Experimental study on preparation of LaMO3 (M = Fe, Co, Ni) nanocrystals and their catalytic activity

The perovskite-type oxides LaMO₃ (M = Fe, Co, Ni) were prepared by a glycine combustion method using La (NO₃)₃·6H₂O and Fe (NO₃)₃·9H₂O, Co (NO₃)₂·6H₂O, Ni (NO₃)₂·6H₂O as the raw materials, respectively, and C₂H₅NO₂ as gelating agent. The products were characterized by XRD, TEM, HRTEM, SEM and BET. The catalytic activity of LaMO₃ (M = Fe, Co, Ni) nanocrystals on thermal decomposition of NH₄CIO₄ (AP) were carried out by DTA and TG. The burning rate of the propellant modified by obtained LaCoO₃ was measured by strand burner method. The experimental results showed that the obtained products can play a catalytic role in the thermal decomposition of AP and combustion of AP-based propellant. The order of the catalytic performance of obtained products on AP thermal decomposition is LaCoO₃ > LaNiO₃ ≈ LaFeO₃. Adding 2% of LaCoO₃ nanocrystals to AP decreases the decomposition temperature by 134 °C and increases the heat of decomposition by 0.8 kJ g⁻¹. Compared with basic propellant, the burning rate of propellant modified by 1% LaCoO₃ nanocrystals increases around 8%.     

A template self-assembly strategy to synthesize free-standing TiO2-MO (M = Mg, Co, Ni, Cu or Zn) colloidal films

A series of binary free-standing colloidal films, TiO₂-MO (M = Mg, Co, Ni, Cu and Zn) have been synthesized via a self-assembly technology, utilizing sodium dodecyl sulfonate (SDS) as template and the mixture of titanium butoxide and MCl₂ as precursor. X-ray diffraction (XRD) and transmission electron microscope (TEM) have been employed to characterize TiO₂-MO samples. Results show that TiO₂-MO films are mainly composed by well-ordered nanostructures (e.g. mesoporous particles and lamellar pieces) and more importantly, the increased Ti(OC₄H₉)₄ precursor have significant effect on modifying the as-synthesized nanostructures. A structural model based on SDS micellar template, the complex metallic oxide precursor and charge-density matching between the template and precursor has been proposed. Remarkably, this template self-assembly method has a potential to design a variety of functional multicomponent materials with high-ordered nanostructures, such as high gas sensing SnO₂-ZnO films.     

Synthesis and characterization of titanates of the formula MTiO3 (M = Mn,Fe, Co,Ni and Cd) by co-precipitation of mixed metal oxalates

When divalent metal chloride solutions of Mn, Fe, Co, Ni and Cd were mixed with potassium titanyl oxalate solution, mixed metal oxalates were obtained in the case of Fe, Co and Ni at room temperature in the pH range 1.5–3. In the case of manganese, heating was found to be necessary to induce precipitation and complete precipitation occurred at 80 °C. Mixed cadmium and titanyl oxalate precipitation was complete at a pH of 3 at room temperature. Various physico-chemical techniques were employed to characterize the as-dried oxalate precursors and the final MTiO₃ oxide powders (M = Mn, Fe, Co, Ni and Cd) obtained on thermal decomposition. All these experimental results relating to the synthesis and characterizations of MTiO₃ oxides are presented in this paper. The results suggest that the reaction of potassium titanyl oxalate and metal chloride solutions may not lead to the formation of a single molecular precursor by direct salt elimination reaction in the pH range 1.5–3.     

