71Ga NMR Research Articles

$$^{71}$$Ga NMR study on hexagonal La$$_3$$FeGaS$$_7$$ with a chiral crystal structure

$$^{71}$$Ga NMR study on hexagonal La$$_3$$FeGaS$$_7$$ with a chiral crystal structure

Transformation of Light Alkanes into High-Value Aromatics

This research work is focused on the transformation of light alkane (propane) into high-value aromatics using gallo-alumino-silicate catalysts. Two sets of gallo-alumino-silicates were synthesized for this study. In the first set, the ratio of Ga/(Al+Ga) was modified, while the Si/(Al+Ga) ratio was held constant. In the subsequent set, the Si/(Al+Ga) ratio was adjusted, while maintaining a consistent Ga/(Al+Ga) ratio. This approach aimed to directly assess the impact of each ratio on catalyst performance. The comprehensive characterization of all catalysts was conducted using various instrumental techniques, i.e., BET surface area, XRD, NH3-TPD, 27Al, 71Ga and 29Si MAS NMR, and XPS. A gradual reduction in the percentage of crystallinity and rise in meso-surface area was noticed with a rise in Ga/(Al+Ga) ratio. The total acidity (NH3-TPD) demonstrated a decline as the Si/(Al+Ga) ratio increased, attributed to an overall decline in Al3+ or Ga3+ species. The XPS intensity of the Ga 2p3/2 peak rose in correlation with an elevated ratio of Ga/(Al+Ga), suggesting the formation of extra-framework Ga species. The propane conversion, aromatic yield, and aromatization/cracking ratio exhibited an increase with an increasing Ga/(Al+Ga) ratio, reaching an optimum value of 0.46 before declining. Conversely, an appreciable drop in the conversion of propane and yield of aromatics was detected with the rise in Si/(Al+Ga) ratio, attributing to the decline in acidity. The catalyst having a Ga/(Al+Ga) ration of 0.46 exhibited the highest propane conversion and aromatic yield of 83.0% and 55.0% respectively.

71Ga NMR Signatures of Lewis and Brønsted Acid Sites in Gallium Silicates Evidenced and Deciphered upon Interaction with Probe Molecules

⁷¹Ga NMR Signatures of Lewis and Brønsted Acid Sites in Gallium Silicates Evidenced and Deciphered upon Interaction with Probe Molecules

H3nota Derivatives Possessing Picolyl and Picolinate Pendants for Ga3+ Coordination and 67Ga3+ Radiolabeling.

We synthesized, thanks to the regiospecific N-functionalization using an orthoamide intermediate, two 1,4,7-triazacyclononane derivatives containing an acetate arm and either a methylpyridine or a picolinic acid group, respectively, Hnoapy and H2noapa, as new Ga3+ chelators for potential use in nuclear medicine. The corresponding Ga3+ complexes were synthesized and structurally characterized in solution by 1H and 13C NMR. The [Ga(noapy)]2+ complex appears to exist in solution as two diasteroisomeric pairs of enantiomers, as confirmed by density functional theory (DFT) calculations, while for [Ga(noapa)]+, a single species is present in solution. Solid-state investigations were possible for the [Ga(noapa)]+ complex, which crystallized from water as a pair of enantiomers. The average length of the N-Ga bonds of 2.090 Å is identical with that found for the [Ga(nota)] complex, showing that the presence of the picolinate arm does not hinder the coordination of the ligand to the metal ion. Protonation constants of noapy- and noapa2- were determined by potentiometric titrations, providing an overall basicity ∑log KiH (i = 1-4) that increases in the order noapy- < noapa2- < nota3- with increases in the negative charge of the ligand. Stability constants determined by pH-potentiometric titrations supplemented with 71Ga NMR data show that the stabilities of [Ga(noapy)]2+ and [Ga(noapa)]+ are lower compared to that of [Ga(nota)] but higher than those of other standards such as [Ga(aazta)]-. 67Ga radiolabeling studies were performed in order to demonstrate the potential of these chelators for 67/68Ga-based radiopharmaceuticals. The labelings of Hnoapy and H2noapa were nearly identical, outperforming H3nota. Stability studies were conducted in phosphate-buffered saline and in the presence of human serum transferrin, revealing no significant decomplexation of [67Ga][Ga(noapy)]2+ and [67Ga][Ga(noapa)]+ compared to [67Ga][Ga(nota)]. Finally, all complexes were found to be highly hydrophilic, with calculated log D7.4 values of -3.42 ± 0.05, -3.34 ± 0.04, and -3.00 ± 0.23 for Hnoapy, H2noapa, and H3nota, respectively, correlating with the charge of each complex and the electrostatic potentials obtained with DFT.

