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The Mizoroki-Heck reaction is one of the most known and best studied catalytic transformations and has provided an outstanding driving force for the development of catalysis and synthetic applications. Three out of four classical Mizoroki-Heck catalytic cycle intermediates contain Pd-C bonds and are well known and studied in detail. However, a simple palladium hydride (which is formed after the product-releasing β-H-elimination step) is a kind of elusive intermediate in the Mizoroki-Heck reaction. In the present study, we performed a combined theoretical and mass spectrometry (MS) study of palladium hydride complexes [PdX2H]- (X = Cl, Br, and I), which are reactive intermediates in the Mizoroki-Heck reaction. Static and molecular dynamic calculations revealed that these species have a T-shaped structure with a trans-arrangement of halogen atoms. Other isomers of [PdX2H]- are unstable and easily rearrange into the T-shaped form or decompose. These palladium hydride intermediates were detected by MS in precatalyst activation using NaBH4, Et3N, and a solvent molecule as reducing agents. Online MS monitoring allowed the detection of [PdX2H]- species in the course of the Mizoroki-Heck reaction.

The product of a revealed transformation-NHC-ethynyl coupling-was observed as a catalyst transformation pathway in the Sonogashira cross-coupling, catalyzed by Pd/NHC complexes. The 2-ethynylated azolium salt was isolated in individual form and fully characterized, including X-ray analysis. A number of possible intermediates of this transformation with common formulae (NHC)n Pd(C2 Ph) (n=1,2) were observed and subjected to collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments to elucidate their structure. Measured bond dissociation energies (BDEs) and IRMPD spectra were in an excellent agreement with quantum calculations for coupling product π-complexes with Pd0 . Molecular dynamics simulations confirmed the observed multiple CID fragmentation pathways. An unconventional methodology to study catalyst evolution suggests the reported transformation to be considered in the development of new catalytic systems for alkyne functionalization reactions.

Catalytic atom-economic hydrothiolation of cyclopropyl acetylenes was developed. Using Pd/NHC complex as a precatalyst, regioselective addition of thiols to cyclopropyl acetylenes was successfully ...

The influence of the accuracy of mass measurements on the number of possible structural compositions and the computation time of computer-aided interpretation of mass spectrometric data has been evaluated. Experimental measurements have been performed for two model objects in the range of small and medium masses using high, ultrahigh, and extreme high resolution electrospray ionization mass spectrometers. The number of possible solutions have been examined and prospects of using machine learning in combination with mass spectrometry for predicting new data on reaction mechanisms and searching for hidden relationships in the chemical space have been demonstrated. It has been shown that there are two types of relationships between the molecular formula and the mass determination error depending on the ion mass: a nonlinear curve is observed for small molecules and a linear relationship is observed for large molecules.

Multiplexing capabilities and sensitivity of surface-enhanced Raman spectroscopy (SERS) nanoparticles (NPs) are strongly dependent on the selected Raman reporter. These Raman-active molecules are responsible for giving each batch of SERS NPs its unique spectral fingerprint. Herein, we studied four types of SERS NPs, namely, AuNPs labeled with trans-1,2-bis(4-pyridyl)ethylene (BPE), 4,4'-bis(mercaptomethyl)biphenyl (BMMBP), 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (PODT), and 5-(4-pyridyl)-1H-1,2,4-triazole-3-thiol (PTT), and demonstrated that the best level of theory could be chosen based on inner products of DFT-calculated and experimental Raman spectra. We also calculated the theoretical spectra of these Raman reporters bound to Au20 clusters to interrogate how SERS enhancement would affect their spectral fingerprint. Importantly, we found a correlation between B3LYP-D3 calculated and experimental enhancement factors, which opens up an avenue toward predicting which Raman reporters could offer improved sensitivity. We observed 0.5 and 3 fM limits of detection for BMMBP- and PTT-labeled 60 nm AuNPs, respectively.