Heteronuclear M(II)-Ln(III) (M = Co, Mn; Ln = La, Pr, Sm, Gd, Dy and Er) coordination polymers: Synthesis, structures and magnetic properties

Thirteen novel 3d-4f heteronuclear coordination polymers based on the pyridine-2,6-dicarboxylic acid (H₂pda) and imidazole ligands, HIm[(pda)₃MLn(Im)₂(H₂O)₂]·3H₂O (Im = imidazole; M = Co, Ln = Pr (1), Gd (2), Dy (3), Er (4); M = Mn, Ln = Pr (5), Sm (6), Gd (7), Dy (8), Er (9)), HIm[(pda)₃CoSm(Im)₂(H₂O)₂]·2H₂O (10), [(Im)₄M(H₂O)₂][(pda)₄La₂(H₂O)₂]·2H₂O (M = Co (11), Mn (12)), and [(pda)₆Co₃Pr₂(H₂O)₆]·6H₂O (13), have been prepared and structurally characterized. X-ray crystallographic analyses revealed that these complexes display four different types of structures. Complexes 1-9 are isostructural, and possess 1-D chain structures constructed by alternately arrayed nine-coordinated Ln(III) (Ln = Pr, Sm, Gd, Dy, Er) and six-coordinated M(II) (M = Mn, Co) ions. Complex 10 exhibits a unique one-dimensional structure, in which two independent chains are parallel viewed down the a-axis and anti-parallel viewed down the c-axis. Complexes 11 and 12 are isostructural and display 1-D homometallic chain structures. Complex 13 is a 3D framework fabricated through PrN₃O₆ and CoO₆ polyhedrons as building blocks. The variable-temperature solid-state dc magnetic susceptibilities of complexes 2, 3, 4, 9 and 13 have been investigated. Antiferromagnetic exchange interactions were determined for these five complexes.     

Synthesis, crystal structure and magnetic characterization of metal(II) coordination polymers based on 2-carboxyethylphosphonic acid and 1,10-phenanthroline (metal=Cu, Co, Cd)

Three non-isostructural metal(II) coordination polymers (metal=copper, cobalt, cadmium) were synthesized under the same mild hydrothermal conditions (T=408 K) by mixture of the corresponding metal acetate with 2-carboxyethylphosphonic acid and 1,10-phenanthroline (1:1:1 M ratio) and their structures were determined by single-crystal X-ray diffraction. Cu₂(HO₃PCH₂CH₂COO)₂(C₁₂H₈N₂)₂(H₂O)₂ and Cd₂(HO₃PCH₂CH₂COO)₂(C₁₂H₈N₂)₂ are triclinic (space group P-1) with a=7.908(5) Å, b=10.373(5) Å, c=11.515(5) Å, α=111.683(5)°, β=95.801(5)°, γ=110.212(5)° (T=120 K), and a=8.162(5) Å, b=9.500(5) Å, c=11.148(5) Å, α=102.623(5)°, β=98.607(5)°, γ=113.004(5)° (T=293 K), respectively. In contrast, [Co₂(HO₃PCH₂CH₂COO)₂(C₁₂H₈N₂)₂(μ-OH₂)](H₂O) is orthorhombic (space group Pbcn) with a=21.1057(2) Å, b=9.8231(1) Å, c=15.4251(1) Å (T=120 K). For these three compounds, structural features, including H-bond network and the π-π stacking interactions, and thermal stability are reported and discussed. None of the materials present a long-range magnetic order in the range of temperatures investigated from 300 K down to 1.8 K.     

Pentacyanido(L)ferrate(III) complexes: Crystal structures, electrochemical and DFT studies (L = pyrazine, 4,4′-bipyridine, azide)

The crystal structures of two pentacyanido(L) ferrate(III) complexes, [P(C₆H₅)₄]₂[Fe(CN)₅(prz)]·4H₂O 1, [P(C₆H₅)₄]₂[Fe(CN)₅(4,4′-bipy)]·3H₂O 2, have been solved. Within the two complex anions the iron atoms are hexacoordinated by five cyanido ligands, the sixth position being occupied by the nitrogen atom arising from pyrazine and, respectively, 4,4′-bipyridine. The electrochemical properties of compounds 1, 2 and of the azido derivative, (Ph₄As)₂[Na(H₂O)₄][Fe(CN)₅(N₃)] 3, have been investigated by cyclic voltammetry. A relatively complicated redox behavior of these complexes was found, due especially to the electron transfer involving the central metallic ion that changes reversibly its oxidation state (Fe^III/Fe^II redox site) and also to the coligand (4,4′-bipyridine, pyrazine or azide) which intervenes in a distinct electron transfer. The experimental data have been rationalized through DFT calculations.     

