Aryl chlorides, due to their affordability and accessibility, are preferred reagents in Pd-catalyzed arylation reactions. However, the reactivity of aryl chlorides is often lower than that of aryl bromides and iodides due to the significantly higher barriers of the oxidative addition stage. This research introduces a novel design for NHC ligands, which notably enhances the efficiency of Pd/NHC catalytic systems in reactions where the oxidative addition of aryl chloride is the rate-limiting step. This design leverages the synergy between specific steric characteristics and the anionic nature of the newly fashioned 1,2,4-triazol-5-ylidene ligands. These ligands, inspired by Nitron-type designs, can be ionized under basic conditions due to their NH-acidic aryl(alkyl)amino groups. Detailed experimental and DFT studies revealed that the deprotonation of these NHCs promotes electron donation to the metal center, promoting the oxidative addition of aryl chloride. The specially optimized ATPr ligand, featuring 2,6-diisopropylphenyl groups, displayed remarkable catalytic efficacy in the Suzuki-Miyaura reaction and improved outcomes in ketone α-arylation and Buchwald-Hartwig reactions with unactivated aryl chlorides. The insights and strategies established in this study provide rational considerations for further advancements in NHC designs and their applications in metal-catalyzed reactions.

Palladium complexes with N-heterocyclic carbenes (Pd/NHC) serve as prominent precatalysts in numerous Pd-catalyzed organic reactions. While the evolution of Pd/NHC complexes, which involves the cleavage of the Pd–C(NHC) bond via reductive elimination and dissociation, is acknowledged to influence the catalysis mechanism and the performance of the catalytic systems, conventional analytic techniques [such as NMR, IR, UV–vis, gas chromatography–mass spectrometry (GC–MS), and high-performance liquid chromatography (HPLC)] frequently fail to quantitatively monitor the transformations of Pd/NHC complexes at catalyst concentrations typical of real-world conditions (below approximately 1 mol %). In this study, for the first time, we show the viability of using electrospray ionization mass spectrometry (ESI-MS). This approach was combined with the use of selectively deuterated H-NHC, Ph-NHC, and O-NHC coupling products as internal standards, allowing for an in-depth quantitative analysis of the evolution of Pd/NHC catalysts within actual catalytic systems. The reliability of this approach was affirmed by aligning the ESI-MS results with the NMR spectroscopy data obtained at greater Pd/NHC precatalyst concentrations (2–5 mol %) in the Mizoroki–Heck, Sonogashira, and alkyne transfer hydrogenation reactions. The efficacy of the ESI-MS methodology was further demonstrated through its application in the Mizoroki–Heck reaction at Pd/NHC loadings of 5, 0.5, 0.05, and 0.005 mol %. In this work, for the first time, we present a methodology for the quantitative characterization of pivotal catalyst transformation processes commonly observed in M/NHC systems.

Readily available air-stable (NHC)NiCp₂ complexes (NHC is N-heterocyclic carbene, Cp is cyclopentadienyl anion ligand) can be used as efficient thermally activated precatalysts for hydroheteroarylation of olefins with various heteroarenes. According to preliminary mechanistic studies, precatalyst activation may involve reductive elimination of two Cp ligands to give 1,1''-bi(cyclopenta-2,4-diene) and (NHC)Ni⁰ active species under elevated temperature.

A new method has been developed for synthesizing nitron- type 1,4-dyaryl-3-arylamino-1,2,4-triazolium salts through a stepwise reaction of N,N'-diarylcarbodiimides with aryl hydrazines, trimethyl orthoformate and Me3SiCl. These salts have been utilized as NHC-proligands to prepare novel (NHC)₂NiCl₂ and (NHC)NiCpCl (Cp = cyclopentadienyl anion) complexes. Catalytic trials of these complexes uncovered a highly effective catalyst for the Suzuki–Miyaura cross-coupling of non-activated aryl chlorides.

