71Ga NMR Spectroscopy Research Articles

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.

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.

Synthesis of the Cationic Gallium Phthalocyanines and Their Catalytic Application in Gallium(III)-Activated Processes for Donor–Acceptor Substrates

The novel class of gallium catalysts based on cationic gallium phthalocyanines (RPcGa+) with SbF6– and Sb2F11– weakly coordinating anions was developed in order to using in the catalytic version for gallium-specific chemistry. Two cycloaddition/annulation processes of styrylmalonates with aromatic aldehydes and styrene were implemented in the catalytic version using this approach. The synthetic strategy to gallium phthalocyanines and their structural features were studied in details, including the use of specially adapted 71Ga NMR spectroscopy in combination with 19F and 121Sb ones, as well as the mechanistic aspects.

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.

Comparative solution studies and cytotoxicity of gallium(III) and iron(III) complexes of 3-hydroxy-2(1H)-pyridinones

The stoichiometry and stability constants of the gallium(III) and iron(III) complexes of two alkoxycarbonylmethyl-3-hydroxy-2(1H)-pyridinone ligands were determined by means of pH-potentiometry, UV–Vis spectrophotometry and 1H and 71Ga NMR spectroscopy in aqueous solution. The cytotoxicity of one of the gallium(III) complexes was also measured in multidrug resistant/non-resistant human colon adenocarcinoma cell lines. Iron(III) forms complexes with the studied 3-hydroxy-2-pyridinones of higher stability than gallium(III), while the obtained pFe values are significantly lower (pFe: 14.95, 15.06; pH 7.4, cM = 1 µM, cL = 10 µM) compared to those of typical iron binders such as deferiprone or transferrin. The moderate gallium(III) and iron(III) binding ability of the compounds stands for lower solution complex stability compared to that of analogous bidentate non-substituted 3-hydroxy-2-pyridinone or 3-hydroxy-4-pyridinone (O,O) donor ligands. Tris-ligand complexes of the general formula [ML3] (M = Ga, Fe) predominate at physiological pH for both ligands. No interaction with cell culture medium components was observed in the millimolar concentration range of gallium(III) complexes, however they can suffer significant decomposition at biologically relevant low concentrations leading to negligible cytotoxic activity. The redox potential of the studied iron–3-hydroxy-2-pyridinone complex (E1/2 = −597 mV at pH 7.4) falls into the range that is typical of iron(III) complexes with conventional bidentate (O,O) donor-containing chelators.

Ionic Ga-Complexes of Alkylidene- and Arylmethylidenemalonates and Their Reactions with Acetylenes: An In-Depth Look into the Mechanism of the Occurring Gallium Chemistry.

A new synthetic concept was suggested in the chemistry of substituted methylidenemalonates that enables their utilization as 1,2-zwitterionic synthons. This strategy is to generate liquid ionic Ga complexes from methylidenemalonates and GaHal3 with a strict 3/4 composition and then use them in further synthesis. A number of complexes with different metal halides have been synthesized and studied in detail. The unique properties of gallium among all metals have been demonstrated and explained. On the basis of the discovered new class of gallium complexes of methylidenemalonates, a number of novel reactions with acetylenes have been elaborated, which are unknown in the conventional chemistry of methylidenemalonates. The main demonstrated process is a three-component addition to a triple bond involving halide anions, leading to the formation of polyfunctional vinyl halides with high E-selectivity. The mechanism has been studied experimentally in fine detail. Application of specially optimized 71Ga NMR spectroscopy makes it possible to take an in-depth look into the gallium chemistry in a new light. In particular, the key participation of GaHal4- anions in the occurring transformations has been established.

Equilibrium, Kinetic and Structural Properties of Gallium(III) and Some Divalent Metal Complexes Formed with the New DATAm and DATA5m Ligands.