Triazole-bridged magnetic M(II) assemblies (M = Co,Ni) capped with the end-on terephthalate dianion involving multi-intermolecular contacts

Two dimeric complexes [M₂(atrz)₃(H₂O)₄(tp)₂] · 11H₂O [M = Co(1 · 11H₂O), Ni(2 · 11H₂O); tp = terephthalate, atrz = 4-aminotriazole] were constructed from triply-bridged triazole ligands and terminal tp groups. Interestingly tp behaves as a capping group in an end-on fashion, which is a rare example for tp-related systems. Among the dinuclear assemblies, three types of intermolecular interactions (hydrogen bond, face-to-face π–π and edge-to-face CH–π contacts) as well as intramolecular hydrogen bonding are observed, eventually leading to the formation of a 3D network. Magnetic data show that moderate antiferromagnetic interactions between the two metal centers communicate through triazole groups, and non-negligible interdimer antiferromagnetic couplings are also involved in this system.     

Syntheses and crystal structures of cadmium(II)X2-hexamethylenetetramine (X = Br/I/SCN) coordination polymers having different dimensionality

Four new coordination polymers of cadmium(II) with hexamethylenetetramine (htm) have been synthesized and characterized by routine physicochemical techniques as well as by X-ray single crystal structure analysis. They are [CdBr(htm)(SCN)(H₂O)₂·CH₃OH]_n (1), [CdI(htm)(SCN)(H₂O)₂·0.5(CH₃OH)]_n (2), [Cd₂(htm)₃(SCN)₄(H₂O)]_n·nH₂O (3) and [Cd₃Br₆(htm)₂(H₂O)₅·(htm)(H₂O)₆]_n (4). Complexes 1, 2 and 3 exhibit 1D polymeric structure and complex 4 shows a 2D undulated layered arrangement, containing Cd₆(htm)₆ hexagonal units as building block, which extended to a 3D supramolecular architecture through hydrogen bonding. Thorough thermal investigation suggest that as far as the thermal stability of Cd(II)-htm bond is concerned it attains the maximum in complex 1 and minimum in complex 4. In case of complex 3 the thermal study inferred that CdS end product was obtained at ∼730 °C, whereas in case of other complexes the thermally stable end product remained unidentified. Solid state fluorescence study shows that all the complexes are luminescent at room temperature except complex 3.     

Three-coordinate bis(phosphinimino)methanide derivatives of ‘open shell’ [M(II)] (M = Mn, Fe, Co) transition metals

An alternative two-step synthesis of the previously reported bis(diphenylphosphinimino)methane [CH₂(Ph₂P = NC₆H₃ⁱPr₂-2,6)₂] involving bromine oxidation of dppm and metathesis with 2,6-diisopropylaniline is described. [CH₂(Ph₂PNC₆H₃ⁱPr₂-2,6)₂] is readily deprotonated by the transition metal silylamides [M{N(SiMe₃)₂}₂] (M = Mn, Fe and Co) to provide a series of exclusively three-coordinate derivatives [{CH(Ph₂PNC₆H₃ⁱPr₂-2,6)₂M{N(SiMe₃)₂}] which have been characterised by elemental analysis, magnetic measurements and, in the case of the Fe derivative, X-ray structural analysis. Reactivity studies have shown that even such sterically demanding bis(phosphinimino)methanide ligands are prone to protonolysis and reductive [P(V) to P(III)] degradation which limits their utility as stable platforms for further M(II) derivitisation.     

Synthesis and structure of the two-dimensional coordination networks [Ln(PDC)(N-HPDC)]∞ (PDC = pyridine-3,4-dicarboxylate,Ln = La,Ce, Pr)

The title compounds, [Ln(PDC)(HPDC)]_∞ (H₂PDC = pyridine-3,4-dicarboxylic acid, Ln = La, Ce, Pr), have been prepared and have been characterised by IR, MS, TGA, X-ray crystallography and powder diffraction. These isostructural complexes display both bridging and chelating carboxylate binding modes including a highly unusual chelation in which both carboxylate substituents bind to the same Pr atom, imparting chirality around the metal despite the non-chiral crystal setting. This system represents the first fully characterised metal complex containing the N-protonated ligand derivative (N-HPDC⁻). A polymorph of the ligand precursor, pyridine-3,4-dicarboxylic acid is also reported.     

