Counteranion Modulated Crystal Growth and Function of One-Dimensional Homochiral Coordination Polymers: Morphology, Structures, and Magnetic Properties

Solvent modulation of the morphology of homochiral gadolinium coordination polymers and its impact on circularly polarized luminescence

Studying the effect of morphology on the circularly polarized luminescence (CPL) of chiral molecular materials is important for the development of CPL-active materials for applications. Herein, we report that the morphology of Gd(NO3)3/R-,S-AnempH2 [AnempH2 = (1-anthrylethylamino)methylphosphonic acid] assemblies can be controlled by solvent modulation to form spiral bundles Gd(R-,S-AnempH)3·2H2O (R-,S-1), crystals Gd(R-,S-AnempH)3·2H2O (R-,S-2) and spindle-shaped particles Gd(R-,S-AnempH)3·3H2O·0.5DMF (R-,S-3) with similar chain structures. Interestingly, R-,S-1 are CPL active and show the highest value of dissymmetric factor among the three pairs of enantiomers (|glum| = 2.1 × 10-3), which is 2.8 times larger than that of R-,S-2, while R-,S-3 are CPL inactive with |glum| ≈ 0. This work provides a new route to control the morphology of chiral coordination polymers and improve their CPL performance.

Macroscopic Helical Assembly of One-Dimensional Coordination Polymers: Helicity Inversion Triggered by Solvent Isomerism

Assembling macroscopic helices with controllable chirality and understanding their formation mechanism are highly desirable but challenging tasks for artificial systems, especially coordination polymers. Here, we utilize solvents as an effective tool to induce the formation of macroscopic helices of chiral coordination polymers (CPs) and manipulate their helical sense. We chose the Ni/R-,S-BrpempH2 system with a one-dimensional tubular structure, where R-,S-BrpempH2 stands for R-,S-(1-(4-bromophenyl)ethylaminomethylphosphonic acid). The morphology of the self-assemblies can be controlled by varying the cosolvent in water, resulting in the formation of twisted ribbons of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-2T) in pure H2O; needle-like crystals of R-,S-Ni(Brpemp)(H2O)2·1/3CH3CN (R-,S-1C) in 20 vol % CH3CN/H2O; nanofibers of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-3F) in 20-40 vol % methanol/H2O or ethanol/H2O; and superhelices of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-4H or 5H) in 40 vol % propanol/H2O. Interestingly, the helicity of the superhelix can be controlled by using a propanol isomer in water. For the Ni/R-BrpempH2 system, a left-handed superhelix of R-4H(M) was obtained in 40 vol % NPA/H2O, while a right-handed superhelix of R-5H(P) was isolated in 40 vol % IPA/H2O. These results were rationalized by theoretical calculations. Adsorption studies revealed the chiral recognition behavior of these compounds. This work may contribute to the development of chiral CPs with a macroscopic helical morphology and interesting functionalities.

Macroscopic handedness inversion of terbium coordination polymers achieved by doping homochiral ligand analogues

Inspired by natural biological systems, chiral or handedness inversion by altering external and internal conditions to influence intermolecular interactions is an attractive topic for regulating chiral self-assembled materials. For coordination polymers, the regulation of their helical handedness remains little reported compared to polymers and supramolecules. In this work, we choose the chiral ligands R-pempH2 (pempH2 = (1-phenylethylamino)methylphosphonic acid) and R-XpempH2 (X = F, Cl, Br) as the second ligand, which can introduce C-H⋯π and C-H⋯X interactions, doped into the reaction system of the Tb(R-cyampH)3·3H2O (cyampH2 = (1-cyclohexylethylamino)methylphosphonic acid) coordination polymer, which itself can form a right-handed superhelix by van der Waals forces, and a series of superhelices R-1H-x, R-2F-x, R-3Cl-x, and R-4Br-x with different doping ratios x were obtained, whose handedness is related to the second ligand and its doping ratio, indicating the decisive role of interchain interactions of different strengths in the helical handedness. This study could provide a new pathway for the design and self-assembly of chiral materials with controllable handedness and help the further understanding of the mechanism of self-assembly of coordination polymers forming macroscopic helical systems.

