Pulsed gradient spin-echo (pgse) diffusion and 1H,19F heteronuclear Overhauser spectroscopy (HOESY) NMR methods in inorganic and organometallic chemistry: something old and something new

Platinum Elimination from Bis(triethylsilylpolyynyl) Complexes (n-Bu2C(CH2PPh2)2)Pt((C≡C)nSiEt3)2 (n=2, 3) and Macrocycles Comprised of Four L2Pt Corners and Four C≡CC≡CC≡C Linkers; An Approach to Cyclo[24]carbon

The overarching goal of this study is to effect the elimination of platinum from adducts with cis -C≡C-Pt-C≡C- linkages, thereby generating novel conjugated polyynes. Thus, the bis(hexatriynyl) complex trans-(p-tol3P)2Pt((C≡C)3H)2 is treated with 1,3-diphosphines R2C(CH2PPh2)2 to generate (R2C(CH2PPh2)2)2Pt((C≡C)3H)2 (14; R=c, n-Bu; e, p-tolCH2). These condense with the diiodide complexes R2C(CH2PPh2)2PtI2 (9 a,c) in the presence of CuI (cat.) and excess HNEt2 to give the title macrocycles [(R2C(CH2PPh2)2)Pt(C≡C)3]4 (16 c,e) as adducts of the byproduct [H2NEt2]+ I- (30-66 %). DOSY NMR experiments establish that this association is maintained in solution, but NaOAc removes the ammonium salt. The bis(triethylsilylpolyynyl) complexes (n-Bu2C(CH2PPh2)2)Pt((C≡C)nSiEt3)2 (n=2, 3) are synthesized analogously to 14 c. They react with I2 at rt to give mainly the diiodide complex 9 c and the coupling product Et3Si(C≡CC≡C)nSiEt3. The possibility of competing reactions giving IC≡C species is investigated. Analogous reactions of the Pt4C24 macrocycle 16 c also give 9 c, but no sp 13C NMR signals or mass spectrometric Cx z+ ions (x=24-100) could be detected. It is proposed that some cyclo[24]carbon is generated, but then rapidly converts to other forms of elemental carbon. No cyclotetracosane (C24H48) is detected when this sequence is carried out in the presence of PtO2 and H2.

Polymer Molecular Weights via DOSY NMR

Diffusion-ordered spectroscopy (DOSY) ¹H nuclear magnetic resonance (¹H NMR) has become a powerful tool to characterize the molecular weights of polymers. Compared to common characterization techniques, such as size exclusion chromatography (SEC), DOSY is faster, uses less solvent, and does not require a purified polymer sample. Poly(methyl methacrylate) (PMMA), polystyrene (PS), and polybutadiene (PB) molecular weights were determined by the linear correlation between the logarithm of their diffusion coefficients (D) and the logarithm of their molecular weights based on SEC molecular weights. Here, we emphasize the importance of the preparation needed to generate the calibration curves, which includes choosing the correct pulse sequence, optimizing parameters, and sample preparation. The limitation of the PMMA calibration curve was investigated by increasing the dispersity of PMMA. Additionally, by accounting for viscosity in the Stokes-Einstein equation, a variety of solvents were used to produce a "universal" calibration curve for PMMA to determine molecular weight. Furthermore, we place a spotlight on the increasing importance of DOSY NMR being incorporated into the polymer chemist's toolbox.

Exploration of Solid-State vs Solution-State Structure in Contact Ion Pair Systems: Synthesis, Characterization, and Solution-State Dynamics of Zinc Diphenyl Phosphate, [Zn{O2P(OPh)2}2], Donor-Base-Supported Complexes

A family of zinc phosphate complexes supported by nitrogen donor-base ligands have been synthesized, and their molecular structures were identified in both the solid (X-ray crystallography) and solution state (DOSY NMR spectroscopy). [Zn{O₂P(OPh)₂}₂]_∞ (1), formed from the reaction of Zn[N(SiMe₃)₂]₂ with HO(O)P(OPh)₂ coordinates to donor-base ligands, i.e., pyridine (Py), 4-methylpyridine (4-MePy), 2,2-bipyridine (bipy), tetramethylethylenediamine (TMEDA), pentamethyldiethylenetriamine (PMDETA), and 1,3,5-trimethyl-1,3,5-triazacyclohexane (Me₃-TAC), to produce polymeric 1D structures, [(Py)₂Zn{O₂P(OPh)₂}₂]_∞ (2) and [(4-MePy)₂Zn{O₂P(OPh)₂}₂]_∞ (3), the bimetalic systems, [(Bipy)Zn{O₂P(OPh)₂}₂]₂ (4), [(TMEDA)Zn{O₂P(OPh)₂}₂]₂ (5), and [(Me₃-TAC)Zn{O₂P(OPh)₂}₂]₂ (7), as well as a mono-nuclear zinc bis-diphenylphosphate complex, [(PMDETA)Zn{O₂P(OPh)₂}₂] (6). ¹H NMR DOSY has been used to calculate averaged molecular weights of the species. Studies are consistent with the disassembly of polymeric 3 into the bimetallic species [(Me-Py)₂·Zn₂{O₂P(OPh)₂}₄], where the Me-Py ligand is in rapid exchange with free Me-Py in solution. Further ¹H DOSY NMR studies of 4 and 5 reveal that dissolution of the complex results in a monomer dimer equilibrium, i.e., [(Bipy)Zn{O₂P(OPh)₂}₂]₂ ⇆ 2[(Bipy)Zn{O₂P(OPh)₂}₂] and [(TMEDA)Zn{O₂P(OPh)₂}₂]₂ ⇆ 2[(TMEDA)Zn{O₂P(OPh)₂}₂], respectively, in which the equilibria lie toward formation of the monomer. As part of our studies, variable temperature ¹H DOSY experiments (223 to 313 K) were performed upon 5 in d₈-tol, which allowed us to approximate the enthalpy [ΔH = -43.2 kJ mol^-1 (±3.79)], entropy [ΔS = 109 J mol^-1 K^-1 (±13.9)], and approximate Gibbs free energy [ΔG = 75.6 kJ mol^-1 (±5.62) at 293 K)] of monomer-dimer equilibria. While complex 6 is shown to maintain its monomeric solid-state structure, ¹H DOSY experiments of 7 at 298 K reveal two separate normalized diffusion coefficients consistent with the presence of the bimetallic species [(TAC)_2-xZn₂{O₂P(OPh)₂}₄], (x = 1 or 0) and free TAC ligand.