Oxalate-2-ethanolamine complexes of some bivalent cations (Mn,Co, Ni,Cu, Zn and Cd)

On the refluxing ofM(II) oxalate (M=Mn, Co, Ni, Cu, Zn or Cd) and 2-ethanolamine in chloroform, the following complexes were obtained: MnC₂O₄·HOCH₂CH₂NH₂·H₂O, CoC₂O₄·2HOCH₂CH₂NH₂, Ni₂(C₂O₄)₂·5HOCH₂CH₂NH₂·3H₂O, Cu₂(C₂O₄)₂·5HOCH₂CH₂NH₂, Zn₂(C₂O₄)₂·5HOCH₂CH₂NH₂·2H₂O and Cd₂(C₂O₄)₂·HOCH₂CH₂NH₂·2H₂O. Following the reaction ofM(II) oxalate with 2-ethanolamine in the presence of ethanolammonium oxalate, a compound with the empirical formula ZnC₂O₄·HOCH₂CH₂NH₂·2H₂O¹ was isolated. The complexes were identified by using elemental analysis, X-ray powder diffraction patterns, IR spectra, and thermogravimetric and differential thermal analysis. The IR spectra and X-ray powder diffraction patterns showed that the complexes obtained were not isostructural. Their thermal decompositions, in the temperature interval between 20 and about 900°C, also take place in different ways, mainly through the formation of different amine complexes. The DTA curves exhibit a number of thermal effects.     

Aqueous syntheses of [(Cp-R)M(CO)3] type complexes (Cp = cyclopentadienyl, M = Mn, Tc, Re) with bioactive functionalities

We describe reactions of [^99mTc(H₂O)₃(CO)₃)]⁺ (1) with Diels-Alder products of cyclopentadiene such as “Thiele’s acid” (HCp-COOH)₂ (2) and derivatives thereof in which the corresponding [(Cp-COOH)^99mTc(CO)₃)] (3) complex did form in water. We propose a metal mediated Diels-Alder reaction mechanism. To show that this reaction was not limited to carboxylate groups, we synthesized conjugates of 2 (HCp-CONHR)₂ (4a-c) (4a, R = benzyl amine; 4b, R = N_α-Boc-l-2,3-diaminopropionic acid and 4c, R = glycine). The corresponding ^99mTc complexes [(4a)^99mTc(CO)₃)] 6a, [(4b)^99mTc(CO)₃)] 6b and [(4c)^99mTc(CO)₃)] 6c have been prepared along the same route as for Thiele’s acid in aqueous media demonstrating the general applicability of this synthetic strategy. The authenticity of the ^99mTc complexes on the no carrier added level have been confirmed by chromatographic comparison with the structurally characterized manganese or rhenium complexes.Studies of the reaction of 1 with Thiele’s acid bound to a solid phase resin demonstrated the formation of [(Cp-COOH)^99mTc(CO)₃)] 3 in a heterogeneous reaction. This is the first evidence for the formation of no carrier added ^99mTc radiopharmaceuticals containing cyclopentadienyl ligands via solid phase syntheses. Macroscopically, the manganese analogue 5a and the rhenium complexes 5b-c have been prepared and characterized by IR, NMR, ESI-MS and X-ray crystallography for 5a (monoclinic, P2₁/c, a = 9.8696(2) Å, b = 25.8533(4) Å, c = 11.8414(2) Å, β = 98.7322(17)°) in order to unambiguously assign the authenticity of the corresponding ^99mTc complexes.     

