Increase of Direct C-C Coupling Reaction Yield by Identifying Structural and Electronic Properties of High-Spin Iron Tetra-azamacrocyclic Complexes

Iron-Catalyzed Asymmetric α-Alkylation of 2-Acylimidazoles via Dehydrogenative Radical Cross-Coupling with Alkanes

The direct α-alkylation of acyclic carbonyls with nonactivated hydrocarbons through C(sp3 )-H functionalization is both extremely promising and notably challenging, especially when attempting to achieve enantioselectivity using iron-based catalysts. We have identified a robust chiral iron complex for the oxidative cross-coupling of 2-acylimidazoles with benzylic and allylic hydrocarbons, as well as nonactivated alkanes. The readily available and tunable N,N'-dioxide catalysts of iron in connection with oxidants exhibit precise asymmetric induction (up to 99 % ee) with good compatibility in moderate to good yields (up to 88 % yield). This protocol provides an elegant and straightforward access to optically active acyclic carbonyl derivatives starting from simple alkanes without prefunctionalization. Density functional theory (DFT) calculations and control experiments were made to gain insight into the nature of C-C bond formation and the origin of enantioselectivity. We propose a radical-radical cross-coupling process enabled by the immediate interconversion between chiral ferric species and ferrous species.

Pyridine modifications regulate the electronics and reactivity of Fe-pyridinophane complexes

12-Membered pyridinophanes are the focus of many studies as biological mimics, chelators, and catalytic precursors. Therefore, the desire to tune the reactivity of pyridinophanes to better control the applications of derivative metal complexes has inspired many structure-activity relationship studies. However, the separation of structural versus electronic changes imparted by ligand modification has made these structure-activity relationship studies of transition metal catalysts challenging to define. In this work we show that 4-substitution of the pyridine ring in 12-membered tetra-aza pyridinophanes successfully provides a regulatory handle on the electronic properties of the metal center and, therefore, the catalytic C-C coupling activity of the respective iron complexes. The C-C coupling reaction catalyzed by Fe(L1-L6) provides a range of yields (32-58%) that directly correlate with iron redox potentials (ΔE1/2 = 152 mV) and metal binding constants (Δlog β = 3.45), while the geometry of the complexes was virtually indistinguishable. These are the first results to definitively show the redox potential and metal binding as independent properties from the coordination chemistry in one ligand series. Adjustments to these chemical properties were then shown to provide a regulatory handle for the C-C coupling reactivity tuned via pyridine substitution in pyridinophanes.

A Bridge too Far? Comparison of Transition Metal Complexes of Dibenzyltetraazamacrocycles with and without Ethylene Cross-Bridges: X-ray Crystal Structures, Kinetic Stability, and Electronic Properties

Tetraazamacrocycles, cyclic molecules with four nitrogen atoms, have long been known to produce highly stable transition metal complexes. Cross-bridging such molecules with two-carbon chains has been shown to enhance the stability of these complexes even further. This provides enough stability to use the resulting compounds in applications as diverse and demanding as aqueous, green oxidation catalysis all the way to drug molecules injected into humans. Although the stability of these compounds is believed to result from the increased rigidity and topological complexity imparted by the cross-bridge, there is insufficient experimental data to exclude other causes. In this study, standard organic and inorganic synthetic methods were used to produce unbridged dibenzyl tetraazamacrocycle complexes of Co, Ni, Cu, and Zn that are analogues of known cross-bridged tetraazamacrocycles and their transition metal complexes to allow direct comparison of molecules that are identical except for the cross-bridge. The syntheses of the known tetraazamacrocycles and the new transition metal complexes were successful with high yields and purity. Initial chemical characterization of the complexes was conducted by UV-Visible spectroscopy, while cyclic voltammetry showed more marked differences in electronic properties from bridged versions. Direct comparison studies of the unbridged and bridged compounds' kinetic stabilities, as demonstrated by decomposition using high acid concentration and elevated temperature, showed that the cyclen-based complex stability did not benefit from cross-bridging. This is likely due to poor complementarity with the Cu2+ ion while cyclam-based complexes benefited greatly. We conclude that ligand-metal complementarity must be maintained in order for the topological and rigidity constraints imparted by the cross-bridge to contribute significantly to complex robustness.

