Diastereoselective Synthesis of Phosphinyl Peptides via Rh-Catalyzed 1,4-Addition in Coparticipation of a P-Chiral Moiety and Difluorphos.

The asymmetric Rh-catalyzed 1,4-addition of aryl/heteroaryl moieties to α,ß-unsaturated esters was achieved in high diastereoselectivity via the coparticipation of a P-chiral phosphinyl moiety at Cß to the prochiral center and (R)- or (S)-Difluorphos. This methodology expands the synthetic toolbox available for the preparation of structurally diverse chiral phosphinyl peptides.

From Coordination to π-Hole Chemistry of Transition Metals: Metalloporphyrins as a Case of Study.

Herein we have evidenced the formation of favorable π-hole Br⋅⋅⋅metal noncovalent interactions (NCIs) involving elements from groups 9, 11 and 12. More in detail, M (M=Co2+, Ni2+, Cu2+ and Zn2+) containing porphyrins have been synthesized and their supramolecular assemblies structurally characterized by means of single crystal X-ray diffraction and Hirshfeld surface analyses, revealing the formation of directional Br⋅⋅⋅M contacts in addition to ancillary hydrogen bond and lone pair-π bonds. Computations at the PBE0-D3/def2-TZVP level of theory revealed the π-hole nature of the Br⋅⋅⋅M interaction. In addition, the physical nature of these NCIs was studied using Quantum Chemistry methodologies, providing evidence of π-hole Spodium and Regium bonds in Zn2+ and Cu2+ porphyrins, in addition to unveiling the presence of a π-hole for group 9 (Co2+). On the other hand, group 10 (Ni2+) acted as both electron donor and acceptor moiety without showing an electropositive π-hole. Owing to the underexplored potential of π-hole interactions in transition metal chemistry, we believe the results reported herein will be useful in supramolecular chemistry, organometallics, and solid-state chemistry by i) putting under the spotlight the π-hole chemistry involving first row transition metals and ii) unlocking a new tool to direct the self-assembly of metalloporphyrins.

Solvent-Switchable Remote C-H Activation via 1,4-Palladium Migration Enables Site-Selective C-P Bond Formation: A Tool for the Synthesis of P-Chiral Phosphinyl Imidazoles.

Solvent-switchable and site-selective phosphorylation of imidazoles at the C2 or C5 position of the imidazole ring was achieved via 1,4-palladium migration. P-Chiral tert-butyl(aryl)phosphine oxides were cross-coupled to 1-(2-bromophenyl)-1H-imidazoles with high enantiospecificity, thereby leading to a novel class of chiral imidazole-based phosphine oxides. As proof of concept, reduction of an analogue yielded the corresponding P-chiral 2-phosphinyl imidazole ligand, which was shown to induce high enantioselectivity in the formation of axially chiral molecules synthesized via Pd-catalyzed Suzuki-Miyaura cross-coupling.

Vibrational dynamics as an essential determinant of the thermal stability of zinc zeolitic imidazolate lattices.

Thermal stability and kinetics of zeolitic imidazolate frameworks (ZIFs) are crucial for their applications as energetic materials. Here, the effect of microscopic vibrational dynamics on the thermal stability of ZIFs is demonstrated by using simple tools. Specifically, we explored the thermal kinetics based on Flynn-Wall-Ozawa and Kissinger's methods. The study comprises a combination of structure-related effects such as topology, density, and alkyl substitution with respect to vibrational dynamics in ZIFs. The results exhibit a linear correlation between the vibrational dynamics of the linkers and activation energy, I.E. stabilization of ZIFs, in the polymorphic Zn(EtIm)2 series. At the same time, thermal destabilization was observed with the growing alkyl chain and was further probed by IR spectroscopy.

In situ monitoring of mechanochemical MOF formation by NMR relaxation time correlation.

In this paper, we present a new approach to monitoring mechanochemical transformations, based on a magnetic resonance (MR) method in which relaxation time correlation maps are used to track the formation of the popular metal-organic framework (MOF) materials Zn-MOF-74 and ZIF-8. The two-dimensional (2D) relaxation correlation measurement employed yields a spectrum which visually and analytically identifies different 1H environments in the sample of interest. The measurement is well-suited to analyzing solid mixtures, and liquids, in complex systems. Application in this work to monitoring MOF formation shows changes in signal amplitudes, and their MR lifetime coordinates, within the 2D plots as the reaction progresses, confirming reaction completion. This new measurement provides a simple way to analyse solid-state reactions without dissolution, and there is a logical pathway to benchtop measurement with a new generation of permanent magnet-based MR instruments. The methodology described permits measurement in an MR compatible milling container, which may be directly transferred from the shaker assembly to the MR magnet for in situ measurement of the entire reaction mixture.

