Five new metal-organic frameworks, [Zn2L2]·2DMF·2MeOH (1), [Zn2L2(py)2] (2), [Cd2L2]·Diox·MeOH·6H2O (3), [Mn2L2]·2DMF·2MeOH (4), and [Co2L2]·2DMF·4H2O (5), were assembled by using a novel 8-hydroxyquinolinate derivative H2L with different metal ions. Complex 1 features a 3D porous network consisting of meso-helical chains ( P + M) built from metal-ligand coordination bonds. The adjacent dinuclear ZnII building blocks in 2 are connected together to generate a 2D grid network. In complex 3, each binuclear motif is bound to four ZnII ions to produce a 2D layer structure that stacks into a 3D porous structure. The framework of complex 4 is isostructural to 5, featuring a 21 helical chain built from [M2L2] units (M = Mn or Co). The adjacent meso-helices associated in parallel are interconnected by the phenolate μ2-O atoms of H2L to give rise to a 2D network. Distinct solid-state luminescence properties of 1-3 were observed, arising from their different metal nodes and frameworks. In particular, complex 1 exhibited excellent stability in both common organic solvents and H2O, thus facilitating its utility as a chemical sensor. Remarkably, luminescent 1 showed highly sensitive detection for nitroaromatic molecules in methanol and Fe3+ ion in H2O even in the presence of other interfering metal cations.

8-Hydroxyquinolinate-Based Metal–Organic Frameworks: Synthesis, Tunable Luminescent Properties, and Highly Sensitive Detection of Small Molecules and Metal Ions

Using two 8-hydroxyquinolinate ligands (L1-MOM and L2-MOM) containing 3-pyridyl or 4-pyridyl groups, five novel coordination polymers, namely, [Zn3(L1)6] (1), [Zn(L1)2]·2MeOH (2), [Zn(L2)2] (3), [Cd(L2)2] (4), and [Cd4(L1)6]·13H2O (5), were synthesized and characterized by a variety of techniques. Single-crystal X-ray structures have revealed that these coordination polymers exhibit a structural diversification due to the different choices of metal salts and the effect of pyridyl nitrogen position. Compounds 1–5 exhibited different fluorescence emissions and lifetimes upon excitation in the solid state. The sensing behavior of these polymers was also investigated upon exposure to vapors of various nitroaromatic molecules (analytes). The results show that all five polymers are capable of sensing these nitroaromatic molecules in the vapor phase through fluorescence quenching. Interestingly, 3 exhibits superior sensitivity to the analytes in comparison with other polymers. 2-Nitrotoluene quenches the emission of 3 by as much as 96%.

Five 8-hydroxyquinolinate-based coordination polymers with tunable structures and photoluminescent properties for sensing nitroaromatics

An 8-hydroxyquinolinate-based metal–organic framework [Zn(L)]·2MeOH·H2O (1) was synthesized for the first time by treating a novel 8-hydroxyquinolinate ligand H2L involving two carboxylate groups with zinc salt. Based on single-crystal X-ray analysis, the Zn-MOF exhibits a porous framework assembled from interlinked nanotubes that are constructed from three 43 helices. Complex 1 possesses excellent stability in both water and common organic solvents, which is conducive to its utility as a chemical sensor. The fluorescent sensing studies indicated that 1 can detect Fe3+ in aqueous solution with high sensitivity and selectivity even in the presence of other interfering metal cations. The recyclability and possible mechanism of 1 in the Fe3+ sensing process have been demonstrated. Additionally, fluorescent 1 is capable of sensing Cr2O72− in aqueous solution and nitroaromatic explosives in DMF. High selectivity and sensitivity were observed by fluorescence quenching effects. The above observations suggested that 1 can be considered as a potential fluorescent sensor for detecting Fe3+, Cr2O72− and nitroaromatic explosives.