Imidazolium salts have found ubiquitous applications as N-heterocyclic carbene precursors and metal nanoparticle stabilizers in catalysis and metallodrug research. Substituents directly attached to the imidazole ring can have a significant influence on the electronic, steric, and other properties of NHC-proligands as well as their metal complexes. In the present study, for the first time, a new type of Pd/NHC complex with the RSO₂ group directly attached to the imidazol-2-ylidene ligand core was designed and synthesized. The electronic properties as well as structural features of the new ligands were evaluated by means of experimental and computational methods. Interestingly, the introduction of a 4-aryl(alkyl)sulfonyl group only slightly decreased the electron donation, but it significantly increased the π-acceptance and slightly enhanced the buried volume (%V_bur) of new imidazol-2-ylidenes. New Pd/NHC complexes were obtained through selective C(2)H-palladation of some of the synthesized 4-RSO₂-functionalized imidazolium salts under mild conditions. Several complexes demonstrated good activity in the catalysis of model cross-coupling reactions, outperforming the activity of similar complexes with non-substituted NHC ligands.

An efficient protocol for the C−N cross-coupling of aryl chlorides with (hetero)aryl- and alkyl amines under nickel catalysis has been developed. The main advantage of the protocol is the use of a self-activated Ni/NHC catalytic system generated in situ from readily available bench-stable air-tolerant precursors: NiCl₂Py₂, IPr ⋅ HCl, and sodium tert-butoxide. A mass spectrometry mechanistic study of the reaction system revealed the dynamics of competitive processes of Ni/NHC active species formation and degradation involving NHC reductive elimination reactions and tert-butoxide base. Optimization of the NiCl₂Py₂/IPr ⋅ HCl/tBuONa ratio and the reaction temperature allowed efficient catalysis to be achieved. The developed simple protocol represents an alternative for methods relying on the use of air-sensitive and unstable Ni(cod)₂ or expensive well-defined Ni/NHC precatalysts.

A new method for selective C(5)–H alkylation of 2-substituted furans with tertiary and secondary alkyl bromides under photoinduced by visible light (∼460 nm) palladium catalysis has been developed. The method is relied on the use of available Pd(PPh₃)₄ catalyst under mild conditions (30 ± 5 °C, K₂CO₃ as base), tolerates to various functional groups in furanic substrates and provides from good to excellent yields of alkylated products.

Glycyrrhiza glabra residue (GR), the fibrous industrial byproduct of glycyrrhizin extraction, and its conversion to sugars are of high potential value in biorefinery. To depolymerize GR and to separate its glucan, xylan and lignin components, three pretreatment agents were applied: PHP (phosphoric acid/hydrogen peroxide), AHP (ammonia/hydrogen peroxide) and DES (deep eutectic solvent). PHP removed the most lignin (81.7 %), while AHP recovered the most sugar (97.2 %). DES pretreatment gave a balanced 78.4 % lignin removal and 83.9 % sugar recovery. GR component separation for each pretreatment was verified by SEM, XRD, and FT-IR. Enzymatic saccharification of DES pretreated GR gave a higher sugar yield than that from PHP or AHP pretreatment, explained by fractal-like kinetic analysis of the rate constant and fractal dimension parameters. Overall, sugar yield was dependent on a combination of component separation and substrate size distribution and morphology.

A simple one-step synthesis of (NHC)₂NiCl₂ complexes containing bulky N-heterocyclic carbene (NHC) ligands relies on the C–H metallation of N,N'-diaryl-substituted azolium salts, NHC-precursors, with NiCl₂Py₂ in the presence of Cs₂CO₃ and pyridine. This procedure allows one to implement all synthetic manipulations in air.

C-Amino-1,2,4-triazoles are challenging polynitrogen substrates for metal-catalyzed arylation due to their multidentate character, enhanced coordinating ability and decreased nucleophilicity of the amino group. In the present study, the Buchwald–Hartwig cross-coupling of diverse 3(5)-amino-1,2,4-triazoles with aryl chlorides and bromides delivering (hetero)arylamino-1,2,4-triazoles in good-to-excellent yields under Pd/NHC catalysis was developed. The use of Pd complexes with bulky NHC ligands such as IPr^*OMe and TPEDO (1,1,2,2-tetraphenylethane-1,2-diol) as an in situ Pd(II) to Pd(0) reductant enabled the selective arylation of the NH₂ group even in acidic NH unprotected substrates and deactivated 1-substituted 5-amino- and 4-substituted 3-amino-1,2,4-triazoles. The reaction mechanism and structure–activity relationships were studied with DFT calculations. A significant effect of the position of the N-substituent in the 1,2,4-triazole ring on the favorable reaction pathways was revealed.