Weak ferromagnetism in 1D coordination polymers: Syntheses,crystal structures,spectroscopic and magnetic properties of [Cu(L)(Memal)]n (Memal = the dianion of methylmalonic acid,L = 1,10-phenanthroline, 2,2′-bipyridine)

Investigation of the CuCl₂/H₂Memal/L (H₂Memal = methylmalonic acid, L = 1,10-phenanthroline or 2,2′-bipyridine) reaction system in MeOH and various molar ratios has lead to the isolation of two one-dimensional coordination polymers presenting the [Cu(L)(Memal)] repeating unit (1, L = 1,10-phen; 2, L = 2,2′-bpy). The Memal²⁻ ligand adopts the bidentate [chelating] + unidentate coordination mode between the Cu^II ions. Magnetic susceptibility measurements on 1 and 2 indicated the existence of weak ferromagnetic intrachain interactions and X-band EPR spectra from powdered samples of 1 and 2 are consistent with the stereochemistry of the Cu^II ions and with the presence of weak exchange interactions.     

Synthesis,structures and NLO properties of five non-centrosymmetric coordination compounds from the copper(II)/dps system (dps = 4,4′-dipyridyl sulfide)

The formation, crystal structure and properties of five copper(II) coordination compounds with the angular ligand, 4,4′-dipyridyl sulfide (dps) are described, {[Cu₃(μ-dps)₄(μ-SO₄)₂(SO₄)(H₂O)₅] · 10H₂O}_∞ (1 · 10H₂O), [Cu(dps)₄(H₂O)₂] · (ClO₄)₂ · H₂O (2 · H₂O), {[Cu(μ-dps)₂(DMF)₂](ClO₄)₂}_∞ (3), {[Cu(μ-dps)₂(H₂O)₂] · (NO₃)₂ · 2H₂O}_∞ (4 · 2H₂O) and {[Cu₃(μ-dps)₆(DMF)₂(H₂O)₄] · (NO₃)₆ · (DMF) · 6H₂O}_∞ (5 · DMF · 6H₂O). The topological architectures of all these coordination compounds are strongly dependent on the counteranions, with the aid of guest solvents, and include a chiral 3D non-interpenetrated structure for 1, an acentric mononuclear structure for 2, acentric 2D undulating networks for 3 and 5, and a chiral 1D double-stranded chain for 4. In particular, all these acentric or chiral coordination architectures are generated from an achiral ligand as a building unit, and their second-order non-linear optical (NLO) properties are also studied in this paper.     

Alkaline earth metal poly(hydrogen fluorides) hexafluoroarsenates(V) and hexafluorophosphate(V): M2(H2F3)(HF2)2(AF6) (M = Ca, A = As; M = Sr, A = As, P)

New compounds of the type M₂(H₂F₃)(HF₂)₂(AF₆) with M = Ca, A = As and M = Sr, A = As, P) were isolated. Ca₂(H₂F₃)(HF₂)₂(AsF₆) was prepared from Ca(AsF₆)₂ with repeated additions of neutral anhydrous hydrogen fluoride (aHF). It crystallizes in a space group P4₃22 with a = 714.67(10) pm, c = 1754.8(3) pm, V = 0.8963(2) nm³ and Z = 4. Sr₂(H₂F₃)(HF₂)₂(AsF₆) was prepared at room temperature by dissolving SrF₂ in aHF acidified with AsF₅ in mole ratio SrF₂:AsF₅ = 2:1. It crystallizes in a space group P4₃22 with a = 746.00(12) pm, c = 1805.1(5) pm, V = 1.0046(4) nm³ and Z = 4. Sr₂(H₂F₃)(HF₂)₂(PF₆) was prepared from Sr(XeF₂)_n(PF₆)₂ in neutral aHF. It crystallizes in a space group P4₁22 with a = 737.0(3) pm, c = 1793.7(14) pm, V = 0.9744(9) nm³ and Z = 4. The compounds M₂(H₂F₃)(HF₂)₂(AF₆) gradually lose HF at room temperature in a dynamic vacuum or during being powdered for recording IR spectra or X-ray powder ray diffraction patterns. All compounds are isotypical with coordination of nine fluorine atoms around a metal center forming a distorted Archimedian antiprism with one face capped. This is the first example of the compounds in which H₂F₃⁻ and HF₂⁻ anions simultaneously bridge metal centers forming close packed three-dimensional network of polymeric compounds with low solubility in aHF. The HF₂⁻ anions are asymmetric with usual F?F distances of 227.3-228.5 pm. Vibrational frequency (ν₁) of HF₂⁻ is close to that in NaHF₂. The anion H₂F₃⁻ exhibits unusually small F?F?F angle of 95.1°-97.6° most probably as a consequence of close packed structure.     

