Selective anion binding by a cofacial binuclear zinc complex of a Schiff-base pyrrole macrocycle

Tying a knot between crown ethers and porphyrins

Porphyrins and crown ether hybrids have emerged as a promising class of molecules composed of elements of a tetrapyrrole macrocycle and an oligo(ethylene glycol) segment. These hybrid systems constitute a broad group of compounds, including crowned porphyrins, crownphyrins, and calixpyrrole-crown ether systems forming Pacman complexes with transition metals. Their unique nature accustoms them as excellent ligands and hosts capable of binding guest molecules/ions, but also to undergo unusual transformations, such as metal-induced expansion/contraction. Depending on the design of the particular hybrid, they present unique features involving intriguing redox chemistry, interesting optical properties, and reactivity towards transition metals. In this perspective article, the overview of both the early designs of porphyrin-crown ether hybrids, as well as the most recent advances in the synthesis and characterisation of this remarkable group of macrocyclic systems, are addressed. The discussion covers the strategies employed in synthesising these systems, including cyclisation reactions, self-assembly, and their remarkable reactivity. The potential applications of porphyrin-crown ether hybrids are also highlighted. Moreover, the discussion identifies the challenges associated with synthesising and characterising hybrids, outlining the possible future directions.

Fluorescent probe sensor based on (R)-(-)-4-phenyl-2-oxazolidone for effective detection of divalent cations

Significant progress attained in sensor science in recent years has resulted in the development of highly efficient fluorescence probes for sensing metal ions. Fluorescent molecular probes based on (R)-(-)-4-phenyl-2-oxazolidone are reported here. Fluorescence studies indicated that the molecular probe could be used successfully to sense divalent metal cations such as Cu²⁺ , Co²⁺ , Pb²⁺ , and Zn²⁺ . The addition of divalent metal cations to the molecular probe produced a specific interaction pattern under UV-visible and fluorescence spectroscopy. These molecules could detect metal cations using fluorescence quenching. Stern-Volmer plots were used to determine quenching rate coefficients, which were calculated to be 2 × 10¹ , 1.06 × 10³ and 7.39 × 10² M^-1 s^-1 for copper, cobalt, and zinc respectively. Calculation of limit of detection for heavy metal cations revealed that the reported molecular probes improved the limit of detection compared with available standard data. Limit of quantitation values were also well within the permissible range. The frontier energy gap of highest occupied molecular orbital to the lowest unoccupied molecular orbital was evaluated using the density functional theory approach and Gaussian 09 W software, which complemented the coordination of azetidinones with divalent metal ions.

New zinc complexes derived from "self-adaptable" acyclic diiminodipyrromethanes as potent catalysts for the reduction of curing temperature of bisphenol-A/F benzoxazines

The simple modification of the Schiff-base ligands often brings significant changes in the coordination properties of the metal-complexes, providing newer prospects for their unexplored applications. In this context, the present work utilized the “self-adaptable” acyclic diiminodipyrromethane Schiff's bases (2a and 2b) for the synthesis of their Zn-based complexes and explored their potential in the ring-opening polymerization of benzoxazines. The two zinc complexes of composition [Zn{(Ph)(CH₃)C(2,6-ⁱPr₂C₆H₃–N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH–C₄H₂N)(2,6-ⁱPr₂C₆H₃–NCH–C₄H₂NH)}₂] (3) and [ZnCl₂{(Ph)(CH₃)C(Ph₃C–NHCH–C₄H₂N)₂}] (4) were synthesized in good yields, and the structures were confirmed by single crystal X-ray diffraction (XRD). Later, zinc complexes (3 & 4) were used as catalysts to reduce the curing (ring-opening polymerization) temperature of benzoxazine monomers such as Bisphenol-A (BA-a) and Bisphenol-F (BF-a) benzoxazines. Dynamic scanning calorimetry (DSC) studies revealed that the on-set curing (T_p) temperatures were reasonably decreased upto 20% for the benzoxazines. Furthermore, the thermal stabilities of the polybenzoxazines (PBzs) derived in the presence of zinc catalysts (3 and 4) were compared with PBz obtained in the absence of catalyst under similar conditions. The thermal studies reveled that there is no significant changes in the initial degradation of polymers. However, the thermal stability in terms of char yields at 800 °C improved upto 10–21% for the bisphenol-A/F benzoxazines.

