Advances and Challenges in Development of Transition Metal Catalysts for Heterogeneous Hydrogenation of Organic Compounds

Actual problems of development of catalysts for hydrogenation of heterocyclic compounds by hydrogen are summarized and discussed. The scope of review covers composites of nanoparticles of platinum group metals and 3d metals for heterogeneous catalytic processes. Such problems include increase of catalyst activity, which is important for reduction of precious metals content; development of new catalytic systems which do not contain metals of platinum group or contain cheaper analogues of Pd; control of factors which make influence on the selectivity of the catalysts; achievement of high reproducibility of the catalyst's performance and quality control of the catalysts. Own results of the authors are also summarized and described. The catalysts were prepared by decomposition of Pd0 and Ni0 complexes, pyrolysis of Ni2+ and Co2+ complexes deposited on aerosil and reduction of Ni2+ in pores of porous support in situ. The developed catalysts were used for hydrogenation of multigram batches of heterocyclic compounds.

Redox Behavior of Cobalt-Phosphine Complexes vs Their Catalytic Activity in Organozinc Compound Formation: Background for Mechanistic Investigations

Catalytic activity in arylzinc compound formation was studied for eight Co complexes with phosphines along with their redox properties for implementing the idea of rational design. It was found that Co(XantPhos)Cl₂ and Co(N-XantPhos)Cl₂ demonstrated distinct reversible Co^II/Co^I redox processes and acted as efficient catalysts of arylzinc compound formation. Meanwhile, for Co(DPEphos)Cl₂, Co(dppf)Cl₂, Co(dppb)Cl₂, Co(PPh₃)₂Cl₂, and Co(XantPhos)(Piv)₂ (the latter one without the addition of LiCl), reversible redox processes were not observed. These catalysts did not act efficiently for the model process of organozinc compound formation. Co₄(dppe)₅Cl₈ was the only exception, explained by a completely different structure (CoP₄Cl and CoPCl₃) of donor sets instead of CoP₂X₂ (X = Cl or O). The stability of complexes in tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) solutions was studied by UV-vis spectroscopy. Previously unknown X-ray structures for Co(XantPhos)(Piv)₂, Co(N-XantPhos)Cl₂, and {Co(DMF)₆}{(CoCl₃)₂(dppb)} were determined. The use of pivalate counterions instead of chloride for Co(XantPhos)²⁺ led to a significant (ca. 20 times) increase of the kinetic solubility in THF compared to Co(XantPhos)Cl₂, preserving high catalytic productivity upon the addition of LiCl. This allowed the latter to be efficiently used in combination with LiCl as the catalyst for arylzinc compound formation on a 2 g scale. The data obtained in this work can be regarded as experimental confirmation of the first and last stages of the plausible reaction pathway of arylzinc compound formation, involving Co^II → Co^I and Co^I → Co^II transformations, which could be a significant framework for further mechanistic investigations.

Screening of Palladium/Charcoal Catalysts for Hydrogenation of Diene Carboxylates with Isolated-Rings (Hetero)aliphatic Scaffold

A series of seven palladium-containing composites, i.e., four Pd/C and three Pd(OH)₂/C (Pearlman's catalysts), was prepared using modified common approaches to deposition of Pd or hydrated PdO on charcoal. All the composites were tested in the catalytic hydrogenation of diene carboxylates with the isolated-ring scaffold, e.g., 5,6-dihydropyridine-1(2H)-carboxylates with 2-(alkoxycarbonyl)cyclopent-1-en-1-yl and hex-1-en-1-yl substituents at the C(4)-position. The performance of the composites was also studied via the hydrogenation of quinoline as a model reaction. The composites were characterized by transmission and scanning electron microscopy (TEM and SEM), powder X-ray diffraction, and low-temperature N₂ adsorption. It was found that the composites containing Pd nanoparticles (NPs) of 5-40 nm size were the most efficient catalysts for the hydrogenation of dienes, providing the reduced products with up to 90% yields at p(H₂) = 100 atm, T = 30 °C for 24 h. The method of Pd NPs formation had more effect on the catalyst performance than the size of the NPs. The catalytic performance of Pearlman's catalysts (Pd(OH)₂/C) in the hydrogenation of dienes was comparable to or lower than the performance of the Pd/C systems, though the Pearlman's catalysts were more efficient in the hydrogenation of quinoline.

