A family of Cd(II) coordination polymers constructed from 6-aminopicolinate and bipyridyl co-linkers: study of their growth in paper and photoluminescence sensing of Fe3+ and Zn2+ ions.

In this work, we report on five novel coordination polymers (CPs) based on the linkage of the [Cd(6apic)2] building block [where 6apic = 6-aminopicolinate] by different bipyridine-type organic spacers, forming different coordination compounds with the following formulae: [Cd(µ-6apic)2]n (1), {[Cd(6apic)2(µ-bipy)]·H2O}n (2), {[Cd(6apic)2(µ-bpe)]·2H2O}n (3), [Cd(6apic)(µ-6apic)(µ-bpa)0.5]n (4) and {[Cd2(6apic)4(µ-tmbp)]·7H2O}n (5) [where bipy = 4,4'-bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, bpa = 1,2-di(4-pyridyl)ethane (bpa) and tmbp = 1,3-di(4-pyridyl)propane]. Most of the synthesized compounds form infinite metal-organic rods through the linkage of the building block by the bipyridine-type linker, except in the case of compound 4 whose assembly forms a densely packed 3D architecture. All compounds were fully characterized and their photoluminescence properties were studied experimentally and computationally through density functional theory (DFT) calculations. All compounds display, upon UV excitation, a similar blue emission of variable intensity depending on the linker employed for the connection of the building units, among which compound 2 deserves to be highlighted for its room temperature phosphorescence (RTP) with an emission lifetime of 32 ms that extends to 79 ms at low temperature. These good photoluminescence properties, in addition to its stability in water over a wide pH range (between 2 and 10), motivated us to study compound 2 as a sensor for the detection of metal ions in water, and it showed high sensitivity to Fe3+ through a fluorescence turn-off mechanism and an unspecific turn-on response to Zn2+. Furthermore, the compound is processed as a paper-based analytical device (PAD) in which the phosphorescence emission is preserved, improving the sensing capacity toward Fe3+ ions.

Cerium(III) and 5-methylisophthalate-based MOFs with slow relaxation of magnetization and photoluminescence emission.

Two novel Ce(III) metal organic frameworks (MOFs) with formulas [Ce(5Meip)(H-5Meip)]nGR-MOF-17 and [CeCl(5Meip)(DMF)]nGR-MOF-18 (5Meip = 5-methylisophthalate, DMF = N,N-dimethylformamide) have been synthesized, forming 3-dimensional frameworks. Magnetic measurements show that both compounds present field-induced slow magnetic relaxation under a small applied dc field. For GR-MOF-17, the temperature dependence of relaxation times is best described by a Raman mechanism, whereas for GR-MOF-18, relaxation occurs through a combination of Raman and local-mode pathways. Moreover, when avoiding short Ce⋯Ce interactions by magnetic dilution in GR-MOF-17@La and GR-MOF-18@La, only the local-mode mechanism is responsible for magnetic relaxation. Photophysical studies show the occurrence of ligand-centred luminescence in both compounds and phosphorescence emission at low temperature for GR-MOF-17.

A Lamellar Zn-Based Coordination Polymer Showing Increasing Photoluminescence upon Dehydration.

The present study reports on a 2D lamellar coordination polymer (CP) of {[Zn(µ3-pmdc)(H2O)]·H2O}n formula (pmdc = pyrimidine-4,6-dicarboxylate). This CP is synthesized under an appropriate acid-base reaction between the gently mortared reagents in the solid state through a solvent-free procedure that avoids the presence of concomitant byproducts. The X-ray crystal structure reveals the occurrence of Zn2 entities connected through carboxylate groups of pmdc, which behave as triconnected nodes, giving rise to six-membered ring-based layers that are piled up through hydrogen bonding interactions. In addition to a routine physico-chemical characterization, the thermal evolution of the compound has been studied by combining thermogravimetric and thermodiffractometric data. The photoluminescence properties are characterized in the solid state and the processes governing the spectra are described using time-dependent density-functional theory (TD-DFT) with two different approaches employing different program packages. The emissive capacity of the material is further analyzed according to the dehydration and decreasing temperature of the polycrystalline sample.

