Hydrosilylation vs. [2 + 2]-cycloaddition: A theoretical study with iron and ruthenium complexes

Recently an exciting new mechanism of hydrosilylation had been found in experiments with the ruthenium-silylene complex [Cp^∗(i-Pr₃P)Ru(H)₂Si(H)Ph · OEt₂][B(C₆F₅)₄] by Glaser and Tilley. The mechanism of the hydrosilylation and possible alternative pathways are investigated with quantum chemical methods utilizing the B3LYP method, a double zeta pseudopotential basis set for iron and ruthenium and the 6-31G^∗ basis set for all other elements. Starting from the model complex [Cp(H₃P)Ru(H)₂Si(H)Ph]⁺ the coordination of ethene at the silicon atom leads preferably to the hydrosilylation of a terminal Si-H-bond. The analysis of the electron density distribution of the catalytic active complex shows surprising bond features between Ru and Si. The Ru-Si bond is bridged by two hydrogen atoms.The [2 + 2]-cycloaddition of the alkene to the Ru-Si-bond, which would be a reasonable alternative reaction pathway, was not observed. It is necessary to make drastic changes in the ligand environment of the transition metal-silicone complex to observe cycloaddition reactions. With complexes of the type (OC)₄MSi(H)Ph (M = Ru, Fe) the cycloaddition could be a serious alternative to the hydrosilylation.     

Platinum-catalyzed double silylations of alkynes with bis(dichlorosilyl)methanes

Bis(dichlorosilyl)methanes 1 undergo the two kind reactions of a double hydrosilylation and a dehydrogenative double silylation with alkynes 2 such as acetylene and activated phenyl-substituted acetylenes in the presence of Speier’s catalyst to give 1,1,3,3-tetrachloro-1,3-disilacyclopentanes 3 and 1,1,3,3-tetrachloro-1,3-disilacyclopent-4-enes 4 as cyclic products, respectively, depending upon the molecular structures of both bis(dichlorosilyl)methanes (1) and alkynes (2). Simple bis(dichlorosilyl)methane (1a) reacted with alkynes [R¹-CC-R²: R¹ = H, R² = H (2a), Ph (2b); R¹ = R² = Ph (2c)] at 80 °C to afford 1,1,3,3-tetrachloro-1,3-disilacyclopentanes 3 as the double hydrosilylation products in fair to good yields (33-84%). Among these reactions, the reaction with 2c gave a trans-4,5-diphenyl-1,1,3,3-tetrachloro-1,3-disilacyclopentane 3ac in the highest yield (84%). When a variety of bis(dichlorosilyl)(silyl)methanes [(Me_nCl_3 − nSi)CH(SiHCl₂)₂: n = 0 (1b), 1 (1c), 2 (1d), 3 (1e)] were applied in the reaction with alkyne (2c) under the same reaction conditions. The double hydrosilylation products, 2-silyl-1,1,3,3-tetrachloro-1,3-disilacyclopentanes (3), were obtained in fair to excellent yields (38-98%). The yields of compound 3 deceased as follows: n = 1 > 2 > 3 > 0. The reaction of alkynes (2a-c) with 1c under the same conditions gave one of two type products of 1,1,3,3-tetrachloro-1,3-disilacyclopentanes 3 and 1,1,3,3-tetrachloro-1,3-disilacyclopent-4-enes (4): simple alkyne 2a and terminal 2b gave the latter products 4ca and 4cb in 91% and 57% yields, respectively, while internal alkyne 2c afforded the former cyclic products 3cc with trans form between two phenyl groups at the 3- and 4-carbon atoms in 98% yield, respectively. Among platinum compounds such as Speier’s catalyst, PtCl₂(PEt₃)₂, Pt(PPh₃)₂(C₂H₄), Pt(PPh₃)₄, Pt[ViMeSiO]₄, and Pt/C, Speier’s catalyst was the best catalyst for such silylation reactions.     

