FTIR spectroscopic evidence for new isomers of 3-aminopyrazine-2-carboxylic acid formed in argon matrices upon UV irradiations

The UV-induced photochemistry and molecular structure of 3-aminopyrazine-2-carboxylic acid were studied in argon matrices by Fourier-transform infrared spectroscopy and B3LYP/6-311++G(2d,2p) calculations. Out of seventeen possible isomers of this molecule located on the singlet potential energy surface the most stable one, APA1 comprising intramolecular O-H···N and N-H···O hydrogen bonds, was detected experimentally in the matrix after deposition. Two new conformers APA2 and APA3 were generated upon irradiation with λ = 280 nm by trans/cis-COOH isomerization and at λ = 360 nm by COOH group rotamerization, respectively, whereas an amino-imino tautomerization leading to IPA1 and IPA2 structures occurred at λ = 305 nm. The reverse reactions were also observed upon irradiation of the matrices at 265, 230 and 400 nm. Simultaneously with the photoisomerizations, a cleavage of the pyrazine ring along with CO₂ elimination was observed leading to the formation of carbodiimide and cyanamide derivatives.

Crystal engineering of a rectangular coordination network to enable xylenes selectivity over ethylbenzene

Separation of the C8 aromatic isomers, p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB), is relevant thanks to their widespread application as chemical feedstocks but challenging because of their similar boiling points and close molecular dimensions. Physisorptive separation could offer an energy-efficient solution to this challenge but sorbents which exhibit strong selectivity for one of the isomers remain a largely unmet challenge despite recent reports of OX or PX selective sorbents with high uptake capacity. For example, the square lattice, sql, topology coordination network [Co(bipy)2(NCS)2] n (sql-1-Co-NCS) exhibits the rare combination of high OX selectivity and high uptake capacity. Herein we report that a crystal engineering approach enabled isolation of the mixed-linker sql coordination network [Co(bipy)(bptz)(NCS)2] n (sql-1,3-Co-NCS, bipy = 4,4'-bipyridine, bptz = 4,4'-bis(4-pyridyl)tetrazine) and study of its C8 vapour and liquid sorption properties. sql-1,3-Co-NCS was found to exhibit high adsorption capacity from liquid xylenes (∼37 wt%) and is to our knowledge the first sorbent to exhibit high selectivity for each of xylene isomer over EB (S OX/EB, S MX/EB, S PX/EB > 5). Insights into the performance of sql-1,3-Co-NCS are gained from structural studies which reveal stacking interactions between electron-deficient bptz linkers and the respective xylenes. sql-1,3-Co-NCS is the first N-donor mixed-linker sql coordination network studied for its gas/vapour sorption properties and represents a large and diverse class of understudied coordination networks.

Synthesis, Structures, Electrochemistry, and Catalytic Activity towards Cyclohexanol Oxidation of Mono-, Di-, and Polynuclear Iron(III) Complexes with 3-Amino-2-Pyrazinecarboxylate

The synthesis and characterization of a set of iron(III) complexes, viz. the mononuclear [Fe(L)3] (1) and [NHEt3][Fe(L)2(Cl)2] (2), the dinuclear methoxido-bridged [Fe(L)2(μ-OMe)]2.DMF.1.5MeOH (3), and the heteronuclear Fe(III)/Na(I) two-dimensional coordination polymer [Fe(N3)(μ-L)2(μ-O)1/2(Na)(μ-H2O)1/2]n (4), are reported. Reactions of 3-amino-2-pyrazinecarboxylic acid (HL) with iron(III) chloride under different reaction conditions were studied, and the obtained compounds were characterized by elemental analysis, Fourier Transform Infrared (FT-IR) spectroscopy, and X-ray single-crystal diffraction. Compound 1 is a neutral mononuclear complex, whereas 2 is mono-anionic with its charge being neutralized by triethylammonium cation. Compounds 3 and 4 display a di-methoxido-bridged dinuclear complex and a two-dimensional heterometallic Fe(III)/Na(I) polynuclear coordination polymer, respectively. Compounds 3 and 4 are the first examples of methoxido- and oxido-bridged iron(III) complexes, respectively, with 3-amino-2-pyrazinecarboxylate ligands. The electrochemical study of these compounds reveals a facile single-electron reversible Fe(III)-to-Fe(II) reduction at a positive potential of 0.08V vs. saturated calomel electrode (SCE), which is in line with their ability to act as efficient oxidants and heterogeneous catalysts for the solvent-free microwave-assisted peroxidative oxidation (with tert-butyl hydroperoxide) of cyclohexanol to cyclohexanone (almost quantitative yields after 1 h). Moreover, the catalysts are easily recovered and reused for five consecutive cycles, maintaining a high activity and selectivity.