The development of 68 Ge/68 Ga generators has made the positron-emitting 68 Ga isotope widely accessible and raised interest in new chelate complexes of Ga3+ . The hexadentate 1,4-di(acetate)-6-methyl[amino(methyl)acetate]perhydro-1,4-diazepane (DATAm ) ligand and its bifunctional analogue, 1,4-di(acetate)-6-pentanoic acid[amino(methyl)acetate]perhydro-1,4-diazepane (DATA5m ), rapidly form complexes with 68 Ga in high radiochemical yield. The stability constants of DATAm and DATA5m complexes formed with Ga3+ , Zn2+ , Cu2+ , Mn2+ and Ca2+ have been determined by using pH potentiometry, spectrophotometry (Cu2+ ) and 1 H and 71 Ga NMR spectroscopy (Ga3+ ). The stability constants of Ga(DATAm ) and Ga(DATA5m ) complexes are slightly higher than those of Ga(AAZTA). The species distribution calculations indicated the predominance of Ga(L)OH mixed-hydroxo complexes at physiological pH. The 1 H and 71 Ga NMR spectroscopy studies provided information about the coordinated functional groups of ligands and on the kinetics of exchange between the Ga(L) and Ga(L)OH complexes. The transmetalation reactions between the Ga(L) complexes and Cu2+ citrate (6<pH<8.5) occur through both spontaneous and OH- -assisted dissociation of the Ga(L)OH species. At pH 7.4 and 25 °C, the half-lives of the dissociation of Ga(DATAm ), Ga(DATA5m ) and Ga(AAZTA) were 11, 44 and 24 h, respectively. Similar half-lives have been obtained for the ligand-exchange reactions between the Ga(L)OH complexes and transferrin. The equilibrium and kinetic data indicate that the Ga(DATA5m ) complex is a good 68 Ga-based radiodiagnostic candidate.

Physical properties of the GaPd2 intermetallic catalyst in bulk and nanoparticle morphology

Intermetallic compound GaPd2 is a highly selective catalyst material for the semi-hydrogenation of acetylene. We have determined anisotropic electronic, thermal and magnetic properties of a GaPd2 monocrystal along three orthogonal orthorhombic directions of the structure. By using 69Ga and 71Ga NMR spectroscopy, we have determined the electric-field-gradient tensor at the Ga site in the unit cell and the Knight shift, which yields the electronic density of states (DOS) at the Fermi energy εF. The DOS at εF was determined independently also from the specific heat. To see the change of electronic properties of the GaPd2 phase on going from the bulk material to the nanoparticles morphology, we have synthesized GaPd2/SiO2 supported nanoparticles and determined their electronic DOS at εF from the 71Ga NMR spin-lattice relaxation rate. The electronic DOS of the GaPd2 was also studied theoretically from first principles. All results are compared to the chemically related compound GaPd. The active–site-isolation concept for an increased catalytic selectivity is discussed in relation to the GaPd2 and GaPd structures.

Local environment and composition of magnesium gallium layered double hydroxides determined from solid-state 1H and 71Ga NMR spectroscopy

Ordering of gallium(III) in a series of magnesium gallium (MgGa) layered double hydroxides (LDHs), [Mg1−xGax(OH)2(NO3)x·yH2O] was investigated using solid-state 1H and 71Ga NMR spectroscopy as well as powder X-ray diffraction. Three different proton environments from Mg3OH, Mg2GaOH and intergallery water molecules were assigned and quantified using {1H,71Ga} HETCOR and 1H MAS NMR. A single 71Ga site originating from the unique Ga site in the MgGa LDH׳s was observed in 71Ga MAS and 3QMAS NMR spectra. Both 1H MAS NMR spectra recorded at 21.1T (900MHz) and elemental analysis show that the synthesized MgGa LDH׳s had a lower Mg:Ga ratio than that of the starting reactant solution. The origin of this is the formation of soluble [Ga(OH)4]− complexes formed during synthesis, and not due to formation of insoluble gallium (oxy)hydroxides. No sign of GaOGa connectivities or defects were detected for the MgGa LDH׳s.

Buffered chlorogallate(iii) ionic liquids and electrodeposition of gallium films

Buffering of Lewis acidic chlorometallate ionic liquids is a useful tool to modify their properties for electrochemical and catalytic applications. Lewis acidic chlorogallate(iii) ionic liquids containing the 1-octyl-3-methylimidazolium cation, buffered with sodium chloride, were studied using (71)Ga NMR spectroscopy and cyclic voltammetry. All the studied Lewis acidic compositions (0.50 < χGaCl3 ≤ 0.75) could be buffered to mild or moderate acidity, but not to neutrality. Electrodeposition of gallium from such buffered systems was possible, yielding deposits of improved morphology over the unbuffered ionic liquids, due to the constant melt composition maintained by the buffer. These findings were in a stark contrast with older studies on chloroaluminate(iii) ionic liquids buffered with sodium chloride.