Be and P NMR analyses on Be complexation with cyclo-tri-μ-imidotriphosphate anions in aqueous solution

Microscopic information on the complexation of Be²⁺ with cyclo-tri-μ-imidotriphosphate anions in aqueous solution has been gained by both ⁹Be and ³¹P NMR techniques at −2.3 °C. Separate NMR signals corresponding to free and complexed species have been observed in both spectra. Based on an empirical additivity rule, i.e., proportionality observed between the ⁹Be NMR chemical shift values and the number of coordinating atoms of ligand molecules, the ⁹Be NMR spectra have been deconvoluted. By precise equilibrium analyses, the formation of [BeX(H₂O)₃]⁺ and [BeX₂(H₂O)₂]⁰ (X = non-bridging oxygen donor as a coordination atom in the phosphate groups) has been verified, and the formation of complexes coordinating with the nitrogen atoms of the cyclic framework in the ligand molecule has been excluded. Instead, the formation of one-to-one (ML) complexes, one-to-two (ML₂), together with two-to-one (M₂L) complexes (L = cP₃O₆(NH)₃) has been disclosed, the stability constants of which have been evaluated as log K_ML = 3.87 ± 0.03 (mol dm⁻³)⁻¹, log K_ML2 = 2.43 ± 0.03 (mol dm⁻³)⁻² and log K_M2L = 1.30 ± 0.02 (mol dm⁻³)⁻², respectively. ³¹P NMR spectra measured concurrently have verified the formation of the complexes estimated by the ⁹Be NMR measurement. Intrinsic ³¹P NMR chemical shift values of the phosphorus atoms belonging to ligand molecules complexed with Be²⁺, together with the ³¹P-³¹P spin-spin coupling constants have been determined.     

Diphenyltin(IV) complexes of the 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olates: Synthesis and multinuclear NMR, Sn Mössbauer, electrospray ionization MS, X-ray characterization and assessment of in vitro cytotoxicity

A series of cis-bis{5-[(E)-2-(aryl)-1-diazenyl]quinolinolato}diphenyltin(IV) complexes have been synthesized and characterized by ¹H, ¹³C, ¹¹⁹Sn NMR, ESI-MS, IR and ^119mSn Mössbauer spectroscopic techniques in combination with elemental analysis. The structures of a ligand L⁶H (i.e., 5-[(E)-2-(4-ethoxyphenyl)-1-diazenyl]quinolin-8-ol) and three diphenyltin(IV) complexes, viz., Ph₂Sn(L¹)₂ · (CH₃)₂CO (1), Ph₂Sn(L⁴)₂ (4) and Ph₂Sn(L⁵)₂ (5) (L = 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-ol: aryl = phenyl - (L¹H); 4′-methylphenyl - (L⁴H) and 4′-bromophenyl - (L⁵H)) were determined by single crystal X-ray diffraction. In general, the complexes were found to adopt a distorted cis-octahedral arrangement around the tin atom. These complexes retain their solid-state structure in non-coordinating solvent as evidenced by ¹¹⁹Sn NMR spectroscopic results. The in vitro cytotoxicity of 1 is reported and compared with Ph₂Sn(Ox)₂ (Ox = deprotonated quinolin-8-ol) against seven well characterized human tumor cell lines.     

Synthesis and characterization of some organotin(IV) adducts containing a related series of pyridines: Crystal structure of [SnMe2Cl2(bu2bpy)]