Are Ar3SbCl2 Species Lewis Acidic? Exploration of the Concept and Pnictogen Bond Catalysis Using a Geometrically Constrained Example

As part of our investigations into the Lewis acidic behavior of antimony derivatives, we have decided to study the properties of 5-phenyl-5,5-dichloro-λ5-dibenzostibole (1), a dichlorostiborane with an antimony atom confined to a five-membered heterocycle. Our work shows that the resulting geometrical constraints elevate the Lewis acidity of the antimony atom, as confirmed by the crystal structure of 1-THF and the solution study of the interaction of 1 with Ph3PO. The enhanced Lewis acidic properties of 1, which exceed those of simple dichlorostiboranes such as Ph3SbCl2, also become manifest in pnictogen bonding catalysis experiments involving the reductions of imines with Hantzsch ester. The influence of geometrical constraints in the chemistry of this compound is also supported by a computational activation strain analysis as well as by an energy decomposition analysis of a model Me3PO adduct.

Solvent Effects Used for Optimal Simultaneous Analysis of Amino Acids via 19F NMR Spectroscopy

¹⁹F NMR has been extensively used in simultaneous analysis of multicomponent due to its 100% natural isotope abundance, high NMR-sensitivity, and wide-range chemical shifts. The solvent effects are usually observed in NMR spectroscopy and cause large changes in ¹⁹F chemical shifts. Herein, we propose that the simultaneous analysis of a complex mixture can be achieved using solvent effects via ¹⁹F NMR spectroscopy, such as a mixture solution of amino acids (AAs). AAs are not only cell-signaling molecules, but are also considered as biomarkers of some diseases. Hence, the analysis of AAs is important for human health and the diagnosis of diseases. In this work, the key to the success of sensing 19 biogenic AAs is the use of 2-fluorobenzaldehyde (2FBA) as a highly sensitive derivatizing agent and solvent effects to produce distinguishable ¹⁹F NMR signals. As a result, the resolution of ¹⁹F NMR spectroscopy of multiple 2FBA-labeled AAs is obviously higher than other methods based on ¹⁹F NMR. Moreover, 14 and 18 AAs can be satisfactorily differentiated and unambiguously identified in different complicated media supporting the growth of mammalian cells. Furthermore, quantification of the concentration of AAs can be made, and the limit of detection reaches 10 μM. Our work provides new insights into the simultaneous analysis of a multicomponent mixture based on solvent effects by ¹⁹F NMR spectroscopy.

Acetylides for the Preparation of Phosphorescent Iridium(III) Complexes: Iridaoxazoles and Their Transformation into Hydroxycarbenes and N,C(sp3),C(sp2),O-Tetradentate Ligands

The preparation of three families of phosphorescent iridium(III) emitters, including iridaoxazole derivatives, hydroxycarbene compounds, and N,C(sp³),C(sp²),O-tetradentate containing complexes, has been performed starting from dimers cis-[Ir(μ²-η²-C≡CR){κ²-C,N-(MeC₆H₃-py)}₂]₂ (R = ^tBu (1a), Ph (1b)). Reactions of 1a with benzamide, acetamide, phenylacetamide, and trifluoroacetamide lead to the iridaoxazole derivatives Ir{κ²-C,O-[C(CH₂^tBu)NC(R)O]}{κ²-C,N-(MeC₆H₃-py)}₂ (R = Ph (2), Me (3), CH₂Ph (4), CF₃ (5)) with a fac disposition of carbons and heteroatoms around the metal center. In 2-methyltetrahydrofuran and dichloromethane, water promotes the C-N rupture of the IrC-N bond of the iridaoxazole ring of 3-5 to form amidate-iridium(III)-hydroxycarbene derivatives Ir{κ¹-N-[NHC(R)O]}{κ²-C,N-(MeC₆H₃-py)}₂{═C(CH₂^tBu)OH} (R = Me (6), CH₂Ph (7), CF₃ (8)). In contrast to 1a, dimer 1b reacts with benzamide and acetamide to give Ir{κ⁴-N,C,C',O-[py-MeC₆H₃-C(CH₂-C₆H₄)NHC(R)O]}{κ²-C,N-(MeC₆H₃-py)}(R = Ph (9), Me (10)), which bear a N,C(sp³),C(sp²),O-tetradentate ligand resulting from a triple coupling (an alkynyl ligand, an amide, and a coordinated aryl group) and a C-H bond activation at the metal coordination sphere. Complexes 2-4 and 6-10 are emissive upon photoexcitation, in orange (2-4), green (6-8), and yellow (9 and 10) regions, with quantum yields between low and moderate (0.01-0.50) and short lifetimes (0.2-9.0 μs).