Non-collinear magnetic structure of the MAX phase Mn2GaC epitaxial films inferred from zero-field NMR study (CE-5:L05)

Zero-field NMR investigations on the 69Ga, 71Ga, and 55Mn nuclei have been performed at 4.2 K on a 100 nm thick epitaxial Mn2GaC(0001) MAX phase film grown on a MgO(111) substrate. This nano-laminated structure consists of ferromagnetic Mn–C–Mn stacks interleaved with monolayers of gallium. A resolved quadrupolar structure of the observed gallium resonances is a fingerprint of a well-defined crystal field confirming high crystal quality. The nuclei of non-magnetic gallium atoms are shown to experience the transferred hyperfine magnetic field of 15.75 T (±0.05 T), which is due to polarization of their 4s electron shell by the magnetic moments of manganese neighbors, evidencing the presence of an uncompensated ferromagnetic moment within the manganese sublattice. The average magnetic moment of manganese was found to be around 2 μB, strongly contrasting with the reported remnant magnetization of only 0.3 μB. Moreover, the 55Mn NMR spectrum indicates the presence of magnetically non-equivalent manganese sites within this structure. The observed features of the 69,71Ga and 55Mn NMR spectra cannot be reconciled with any of the hitherto proposed collinear arrangements of manganese moments and suggest their non-collinear arrangement across a gallium layer. Nevertheless a more advanced study is required to elucidate the detailed nature of magnetic structure in this material.

Catalytic Enhancement of Cyclohexene Hydration by Ga-Doped ZSM-5 Zeolites

Ga-doped ZSM-5 zeolites were directly synthesized by a facile one-step hydrothermal method without organic templates and calcination and then investigated in the cyclohexene hydration reaction. The structure, component, textural properties, and acidity of the as-prepared samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), Brunauer–Emmett–Teller (BET), ammonia temperature-programmed desorption (NH3-TPD), pyridine-chemisorbed IR (Py-IR), and 71Ga, 27Al, 29Si, and 1H magic-angle spinning (MAS) NMR techniques. The characterization results showed that the introduction of Ga atoms into the ZSM-5 zeolite framework is much easier than Al atoms and beneficial to promote the formation of small-sized crystals. The number of Brønsted acid sites of Ga-doped ZSM-5 samples obviously increased compared with Ga0-ZSM-5. Additionally, the highest cyclohexanol yield (10.1%) was achieved over the Ga3-ZSM-5 sample, while the cyclohexanol yield of the Ga0-ZSM-5 sample was 8.6%. This result indicated that the improved catalytic performance is related to its larger external surface area, smaller particle size, and more Brønsted acid sites derived from Si–OH–Al and Si–OH–Ga of Ga3-ZSM-5. Notably, the green route reduces harmful gas emission and provides a basis for doping other heteroatoms to regulate the catalytic performance of zeolites, especially in industrial production.

P-Block metal complexes with bis(pyrazol-1-yl)acetato ligands.

The hetereoscorpionate ligands bis(pyrazol-1-yl)acetic acid (Hbpza) and bis(3,5-dimethylpyrazol-1-yl)acetic acid (Hbdmpza) are reacted with [Sn(OAc)2] or [Sn(acac)2] to yield the corresponding Sn(II) complexes. A single crystal X-ray determination for the in solution monomeric complex [Sn(bpza)2] (1a) revealed a dinuclear molecular structure [Sn2(bpza)4] (1b), with κ3-N,N,O-coordinated bpza ligands at each of the Sn(II) and two bpza ligands μ-bridging between the Sn(II) centres. The molecular structure of [Sn(bdmpza)2] (2) exhibits a homoleptic bisligand complex with both ligands displaced by the free electron pair, which is proven by DFT calculations. Oxidation of complex 2 in an attempt to synthesize a homoleptic Sn(IV) complex leads to the formation of [Sn(bdmpza)F3] (3). In addition, homoleptic bisligand gallium complexes [Ga(bdmpza)2][ClO4] (4) and [Ga(bpza)2][GaCl4] (5) were prepared and characterized by 71Ga NMR and IR spectroscopy as well as by X-ray crystallography.