Catalytic Oxidation of Benzoins by Hydrogen Peroxide on Nanosized HKUST-1: Influence of Substituents on the Reaction Rates and DFT Modeling of the Reaction Path

In this research, the oxidation of a series of benzoins, R-C(=O)-CH(OH)-R, where R = phenyl, 4-methoxyphenyl, 4-bromophenyl, and 2-naphthyl, by hydrogen peroxide in the presence of nanostructured HKUST-1 (suspension in acetonitrile/water mixture) was studied. The respective benzoic acids were the only products of the reactions. The initial average reaction rates were experimentally determined at different concentrations of benzoin, H₂O₂ and an effective concentration of HKUST-1. The sorption of the isotherms of benzoin, dimethoxybenzoin and benzoic acid on HKUST-1, as well as their sorption kinetic curves, were measured. The increase in H₂O₂ concentration expectedly led to an acceleration of the reaction. The dependencies of the benzoin oxidation rates on the concentrations of both benzoin and HKUST-1 passed through the maxima. This finding could be explained by a counterplay between the increasing reaction rate and increasing benzoin sorption on the catalyst with the increase in the concentration. The electronic effect of the substituent in benzoin had a significant influence on the reaction rate, while no relation between the size of the substrate molecule and the rate of its oxidation was found. It was confirmed by DFT modeling that the reaction could pass through the Baeyer-Villiger mechanism, involving an attack by the HOO^- anion on the C atom of the activated C=O group.

Third Generation Buchwald Precatalysts with XPhos and RuPhos: Multigram Scale Synthesis, Solvent-Dependent Isomerization of XPhos Pd G3 and Quality Control by 1H- and 31P-NMR Spectroscopy

The third generation Buchwald precatalysts Pd(ABP)(Phos)(OMs) (also known as Phos Pd G3)) with XPhos and RuPhos were prepared in multigram scale by a modified procedure (ABP = fragment of C-deprotonated 2-aminobiphenyl, XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl, RuPhos = 2-dicyclohexylphosphino-2′,6′-diisopropoxybiphenyl, OMs⁻ = CH₃SO₃⁻). The ¹H- and ³¹P-NMR spectra of the title complexes and some impurities, measured by various 1D and 2D techniques, were analyzed in detail. The solvent-dependent isomerization of Pd(ABP)(XPhos)(OMs) was studied by NMR, and the X-ray structures of two isomers were determined. The impurities in precatalysts, such as Pd(ABP)(HABP)(OMs) (HABP—neutral 2-aminobiphenyl coordinated to Pd²⁺ in N-monodentate mode) and PdCl₂(XPhos)₂, were identified and characterized by single crystal X-ray diffraction. A simple method for the quick quality control (QC) of the precatalysts, suitable for routine use, was proposed. The method was based on the assessment of the impurity content on the basis of the ¹H-NMR spectra analysis.