A new supramolecular bonding motif involving NH bonds of ammonium salts and macrocycles derived from platinum corners and butadiynediyl linkers

The title Pt4C16 species can give roughly planar geometries in crystals, but afford folded forms with ammonium cations, which are captured like unsuspecting prey and potentially fossilized as with the ancient creatures who once roamed the galaxy.

A generalized kinetic model for compartmentalization of organometallic catalysis

Compartmentalization is an attractive approach to enhance catalytic activity by retaining reactive intermediates and mitigating deactivating pathways. Such a concept has been well explored in biochemical and more recently, organometallic catalysis to ensure high reaction turnovers with minimal side reactions. However, the scarcity of theoretical frameworks towards confined organometallic chemistry impedes broader utility for the implementation of compartmentalization. Herein, we report a general kinetic model and offer design guidance for a compartmentalized organometallic catalytic cycle. In comparison to a non-compartmentalized catalysis, compartmentalization is quantitatively shown to prevent the unwanted intermediate deactivation, boost the corresponding reaction efficiency (γ), and subsequently increase catalytic turnover frequency (TOF). The key parameter in the model is the volumetric diffusive conductance (F_V) that describes catalysts' diffusion propensity across a compartment's boundary. Optimal values of F_V for a specific organometallic chemistry are needed to achieve maximal values of γ and TOF. As illustrated in specific reaction examples, our model suggests that a tailored compartment design, including the use of nanomaterials, is needed to suit a specific organometallic catalytic cycle. This work provides justification and design principles for further exploration into compartmentalizing organometallics to enhance catalytic performance. The conclusions from this work are generally applicable to other catalytic systems that need proper design guidance in confinement and compartmentalization.

Compartmentalization is an attractive approach to enhance catalytic activity by retaining reactive intermediates and mitigating deactivating pathways.

Unveiling the mechanisms of organic room-temperature phosphorescence in various surrounding environments: a computational study

Room-temperature phosphorescence (RTP) from pure organic materials has been promising in next-generation OLEDs. Understanding the photophysical properties of RTP molecules is attractive but challenging. In this study, through a combined quantum mechanics and molecular mechanics (QM/MM) method taking 2-(3,4-dimethoxybenzyl)isoindoline-1,3-dione (complex b) as an example, we comparatively investigate the photophysical properties of complex b in diverse environments (solution, crystal, and amorphous). From solution to amorphous to crystal phase, the excited-state decay rates for the molecule indicate that the AIE phenomenon of complex b is mainly induced by the increased phosphorescence rates. However, the increased nonradiative decay rate k_nr of T₁ → S₀ from the solution to the crystal phase could be attributed to the different electron coupling in the crystal phase. Moreover, the theoretical results also show that the small energy gap between the lowest singlet excited state (S₁) and triplet excited state (T₁) and low reorganization energy can help enhance intersystem crossing to facilitate a more competitive radiative process from the T₁ state to ground state (S₀). Additionally, the stronger intermolecular π-π interaction can cause high phosphorescence quantum efficiency in the crystalline phase. Our study presents a rational explanation for aggregation-induced RTP, which is beneficial for the design of new organic RTP materials in the future.

ThIV-Desferrioxamine: characterization of a fluorescent bacterial probe

Diversifying our ability to guard against emerging pathogenic threats is essential for keeping pace with global health challenges, including those presented by drug-resistant bacteria. Some modern diagnostic and therapeutic innovations to address this challenge focus on targeting methods that exploit bacterial nutrient sequestration pathways, such as the desferrioxamine (DFO) siderophore used by Staphylococcus aureus (S. aureus) to sequester FeIII. Building on recent studies that have shown DFO to be a versatile vehicle for chemical delivery, we show proof-of-principle that the FeIII sequestration pathway can be used to deliver a potential radiotherapeutic. Our approach replaces the FeIII nutrient sequestered by H4DFO+ with ThIV and made use of a common fluorophore, FITC, which we covalently bonded to DFO to provide a combinatorial probe for simultaneous chelation paired with imaging and spectroscopy, H3DFO_FITC. Combining insight provided from FITC-based imaging with characterization by NMR spectroscopy, we demonstrated that the fluorescent DFO_FITC conjugate retained the ThIV chelation properties of native H4DFO+. Fluorescence microscopy with both [Th(DFO_FITC)] and [Fe(DFO_FITC)] complexes showed similar uptake by S. aureus and increased intercellular accumulation as compared to the FITC and unchelated H3DFO_FITC controls. Collectively, these results demonstrate the potential for the newly developed H3DFO_FITC conjugate to be used as a targeting vector and bacterial imaging probe for S. aureus. The results presented within provide a framework to expand H4DFO+ and H3DFO_FITC to relevant radiotherapeutics (like 227Th).

Correlations between ligand field Δ o, spin crossover T 1/2 and redox potential E pa in a family of five dinuclear helicates

A family of five new bis-bidentate azole-triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [FeII 2L3]4+, has been prepared and characterised. X-ray crystal structures show all five Fe(ii) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D3-MeCN solution, regardless of HS fraction: γ HS(298 K) = 0-0.81. Variable temperature 1H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe2L3]4+ tunes: (a) solution SCO T 1/2 from 247 to 471 K, and (b) reversible redox potential, E m(FeII/III), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, E pa = 0.78 V, vs. 0.01 M AgNO3/Ag. For the four reversible redox systems, a strong correlation (R 2 = 0.99) is observed between T 1/2 and E pa. Finally, the analogous Ni(ii) helicates have been prepared to obtain Δ o, establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm-1), (b) that Δ o ([NiII 2L3]4+) strongly correlates (R 2 = 0.87) with T 1/2 ([FeII 2L3]4+), and (c) interestingly that Δ o strongly correlates (R 2 = 0.98) with E pa for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(ii) to Fe(iii).