Key similarities and differences of Pd and Ni in catalytic systems are discussed. Overall, Ni and Pd catalyze a vast number of similar C–C and C–heteroatom bond-forming reactions. However, the smaller atomic radius and lower electronegativity of Ni, as well as the more negative redox potentials of low-valent Ni species, often provide higher reactivity of Ni systems in oxidative addition or insertion reactions and higher persistence of alkyl-Ni intermediates against β-hydrogen elimination, thus enabling activation of more reluctant electrophiles, including alkyl electrophiles. Another key point relates to the higher stability of the open-shell electronic configurations of Ni(I) and Ni(III) compared with Pd(I) and Pd(III). Nickel systems very often involve a number of interconvertible Ni(n+) active species of variable oxidation states (Ni(0), Ni(I), Ni(II), and Ni(III)). In contrast, catalytic reactions involving Pd(I) or Pd(III) active species are still relatively less developed and may require facilitation by special ligands or merging with photo- or electrocatalysis. However, the relatively high redox potentials of Pd(n+) species ensure their facile reduction to Pd(0) species under the assistance of numerous reagents or solvents, providing relatively high concentrations of molecular Pd1(0) complexes that can reversibly aggregate into active Pdn clusters and nanoparticles to form a cocktail of interconvertible Pdn(0) active species of various nuclearities (i.e., various values of "n"). Nickel systems involving Ni(0) complexes often require special strong reductants; they are more sensitive to deactivation by air and other oxidizers and, as consequence, often operate at higher catalyst loadings than palladium systems in the same reactions. The ease of activation and relatively high stability of low-valent active Pd species provide high robustness and versatility for palladium catalysis, whereas a variety of Ni oxidation states enables more diverse and uncommon reactivity, albeit requiring higher efforts in the activation and stabilization of nickel catalytic systems. As a point for discussion, we may note that Pd catalytic systems may easily form a "cocktail of particles" of different nuclearities but similar oxidation states (Pd1, Pdn, Pd NPs), whereas nickel may behave as a "cocktail of species" in different oxidation states but is less variable in stable nuclearities. Undoubtedly, there is stronger demand than ever not only to develop improved efficient catalysts but also to understand the mechanisms of Pd and Ni catalytic systems.

Imidazolium salts have ubiquitous applications in energy research, catalysis, materials and medicinal sciences. Here, we report a new strategy for the synthesis of diverse heteroatom-functionalized imidazolium and imidazolinium salts from easily available 1,4-diaza-1,3-butadienes in one step. The strategy relies on a discovered family of unprecedented nucleophilic addition/cyclization reactions with trialkyl orthoformates and heteroatomic nucleophiles. To probe general areas of application, synthesized N-heterocyclic carbene (NHC) precursors were feasible for direct metallation to give functionalized M/carbene complexes (M = Pd, Ni, Cu, Ag, Au), which were isolated in individual form. The utility of chloromethyl function for the postmodification of the synthesized salts and Pd/carbene complexes was demonstrated. The obtained complexes and imidazolium salts demonstrated good activities in Pd- or Ni-catalyzed model cross-coupling and C-H activation reactions.

An efficient selective C(3)–H arylation of furan ring in 2-(furan-2-yl)benzimidazoles, derivatives of fuberidazole fungicide, with aryl bromides catalyzed by [RuCl2(cymene)]2/ pivalic acid system has been accomplished. High selectivity of the process may be accounted for by the action of benzimidazol-2-yl substituent as the directing group.

Ruthenium(ii) complexes with chelating N-heterocyclic carbene (NHC) ligands were studied in the arylation of phenyl group in 2-phenylpyridine and 1-phenylpyrazole with aryl chlorides in water. Complexes with NHC-ligands containing a hemilabile coordinating N-carboxylatomethyl group enable fast and selective ortho-CH-diarylation in the absence of carboxylate-assisting additives.