The processes involving the capture of free radicals were explored by performing DFT molecular dynamics simulations and modeling of reaction energy profiles. We describe the idea of a radical recognition assay, where not only the presence of a radical but also the nature/reactivity of a radical may be assessed. The idea is to utilize a set of radical-sensitive molecules as tunable sensors, followed by insight into the studied radical species based on the observed reactivity/selectivity. We utilize this approach for selective recognition of common radicals—alkyl, phenyl, and iodine. By matching quantum chemical calculations with experimental data, we show that components of a system react differently with the studied radicals. Possible radical generation processes were studied involving model reactions under UV light and metal-catalyzed conditions.

It has recently been shown that palladium-catalyzed reactions with N-heterocyclic carbene (NHC) ligands involve R-NHC coupling accompanied by transformation of the molecular catalytic system into the nanoscale catalytic system. An important question appeared in this regard is whether such a change in the catalytic system is irreversible. More specifically, is the reverse nano-to-molecular transformation possible? In view of the paramount significance of this question to the area of catalyst design, we studied the capability of 2-substituted azolium salts to undergo the breakage of C-C bond and exchange substituents on the carbene carbon with corresponding aryl halides in the presence of Pd nanoparticles. The study provides important experimental evidence of possibility of the reversible R-NHC coupling. The observed behavior indicates that the nanosized metal species are capable of reverse transition to molecular species. Such an option, known for phosphine ligands, was previously unexplored for NHC ligands. The present study for the first time demonstrates bidirectional dynamic transitions between the molecular and nanostructured states in Pd/NHC systems. As a unique feature, surprisingly small activation barriers (18 kcal/mol) and noticeable thermodynamic driving force (-5 to -7 kcal/mol) were calculated for C-C bond oxidative addition to Pd(0) centers in the studied system. The first example of NHC-mediated Pd leaching from metal nanoparticles to solution was observed and formation of Pd/NHC complex in solution was detected by ESI-MS.

Magnetic stir bars are routinely used by every chemist doing synthetic or catalytic transformations in solution. Each bar lasts for months or years, as the regular PTFE (polytetrafluoroethylene) co...

The mercury test is a rapid and widely used method for distinguishing truly homogeneous molecular catalysis from nanoparticle metal catalysis. In the current work, using various M0 and MII complexe...

Trichomonas vaginalis causes the most common, nonviral sexually transmitted infection. Only metronidazole (Mz) and tinidazole are approved for treating trichomoniasis, yet resistance is a clinical problem. The gold(I) complex, auranofin, is active against T. vaginalis and other protozoa but has significant human toxicity. In a systematic structure-activity exploration, we show here that diversification of gold(I) complexes, particularly as halides with simple C1-C3 trialkyl phosphines or as bistrialkyl phosphine complexes, can markedly improve potency against T. vaginalis and selectivity over human cells compared to that of the existing antirheumatic gold(I) drugs. All gold(I) complexes inhibited the two most abundant isoforms of the presumed target enzyme, thioredoxin reductase, but a subset of compounds were markedly more active against live T. vaginalis than the enzyme, suggesting that alternative targets exist. Furthermore, all tested gold(I) complexes acted independently of Mz and were able to overcome Mz resistance, making them candidates for the treatment of Mz-refractory trichomoniasis.

Solar energy is one of the main renewable energy sources currently being researched, with commercial thin film solar cells currently made of CdTe or CuIn(1-x)GaxSe2 (CIGS) absorbers. However, whilst these materials make up the majority of the thin film commercial market, these solar cells have various problems relating to materials cost, and toxicity of constituent elements. Kesterite (Cu2ZnSn(S,Se)4) solar cells are becoming increasingly popular due to their tuneable band gap, relative affordability of the constituent elements, and the ability to produce high efficiency devices from solution processes. However, often expensive and toxic materials are used in production. In this paper we report on a newly developed amine-thiol solvent system based on 10% cysteamine in ethanolamine, which has low toxicity, is user-friendly and is able to readily dissolve all kesterite constituent elements, including metals and their oxides. The dissolution process and the structures of the prevalent metal complexes formed were investigated with the aid of spectroscopic methods, such as electrospray ionization mass spectrometry (ESI-MS) and infrared multiple photon dissociation (IRMPD). In most cases, two molecules of cysteamine were bound to the metals as bidentate ligands. By employing spin coating of the resulting inks, devices of up to 8.1% power conversion efficiency were fabricated.

