Long Range Electronic Effects on the Host-Guest Complexation within the Oxygen Depleted 5,5'-Bicalixarene Cavities.

We report the synthesis of a series of the oxygen-depleted conjugated 5,5'-Bicalix[4]arene compounds bearing various substituents at the terminal positions of the conjugated chain and their fluorescence response to the presence of a cationic N-methylpyridinium guest. The complexation of this cation within the bicalixarene cavity results in the fluorescence quenching, with the host molecules bearing electron-donating groups demonstrating a stronger fluorescence response. These results show the importance of the electronic effects on the host-guest complexation within the hydrophobic calixarene scaffolds.

Polymeric architecture as a tool for controlling the reactivity of palladium(II) loaded nanoreactors.

Self-assembled systems, like polymeric micelles, have become great facilitators for conducting organic reactions in aqueous media due to their broad potential applications in green chemistry and biomedical applications. Massive strides have been taken to improve the reaction scope of such systems, enabling them to perform bioorthogonal reactions for prodrug therapy. Considering these significant advancements, we sought to study the relationships between the architecture of the amphiphiles and the reactivity of their PdII loaded micellar nanoreactors in conducting depropargylation reactions. Towards this goal, we designed and synthesized a series of isomeric polyethylene glycol (PEG)-dendron amphiphiles with different dendritic architectures but with an identical degree of hydrophobicity and hydrophilic to lipophilic balance (HLB). We observed that the dendritic architecture, which serves as the main binding site for the PdII ions, has greater influence on the reactivity than the hydrophobicity of the dendron. These trends remained constant for two different propargyl caged substrates, validating the obtained results. Density functional theory (DFT) calculations of simplified models of the dendritic blocks revealed the different binding modes of the various dendritic architectures to PdII ions, which could explain the observed differences in the reactivity of the nanoreactors with different dendritic architectures. Our results demonstrate how tuning the internal architecture of the amphiphiles by changing the orientation of the chelating moieties can be used as a tool for controlling the reactivity of PdII loaded nanoreactors.

Highly Chemoselective Zn+2 -Catalyzed Hydrosilylation of Alkynes.

Hydrosilylation of C=C double and C≡C triple bonds is one of the most widely used processes in organosilicon chemistry, mostly catalyzed by Pt-based complexes. Herein, the synthesis of a dicationic Zn+2 -based complex with a tripodal tris(2-pyridylmethyl)amine (TPA) ligand is reported which was found to be a highly chemoselective catalyst for hydrosilylation reactions of alkynes. Mechanistic studies revealed that unlike typical Zn-catalyzed hydrosilylation reactions where the key step is the activation of the Si-H bond, this system catalyzes the hydrosilylation reaction through the activation of C≡C triple bonds, which presumably is the reason for its high chemoselectivity. Remarkably, the hydrosilylation of alkynes could be performed in the presence of alkenes and other functional groups that remained intact in this reaction.

Metallomimetic Chemistry of a Cationic, Geometrically Constrained Phosphine in the Catalytic Hydrodefluorination and Amination of Ar-F Bonds.

The synthesis, isolation, and reactivity of a cationic, geometrically constrained σ3-P compound in the hexaphenyl-carbodiphosphoranyl-based pincer-type ligand (1+) are reported. 1+ reacts with electron-poor fluoroarenes via an oxidative addition-type reaction of the C-F bond to the PIII-center, yielding new fluorophosphorane-type species (PV). This reactivity of 1+ was used in the catalytic hydrodefluorination of Ar-F bonds with PhSiH3, and in a catalytic C-N bond-forming cross-coupling reactions between fluoroarenes and aminosilanes. Importantly, 1+ in these catalytic reactions closely mimics the mode of action of the transition metal-based catalysts.

Carbon-Nitride Popcorn-A Novel Catalyst Prepared by Self-Propagating Combustion of Nitrogen-Rich Triazenes.

