Copper(II) complexes of 2-(pyrrolidinomethyl)- and 2-(3-pyrrolinomethyl)pyridine N-oxides

Summary This title pyridine N-oxides have been prepared and their copper(II) complexes isolated as perchlorate, tetrafluoroborate and nitrate salts. The ligands coordinatevia both the pyridine N-oxide oxygen and the amine nitrogen to give bis(ligand) complexes for the perchlorate and tetrafluoroborate salts. The nitrate solids have [CuL(NO₃)₂] stoichiometry with monodentate nitrato-ligands. The spectral properties of these complexes are compared to those of N-alkyl-and N,N-dialkyl2-picolinamine N-oxides as well as other 2-substituted pyridine N-oxides.NATO Fellow on leave from Istanbul Medical Faculty, Istanbul University.     

Pentacoordinate Silicon Complexes with N‐(2‐pyridylmethyl)­salicylamide as a Dianionic (ONN′) Tridentate Chelator

The title compound, N‐(2‐pyridylmethyl)salicylamide ( 1 ), was synthesized by ester aminolysis of methyl salicylate and 2‐picolylamine. In the presence of triethylamine as a supporting base, the salicylamide moiety reacts with the organodichlorosilanes RR′SiCl₂ to form the desired six‐membered heterocycles of the type RR′Si–O–(o‐C₆H₄)–C(=O)N(pic), with pic being the 2‐pyridylmethyl (i.e., 2‐picolyl) moiety and RR′ = Me, Me ( 2a ); Me, Ph ( 2b ); Ph, Ph ( 2c ); Bn, Bn ( 2d ); All, Ph ( 2e ) and Ph, H ( 2f ). Despite the absence of notable ring strain release Lewis acidity (i.e., only a six‐membered chelate is formed by the dianion, and smaller rings are not present in the compound), the poor electron withdrawal from silicon by its C– or H– substituents and the flexible methylene bridge between the salicylamide and the pyridine moiety, the pyridine N donor atom furnishes pentacoordinate silicon coordination spheres in all of these compounds 2a – 2f . The coordination number of the silicon atom was confirmed by single‐crystal X‐ray diffraction analysis for the solid state and by ²⁹Si NMR spectroscopy for the solution state.     

Synergistic extraction studies of uranium(VI) and plutonium(VI) with some β-diketones and heterocyclic N-oxides

The equilibrium constants for coordination of methyl substituted pyridine N-oxides with plutonium(VI) thenoyl trifluoroacetonate in chloroform (K_s) follow an order similar to those of the analogous uranium(VI) complexes indicating steric hindrance to bonding in the case of ortho substituted pyridine N-oxides. The extraction constants (k) of Pu(VI) chelates with various β-diketones are found to be only marginally higher than the values for the corresponding uranium(VI) chelates which is in conformity with the close similarity of the ionic radii of PuO ₂ ²⁺ and UO ₂ ²⁺ .     

N16-oxides of sparteine, 2-methylsparteine and 2-phenylsparteine as ligands; spectroscopic and DFT studies of complexes with ZnX2 (X=Cl,Br)

Six new ZnX₂ (X=Cl, Br) complexes with N16-oxides of sparteine, 2-methylsparteine and 2-phenylsparteine as ligands have been synthesized and characterized by MS, IR, NMR and DFT methods. All complexes have 1 : 1 stoichiometry. Complexation with N16-oxides involves inversion of the configuration at N16, converting ring C from a boat into a chair with the oxygen engaged in coordination. All complexes investigated are of composition [(L–H)⁺(ZnX₃)^?] (where L is N-oxide). The structures of the complexes obtained have been compared with those of the monoperchlorate salts of the N-oxides.     

Halogen Bonding Adsorbent Pyridine N-oxides for Iodine Capture in Water

Rapid capture of ¹²⁹I with high volatility and toxicity in the environment has attracted much attention. Herein we reported a firstly synthesized nonporous material: pyridine N-oxides (NTPO and ATPO) as iodine adsorbent. Both of NTPO and ATPO exhibit remarkable performance on the adsorption of iodine in aqueous solution, vapor state and organic solvents. Upon the capture of iodine, pyridine N-oxides were transformed to binary cocrystals combined with the pyridine N-oxides and iodine which is driven by halogen bond between iodine and oxygen atoms. Moreover, pyridine N-oxides shows high chemical, thermal and moisture stability.     

