Variable Temperature IR Research Articles

The further development of the borate nitrate family: Synthesis and characterization of Ag12(B9O18)(NO3)3 with unique isolated B9O18 clusters, 9B:6Δ3□:3(–)

Single crystals of a novel silver borate nitrate, Ag12(B9O18)(NO3)3 (1), were produced as a byproduct upon preparation of Ag3B6O10(NO3). 1 is hexagonal, (P63/m, a = 11.2896(3) Å, c = 11.6693(3) Å); its structure exhibits just a second example of unique [B9O18]9– nonaborate anions [9B:6Δ3□:3(<2Δ□>–)<3□>] which can be described as three triborate groups linked in a large cycle. As commonly observed among silver borates, the Ag+ cations exhibit strongly anharmonic vibrations which were described using Gram–Chariler series up to 4th order. 1 is characterized by single-crystal X-ray diffraction, variable-temperature powder X-ray diffraction, IR, Raman, and UV–vis-NIR spectroscopy, complex thermal analysis, and DFT calculations. The thermal expansion of 1 is slightly anisotropic (αa = 15.0, αc = 17.7 × 10−6 °C−1 at 200 °C). Upon heating, the compound decomposes with formation of metallic silver and another borate, AgBO2, which could be earlier prepared only at high oxygen pressure.

Substituent effect on the spin transition of Mixed-Valence Cyanide-Bridged {FeIII2FeII} complexes

We synthesized three isostructural mixed-valence {FeIII2FeII} trinuclear complexes {[(Tp4-Me)FeIII(CN)3]2FeII(1-(4-R-phenyl)-1H-imidazole)}·Sol (1: R = Br, Sol = 10·H2O; 2: R = Cl, Sol = 2 CH3OH; 3: R = F, Sol = 2 CH3OH; Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate) and characterized their structures and magnetic and spectroscopic properties. The linear trinuclear structures of complexes 1–––3 are isomorphic. Magnetic susceptibility measurements and variable-temperature IR analyses revealed that the magnetic behavior of the complexes can be finely tuned by the substituents, which significantly influence the ligand field strength. The complexes exhibit complete one-step spin-crossover (SCO) behavior with T1/2↑ = 146 K, T1/2↓ = 142 K for 1; T1/2↑ = 155 K, T1/2↓ = 150 K for 2; T1/2↑ = 147 K, T1/2↓ = 144 K for 3, respectively. Photomagnetic study indicates that all of them show the light-induced spin state transition from low-spin to high spin states by applying 946 nm light for 1 and 808 nm light for 2 and 3. Variable-temperature structural analysis demonstrates that the three complexes exhibit π···π interactions between the intermolecular benzene ring and the pyrazole ring. Our study demonstrates that the modification of substituent ligand can significantly affect the intermolecular interactions of these trinuclear complexes and the SCO behaviour of them.

Fluorosumanenes as building blocks for organic crystalline dielectrics

Abstract Organic crystalline dielectric materials are attractive target in the field of materials chemistry. In their designing strategy, the molecular motion induced by the external dielectric field is required to maximize the polarization effect in the materials to realize a large dielectric constant. Especially, the molecular motion of curved-π aromatics such as C60 and their supramolecular complexes are the known to show characteristic smoothness in the solid state. In this context, we focused on the in-plane motion of the one of the representative buckybowl, sumanene (Sum) in its curve-to-curve-contacted columnar structure as the new platform of the motion for the emergence of dielectric response. The newly designed and synthesized fluorinated sumanene, 1,1-difluorosumanene (F2-Sum), which possesses two fluorine atoms on the same benzylic carbon of pristine sumanene showed a large dipole moment along the in-plane direction. Thermal analyses, variable temperature X-ray diffraction and IR measurements indicated the presence of in-plane motion of F2-Sum although no clear phase transition was involved. This thermal property of F2-Sum realized an anisotropic dielectric response with a Debye-type dielectric relaxation in the single crystalline state. Further trials to form the solid solution of Sum and F2-Sum in various mixing ratio realized the tuning of the dielectric property.

