancerous (MCF-12A) cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and Apopercentage assays, respectively. C5 demonstrated strong DNA-binding affinity for calf thymus DNA (CT-DNA) with a binding constant of 8.049 × 104 M-1. C3 reduced cell viability of all the six cell lines, which included five cancerous cell lines, by more than 80%. The C5 complex also demonstrated remarkably high selectivity with no cytotoxic activity toward the noncancerous breast cell line but reduced the viability of the five cancerous cell lines, which included one breast cancer cell line, by more than 60%. Further studies are required to evaluate the selective toxicity of these two complexes and to fully understand their mechanism of action.

Serendipity: Werner's argument that the `two-only forms' (green and violet) of [CoCl2(en)2]+ salts demanded his octahedral model was correct. True?

Chemists of the late 19th century, including Alfred Werner, prepared salts containing either green or violet cations of composition [CoCl2(en)2]+ (en is ethylenediamine, C2H8N2); we now refer to these as trans-dichloro and cis-dichloro species. We have discovered a third salt, purple in color, containing cations of the same elemental composition and whose asymmetric unit composition is [CoCl2(en)2]2Cl2·3H2O, in which the cobalt cations are a cis:trans dichloro pair. Such a discovery would undermine Werner's argument that if only two forms can be prepared, his octahedral theory was proven. Probably because his students never examined their crystals under a microscope, they failed to observe the `third' species, thereby ruining Werner's argument since he relied strictly on color to identify them. That was fortunate since our purple salt would have led him to abandon, or certainly delay, his momentous discovery. Our crystals consist of a 1:1 mixture of the cis and trans cations, thereby sharing the same elemental analysis and conductivity as the single salts, but not their crystal structure, inasmuch as X-ray diffraction had not even been discovered then. Serendipitously, our discovery would have been a great boon to his theoretical acumen, while his `two-color' argument may have doomed him.

Enhancing the Acceptor Character of Conjugated Organoborane Macrocycles: A Highly Electron-Deficient Hexaboracyclophane.

We introduce a new boron-doped cyclophane, the hexabora[16 ]cyclophane B6-F Mes, in which six tricoordinate borane moieties alternate with short conjugated p-phenylene linkers. Exocyclic 2,4,6-tris(trifluoromethyl)phenyl (F Mes) groups serve not only to further withdraw electron density but at the same time sterically shield the boron atoms, resulting in a macrocycle that is both highly electron-deficient and stable. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies uncover unique properties of B6-F Mes, including a low-lying and extensively delocalized LUMO and a wide HOMO-LUMO gap, which arise from the combination of a cyclic π-system, strong electronic communication between the closely spaced borons, and the attachment of electron-deficient pendent groups. The binding of small anions to the electron-deficient macrocycle and molecular model compounds is investigated and emissive exciplexes are detected in aromatic solvents.

Lewis Pairs as Highly Tunable Dynamic Cross-Links in Transient Polymer Networks.

Classical Lewis pairs (LPs) between unhindered electron-poor Lewis acids (LAs) and electron-rich Lewis bases (LBs) present an overlooked motif with tremendous potential as dynamic cross-links in transient polymer networks (TPNs) for self-healing and stimuli-responsive applications. We demonstrate that simple and intuitive matching of weak/strong organoborane LA and amine LB pairs offers access to a large set of binding equilibrium constants, Keq, that span ∼6 orders and dissociation rate constants, kdiss, that span ∼7 orders of magnitude. The implementation in polystyrene (PS)/polydimethylsiloxane (PDMS) blends results in dynamically cross-linked networks with bulk thermomechanical properties that are directly correlated with the strength and kinetic parameters for the LP interactions. The LP dynamic cross-link design is highly versatile and broadly applicable to different polymer architectures as demonstrated in the formation of reprocessable elastomers from Lewis base-decorated high molecular weight PDMS in combination with Lewis acid-decorated PS when reinforced with fumed silica as a filler.

Diferrocenylmercury diphosphine diastereomers with unique geometries: trans-chelation at Pd(ii) with short Hg(ii)Pd(ii) contacts.

Diphosphine chelate ligands are essential in many catalytic processes with both the electronic structure and bite angle having a dramatic influence on the coordination behavior and catalytic performance. The synthesis of a new class of diferrocenylmercury-supported diphosphine chelate ligands was accomplished by the reaction of (ortho-diphenylphosphino)ferrocenyl sulfinate (2) with t-BuLi, followed by treatment with mercury(ii) chloride. Two diastereomers, 4a (pSpR-, meso-isomer) and 4b (pSpS-isomer), differ in the orientation of the ferrocene moiety relative to the central Ph2PC5H3-Hg-C5H3PPh2 bridging entity. They were isolated independently and fully characterized in solution and in the solid state by single crystal X-ray diffraction analysis. Key characteristics of these ligands are their exceptionally wide and flexible bite angles and the unique stereochemical environment that is achieved upon coordination to transition metals. Complexation to Pd(ii)Cl2 gives rise to unusual square-planar trans-chelate complexes 5a (meso) and 5b (pSpS). In competition reactions, 4a and 4b show similar reactivity toward Pd(ii)Cl2. The molecular structures of 5a and 5b exhibit short PdHg contacts, possibly indicating secondary metallophilic interactions as further evidenced by bond-critical points between Pd and Hg that were identified by AIM analyses.

Electron-Deficient Borinic Acid Polymers: Synthesis, Supramolecular Assembly, and Examination as Catalysts in Amide Bond Formation.

The Lewis acidic character of borinic-acid-functionalized polymers suggests broad potential applications in supramolecular materials, chemo- and biosensors, as well as supported catalysts. Two highly electron-deficient borinic acid copolymers (3 a and 3 b) with variable steric hindrance at the boron center were prepared by reaction of aryldibromoboranes ArBBr2 (2, Ar=2,4-Cl2 Ph, 3,5-Cl2 Ph) with a 10 % stannylated polystyrene random copolymer, followed by conversion to the desired PS-B(Ar)OH functionalities. The supramolecular assembly of these polymers through Lewis acid-Lewis base interactions and reversible covalent B-O-B bond formation was investigated. Exposure of a polymer solution of 3 a to pyridine triggered spontaneous gelation, whereas 3 b only gelled upon addition of molecular sieves to favor formation of boroxane crosslinks. The crosslinking process was readily reversed by addition of small amounts of water or wet solvent. The dynamic processes were studied in detail by variable-temperature (VT) NMR by using molecular model compounds. The polymers and their corresponding model compounds were also examined as catalysts in the amide bond formation reaction between phenylacetic acid and benzylamine. The 3,5-dichlorophenyl borinic acid derivatives proved to be the more effective catalysts. Mechanistic studies suggested that the borane Lewis acid-catalyzed coupling involves initial acid-induced protodeboronation to release the dichlorophenyl boronic acid as the active catalyst.