Polyol-mediated low-temperature synthesis of crystalline tungstate nanoparticles MWO4 (M = Mn,Fe, Co,Ni, Cu,Zn)

A polyol-mediated synthesis is presented as a general access to nanoscaled transition-metal tungstates MWO₄ (M = Mn, Fe, Co, Ni, Cu, Zn). Using simple inorganic salts as starting materials, uniform and readily crystalline nanoparticles are prepared under mild conditions (T < 220 °C). The nanoparticles are of high quality in terms of small diameter (<20 nm), high surface area (up to 200 m² g⁻¹), phase purity and yield (>85%). Size, morphology and composition can be adjusted by precise variation of the reaction parameters, including type of starting material, duration and temperature of reaction. The transition-metal tungstate nanoparticles are fully functional, exhibiting typical properties of this class of materials, for instance, superparamagnetism (CoWO₄), luminescence (ZnWO₄) and photocatalytic activity (CuWO₄).     

Synthesis and characterization of Li(I)-M(II) (M = Co, Ni) heterometallic complexes as molecular precursors for LiMO2

Lithium-containing heterometallic complexes with cobalt (Li₂Co₂(Piv)₆(2,4-Lut)₂ (2, Piv is the pivalate anion) and Li₂Co₂(O₂CCH₂Bu^t)₆(2,4-Lut)₂ (3)) and with nickel (Li₂Ni₂(Piv)₆(DME)₂ (4) and Li₂Ni₂(Piv)₆(2,2′-bpy)₂ (5)) were synthesized. The structures of the complexes were established by X-ray diffraction. The magnetic properties of complexes 2 and 4 were studied. The thermal behavior of compounds 2, 3, and 5 was investigated. It was shown that the compounds under study can be used as molecular precursors for the synthesis of lithium cobaltate and nickelate.     

Quantum chemical study of ethylene addition to group-7 oxo complexes MO2(CH3)(CH2) (M = Mn, Tc, Re)

Quantum chemical calculations using DFT at the B3LYP level have been carried out for the reaction of ethylene with the group-7 compounds ReO₂(CH₃)(CH₂) (Re1), TcO₂(CH₃)(CH₂) (Tc1) and MnO₂(CH₃)(CH₂) (Mn1). The calculations suggest rather complex scenarios with numerous pathways, where the initial compounds Re1-Mn1 may either engage in cycloaddition reactions or numerous addition reactions with concomitant hydrogen migration. There are also energetically low-lying rearrangements of the starting compounds to isomers which may react with ethylene yielding further products. The [2 + 2]_Re,C cycloaddition reaction of the starting molecule Re1 is kinetically and thermodynamically favored over the [3 + 2]_C,O and [3 + 2]_O,O cycloadditions. However, the reaction which leads to the most stable product takes place with initial rearrangement to the dioxohydridometallacyclopropane isomer Re1a that adds ethylene with concomitant hydrogen migration yielding Re1a-1. The latter reaction has a slightly higher barrier than the [2 + 2]_Re,C cycloaddition reaction. The direct [3 + 2]_C,O cycloaddition becomes more favorable than the [2 + 2]_M,C reaction for the starting compounds Tc1 and Mn1 of the lighter metals technetium and manganese but the calculations predict that other reactions are kinetically and thermodynamically more favorable than the cycloadditions. The reactions with the lowest activation barriers lead after rearrangement to the ethyl substituted dioxometallacyclopropanes Tc1a-1 and Mn1a-1. The manganese compound exhibits an even more complex reaction scenario than the technetium compounds. The thermodynamically most stable final product of ethylene addition to Mn1 is the ethoxy substituted metallacyclopropane Mn1a-2 which has, however, a high activation barrier.