Dynamics of Organic Cations in Switchable Quinuclidinium Metal Chloride Dielectrics

The synthesis and structure of a series of quinuclidinium-based organic–inorganic hybrids of Q2CuCl4, Q2CoCl4, and Q4Pb3Cl10 are reported (Q = quinuclidinium, C7H13NH+). The dynamic disorder observed in the structure of quinuclidinium metal chlorides was investigated using variable-temperature single-crystal X-ray diffraction, IR, Raman, NMR, and dielectric spectroscopy. The crystals undergo phase transitions (PTs) where an order–disorder process takes place. However, it is assumed that in the case of Q2CoCl4 and Q2CuCl4, the reorientation of Q cations is a major force driving PT, while in Q4Pb3Cl10, the reorientation of inorganic ions (Pb–Cl moieties) is at least equally important. All three hybrid compounds exhibit switchable and tunable properties of the dielectric function between two stable ON/OFF states. The magnetic properties were also investigated over a wide temperature range by measuring the magnetic susceptibility and magnetization as a function of temperature and external magnetic field, which allowed us to demonstrate the competition between the paramagnetic and diamagnetic signals in these compounds. The dynamics probed by NMR has shown a nonequivalence of Q cations in all studied compounds.

Оценка бренстедовской кислотности протонных цеолитов с помощью VTIR-спектроскопии --- обзор текущих исследований

Many industrial applications of protonic zeolites as solid-acid heterogeneous catalysts rely on the strength or their Brønsted acidity, which (together with zeolite topology) affects both, catalytic activity and selectivity. Therefore, the convenience to have an accurate (and simple) experimental technique for measuring Brønsted acid strength. The enthalpy change, ΔH0, corresponding to the hydrogen bonding interaction of a weak base (such as CO or dinitrogen) with their Brønsted acid [Si(OH)Al] hydroxyl groups should directly correlate with the zeolite acid strength. Nevertheless, because of simplicity, the bathochromic shift of the O–H stretching frequency, Δν(OH), is usually measured by IR spectroscopy at a low temperature, and correlated with acid strength, for ranking zeolite acidity. Herein the use of variable-temperature IR (VTIR) spectroscopy to determine simultaneously ΔH0 and Δν(OH) is demonstrated; followed by an abridged overview showing that direct correlation between Δν(OH) and Brønsted acid strength can be misleading when ranking zeolite acidity.

Ranking the Bronsted Acid Strength of Protonic Zeolites with VTIR Spectroscopy --- An Overview of Current Research

Many industrial applications of protonic zeolites as solid-acid heterogeneous catalysts rely on the strength or their Br nsted acidity, which (together with zeolite topology) affects both, catalytic activity and selectivity. Therefore, the convenience to have an accurate (and simple) experimental technique for measuring Br nsted acid strength. The enthalpy change, Delta H0, corresponding to the hydrogen bonding interaction of a weak base (such as CO or dinitrogen) with their Br nsted acid [Si(OH)Al] hydroxyl groups should directly correlate with the zeolite acid strength. Nevertheless, because of simplicity, the bathochromic shift of the O-H stretching frequency, Deltaν(OH), is usually measured by IR spectroscopy at a low temperature, and correlated with acid strength, for ranking zeolite acidity. Herein the use of variable-temperature IR (VTIR) spectroscopy to determine simultaneously Delta H0 and Deltaν(OH) is demonstrated; followed by an abridged overview showing that direct correlation between Deltaν(OH) and Br.nsted acid strength can be misleading when ranking zeolite acidity. Keywords: VTIR spectroscopy, br nsted acidity, H-zeolites

Stepwise Dielectric Switching Occurs in Two Photo-Responsive Complexes Possessing Two-Dimensional Structures.