The present work utilized the “self-adaptable” acyclic diiminodipyrromethane Schiff's bases (2a and 2b) for the synthesis of their Zn-based complexes and explored their potential in the ring-opening polymerization of BA-a and BF-a benzoxazines.

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.

Molecular recognition of planar and non-planar aromatic hydrocarbons through multipoint Ag-π bonding in a dinuclear metallo-macrocycle

Exploration of a novel structural motif of host-guest interactions is one of the most fundamental topics to develop macrocycle-based host-guest/supramolecular systems. Herein, we present an unprecedented mode of inclusion of aromatic hydrocarbons into a macrocyclic cavity via multipoint Ag-π bonding as a driving force. A dinuclear Ag^I-macrocycle encapsulated one molecule of anthracene, a typical planar aromatic hydrocarbon, in solution and in the solid state. Single-crystal X-ray diffraction analysis of the host-guest inclusion complex revealed the binding of anthracene via multipoint Ag-π bonding to both Ag^I ions arranged within the open-ended nano-cavity of the dinuclear Ag^I-macrocycle. Notably, this binding motif based on Ag-π bonding was also applied to the inclusion of triptycene, a non-planar aromatic hydrocarbon with a steric tripodal structure, to evaluate the rotational motion of the molecular paddle-wheel in the Ag^I-macrocycle.

Polynuclear alkoxy-zinc complexes of bowl-shaped macrocycles and their use in the copolymerisation of cyclohexene oxide and CO2

The reactions between alcohols and the tetranuclear ethyl-Zn complexes of an ortho-phenylene-bridged polypyrrole macrocycle, Zn4Et4(L1) 1 and the related anthracenyl-bridged macrocyclic complex, Zn4Et4(THF)4(L2) 2 have been studied. With long-chain alcohols such as n-hexanol, the clean formation of the tetranuclear hexoxide complex Zn4(OC6H13)4(L1) 3 occurs. In contrast, the use of shorter-chain alcohols such as i-propanol results in the trinuclear complex Zn3(μ2-OiPr)2(μ3-OiPr)(HL1) 4 that arises from demetalation; this complex was characterised by X-ray crystallography. The clean formation of these polynuclear zinc clusters allowed a study of their use as catalysts in the ring-opening copolymerisation (ROCOP) reaction between cyclohexene oxide and CO2. In situ reactions involving the pre-catalyst 1 and n-hexanol formed the desired polymer with the best selectivity for polycarbonate (90%) at 30 atm CO2, whilst the activity and performance of pre-catalyst 2 was poor in comparison.

A square planar gold(iii) bis-(1,1′-dimethyl-3,3′-methylene-diimidazol-2,2′-diylidene) trication as an efficient and selective receptor towards halogen anions: the cooperative effect of Au⋯X and X⋯HC interactions

A tetra-NHC gold(iii) complex acts as anion sensor.

Fe(2+) binding on amyloid β-peptide promotes aggregation

The metal ions Zn(2+) , Cu(2+) , and Fe(2+) play a significant role in the aggregation mechanism of Aβ peptides. However, the nature of binding between metal and peptide has remained elusive; the detailed information on this from the experimental study is very difficult. Density functional theory (dft) (M06-2X/6-311++G (2df,2pd) +LANL2DZ) has employed to determine the force field resulting due to metal and histidine interaction. We performed 200 ns molecular dynamics (MD) simulation on Aβ1-42 -Zn(2+) , Aβ1-42 -Cu(2+) , and Aβ1-42 -Fe(2+) systems in explicit water with different combination of coordinating residues including the three Histidine residues in the N-terminal. The present investigation, the Aβ1-42 -Zn(2+) system possess three turn conformations separated by coil structure. Zn(2+) binding caused the loss of the helical structure of N-terminal residues which transformed into the S-shaped conformation. Zn(2+) has reduced the coil and increases the turn content of the peptide compared with experimental study. On the other hand, the Cu(2+) binds with peptide, β sheet formation is observed at the N-terminal residues of the peptide. Fe(2+) binding is to promote the formation of Glu22-Lys28 salt-bridge which stabilized the turn conformation in the Phe19-Gly25 residues, subsequently β sheets were observed at His13-Lys18 and Gly29-Gly37 residues. The turn conformation facilitates the β sheets are arranged in parallel by enhancing the hydrophobic contact between Gly25 and Met35, Lys16 and Met35, Leu17 and Leu34, Val18 and Leu34 residues. The Fe(2+) binding reduced the helix structure and increases the β sheet content in the peptide, which suggested, Fe(2+) promotes the oligomerization by enhancing the peptide-peptide interaction. Proteins 2016; 84:1257-1274. © 2016 Wiley Periodicals, Inc.