Cu-Catalyzed Pyridine Synthesis via Oxidative Annulation of Cyclic Ketones with Propargylamine

A Cu-catalyzed, easily scalable one-pot synthesis of fused pyridines by the reaction of cyclic ketones with propargylamine is described. The protocol was optimized based on the results of more than 30 experiments. The highest product yields were achieved in i-PrOH as a solvent in the presence of 5.0 mol % CuCl₂ in air. In contrast to the well-known Au-catalyzed protocol, our procedure is "laboratory friendly", cost-effective, and suitable for preparing dozens of grams of fused pyridine-based building blocks and does not require a high-pressure autoclave technique. Decreasing the catalyst amount in the reaction to 1.25 mol % CuCl₂ provided a yield comparable to that achieved with 5 mol % catalyst, though a longer reaction time was required. A plausible reaction mechanism was proposed. The scope and limitation of the reaction were studied using 24 different cyclic ketones as starting materials. The fused pyridine yield decreased among cyclic ketones in the following order: six-membered ≫ eight-membered > five-membered ∼ seven-membered. The elaborated reaction conditions demonstrated tolerance to a number of protective functional groups in ketone such as ester, tert-butoxycarbonyl (Boc)-protected amine, and acetal moieties.

Structures and Spectral and Magnetic Properties of a Series of Carbacylamidophosphate Pentanuclear Lanthanide(III) Hydroxo Complexes

A series of pentanuclear lanthanide complexes Ln₅L₆(μ-L)₄(μ₃-OH)₄(μ₄-OH) (Ln^III = Nd, Dy, Ho, Er, Yb; L^- = dimethyl N-benzoylamidophosphate ion, [C₆H₅C(O)-N-P(O)(OCH₃)₂]^-) was obtained by the reaction of sodium dimethyl N-benzoylamidophosphate with the corresponding lanthanide nitrates. The pentanuclear cores formed as a result of self-arrangement and their composition did not depend on the lanthanide ion. The complexes and sodium dimethyl N-benzoylamidophosphate have been characterized by single-crystal X-ray diffraction. The absorption spectra of the complexes were measured at 300 and 4 K. The dysprosium and ytterbium complexes exhibited weak emission in the visible and IR regions, respectively. Temperature dependences of magnetic susceptibility (χ_M) of the dysprosium, holmium, and erbium compounds were studied. It was found that χ_M vs T dependences were governed by the crystal field splitting effects with the Δ parameter being in the range 5-17 cm^-1. Slow magnetic relaxation was found for the dysprosium complex by ac magnetic measurements, while no significant out-of-phase signals were detected for holmium and erbium complexes.

CoII Complexes with a Tripyridine Ligand, Containing a 2,6-Di-tert-butylphenolic Fragment: Synthesis, Structure, and Formation of Stable Radicals

Interaction of a tripyridine ligand bearing a 2,6-di-tert-butylphenolic fragment (L, 2,6-di-tert-butyl-4-(3,5-bis(4-pyridyl)pyridyl)phenol) with Co^II pivalate or chloride led to the formation of one-dimensional coordination polymers [Co(L)Cl₂] _n ·nEtOH (1) and [Co₃(L)₂(OH)(Piv)₅] _n (2) or a trinuclear complex Co₃(H₂O)₄(L)₂Cl₆ (3) (Piv^- = pivalate). Chemical oxidation of L and 1-3 by PbO₂ or K₃[Fe(CN)₆], as well as exposure of L (in solution or solid state) and 2 (in solid state) to UV irradiation, led to the formation of free radicals with g = 2.0024, which probably originated because of oxidation of 2,6-di-tert-butylphenolic groups. These radicals were stable for several days in solutions and more than 1 month in solid samples. Irradiation and oxidation of the solid samples probably caused formation of the phenoxyl radical only on their surface. It was shown by density functional theory calculations that exchange coupling between the unpaired electron of the phenoxyl radical and Co^II ions was negligibly weak and could not affect the electron paramagnetic resonance signal of the radical, as well as exchange coupling of Co^II ions could not be transmitted by L. The latter conclusion was confirmed by the analysis of magnetic properties of 1: temperature dependency of magnetic susceptibility (χ_M) of 1 could be simulated by a simple model for isolated Co^II ions.

Formation of hierarchically-ordered nanoporous silver foam and its electrocatalytic properties in reductive dehalogenation of organic compounds

Nanostructuring of silver notably improved its electrocatalytic activity in reductive dehalogenation of a variety of aryl and alkyl bromides.