Computational Exploration of Mechanistic Avenues in Metal-Free CO2 Reduction to CO by Disilyne Bisphosphine Adduct and Phosphonium Silaylide

Recent years have seen a growing interest in metal-free CO₂ activation by silylenes, silylones, and silanones. However, compared to mononuclear silicon species, CO₂ reduction mediated by dinuclear silicon compounds, especially disilynes, has been less explored. We have carried out extensive computational investigations to explore the mechanistic avenues in CO₂ reduction to CO by donor-stabilized disilyne bisphosphine adduct ( R1^M ) and phosphonium silaylide ( R2 ) using density functional theory calculations. Theoretical calculations suggest that R1^M exhibits donor-stabilized bis(silylene) bonding features with unusual Si−Si multiple bonding. Various modes of CO₂ coordination to R1^M have been investigated and the coordination of CO₂ by the carbon center to R1^M is found to be kinetically more facile than that by oxygen involving only one or both the silicon centers. Both the theoretically predicted reaction mechanisms of R1^M and R2 -mediated CO₂ reduction reveal the crucial role of silicon-centered lone pairs in CO₂ activations and generation of key intermediates possessing enormous strain in the Si−C−O ring, which plays the pivotal role in CO extrusion.     

Synthesis and Characterization of New N‐Heterocyclic Silylazides

Three novel pyridine functionalized N‐heterocyclic silanes, bearing chloride and azide moieties, were synthesized and characterized by NMR spectroscopy (¹H, ¹³C, ²⁹Si), mass spectrometry, elemental analysis, and single‐crystal XRD. The molecular structures show a comparably strong dative interaction of the pyridine‐N with the Si center, formally inducing a penta‐coordination arrangement at the silicon(IV). Under appropriate conditions, the silylazides, presented in this work, might be able to thermo‐ or photolytically liberate gaseous nitrogen giving rise to a promising synthetic option to access a variety of new transition metal silylene complexes with potential applications in various catalytic reactions.     

Synthesis of N-(trifluorosilylmethyl)glutarimide. Alternate intramolecular coordination of the silicon atom to two oxygen atoms

A new representative of draconoids, N-(trifluorosilylmethyl)glutarimide, was synthesized by reaction of N-(trimethyoxysilylmethyl)glutarimide with boron trifluoride-ether complex. According to the IR and ¹H, ¹³C, ¹⁵N, ¹⁹F, and ²⁹Si NMR data, this compound molecule is characterized by intramolecular coordination between the silicon and carbonyl oxygen atoms (alternate coordination of the silicon atom to each oxygen atom).     

Physicochemical and catalytic properties of SAPO-31 materials prepared from the silica sources characterized by a different degree of polymerization

Two series of microporous silicoaluminophosphates SAPO-31 with different Si content (Si/Al = 0—0.5) were synthesized using silica sources characterized by various degree of polymerization (fumed silica and tetraethoxysilane). Physicochemical properties of the resulted materials were studied by X-ray diffraction, XPS, scanning (SEM) and transmission (TEM) electron microscopy. The nature of the silicon species was identified by ²⁹Si NMR method and the acidity was measured by IR-spectroscopy of adsorbed pyridine. The activity and selectivity of Pd modified SAPO-31 samples from both series were evaluated in the isomerization of n-decane. It was found that the use of the monomer as a silicon source ensured more efficient silicon incorporation, which resulted in the higher activity of Pd/SAPO-31 catalysts. Moreover, the selectivity of these samples for isomerization was found to be independent of the silicon content and its distribution.     

Determination of water content in silica nanopowder using wavelength-dispersive X-ray fluorescence spectrometer