Piperazine linked salicylaldoxime and salicylaldimine-based dicopper(ii) receptors for anions

The syntheses and single crystal X-ray analyses of five strapped salicylaldoxime/salicylaldimine based dicopper(ii) receptors utilising a new piperazine linker are described. The complexes form 2 + 2 metallocycles and the molecular structures of all four complexes possess a small internal cavity with the utilisation of a short piperazine linker. The molecular structures of complexes [Cu2(L(4) - H)(L(4) - 2H) ⊂ DMF]BF4·DMF, and [Cu2(L(4) - H)2Br]Br·1.25DMSO·H2O·MeOH, show that intramolecular H-bonding interactions due to the presence of -OH (oxime moiety) groups lead to a Pacman-like cleft arrangement of the two metal coordinating subunits in the metallo-macrocycle. The geometrical constraints brought about by this constrained cleft make the receptor coordinate strongly to a bromide anion involving both metal centres as evidenced by whereas in the larger tetrafluroborate anion is excluded. Absence of the oxime moiety around the metal coordination site of the ligand as demonstrated in the complexes [Cu2(L(5))2BF4](BF4)3, and [Cu2(L(5))2Br]Br3·2MeOH, resulted in less constrained dicopper(ii) helicate forms. For these complexes no anion size discrimination was observed. The addition of pyridine solvent to a slightly modified piperazine-linked ligand produces an expanded 3 + 3 tube-like tricopper complex [Cu3(L(4a) - H)3Py3](BF4)2·(MeOH)3·PF6·(H2O)3, , with two coordinated pyridine molecules occupying the newly formed cavity.

Structural diversity of alkylzinc complexes with pyrrole-based N,O-ligands: from molecular complexes to coordination polymers

The equimolar reaction of R2Zn (where R = Et or (t)Bu) with pyrrole-based N,O-proligands afforded a series of alkylzinc compounds with a variety of intriguing structures including a hexanuclear macrocyclic complex and 1D coordination polymers with versatile intramolecular or intermolecular bonding modes.

New developments in porphyrin-like macrocyclic chemistry: a novel family of dibenzotetraaza[14]annulene-based cofacial dimers

The first known cofacial dimers, based on covalently linked dibenzotetraaza[14]annulenes, were synthesized in reasonable 35–40% yields, without recourse to high-dilution techniques.

Capsoplexes: encapsulating complexes via guest recognition

A new dinuclear Ni–NHC complex is able to selectively recognise and self-assemble with guests via tennis-ball like encapsulation, exemplarily demonstrated employing halides.

A new high-capacity metal ion-complexing gel containing cyclen ligands

“Hairy” nano-scale objects cross-linked into a high-capacity metal-binding hydrogel.

Relativistic DFT and experimental studies of mono- and bis-actinyl complexes of an expanded Schiff-base polypyrrole macrocycle

Relativistic DFT calculations present accurate geometries of complexes and redox properties, confirmed by the newly-developed experimental syntheses.

Towards dipyrrins: oxidation and metalation of acyclic and macrocyclic Schiff-base dipyrromethanes

Oxidation of acyclic Schiff-base dipyrromethanes cleanly results in dipyrrins, whereas the macrocyclic 'Pacman' analogues either decompose or form new dinuclear copper(ii) complexes that are inert to ligand oxidation; the unhindered hydrogen substituent at the meso-carbon allows new structural motifs to form.

Solid nanoarchitecture – Cu(ii) solution: dynamics of the chemical communication

The response of a porphyrin nanoarchitecture, covalently supported on quartz, was investigated upon the external chemical Cu(ii) stimulus. The overall structure behaves as a ternary optical system.

Multipoint recognition of ditopic aromatic guest molecules via Ag-π interactions within a dimetal macrocycle

A macrocyclic host molecule possessing a nanocavity with two Ag(I) centers for guest binding and four anthracene walls has been developed. This dimetal-macrocycle forms stable inclusion complexes with ditopic aromatic guest molecules, [2.2]paracyclophane, and ferrocene, in solution and/or in the solid state through Ag-π interactions within the nanocavity. The binding constants for the inclusion complexes were found to range roughly from 10(4) to 10(9) M(-1). Electrochemical measurement revealed that the oxidized form of the included cationic ferrocene was less stabilized due to the direct binding to the cationic two Ag(I) centers.