Supramolecular "baking powder": a hexameric halogen-bonded phosphonium salt cage encapsulates and functionalises small-molecule carbonyl compounds.

We report a hexameric supramolecular cage assembled from the components of a Wittig-type phosphonium salt, held together by charge-assisted halogen bonds. The cage reliably encapsulates small polar molecules, including aldehydes and ketones, to provide host-guest systems where components are pre-formulated in a near-ideal stoichiometry for a mechanochemical base-activated Wittig olefination. These pre-formulated solids represent a proof-of-principle for a previously not reported supramolecular design of solid-state reactivity in which the host for molecular inclusion also acts as a complementary reagent for the subsequent chemical transformation of an array of guests. The host-guest solid-state complexes can act as supramolecular surrogates to their Wittig olefination vinylbromide products in a Sonogashira-type coupling that enables one-pot mechanochemical conversion of an aldehyde to an enediyne.

Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response.

Cocrystallization of a cis-azobenzene dye with volatile molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light-intensity-dependent responses upon irradiation with low-power visible light. The halogen-bond-driven assembly of the dye cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behavior varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW·cm-2) light irradiation leads to a color change associated with low levels of cis → trans isomerization. Irradiation at higher intensities (150 mW·mm-2) produces photomechanical bending, caused by more extensive isomerization of the dye. At still higher irradiation intensities (2.25 W·mm-2) the cocrystals undergo cold photocarving; i.e., they can be cut and written on with micrometer precision using laser light without a major thermal effect. Real-time Raman spectroscopy shows that this novel photochemical behavior differs from what would be expected from thermal energy input alone. Overall, this work introduces a rational blueprint, based on supramolecular chemistry in the solid state, for new types of crystalline light-responsive materials, which not only respond to being exposed to light but also change their response based on the light intensity.

Effect of Photoinitiators Doped in PDMS for Femtosecond-Laser Writing: Characterization and Outcomes.

Herein, we have characterized in depth the effect of femtosecond (fs)-laser writing on various polydimethylsiloxane (PDMS)-based composites. The study combines systematic and nanoscale characterizations for the PDMS blends that include various photoinitiators (organic and inorganic agents) before and after fs-laser writing. The results exhibit that the photoinitiators can dictate the mechanical properties of the PDMS, in which Young's modulus of PDMS composites has higher elasticity. The study illustrates a major improvement in refractive index change by 15 times higher in the case of PDMS/BP-Ge [benzophenone (BP) allytriethylgermane] and Irgacure 184. Additional enhancement was achieved in the optical performance levels of the PDMS composites (the PDMS composites of Irgacure 184/500, BP-Ge, and Ge-ATEG have a relative difference of less than 5% in comparison with pristine PDMS), which are on par with glasses. This insightful study can guide future investigators in choosing photoinitiators for particular applications in photonics and polymer chemistry.

In situ study of Au nanoparticle formation in a mechanochemical-aging-based method.

As we strive to perform chemical transformations in a more sustainable fashion, enabling solid-state reactions through mechanochemistry has emerged as a highly successful approach. Due to the wide-ranging applications of gold nanoparticles (AuNPs), mechanochemical strategies have already been employed for their synthesis. However, the underlying processes surrounding gold salt reduction, nucleation and growth of AuNPs in the solid state are yet to be understood. Here, we present a mechanically activated aging synthesis of AuNPs, through a solid-state Turkevich reaction. Solid reactants are only briefly exposed to input of mechanical energy before being aged statically over a period of six weeks at different temperatures. This system offers an excellent opportunity for an in situ analysis of both reduction and nanoparticle formation processes. During the aging period, the reaction was monitored using a combination of X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction and transmission electron microscopy, to gain meaningful insights into the mechanisms of solid-state formation of gold nanoparticles. The acquired data allowed for the establishment of the first kinetic model for solid-state nanoparticle formation.

The Effect of Linker-to-Metal Energy Transfer on the Photooxidation Performance of an Isostructural Series of Pyrene-Based Rare-Earth Metal-Organic Frameworks.