The interest in development of non-graphitic polymeric carbon nitrides (PCNs), with various C-to-N ratios, having tunable electronic, optical, and chemical properties is rapidly increasing. Here the first self-propagating combustion synthesis methodology for the facile preparation of novel porous PCN materials (PCN3-PCN7) using new nitrogen-rich triazene-based precursors is reported. This methodology is found to be highly precursor dependent, where variations in the terminal functional groups in the newly designed precursors (compounds 3-7) lead to different combustion behaviors, and morphologies of the resulted PCNs. The foam-type highly porous PCN5, generated from self-propagating combustion of 5 is comprehensively characterized and shows a C-to-N ratio of 0.67 (C3 N4.45 ). Thermal analyses of PCN5 formulations with ammonium perchlorate (AP) reveal that PCN5 has an excellent catalytic activity in the thermal decomposition of AP. This catalytic activity of PCN5 is further evaluated in a closer-to-application scenario, showing an increase of 18% in the burn rate of AP-Al-HTPB (with 2 wt% of PCN5) solid composite propellant. The newly developed template- and additive-free self-propagating combustion synthetic methodology using specially designed nitrogen-rich precursors should provide a novel platform for the preparation of non-graphitic PCNs with a variety of building block chemistries, morphologies, and properties suitable for a broad range of technologies.

Intramolecular C-N bond activation by a geometrically constrained PIII-centre.

In this work the first examples of C-N bond activation by insertion into a geometrically constrained PIII-centre are shown. The mechanisms of these activation processes leading to new PV species were studied both experimentally and computationally. Interestingly, in the case of insertion of the PIII-centre into an N-C(O)H bond, an unstable phosphoranyl-formaldehyde intermediate is probably formed, which undergoes decarbonylation in the presence of a catalytic amount of HCl producing a hydrophosphorane.

Dual Reactivity of a Geometrically Constrained Phosphenium Cation.

A geometrically constrained phosphenium cation in bis(pyrrolyl)pyridine based NNN pincer type ligand (1+ ) was synthesized, isolated and its preliminary reactivity was studied with small molecules. 1+ reacts with MeOH and Et2 NH, activating the O-H and N-H bonds via a P-center/ligand assisted path. The reaction of 1+ with one equiv. of H3 NBH3 leads to its dehydrogenation producing 5. Interestingly, reaction of 1+ with an excess H3 NBH3 leads to phosphinidene (PI ) species coordinating to two BH3 molecules (6). In contrast, [1+ ][OTf] reacts with Et3 SiH by hydride abstraction yielding 1-H and Et3 SiOTf, while [1+ ][B(C6 F5 )4 ] reacts with Et3 SiH via an oxidative addition type reaction of Si-H bond to P-center, affording a new PV compound (8). However, 8 is not stable over time and degrades to a complex mixture of compounds in matter of minutes. Despite this, the ability of [1+ ][B(C6 F5 )4 ] to activate Si-H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1+ closely mimics transition metal behaviour.

Geometrically constrained square pyramidal phosphoranide.

Geometrical constriction of main group elements leading to a change in the reactivity of these main group centers has recently become an important tool in main group chemistry. A lot of focus on using this modern method is dedicated to group 15 elements and especially to phosphorus. In this work, we present the synthesis, isolation and preliminary reactivity study of the geometrically constrained, square pyramidal (SP) phosphoranide anion (1-). Unlike, trigonal bipyramidal (TBP) phosphoranides that were shown to react as nucleophiles while their redox chemistry was not reported, 1- reacts both as a nucleophile and reductant. The chemical oxidation of 1- leads to a P-P dimer (1-1) that is formed via the dimerization of unstable SP phosphoranyl radical (1˙), an unprecedented decay pathway for phosphoranyl radicals. Reaction of 1- with benzophenone leads via a single electron transfer (SET) to 1-OK and corresponding tetraphenyl epoxide (4).

Benzylic Dehydroxylation of Echinocandin Antifungal Drugs Restores Efficacy against Resistance Conferred by Mutated Glucan Synthase.

Each year, infections caused by fungal pathogens claim the lives of about 1.6 million people and affect the health of over a billion people worldwide. Among the most recently developed antifungal drugs are the echinocandins, which noncompetitively inhibit ß-glucan synthase, a membrane-bound protein complex that catalyzes the formation of the main polysaccharide component of the fungal cell wall. Resistance to echinocandins is conferred by mutations in FKS genes, which encode the catalytic subunit of the ß-glucan synthase complex. Here, we report that selective removal of the benzylic alcohol of the nonproteinogenic amino acid 3S,4S-dihydroxy-l-homotyrosine of the echinocandins anidulafungin and rezafungin, restored their efficacy against a large panel of echinocandin-resistant Candida strains. The dehydroxylated compounds did not significantly affect the viability of human-derived cell culture lines. An analysis of the efficacy of the dehydroxylated echinocandins against resistant Candida strains, which contain mutations in the FKS1 and/or FKS2 genes of the parental strains, identified amino acids of the Fks proteins that are likely to reside in proximity to the l-homotyrosine residue of the bound drug. This study describes the first example of a chemical modification strategy to restore the efficacy of echinocandin drugs, which have a critical place in the arsenal of antifungal drugs, against resistant fungal pathogens.

