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.

Crownphyrins: Metal-Mediated Transformations of the Porphyrin-Crown Ether Hybrids

Crownphyrins are hybrid macrocycles combining structural features of porphyrin and crown ethers. The molecular architecture renders them an intriguing class of hosts capable of binding neutral, and ionic guests. The presence of dynamic covalent imine linkages connecting the dipyrrin segment with the ether chain enables unusual coordination behavior of crownphyrins, as demonstrated by the formation of two classes of strikingly different complexes. The remarkable metal-mediated expansion to the helical [2+2] macrocyclic complex is reversible. The reaction of the figure-eight mercury(II) assembly with [2.2.2]cryptand results in ring contraction providing the metal-free crownphyrin macrocycle.

First principles studies of mononuclear and dinuclear Pacman complexes for electrocatalytic reduction of CO2

An iron-containing Pacman complex exhibited high activity and selectivity for the reduction of CO₂ to CH₄.

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.

Macrocyclic Dizinc(II) Alkyl and Alkoxide Complexes: Reversible CO2 Uptake and Polymerization Catalysis Testing

The synthesis of three new dizinc(II) complexes bearing a macrocyclic [2 + 2] Schiff base ligand is reported. The bis(anilido)tetraimine macrocycle reacts with diethylzinc to form a bis(ethyl)dizinc(II) complex, [L(Et)Zn2Et2] (1). The reaction of complex 1 with isopropyl alcohol is reported, forming a bis(isopropyl alkoxide)dizinc complex, [L(Et)Zn2((i)PrO)2] (2). Furthermore, complex 1, with 2 equiv of alcohol, is applied as an initiator for racemic lactide ring-opening polymerization. It shows moderately high activity, resulting in a pseudo-first-order rate coefficient of 9.8 × 10(-3) min(-1), with [LA] = 1 M and [initiator] = 5 mM at 25 °C and in a tetrahydrofuran solvent. Polymerization occurs with good control, as evidenced by the linear fit to a plot of molecular weight versus conversion, the narrow dispersities, and the limited transesterification. The same initiating system is inactive for the ring-opening copolymerization of carbon dioxide (CO2) and cyclohexene oxide at 80 °C and 1 bar of CO2 pressure. However, stoichiometric reactions between complex 2 and CO2, at 1 bar pressure, result in the reversible formation of new dizinc carbonate species, [L(Et)Zn2((i)PrO)((i)PrOCO2)] (3a) and [L(Et)Zn2((i)PrOCO2)2] (3b), and the reaction was studied using density functional theory calculations. All of the new complexes, 1-3b, are fully characterized, including NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.

Exhibition of the Brønsted acid–base character of a Schiff base in palladium(ii) complex formation: lithium complexation, fluxional properties and catalysis of Suzuki reactions in water

The Brønsted acid–base character of bis(iminopyrrolylmethyl)amine was shown through the X-ray structures of palladium complexes. The bischelated palladium complex is fluxional as studied by the VT ¹H NMR method and effectively catalyzes Suzuki reactions in water.