The tetratopic linker, 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4 TBAPy) along with rare-earth (RE) ions is used for the synthesis of 9 isostructures of a metal-organic framework (MOF) with shp topology, named RE-CU-10 (RE = Y(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III)). The synthesis of each RE-CU-10 analogue requires different reaction conditions to achieve phase pure products. Single crystal X-ray diffraction indicates the presence of a RE9 -cluster in Y- to Tm-CU-10, while a RE11 -cluster is observed for Yb- and Lu-CU-10. The photooxidation performance of RE-CU-10 analogues is evaluated, observing competition between linker-to-metal energy transfer versus the generation of singlet oxygen. The singlet oxygen produced is used to detoxify a mustard gas simulant 2-chloroethylethyl sulfide, with half-lives ranging from 4.0 to 5.8 min, some of the fastest reported to date using UV-irradiation and < 1 mol% catalyst, in methanol under O2 saturation.

Experimentally Validated Ab Initio Crystal Structure Prediction of Novel Metal-Organic Framework Materials.

First-principles crystal structure prediction (CSP) is the most powerful approach for materials discovery, enabling the prediction and evaluation of properties of new solid phases based only on a diagram of their underlying components. Here, we present the first CSP-based discovery of metal-organic frameworks (MOFs), offering a broader alternative to conventional techniques, which rely on geometry, intuition, and experimental screening. Phase landscapes were calculated for three systems involving flexible Cu(II) nodes, which could adopt a potentially limitless number of network topologies and are not amenable to conventional MOF design. The CSP procedure was validated experimentally through the synthesis of materials whose structures perfectly matched those found among the lowest-energy calculated structures and whose relevant properties, such as combustion energies, could immediately be evaluated from CSP-derived structures.

Efficient room-temperature phosphorescence of covalent organic frameworks through covalent halogen doping.

Organic room-temperature phosphorescence, a spin-forbidden radiative process, has emerged as an interesting but rare phenomenon with multiple potential applications in optoelectronic devices, biosensing and anticounterfeiting. Covalent organic frameworks (COFs) with accessible nanoscale porosity and precisely engineered topology can offer unique benefits in the design of phosphorescent materials, but these are presently unexplored. Here, we report an approach of covalent doping, whereby a COF is synthesized by copolymerization of halogenated and unsubstituted phenyldiboronic acids, allowing for random distribution of functionalized units at varying ratios, yielding highly phosphorescent COFs. Such controlled halogen doping enhances the intersystem crossing while minimizing triplet-triplet annihilation by diluting the phosphors. The rigidity of the COF suppresses vibrational relaxation and allows a high phosphorescence quantum yield (ΦPhos ≤ 29%) at room temperature. The permanent porosity of the COFs and the combination of the singlet and triplet emitting channels enable a highly efficient COF-based oxygen sensor, with an ultra-wide dynamic detection range (~103-10-5 torr of partial oxygen pressure).

The "η-sweet-spot" (ηmax) in liquid-assisted mechanochemistry: polymorph control and the role of a liquid additive as either a catalyst or an inhibitor in resonant acoustic mixing (RAM).

Resonant acoustic mixing (RAM) offers a simple, efficient route for mechanochemical synthesis in the absence of milling media or bulk solvents. Here, we show the use of RAM to conduct the copper-catalysed coupling of sulfonamides and carbodiimides. This coupling was previously reported to take place only by mechanochemical ball milling, while in conventional solution environments it is not efficient, or does not take place at all. The results demonstrate RAM as a suitable methodology to conduct reactions previously accessed only by ball milling and provide a detailed, systematic overview of how the amount of liquid additive, measured by the ratio of liquid volume to weight of reactants (η, in µL mg-1), can affect the course of a mechanochemical reaction and the polymorphic composition of its product. Switching from ball milling to RAM allowed for the discovery of a new polymorph of the model sulfonylguanidine obtained by catalytic coupling of di(cyclohexyl)carbodiimide (DCC) and p-toluenesulfonamide, and the ability to control reaction temperature in RAM enabled in situ control of the polymorphic behaviour of this nascent product. We show that the reaction conversion for a given reaction time does not change monotonically but, instead, achieves a maximum for a well-defined η-value. This "η-sweet-spot" of conversion is herein designated ηmax. The herein explored reactions demonstrate sensitivity to η on the order of 0.01 µL mg-1, which corresponds to an amount of liquid additive below 5 mol% compared to the reactants, and is at least one to two orders of magnitude lower than the η-value typically considered in the design of liquid-assisted ball milling mechanochemical reactions. Such sensitivity suggests that strategies to optimise liquid-assisted mechanochemical reactions should systematically evaluate η-values at increments of 0.01 µL mg-1, or even finer. At η-values other than ηmax the reaction conversion drops off, demonstrating that the same liquid additive can act either as a catalyst or an inhibitor of a mechanochemical reaction, depending on the amount.