Quantitative quadrupolar NMR (qQNMR) using nitrogen-14 for the determination of choline in complex matrixes

NMR offers the unique potential to selectively excite the chosen nuclei avoiding in an extraordinary way the matrix effect. Quantitative Nitrogen-14 NMR (¹⁴N qNMR) spectroscopy has been introduced for the first time as a robust and validated method to determine choline in a variety of matrixes including quinoa grains, instant coffee and food supplements. A study about the ion pairing of choline bitartrate in aqueous solution by means of diffusion PGSE, NOESY and HOESY NMR have been also provided. Validation of the method within eight concentrations levels (from 1.58 to 79.0 mM) afforded a limit of detection of 400 μg/mL (1.58 mM), a quantification limit of 1000 μg/mL (3.95 mM), excellent linearity (R² higher than 0.999), intra-/inter-day precisions lower than 1.24% (CV), recoveries of 93.5%-102.5%, and complete absence of matrix effect. The fast and reliable quantification of choline together with the accuracy and simplicity of this new approach make it useful in the development of analytical procedures that could dramatically affect traditional analysis.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) have recently garnered enhanced development efforts from research disciplines owing to their superior optical and optoelectronic properties. These materials, however, are unlike conventional quantum dots, because they possess strong ionic character, labile ligand coverage, and overall stability issues. As a result, the system as a whole is highly dynamic and can be affected by slight changes of particle surface environment. Specifically, the surface ligand shell of LHP NCs has proven to play imperative roles throughout the lifetime of a LHP NC. Recent advances in engineering and understanding the roles of surface ligand shells from initial synthesis, through postsynthetic processing and device integration, finally to application performances of colloidal LHP NCs are covered here.

New Insights into the Binding Site and Affinity of the Interaction between Biotin and PAMAMs-NH2 via NMR Studies

Biotin-dendrimer conjugates (such as biotin-PAMAMs-NH₂) are important macromolecules in the field of host-guest chemistry and widely used systems for delivery. The similar chemical structures of the inner and outer layers of PAMAM-NH₂ make it difficult to illuminate the interaction and the binding affinity of biotin-PAMAMs-NH₂. By utilizing NMR techniques including ¹H NMR titration, CSSF-TOCSY, STDD methods, and 2D DOSY analysis, we demonstrate a method to sort out these interactions. The methylene protons of the inner and outer layers of PAMAM-NH₂ are successfully identified and accurately positioned so that the carboxylic acid groups of biotins are having ionic interactions with the outermost amine groups of PAMAM-NH₂. The inner PAMAM-NH₂ is protonated when reaching the isoelectric point of PAMAM-NH₂, increasing the hydrodynamic radius. On the basis of the NMR experiments, a model is proposed, where the carboxylic acid groups and heterocyclic skeleton of biotin arched over the outer layers of PAMAM-NH₂ like a bridge. Furthermore, using STDD epitope mapping, the binding affinity between biotin and PAMAM-NH₂ was quantified. The diffusion behavior of biotin-G5 PAMAM-NH₂ complex is more complicated than that of biotin-G3 PAMAM-NH₂ complex due to steric hindrance. The results provide a theoretical basis for understanding these complicated drug delivery systems.

Reactions of POP-pincer rhodium(I)-aryl complexes with small molecules: coordination flexibility of the ether diphosphine

Reactions of the aryl complexes Rh(aryl){κ3-P,O,P-[xant(PiPr2)2]} (1; aryl = 3,5-Me2C6H3 (a), C6H5 (b), 3,5-Cl2C6H3 (c), 3-FC6H4 (d); xant(PiPr2)2 = 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene) with O2, CO, and MeO2CC≡CCO2Me have been performed. Under 1 atm of O2, the pentane solutions of complexes 1 afford the dinuclear peroxide derivatives [Rh(aryl){κ2-P,P-xant(PiPr2)2}]2(μ-O2)2 (2a–2d) as yellow solids. In solution, these species are unstable. In dichloromethane, at room temperature, they are transformed into the dioxygen adducts Rh(aryl)(η2-O2){κ3-P,O,P-[xant(PiPr2)2]} (3a–3d), as a result of the rupture of the double peroxide bridge and the reduction of the metal center. Complex 3b decomposes in benzene, at 50 °C, to give diphosphine oxide, phenol, and biphenyl. Complexes 1 react with CO to give the square-planar mono carbonyl derivatives Rh(aryl)(CO){κ2-P,P-[xant(PiPr2)2]} (4a–4d), which under carbon monoxide atmosphere evolve to benzoyl species Rh{C(O)aryl}(CO){κ2-P,P-[xant(PiPr2)2]} (5a–5d), resulting from the migratory insertion of CO into the Rh-aryl bond and the coordination of a second CO molecule. The transformation is reversible; under vacuum, complexes 5 regenerate the precursors 4. The addition of the activated alkyne to complexes 1b and 1d initially leads to the π-alkyne intermediates Rh(aryl){η2-C(CO2Me)≡C(CO2Me)}{κ3-P,O,P-[xant(PiPr2)2]} (6b, 6d), which evolve to the alkenyl derivatives Rh{(E)-C(CO2Me)=C(CO2Me)aryl}{κ3-P,O,P-[xant(PiPr2)2]} (7b, 7d). The diphosphine adapts its coordination mode to the stability requirements of the different complexes, coordinating cis-κ2-P,P in complexes 2, fac-κ3-P,O,P in compounds 3, trans-κ2-P,P in the mono carbonyl derivatives 4 and 5, and mer-κ3-P,O,P in products 6 and 7.