Phosphorus mediated imidazolinium to oxazolium ring rearrangement.

An unexpected rearrangement occurred when an imidazolinium based OCO pincer-type ligand (1) reacted with PCl3 producing a chlorophosphine with a pendant oxazolium "arm" (3). The mechanism of this rearrangement was studied both experimentally and by density functional theory (DFT) computations. The deprotonation of 3 led to further unexpected results.

Low-Power Laser Ignition of an Antenna-Type Secondary Energetic Copper Complex: Synthesis, Characterization, Evaluation, and Ignition Mechanism Studies.

In recent years, development of new energetic compounds and formulations, suitable for ignition with relatively low-power lasers, is a highly active and competitive field of research. The main goal of these efforts is focused on achieving and providing much safer solutions for various detonator and initiator systems. In this work, we prepared, characterized, and studied thermal and ignition properties of a new laser-ignitable compound, based on the 5,6-bis(ethylnitroamino)-N'2,N'3-dihydroxypyrazine-2,3-bis(carboximidamide) (DS3) proligand. This new energetic proligand was prepared in three steps, starting with 5,6-bis(ethylamino)-pyrazine-2,3-dicarbonitrile. Crystallography studies of the DS3-derived Cu(II) complex (DS4) revealed a unique stacked antenna-type structure of the latter compound. DS4 has an exothermal temperature of 154.5 °C and was calculated to exhibit a velocity of detonation of 6.36 km·s-1 and a detonation pressure of 15.21 GPa. DS4 showed properties of a secondary explosive, having sensitivity to impact, friction, and electrostatic discharge of 8 J, 360 N, and 12 mJ, respectively. In order to study the mechanism of ignition by a laser (using a diode laser, 915 nm), we conducted a set of experiments that enabled us to characterize a photothermal ignition mechanism. Furthermore, we found that a single pulse, with a time duration of 1 ms and with a total energy of 4.6 mJ, was sufficient for achieving a consistent and full ignition of DS4. Dual-pulse experiments, with variable time intervals between the laser pulses, showed that DS4 undergoes ignition via a photothermal mechanism. Finally, calculating the chemical mechanism of the formation of the complex DS4 and modeling its anhydrous and hydrated crystal structures (density functional theory calculations using Gaussian and HASEM software) allowed us to pinpoint a more precise location of water molecules in experimental crystallographic data. These results suggest that DS4 has potential for further development to a higher technology readiness level and for integration into small-size safe detonator systems as for many civil, aerospace, and defense applications.

Allenic phosphonium borate zwitterions via a phosphonium allenylidene intermediate.

We describe the synthesis of alkynyl phosphanes of the type R2P-C[triple bond, length as m-dash]C-C(OCH3)Ph2 (R = Ph, Cy) and investigate their transformation to geminally substituted phosphonium borato-allene zwitterions upon their reaction with B(C6F5)3. The mechanism for this transformation was studied experimentally and by density functional theory computations (DFT), suggesting the intermediacy of an unsaturated 3-coordinate phosphonium electrophile akin to a methylene phosphonium cation.

Facile proton-coupled electron transfer enabled by coordination-induced E-H bond weakening to boron.

We report the facile activation of aryl E-H (ArEH; E = N, O, S; Ar = Ph or C6F5) or ammonia N-H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1-). Substantial decreases in E-H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol-1 in H2N-H BDFE upon coordination was experimentally determined.

Carborane Stabilized "19-Electron" Molybdenum Metalloradical.

Paramagnetic metal complexes gained a lot of attention due to their participation in a number of important chemical reactions. In most cases, these complexes are dominated by 17-e metalloradicals that are associatively activated with highly reactive paramagnetic 19-e species. Molybdenum paramagnetic complexes are among the most investigated ones. While some examples of persistent 17-e Mo-centered radicals have been reported, in contrast, 19-e Mo-centered radicals are illusive species and as such could rarely be detected. In this work, the photodissociation of the [Cp(CO)3Mo]2 dimer (1) in the presence of phosphines was revisited. As a result, the first persistent, formally 19-e Mo radical with significant electron density on the Mo center (22%), Cp(CO)3Mo•PPh2(o-C2B10H11) (5b), was generated and characterized by EPR spectroscopy and MS as well as studied by DFT calculations. The stabilization of 5b was likely achieved due to a unique electron-withdrawing effect of the o-carboranyl substituent at the phosphorus center.