Magnetic and luminescent coordination networks based on imidazolium salts and lanthanides for sensitive ratiometric thermometry

The synthesis and characterization of six new lanthanide networks [Ln(L)(ox)(H₂O)] with Ln = Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Yb³⁺ is reported. They were synthesized by solvo-ionothermal reaction of lanthanide nitrate Ln(NO₃)₃·xH₂O with the 1,3-bis(carboxymethyl)imidazolium [HL] ligand and oxalic acid (H₂ox) in a water/ethanol solution. The crystal structure of these compounds has been solved on single crystals and the magnetic and luminescent properties have been investigated relying on intrinsic properties of the lanthanide ions. The synthetic strategy has been extended to mixed lanthanide networks leading to four isostructural networks of formula [Tb_{1- x} Eu _x (L)(ox)(H₂O)] with x = 0.01, 0.03, 0.05 and 0.10. These materials were assessed as luminescent ratiometric thermometers based on the emission intensities of ligand, Tb³⁺ and Eu³⁺. The best sensitivities were obtained using the ratio between the emission intensities of Eu³⁺ (⁵D₀→⁷F₂ transition) and of the ligand as the thermometric parameter. [Tb_0.97Eu_0.03(L)(ox)(H₂O)] was found to be one of the best thermometers among lanthanide-bearing coordination polymers and metal-organic frameworks, operative in the physiological range with a maximum sensitivity of 1.38%·K^-1 at 340 K.

Effect of the Composition of Lanthanide Complexes on Their Luminescence Enhancement by Ag@SiO₂ Core-Shell Nanoparticles

Metal-enhanced luminescence of lanthanide complexes by noble metal nanoparticles has attracted much attention because of its high efficiency in improving the luminescent properties of lanthanide ions. Herein, nine kinds of europium and terbium complexes—RE(TPTZ)(ampca)₃·3H₂O, RE(TPTZ)(BA)₃·3H₂O, RE(phen)(ampca)₃·3H₂O, RE(phen)(PTA)_1.5·3H₂O (RE = Eu, Tb) and Eu(phen)(BA)₃·3H₂O (TPTZ = 2,4,6-tri(2-pyridyl)-s-triazine, ampca = 3-aminopyrazine-2-carboxylic acid, BA = benzoic acid, phen = 1,10-phenanthroline, PTA = phthalic acid)—have been synthesized. Meanwhile, seven kinds of core-shell Ag@SiO₂ nanoparticles of two different core sizes (80–100 nm and 40–60 nm) and varied shell thicknesses (5, 12, 20, 30 and 40 nm) have been prepared. The combination of these nine types of lanthanide complexes and seven kinds of Ag@SiO₂ nanoparticles provides an opportunity for a thorough investigation of the metal-enhanced luminescence effect. Luminescence spectra analysis showed that the luminescence enhancement factor not only depends on the size of the Ag@SiO₂ nanoparticles, but also strongly relates to the composition of the lanthanide complexes. Terbium complexes typically possess higher enhancement factors than their corresponding europium complexes with the same ligands, which may result from better spectral overlap between the emission bands of Tb complexes and surface plasmon resonance (SPR) absorption bands of Ag@SiO₂. For the complexes with the same lanthanide ion but varied ligands, the complexes with high enhancement factors are typically those with excitation wavelengths located nearby the SPR absorption bands of Ag@SiO₂ nanoparticles. These findings suggest a combinatorial chemistry strategy is necessary to obtain an optimal metal-enhanced luminescence effect for lanthanide complexes.