Complexes of palladium and nickel with N-heterocyclic carbene ligands (M/NHC, M = Pd, Ni) are widely used as effective catalysts for various amination reactions. A previously unaddressed transformation of M/NHC complexes under typical conditions of the Buchwald–Hartwig amination is disclosed. MII/NHC complexes react with primary aromatic and aliphatic amines in the presence of strong bases to give azol-2(5)-imines and M(0) species via a reductive elimination of NHC and azanide (N-deprotonated amine) ligands. Depending on the structures of the NHC and azanide, the N–NHC coupling can make a significant contribution to the M/NHC catalyst decomposition in the Buchwald–Hartwig and other amination reactions conducted in the presence of strong bases. The discovery of the N–NHC coupling reaction has been shown to be critically influenced by the steric bulkiness of N-substituents on the NHC ligand. The high steric bulkiness of the NHC is an important factor in suppressing the N–NHC coupling deactivation pathway.

Complexes of Pd(II) with NHC ligands can suffer facile decomposition in the presence of alkali metal hydroxides, alkoxydes and other strong oxygen-containing bases via the reductive elimination of the NHC and Pd-coordinated base anion, the so-called O–NHC coupling. O–NHC coupling can represent a serious problem for the stability of Pd/NHC catalytic systems in numerous practically important reactions conducted in the presence of bases. In the present study, a new approach to stabilizing the Pd–NHC bond against cleavage by strong bases was developed. The approach relies on the installation of an NH–acidic RNH substituent at position 3 of the triazole ring of the 1,2,4-triazol-5-ylidene ligand. A series of new Pd/NHCs containing RNH substituents (R = Ac, Ph, alkyl) in triazole NHC ligands were synthesized. These complexes undergo reversible deprotonation of the RNH group in strong alkaline media and demonstrate superior stability of the Pd–NHC bond, significantly higher than complexes of similar structure without the RNH group. DFT calculations revealed that the anionic Pd/NHC complex containing an N-deprotonated acetamido group (R = Ac) is more kinetically stable against O–NHC coupling and less prone to lose NHC via heterolytic dissociation of the Pd–NHC bond than the neutral complex. The new complexes with RNH-functionalized NHC ligands were tested as precatalysts in the Suzuki–Miyaura coupling of p-tolyl bromide with phenylboronic acid in the presence of KOH and revealed more than 2 times higher TONs than similar complexes without the RNH group or ligandless Pd system.

C–H functionalization in the area of fine organic synthesis is dominated by noble metal catalysts, which represent the most expensive and least sustainable options. Sustainable C–H functionalization may involve Ni catalysts. However, Ni(0) complexes are unstable under regular conditions and are more difficult to obtain as compared to Pd(0) or Rh(I). In the present study, a facile method for Ni0/NHC-catalyzed C–H alkylation and alkenylation of heteroarenes with alkenes and internal alkynes is presented. This method relies on the in situ generation of Ni0/NHC complexes from air-tolerant bench-stable precursors, Ni(Cp)2, NHC·HCl salts and sodium formate. The optimized catalytic system demonstrates broad substrate scope and high selectivity (>60 products were obtained in up to 99% isolated yield). The approach represents a user-friendly alternative for air-sensitive and labile (NHC)Ni0 and Ni(COD)2 precatalysts or complexes. The intermediates involved in the catalytic system were investigated and possible decomposition routes were mapped with NMR and ESI-MS. Rational control over the catalyst decomposition pathways further strengthens the sustainability of the procedure.

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.