Two organic-inorganic hybrid complexes, (CH3NH3)Na[Fe(CN)5NO]·H2O (1) and (CH3NH3)2[Fe(CN)5NO] (2), which exhibit stepwise dielectric switching as well as photo-induced structural transformation, are synthesized and examined. In these two compounds, the photo-responsive complex anions, [Fe(CN)5NO]2-, connected by Na+ through N-Na coordination bonds or CH3NH3+ through N···H-N hydrogen bonds, form two-dimensional structures. One organic cation, CH3NH3+, that resides in the intralaminar cavity and plays a role as a template, undergoes a temperature-controlled order-disorder structural phase transition. As the frozen-thawed state change of the polar organic cations modifies the polarizability of materials, stepwise dielectric switching is observed at the phase transition temperature. Furthermore, the photo-induced linkage isomerism of [Fe(CN)5NO]2- building block survives in the new compounds at the low-temperature range, which is verified by variable-temperature IR spectra after photo-irradiation. The coexistence of switchable dielectric properties and photo-induced structural variation suggests multiple optical-electric roles of the present materials.

The Eight Hydrates of Strychnine Sulfate.

Commercial samples of strychnine sulfate were used as the starting material in crystallization experiments accompanied by stability studies. Eight hydrate forms (HyA–HyG), including five novel hydrates, were verified. The crystal structures of HyA (“pentahydrate”) and HyF (“hexahydrate”) were determined from single-crystal X-ray diffraction data. HyF was identified as the most stable hydrate at high water activities at room temperature (RT), and HyA and HyC were also found to be stable at ambient conditions. Long-time storage experiments over nearly two decades confirm that these three hydrates are stable at ambient conditions (20–60% relative humidity). The other five hydrates, HyB (“dihydrate”), HyD, HyE, HyG, and HyH, are only observable at the low(est) relative humidity (RH) levels at RT. Some of these latter forms can only exist within a very narrow RH range and are therefore intermediate phases. By applying a range of complementary experimental techniques such as gravimetric moisture sorption analysis, thermal analysis, moisture controlled PXRD measurements, and variable temperature IR spectroscopy in combination with principal component analysis, it was possible to identify the distinct hydrate phases and elucidate their stability and dehydration pathways. The observed (de)hydration routes, HyA ↔ HyB, HyC ↔ HyD ↔ HyE, HyF ↔ HyG ↔ HyH and HyF → HyA ↔ HyB, depended on the initial hydrate form, particle size, and atmospheric conditions. In addition, a transformation from HyC/HyA to HyF occurs at high RH values at RT. The specific moisture and temperature conditions of none of the applied drying regimes yielded a crystalline water-free form, which highlights the essential role of water molecules for the formation and stability of the crystalline strychnine sulfate phases.

Molar Entropy and Enthalpy of CO Adsorbed in Zeolites as Derived from VTIR Data: Role of Intermolecular Modes.

Detailed analysis of recently reported variable‐temperature IR (VTIR) spectra of carbon monoxide adsorbed in alkaline zeolites shows how, not only the corresponding values of standard adsorption enthalpy (ΔH0 ) and entropy (ΔS0 ) can be obtained, but also the thermodynamic values of molar entropy and enthalpy which characterize the adsorbed gas phase. In addition, it is shown that the so obtained molar entropy data can lead to new insights into soft molecular modes, which would be hardly accessible by conventional IR spectroscopic techniques.

Brønsted acidity of H-[Ga]-ZSM-5 zeolites as determined by variable-temperature IR spectroscopy

Abstract Protonic gallosilicates H-[Ga]-ZSM-5 were synthesized, following a hydrothermal procedure, from gels having Si/Ga ratios of 25, 50 and 75. Likewise, for comparison, protonic zeolites H-[Al]-ZSM-5 having Si/Al ratios of 25 and 50 were also prepared. Bronsted acidity of the structural Si(OH)Ga groups in the gallosilicates was studied by means of IR spectroscopy at a variable temperature (VTIR) using CO and N2 as probe molecules. This instrumental technique enables one simultaneous measurement of the bathochromic shift of the stretching O H mode, Δυ(OH), of the Bronsted acid group interacting (through hydrogen bonding) with the probe molecule and the corresponding standard enthalpy change, ΔH⁰, in the adsorption process. The results obtained clearly showed that the gallosilicates are distinctively less acidic than the aluminosilicates, whichever acidity indicator is used: Δυ(OH) either or ΔH⁰. Nevertheless, no change of Bronsted acid strength was found when changing the Si/Ga ratio.