A wavelength-dispersive X-ray fluorescence (WDXRF) technique that uses the scattered radiation of the X-ray tube lines and the fluorescence radiation of an element present in a powder sample is proposed as a non-destructive method for the determination of the water content in silica powder. Although direct X-ray fluorescence analysis of water using WDXRF is not adequate for the quantitative determination of water in powder, due to the very low fluorescence yield for hydrogen and oxygen, the fluorescence signal of silicon (Si) in silica powder is attenuated by water, and is shown to decrease in proportion to the water content in silica powder. In addition, it is demonstrated that the Compton- and Rayleigh-scattering of the X-ray tube lines is proportional to the water content. The coefficients of determination, R², of the linear regression equations obtained from the calibration curves for all individual scattered radiations and for the fluorescence radiation of Si were > 0.90. The sum of the peak intensities of the four scattering signals, i.e. the Rayleigh-scattered Rh K–L_2,3 and Rh K–M_2,3 lines, and Compton-scattered Rh K–L_2,3 and Rh K–M_2,3 lines, also showed fairly good linearity and sensitivity over a very wide range of water content from 0 wt.% to 61.5 wt.%. However, porosity had a significant effect on the X-ray signal at low water content, in the range from 0 wt.% to 7.5 wt.%, where the sensitivity for the silica nanopowder with well-defined mesopores (~ 3 nm in diameter) decreased to 0.40 kcps/wt.%, from 0.99 kcps/wt.% for the non-porous silica nanopowder. The use of the Si fluorescence signal along with the scattered radiation of the X-ray tube lines expands the applicability of conventional XRF spectrometers to the quantitative determination of water content in silica powder.     

A CP-MAS NMR study of some deactivation methods in capillary gas chromatography

The effect of temperature, water content, and the type of reagent on the silylation of fused silica capillaries was studied by ²⁹Si and ¹³C CP-MAS NMR. Fumed silica (Cab-O-Sil M5), which is essentially a highly dispersed vitreous quartz with a surface comparable to that of fused silica capillary columns, was selected as a model material. Hexamethyldisilazane (HMDS) and 1,2-diphenyl-1,1,3,3-tetraphenyldisilazane (DPTMDS), which were used as silylation reagents, yielded trimethyl- and dimethylphenylsilyl surface groups respectively at lower temperatures (< 350°C and <250°C respectively). At higher temperatures, increasingly more dimethylsilyl groups are formed, with the silicon bound to two oxygen atoms. This process occurs for DPTMDS at a considerably lower temperature than for HMDS. The formation of silyl groups on the surface and the disappearance of hydroxyl groups are followed independently. The ¹³C NMR and GC-MS of the reaction products showed that with DPTMDS, the formation of two Si-O-Si links is accompanied by a loss of phenyl groups rather than of methyl groups. After the Cab-O-Sil had been dried over P₂O₅, the formation of these double links occurred for HMDS only at temperatures above 460°C and for DPTMDS at 400°C. Thus we concluded that water supplies oxygen atoms for double Si-O-Si links (possibly crosslinks) necessary for efficient deactivation. This may explain the less successful silanization of fused silica capillaries because their water content is lower than that of glass capillaries.     

Transport Processes at α‐Quartz–Water Interfaces: Insights from First‐Principles Molecular Dynamics Simulations

Car–Parrinello molecular dynamics (CP–MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the α‐quartz–water interface. The model system initially consists of a periodically repeated quartz slab with O‐terminated and Si‐terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H₂O molecules into H⁺ and OH^? is observed at the Si‐terminated surface. The OH^? fragments immediately bind chemically to the Si‐terminated surface while Grotthus‐type proton diffusion through the water film leads to protonation of the O‐terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO₂ present as Si(OH)₄ in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)₄ molecule as would have been indicated by a substantial lowering of the Si(OH)₄ diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP–MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si? O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si? O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si? O bond. The final solute species is Si(OH)₄.     

Acid-base equilibria of substituted pyridine N-oxides in methanol

Acid dissociation constants in methanol for eight substituted pyridine N-oxides having a wide range of acid-base properties, [quinoline N-oxide (bi-cyclic amine N-oxide) and pyridine (heterocyclic amine)] have been determined using the potentiometric titration method. A linear correlation between ourmethanol data and aqueous pK _a values from the literature has been found. As in polar aprotic solvents cationic homoconjugation phenomenon has been found to be present for sufficiently basic N-oxides. The tendency of substituted pyridine N-oxides towards cationic homoconjugation in methanol is weaker than in polar aprotic solvents and increases with increasing basicity of N-oxides. It has also been found that, in contrast to polar aprotic solvents, the cationic homoconjugation phenomenon in methanol is much more pronounced for heterocyclic amines than their N-oxides.     