Recently, the dynamic nature of the metal-NHC bond has been proposed and the key role of chemical evolution in changing the nature of catalytically active sites is now an emerging topic. A comparative analysis of the ketone α-arylation reaction with aryl halides, catalyzed by M/NHC complexes, was carried out in the present study and showed a fundamental difference in the behavior of the catalytic system for M = Ni and Pd. In situ evolution of Ni/NHC complexes with cleavage of the Ni-NHC bond leads to complete deactivation of catalytic systems, regardless of the nature of the aryl halide ArX (X = Cl, Br, I). However, upon Pd/NHC catalysis, the cleavage of the Pd-NHC bond causes deactivation only in the case of aryl chlorides. In the reactions of more active aryl iodides and aryl bromides, NHC-disconnected Pd species, formed as a result of the chemical transformation of Pd/NHC complexes, can provide effective catalysis in the arylation reaction under study. New catalytic systems based on Pd/NHC and Ni/NHC complexes generated in situ from stable imidazolium salts, IPr·HCl and IPr*OMe·HCl, and Pd(OAc)2 (0.1 mol%) or NiCl2Py2 (5 mol%) were developed for the selective α-arylation of methylaryl ketones (Pd-catalysis) and other ketones less prone to aldol-crotonic condensation (Ni-catalysis). The present study has shown that the different effects of the metal-NHC bond cleavage should be taken into account for the efficient choice and optimization of catalytic systems to carry out arylation reaction with various aryl halides.

Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal−ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

Many reactions catalyzed by Pd complexes with N-heterocyclic carbene (NHC) ligands are performed in the presence of amines which usually act as coupling reagents or mild bases. However, amines can react with Pd/NHC complexes in a number of ways: enhancing molecular catalysis, causing the catalyst deactivation or triggering the ligandless modes of catalysis by producing NHC-free active palladium species. This study gains insight into conditions required for the efficient use of amines as activators of molecular Pd/NHC catalysis and preventing the undesirable reductive cleavage of the Pd-NHC bond in catalytic systems. Reactions of Pd/NHC complexes with various amines within a temperature range of 25–140 °C and thermal stability of the resulting amino-complexes are examined. The results indicate the major influence of the amine structure and reaction temperature on the catalyst transformations. In particular, thermal decomposition of Pd/NHC complexes with aliphatic amine ligands predominantly leads to the reductive Pd-NHC bond cleavage, while deprotonation of the complexes with primary and secondary aliphatic amine ligands in the presence of strong bases at 25–60 °C promotes the activation of molecular Pd/NHC catalysis. Efficient Pd-PEPPSI complex – amine systems suitable for the strong-base-promoted C-S cross-coupling reactions between aryl halides and thiols are suggested on the basis of these findings.

Complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via typically proposed M0/MII catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, MII/MIV catalytic cycles have also been proposed for a number of reactions. Catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling, the side reductive elimination of NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R]+ cations. In this study, relative stability of (NHC)MII(R)(X)L and (NHC)MIV(R)(X)3L intermediates (X = Cl, Br, I; L = NHC, pyridine) against the R-NHC coupling and other decomposition pathways via reductive elimination reactions is evaluated theoretically. The study reveals that R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (MII and MIV), while it is thermodynamically and kinetically more facile for the MIV complexes. Relative effects of the metal M (Ni, Pd, Pt), ligands L and X on the R-NHC coupling for the MIV complexes are significantly stronger than for the MII complexes. In particular, for (NHC)2MIV(Ph)(Br)3 complexes, Ph-NHC coupling is facilitated dramatically from Pt (ΔG = -36.9 kcal/mol, ΔG≠ = 37.5 kcal/mol) to Pd (ΔG = -61.5 kcal/mol, ΔG≠ = 18.3 kcal/mol) and Ni (ΔG = -80.2 kcal/mol, ΔG≠ = 4.7 kcal/mol). For the MII oxidation state of the metal, bis-NHC complexes (L = NHC) are kinetically and thermodynamically slightly more stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). The inverse relation is observed for the MIV oxidation state of the metal, as (NHC)2MIV(R)(X)3 complexes are kinetically (4.3-15.9 kcal/mol) and thermodynamically (8.0-23.2 kcal/mol) significantly less stable than the (NHC)MIV(R)(X)3L (L =pyridine) complexes. For NiIV and PdIV complexes, additional decomposition pathways via the reductive elimination of NHC and X ligand to give [NHC-X]+ cation (X-NHC coupling) or reductive elimination of X-X molecule are found to be thermodynamically and kinetically probable. In overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not stabilized additionally by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising MIV intermediates.