Self-Assembly of Benzimidazole-Derived Tris-NHC Ligands and AgI -Ions to Hexanuclear Organometallic Cages and Their Unusual Transmetalation Chemistry

Multi-ligand self-assembly to attain the Ag^I -N-heterocyclic carbene (NHC)-built hexanuclear organometallic cages of composition [Ag₆ (3 a,b)₄ ](PF₆ )₆ from the reaction of benzimidazole-derived tris(azolium) salts [H₃ -3 a,b](PF₆ )₃ with Ag₂ O was achieved. The molecular structures of the cages were established by X-ray diffraction studies along with NMR and MS analyses. The existence of a single assembly in solution was supported by diffusion-ordered spectroscopy (DOSY) ¹ H NMR spectra. Further, transmetalation reactions of these self-assembled complexes, [Ag₆ (3 a,b)₄ ](PF₆ )₆ , with Cu^I /Au^I -ions provided various coinage metal-NHC complexes having diverse molecular compositions, which included the first example of a hexanuclear Cu^I -dodecacarbene complex, [Cu₆ (3 b)₄ ](PF₆ )₆ .

Hierarchical self-assembly and controlled disassembly of a cavitand-based host–guest supramolecular polymer

Two hierarchical aggregation modes of cavitand-based supramolecular polymers allow implementing orthogonal disassembly procedures: electrochemical reduction for linear chains and solvent-driven dissolution for bundles.

Analysis of Ion Pairing in Solid State and Solution in p-Cymene Ruthenium Complexes

The importance of ion pairing in different fields of chemistry is widely recognized. In this work, we have synthesized a set of cationic p-cymene ruthenium complexes of general formula [(p-cym)Ru(L')(κ²-O^N-L)]X (p-cym = p-cymene; L' = N-methylimidazole (MeIm), N-ethylpiperidylimidazole (EpipIm), 1,3,5-triaza-7-phosphaadamantane (PTA); L = 2-(1H-benzimidazol-2-yl)phenolato (L1), 2-(1,3-benzothiazol-2-yl)phenolato (L2); X = Cl^-, BF₄^-, OTf^-, BPh₄^-). X-ray diffraction studies on selected complexes revealed relatively strong anion-cation interactions in the solid state mainly based on N-H···X (X = Cl, F, O) and C-H···π interactions, also observed in the DFT-modeled complexes in the gas phase. Moreover, NMR studies showed that they exist as intimate ion pairs in solution and, remarkably, as head-to-tail quadruples in the particular case of the cation [(p-cym)Ru(MeIm)(κ²- O^N-L1)]⁺ ([1]⁺) with Cl^- and BPh₄^- as counteranions. Furthermore, a value of ΔG = -2.9 kcal mol^-1 at 299 K has been estimated for the equilibrium {[1]BPh₄···[1]BPh₄} ⇆ 2{[1]⁺···BPh₄^-} in concentrated CDCl₃ solutions. In addition, preliminary studies concerning the cytotoxic properties against HeLa cell lines of the derivatives suggested a positive effect derived from the presence of the lipophilic BPh₄^- anion and also from the NH group of the benzimidazolyl fragment.

The extent of counterion dissociation at the interface of cationic diblock copolymer nanoparticles in non-polar solvents

HYPOTHESIS

Diblock copolymer nanoparticles prepared in non-polar solvents that are sterically stabilized but possess ionic functionality from the inclusion of cationic comonomers in the stabilizer shell are known to exhibit complex electrokinetic behavior (Chem. Sci. 9 (2018) 922-934). For example, core-shell nanoparticles with cationic comonomers located solely within the shell layer have lower magnitude electrophoretic mobilities than nanoparticles containing the same cationic comonomers located within the core, whereas nanoparticles prepared using a minor fraction of steric stabilizer chains containing cationic comonomer repeat units have comparable electrophoretic mobilities to nanoparticles prepared with this cationic comonomer solely located within the core. We hypothesize that these observations can be explained in terms of the strength of the Coulombic interaction between counterions and the nanoparticle interface.

EXPERIMENTS

The highly-fluorinated anionic counterion associated with these cationic nanoparticles is studied by ¹⁹F nuclear magnetic resonance (NMR) spectroscopy in n-dodecane. This revealed only one type of ¹⁹F environment for a soluble macromolecular cation (the oil-soluble steric stabilizer chains used to prepare the nanoparticles), whereas two distinct environments were observed for the sterically-stabilized cationic nanoparticles. Both ¹⁹F diffusion NMR and ¹⁹F-¹³C heteronuclear single quantum correlation (HSQC) measurements support the existence of two environments for this counterion.

FINDINGS

The existence of two distinct ¹⁹F environments for the highly-fluorinated anion associated with the sterically-stabilized nanoparticles demonstrates the presence of spectroscopically distinguishable populations of ion pairs and of fully dissociated free anions. ¹⁹F NMR spectra recorded for sterically-stabilized nanoparticles with a fully ionic shell (all stabilizer chains containing the cationic comonomer) and those with a partly ionic shell (10% of stabilizer chains containing the cationic comonomer) reveal a higher proportion of dissociated anions in the partly ionic case. This suggests a stronger Coulombic interaction between counterions and the cationic interface when the shell is fully ionic, which accounts for the observed reduction in the magnitude of the electrophoretic mobility.