Revisiting structure of silica gels from water glass: an 1H and 29Si MAS and CP-MAS NMR study

The silica gels, derived from water glass solution with pH adjusted at 3.0 and 9.9, were revisited to investigate their constitution, although water glass has been studied for last tens of decades on gelation. Solid-state nuclear magnetic resonance spectroscopy was applied to the nuclei ¹H and ²⁹Si, by the use of magic angle spinning (MAS), ¹H → ²⁹Si CP-MAS (CP: cross-polarization), and modern techniques such as 2D HETCOR (two dimensional heteronuclear correlation), and variable-contact time CP techniques. Gelation time (t_gel) showed U-letter shape dependence on pH. All gels consisted of Qⁿ groups (n: 2, 3, and 4), where Qⁿ stands for a silicate unite [(O_1/2)_nSi (–O^?)_4?n] (n: 0–4). The analysis of the ¹H → ²⁹Si CP kinetics and ¹H-²⁹Si HETCOR spectra elucidated the presence of four kinds of ¹H nuclei, i.e., those giving a peak at 6.9 ppm in chemical shift δ: ¹H–OSi hydrogen bonded to H₂O molecules; one at 4.3 ppm: ¹H of adsorbed water molecules, hydrogen-bonded to the silanol groups; one at 1.7 ppm: ¹H–OSi confined in the gel lattice, including that forming aggregations like Si–O^H/_NaO–Si; and one at 4.2 ppm: ¹H of water molecules on the outermost hydration layer.     

Pyridine-Stabilized Cationic Silanethione Tungsten Complexes: Synthesis,Structures, and Reactivity

Silanethione compounds, R₂Si=S, have been recognized as highly reactive species. One reliable way to stabilize silanethione is its coordination to transition metal fragments to convert silanethione-coordinated transition metal complexes. Herein, we report the synthesis, structure, and reactivity of a second cationic silanethione tungsten complex [Cp*(OC)₃W{S=SiR₂(py)}]TFPB (R=Me ( 5 a ), Ph ( 5 b ), Cp*: η⁵-C₅Me₅, py: pyridine, and TFPB⁻: [B{3,5-(CF₃)₂C₆H₃}₄]⁻). Complex 5 was obtained by H⁻ abstraction from the Si atom in the corresponding silylsulfanyl complex Cp*(OC)₃W(SSiR₂H) ( 4 ) with Ph₃CTFPB, followed by the addition of pyridine. The reaction of 5 with PhNCS and PMe₃ produced [Cp*(OC)₃W{SSiR₂N(Ph)C(PMe₃)₂}]TFPB (R=Me ( 6 a ), Ph ( 6 b )) via the elimination of pyridine and the addition of the 1,3-dipolar species PhNC(PMe₃)₂ ( A ) to the Si atom.     

Reaktionen subvalenter Verbindungen des Siliciums mit Stickstoffheterocyclen

Reactions of Subvalent Compounds of Silicon with Nitrogenheterocycles Subvalent compounds of silicon were prepared from dichloro-bis(diethylamino)silane with Na/K alloy in hexane-dimethyloxyethane containing pyridine, 4, 4′-bipyridine, indole, chinoline, lepidine, isochinoline, mono-, di- or trimethylpyridine, N-methylindole or benzofurane. Products isolated from the reaction mixtures are formed either by the insertion of a Si(NEt₂)₂ unit (i) into a CH bond of methyl groups to give silanes RSi(NEt₂)₂H (with N-methylindole, sym-collidine), (ii) into a CN bond of the heterocycle expanding the six membered ring by one unit to give silazepines, or (iii) by the addition of a ClSi(NEt₂)₂ unit to the nitrogen atom of the aromatic heterocycle, which results in 1, 4-dihydro(chlorosilyl)pyridine derivatives. In the silazepines lithiation and subsequent reaction with (CH₃)₃SiCl occurs on a CH- or CH₂ group of the ring. Most compounds are characterized by elemental analyses, n.m.r. (¹H, ¹³C and ²⁹Si) and mass spectra; some have been identified by high resolution mass spectrometry and (or) characteristic signals in their n.m.r. spectra. X-ray structure analyses are provided for V and XX .     

Studies of Silicon Podand Solvents

Three types of silicon podands, Me₂Si(OR)₂, EtSi(OR)₃ and PhSi(OR)₃, where R is a polyoxaethylene chain with different numbers of oxygen atoms (two, three or four), were obtained and studied by ¹H, ¹³C and ²⁹Si NMR methods. NMR spectra of ¹H, ⁷Li, ¹³C, ²³Na and ²⁹Si nuclei were also used for the study of lithium, sodium and rubidium complexes with the silicon podands. Theoretical calculations were performed using the PM3 hamiltonian. The heats of reactions between the compounds obtained and SbCl₅ were determined.     