Crystallographic and solution NMR structural analyses of four hexacoordinated gallium(iii) complexes based on ligands derived from 6-amino-perhydro-1,4-diazepine

The crystal structures are reported of four gallium(III) complexes based on two pairs of 6-substituted-6-amino-perhydro-1,4-diazepine ligands, together with a study of their solution structures examined by (1)H and (71)Ga NMR spectroscopy. In each case, the ligand adopts a twisted chair conformation that creates a facial array of the three ligand nitrogen atoms. The coordination geometry about each gallium(III) ion is a slightly distorted octahedron, with the 6-phenyl series being slightly more distorted than the 6-methyl analogues. Pulsed NMR experiments allow solution NMR structures to be assessed, revealing good agreement with the solid-state structures.

Order/disorder in YbNi1±xGa2∓x (x ≤ 0.08): Crystal structure, thermal expansion and magnetic properties

The ternary phase YbNi1±xGa2∓x (x ≤ 0.08) has been synthesised from the elements by high-frequency melting in argon atmosphere with subsequent annealing at 600 °C or 800 °C. From X-ray powder and single crystal diffraction investigations the phase is found to be isotypic with MgCuAl2: space group Cmcm, Z = 4, a = 4.0987–4.1147 Å, b = 9.8516–9.8620 Å, c = 6.5995–6.5916 Å for the specimens annealed at 800 °C. The quantum chemical analysis of atomic interactions in YbNiGa2 shows Ga and Ni forming a network polyanion with covalent bonds, while ytterbium cations are located within the cavities of the polyanion. From the magnetic susceptibility data and LIII XAS spectra, the 4f13 electronic configuration of ytterbium (Yb3+) was established in samples with nickel excess (comparing to x = 0), whereas an additional small contribution of the 4f14 configuration is found for compositions with the gallium excess. No change in the ytterbium valence state was detected down to 5 K. In situ low- and high-temperature powder diffraction investigations applying both X-ray synchrotron radiation and neutrons revealed no change in structure within the broad temperature range 3.4–1173 K. Both synchrotron and neutron diffraction data show smooth, but strongly anisotropic variation of lattice parameters below 200 K: the thermal contraction ratio in [100], [010] and [001] directions is 4.5:3.2:1. The temperature dependence of atomic displacement parameters indicates a certain amount of static disorder for all atomic species in the crystal. At least two distinct sites with varying charge distributions in the vicinity of the Ga atoms are confirmed by 69,71Ga NMR spectroscopy.

Gallium labeled NOTA-based conjugates for peptide receptor-mediated medical imaging

We report a straightforward and efficient synthetic strategy for the synthesis of three model glycine-arginine-glycine-aspartic acid-glycine (GRGDG) conjugates based on derivatives of NOTA and of their Ga(III) complexes targeted to the integrin α(ν)β(3) receptor. (71)Ga NMR spectroscopy showed that the Ga(III)-labeled conjugates are highly stable in aqueous solution. The (67)Ga-labeled conjugates proved to have high kinetic stability and showed a weak but specific binding to the receptors in a U87MG-glioblastoma cell line.

Bidentate Phosphine Oxides as Ligands to form GaIII Shell Complexes. [Ga(CH2(P(O)Ph)2)3]3+: Synthesis, Structural and Spectroscopic Study

AbstractThe tri‐cationic shell gallium complex [Ga(CH2(P(O)Ph)2)3]3+ containing three neutral diphosphine oxide ligands has been synthesized and structurally characterized by using X‐ray crystallography, mass spectrometry, and 1H, 13C, 31P, and 71Ga NMR spectroscopy. The ligands form three equivalent six‐membered chelate cycles to give an idealized octahedral environment around the Ga3+ ion. This compound is the first example of a mononuclear homoleptic octahedral structure formed by neutral phosphine oxide ligands in the chemistry of the 13 group elements in the +3 oxidation state.

Investigation of the Mn-doped compound CuGaTe2 by 63Cu AND 69Ga nuclear magnetic resonance techniques

The synthesized base compound CuGaTe 2 and the Mn-doped compounds Cu 0.97 Ga 0.97 Mn 0.06 Te 2 and Cu 0.93 Ga 0.93 Mn 0.14 Te 2 have been investigated by using 63 Cu and 69 Ga NMR spectroscopy. The NMR spectra obtained testify to substantial structural distortions near the cationic positions in the crystal lattice of Mndoped samples. The constants of quadrupole coupling between 63 Cu and 69 Ga nuclei in the compounds investigated have been estimated.