The high activity of mesoporous Ga-SiO2 catalysts in the upgrading of glycerol to solketal explained by in-depth characterization

Mesoporous Ga-silicates were prepared using a sustainable and continuous aerosol synthesis procedure. The novel solids showed record activity in the synthesis of solketal from glycerol and acetone. Three different Si/Ga ratios of 34, 74 and 148 were investigated and all materials displayed favorable features for catalytic applications such as high surface area and calibrated mesoporosity. An in-depth characterization allowed to elucidate the remarkable catalytic performance. The insertion of Ga predominantly as single site in tetrahedral coordination in the silica framework was highlighted by X-ray photoelectron spectroscopy (XPS), using the Auger parameter of Ga in a Wagner plot representation. The speciation of Ga was further clarified using solid state 71Ga NMR spectroscopy, confirming the formation of mainly isolated Ga species. Consistently, the aerosol-made Ga-silicates displayed outstanding turnover frequencies (up to 677 h−1) and selectivity, markedly outcompeting other reference metallosilicate catalysts reported in literature. Moreover, the most active catalyst was successfully reused in multiple catalytic cycles thus proving its stability under the selected reaction conditions.

Macroscopic Orientation Domains Grown via Aerodynamic Levitation: A Path toward Single Crystals

Powder pellets of ca. 30 mg with the mol % composition 0.61Ga2O30.39La2O3 are levitated in a gas stream and melted using a pair of CO2 lasers. The levitating melt droplets crystallize to form La2Ga3O7.5 or LaGaO3 after the lasers are turned off. The microstructure of cross sections prepared from them is analyzed using scanning electron microscopy and electron backscatter diffraction. One of the beads containing La2Ga3O7.5 shows a fully crystallized, polycrystalline microstructure, whereas the other broke apart during cooling, and its cross section shows only a single orientation domain which is more than 7 mm2 in area. Solid-state 71Ga NMR spectroscopy was applied to this sample and showed that it was not a single crystal but contains only about 3 orientation domains. The bead containing LaGaO3 showed dendritic growth originating from multiple nuclei. Although a single crystal was not achieved by the applied experiments so far, a detailed correlation of measured cooling curves with the observed microstructures enables devising steps along the path toward single crystal production in future experiments.

Thermal Dehydrofluorination of GaPO-34 Revealed by NMR Crystallography

Using a combination of solid-state NMR spectroscopy, powder X-ray diffraction (pXRD), thermogravimetry, and periodic density functional theory (DFT) calculations, we investigate the calcination of the chabazite-type gallophosphate, GaPO-34, prepared with either 1-methylimidazole (mim) or pyridine (py) as the structure-directing agent (SDA) and fluoride as the charge-balancing anion. We demonstrate that, prior to SDA combustion, there is an unusual low-temperature dehydrofluorination step at ∼330 °C for the mim material, but not for the py form. The DFT-derived structure for the dehydrofluorinated intermediate contains pentacoordinate Ga species with Ga–N bonds of 2.04 A to the mim nitrogen atom, in addition to four Ga–O bonds to neighboring PO4 tetrahedra. This observation is consistent with 71Ga NMR spectroscopy, which shows that one-third of the Ga is pentacoordinate with a large quadrupolar coupling constant of ∼11 MHz. Powder X-ray diffraction measured in situ on heating shows the transient appearance of a distinct crystalline phase between 325 and 425 °C before the characteristic chabazite structure is seen, which is consistent with dehydrofluorination prior to loss of the organic SDA. No such dehydrofluorinated intermediate structure is observed for the py form of GaPO-34, which is ascribed to the lower pKa of pyridinium relative to 1-methylimidazolium.

Quaternary Wurtzitic Nitrides (1 - x)ZnGeN2-2xGaN (x = 0.02, 0.05): Disorder-Induced Band-Gap Narrowing and Potentiality as a Solar-Active Photocatalyst.

We examined the ZnGeN2-GaN solid-solution system (Zn1-xGe1-xGa2xN2) in the unexplored compositional region of x < 0.10 to reveal the transitional structural and optical properties caused by the introduction of Ga. Fairly stoichiometric fine powder specimens with compositions of x = 0.02 and 0.05 were prepared by the gas-reduction-nitridation method, and their partially ordered Pna21 structure was identified by solid-state 71Ga NMR spectroscopy and time-of-flight neutron powder diffraction. The Rietveld refinement results of the neutron diffraction data showed that the introduction of 2 atom % Ga readily retards the cation ordering in ZnGeN2, and this composition-induced transition to the wurtzite disordered phase proceeds mostly in the range of x < 0.10. The synthesized samples showed gradual red shifts of the absorbance and photoluminescence excitation spectra with their x value, consistent with their degree of disorder, indicating that the narrowing of the band gap achieved in the current system results primarily from the disorder of the cation sublattice accompanied by octet-rule violation, as has been predicted theoretically. The test reactions for photocatalytic water splitting resulted in improved H2 evolution rates of 6.1-72.6 μmol/h under UV-visible-light irradiation, and stable solar H2 evolution of up to 5 days was demonstrated.