Co-linear, double-uranyl coordination by an expanded Schiff-base polypyrrole macrocycle

Expansion of a Schiff-base polypyrrolic macrocycle allows the formation of a binuclear uranyl complex with co-linear uranyl ions and a very short oxo-oxo distance.

The unprecedented preparation of dinuclear zinc(II) complexes from 4-halido-2-[(3-cyclohexylaminopropylimino)methyl]phenol

Three new centrosymmetric trinuclear nickel(II) and manganese(II) complexes, Ni[Ni(CH(3)COO)(CPA)](2) (1), Ni[Ni(CH(3)COO)(BPA)](2) (2), Mn[Mn(CH(3)COO)(BPA)](2) (3), where H(2)CPA = N,N'-bis(5-chlorosalicylidene)-1,3-propanediamine, H(2)BPA = N,N'-bis(5-bromosalicylidene)-1,3-propanediamine, and two new centrosymmetric dinuclear zinc(II) complexes, [Zn(2)(CMP)(2)] (4) and [Zn(2)(BMP)(2)] (5), where H(2)CMP = 4-chloro-2-{[3-(5-chloro-2-hydroxybenzyl)aminopropylimino]methyl}phenol, and H(2)BMP = 4-bromo-2-{[3-(5-bromo-2-hydroxybenzyl)aminopropylimino]methyl}phenol, have been prepared from the Schiff bases derived from 5-halido-substituted salicylaldehydes with N-hexylpropane-1,3-diamine under solvothermal conditions. The complexes have been characterised by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction studies. The complexes 1, 2, and 3 crystallise in the monoclinic space group P2(1)/c with cell dimensions a = 9.347(1), b = 11.507(2), c = 18.539(2) Å, β = 93.774(2)°, Z = 2 (for 1), a = 9.111(4), b = 12.089(6), c = 18.724(8) Å, β = 92.117(7)°, Z = 2 (for 2), and a = 11.328(2), b = 22.468(5), c = 8.270(2) Å, β = 93.74(3)°, Z = 2 (for 3), while complexes 4 and 5 crystallise in the triclinic space group P1, with cell dimensions a = 7.483(1), b = 9.990(2), c = 12.155(2) Å, α = 75.27(3), β = 85.00(3), γ = 73.82(3)°, Z = 1 (for 4), and a = 7.008(1), b = 10.081(2), c = 13.095(3) Å, α = 100.62(3), β = 95.51(3), γ = 104.03(3)°, Z = 1 (for 5). It is interesting that the mono-Schiff bases 4-chloro-2-[(3-cyclohexylaminopropylimino)methyl]phenol (HCCP) and 4-bromo-2-[(3-cyclohexylaminopropylimino)methyl]phenol (HBCP) used to prepare the nickel(II) and manganese(II) complexes were transferred to bis-Schiff bases H(2)CPA and H(2)BPA in the complexes 1, 2, and 3, while the mono-Schiff bases HCCP and HBCP used to prepare the zinc(II) complexes were transferred to novel ligands H(2)CMP and H(2)BMP, bearing the unexpected, newly formed carbon-nitrogen single bond.

Highly selective colorimetric chemosensor for Co2+

A new highly selective colorimetric chemosensor for Co(2+) was developed based on coumarin-conjugated thiocarbanohydrazone. The ligand senses Co(2+) in solution by changing its color from light yellow to deep pink. The sensor has been used in the development of practically viable colorimetric kits and as a staining agent for Co(2+) in microorganisms.

Anion-driven conformation control and enhanced sulfate binding utilising aryl linked salicylaldoxime dicopper helicates

The synthesis and spectroscopic analysis of both "metal-only" and anion encapsulated dicopper(II) double helicates utilising a new 1,4-aryl spacer is described. X-Ray crystallographic analysis of the complexes reveal that the aromatic spacer increases rigidity in the complex, yet the complexes are still able to undergo a dramatic "coiling up" to bind anions. Spectroscopic analysis has shown a clear enhancement in the binding strength of SO(4)(2-) over the non-coordinating anions ClO(4)(-), NO(3)(-) and Br(-) which has been attributed to a combination of enhanced rigidity in the complex and an increased electrostatic interaction between the complex and the dianion.