Binary Donor-Acceptor Adducts of Tetrathiafulvalene Donors with Cyclic Trimetallic Monovalent Coinage Metal Acceptors

Reactions between the π-acidic cyclic trimetallic coinage metal(I) complexes {[Cu(μ-3,5-(CF₃)₂pz)]₃, {[Ag(μ-3,5-(CF₃)₂pz)]₃, and {[Au(μ-3,5-(CF₃)₂pz)]₃ with TTF, DBTTF and BEDT-TTF give rise to a series of coinage metal(I)-based new binary donor-acceptor adducts {[Cu(μ-3,5-(CF₃)₂pz)]₃DBTTF} (1), {[Ag(μ-3,5-(CF₃)₂pz)]₃DBTTF} (2), {[Au(μ-3,5-(CF₃)₂pz)]₃DBTTF} (3), {[Cu(μ-3,5-(CF₃)₂pz)]₃TTF} (4), {[Ag(μ-3,5-(CF₃)₂pz)]₃TTF} (5), {[Au(μ-3,5-(CF₃)₂pz)]₃TTF} (6), {[Cu(μ-3,5-(CF₃)₂pz)]₃BEDT-TTF} (7), {[Ag(μ-3,5-(CF₃)₂pz)]₃BEDT-TTF} (8), and {[Au(μ-3,5-(CF₃)₂pz)]₃BEDT-TTF} (9), where pz = pyrazolate, TTF = tetrathiafulvalene, DBTTF = dibenzotetrathiafulvalene, and BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene. This series of binary donor-acceptor adducts has been found to exhibit remarkable supramolecular structures in both the solid state and solution, whereby they exhibit supramolecular stacked chains and oligomers, respectively. The supramolecular solid-state and solution binary donor-acceptor adducts also exhibit superior shelf stability under ambient laboratory storage conditions. Structural and other electronic properties of solids and solutions of these adducts have been characterized by single-crystal X-ray diffraction (XRD) structural analysis, ¹H and ¹⁹F NMR, UV-vis-near-IR spectroscopy, Fourier transform infrared, and computational investigations. The combined results of XRD structural data analysis, spectroscopic measurements, and theoretical studies suggest sustenance of the donor-acceptor stacked structure and electronic communication in both the solid state and solution. These properties are discussed in terms of potential applications for this new class of supramolecular binary donor-acceptor adducts in molecular electronic devices, including solar cells, magnetic switching devices, and field-effect transistors.

Relaxation Dispersion NMR to Reveal Fast Dynamics in Brønsted Acid Catalysis: Influence of Sterics and H-Bond Strength on Conformations and Substrate Hopping

NMR provides both structural and dynamic information, which is key to connecting intermediates and to understanding reaction pathways. However, fast exchanging catalytic intermediates are often inaccessible by conventional NMR due its limited time resolution. Here, we show the combined application of the 1H off-resonance R1ρ NMR method and low temperature (185-175 K) to resolve intermediates exchanging on a μs time scale (ns at room temperature). The potential of the approach is demonstrated on chiral phosphoric acid (CPA) catalysts in their complexes with imines. The otherwise inaccessible exchange kinetics of the E-I ⇌ E-II imine conformations and thermodynamic E-I:E-II imine ratios inside the catalyst pocket are experimentally determined and corroborated by calculations. The E-I ⇌ E-II exchange rate constants (kex185 K) for different catalyst-substrate binary complexes varied between 2500 and 19 000 s-1 (τex = 500-50 μs). Theoretical analysis of these exchange rate constants revealed the involvement of an intermediary tilted conformation E-III, which structurally resembles the hydride transfer transition state. The main E-I and E-II exchange pathway is a hydrogen bond strength dependent tilting-switching-tilting mechanism via a bifurcated hydrogen bond as a transition state. The reduction in the sterics of the catalyst showed an accelerated switching process by at least an order of magnitude and enabled an additional rotational pathway. Hence, the exchange process is mainly a function of the intrinsic properties of the 3,3'-substituents of the catalyst. Overall, we believe that the present study opens a new dimension in catalysis via experimental access to structures, populations, and kinetics of catalyst-substrate complexes on the μs time scale by the 1H off-resonance R1ρ method.

Extraction of Reliable Molecular Information from Diffusion NMR Spectroscopy: Hydrodynamic Volume or Molecular Mass?

Measuring accurate translational self-diffusion coefficients (D_t ) by NMR techniques with modern spectrometers has become rather routine. In contrast, the derivation of reliable molecular information therefrom still remains a nontrivial task. In this paper, two established approaches to estimating molecular size in terms of hydrodynamic volume (V_H ) or molecular weight (M) are compared. Ad hoc designed experiments allowed the critical aspects of their application to be explored by translating relatively complex theoretical principles into practical take-home messages. For instance, comparing the D_t values of three isosteric Cp₂ MCl₂ complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that V_H is the critical molecular property affecting D_t . This central concept served to clarify the assumptions behind the derivation of D_t =ƒ(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (D_t ) versus log (M) linear correlations for a set of species have been highlighted by further investigations of selected examples. The effectiveness of the Stokes-Einstein equation itself in describing the aggregation or polymerization of differently shaped species has been explored by comparing, for example, a ball-shaped silsesquioxane cage with its cigar-like dimeric form, or styrene with polystyrene macromolecules.

