Revealing the Mechanism of Combining Best Properties of Homogeneous and Heterogeneous Catalysis in Hybrid Pd/NHC Systems.

The formation of transient hybrid nanoscale metal species from homogeneous molecular precatalysts has been demonstrated by in situ NMR studies of catalytic reactions involving transition metals with N-heterocyclic carbene ligands (M/NHC). These hybrid structures provide benefits of both molecular complexes and nanoparticles, enhancing the activity, selectivity, flexibility, and regulation of active species. However, they are challenging to identify experimentally due to the unsuitability of standard methods used for homogeneous or heterogeneous catalysis. Utilizing a sophisticated solid-state NMR technique, we provide evidence for the formation of NHC-ligated catalytically active Pd nanoparticles (PdNPs) from Pd/NHC complexes during catalysis. The coordination of NHCs via C(NHC)-Pd bonding to the metal surface was first confirmed by observing the Knight shift in the 13C NMR spectrum of the frozen reaction mixture. Computational modeling revealed that as little as few NHC ligands are sufficient for complete ligation of the surface of the formed PdNPs. Catalytic experiments combined with in situ NMR studies confirmed the significant effect of surface covalently bound NHC ligands on the catalytic properties of the PdNPs formed by decomposition of the Pd/NHC complexes. This observation shows the crucial influence of NHC ligands on the activity and stability of nanoparticulate catalytic systems.

Quantitative Determination of Active Species Transforming the R-NHC Coupling Process under Catalytic Conditions.

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.

New class of RSO2-NHC ligands and Pd/RSO2-NHC complexes with tailored electronic properties and high performance in catalytic C-C and C-N bonds formation.

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 (%Vbur) 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.

One-Step Access to Heteroatom-Functionalized Imidazol(in)ium Salts.

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 the 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.

The key role of R-NHC coupling (R = C, H, heteroatom) and M-NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species.

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.

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki-Heck Reaction.

Invited for the cover of this issue are Valentine P. Ananikov and co-workers. The image depicts the dynamic behaviour of a Pd/NHC catalytic system with easy transition from molecular to ionic complex. Read the full text of the article at 10.1002/chem.201903221.

Relative stabilities of M/NHC complexes (M = Ni, Pd, Pt) against R-NHC, X-NHC and X-X couplings in M(0)/M(ii) and M(ii)/M(iv) catalytic cycles: a theoretical study.

The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M0/MII catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, MII/MIV catalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R]+ cations. In this study, the relative stabilities of (NHC)MII(R)(X)L and (NHC)MIV(R)(X)3L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R-NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the 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. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R-NHC coupling for the MIV complexes were significantly stronger than that for the MII complexes. In particular, for the (NHC)2MIV(Ph)(Br)3 complexes, Ph-NHC coupling was facilitated dramatically from Pt (ΔG = -36.9 kcal mol-1, ΔG≠ = 37.5 kcal mol-1) to Pd (ΔG = -61.5 kcal mol-1, ΔG≠ = 18.3 kcal mol-1) and Ni (ΔG = -80.2 kcal mol-1, ΔG≠ = 4.7 kcal mol-1). For the MII oxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the MIV oxidation state of the metal as the (NHC)2MIV(R)(X)3 complexes were kinetically (4.3-15.9 kcal mol-1) and thermodynamically (8.0-23.2 kcal mol-1) significantly less stable than the (NHC)MIV(R)(X)3L (L = pyridine) complexes. For the NiIV and PdIV complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC-X]+ cation (X-NHC coupling) or reductive elimination of the X-X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising MIV intermediates.

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki-Heck Reaction.

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.

Sustainable Utilization of Biomass Refinery Wastes for Accessing Activated Carbons and Supercapacitor Electrode Materials.

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.

Revealing the unusual role of bases in activation/deactivation of catalytic systems: O-NHC coupling in M/NHC catalysis.

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.

Selective Synthesis of 2,5-Diformylfuran by Sustainable 4-acetamido-TEMPO/Halogen-Mediated Electrooxidation of 5-Hydroxymethylfurfural.

A new method was developed for the selective gram-scale synthesis of 2,5-diformylfuran (DFF), which is an important chemical with a high application potential, via oxidation of biomass-derived 5-hydroxylmethylfurfural (HMF) catalyzed by 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-AcNH-TEMPO) in a two-phase system consisting of a methylene chloride and aqueous solution containing sodium hydrogen carbonate and potassium iodide. The key feature of this method is the generation of the I2 (co-)oxidant by anodic oxidation of iodide anions during pulse electrolysis. In addition, the electrolyte can be successfully recycled five times while maintaining a 62-65 % yield of DFF. This novel method provides a sustainable pathway for waste-free production of DFF without the use of metal catalysts and expensive oxidants. An advantage of electrooxidation is utilized in the preparation of demanding chemical.

Diversity Oriented Synthesis of Polycyclic Heterocycles through the Condensation of 2-Amino[1,2,4]triazolo[1,5-a]pyrimidines with 1,3-Diketones.