Synthesis and Characterization of Late Transition Metal Complexes of Mono-Acetate Pendant Armed Ethylene Cross-Bridged Tetraazamacrocycles with Promise as Oxidation Catalysts for Dye Bleaching

Ethylene cross-bridged tetraazamacrocycles are known to produce kinetically stable transition metal complexes that can act as robust oxidation catalysts under harsh aqueous conditions. We have synthesized ligand analogs with single acetate pendant arms that act as pentadentate ligands to Mn, Fe, Co, Ni, Cu, and Zn. These complexes have been synthesized and characterized, including the structural characterization of four Co and Cu complexes. Cyclic voltammetry demonstrates that multiple oxidation states are stabilized by these rigid, bicyclic ligands. Yet, redox potentials of the metal complexes are modified compared to the "parent" ligands due to the pendant acetate arm. Similarly, gains in kinetic stability under harsh acidic conditions, compared to parent complexes without the pendant acetate arm, were demonstrated by a half-life seven times longer for the cyclam copper complex. Due to the reversible, high oxidation states available for the Mn and Fe complexes, the Mn and Fe complexes were examined as catalysts for the bleaching of three commonly used pollutant model dyes (methylene blue, methyl orange, and Rhodamine B) in water with hydrogen peroxide as oxidant. The efficient bleaching of these dyes was observed.

A macrocyclic molecule with multiple antioxidative activities protects the lens from oxidative damage

Growing evidence links oxidative stress to the development of a cataract and other diseases of the eye. Treatments for lens-derived diseases are still elusive outside of the standard surgical interventions, which still carry risks today. Therefore, a potential drug molecule OHPy2N2 was explored for the ability to target multiple components of oxidative stress in the lens to prevent cataract formation. Several pathways were identified. Here we show that the OHPy2N2 molecule activates innate catalytic mechanisms in primary lens epithelial cells to prevent damage induced by oxidative stress. This protection was linked to the upregulation of Nuclear factor erythroid-2-related factor 2 and downstream antioxidant enzyme for glutathione-dependent glutaredoxins, based on Western Blot methods. The anti-ferroptotic potential was established by showing that OHPy2N2 increases levels of glutathione peroxidase, decreases lipid peroxidation, and readily binds iron (II) and (III). The bioenergetics pathway, which has been shown to be negatively impacted in many diseases involving oxidative stress, was also enhanced as evidence by increased levels of Adenosine triphosphate product when the lens epithelial cells were co-incubated with OHPy2N2. Lastly, OHPy2N2 was also found to prevent oxidative stress-induced lens opacity in an ex vivo organ culture model. Overall, these results show that there are multiple pathways that the OHPy2N2 has the ability to impact to promote natural mechanisms within cells to protect against chronic oxidative stress in the eye.

A general and stereoselective approach to 14-membered cyclic bis-semicarbazones involving BF3-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene

A general and stereoselective five-step approach to 14-membered cyclic bis-semicarbazones, 5,12-diaryl-7,14-dimethyl-1,2,4,8,9,11-hexaazacyclotetradeca-7,14-diene-3,10-diones, starting from aldehyde semicarbazones has been developed. The key intermediates, 4-(3-oxobut-1-yl)semicarbazones, were prepared by BF₃-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene with 4-[(aryl)(methoxy)methyl]- or 4-[(aryl)(tosyl)methyl]semicarbazones. Treatment of these intermediates with excess of hydrazine gave hydrazones of 4-(3-oxobut-1-yl)semicarbazones or 4-(3-oxobut-1-yl)semicarbazides, which in the presence of TsOH were converted into the target macrocycles. All steps of this approach could be scaled up easily to the multi-gram level.

Expanding and quantifying the crystal chemistry of the flexible ligand 15aneN5

15aneN5 is structurally characterized complexed to Cr3+, Mn3+, Fe3+, Co3+, and Cu2+ for the first time. Ru3+ complexation yields a Ru2+ diimine structure. A geometric factor quantifies the coordination geometry of the ligand in its complexes.

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...

An ethylene cross-bridged pentaazamacrocycle and its Cu2+ complex: constrained ligand topology and excellent kinetic stability

Rigid and topologically constrained ethylene cross-bridged tetraazamacrocycles have been increasingly utilised for thirty years as they form remarkably stable transition metal complexes for catalysis, biomedical imaging, and inorganic drug molecule applications. Extending these benefits to pentaazamacrocycles has been achieved and a first transition metal complex prepared and structurally characterized.