Organotin(IV) complexes of [SnR_(4−n)Cl_n] (n = 2, R = Me, ⁿBu; n = 1, R = Ph) react with the bidentate pyridyl ligand 4,4′-di-tert-butyl-2,2′-bipyridine (bu₂bpy) to give hexa-coordinated adducts with the general formula [SnR_(4−n)Cl_n(bu₂bpy)]. However, the reaction of these organotin(IV) complexes with the corresponding monodentate ligand 4-tert-butylpyridine (bupy) resulted in the formation of the hexa-coordinated complex [SnMe₂Cl₂(bupy)₂] and the penta-coordinated complexes [SnR_(4−n)Cl_n(bupy)] (n = 2, R = ⁿBu; n = 1, R = Ph). Moreover, the reaction of the above organotin(IV) complexes with 4,4′-trimethylenedipyridine (tmdp) yields hexa-coordinated adducts with the general formula [SnR₂Cl₂(tmdp)] (R = Me, ⁿBu) and the penta-coordinated complex [ClPh₃Sn-μ-(tmdp)SnPh₃Cl] in the solid state. The resulting complexes have been characterized by multinuclear NMR (¹H, ¹³C, ¹¹⁹Sn) spectroscopy and elemental analysis. NMR data shows that the triphenyltin(IV) adducts are not stable in solution and dissociate to give tetra-coordinated tin(IV) complexes. The X-ray crystal structure determination of [SnMe₂Cl₂(bu₂bpy)] reveals that the tin atom is hexa-coordinated in an octahedral geometry with a trans-[SnMe₂] configuration.     

“Constrained geometry” catalysts of the rare-earth metals for the hydrosilylation of olefins

A series of yttrium and lutetium alkyl complexes [Ln(η⁵-C₅Me₄ZNR′-κN)(CH₂SiMe₃)(THF)_n] (Ln = Y, Lu) was prepared by reacting the tris(trimethylsilylmethyl) precursor [Ln(CH₂SiMe₃)₃(THF)₂] with different linked amino-cyclopentadienes of the type (C₅Me₄H)ZNHR′ (Z = SiMe₂, CH₂SiMe₂; R′ = tBu, Ph, C₆H₄-tBu-4, C₆H₄-nBu-4). The catalytic activity of these alkyl complexes in the hydrosilylation of 1-decene and styrene using PhSiH₃ as reagent was examined under standard conditions. A significant influence of the ligand structure on the catalytic property (turnover frequency, regioselectivity) was observed with the yttrium complex [Y(η⁵-C₅Me₄CH₂SiMe₂NtBu-κN)(CH₂SiMe₃)(THF)] being the most active for 1-decene hydrosilylation.     

Conformational fluxionality in a palladium(II) complex of flexible click chelator 4-phenyl-1-(2-picolyl)-1,2,3-triazole: A dynamic NMR and DFT study

An experimental and theoretical DFT study was carried out on the solution behavior in [D₇]DMF for bis-chelate complex [Pd(L)₂](BF₄)₂·2CH₃CN (L = 4-phenyl-1-(2-picolyl)-1,2,3-triazole). In structure of [Pd(L)₂]²⁺, the central square-planar palladium(II) cation is trans-chelated by two L substrates, each through the pyridine and the triazole N2 nitrogen atoms, forming two six-membered metallacycles. These can adopt boat-like conformations anti-trans-[Pd(L)₂]²⁺ and syn-trans-[Pd(L)₂]²⁺ in which the picolyl methylene carbons are anti or syn, respectively, relative to the palladium coordination plane. In solution, the boat-to-boat inversion at both metallacycles takes place. The conformers are in a dynamic equilibrium, which was monitored by variable-temperature (VT) ¹H NMR spectroscopy in the temperature range of 223-353 K. The equilibrium lies on the side of the anti-trans-[Pd(L)₂]²⁺ conformer and the corresponding reaction enthalpy and entropy is estimated to be 0.6 ± 0.5 kcal mol⁻¹ and 0.8 ± 1 cal mol⁻¹ K⁻¹, respectively. From the full-line-shape analysis of resonances in the VT ¹H NMR spectra, the activation enthalpy and activation entropy was determined to be 13.0 ± 0.4 kcal mol⁻¹ and 2.7 ± 1.6 cal mol⁻¹ K⁻¹, respectively. The activation entropy close to zero suggests a nondissociative mechanism for the isomerisation. DFT investigation revealed that the isomerisation proceeds through a one step mechanism with a barrier of 11.40 kcal mol⁻¹. The structures of the syn and anti conformers as well as that of the transition state were characterized. Energy decomposition analysis was carried out in order to explore the origins of the stability difference between the syn and anti isomers.     