Polymorphs and solid solutions: materials with new luminescent properties obtained through mechanochemical transformation of dicyanoaurate(I) salts.

We report the use of mechano- and thermochemical methods to create new solid-state luminescent materials from well-known inorganic salts, potassium dicyanoaurate(I) KAu(CN)2, and potassium dicyanocuprate(I) KCu(CN)2. In particular, manual grinding or ball milling of commercial samples of KAu(CN)2 led to the formation of a novel polymorph of the salt, herein termed m-KAu(CN)2, evident by a significant change in color of the fluorescence emission of the solid material from orange to violet. The formation of m-KAu(CN)2 is reversible upon addition of small amounts of solvents, and powder X-ray diffraction analysis indicates that the structure of m-KAu(CN)2 might be related to that of pristine KAu(CN)2 through a change in ordering of Au(CN)2- ions in a layered structure. Thermal treatment of KAu(CN)2 led to the discovery of another polymorph of this well-known salt, herein termed t-KAu(CN)2, making KAu(CN)2 a rare example of a system in which thermochemical and mechanochemical treatments lead to the formation of different, in each case previously not reported, polymorphic forms. The thermally-induced transformation from KAu(CN)2 to t-KAu(CN)2 takes place around 250 °C and proceeds in a crystal-to-crystal fashion, which enabled the preliminary structural characterisation through single crystal X-ray diffraction, revealing the retention of the layered structure and a change in ordering of Au(CN)2- ions. Milling of the simple salt KAu(CN)2 in the presence of equimolar amounts or less of its copper(I)-based analogue coordination polymer KCu(CN)2 leads to the formation of a series of solid solution materials, isostructural to m-KAu(CN)2 and with visible fluorescence emission distinct from KCu(CN)2 or any herein investigated forms of KAu(CN)2.

Direct observation by high resolution transmission electron microscopy of gold(III) particle transformation during aging reduction reaction.

We use a gold nanoparticle synthesis as a model system to study the morphological and compositional changes in gold(III) precursor particles, while reduction is taking place during aging after mechanical activation. Scanning transmission electron microscopy coupled with a high-angle annular dark field detector revealed the nanoscale changes in particle morphology, while electron energy loss spectroscopy mapped the changes in the chemical landscape during the reduction process. Tracking a specific region of interest on the sample grid allowed for comparisons to be made of the same particles across a two day monitoring period. High-angle annular dark field images permitted the visualization of particle size reduction of the gold salt while electron energy loss spectroscopy captured the surprising mobility of the lighter chlorine and sodium ions in a solid matrix during the reduction process. This system offers unique insight into precursor particle reactivity in the solid phase, which is relevant for many mechanochemical and aging-based reactions.

Unravelling the synthesis of a rare-earth cluster-based metal-organic framework with spn topology.

Y-CU-45, an analogue of Zr-MOF-808, is synthesized for the first time. Several reaction conditions are tested demonstrating that two fluorinated modulators are required for a reproducible synthesis yielding high quality material. Y-CU-45 shows high crystallinity and surface area, shining light on the potential for rare-earth cluster-based MOFs with open metal sites.

Electrochemical, Photophysical, Morphological and DFT Study of Polymorphic Sn(IV)-Porphyrins Containing Fluorinated Axial Ligand.