Stable Lanthanide Metal-Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

Four new isostructural lanthanide metal-organic frameworks (MOFs), namely {[Ln(DMTP-DC)_1.5(H₂O)₃]·DMF}_n [H₂DMTP-DC = 2',5'-dimethoxytriphenyl-4,4″-dicarboxylic acid; Ln^III = Eu^III (1), Gd^III (2), Tb^III (3), and Dy^III (4)], have been synthesized and characterized. Single-crystal structure analysis reveals that 1-4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H₂O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of Ln^III ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 μM for arginine and 9.31 μM for lysine. All 1-4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1-4 at 55 °C and 100% RH are 9.94 × 10^-5, 1.62 × 10^-4, 1.71 × 10^-4, and 2.67 × 10^-4 S·cm^-1, respectively. The value of σ increases with the decrease in ionic radius, indicating that the radius of the Ln^III ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (-ΔS_m) of 18.86 J kg^-1 K^-1 for ΔH = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1-4 are potentially multifunctional MOF materials.

Controllable Macroscopic Chirality of Coordination Polymers through pH and Anion-Mediated Weak Interactions

Helical architectures with controllable helical sense bias have recently attracted considerable interest for mimicking biological helices and developing novel chiral materials. Coordination polymers (CPs), composed of metal ion nodes and organic linkers, are intriguing systems showing tunable structures and functions. However, CPs with helical morphologies have rarely been explored so far. Particularly, chirality inversion through external stimulus has not been achieved in helical CPs. In this work, we carried out an in-depth investigation on the self-assembly of 1D gadolinium(III) phosphonate CPs using GdX₃ (X=Cl, Br, I) and Gd(RSO₃ ) (R=CH₃ , C₆ H₅ , CF₃ ) as metal sources and R-(1-phenylethylamino)methyl phosphonic acid (R-pempH₂ ) as ligand. Superhelices were formed by precise control of the interchain interactions through different intercalated anions. Furthermore, the twisting direction of superhelices could be controlled by synergistic effect of anions and pH. This study may provide a new route to fabricate helical nanostructures of CPs with a desirable chiral sense and help understand the inner mechanism of the self-assembly process of macroscopic helical structures of molecular systems.

From helices to superhelices: hierarchical assembly of homochiral van der Waals 1D coordination polymers

Chiral transcription from the molecular level to the macroscopic level by self-organization has been a topic of considerable interest for mimicking biological systems. Homochiral coordination polymers (CPs) are intriguing systems that can be applied in the construction of artificial helical architectures, but they have scarcely been explored to date. Herein, we propose a new strategy for the generation of superhelices of 1D CPs by introducing flexible cyclohexyl groups on the side chains to simultaneously induce interchain van der Waals interactions and chain misalignment due to conformer interconversion. Superhelices of S- or R-Tb(cyampH)3·3H2O (S-1H, R-1H) [cyampH2 = S- or R-(1-cyclohexylethyl)aminomethylphosphonic acid] were obtained successfully, the formation of which was found to follow a new type of "chain-twist-growth" mechanism that had not been described previously. The design strategy used in this work may open a new and general route to the hierarchical assembly and synthesis of helical CP materials.

Homochiral Dysprosium Phosphonate Nanowires: Morphology Control and Magnetic Dynamics

Controllable synthesis of uniformly distributed nanowires of coordination polymers with inherent physical functions is highly desirable but challenging. In particular, the combination of chirality and magnetism into nanowires has potential applications in multifunctional materials and spintronic devices. Herein, we report four pairs of enantiopure coordination polymers with formulae S-, R-Dy(cyampH)₃  ⋅ CH₃ COOH ⋅ 2H₂ O (S-1, R-1), S-, R-Dy(cyampH)₃  ⋅ 3H₂ O (S-2, R-2), S-, R-Dy(cyampH)₂ (C₂ H₅ COO) ⋅ 3H₂ O (S-3, R-3) and S-, R-Dy(cyampH)₃  ⋅ 0.5C₂ H₅ COOH ⋅ 2H₂ O (S-4, R-4) [cyampH₂ =S-, R-(1-cyclohexylethyl)aminomethylphosphonic acids], which were obtained depending on the pH of the reaction mixtures and the specific carboxylic acid used as pH regulator. Interestingly, compounds 3 were obtained as superlong nanowires, showing 1D neutral chain structure which contains both phosphonate and propionate anion ligands. While compounds 1, 2 and 4 appeared as block-like crystals, superhelices and nanorods, respectively, and exhibited similar neutral chain structures containing only phosphonate ligand. Slow magnetization relaxation characteristic of single-molecule magnet (SMM) behavior was observed for compounds S-1 and S-3. Theoretical calculations were performed to rationalize the magneto-structural relationships.

Regulating the solution structural integrity and slow magnetic relaxation behavior of two Dy6 clusters with a pyridine–triazole ligand

Two {Dy₆} skeletons consolidated by nitrate (1) and hydroxide (2) have been isolated. HRESI-MS reveals that 1 retains a higher structural integrity and fragment stability. While 2 has a typical slow magnetic relaxation of SMM behavior.