Comparison of pyrimethanil-imprinted beads and bulk polymer as stationary phase by non-linear chromatography

A pyrimethanil-imprinted polymer (P1) was prepared by iniferter-mediated photografting a mixture of methacrylic acid and ethylene dimethacrylate onto homemade near-monodispersed chloromethylated polydivinylbenzene beads. The chromatographic behaviour of a column packed with these imprinted beads was compared with another column packed with irregular particles obtained by grinding a bulk pyrimethanil-imprinted polymer (P2). The comparison was made using the kinetic model of non-linear chromatography, studying the elution of the template and of two related substances, cyprodinil and mepanipyrim. Extension of the region of linearity, capacity factors for the template and the related substances, column selectivity, binding site heterogeneity, apparent affinity constant (K) and lumped kinetic association (k_a) and dissociation rate constant (k_d) were studied during a large interval of solute concentration, ranging between 1 and 2000 μg/ml. From the experimental results obtained, in the linearity region of solute concentration column selectivity and binding site heterogeneity remained essentially the same for the two columns, while column capacity (at 20 μg/ml, P1 = 23.1, P2 = 11.5), K (at 20 μg/ml, P1 = 8.3 × 10⁶ M⁻¹, P2 = 2.5 × 10⁶ M⁻¹) and k_a (at 20 μg/ml, P1 = 3.5 μM⁻¹ s⁻¹, P2 = 0.47 μM⁻¹ s⁻¹) significantly increased and k_d (at 20 μg/ml, P1 = 0.42 s⁻¹, P2 = 0.67 s⁻¹) decreased for the column packed with the imprinted beads. These results are consistent with an influence of the polymerisation method on the morphology of the resulting polymer and not on the molecular recognition properties due to the molecular imprinting process.     

X-ray crystallographic structure and physical properties of the pentacoordinated [TPAFeCl] complex

A new pentacoordinated ferrous compound [TPAFeCl]⁺ (TPA = tris(2-pyridylmethyl)amine) was synthesized from the reaction between H₃TPA(ClO₄)₃ and Fe(PⁿPr₃)₂Cl₂ in MeCN. The unique trigonal bipyramidal [TPAFeCl]⁺ complex was characterized as a S = 2 high spin complex based on the crystallographic structure, magnetic susceptibility, ¹H NMR spectrum and semi-empirical ZINDO/S calculations. Crystal of [TPAFeCl]₂(FeCl₄)(MeCN)₂ was monoclinic with a = 12.019(2) Å, b = 27.550(5) Å, c = 14.138(2) Å, β = 94.168(3)°, V = 4668.9(13) Å³, space group C/c, and the unit cell contained a racemic mixture of Δ and Λ isomers with ferrous tetrachloride anion.     

Synthesis of the first examples of stable heterometallic isopropoxides of an organometallic RSn(IV) moiety

Five-, six-, and seven-coordinate volatile butyltin(IV) heterobimetallic derivatives, respectively of the types, [BuSn{(μ-OPrⁱ)₂Al(OPrⁱ)₂}Cl₂] (1), [BuSn{(μ-OPrⁱ)₂Al(OPrⁱ)₂}₂Cl] (2), and BuSn{(μ-OPrⁱ)₂M(OPrⁱ)_x − 2}₃ (3:M = Al (x = 4); 4:M = Ga (x = 4); 5:M = Nb (x = 6)) have been synthesized by the reactions of BuSnCl₃ with potassium tetraisopropoxoaluminate in 1:1, 1:2, and 1:3 molar ratios. Replacement reactions of chloride in (1) and (2) with appropriate alkoxometallate (tetraisopropoxoaluminate, tetraisopropoxogallate, or hexaisopropoxoniobate) ligands result in the formation of novel BuSn(IV) heterotri- and tetra-metallic derivatives. All of these derivatives have been characterized by elemental analyses, molecular weight measurements, and spectroscopic (IR, ¹H, ²⁷Al, and ¹¹⁹Sn NMR) studies. Based on these studies, plausible structures for the new derivatives involving bidentate ligation of the alkoxometallate ligands have been suggested.     