N‐Heterocyclic carbene ligands (NHC) 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 cation 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 via either Ph‐NHC coupling or reversible H‐NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

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 complexes of palladium and platinum that are often used in homogeneous catalysis as examples, we demonstrated that the mercury test is generally inadequate as a method for distinguishing between homogeneous and cluster/nanoparticle catalysis mechanisms for the following reasons: (i) the general and facile reactivity of both molecular M0 and MII complexes toward metallic mercury and (ii) the very high and often unpredictable dependence of the test results on the operational conditions and the inability to develop universal quantitatively defined operational parameters. Two main types or mercury-induced transformations, the cleavage of M0 complexes and the oxidative–reductive transmetalation of MII complexes, including a reaction of highly popular MII/NHC complexes, were elucidated using NMR, ESI-MS, and EDXRF techniques. A mechanistic picture of the reactions involving metal complexes was revealed with mercury, and representative metal species were isolated and characterized. Even in an attempt to not overstate the results, one must note that the use of the mercury tests often leads to inaccurate conclusions and complicates the mechanistic studies of these catalytic systems. As a general concept, distinguishing reaction mechanisms (homogeneous vs cluster/nanoparticle) by using catalyst poisoning requires careful rethinking in the case of dynamic catalytic systems.

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 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)2X2were 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 of M/NHC complexes, which balances the activity and durability of a catalytic system. The fast release of catalytically active species from M(NHC)X2L complexes can rapidly initiate catalytic transformation, while the slow release of catalytically active species from M(NHC)2X2 complexes can compensate for degradation of catalytically active species and help to maintain a reliable amount of catalyst. The study clearly shows an outstanding potential of dynamic catalytic systems, where the key roles are played by the lability of the M–NHC framework rather than its stability.

Biomass processing wastes (humins) are anticipated to become a large‐tonnage solid waste in the near future, owing to the accelerated development of renewable technologies based on utilization of carbohydrates. In this work, the utility of humins as a feedstock for the production of activated carbon by various methods (pyrolysis, physical and chemical activation, or combined approaches) was evaluated. The obtained activated carbons were tested as potential electrode materials for supercapacitor applications and demonstrated combined micro‐ and mesoporous structures with a good capacitance of 370 F g−1 (at a current density of 0.5 A g−1) and good cycling stability with a capacitance retention of 92 % after 10 000 charge/discharge cycles (at 10 A g−1 in 6 m aqueous KOH electrolyte). The applicability of the developed activated carbon for practical usage as a supercapacitor electrode material was demonstrated by its successful utilization in symmetric two‐electrode cells and by powering electric devices. These findings provide a new approach to deal with the problem of sustainable wastes utilization and to advance challenging energy storage applications.

The Pt/C catalysts with various Pt content (5-30 wt%) synthesized viaelectrochemical pulse alternating current technique have been evaluated for the base-free aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. The higher Pt content in the catalyst (30 wt%) provides the product yield up to 65% upon performing the process in concentrated (∼0.1 M) aqueous solutions of the substrate.

Oxidative addition of organic halides (R–X) to (NHC)Pd0L complexes is involved in numerous metal-catalyzed reactions, and this step is expected to afford (NHC)PdII(R)(X)L intermediate complexes. However, these complexes may undergo further transformation via R–NHC coupling, which removes the NHC ligands from the metal and results in the generation of "bare" NHC-free metal species. The comparative theoretical study carried out in the present work revealed that the kinetic and thermodynamic stability of the (NHC)PdII(R)(X)L oxidative addition intermediates depends strongly on the nature of the organic group R. The predicted reactivity in the R–NHC coupling process decreases in the following order: R = Vinyl > Ethynyl > Ph > Me. Accordingly, for R = Me, a classical (NHC)PdII(R)(X)L intermediate can be expected as a product of the oxidative addition step, whereas for R = Ph, the outcome of the oxidative addition may already contain the NHC-free palladium complex. For R = Ethynyl, comparable amounts of both complexes should be formed, while for R = Vinyl, the NHC-free palladium complex can be the major product of the oxidative addition process. Unusual thermodynamic and kinetic instability of the (NHC)Pd(vinyl)(X)L complex and the tendency to vinyl–NHC coupling predicted by the computational modeling has been confirmed by experimental measurements with online mass spectrometric reaction monitoring. Thus, the outcome of the oxidative addition strongly depends on the type of organic group R and the R–NHC coupling process greatly influences the activity and stability of metal catalysts.