The behavior of ubiquitously used nickel, palladium, and platinum complexes containing N-heterocyclic carbene ligands was studied in solution in the presence of aliphatic amines. Transformation of M(NHC)X2L complexes readily occurred according to the following reactions: (i) release of the NHC ligand in the form of azolium salt and formation of metal clusters or nanoparticles and (ii) isomerization of mono-NHC complexes M(NHC)X2L to bis-NHC derivatives M(NHC)2X2. Facile cleavage of the M–NHC bond was observed and provided the possibility for fast release of catalytically active NHC-free metal species. Bis-NHC metal complexes M(NHC)2X2 were found to be significantly more stable and represented a molecular reservoir of catalytically active species. Slow decomposition of the bis-NHC complexes by removal of the NHC ligands (also in the form of azolium salts) occurred, generating metal clusters or nanoparticles. The observed combination of dual fast- and slow-release channels is an intrinsic latent opportunity...

Palladium complexes with fluorinated acetylacetonate chelating ligands were studied as catalysts for alkyne hydrothiolation. A ten-fold increase in the catalytic efficiency was achieved by using 0.1 mol% of Pd(hfpd)2 complex (hfpd = hexafluoroacetylacetonate) with a variety of thiol–yne coupling partners. The principal possibility of a hundred-fold increase in the efficiency of Pd-catalyzed Markovnikov-type RSH addition with 0.01 mol% of the catalyst was successfully achieved with the hfpd ligand for the first time. The hexafluoroacetylacetonate chelating ligand not only enhanced the affinity of palladium centers to the triple bond of acetylene, but also stabilized the catalytic system against formation of insoluble polymeric [Pd(SPh)2]n species, thus ensuring that the reaction operates homogeneously. Utilizing other diketonate ligands resulted in cocktail-type catalysis with variable and poorly predictable contributions of homogeneous and heterogeneous pathways.

Actual palladium catalysts in synthetic transformations in reaction mixtures are usually represented by dynamic catalytic systems that contain various interconvertible forms of metal particles, including molecular complexes, metal clusters, and nanoparticles. The low thermodynamic stability of Pd nanoparticles can lead to their aggregation and, as a consequence, to the deactivation of the catalytic systems. Therefore, stabilization of nanosized Pd particles is of key importance to ensure efficient catalysis. This review discusses the main pathways for the formation of Pd nanoparticles and clusters from various precatalysts in catalytic systems, as well as current views on the mechanisms of stabilization of these nanosized Pd particles using various types of ionic nitrogen compounds, such as ammonium, amidinium, azolium, and pyridinium salts. The use of ionic nitrogen compounds as specially added or in situ formed stabilizers, ligands, catalytic promoters, heterogenized catalysts (supported ionic liquid phase, SILP) and reaction media (ionic liquids) is exemplified by several important catalytic reactions. The main effects of ionic nitrogen compounds on catalytic processes are also discussed, including possible involvement in catalytic cycles and unwanted side reactions.

Organic systems still dominate several traditional areas of chemical science and well-known applications, such as the synthesis of pharmaceutical compounds and drugs. However, a fascinating trend has appeared recently to combine pure organic systems into advanced molecular architectures and to create hybrid molecular systems. These interdisciplinary areas, where possible connections between organic and hybrid systems may be developed, are the focus of the present review to develop valuable practical applications.

In the present review, we consider the transformations of molecular catalysts, leaching, aggregation and various interconversions of metal complexes, clusters and nanoparticles that occur during catalytic processes. The role of catalyst evolution and the mechanistic picture of “Cocktail”-type systems are considered from the perspective of the development of a new generation of efficient, selective and re-usable catalysts for synthetic applications. Rational catalyst development and the improvement of catalyst performance cannot be achieved without an understanding of the dynamic nature of catalytic systems.