Organotin compounds: an ionophore system for fluoride ion recognition

Ion-selective properties were established for membrane electrodes prepared by using organotin compounds of type (L^CNRSnF₂)_n, (R = n-Bu (I), = Ph (II)) and (L^CNSnF₃)_n (III) (L^CN = C₆H₄(CH₂NMe₂)-2). Electrodes formulated with the optimized membranes containing the organotin compounds I-III as ionophores and sodium tetraphenylborate (10-30%) exhibited high selectivity for fluoride over other anions. An electrode prepared with ionophore II using dibutyl phthalate as the plasticizer and 15% sodium tetraphenylborate (NaTPB) as anion additive, possesses the best potentiometric response characteristics. It shows a detection limit of 7.9 × 10⁻⁷ M with a slope of 62.7 mV decade⁻¹ of activity in buffer solutions of pH 5.5. The interference from other anions is suppressed under this optimized measurement conditions. An entirely non-Hofmeister selectivity sequence (F⁻ > CH₃COO⁻ > Cl⁻ > I⁻ ∼ Br⁻ >ClO₄⁻ > NO₂⁻ > NO₃⁻ > SCN⁻) with remarkable preference towards fluoride is obtained. The influence on the electrode performances by anion additive was studied, and the possible response mechanism was investigated by UV-vis spectra. The electrode has been used for direct determination of fluoride in drinking mineral water with satisfactory results.     

Synthesis and characterization of a new high-Tg photorefractive material

The synthesis and characterization of a new high-T_g photorefractive polymer, polyphosphazene P2, was described. It was obtained via a post-azo coupling reaction. The resulting material has been characterized by means of ¹H NMR, ³¹P NMR, FT-IR, UV-vis, GPC and DSC. Chromophore contents up to 14 mol% have been realized. The molecular weights of polyphosphazene P2 are 3.3 × 10⁴ (M_n) and 4.3 × 10⁴ (M_w), respectively; and the glass-transition temperature is high (170 °C). Measurements reveal that it possesses relatively large magnitude of photoinduced birefringence (4.2-4.7 × 10⁻³).     

(Mesitylene)ruthenium π-complexes with benzo-15-crown-5 and dibenzo-18-crown-6

The reactions of benzo-15-crown-5 and dibenzo-18-crown-6 with 1 equiv. of [(mes)Ru(MeNO₂)₃]²⁺ (mes = 1,3,5-C₆H₃Me₃) give the mononuclear complexes [(mes)Ru(η⁶-benzo-15-crown-5)]²⁺ (1) and [(mes)Ru(η⁶-dibenzo-18-crown-6)]²⁺ (2) in 50% yield. Similar reaction with 2 equiv. of [(mes)Ru(MeNO₂)₃]²⁺ produces the dinuclear complex [(μ-η⁶:η⁶-dibenzo-18-crown-6)Ru₂(mes)₂]⁴⁺ (3) in 96% yield as a 2:3 mixture of cis- and trans-isomers. Structures of 2(OTf)₂ and trans-3(OTf)₄ were confirmed by X-ray diffraction. The NMR titration showed that mononuclear dications 1 and 2 bind Na⁺ ion less effective (K_a = 600 and 250 M^-1) than free benzo-15-crown-5 and dibenzo-18-crown-6 (K_a = 2 × 10⁵ and 5 × 10⁶ M⁻¹). The dinuclear tetracation 3 does not bind Na⁺ within measurable limits of NMR titration method. The electrochemical behaviour of complexes 1-3 was studied in propylene carbonate solution. They exhibit a partially chemically reversible Ru(II)/Ru(I) reduction, which in the case of the dinuclear complex 3 proceeds through two slightly separated steps. The redox activity of the complexes is substantially unaffected by the presence of sodium ion.     