An air-stable, Zn2+-based catalyst for hydrosilylation of alkenes and alkynes.

Hydrosilylation of C[double bond, length as m-dash]C double and C[triple bond, length as m-dash]C triple bonds is one of the most widely used processes in organosilicon chemistry, mostly catalyzed by Pt-based complexes. We report here the synthesis of an air-stable dicationic Zn2+-based complex in a hemilabile tris(2-methyl-6-pyridylmethyl) phosphine (TmPPh) ligand, 12+[B(C6F5)4]2. When heated, 12+[B(C6F5)4]2 activates Si-H bonds reversibly via ligand/metal cooperation between Lewis acidic Zn2+ and Lewis basic N centers in a frustrated Lewis pair (FLP) type fashion. Consequently, 12+[B(C6F5)4]2 was found to be an effective catalyst for hydrosilylation reactions of C[double bond, length as m-dash]C double and C[triple bond, length as m-dash]C triple bonds. Remarkably, these hydrosilylation reactions can be loaded under aerobic conditions, as well as, in some cases, work under neat conditions. The mechanism of the activation of the Si-H bond and the hydrosilylation reaction is proposed based on experiments and density functional theory (DFT) calculations.

Fluorophore-Appendant 5,5'-Bicalixarene Scaffolds for Host-Guest Sensing of Nitric Oxide.

Conjugated 5,5'-Bicalixarene scaffolds having fluorophores at the chain termini have been prepared and tested in the supramolecular detection of nitric oxide. Scaffolds bearing electron-rich fluorophores demonstrated a stronger turn-off response to the presence of NO than the fluorophore-free analogue in both organic and aqueous media, while no fluorescence quenching happened when the electron-deficient fluorophores were employed. Unprecedented ratiometric supramolecular sensing was observed when fluorophores of the opposite electronic demands were placed at the scaffold's termini.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry.

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph2N)3V(µ-N)B(C6F5)2 (1a) and (N(CH2CH2N(C6F5))3)V(µ-N)B(C6F5)2 (1b). Redox control of the VIV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the VV complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the VIV complex, [CoCp2*][(N(CH2CH2N(C6F5))3)V(µ-N)B(C6F5)2] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the VIV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

Base- and Catalyst-Induced Orthogonal Site Selectivities in Acylation of Amphiphilic Diols.

Seeking to selectively functionalize natural and synthetic amphiphiles, we explored acylation of model amphiphilic diols. The use of a nucleophilic catalyst enabled a remarkable shift of the site selectivity from the polar site, preferred in background noncatalyzed or base-promoted reactions, to the apolar site. This tendency was significantly enhanced for organocatalysts comprising an imidazole active site surrounded by long/branched tails. An explanation of these orthogonal modes of selectivity is supported by competitive experiments with monoalcohol substrates.

Phosphorylation Organocatalysts Highly Active by Design.

The activity of nucleophilic organocatalysts for alcohol/phenol phosphorylation was enhanced through attaching oligoether appendages to a benzyl substituent on imidazole- or aminopyridine-based active units, presumably because of stabilizing n-cation interactions of the ethereal oxygens with the positively charged aza-heterocycle in the catalytic intermediates, and was substantially higher than that of known benchmark catalysts for a range of substrates. Density functional theory calculations and the study of analogues having a lower potential for such stabilizing interactions support our hypothesis.

Redox-switchable carboranes for uranium capture and release.

The uranyl ion (UO22+; U(VI) oxidation state) is the most common form of uranium found in terrestrial and aquatic environments and is a central component in nuclear fuel processing and waste remediation efforts. Uranyl capture from either seawater or nuclear waste has been well studied and typically relies on extremely strong chelating/binding affinities to UO22+ using chelating polymers1,2, porous inorganic3-5 or carbon-based6,7 materials, as well as homogeneous8 compounds. By contrast, the controlled release of uranyl after capture is less established and can be difficult, expensive or destructive to the initial material2,9. Here we show how harnessing the redox-switchable chelating and donating properties of an ortho-substituted closo-carborane (1,2-(Ph2PO)2-1,2-C2B10H10) cluster molecule can lead to the controlled chemical or electrochemical capture and release of UO22+ in monophasic (organic) or biphasic (organic/aqueous) model solvent systems. This is achieved by taking advantage of the increase in the ligand bite angle when the closo-carborane is reduced to the nido-carborane, resulting in C-C bond rupture and cage opening. The use of electrochemical methods for uranyl capture and release may complement existing sorbent and processing systems.