Packing polymorphism in 3-amino-2-pyrazinecarboxylate based tin(ii) complexes and their catalytic activity towards cyanosilylation of aldehydes

3-Amino-2-pyrazinecarboxylic acid is used to synthesize two new mononuclear interconvertible packing polymorphs of tin(ii) which act as heterogeneous catalysts for the cyanosilylation of aldehydes with trimethylsilyl cyanide.

Variable coordination of carbazolyl-bis(tetrazole) ligands in lanthanide chemistry

Lanthanide complexes of 1,8-bis(2′-isopropyltetrazo-5′-yl)-3,6-di-tert-butylcarbazolide adopt geometries that include planar, buckled and tetrazolyl-bridged tridentate, as well as bidentate, bonding modes.

The influence of an external magnetic field and magnetic-site dilution on the magnetization dynamics of a coordination network based on ferromagnetic coupled dinuclear dysprosium(iii) units

A 2D coordination network was synthesized and structurally characterized. Ferromagnetic interactions, multi-process slow relaxation of magnetization and hysteresis loops were also studied.

Multi-shelled ceria hollow spheres with a tunable shell number and thickness and their superior catalytic activity

Multi-shelled ceria hollow spheres with tunable shell number and thickness have been prepared via a coordination polymer precursor method. Besides, this method was extended to the preparation of other rare earth oxides. Au and AuPd loaded ceria hollow spheres composites display superior catalytic activity.

Field-Induced Relaxation of Magnetization in a Three-Dimensional LnMOF with the Second Bridging Ligand Squarate

A three-dimensional (3D) dysprosium(III) metal-organic framework with nicotinate N-oxide (NNO^-) and squarate (C₄O₄ ^2-) mixed bridging ligands, [Dy(NNO)(C₄O₄)(H₂O)] _n (1), has been hydrothermally synthesized. The dysprosium(III) ions are linked to each other by the squarate anions to form a unique dysprosium(III) squarate double-layered network; the NNO^- anions then bridge such layers to complete the 3D framework. Complex 1 exhibits a two-step relaxation of magnetization under a dc field of 1000 Oe, with effective energy barrier values of 8.5 and 14.3 K, respectively.

Lanthanide contraction for helicity fine-tuning and helix-winding control of single-helical metal complexes

Lanthanide contraction was used for helicity fine-tuning and helix winding control of single-helical tetranuclear complexes LZn₃Ln (Ln = La–Lu).

A 3D MOF constructed from dysprosium(iii) oxalate and capping ligands: ferromagnetic coupling and field-induced two-step magnetic relaxation

Field-induced two-step magnetic relaxation was observed in the first ferromagnetic 3D lanthanide(iii) oxalate framework with a capping ligand.

Synthesis, molecular and supramolecular structure of a new dinuclear aluminium(III) complex derived from 3-aminopyrazine- 2-carboxylic acid

Solvothermal reaction of aluminum(III) nitrate nonahydrate with 3-aminopyrazine-2-carboxylic acid (HL) gave rise to a bis(μ2-hydroxo)-tetrakis(3-aminopyrazine-2-carboxylato)-dialuminium(III) complex [Al(L)2(OH)]2 (1) (L = 3-aminopyrazine-2-carboxylate). Crystal structure analysis reveals that each hexacoordinated Al(III) centre adopts a distorted octahedral geometry occupied by two Ocarboxylate, two Npyrazine and two Ohydroxo atoms. The L– ligand binds the metal cation by means of one pyrazine N-atom and carboxylate O-atoms via chelating fashion. Two hydroxo groups bridge between two aluminum(III) centres, assisting to construct a dinuclear structure. In the crystal lattice of 1 two symmetry non-equivalent dinuclear complexes are present. A 3D supramolecular associate in the crystal lattice of 1 has been stabilized by a number of non-covalent H-bonding interactions. Topology of the hydrogen bonded network has been also analyzed.