Sorption discrimination between secondary alcohol enantiomers by chiral alkyl-dicarboxylate MOFs

The coordination polymers withtrans-(S,S)-1,2-cyclopropane dicarboxylate or (1R,3S)-camphorate contain only one polar group in close proximity to the asymmetric C atom but show different sorption of (R) or (S) isomers of 2-butanol.

COMPARISON OF SPECTROSCOPIC PROPERTIES OF INTRAOCULAR FLUID IN PATIENTS WITH CATARACT AND PRIMARY OPEN-ANGLE GLAUCOMA

This paper presents a comparison ofspectroscopic properties of aqueous humour in patients with cataract (60 patients, 60 eyes) and combination of cataract and primary open angle glaucoma (POAG, 36 patients, 36 eyes). The samples of aqueous humour were collected during the surgery. Both groups of patients were treated with different types of local drug antihypertensive therapy with different multiplicity and duration of instillation during the preoperative period. The absorption spectrum of aqueous humour were measured in the wavelength range 190-1100 nm. There were two absorption bands with maximums at 202 and 267 nm in the absorption spectrum (2nd maximum of some samples was shifted to larger wavelengths). The mean absorbance of aqueous humour at 202 nm in patients without POAG was significantly lower than in patients with POAG. This difference could be explained with accumulation of metabolites in aqueous humour of patients with POAG, which in turn may be caused by deceleration of aqueous humour flow in patients with POAG. Meanwhile, there were no statistically significant differences of mean absorbance of aqueous humour at 267 nm between the patients of both groups.

Solvent-Induced Change of Electronic Spectra and Magnetic Susceptibility of Co(II) Coordination Polymer with 2,4,6-Tris(4-pyridyl)-1,3,5-triazine

One-dimensional coordination polymer [Co(Piv)2(4-ptz)(C2H5OH)2]n (compound 1, Piv(-) = pivalate, 4-ptz = 2,4,6-tris(4-pyridyl)-1,3,5-triazine) was synthesized by interaction of Co(II) pivalate with 4-ptz. Desolvation of 1 led to formation of [Co(Piv)2(4-ptz)]n (compound 2), which adsorbed N2 and H2 at 78 K as a typical microporous sorbent. In contrast, absorption of methanol and ethanol by 2 at 295 K led to structural transformation probably connected with coordination of these alcohols to Co(II). Formation of 2 from 1 was accompanied by change of color of sample from orange to brown and more than 2-fold decrease of molar magnetic susceptibility (χM) in the temperature range from 2 to 300 K. Resolvation of 2 by ethanol or water resulted in restoration of spectral characteristics and χM values almost to the level of that of 1. χMT versus T curves for 1 and samples, obtained by resolvation of 2 by H2O or C2H5OH, were fitted using a model for Co(II) complex with zero-field splitting of this ion.

Modeling of catalytically active metal complex species and intermediates in reactions of organic halides electroreduction