Solid-State Hydrogen-Bond Alterations in a [Co2 Fe2 ] Complex with Bifunctional Hydrogen-Bonding Donors.

A hydrogen-bonding donor-acceptor system, [Co2 Fe2 (bpy*)4 (CN)6 (tp*)2 ](PF6 )2 ⋅2ABA⋅4BN⋅2PE (1 solv ), was prepared by co-crystallization of an external stimuli-responsive cyanide-bridged tetranuclear [Co2 Fe2 ] complex and bifunctional hydrogen-bonding donors, p-aminobenzoic acid. Compound 1 solv exhibited a gradual electron-transfer-coupled spin transition (ETCST), and the removal of solvent molecules led to an abrupt thermal ETCST behavior with increased transition temperature. X-ray structural analysis revealed that the modification of ETCST was caused by a significant alteration of a hydrogen-bonding mode between the tetranuclear [Co2 Fe2 ]2+ cations and ABA molecules. Variable temperature IR measurements indicated that the desolvated form, 1 desolv , showed dynamic alteration of hydrogen-bonding interactions coupled with thermal ETCST behavior. These results suggested that the tetranuclear [Co2 Fe2 ] complex shows solid-state modulations of hydrogen-bond strengths by external stimuli.

The partial dehydrogenation of aluminium dihydrides.

The reactions of a series of β-diketiminate stabilised aluminium dihydrides with ruthenium bis(phosphine), palladium bis(phosphine) and palladium cyclopentadienyl complexes is reported. In the case of ruthenium, alane coordination occurs with no evidence for hydrogen loss resulting in the formation of ruthenium complexes with a pseudo-octahedral geometry and cis-relation of phosphine ligands. These new ruthenium complexes have been characterised by multinuclear and variable temperature NMR spectroscopy, IR spectroscopy and single crystal X-ray diffraction. In the case of palladium, a series of structural snapshots of alane dehydrogenation have been isolated and crystallographically characterised. Variation of the palladium precursor and ligand on aluminium allows kinetic control over reactivity and isolation of intermetallic complexes that contain new Pd-Al and Pd-Pd interactions. These complexes differ by the ratio of H : Al (2 : 1, 1.5 : 1 and 1 : 1) with lower hydride content species forming with dihydrogen loss. A combination of X-ray and neutron diffraction studies have been used to interrogate the structures and provide confidence in the assignment of the number and position of hydride ligands. 27Al MAS NMR spectroscopy and calculations (DFT, QTAIM) have been used to gain an understanding of the dehydrogenation processes. The latter provide evidence for dehydrogenation being accompanied by metal-metal bond formation and an increased negative charge on Al due to the covalency of the new metal-metal bonds. To the best of our knowledge, we present the first structural information for intermediate species in alane dehydrogenation including a rare neutron diffraction study of a palladium-aluminium hydride complex. Furthermore, as part of these studies we have obtained the first SS 27Al NMR data on an aluminium(i) complex. Our findings are relevant to hydrogen storage, materials chemistry and catalysis.

Studies on Silver Ions Releasing Processes and Mechanical Properties of Surface-Modified Titanium Alloy Implants.