In this study, we report the polymorphism of six coordinated Sn(IV)- tetrabromophenyl porphyrins axially armed with fluorine-substituted phenolate ligands (structural formula [Sn(TBrPP)2+ (A- )2 ], where A is the axial ligand=3,5-difluoro phenol, compound 1). One form stabilizes in triclinic system (namely, 1α), and the other stabilizes in monoclinic system (namely, 1ß). The two 1α and 1ß polymorphs display distinct photophysical and morphological properties in the solid state. X-ray diffraction study reveals that these polymorphs 1α and 1ß significantly differ in their supramolecular architecture, different axial phenolate conformations, and noncovalent interactions, which are responsible for their distinct solid-state properties. The crystal packing of these polymorphs dominates by intermolecular C-H⋅⋅⋅F, C-H⋅⋅⋅π and C-Br⋅⋅⋅F interhalogen interactions. Furthermore, the solid-state emission spectra of 1α showed red-shifted emission bands with respect to 1ß, in addition the redox behavior of 1α is slightly different in comparison to 1ß. Complementary theoretical studies with Hirshfeld surface analysis show the definite role of Br⋅⋅⋅F interhalogen interactions in the overall stability. Mapping the electrostatic potential isosurfaces with the aid of density functional theory in compound 1 clearly shows the presence of σ-hole, a requisite feature to show halogen interactions in the crystalline state. In addition, lattice energy and single point energy calculation shows that 1α was found to be energetically more favorable and thermodynamically more stable compare to 1ß.

SpeedMixing: Rapid Tribochemical Synthesis and Discovery of Pharmaceutical Cocrystals without Milling or Grinding Media.

We present SpeedMixing, a rapid blending technology, as an approach for fast mechanosynthesis and discovery of model pharmaceutical cocrystals through rapid spinning in the absence of bulk solvents and milling/grinding media. Syntheses of pharmaceutical cocrystals based on the active pharmaceutical ingredients (APIs) carbamazepine, dihydrocarbamazepine, and nicotinamide demonstrate SpeedMixing as a method for rapid, scalable, as well as controllable and selective synthesis of cocrystals, cocrystal polymorphs and stoichiomorphs, including the discovery of an unexpected methanol solvate of the archetypal cocrystal of carbamazepine and saccharin, which has eluded extensive screens over 20 years.

Metal-organic frameworks as hypergolic additives for hybrid rockets.

Hybrid rocket propulsion can contribute to reduce launch costs by simplifying engine design and operation. Hypergolic propellants, i.e. igniting spontaneously and immediately upon contact between fuel and oxidizer, further simplify system integration by removing the need for an ignition system. Such hybrid engines could also replace currently popular hypergolic propulsion approaches based on extremely toxic and carcinogenic hydrazines. Here we present the first demonstration for the use of hypergolic metal-organic frameworks (HMOFs) as additives to trigger hypergolic ignition in conventional paraffin-based hybrid engine fuels. HMOFS are a recently introduced class of stable and safe hypergolic materials, used here as a platform to bring readily tunable ignition and combustion properties to hydrocarbon fuels. We present an experimental investigation of the ignition delay (ID, the time from first contact with an oxidizer to ignition) of blends of HMOFs with paraffin, using White Fuming Nitric Acid (WFNA) as the oxidizer. The majority of measured IDs are under 10 ms, significantly below the upper limit of 50 ms required for functional hypergolic propellant, and within the ultrafast ignition range. A theoretical analysis of the performance of HMOFs-containing fuels in a hybrid launcher engine scenario also reveals the effect of the HMOF mass fraction on the specific impulse (I sp) and density impulse (ρI sp). The use of HMOFs to produce paraffin-based hypergolic fuels results in a slight decrease of the I sp and ρI sp compared to that of pure paraffin, similar to the effect observed with Ammonia Borane (AB), a popular hypergolic additive. HMOFs however have a much higher thermal stability, allowing for convenient mixing with hot liquid paraffin, making the manufacturing processes simpler and safer compared to other hypergolic additives such as AB.

Multivariate metal-organic framework MTV-MIL-101 via post-synthetic cation exchange: is it truly achievable?

The post-synthetic exchange (PSE) method is a well-proven route to replace, modify, and add different functionalities to metal-organic frameworks (MOFs). Particularly, the solvent-assisted cation substitution (SACS) technique has been reported to prepare mixed-metal multivariate metal-organic frameworks (MTV-MOFs). However, such a technique does not apply to all types of MOFs. In 2013, Szilágyi et al. reported the achievement of the mixed-metal MTV-MIL-101 framework via PSE. Since then, a debate has been taking place about the validity of these findings. On the other hand, the attainment of the mixed-metal MIL-101 was reported to be obtainable through the direct synthesis, which is, to some, the only way to achieve it. Here, we settle this dispute by investigating Szilágyi's method not only as described, but also at extended conditions of time and different metal precursors: all attempts were vain. However, by reconsidering the refluxing solvent (dimethylformamide "DMF" instead of water) and the applied reaction conditions (110 °C-20 h), mixed-metal MIL-101(Cr/Fe) was achieved via a simple PSE method.