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes.

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga3+ ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and 71Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2–3) a polyoxogallate structure is identified as either Ga30 or Ga32 in contradiction with the previously proposed Ga13 Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga3+ ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.

Insights into the Composition and Structural Chemistry of Gallium(I) Triflate

Abstract“GaOTf” is a simple, convenient source of low‐valent gallium for synthetic chemistry and catalysis. However, little is currently known about its composition or reactivity. In this work, 71Ga NMR spectroscopy shows the presence of [Ga(arene)n]+ salts on oxidation of Ga metal with AgOTf in arene solvents. However, a more complex picture of speciation is uncovered by X‐ray diffraction studies. In all cases, mixed‐valence compounds containing Ga‐arene and Ga‐OTf coordination motifs, in addition to an unusual “naked” [Ga]+ ion, are found. Addition of 18‐crown‐6 allows for the isolation of a discrete GaI crown complex. Evidence of a potential intermediate in the formation of “GaOTf” has been isolated in the form of the bimetallic silver(I)/gallium(I) cluster anion [Ag4{Ga(OTf)3}4(μ‐Ga)6(OTf)4]2−.

Insights into the Composition and Structural Chemistry of Gallium(I) Triflate.

“GaOTf” is a simple, convenient source of low‐valent gallium for synthetic chemistry and catalysis. However, little is currently known about its composition or reactivity. In this work, 71Ga NMR spectroscopy shows the presence of [Ga(arene)n]+ salts on oxidation of Ga metal with AgOTf in arene solvents. However, a more complex picture of speciation is uncovered by X‐ray diffraction studies. In all cases, mixed‐valence compounds containing Ga‐arene and Ga‐OTf coordination motifs, in addition to an unusual “naked” [Ga]+ ion, are found. Addition of 18‐crown‐6 allows for the isolation of a discrete GaI crown complex. Evidence of a potential intermediate in the formation of “GaOTf” has been isolated in the form of the bimetallic silver(I)/gallium(I) cluster anion [Ag4{Ga(OTf)3}4(μ‐Ga)6(OTf)4]2−.

Application of NMR Crystallography to Highly Disordered Templated Materials: Extensive Local Structural Disorder in the Gallophosphate GaPO-34A.

We present an NMR crystallographic investigation of two as-made forms of the recently characterized gallophosphate GaPO-34A, which has an unusual framework composition with a Ga:P ratio of 7:6 and contains both hydroxide and fluoride anions and either 1-methylimidazolium or pyridinium as the structure-directing agent. We combine previously reported X-ray crystallographic data with solid-state NMR spectroscopy and periodic density functional theory (DFT) calculations to show that the structure contains at least three distinct types of disorder (occupational, compositional, and dynamic). The occupational disorder arises from the presence of six anion sites per unit cell, but a total occupancy of five of these, leading to full occupancy of four sites and partial occupancy of the fifth and sixth (which are related by symmetry). The mixture of OH and F present leads to compositional disorder on the occupied anion sites, although the occupancy of some sites by F is calculated to be energetically unfavorable and signals relating to F on these sites are not observed by NMR spectroscopy, confirming that the compositional disorder is not random. Finally, a combination of high-field 71Ga NMR spectroscopy and variable-temperature 13C and 31P NMR experiments shows that the structure directing agents are dynamic on the microsecond time scale, which can be supported by averaging the 31P chemical shifts calculated with the SDA in different orientations. This demonstrates the value of an NMR crystallographic approach, particularly in the case of highly disordered crystalline materials, where the growth of large single crystals for conventional structure determination may not be possible owing to the extent of disorder present.