Dispersed silver nanoparticles (AgNPs) on the surface of titanium alloy (Ti6Al4V) and titanium alloy modified by titania nanotube layer (Ti6Al4V/TNT) substrates were produced by the chemical vapor deposition method (CVD) using a novel precursor of the formula [Ag5(O2CC2F5)5(H2O)3]. The structure and volatile properties of this compound were determined using single crystal X-ray diffractometry, variable temperature IR spectrophotometry (VT IR), and electron inducted mass spectrometry (EI MS). The morphology and the structure of the produced Ti6Al4V/AgNPs and Ti6Al4V/TNT/AgNPs composites were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Moreover, measurements of hardness, Young’s modulus, adhesion, wettability, and surface free energy have been carried out. The ability to release silver ions from the surface of produced nanocomposite materials immersed in phosphate-buffered saline (PBS) solution has been estimated using inductively coupled plasma mass spectrometry (ICP-MS). The results of our studies proved the usefulness of the CVD method to enrich of the Ti6Al4V/TNT system with silver nanoparticles. Among the studied surface-modified titanium alloy implants, the better nano-mechanical properties were noticed for the Ti6Al4V/TNT/AgNPs composite in comparison to systems non-enriched by AgNPs. The location of silver nanoparticles inside of titania nanotubes caused their lowest release rate, which may indicate suitable properties on the above-mentioned type of the composite for the construction of implants with a long term antimicrobial activity.

Temperature Dependence of Carbon Monoxide Adsorption on a High-Silica H-FER Zeolite

The temperature dependence of the isosteric heat of the CO adsorption on a high-silica protonic ferrierite (H-FER) zeolite was investigated using microcalorimetry and density functional theory/coupled-cluster (DFT/CC) atomistic simulations. A large change in the experimental heat of adsorption was observed at the zero-coverage limit for the CO/H-FER system (from 32.2 kJ/mol at 200 K to 25.4 kJ/mol at 300 K). This can be explained by a dramatic change in the CO dynamics in the 200–300 K temperature range. During our ab initio molecular dynamics simulation at 200 K, the CO molecule is localized; at 300 K, the molecule jumps between adjacent Brønsted sites. The only exception has been found for the T4″ site, where the fast desorption of the CO molecule is prevented by a curved ferrierite wall enclosing this site. The previously reported variable temperature IR heat of adsorption of the CO/H-FER with Si/Al 27.5 (28.4(±2) kJ/mol) is consistent with the Perdew–Burke–Ernzerhof/CC predictions for individual sites when the statistical distribution of adsorption sites and temperature effects are taken into account.

Probing Brønsted Acidity of Protonic Zeolites with Variable-Temperature Infrared Spectroscopy

Most industrial applications of zeolites as solid-acid catalysts rely on their high Brønsted acidity, which affects both catalytic activity and selectivity, and hence the convenience to find an accurate experimental technique for measuring the acid strength. The enthalpy change, ΔH0, involved in the hydrogen bonding interaction between a weak base (such as carbon monoxide) and the Brønsted acid [Si(OH)Al] hydroxyl groups should correlate directly with the zeolite acid strength. However, on account of simplicity, the bathochromic shift of the O–H stretching frequency, Δv(OH), is usually measured by IR spectroscopy at a (fixed) low temperature in-stead of ΔH0 and correlated with the acid strength for ranking the zeolite acidity. Herein, the use of variable-temperature IR spectroscopy to determine simultaneously ΔH0 and Δv(OH) is demonstrated, followed by a review of recent experimental results showing that the practice of ranking the acid strength by the corresponding O–H frequency shift probed by a weak base could be misleading; and that can be so much the case of zeolites showing a wide range of structure types.

