Influence of the Pendant Arm, Halide, and Solvent on High-Efficient-Tuning [1 + 1] and [2 + 2] Schiff-Base Macrocyclic Complexes via a Zinc-Ion Template

Synthesis, structural characterization, and theoretical analysis of novel zinc(ii) schiff base complexes with halogen and hydrogen bonding interactions

In this article, we present the synthesis and characterization of three zinc(ii) complexes, [ZnII(HL1)2] (1), [ZnII(HL2)2]·2H2O (2) and [ZnII(HL3)2] (3), with three tridentate Schiff base ligands, H2L1, H2L2, and H2L3. The structures of the complexes were confirmed by single-crystal X-ray diffraction analysis. DFT calculations were performed to gain insights into the self-assembly of the complexes in their solid-state structures. Complex 1 exhibits dual halogen-bonding interactions (Br⋯Br and Br⋯O) in its solid-state structure, which have been thoroughly investigated through molecular electrostatic potential (MEP) surface calculations, alongside QTAIM and NCIPlot analyses. Furthermore, complex 2 features a fascinating hydrogen-bonding network involving lattice water molecules, which serves to link the [ZnII(HL2)2] units into a one-dimensional supramolecular polymer. This network has been meticulously examined using QTAIM and NCIplot analyses, allowing for an estimation of the hydrogen bond strengths. The significance of H-bonds and CH⋯π interactions in complex 3 was investigated, as these interactions are crucial for the formation of infinite 1D chains in the solid state.

Biasing the Formation of Solution-Unstable Intermediates in Coordination Self-Assembly by Mechanochemistry

Due to the reversible nature of coordination bonds and solvation effect, coordination self-assembly pathways are often difficult to elucidate experimentally in solution, as intermediates and products are in constant equilibration. The present study shows that some of these transient and high-energy self-assembly intermediates can be accessed by means of ball-milling approaches. Among them, highly aqueous-unstable Pd3 L11 and Pd6 L14 open-cage intermediates of the framed Fujita Pd6 L14 cage and Pd2 L22 , Pd3 L21 and Pd4 L22 intermediates of Mukherjee Pd6 L24 capsule are successfully trapped in solid-state, where Pd=tmedaPd2+ , L1=2,4,6-tris(4-pyridyl)-1,3,5-triazine and L2=1,3,5-tris(1-imidazolyl)benzene). Their structures are assigned by a combination of solution-based characterization tools such as standard NMR spectroscopy, DOSY NMR, ESI-MS and X-ray diffraction. Collectively, these results highlight the opportunity of using mechanochemistry to access unique chemical space with vastly different reactivity compared to conventional solution-based supramolecular self-assembly reactions.

Reversible Macrocycle-to-Macrocycle Interconversion Driven by Solvent Selection

Macrocycle-to-macrocycle interconversions are of interest because they can allow access to a variety of structures. However, reversible interconversion between different sized macrocycles remains challenging to control. Herein, we report a facile one-pot synthesis of a series of self-assembled macrocycles from readily prepared α,α'-linked oligopyrrolic dialdehydes and various alkyl diamines. The condensation of pyridine-bridged oligopyrrolic dialdehyde 3 and simple alkyl diamines proved independent of solvent, always yielding the [2 + 2] macrocyclic products. However, when 3 was condensed with 2,2'-oxybis(ethylamine) 14, either ([1 + 1] or [2 + 2]) products are obtained depending on the choice of solvent. Reaction of 3 and 14 in methanol, ethanol, or chloroform gave the [1 + 1] macrocycle as the sole product. In contrast, condensation of 3 and 14 in dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), or acetonitrile (MeCN) yielded the [2 + 2] macrocycle as the major product in the form of a precipitate. Reversible interconversion between the [1 + 1] and [2 + 2] macrocycles could be achieved by tuning the solvent, with the ratio driven by thermodynamic and solubility considerations.