DyIII single-molecule magnets from ligands incorporating both amine and acylhydrazine Schiff base groups: the centrosymmetric {Dy2} displaying dual magnetic relaxation behaviors

The novel multidentate chelating ligands N'-(2-pyridylmethylidene)-2-(2-pyridylmethylideneamino)benzohydrazide (Hpphz) and N'-(2-salicylmethylidene)-2-(2-salicylmethylideneamino)benzohydrazide (H₃sshz), which incorporate both amine and acylhydrazine Schiff base groups, were synthesized and investigated in Dy^III coordination chemistry. The reactions of Hpphz and Dy(OAc)₃·4H₂O have yielded two {Dy₂} featuring double OAc^- bridges: [Dy₂(H₂aphz)₂(OAc)₄(ROH)₂] [R = Me (1) and Et (2)], where the Hpphz ligands were in situ hydrolyzed into 2-amino-(2-pyridylmethylideneamino)benzohydrazide ions (H₂aphz^-). Besides, the reaction between H₃sshz and Dy(NO)₃·6H₂O afforded a [Dy₆(sshz)₄(μ₃-OH)₄(μ₄-O)(MeOH)₄]₂·17.5MeOH·2H₂O cluster (3). This cluster contained two discrete {Dy₆} cores, each of which consisted of a pair of {Dy₃} triangular units. All the complexes displayed a single relaxation process of single-molecule magnet (SMM) behaviors under a zero dc field. Both 1 and 2 showed field-induced dual magnetic-relaxation behaviors. However, their diluted samples (1@Y and 2@Y) only showed one-step relaxation behaviors whether under a zero or applied dc field, indicating that the dual magnetic-relaxation behaviors of 1 and 2 were absent after the dilution. Combined with ab initio calculations, it could be infered that the dual magnetic-relaxation behaviors of 1 and 2 might be ascribled to the joint contributions of the single ion anisotropy and magnetic interactions. Examples of this type are rather rare in previous studies. Ab initio calculations also suggested that the discrepancy between the relaxation processes of 1 and 2 may be caused by the small difference between their magnetic interactions.

Reticular Chemistry of Uranyl Phosphonates: Sterically Hindered Phosphonate Ligand Method is Significant for Constructing Zero-Dimensional Secondary Building Units

Designability is an attractive feature for metal-organic frameworks (MOFs) and essential for reticular chemistry, and many ideas are significantly useful in the carboxylate system. Bi-, tri-, and tetra-topic phosphonate ligands are used to achieve framework structures. However, an efficient method for designing phosphonate MOFs is still on the way, especially for uranyl phosphonates, owing to the complicated coordination modes of the phosphonate group. Uranyl phosphonates prefer layer or pillar-layered structures as the topology extension for uranyl units occurs in the plane perpendicular to the linear uranium-oxo bonds and phosphonate ligands favor the formation of compact structures. Therefore, an approach that can construct three-dimensional (3D) uranyl phosphonate MOFs is desired. In this paper, a sterically hindered phosphonate ligand method (SHPL) is described and is successfully used to achieve 3D framework structures of uranyl phosphonates. Four MOF compounds ([AMIM]₂ (UO₂ )(TppmH₄ )⋅H₂ O (UPF-101), [BMMIM]₂ (UO₂ )₃ (TppmH₄ )₂ ⋅H₂ O (UPF-102), [Py14]₂ (UO₂ )₃ (TppmH₄ )₂ ⋅3 H₂ O (UPF-103), and [BMIM](UO₂ )₃ (TppmH₃ )F₂ ⋅2 H₂ O (UPF-104); [AMIM]=1-allyl-3-methylimidazolium, [BMMIM]=1-butyl-2,3-dimethylimidazolium, [Py14]=N-butyl-N-methylpyrrolidinium, and [BMIM]=1-butyl-3-methylimidazolium) are obtained by ionothermal synthesis, with zero-dimensional nodes of uranyl phosphonates linked by steric tetra-topic ligands, namely tetrakis[4-(dihyroxyphosphoryl)phenyl]methane (TppmH₈ ), to give 3D framework structures. Characterization by PXRD, UV/Vis, IR, Raman spectroscopy, and thermogravimetry (TG) were also performed.

Cooperative proton transportation based on the reversible single crystal–single crystal transformation in a highly water-stable Cu-MOF with its facile and scalable preparation

A Cu-MOF with excellent acid–base stability in boiling water was constructed under mild conditions. This MOF was very suitable for scaled-up preparation, and exhibits distinct proton conductivity at temperatures above and below 75 °C.