The results of quantum chemical modeling of organic and metal-containing intermediates that occur in electrocatalytic dehalogenation reactions of organic chlorides are presented. Modeling of processes that take place in successive steps of the electrochemical reduction of representative C1 and C2 chlorides - CHCl3 and Freon R113 (1,1,2-trifluoro-1,2,2-trichloroethane) - was carried out by density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). It was found that taking solvation into account using an implicit solvent model (conductor-like screening model, COSMO) or considering explicit solvent molecules gave similar results. In addition to modeling of simple non-catalytic dehalogenation, processes with a number of complexes and their reduced forms, some of which were catalytically active, were investigated by DFT. Complexes M(L1)2 (M = Fe, Co, Ni, Cu, Zn, L1H = Schiff base from 2-pyridinecarbaldehyde and the hydrazide of 4-pyridinecarboxylic acid), Ni(L2) (H2L2 is the Schiff base from salicylaldehyde and 1,2-ethylenediamine, known as salen) and Co(L3)2Cl2, representing a fragment of a redox-active coordination polymer [Co(L3)Cl2]n (L3 is the dithioamide of 1,3-benzenedicarboxylic acid), were considered. Gradual changes in electronic structure in a series of compounds M(L1)2 were observed, and correlations between [M(L1)2](0) spin-up and spin-down LUMO energies and the relative energies of the corresponding high-spin and low-spin reduced forms, as well as the shape of the orbitals, were proposed. These results can be helpful for determination of the nature of redox-processes in similar systems by DFT. No specific covalent interactions between [M(L1)2](-) and the R113 molecule (M = Fe, Co, Ni, Zn) were found, which indicates that M(L1)2 electrocatalysts act rather like electron transfer mediators via outer-shell electron transfer. A relaxed surface scan of the adducts {M(L1)2·R113}(-) (M = Ni or Co) versus the distance between the chlorine atom leaving during reduction and the corresponding carbon atom showed an energy barrier to electron transfer (the first stage of R113 catalytic reduction), while DFT optimization of the {Ni(L2)·R113}(-) adduct showed barrier-free decomposition. The difference between the stabilities of the {Ni(L1)2·R113}(-) and {Ni(L2)·R113}(-) adducts correlates with the difference between the catalytic activities of Ni(L1)2 and Ni(L2) in the electrochemical reduction of R113.

Redox-active porous coordination polymers prepared by trinuclear heterometallic pivalate linking with the redox-active nickel(II) complex: synthesis, structure, magnetic and redox properties, and electrocatalytic activity in organic compound dehalogenation in heterogeneous medium

Linking of the trinuclear pivalate fragment Fe2CoO(Piv)6 by the redox-active bridge Ni(L)2 (compound 1; LH is Schiff base from hydrazide of 4-pyridinecarboxylic acid and 2-pyridinecarbaldehyde, Piv(-) = pivalate) led to formation of a new porous coordination polymer (PCP) {Fe2CoO(Piv)6}{Ni(L)2}1.5 (2). X-ray structures of 1 and 2 were determined. A crystal lattice of compound 2 is built from stacked 2D layers; the Ni(L)2 units can be considered as bridges, which bind two Fe2CoO(Piv)6 units. In desolvated form, 2 possesses a porous crystal lattice (SBET = 50 m(2) g(-1), VDR = 0.017 cm(3) g(-1) estimated from N2 sorption at 78 K). At 298 K, 2 absorbed a significant quantity of methanol (up to 0.3 cm(3) g(-1)) and chloroform. Temperature dependence of molar magnetic susceptibility of 2 could be fitted as superposition of χMT of Fe2CoO(Piv)6 and Ni(L)2 units, possible interactions between them were taken into account using molecular field model. In turn, magnetic properties of the Fe2CoO(Piv)6 unit were fitted using two models, one of which directly took into account a spin-orbit coupling of Co(II), and in the second model the spin-orbit coupling of Co(II) was approximated as zero-field splitting. Electrochemical and electrocatalytic properties of 2 were studied by cyclic voltammetry in suspension and compared with electrochemical and electrocatalytic properties of a soluble analogue 1. A catalytic effect was determined by analysis of the catalytic current dependency on concentrations of the substrate. Compound 1 possessed electrocatalytic activity in organic halide dehalogenation, and such activity was preserved for the Ni(L)2 units, incorporated into the framework of 2. In addition, a new property occurred in the case of 2: the catalytic activity of PCP depended on its sorption capacity with respect to the substrate. In contrast to homogeneous catalysts, usage of solid PCPs may allow selectivity due to porous structure and simplify separation of product.