Synthesis, structure and catalytic application of lead(II) complexes in cyanosilylation reactions

Hydrothermal reactions of a lead(II) salt with 3-aminopyrazine-2-carboxylic acid (HL1) gave rise to a series of lead(II) coordination compounds (1–6) having zero, one, two and three dimensional structures. X-ray diffraction structural analyses reveal that complexes [Pb(L1)2]2 (1) and [Pb(L1)(OCHNH2)(η-OCHO)]2 (2) possess dinuclear structures, containing a centre of symmetry. Complexes [Pb2(L1)4(NHMeCHO)2]n (3) and [Pb2(L1)4]n·(H2O)n·(2.5DMF)n (4) have 1D chain like structures, and [Pb5(L1)7(η-NO3)(μ-HCOO)(η-HCOO)]n·(DMF)n·(MeOH)n (5) shows a 2D sheet like structure constructed by the [Pb5O5(HCOO)] cluster and 3-aminopyrazine-2-carboxylate anions. The hydrothermal reaction of lead(II) nitrate with HL1 in DMF led to in situ formation of 3,3′-(methylenebis(azanediyl))bis(pyrazine-2-carboxylic acid) [H2L2] which produces the 3D framework [Pb2(L2)2]n·(2DMF)n·(H2O)n (6). The L1(−) and L2(2−) ligands bind the metal cations by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. The carboxylate group of L1(−) presents a diversity of coordination modes, viz., monodentate (1 and 3), bridging μ2 (3) and bridging μ3 (4), monodentate bridging μ2 (1, 2, 4, 5 and 6) and bridging chelate μ2 (5). The carboxylate moiety of L2(2−) in 6 binds the metal in a bridging μ2 fashion. The Pb(II) ions display coordination numbers from 5 to 8 with hemi- or holodirected coordination environments. The Pb(II) complexes act as heterogeneous catalysts for the cyanosilylation reaction, at 15 °C, of different aldehydes with trimethylsilyl cyanide (TMSCN) and can be recycled at least three times without losing activity.

Synthesis, structure and thermal study of a new 3-aminopyrazine-2-carboxylate based zinc(II) coordination polymer

A two-dimensional coordination polymer [Zn(L)2]n (1) (L = 3-aminopyrazine-2-carboxylate) synthesized from the solvothermal reaction of zinc(II) salt with 3-aminopyrazine-2-carboxylic acid (HL) is described. Compound 1 has been characterized by single X-ray diffraction, IR spectra and thermogravimetric analyses. Crystal structure analysis reveals that each hexacoordinated zinc(II) center adopts a distorted octahedral geometry occupied by three Ocarboxylate and three Npyrazine atoms. The L– ligand binds the metal cation by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. A three-dimensional supramolecular associate in the crystal lattice of 1 has been stabilized by a number of non-covalent interactions. The IR spectroscopic and TGA properties are investigated in this work. Topological analysis of the two-dimensional network has been also discussed.

Homochiral luminescent lanthanide dinuclear complexes derived from a chiral carboxylate

A family of chiral Ln₂ clusters based on a chiral monocarboxylate ligand has been successfully constructed via a diffusion method, and Eu, Tb and Dy analogues display good luminescent properties.

Luminescence and slow magnetic relaxation of isostructural 2D lanthanide metal–organic frameworks derived from both nicotinate N-oxide and glutarate

The isostructural 2D lanthanide coordination polymers may exhibit either luminescence or slow magnetic relaxation, depending on the lanthanide ions.

Slow magnetic relaxation of a three-dimensional metal–organic framework featuring a unique dysprosium(iii) oxalate layer

Two-step thermal magnetic relaxation was observed in a novel three-dimensional dysprosium(iii) metal–organic framework.

Three sra topological lanthanide–organic frameworks built from 2,2′-dimethoxy-4,4′-biphenyldicarboxylic acid

The first neutral 3D sra-LOF built from a 1D inorganic rod-shaped chain [Ln(CO₂)₂(HCO₂)]_n (Ln = Eu, Gd, Dy) was synthesized.