Tunable Construction of Sandwich-Type Double-[1 + 1] and Half-Folded [2 + 2] Schiff-Base Complexes Controlled by the Combination of Primary and Secondary Template Effects

The first-row transition-metal ions Mn²⁺-Cu²⁺ could serve as effective templates to construct three types of double-[1 + 1], [2 + 2], and [1 + 1] Schiff-base dinuclear macrocyclic complexes in the presence of dialdehydes with different pendant arms and a common 1,8-diamine. The extremely flexible nature of macrocyclic ligands allows for the multiple template-directed syntheses, but the final products could be finely tuned by the subtle variations of Mn²⁺-Cu²⁺ ions in a 3d-electronic configuration, radius, and coordination number/geometry as well as the auxiliary (pendant-armed and anionic) template effect at the same time. Two borderlines are observed at the Co²⁺ ion for forming double-[1 + 1] and [2 + 2] metallacycles involving the H₂pdd precursor and the [1 + 1] Cu²⁺ complex for double-[1 + 1] and [2 + 2] macrocycles containing the H₂hpdd unit, respectively. The structural diversity is originated from the non-perfect match between [1 + 1]/[2 + 2] Schiff-base macrocycles and dinuclear metal centers; hence, a compromise between the metal coordination modes and alterations of the ligand conformation takes place.

Self-assembly and luminescence of trinuclear lanthanide based supramolecular circular helicates

A series of trinuclear lanthanide circular helicates [Ln₃L₃]Cl₃ have been synthesized via supramolecular subcomponent self-assembly. Photophysical studies demonstrate that the unique helical bis-tetradentate 1,29-dialdehyde ligand L^2- could serve as an effective sensitizer for Sm^III, Eu^III, Nd^III and Yb^III ions exhibiting efficient characteristic fluorescence emission because of the shell-type blocking of the ligand, which makes them the first circular helicates exhibiting near-infrared lanthanide fluorescence.

Transmetalation and Demetallization for Open-Oyster-like Non-Ionic Cd(II) Macrocycles

This work designed a nonionic extended dialdehyde 6,6'-(phenylazanediyl)dipicolinaldehyde (PDPA) for constructing Schiff-base macrocyclic complexes with weaker metal-ligand interactions, so as to solve the long-standing challenges of transmetalation and demetallization in macrocyclic complexes. An enantiomeric pair of open-oyster-like 26-membered [2 + 2] Schiff-base macrocyclic dinuclear Cd(II) complexes (S,S-1a, R,R-1b) could be obtained, having S,S/R,R-1,2-diaminocyclohexane (S,S/R,R-DACH) precursors, while Cu(II) ion template only resulted in a mononuclear Schiff-base Cu(II) acyclic complex (S,S-2) accompanied by the half-oxidation of PDPA instead of expected [2 + 2] Cu(II) macrocyclic complexes. It is suggested that the weak oxidization capability of Cu(II) ion is responsible for the formation of S,S-2 because X-ray photoelectron spectroscopy (XPS) for the solid powder of reaction mixture of direct Cu(II) ion template synthesis implies that both Cu(I) and Cu(II) species are present. In fact, corresponding [2 + 2] dinuclear Cu(II) macrocycles and even metal-free macrocycles unsuitable for direct synthesis can be obtained via Cd(II) → Cu(II) transmetalation and Na₂S demetalation verified by ESI-MS and UV-vis spectra. In addition, control experiments indicate that the synthesis of metal-free macrocycles via the direct nontemplate method merely results in the mixture of multiple components of [1 + 1], [2 + 2], and [3 + 3] Schiff-base macrocycles, and they are difficult to isolate.

Luminescent Sm(III) complex bearing dynamic imine bonds as a multi-responsive fluorescent sensor for F- and PO43- anions together with Zn2+ cation in water samples