Potentiometric and ab initio studies of acid-base interactions of substituted 4-halo(Cl, Br)pyridine N-oxide systems

By using the potentiometric method, acidity constants have been determined in systems of tri- and tetra-substituted pyridine N-oxides. The potentiometric measurements in systems of four 4-chloropyridine N-oxide derivatives containing the chlorine atom at position 4 to the NO₂ group and four bromine counterparts were carried out in polar non-aqueous solvents, viz. amphiprotic methanol (MeOH) and aprotic protophilic dimethyl sulfoxide (DMSO). It was found that in all the systems studied the pK_a values were readily determinable (as indicated by small standard deviations) in MeOH, whereas in DMSO large standard deviations were obtained making the pK_a values either hardly determinable or indeterminable from potentiometric measurements. Furthermore, it was demonstrated that the acidity constants of protonated N-oxides studied in MeOH changed according to the sequence of their acidity constants in water. It was also found that in the polar solvents studied, i.e. in the amphiprotic methanol and the highly basic aprotic dimethyl sulfoxide, the cationic homo-conjugation equlibrium constants could not be determined using potentiometric method. Also, by using ab initio methods at the RHF and MP2 levels and the PCM model, utilizing the Gaussian 6-31++G^∗∗ basis set, energies and Gibbs free energies of the protonation reactions of the N-oxides have been determined. The energy parameters have been compared with acidity constants of the protonated N-oxides determined by potentiometric titration in methanol to establish a correlation between these approaches.     

Silver(I) Cation Complexation with 3α,3'α-Bis(pyridine-n-carboxy) Lithocholic Acid 1,2-Ethanediol Diesters (n = 2–4): 1H, 13C and 15N NMR Spectral Studies and Molecular Orbital Calculations

Three isomeric molecular clefts: 3,3'-bis(pyridine-n-carboxy) lithocholic acid 1,2-ethanediol diesters (n = 2–4) 1–3 have been synthesized and their structures ascertained by ¹H, ¹³C NMR and MALDI TOF MS. Their complex formation with Ag⁺-cation (added as AgO₃ SCF₃) have been investigated by means of NMR and molecular orbital calculations. The coordination behaviour of the silver(I) cation is dependent on the isomerism of the pyridine-n-carboxy moiety. In 1 (pyridine-2-carboxylato = picolinato) both NMR and theoretical calculations strongly suggest that the coordination occurs with the lone electron pairs of the pyridine nitrogen and carbonyl oxygen in both of the arms of the molecular cleft separately. In 2 and 3 (pyridine-3-carboxylato = nicotinato and pyridine-4-carboxylato = isonicotinato) where the distance between the pyridine nitrogen and carbonyl oxygen is too large to allow the same type of coordination as in 1, ₃-complexation with the pyridine ring and carbonyl oxygen of the different arms of the molecular cleft simultaneously is suggested by molecular orbital calculations and supported also by NMR. No Ag⁺-cation coordination was observed with the 1,2-ethanediol oxygens in 1–3.     

A hydrogen-1, nitrogen-15, and chlorine-35 NMR coordination study of Lu(ClO4)3 and Lu(NO3)3 in aqueous solvent mixtures

A coordination study of Lu(III) has been carried out for the nitrate and perchlorate salts in aqueous mixtures of acetone-d₆ and Freon-12 by¹H,¹⁵N and³⁵Cl NMR spectroscopy. At temperatures lower than –90°C, proton and ligand exchange are slow enough to permit the direct observation of¹H resonance signals for coordinated and free water molecules, leading to an accurate measure of the Lu(III) hydration number. In perchlorate solution, in the absence of inner-shell ion-pairing, Lu(III) exhibits a maximum coordination number of six over the allowable concentration range of study, contrasting markedly with the report of values of six to nine or greater as determined by a similar NMR method. The absence of contact ion-pairing was confirmed by³⁵Cl NMR chemical shift and linewidth measurements. Extensive ion-pairing was observed in the nitrate solutions as reflected by the lower Lu(III) hydration numbers of two to three in these systems, the observation of two coordinated water signals, and¹⁵N NMR signals for two complexes. The¹H and¹⁵N NMR spectra and the hydration number could be accounted for by the presence of (H₂O)₄Lu(NO₃)²⁺ and (H₂O)₂Lu(NO₃) ₂ ¹⁺ .