Brønsted acid catalysis - the effect of 3,3'-substituents on the structural space and the stabilization of imine/phosphoric acid complexes

BINOL derived chiral phosphoric acids (CPAs) are widely known for their high selectivity. Numerous 3,3'-substituents are used for a variety of stereoselective reactions and theoretical models of their effects are provided. However, experimental data about the structural space of CPA complexes in solution is extremely rare and so far restricted to NMR investigations of binary TRIP/imine complexes featuring two E- and two Z-imine conformations. Therefore, in this paper the structural space of 16 CPA/imine binary complexes is screened and 8 of them are investigated in detail by NMR. For the first time dimers of CPA/imine complexes in solution were experimentally identified, which show an imine position similar to the transition state in transfer hydrogenations. Furthermore, our experimental and computational data revealed an astonishing invariance of the four core structures regardless of the different steric and electronic properties of the 3,3'-substituent. However, a significant variation of E/Z-ratios is observed, demonstrating a strong influence of the 3,3'-substituents on the stabilization of the imine in the complexes. These experimental E/Z-ratios cannot be reproduced by calculations commonly applied for mechanistic studies, despite extensive conformational scans and treatment of the electronic structure at a high level of theory with various implicit solvent corrections. Thus, these first detailed experimental data about the structural space and influence of the 3,3'-substituent on the energetics of CPA/imine complexes can serve as basis to validate and improve theoretical predictive models.

Diffusion NMR for the characterization, in solution, of supramolecular systems based on calixarenes, resorcinarenes, and other macrocyclic arenes

The use of diffusion NMR in studying calixarenes and other arene-based supramolecular systems is described, emphasizing the pivotal role played by the calixarene community in transforming the methods into a routine tool used in supramolecular chemistry.

Imidazolyl-phenyl (IMP) anions: a modular structure for tuning solubility and coordinating ability

The effect of counteranion upon a cation's solution-phase reactivity depends on a subtle interplay of weak interactions.

Multinuclear Ru(ii) and Ir(iii) decorated tetraphenylporphyrins as efficient PDT agents

Two novel multi-metallic porphyrin complexes were synthesised and evaluated as effective PDT agents against human breast epithelial cells (SKBR-3).

Mannoside and 1,2-mannobioside β-cyclodextrin-scaffolded NO-photodonors for targeting antibiotic resistant bacteria

Two β-cyclodextrin derivatives randomly appended on the primary face with both the nitric oxide (NO) photodonor 4-nitro-3-(trifluoromethyl)aniline and a mannose or α(1→2)mannobioside residue are reported to construct targeted NO photoreleasing nanocarriers. 2D ROESY and PGSE NMR suggested supramolecular homodimerization in water by inclusion of the nitroaniline group into the facing macrocycle cavities. Isothermal titration calorimetry on their concanavalin A lectin binding showed an exothermic binding event to the lectin and an endothermic process during the dilution of the conjugates. Both α(1→2)mannobioside and the nitroaniline moieties significantly enhanced the binding to the lectin. These effects might arise from a better fit within the carbohydrate-recognition site in the former case and a multivalent effect caused by homodimerization in the latter. Direct detection of NO by amperometric technique shows that both β-cyclodextrin derivatives release this radical upon excitation with visible light with higher efficiency than the unfunctionalized NO photodonor.

High-Voltage Lithium-Metal Batteries Enabled by Localized High-Concentration Electrolytes

Rechargeable lithium-metal batteries (LMBs) are regarded as the "holy grail" of energy-storage systems, but the electrolytes that are highly stable with both a lithium-metal anode and high-voltage cathodes still remain a great challenge. Here a novel "localized high-concentration electrolyte" (HCE; 1.2 m lithium bis(fluorosulfonyl)imide in a mixture of dimethyl carbonate/bis(2,2,2-trifluoroethyl) ether (1:2 by mol)) is reported that enables dendrite-free cycling of lithium-metal anodes with high Coulombic efficiency (99.5%) and excellent capacity retention (>80% after 700 cycles) of Li||LiNi_1/3 Mn_1/3 Co_1/3 O₂ batteries. Unlike the HCEs reported before, the electrolyte reported in this work exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability that make LMBs closer to practical applications. The fundamental concept of "localized HCEs" developed in this work can also be applied to other battery systems, sensors, supercapacitors, and other electrochemical systems.

Decaborane anion tautomerism: ion pairing and proton transfer control

The reaction of 1,8-bis(dimethylamino)naphthalene-which is often referred by the trade name Proton-Sponge (PS)-with decaborane in hexane afforded [HPS][B10H13] (1) salt as a pale-yellow precipitate. Variable-temperature NMR studies allowed the full assignment of 1H and 11B spectra for this familiar ten-vertex polyhedral anion. In addition, this work reveals that an increase in the temperature leads to the intramolecular exchange of three B-H-B hydrogen atoms around the hexagonal face of the boat-shaped cluster. This previously unrecognised H-tautomerism complements the long-known low-energy proton exchange of only one of the bridging hydrogen atoms. The temperature dependent proton fluxional behaviour controls the molecular environment of the polyhedral cage, averaging the negative charge of the anion. The result is a debilitation of the cation-anion interactions in solution, favouring the transfer of the proton from the organic aromatic cation, [HPS]+, to the polyhedral anion, [B10H13]-. This proton transfer affords Proton-Sponge and decaborane, increasing the entropy of the system and sustaining an equilibrium which at high temperatures shifts toward the neutral reactants and at low temperatures moves toward the ionic products. A single X-ray diffraction analysis of 1 is discussed.