The acid-catalyzed condensation between 2-aminosubstituted [1,2,4]triazolo[1,5-a]pyrimidines and their analogues with various saturation of the pyrimidine ring and 1,3-diketones or 1,1,3,3-tetramethoxypropane was evaluated as a new approach for the synthesis of diversely substituted polycyclic derivatives of triazolopyrimidine. The reaction of 4,5,6,7-tetrahydro- or aromatic aminotriazolopyrimidines results in selective formation of the corresponding [1,2,4]triazolo[1,5-a:4,3-a']dipyrimidin-5-ium salts, and the condensation of substrates containing the 4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine fragment is accompanied by a cascade rearrangement with unusual recyclization of the dihydropyrimidine ring to yield partially hydrogenated [1,2,4]triazolo[1,5-a:4,3-a']dipyrimidin-5-ium or pyrimido[1',2':1,5][1,2,4]triazolo[3,4-b]quinazolin-5-ium salts. The proposed methodology exhibits a wide scope, providing rapid access to polycondensed derivatives of the [1,2,4]triazolo[1,5-a]pyrimidine scaffold. DFT calculations of the Gibbs free energies of possible isomers were performed to rationalize the experimentally observed reactivity and selectivity.

Reactivity of C-Amino-1,2,4-triazoles toward Electrophiles: A Combined Computational and Experimental Study of Alkylation by Halogen Alkanes.

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.

Crystal structure of bis-[(5-amino-1H-1,2,4-triazol-3-yl-κN (4))acetato-κO]di-aqua-nickel(II) dihydrate.

The title compound, [Ni(C4H5N4O2)2(H2O)2]·2H2O, represents the first transition metal complex of the novel chelating triazole ligand, 2-(5-amino-1H-1,2,4-triazol-3-yl)acetic acid (ATAA), to be structurally characterized. In the mol-ecule of the title complex, the nickel(II) cation is located on an inversion centre and is coordinated by two water mol-ecules in axial positions and two O and two N atoms from two trans-oriented chelating anions of the deprotonated ATAA ligand, forming a slightly distorted octa-hedron. The trans angles of the octa-hedron are all 180° due to the inversion symmetry of the mol-ecule. The cis-angles are in the range 87.25 (8)-92.75 (8)°. The six-membered chelate ring adopts a slightly twisted boat conformation with puckering parameters Q = 0.542 (2) Å, Θ = 88.5 (2) and ϕ = 15.4 (3)°. The mol-ecular conformation is stabilized by intra-molecular N-H⋯O hydrogen bonds between the amino group and the chelating carboxyl-ate O atom of two trans-oriented ligands. In the crystal, the complex mol-ecules and lattice water mol-ecules are linked into a three-dimensional framework by an extensive network of N-H⋯O, O-H⋯O and O-H⋯N hydrogen bonds.

A direct approach to a 6-hetarylamino[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine library.

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.

2-Amino-5-methyl-3-(2-oxo-2-phenyl-eth-yl)-7-phenyl-4,5,6,7-tetra-hydro-3H-[1,2,4]triazolo[1,5-a]pyrimidin-8-ium bromide ethanol monosolvate.

In the title compound, C20H22N5O(+)·Br(-)·C2H6O, the tetra-hydro-pyrimidine ring of the bicyclic cation adopts a half-chair conformation with an equatorial orientation of the phenyl and methyl substituents. The amino group is nearly coplanar with the 1,2,4-triazole ring [interplanar angle = 4.08 (8)°] and has a slightly pyramidal configuration. The mean planes of the triazole ring and the benzene ring of the phenacyl group form a dihedral angle of 88.58 (7)°. In the crystal, N-H⋯Br, N-H⋯O and O-H⋯Br hydrogen bonds link the cations, anions and ethanol mol-ecules into layers parallel to the bc plane.

6-(3,5-Dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetra-zin-3(2H)-one.

The title compound, C7H8N6O, represents the keto form and adopts a nearly planar structure (r.m.s. deviation of the non-H atoms = 0.072 Å). In the crystal, mol-ecules form spiral chains along the c axis by N-H⋯N hydrogen bonds. The chains are linked to each other by weak C-H⋯O hydrogen bonds, forming a three-dimensional framework.

3-Methyl-4-(2-phenyl-1,2,4-triazolo[1,5-a]pyrimidin-7-yl)furazan.

In the title mol-ecule, C14H10N6O, the planes of the methyl-furazan fragment and the phenyl ring attached to the triazolo-pyrimidine bicycle are twisted from the mean plane of the bicycle at angles of 45.92 (5) and 5.45 (4)°, respectively. In the crystal, π-π inter-actions, indicated by short distances [in the range 3.456 (3)-3.591 (3) Å] between the centroids of the five- and six-membered rings of neighbouring mol-ecules, link the mol-ecules into stacks propagating along the c-axis direction.

4-Benzyl-3-[(1-oxidoethylidene)amino]-1-phenyl-4,5-dihydro-1H-1,2,4-triazol-5-iminium.

The title compound, C(17)H(17)N(5)O, exists in the zwitterionic form with the amide group deprotonated. The mean planes of the 1,2,4-triazole and N-phenyl rings form a dihedral angle of 39.14 (8)°. The N atom of the amino group adopts a trigonal configuration. Inter-moleculat C-H⋯O and C-H⋯N hydrogen bonds occur. In the crystal, mol-ecules are linked into a two-dimensional network parallel to (10[Formula: see text]) by N-H⋯O and N-H⋯N hydrogen bonds. C-H⋯N contacts are also observed.

4-(5-Amino-1H-1,2,4-triazol-3-yl)pyridinium chloride monohydrate.

In the cation of the title compound, C(7)H(8)N(5) (+)·Cl(-)·H(2)O, the mean planes of the pyridine and 1,2,4-triazole rings form a dihedral angle of 2.3 (1)°. The N atom of the amino group adopts a trigonal-pyramidal configuration. The N atom of the pyridine ring is protonated, forming a chloride salt. In the crystal, inter-molecular N-H⋯O, N-H⋯N, N-H⋯Cl and O-H⋯Cl hydrogen bonds link the cations, anions and water mol-ecules into layers parallel to the (1, 0, ) plane.