A highly stable 8-hydroxyquinolinate-based metal–organic framework as a selective fluorescence sensor for Fe3+, Cr2O72− and nitroaromatic explosives

Treatment of N,N′-bis(2,6-dimethylphenyl)formamidine (L1), N,N′-bis(2,6-diisopropylphenyl)formamidine (L2), and N,N′-dimesitylformamidine (L3) with Zn(OAc)2·2H2O or Cu(OAc)2·H2O produced the corresponding Zn(II) and Cu(II) N,N′-diarylformamidine complexes [Zn3(L1)2(OAc)6] (1), [Zn2(L2)2(OAc)4] (2), [Zn2(L3)2(OAc)4] (3) and [Cu2(L2)2(OAc)4] (4), respectively. While complex 1 is trinuclear, compounds 2–4 are dimeric in the solid state. The X-band EPR spectra of complex 4 in solid and solution states are consistent with perfect axial symmetry and confirm retention of the dinuclear paddle-wheel core in the solution state. Complexes 1–4 formed active catalysts in the ring opening polymerization (ROP) of e-caprolactone (e-CL) and lactides (LA). Complexes 1 and 3 exhibited higher rate constants of 0.1009 h−1 and 0.0963 h−1 compared to the rate constants of 0.0479 h−1 and 0.0477 h−1 observed for 2 and 4, respectively, in the ROP of e-CL at 110 °C. Higher rate constants of 0.5963 h−1 and 1.2962 h−1 were obtained for complexes 1 and 3 in the ROP of LAs compared to those reported in the ROP of e-CL at 110 °C. Activation parameters were determined as ΔH‡ = 25.08 kJ mol−1 and ΔS‡ = −201.7 J K−1 mol−1 for the ROP of e-CL using 3. Investigation of the kinetics of polymerization of e-CL and LAs revealed first order dependence of the polymerization reactions on monomer concentration. Moderate molecular weight polymers of up to 21 286 g mol−1 exhibiting relatively moderate molecular weight distributions and moderately heterotactic PLAs with Pr up to 0.65 were obtained.

Zn(II) and Cu(II) formamidine complexes: structural, kinetics and polymer tacticity studies in the ring-opening polymerization of ε-caprolactone and lactides

Tuning the Fluorescence Emission and HOMO‐LUMO Band Gap in Homoleptic Zinc(II) Complexes with N,O‐Bidentate (Imidazo[1,5‐a]pyrid‐3‐yl)phenols

We reported here the self-assembly of two supramolecular structures based on similar trimeric Zn(II) units that are built from novel 2-substituted 8-hydroxyquinoline ligands and coordination Zn(II) ions. The aggregation behavior of zinc salt and ligand in solution was investigated by a variety of techniques, including 1H NMR, UV-vis and photoluminescence (PL). In the solid state, the supramolecular structures can be controlled by the substituted groups (–NO2 and –F) via intermolecular interaction, such as π⋯π stacking, C–H⋯O, and C–F⋯F–C interactions. As a result, the two trimeric Zn(II) complexes exhibit disparate photophysical properties. The present research holds great promise in the development of novel multinuclear Zn(II) materials, and may contribute to the understanding of structure–property relationships.

Controllable supramolecular structures and luminescent properties of unique trimeric Zn(II) 8-hydroxyquinolinates tuned by functional substituents

Three new Mn(II) coordination polymers, namely [Mn3L6·2H2O] (1), [MnL2] (2), and [MnL2·2H2O] (3), were prepared by solvothermal reactions of Mn(II) salts with a bispyridyl-based quinolinate ligand. All complexes were characterized by elemental analysis, IR spectra, powder and single-crystal X-ray crystallography. Single crystal X-ray studies show that these coordination polymers exhibit a structural diversification due to the different counteranions (OAc−, Cl−, and NO3−). Complex 1 has a 2D supramolecular structure with a cyclic tetramer Mn3L6 secondary building unit. Complex 2 possesses a rhombohedral grid network containing a type of meso-helical chain (P + M) constructed via the metal–ligand coordination interaction. Complex 3 features a 3D non-porous structure based on the arrangement of 2D grids. Magnetic susceptibility measurements indicate that the three Mn(II) polymers 1–3 show disparate magnetic properties due to their different supramolecular structures.

Three Mn(II) coordination polymers with a bispyridyl-based quinolinate ligand: the anion-controlled tunable structural and magnetic properties

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Controllable supramolecular structures and luminescent properties of unique trimeric Zn(II) 8-hydroxyquinolinates tuned by functional substituents

Three Mn(II) coordination polymers with a bispyridyl-based quinolinate ligand: the anion-controlled tunable structural and magnetic properties