Lanthanide(III) Ions and 5-Methylisophthalate Ligand Based Coordination Polymers: An Insight into Their Photoluminescence Emission and Chemosensing for Nitroaromatic Molecules.

The work presented herein reports on the synthesis, structural and physico-chemical characterization, luminescence properties and luminescent sensing activity of a family of isostructural coordination polymers (CPs) with the general formula [Ln2(µ4-5Meip)3(DMF)]n (where Ln(III) = Sm (1Sm), Eu (2Eu), Gd (3Gd), Tb (4Tb) and Yb (5Yb) and 5Meip = 5-methylisophthalate, DMF = N,N-dimethylmethanamide). Crystal structures consist of 3D frameworks tailored by the linkage between infinite lanthanide(III)-carboxylate rods by means of the tetradentate 5Meip ligands. Photoluminescence measurements in solid state at variable temperatures reveal the best-in-class properties based on the capacity of the 5Meip ligand to provide efficient energy transfers to the lanthanide(III) ions, which brings intense emissions in both the visible and near-infrared (NIR) regions. On the one hand, compound 5Yb displays characteristic lanthanide-centered bands in the NIR with sizeable intensity even at room temperature. Among the compounds emitting in the visible region, 4Tb presents a high QY of 63%, which may be explained according to computational calculations. At last, taking advantage of the good performance as well as high chemical and optical stability of 4Tb in water and methanol, its sensing capacity to detect 2,4,6-trinitrophenol (TNP) among other nitroaromatic-like explosives has been explored, obtaining high detection capacity (with Ksv around 105 M-1), low limit of detection (in the 10-6-10-7 M) and selectivity among other molecules (especially in methanol).

An experimental and theoretical study of the magnetic relaxation in heterometallic coordination polymers based on 6-methyl-2-oxonicotinate and lanthanide(III) ions with square antriprismatic environment.

Two new isostructural compounds based on 6-methyl-2-oxonicotinate (6m2onic) ligand and sodium and lanthanide(III) ions are reported. The structural and chemical characterization reveals the following chemical formula: {[Ln(6m2onic)2(µ-6m2onic)2Na(H2O)3]·8H2O}n [where Ln(III) = Dy (1Dy) and Er (2Er)]. These compounds crystallize in the form of one-dimensional arrays held together into a hydrogen-bonded structure, in which 6m2onic ligands establish four O,O' chelating rings with the lanthanide to render a distorted square antiprism (SAPR) geometry. Magnetic dc and ac susceptibility measurements confirm that 1Dy and 2Er behave as SIMs. Magnetic dilutions using Y(III) matrices have been made to achieve a Dy(III) counterpart (1Y/Dy) that presents slow magnetic relaxation under zero dc field. Under an optimized Hdc field (of 1000 Oe and 1500 Oe for 1Y/Dy and 2Y/Er, respectively), 1Y/Dy reveals the occurrence of two well-separated maxima, attributed to SR (Ueff = 65.2 K (45.3 cm-1) and τ0 = 2.76 × 10-9 s) and FR processes (Ueff = 23.2 K (16.1 cm-1) and τ0 = 1.40 × 10-8 s), whereas 2Y/Er shows a multiple relaxation pathway that considers quantum tunnelling of the magnetization (QTM), Orbach, Raman and direct mechanisms. Ab initio calculations have been carried out to support the experimental evidence and to explain the lanthanide ion-dependent behaviour deepen the understanding of the magneto-structural relationship of the SAPR environment.

Combined experimental and theoretical investigation on the magnetic properties derived from the coordination of 6-methyl-2-oxonicotinate to 3d-metal ions.