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)2complex (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]nspecies, 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.

An efficient synthesis of 2-amino-1-R-[1,2,4]triazolo[1,5-a]-pyrimidinium or 3-amino-2-R-[1,2,4]triazolo[4,3-a]pyrimidi- nium chloride derivatives by heterocyclization of 3,5-diamino-1-R-1,2,4-triazoles (R = Alk or Ar) with pentane-2,4-diones was developed. The process is promoted by chlorotrimethyl- silane which plays the dual role of carbonyl-activating agent and water scavenger.

An efficient method for the oxidation of alcohols to aldehydes or ketones in a two-phase CH2Cl2/NaHCO3 (aq.) system, using iodine and catalytic amounts of 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl and 2,4,6-trimethylpyridine, was developed. The performance of the method was demonstrated by the selective oxidation of 37 variously substituted alcohols in ≥90% yield, including the gram-scale synthesis of the important chemical 2,5-diformylfuran from biomass-derived 5-hydroxylmethylfurfural.

Metal complexes bearing N-heterocyclic carbene (NHC) ligands are typically considered the system of choice for homogeneous catalysis with well-defined molecular active species due to their stable metal–ligand framework. A detailed study involving 19 different Pd-NHC complexes with imidazolium, benzimidazolium, and triazolium ligands has been carried out in the present work and revealed a new mode of operation of metal-NHC systems. The catalytic activity of the studied Pd-NHC systems is predominantly determined by the cleavage of the metal–NHC bond, while the catalyst performance is strongly affected by the stabilization of in situ formed metal clusters. In the present study, the formation of Pd nanoparticles was observed from a broad range of metal complexes with NHC ligands under standard Mizoroki–Heck reaction conditions. A mechanistic analysis revealed two different pathways to connect Pd-NHC complexes to "cocktail"-type catalysis: (i) reductive elimination from a Pd(II) intermediate and the release of NHC-containing byproducts and (ii) dissociation of NHC ligands from Pd intermediates. Metal-NHC systems are ubiquitously applied in modern organic synthesis and catalysis, while the new mode of operation revealed in the present study guides catalyst design and opens a variety of novel opportunities. As shown by experimental studies and theoretical calculations, metal clusters and nanoparticles can be readily formed from M-NHC complexes after formation of new M–C or M–H bonds followed by C–NHC or H–NHC coupling. Thus, a combination of a classical molecular mode of operation and a novel cocktail-type mode of operation, described in the present study, may be anticipated as an intrinsic feature of M-NHC catalytic systems.

5-Hydroxymethylfurfural (HMF) is an important versatile reagent, a so-called platform chemical, that can be produced from plant biomass compounds: hexose carbohydrates and lignocellulose. In the near future, HMF and its derivatives could become an alternative feedstock for the chemical industry and replace, to a great extent, non-renewable sources of hydrocarbons (oil, natural gas and coal). This review analyzes recent advances in the synthesis of HMF from plant feedstocks and considers the prospects for the use of HMF in the production of monomers and polymers, porous carbon materials, engine fuels, solvents, pharmaceuticals, pesticides and chemicals. The most important HMF derivatives considered in the review include 2,5-furandicarboxylic acid, 2,5-diformylfuran, 2,5-bis(hydroxymethyl)furan, 2,5-bis(aminomethyl)furan, 2,5-dimethylfuran, 2,5-dimethyltetrahydrofuran, 2,5-bis(methoxymethyl)furan, and 5-ethoxymethylfurfural. In the nearest future, a significant extension of the HMF application is expected, and this platform chemical may be considered a major source of carbon and hydrogen for the chemistry of the 21st century.