Open Metal Sites in the Metal–Organic Framework CPO-27-Cu: Detection of Regular and Defect Copper Species by CO and NO Probe Molecules

The open copper metal sites in CPO-27-Cu were studied by means of IR spectroscopy of adsorbed CO and NO, and density functional theory calculations. Very low Lewis acidity of the Cu2+ sites was established by CO (IR band at 2153–2149 cm–1). Variable-temperature IR experiments indicate adsorption enthalpy of ca. −20 kJ mol–1. It was also found that CO is a sensitive probe of the occupation of the neighboring copper sites. In contrast to the general expectations, NO is very weakly adsorbed on the Cu2+ sites (−14.5 kJ mol–1, IR band at 1888 cm–1). The effect is attributed to the particular Cu2+ ion coordination and electronic state, leading to a large Jan–Teller deformation and low effective charge, preventing significant charge transfer effects between the metal center and the guest molecules as well as any significant electrostatic interactions. Thus, dominating are van der Waals interactions which position the adsorbed molecule relatively far away at about 2.7–3.0 A. Adsorption of CO also revealed that a ...

Steric and Acidity Control in Hydrogen Bonding and Proton Transfer to trans-W(N2)2(dppe)2.

The interaction of trans-W(N2)2(dppe)2 (1; dppe = 1,2-bis(diphenylphosphino)ethane) with relatively weak acids (p-nitrophenol, fluorinated alcohols, CF3COOH) was studied by means of variable temperature IR and NMR spectroscopy and complemented by DFT/B3PW91-D3 calculations. The results show, for the first time, the formation of a hydrogen bond to the coordinated dinitrogen, W-N≡N···H-O, that is preferred over H-bonding to the metal atom, W···H-O, despite the higher proton affinity of the latter. Protonation of the core metal-the undesirable side step in the conversion of N2 to NH3-can be avoided by using weaker and, more importantly, bulkier acids.

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research.

The current state of the art in the application of variable-temperature IR (VTIR) spectroscopy to the study of (i) adsorption sites in zeolites, including dual cation sites; (ii) the structure of adsorption complexes and (iii) gas-solid interaction energy is reviewed. The main focus is placed on the potential use of zeolites for gas separation, purification and transport, but possible extension to the field of heterogeneous catalysis is also envisaged. A critical comparison with classical IR spectroscopy and adsorption calorimetry shows that the main merits of VTIR spectroscopy are (i) its ability to provide simultaneously the spectroscopic signature of the adsorption complex and the standard enthalpy change involved in the adsorption process; and (ii) the enhanced potential of VTIR to be site specific in favorable cases.

Synthesis and Structural Characterization of a CHA-type AlPO4 Molecular Sieve with Penta-Coordinated Framework Aluminum Atoms.

The structure-directing effects of a series of polymethylimidazolium cations with different numbers of methyl groups as organic structure-directing agents (OSDAs) in the synthesis of aluminophosphate (AlPO4)-based molecular sieves in both fluoride and hydroxide media are investigated. On the one hand, among the OSDAs studied here, the smallest 1,3-dimethylimidazolium and the largest 1,2,3,4,5-pentamethylimidazolium cations were found to direct the synthesis of a new variant of the triclinic chabazite (CHA)-type AlPO4 material, designated AlPO4-34(t)V, and the one-dimensional small-pore silicoaluminophosphate (SAPO) molecular sieve STA-6 in hydroxide media, respectively. On the other hand, the intermediate-sized 1,2,3,4-tetramethylimidazolium cation gave SSZ-51, a two-dimensional large-pore SAPO material, in fluoride media. Synchrotron powder X-ray diffraction and Rietveld analyses reveal that as-made AlPO4-34(t)V contains penta-coordinated framework Al species connected by hydroxyl groups, as well as tetrahedral framework Al, which contrasts with the distortions arising from the two F- or OH- bridges between octahedral Al atoms in all already known AlPO4-34 materials. The presence of Al-OH-Al linkages in this triclinic AlPO4-34 molecular sieve has been further corroborated by thermal analysis, variable-temperature IR,27Al magic-angle spinning NMR, and dispersion-corrected density functional theory calculations.

Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (tBuPCP)IrH(Cl) (1; tBuPCP = κ3-2,6-(CH2PtBu2)2C6H3) appeared to be moderately active in NHMe2·BH3 (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (tBuPCP)IrHCl(η1-BH3·NHMe2) (3) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)IrH2(η1-BH3·NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and tert-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of 1) the h...