Vanadiumoxo–aroylhydrazone complexes: Synthesis,structure and DFT calculations

The aroylhydrazone Schiff base ligands (E)-N’-(2-hydroxybenzylidene)benzohydrazide = H₂L¹, (E)-N’-(2-hydroxy-3-methoxybenzylidene)benzohydrazide = H₂L² and = (E)-N’-(5-bromo-2-hydroxybenzylidene)benzohydrazide = H₂L³ gave the vanadium(V)oxo-aroylhydrazone complexes [V^VOL¹(OCH₃)(OHCH₃] (1), [V^VOL²(OCH₃)(OHCH₃]·CH₃OH (2) and [V^VOL³(OCH₃)(OHCH₃] (3) on reaction with vanadium(IV) oxide acetylacetonate. The complexes were characterized by spectroscopic methods in the solid state (IR) and in solution (UV–Vis, ¹H NMR). Single crystal X-ray analysis was performed with 3. In methanol solution six-coordinated V^VOL³(OCH₃)(OHCH₃) was formed. V^IV was oxidized to V^v by aerial oxygen in the synthesis. In the VO₅N coordination sphere the alcohol oxygen lies trans to the oxo oxygen. The general V–O bond length order is oxo < methoxylato < phenoxidic < enolato < alcoholic. The complexes are mononuclear, but intermolecular O–H?N hydrogen bonding affords a zigzag chain. DFT calculations on complex 3 reproduced the geometric parameters, IR and UV–Vis spectroscopic data well in a reasonable range.     

Synthesis and biological activity of triorganogermanium substituted heteropolytungstates

Four new compounds, [Me₄N]₄H₅[(RGe)₃(XW₉O_34−n)₂] (R = HOOCCH₂CH₂, HOOCCH₂(m-NO₂C₆H₄)CH, X = P, n = 0; X = Sb, n = 1) have been prepared from trichlorogermanium precursors and lacunary polyoxometallates ([PW₉O₃₄]⁹⁻ and [SbW₉O₃₃]⁹⁻). The products were characterized by means of elemental analysis, IR spectroscopy, ¹H NMR and ¹⁸³W NMR spectroscopy. ¹⁸³W NMR spectra of the complexes support the stoichiometry of the new heteropolyanions and the probable retention of the XW₉ units in water. The organogermanium substituted complexes showed promising activity against two human tumor cell lines (leucocythemia Hela cells and S180 cells) in vitro and [Me₄N]₄H₅[(HOOCCH₂CH₂Ge)₃(SbW₉O₃₃)₂] exhibited some certain antitumoral activity in vivo and can increase the immune ability of the spleen.     

Synthesis, characterization and catalytic behaviors of water-soluble phosphine-sulfonato nickel methyl complexes bearing PEG-amine labile ligand

Two novel water-soluble phosphine-sulfonato nickel (II) methyl complexes [(P^O)NiMeL] (P^O = κ²-P,O-2-(2-MeO-C₆H₄)₂PC₆H₄SO₃, L = H₂N(CH₂CH₂O)_nMe, n = ca. 52, 2a; n = ca. 16, 2b) have been prepared and characterized by ¹H, ³¹P NMR and elemental analysis, and their reactivity towards ethylene was studied.     

Bis-phosphine monoxide platinum(II) and palladium(II) cationic complexes as Lewis acid catalysts in Diels-Alder and sulfoxidation reactions

A series of bis-phosphine monoxide (BPMO) palladium(II) and platinum(II) cationic complexes of the type [M(BPMO-κ²-P,O)₂][X]₂ (M = Pd, Pt; BPMO = Ph₂P-(CH₂)_n-P(O)Ph₂ with n = 1 (dppmO), 2 (dppeO), 3 (dpppO); X = BF₄, TfO) were prepared from the corresponding chlorides [MCl₂(BPMO-κ¹-P)₂] upon treatment with 2 equiv. of AgX in wet acetone/CH₂Cl₂ or MeOH solutions. They were characterized by ¹H and ³¹P{¹H} NMR spectroscopies and, in the case of the complex [Pt(dppeO-κ²-P,O)₂][BF₄]₂, also by X-ray crystallography. These complexes were tested as catalysts in some Diels-Alder and oxidation reactions with different substrates. In the latter reaction Pt(II) complexes showed moderate activity, while for the former one, both classes of complexes were active in the C-C coupling, in particular the Pt(II) species showed interesting high endo/exo diasteroselectivity depending on the counteranion.     

