Hydrogen Peroxide Disproportionation with Manganese Macrocyclic Complexes of Cyclen and Pyclen

Hydrogen Peroxide Disproportionation Activity Is Sensitive to Pyridine Substitutions on Manganese Catalysts Derived from 12-Membered Tetra-Aza Macrocyclic Ligands

The abundance of manganese in nature and versatility to access different oxidation states have made manganese complexes attractive as catalysts for oxidation reactions in both biology and industry. Macrocyclic ligands offer the advantage of substantially controlling the reactivity of the manganese center through electronic tuning and steric constraint. Inspired by the manganese catalase enzyme, a biological catalyst for the disproportionation of H2O2 into water and O2, the work herein employs 12-membered tetra-aza macrocyclic ligands to study how the inclusion of and substitution to the pyridine ring on the macrocyclic ligand scaffold impacts the reactivity of the manganese complex as a H2O2 disproportionation catalyst. Synthesis and isolation of the manganese complexes was validated by characterization using UV-vis spectroscopy, SC-XRD, and cyclic voltammetry. Potentiometric titrations were used to study the ligand basicity as well as the thermodynamic equilibrium with Mn(II). Manganese complexes were also produced in situ and characterized using electrochemistry for comparison to the isolated species. Results from these studies and H2O2 reactivity showed a remarkable difference among the ligands studied, revealing instead a distinction in the reactivity regarding the number of pyridine rings within the scaffold. Moreover, electron-donating groups on the 4-position of the pyridine ring enhanced the reactivity of the manganese center for H2O2 disproportionation, demonstrating a handle for control of oxidation reactions using the pyridinophane macrocycle.

Synthesis and characterization of two piperazine containing macrocycles and their transition metal complexes

Rigidification of the ligand scaffolds has been a particular mechanism of interest employed to achieve properties suitable for MRI contrast, catalysis, or other applications of metal complexes. Towards the goal of targeting a 15-anePyN5Pip type ligand, a serendipitous isolation of a 30-anePy2N10Pip2 aza-macrocycle was achieved, instead. X-ray diffraction and determination of pKa events were carried out and compared to 17-anePyN5Pip. Furthermore, the X-ray diffraction of the Cu(II) and Zn(II) complexes of 17-anePyN5Pip was achieved and compared to previous reports of other first-row transition metal derivatives of this ligand. Determination of the log β with both 30-anePy2N10Pip2 and 17-anePyN5Pip with the divalent MnZn metal-ion series was used to demonstrate the impact that the piperazine ring plays compared to other, less rigid macrocycles reported to date.

Magnetic porphyrin-based metal organic gel for rapid RhB removal and enhanced antibacterial activity by heterogeneous Photo-Fenton reaction under visible light

Nanomaterials with visible light-driven catalytic ability are beneficial in controlling environmental pollutants. Porphyrin-based metal organic gel (MOG) was herein synthesized in one step and magnetic metal organic gel (MMOG) was successfully prepared via in-situ reaction of MOG and Fe₃O₄. This MMOG was developed as a novel visible light assisted Fenton-like catalyst. The catalytic experiments showed the high photo-Fenton activity of MMOG in the degradation of Rhodamine B (RhB) in the presence of visible light and H₂O₂ with a RhB degradation efficiency of 94.2% within 40 min. Notably, the obtained MMOG can kill E. coli and S. aureus with high killing rate (>99.999%) under visible light. Importantly, the MMOG can be recovered simply by an external magnetic field due to the unique magnetic property. This easily synthesized MMOG with photo-Fenton activity under visible light and magnetic property makes MOG based on the photo-Fenton reaction a prospective material for the environmental and biomedical applications.

Recent progresses in the chemistry of 12-membered pyridine-containing tetraazamacrocycles: From synthesis to catalysis

This article provides an overview (non-comprehensive) on recent developments regarding pyridine-containing 12-membered tetraazamacrocycles with pyclen or Py2N2 backbones and their metal complexes from 2017 to the present. Firstly, the synthesis...

Heterometal-Organic Cages as Photo-Fenton-like Catalysts

Metal-organic cages, a class of supramolecular containers constructed by the self-assembly of metal ions and organic ligands, show great promise as catalytic agents. In this work, we designed and synthesized a series of rhombic dodecahedral Ni-Cu heterometal imidazolate cages (Ni₈Cu₆L₂₄) that can act as highly active photo-Fenton-like catalysts. These cages possess a high ability to generate hydroxyl radicals (^•OH) under visible light in the presence of H₂O₂, which can rapidly degrade organic pollutants (e.g., rhodamine B, methylene blue, and methyl orange) into CO₂ and H₂O. Besides, they are robust catalysts, with high catalytic activity and reusability under conditions in high H₂O₂ concentration, providing potentially advanced materials for degrading persistent organic pollutants.

Sustainable hydrogen production from water using tandem dye-sensitized photoelectrochemical cells

If generated from water using renewable energy, hydrogen could serve as a carbon-zero, environmentally benign fuel to meet the needs of modern society. Photoelectrochemical cells integrate the absorption and conversion of solar energy and chemical catalysis for the generation of high value products. Tandem photoelectrochemical devices have demonstrated impressive solar-to-hydrogen conversion efficiencies but have not become economically relevant due to high production cost. Dye-sensitized solar cells, those based on a monolayer of molecular dye adsorbed to a high surface area, optically transparent semiconductor electrode, offer a possible route to realizing tandem photochemical systems for H₂ production by water photolysis with lower overall material and processing costs. This review addresses the design and materials important to the development of tandem dye-sensitized photoelectrochemical cells for solar H₂ production and highlights current published reports detailing systems capable of spontaneous H₂ formation from water using only dye-sensitized interfaces for light capture.

Manganese Complex of a Rigidified 15-Membered Macrocycle: A Comprehensive Study

Owing to the increasing importance of manganese(II) complexes in the field of magnetic resonance imaging (MRI), large efforts have been devoted to find an appropriate ligand for Mn(II) ion encapsulation by providing balance between the seemingly contradictory requirements (i.e., thermodynamic stability and kinetic inertness vs low ligand denticity enabling water molecule(s) to be coordinated in its metal center). Among these ligands, a large number of pyridine or pyridol based open-chain and macrocyclic chelators have been investigated so far. As a next step in the development of these chelators, 15-pyN₃O₂Ph and its transition metal complexes were synthesized and characterized using established methods. The 15-pyN₃O₂Ph ligand incorporates both pyridine and ortho-phenylene units to decrease ligand flexibility. The thermodynamic properties, protonation and stability constants, were determined using pH-potentiometry; the solid-state structures of two protonation states of the free ligand and its manganese complex were obtained by single crystal X-ray diffractometry. The results show a seven-coordinate metal center with two water molecules in the first coordination sphere. The longitudinal relaxivity of [Mn(15-pyN₃O₂Ph)]²⁺ was found to be 5.16 mM^-1 s^-1 at 0.49 T (298 K). Furthermore, the r_2p value of 11.72 mM^-1 s^-1 (0.49 T), which is doubled at 1.41 T field, suggests that design of this Mn(II) complex does achieve some characteristics required for contrast imaging. In addition, ¹⁷O NMR measurements were performed in order to access the microscopic parameters governing this key feature (e.g., water exchange rate). Finally, manganese complexes of ligands with analogous polyaza macrocyclic scaffold have been investigated as low molecular weight Mn(CAT) mimics. Here, we report the H₂O₂ disproportionation study of [Mn(15-pyN₃O₂Ph)]²⁺ to demonstrate the versatility of this ligand scaffold as well.