The influence of doping of Co/SiO2 catalysts with alumina on a performance of Fischer-Tropsch synthesis (FTS) was studied by extended FTS trials in a fixed-bed tubular pilot-scaled reactor. The addition of small amount of Al2O3 causes apparent promotional effect on the catalysts activity and C5+ hydrocarbons selectivity. The largest promotion effect was observed for the catalysts with 1 wt.% of alumina loading. The modification of the catalyst with alumina (1 wt.%) changes molecular weight distribution of the resultant C5+ paraffins with increasing the fraction of C8–C25 and decreasing the fraction of longer chain hydrocarbons. The addition of a proper amount of alumina into Co/SiO2 catalyst alters Co° particle size distribution making it narrower with the maximum at 8 nm and the same mean value for Co° particle size. A volcano-like dependence of CO chemisorption on alumina loadings with a maximum at 1 wt.% was observed. Relatively high CO chemisorption at the proper amount of alumina decreases the ratio of surface hydrogen to carbon monoxide and in such a way promotes formation of C5+ hydrocarbons.

A versatile approach for the synthesis of [1,2,4]triazolodipyrimidinones with various annulations of the triazole and pyrimidine rings was developed. The isomeric triazolodipyrimidinones were obtained by the stepwise condensation of partially hydrogenated [1,2,4]triazolo[1,5-a]pyrimidin-2-amines with β-ketoesters or diethyl ethoxymethylenemalonate, alkoxy base-mediated cyclization of the enamines, and subsequent cascade rearrangement of the 10-oxo-[1,2,4]triazolo[1,5-a:4,3-a′]dipyrimidines.

A combination of computational and experimental methods was used to examine the structure–reactivity relationships in the reactions of C-amino-1H-1,2,4-triazoles with electrophiles. The global nucleophilicity of 3-amino- and 3,5-diamino-1H-1,2,4-triazoles was predicted to be higher than that of 5-amino-1H-1,2,4-triazoles. Fukui functions and molecular electrostatic potential indicate that reactions involving an amino group should occur more easily for the 3-amino- than for the 5-amino-1H-1,2,4-triazoles. Increasing electrophile hardness should increase the probability of attack at the N-4 atom of the triazole ring, whereas increasing softness should enhance the probability of attack at the N-2 atom and 3-NH2 group. Calculated transition state energies of model SN2 reactions and experimental studies showed that quaternization of 1-substituted 3-amino- and 3,5-diamino-1H-1,2,4-triazoles by many alkyl halides proceeds with low selectivity and can involve the N-2 and N-4 atoms as well as the 3-NH2 group as reaction centers. A new method for the selective synthesis of 1,4-disubstituted 3-amino- and 3,5-diamino-1,2,4-triazoles based on quaternization of readily available 1-substituted 3-acetylamino-1,2,4-triazoles with subsequent removal of the acetyl protecting group by acid hydrolysis was developed.

The synthesis of 6-hetarylamino[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines is reported. The functionalized secondary amines were constructed via a K2CO3-mediated SNAr reaction of weakly basic hetarylamines with 3-(3,5-dimethylpyrazol-1-yl)[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines, which allowed displacement 3,5-dimethylpyrazolyl leaving group. Significantly, the reaction exhibited a broad substrate scope and proceeded in good yields.

Partially hydrogenated 2-amino[1,2,4]triazolo[1,5-a]pyrimidines and 2-amino[1,2,4]triazolo[5,1-b]quinazolines react with the chlorides of chloroacetic, 3-chloropropanoic, and 4-chlorobutanoic acids at 0–5 °C to give amides through acylation of the 2-amino group. Heating the corresponding 3-chloropropanoyl derivatives at 80–90 °C in DMF leads to selective intramolecular alkylation at N-3 to form the chlorides of partially hydrogenated [1,2,4]triazolo[1,5-a:4,3-a′]dipyrimidin-5-ones and pyrimido[2′,1′:3,4][1,2,4]triazolo[5,1-b]quinazolin-12-ones. It may be more convenient to prepare such compounds through one-pot processes. Some reactions of the synthesized chlorides of polycondensed heterocycles have been studied. Conditions have been found to effect the selective synthesis of free bases, oxidative aromatization or hydrolysis of the dihydropyrimidine cycle, and the selective hydrolytic cleavage or elimination of the pyrimidone ring. Some of the resulting compounds represent new mesoionic heterocycles.