Mechanistic insights into the tropo-inversion of the biphenyl moiety in chiral bis-amido phosphites and in their palladium(ii) complexes

Chiral bis-amido phosphites L1 and L2 containing a diaminobiphenyl unit and a chiral alkoxy group derived from either (-)-menthol or 3-acetoxy deoxycholic methyl ester have been synthesised. Both L1 and L2 react with PdCl₂(NCPh)₂ affording di- or mononuclear derivatives with formula trans-[Pd(μ-Cl)Cl(L)]₂ (1a, L = L1; 1b, L = L2) or trans-PdCl₂(L)₂ (2a, L = L1; 2b, L = L2) depending on the Pd : L molar ratio. The crystal structure of (M,P)-1a confirms the trans arrangement of the ligand L1 and shows an unusual puckering of the Pd₂(μ-Cl)₂ core (θ 46°). Both the ligands L1 and L2 and their complexes (1 and 2) are fluxional in solution as a consequence of the tropo-inversion of the diaminobiphenyl unit. For L1, L2, 1a and 2a a combined study including variable temperature ³¹P{¹H} NMR spectroscopy and line shape analysis, Eyring plots and DFT calculations have shed light on the mechanism of the tropo-inversion.

Intramolecular Hydrogen Bonding Appetency for Conformational Penchants in Oxalohydrazide Fluoro Derivatives: NMR, MD, QTAIM, and NCI Studies

The conformational stability of synthesized diphenyloxalohydrazide and dibenzoyloxalohydrazide fluoro derivatives has been investigated by extensive NMR studies that are ascertained by various levels of theoretical calculations. Two-dimensional ¹H-¹⁹F HOESY NMR experiments revealed the close spatial proximity between two NMR-active nuclei, confirming the hydrogen bond (HB)-mediated interaction between them, further aiding in establishing the probable stable conformations of these molecules. The relaxed potential energy scan disclosed the energy-minimized most stable structure among the several possible multiple conformations, which is in concurrence with NMR interpretations. Atomistic molecular dynamics simulations have been employed to unequivocally establish the conformational stability and the nature of HB formation at varied temperatures. With the possibility of occurrence of a number of probable conformations, the percentage of occurrences of different types of HBs in them was determined by MD simulations. Their population analysis was carried out using a Boltzmann distribution, in addition to deriving their Gibbs free energies. The molecular interactions governing the stable conformations have not only been ascertained by experimental NMR interpretations but also corroborated by other theoretical computations, viz., quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI).

An ambiphilic phosphine/H-bond donor ligand and its application to the gold mediated cyclization of propargylamides

We describe the synthesis of an ambiphilic phosphine/H-bond donor ligand featuring a trifluoroacetamide functionality, its coordination to gold(i) chloride, and its application as a self-activating catalyst for the cyclization of propargylamides.

The Stepwise Reaction of Rhodium and Iridium Complexes of Formula [MCl2 (κ4 C,N,N',P-L)] with Silver Cations: A Case of trans-Influence and Chiral Self-Recognition

Acetonitrile suspensions of the dichlorido complexes [MCl₂ (κ⁴ C,N,N',P-L)] [M=Rh (1), Ir (2)] react with AgSbF₆ in a 1:2 molar ratio affording the bis-acetonitrile complexes [M(κ⁴ C,N,N',P-L)(NCMe)₂ ][SbF₆ ]₂ (3 and 4). The reaction takes place in a sequential manner and the intermediates can be isolated varying the M:Ag molar ratio. In a 2:1 molar ratio, it affords the dimetallic monochlorido-bridged compounds [{MCl(κ⁴ C,N,N',P-L)}₂ (μ-Cl)][SbF₆ ] (5 and 6). In a 1:1 molar ratio, the monosubstituted solvato-complexes [MCl(κ⁴ C,N,N',P-L)(Solv)][SbF₆ ] (Solv=H₂ O, MeCN, 7-10) were obtained. Finally, in a 2:3 molar ratio, it gives complexes 11 and 12 of formula [{M(κ⁴ C,N,N',P-L)(NCMe)(μ-Cl)}₂ Ag][SbF₆ ]₃ in which a silver cation joints two cationic monosubstituted acetonitrile-complexes [MCl(κ⁴ C,N,N',P-L)(NCMe)]⁺ through the remaining chlorido ligands and two Ag⋅⋅⋅C interactions with one of the phenyl rings of each PPh₂ group. In all the complexes, the aminic nitrogen and the central metal atom are stereogenic centers. In the trimetallic complexes 11 and 12, the silver atom is also a stereogenic center. The formation of the cation of the dimetallic complexes 5 and 6, as well as that of the trimetallic complexes 11 and 12, takes place with chiral molecular self-recognition. Experimental data and DFT calculations provide plausible explanations for the observed molecular recognition. The new complexes have been characterized by analytical, spectroscopic means and by X-ray diffraction methods.

Molecular weight prediction in polystyrene blends. Unprecedented use of a genetic algorithm in pulse field gradient spin echo (PGSE) NMR

A genetic algorithm that uses boxcar functions (diffGA) has been applied for the first time in PGSE NMR. It reconstructs accurate diffusion coefficients for all the components of the mixture, and therefore predicts correct weight-average molecular weights for all of them. The results reported herein complement those obtained with established methods such as ITAMeD, CONTIN and TRAIn algorithms, and provide a detailed solution picture. Its robustness and limits have been stretched in order to ascertain the minimum separation within diffusion coefficients or relative proportion between components. In addition, the new genetic algorithm has been also applied to a mixture of small molecules, providing excellent results at very low computational times.