Cs4P2Se10: A new compound discovered with the application of solid-state and high temperature NMR

The new compound Cs₄P₂Se₁₀ was serendipitously produced in high purity during a high-temperature synthesis done in a nuclear magnetic resonance (NMR) spectrometer. ³¹P magic angle spinning (MAS) NMR of the products of the synthesis revealed that the dominant phosphorus-containing product had a chemical shift of −52.8 ppm that could not be assigned to any known compound. Deep reddish brown well-formed plate-like crystals were isolated from the NMR reaction ampoule and the structure was solved with X-ray diffraction. Cs₄P₂Se₁₀ has the triclinic space group P-1 with a=7.3587(11) Å, b=7.4546(11) Å, c=10.1420(15) Å, α=85.938(2)°, β=88.055(2)°, and γ=85.609(2)° and contains the [P₂Se₁₀]⁴⁻ anion. To our knowledge, this is the first compound containing this anion that is composed of two tetrahedral (PSe₄) units connected by a diselenide linkage. It was also possible to form a glass by quenching the melt in ice water, and Cs₄P₂Se₁₀ was recovered upon annealing. The static ³¹P NMR spectrum at 350 °C contained a single peak with a −35 ppm chemical shift and a ∼7 ppm peak width. This study highlights the potential of solid-state and high-temperature NMR for aiding discovery of new compounds and for probing the species that exist at high temperature.     

Synthesis and spectroscopic properties of Schiff bases derived from 3-hydroxy-4-pyridinecarboxaldehyde

A series of six new Schiff bases has been prepared by reacting aniline and 4-R-substituted anilines (R=CH₃, OCH₃, Br, Cl, NO₂) with 3-hydroxy-4-pyridinecarboxaldehyde. The ¹H, ¹³C, ¹⁵N and ¹⁷O NMR data of these compounds are used to discuss the tautomerism. ¹⁵N NMR and ¹⁷O NMR chemical shifts established the tautomer existing in solution as the hydroxy/imino. ¹³C CPMAS NMR confirms that the same tautomer is found in the solid state. The stabilities of the tautomeric forms have been approached using density functional calculations (B3LYP/6-31G**) in the gas phase. In all cases the neutral hydroxy/imino with E configuration is more stable than the oxo/enamino form (by ∼22 kJ mol⁻¹) and significantly more stable than the betaine (by ∼75 kJ mol⁻¹).     

Phase transitions, structural changes and molecular motions in [Zn(NH3)4](BF4)2 studied by neutron scattering, X-ray powder diffraction and nuclear magnetic resonance

Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) measurements performed at 90-295 K, inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) patterns registered at 22-190 K, and X-ray powder diffraction (XRPD) measurements performed at 86-293 K, provided evidence that the crystal of [Zn(NH₃)₄](BF₄)₂ has four solid phases. The phase transitions occurring at: T_C3=101 K, T_C2=117 K and T_C1=178 K, as were detected earlier by differential scanning calorimetry (DSC), were connected on one hand only with an insignificant change in the crystal structure and on the other hand with a drastic change in the speed of the anisotropic, uniaxial reorientational motions of the NH₃ ligands and BF₄⁻ anions (at T_C3 and at T_C2) and with the dynamical orientational order-disorder process (“tumbling”) of tetrahedral [Zn(NH₃)₄]²⁺ and BF₄⁻ ions (at T_C1). The crystal structure of [Zn(NH₃)₄](BF₄)₂ at room temperature was determined by XRPD as orthorhombic, space group Pnma (No. 62), a=10.523 Å, b=7.892 Å, c=13.354 Å and Z=4. Unfortunately, it was not possible to determine the structure of the intermediate and the low-temperature phase. However, we registered the change of the lattice parameters and unit cell volume as a function of temperature and we can observe only a small deviation from near linear dependence of these parameters upon temperature in the vicinity of the T_C1 phase transition.