Experimental and theoretical evidence for hydrogen doping in polymer solution-processed indium gallium oxide

The field-effect electron mobility of aqueous solution-processed indium gallium oxide (IGO) thin-film transistors (TFTs) is significantly enhanced by polyvinyl alcohol (PVA) addition to the precursor solution, a >70-fold increase to 7.9 cm2/Vs. To understand the origin of this remarkable phenomenon, microstructure, electronic structure, and charge transport of IGO:PVA film are investigated by a battery of experimental and theoretical techniques, including In K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS); resonant soft X-ray scattering (R-SoXS); ultraviolet photoelectron spectroscopy (UPS); Fourier transform-infrared (FT-IR) spectroscopy; time-of-flight secondary-ion mass spectrometry (ToF-SIMS); composition-/processing-dependent TFT properties; high-resolution solid-state 1H, 71Ga, and 115In NMR spectroscopy; and discrete Fourier transform (DFT) analysis with ab initio molecular dynamics (MD) liquid-quench simulations. The 71Ga{1H} rotational-echo double-resonance (REDOR) NMR and other data indicate that PVA achieves optimal H doping with a Ga···H distance of ∼3.4 Å and conversion from six- to four-coordinate Ga, which together suppress deep trap defect localization. This reduces metal-oxide polyhedral distortion, thereby increasing the electron mobility. Hydroxyl polymer doping thus offers a pathway for efficient H doping in green solvent-processed metal oxide films and the promise of high-performance, ultra-stable metal oxide semiconductor electronics with simple binary compositions.

Approaching Dissolved Species in Ammonoacidic GaN Crystal Growth: A Combined Solution NMR and Computational Study.

Solutions of gallium trihalides GaX 3 (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (71Ga, 35Cl, 81Br) NMR spectroscopy. To unravel the role of pH, the analyses were done both in absence and in presence of ammonium halides, which are employed as mineralizers during ammonoacidic gallium nitride crystal growth. While gallium trifluoride and its ammoniate were found to be too sparingly soluble to give rise to a NMR signal, the spectra of solutions of the heavier halides reveal the presence of a single gallium‐containing species in all cases. DFT calculations and molecular dynamics simulations suggest the identification of this species as consisting of a [Ga(NH3)6]3+ cation and up to six surrounding halide anions, resulting in an overall trend towards negative complex charge. Quantitative 71Ga NMR studies on saturated solutions of GaCl3 containing various amounts of additional NH4Cl revealed a near linear increase of GaCl3 solubility with mineralizer concentration of about 0.023 mol GaCl3 per mol NH4Cl at room temperature. These findings reflect the importance of Coulombic shielding for the inhibition of oligomerization and precipitation processes and help to rationalize both the low solubility of gallium halides in neutral ammonia solution and, in turn, the proliferating effect of the mineralizer during ammonoacidic gallium nitride formation.

Synthesis, properties and structures of gallium(III) and indium(III) halide complexes with neutral pnictine coordination

A series of Group 13 halide complexes of SbnBu3, [GaX3(SbnBu3)] and [InX3(SbnBu3)] (X = Cl, Br, I) have been prepared by reaction of the appropriate trihalide with SbnBu3 in n-hexane or CH2Cl2 solution, and characterised by microanalysis, IR, 1H, 13C{1H} and 71Ga NMR spectroscopy. X-ray crystal structures are reported for [InX3(SbnBu3)] (X = Cl, Br) and [GaX3(SbnBu3)]. Similar pseudo-tetrahedral complexes of AsEt3, [GaCl3(AsEt3)], [InCl3(AsEt3)] and the five-coordinate [InCl3(AsEt3)2] were also obtained and their structures determined. Attempts to use [InCl3(SbR3)] and [GaCl3(SbR3)] (R = nBu, Et) as single source precursors for low pressure CVD growth of InSb or GaSb films were unsuccesful, instead producing elemental antimony, while [GaCl3(AsEt3)] and [InCl3(AsEt3)] failed to produce any deposition under similar conditions.

Al/Ga-Doped Li7La3Zr2O12 Garnets as Li-Ion Solid-State Battery Electrolytes: Atomistic Insights into Local Coordination Environments and Their Influence on 17O, 27Al, and 71Ga NMR Spectra.

Li7La3Zr2O12 (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state 17O, 27Al, and 71Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both 27Al and 71Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp 27Al and 71Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO4 neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO4 neighbors. A correlation between the size of the 27Al/71Ga quadrupolar coupling and the distortion of the doping sites (viz. XO4/XO5/XO6 with X = {Al/Ga}) is established. 17O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: 17O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.

NMR Study of Synthetic Gallosilicate Natrolite

Solid-state 29Si and 71Ga NMR was used to study the synthetic gallosilicate Na16Ga16Si24O80 ·16H2O (Ga–natrolite). It has been shown that Ga–natrolite contains mainly Si(GaO4)3(SiO4) and Si(GaO4)2(SiO4)2-structural units and has sufficiently ordered structure. Temperature dependence of the spin–lattice relaxation time T1 of 71Ga nuclei has also been studied using solid-state NMR. Spin–lattice relaxation of the 71Ga was determined to be governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H2O molecules in the Ga–natrolite pores.