Synthesis, structure and magnetic properties of Nd3+ and Pr3+ 2D polymers with tetrafluoro-p-phthalate

Two lanthanide tetrafluoro-p-phthalate (L(2-)) complexes, Ln(L)(1.5)·DMF·H(2)O (Ln = Pr(3+) (1), Nd(3+) (2)), were synthesized using pyridine as a base. The compounds were found to be isostructural, and the structure of 1 has been determined by single crystal X-ray diffraction (monoclinic, space group C2, a = 22.194(2) Å, b = 11.4347(12) Å, c = 11.7160(12) Å, β = 94.703(2)°, V = 2963.3(5) Å(3), Z = 4). The crystal structure of 1 consists of dinuclear Pr(3+) units, which are connected by tetrafluoro-p-phthalate, forming separate 2D polymeric layers. The Ln(3+) ions in the dinuclear Ln(2) units are linked by two μ-O atoms and by two bridging O-C-O groups. The structure is porous with DMF and water molecules located between layers. Non-coordinated DMF molecules occupy about 27% of the unit cell volume. A systematic analysis of reported structures of Ln(III) polymers with p-phthalate and its derivatives shows that the ca. known 60 structures can be divided into six possible structural types depending on the presence of certain structural motifs. The magnetic properties of compounds 1 and 2 were studied. The dependence of χ(M)T on T (where χ(M) is magnetic susceptibility per dinuclear lanthanide unit) for 1 and 2 was simulated using two different models, based on: (i) the Hamiltonian Ĥ = ΔĴ(z)(2)+ μ(B)g(J)HĴ, which utilises an axial splitting parameter Δ and temperature-independent paramagnetism (tip) and (ii) crystal field splitting. It was found that both models gave satisfactory fits, indicating that the Ln-Ln exchange interactions are small and the symmetry of the coordination environment is the main factor influencing the magnetic properties of these compounds.

Porous 2D coordination polymeric formate built up by Mn(II) linking of Fe3O units: influence of guest molecules on magnetic properties

A first representative of coordination polymers, built from polynuclear carboxylate bridged by mononuclear carboxylate, has been synthesised and structurally characterized. The compound [{Fe(3)O(HCOO)(6)}{Mn(HCOO)(3)(H(2)O)(3)}].3.5HCOOH crystallised in the hexagonal system (space group P6(3)), a = b = 12.369(5) A, c = 12.992(5) A, alpha = beta = 90 degrees , gamma = 120 degrees , V = 1721.4(12) A(3). The crystal structure is built from 2D layers, which are formed by the linking of trinuclear {Fe(3)O(HCOO)(6)}(+) units by mononuclear {Mn(HCOO)(3)(H(2)O)(3)}(-) bridges. These 2D honeycomb layers are connected by molecules of formic acid through a hydrogen bond network. The compound has the voids at the centre of the hexagons. These voids are filled by captured solvent molecules. Solvate molecules are also located between the 2D layers, linking them through a system of H-bonds. All these solvates can be easily removed and/or exchanged, which results in the formation of [{Fe(3)O(HCOO)(6)}{Mn(HCOO)(3)(H(2)O)(3)}].0.5HCOOH.H(2)O and [{Fe(3)O(HCOO)(6)}{Mn(HCOO)(3)(H(2)O)(3)}]. Magnetic properties of the compounds can be considered as the superposition of magnetism of Fe(3)O trinuclear units, for which magnetism is governed by the superexchange interaction between Fe(3+) spins (with one Fe-Fe superexchange parameter J) and the magnetism of spins of Mn(2+) ions, the interactions between these units can be taken into account by molecular field, zJ'. The destruction of the hydrogen bond network caused by evacuation of solvent molecules significantly modifies the overall magnetism. It was found that zJ' depends on the presence of solvent molecules in the structure.

A new class of macrocyclic complexes formed via nickel-promoted macrocyclisation of dioxime with dinitrile

o-Phthalonitrile couples with chelating dioxime on nickel(II), with formation of a dinuclear nickel(II) macrocyclic complex--the first representative of a new class of imine-appended macrocycles.