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M-NHC binding in this metal-ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M-NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal-NHC binding and [PdX3 ]- [NHC-R]+ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]+ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph-NHC coupling or reversible H-NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

Chiral copper(II) and cobalt(III) complexes (1–5 and 6, respectively) derived from Schiff bases of (S)-2-(aminomethyl)pyrrolidine and salicylaldehyde derivatives were employed in a mechanistic stud...

A catalytic system based on OX-1 metal–organic framework nanosheets is reported, incorporating catalytically active palladium (Pd) species. The Pd@OX-1 guest@host system is rapidly synthesized via a one-step single-pot supramolecular assembly, with the possibility of controlling the Pd loading. The structures of the resulting framework and of the active Pd species before and after catalytic reactions are studied in detail using a wide variety of techniques including synchrotron radiation infrared spectroscopy, inelastic neutron scattering, and X-ray absorption spectroscopy. Crystals of the resulting Pd@OX-1 composite material contain predominantly atomic and small cluster Pd species, which selectively reside on benzene rings of the benzenedicarboxylate (BDC) linkers. The composites are shown to efficiently catalyze the Suzuki coupling and Heck arylation reactions under a variety of conditions. Pd@OX-1 further shows potential to be recycled for at least five cycles of each reaction as well as an ability to recapture active Pd species during both catalytic reactions.

Usual oxygen bases decompose M/NHC complexes to give “NHC-free” metal species and azolones., Numerous reactions are catalyzed by complexes of metals (M) with N-heterocyclic carbene (NHC) ligands, typically in the presence of oxygen bases, which significantly shape the performance. It is generally accepted that bases are required for either substrate activation (exemplified by transmetallation in the Suzuki cross-coupling), or HX capture (e.g. in a variety of C–C and C-heteroatom couplings, the Heck reaction, C–H functionalization, heterocyclizations, etc.). This study gives insights into the behavior of M(ii)/NHC (M = Pd, Pt, Ni) complexes in solution under the action of bases conventionally engaged in catalysis (KOH, NaOH, t-BuOK, Cs2CO3, K2CO3, etc.). A previously unaddressed transformation of M(ii)/NHC complexes under conditions of typical base-mediated M/NHC catalyzed reactions is disclosed. Pd(ii) and Pt(ii) complexes widely used in catalysis react with the bases to give M(0) species and 2(5)-oxo-substituted azoles via an O–NHC coupling mechanism. Ni(NHC)2X2 complexes hydrolyze in the presence of aqueous potassium hydroxide, and undergo the same O–NHC coupling to give azolones and metallic nickel under the action of t-BuOK under anhydrous conditions. The study reveals a new role of NHC ligands as intramolecular reducing agents for the transformation of M(ii) into “ligandless” M(0) species. This demonstrates that the disclosed base-mediated O–NHC coupling reaction is integrated into the catalytic M/NHC systems and can define the mechanism of catalysis (molecular M/NHC vs. “NHC-free” cocktail-type catalysis). A proposed mechanism of the revealed transformation includes NHC-OR reductive elimination, as implied by a series of mechanistic studies including 18O labeling experiments.

The great impact of the nanoscale organization of reactive species on their performance in chemical transformations creates the possibility of fine-tuning of reaction parameters by modulating the nano-level properties. This methodology is extensively applied for the catalysts development whereas nanostructured reactants represent the practically unexplored area. Here we report the palladium- and copper-catalyzed cross-coupling reaction involving nano-structured nickel thiolate particles as reagents. On the basis of experimental findings we propose the cooperative effect of nano-level and molecular-level properties on their reactivity. The high degree of ordering, small particles size, and electron donating properties of the substituents favor the product formation. Reactant particles evolution in the reaction is visualized directly by dynamic liquid-phase electron microscopy including recording of video movies. Mechanism of the reaction in liquid phase is established using on-line mass spectrometry measurements. Together the findings provide new opportunities for organic chemical transformations design and for mechanistic studies., Direct visualization and characterization of the influence of nanoscale organization on the reactivity of organic functional groups in solution remain a challenge. Here, the authors report the real-time observation of the involvement of a nanostructured reagent in a cross-coupling reaction using advanced liquid-phase electron microscopy.