We have designed and synthesized a new luminescent mononuclear samarium (III) complex Sm-2_h based on the [1 + 1] Schiff-base macrocycle H₂L2_h, derived from the cyclocondensation reaction between dialdehyde and diamine precursors, and its exact architecture is determined to be [Sm(HL2_h) (NO₃)₂]. The sensing ability of complex Sm-2_h is carefully evaluated for various common inorganic ions in solution. It is shown that complex Sm-2_h is a multi-responsive fluorimetric sensor with high selectivity for F^- and PO₄^3- anions together with Zn²⁺ cation. The sensing process is rapid within 60 s for F^- and PO₄^3- ions and 300 s for Zn²⁺ ion. Further detailed responsive investigations suggest that its sensing behavior has excellent linear relationship between the fluorescence intensity (or absorption value) and ion concentration. The limit of detection (LOD) for sensing F^-, PO₄^3- and Zn²⁺ ions are as low as 2.61 μM (2.94 μM), 1.92 μM (1.64 μM) and 5.67 μM (3.53 μM), respectively, verified by fluorimetric (or colorimetric) titration experiments. ESI mass spectra prove that these efficient detections originate from the structure collapse of sensor Sm-2_h because of the ion-induced imine bond breakage. Moreover, sensor Sm-2_h shows excellent sensing performances for F^-, PO₄^3- and Zn²⁺ ions in real water samples, and we also have developed a convenient method to detect these three ions by use of the sensor impregnated test paper strips, providing rapid and distinguishable fluorimetric color changes. Therefore, the macrocyclic Sm(III) complex Sm-2_h could be regarded as a valuable candidate for monitoring F^-, PO₄^3- and Zn²⁺ ions in practical applications.

Schiff-base macrocyclic ZnII complexes based upon flexible pendant-armed extended dialdehydes

This Frontier article reviews diverse structures of Schiff-base macrocyclic ZnII complexes derived from a family of flexible pendant-armed extended dialdehydes via a preorganized synthetic strategy and the corresponding structure-determining factors, including primary cationic template effects and secondary complementary template effects. In addition, further post-modification and supramolecular self-assembly of Schiff-base ZnII macrocycles and their possible applications in a variety of areas have been discussed.

An excellent colorimetric and “turn off” fluorescent probe for tetrahydrofuran based on a luminescent macrocyclic samarium(iii) complex

We designed and prepared a macrocyclic samarium(iii) complex which served as an excellent colorimetric and “turn-off” fluorescent probe for sensing tetrahydrofuran.

Transmetalation for Flexible Pendant-Armed Schiff-Base Macrocyclic Complexes Influenced by Halide Effects

Three 46-membered [2 + 2] pendant-armed Schiff-base macrocyclic dinuclear Cd^II and Cu^II complexes (2a, 2b, and 3b) and one 23-membered [1 + 1] Cu^II macrocycle 4a were prepared from the template-directed condensation reactions between 1,2-bis(2-aminoethoxy)-ethane and extended Cl-dialdehyde in the presence of CdX₂ and CuX₂ (X = Cl and Br), in which halide effects play important roles in the organization of final macrocyclic complexes in addition to the dominant influence of metal cations. Transmetalation was intensively studied among these Cd^II and Cu^II complexes with large and flexible macrocyclic ligands, including two previously synthesized dinuclear Zn^II macrocycles (1a and 1b). Our results indicate that Zn^II → Cu^II and Cd^II → Cu^II transmetalation proceeds more quickly than that from Cd^II to Zn^II, and all the processes are found to be irreversible. It is noted that a [2 + 2] heterodinuclear Cd^IIZn^II macrocyclic intermediate could be detected by ESI-MS together with [2 + 2] homodinuclear Cd^II and Zn^II macrocyclic complexes. Furthermore, distinct halide behavior was observed in the in situ Cd^II → Cu^II and Zn^II → Cu^II metal-ion exchange. That is to say, [2 + 2] macrocycles (1a and 2a) could be converted to [1 + 1] macrocycles 4a and 4b under the reflux condition in the case of CuCl₂, accompanied by the configurational transformation from half-folded dinuclear Zn^II and Cd^II to unfolded Cu^II macrocyclic skeleton. In contrast, CuBr₂ leads to a highly efficient transmetalation to corresponding [2 + 2] dinuclear Cu^II complex 3b from both 1b and 2b no matter the experimental condition used.

Zebra reaction or the recipe for the synthesis of heterodimeric zinc complexes

A series of asymmetric heterodimeric zinc complexes have been synthesized in a direct reaction between conformationally flexible chiral/achiral homodimers. The cooperative activity of steric factors and coordination codes resulted in an intriguing chiral self-sorting process. Herein, we are reporting our recent exploration of the first example of such a type of reaction.