Five new compounds are reported herein starting from 2-hydroxy-6-methylnicotinic acid (H2h6mnic) and first-row transition metal ions, although H2h6mnic shows a prototropy in solution to lead to the 6-methyl-2-oxonicotinate (6m2onic) ligand that is the molecule eventually present in the compounds. The structural and chemical characterization reveals the following chemical formulae: {[MnNa(µ3-6m2onic)2(µ-6m2onic)(MeOH)]·H2O·MeOH}n (1Mn), {[M2Na2(µ3-6m2onic)2(µ-6m2onic)2(µ-H2O)(H2O)6](NO3)2}n [MII = Co (2Co) and Ni (3Ni)], 2[Cu2(6m2onic)3(µ-6m2onic)(MeOH)]·[Cu2(6m2onic)2(µ-6m2onic)2]·2[Cu(6m2onic)2(MeOH)]·32H2O (4Cu) and {[Cu(µ-6m2onic)2]·6H2O}n (5Cu) (where 6m2onic = 6-methyl-2-oxonicotinate). An unusual structural diversity is observed for the compounds, ranging from isolated complexes (in 4Cu), 1D arrays (in 1Mn and 5Cu) and 3D frameworks (in 2Co and 3Ni). Magnetic properties have been studied for all compounds. Analysis of the magnetic dc susceptibility and magnetization data for 4Cu and 5Cu suggests the occurrence of ferromagnetic exchange, which is well explained by broken-symmetry and CASSCF calculations. The sizeable easy-plane magnetic anisotropy present in compound 2Co allows for a field-induced magnet behaviour with an experimental effective energy barrier of 16.2 cm-1, although the slow relaxation seems to be best described through Raman and direct processes in agreement with the results of ab initio calculations.

Modulating Magnetic and Photoluminescence Properties in 2-Aminonicotinate-Based Bifunctional Coordination Polymers by Merging 3d Metal Ions.

Herein, the synthesis and study of bifunctional coordination polymers (CPs) with both magnetic and photoluminescence properties, derived from a heterometallic environment, are reported. As a starting point, three isostructural monometallic CPs with the formula [M(µ-2ani)2 ]n (MII =Mn (1Mn ), Co (3Co ) and Ni (4Ni ); 2ani=2-aminonicotinate), crystallise as chiral 2D-layered structures stacked by means of supramolecular interactions. These compounds show high thermal stability in the solid state (above 350 °C), despite which, in aqueous solution, compound 1Mn is shown to partially transform into a novel 1D chain CP with the formula [Mn(2ani)2 (µ-H2 O)2 ]n (2Mn ). A study of the direct current (dc) magnetic properties of 1Mn , 3Co and 4Ni reveals a spin-canted structure derived from antisymmetric antiferromagnetic weak exchanges along the chiral network (as confirmed by DFT calculations) and magnetic anisotropy of the ions, in such a way that long-range ordering is observed with variable magnitude for the spin carriers. Moreover, compounds 3Co and 4Ni show no frequency-dependent alternating current (ac) susceptibility curves under zero dc field; this is characteristic behaviour of a glassy state that may be partially supressed for 3Co by applying an external dc field. To overcome long-range magnetic ordering, CoII ions are diluted in a diamagnetic ZnII -based matrix, which enables single-molecule magnet behaviour. Interestingly, this strategy allows a bifunctional Cox Zn1-x 2ani material, which is imbued with a strong photoluminescent emitting capacity, as characterised by an intense blue light followed by a green afterglow, to be obtained.

Chiral coordination polymers based on d10 metals and 2-aminonicotinate with blue fluorescent/green phosphorescent anisotropic emissions.

Herein, we report the detailed structural characterization and photoluminescence (PL) properties of two new isostructural coordination polymers (CPs) based on 2-aminonicotinate (2ani) ligand and zinc(ii) or cadmium(ii) metal ions, namely, [M(µ-2ani)2]n [MII = Zn(1) and Cd(2)]. These compounds are characterized as chiral 2D-layered structures, resulting from the bridging of the 2ani ligands in the MN2O4 coordination shell. Also, strong supramolecular interactions are found to sustain the overall crystal structure, which endows these materials with high stability. The single crystals of 1 exhibit waveguiding behaviour along some preferred orientations. The examination of the polycrystalline samples of both compounds under UV light also shows that the samples exhibit remarkably strong PL emissions, which consist of blue fluorescence and green phosphorescence involving long-lasting phosphorescence (LLP) at low temperature. Fitting of the decay curves confirms the occurrence of short (in the order of nanoseconds) and very large lifetimes (τ ≈ 320 and 560 ms for 1 and 2, respectively) of the wavelengths responsible for their blue and green emissions, respectively, which have been studied using DFT calculations.