Stabilisation of Exotic Tribromide (Br3-) Anions via Supramolecular Interaction with A Tosylated Macrocyclic Pyridinophane. A Serendipitous Case

Tetraaza-macrocyclic pyridinophane L-Ts, decorated with a p-toluenesulfonyl (tosyl; Ts) group, appear to be a useful tool to provide evidence on how the interplay of various supramolecular forces can help stabilise exotic anionic species such as tribromide (Br₃⁻) anions. Indeed, crystals of (H₂L-Ts)(Br₃)_1.5(NO₃)_0.5 unexpectedly grew from an acidic (HNO₃) aqueous solution of L-Ts in the presence of Br⁻ anions. The crystal structure of this compound was determined by single crystal XRD analysis. Hydrogen bonds, salt-bridges, anion-π, π-π stacking, and van der Waals interactions contribute to stabilising the crystal lattice. The observation of two independent Br₃⁻ anions stuck over the π-electron densities of pyridine and tosyl ligand groups, one of them being sandwiched between two pyridine rings, corroborates the significance of anion-π interactions for N-containing heterocycles. We show herein the possibility of detecting anion-π contacts from fingerprint plots generated by Hirshfeld surface analysis, demonstrating the effective usage of this structural investigation technique to further dissect individual contributions of stabilising supramolecular forces.

Synthesis of 12-Membered Tetra-aza Macrocyclic Pyridinophanes Bearing Electron-Withdrawing Groups

The number of substituted pyridine pyridinophanes found in the literature is limited due to challenges associated with 12-membered macrocycle and modified pyridine synthesis. Most notably, the electrophilic character at the 4-position of pyridine in pyridinophanes presents a unique challenge for introducing electrophilic chemical groups. Likewise, of the few reported, most substituted pyridine pyridinophanes in the literature are limited to electron-donating functionalities. Herein, new synthetic strategies for four new macrocycles bearing the electron-withdrawing groups CN, Cl, NO₂, and CF₃ are introduced. Potentiometric titrations were used to determine the protonation constants of the new pyridinophanes. Further, the influence of such modifications on the chemical behavior is predicted by comparing the potentiometric results to previously reported systems. X-ray diffraction analysis of the 4-Cl substituted species and its Cu(II) complex are also described to demonstrate the metal binding nature of these ligands. DFT analysis is used to support the experimental findings through energy calculations and ESP maps. These new molecules serve as a foundation to access a range of new pyridinophane small molecules and applications in future work.

Sensing Zn2+ in Aqueous Solution with a Fluorescent Scorpiand Macrocyclic Ligand Decorated with an Anthracene Bearing Tail

Synthesis of the new scorpiand ligand L composed of a [9]aneN3 macrocyclic ring bearing a CH2CH2NHCH2-anthracene tail is reported. L forms both cation (Zn2+) and anion (phosphate, benzoate) complexes. In addition, the zinc complexes of L bind these anions. The equilibrium constants for ligand protonation and complex formation were determined in 0.1 M NaCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric (pH-metric) titrations. pH Controlled coordination/detachment of the ligand tail to Zn2+ switch on and off the fluorescence emission from the anthracene fluorophore. Accordingly, L is able to sense Zn2+ in the pH range 6-10 down to nM concentrations of the metal ion. L can efficiently sense Zn2+ even in the presence of large excess of coordinating anions, such as cyanide, sulphide, phosphate and benzoate, despite their ability to bind the metal ion.

Functionalized pyridine in pyclen-based iron(iii) complexes: evaluation of fundamental properties

Six iron(iii) pyridinophane complexes were evaluated to determine whether functionalizing the pyridine ring could introduce a handle by which electrochemical properties and thermodynamic stability can be tuned.

Acetate as a model for aspartate-based CXCR4 chemokine receptor binding of cobalt and nickel complexes of cross-bridged tetraazamacrocycles

A number of disease states including WHIM syndrome, HIV infection and cancer have been linked to the chemokine receptor CXCR4. High-affinity CXCR4 antagonist transition metal complexes of configurationally restricted bis-tetraazamacrocyclic ligands have been identified in previous studies. Recently synthesised and structurally characterised Co2+/Co3+ and Ni2+ acetate complexes of mono-macrocycle cross-bridged ligands have been used to mimic their known coordination interaction with the aspartate side chains on binding to CXCR4. Here, X-ray crystal structures for three Co2+/Co3+ acetate complexes and five Ni2+ acetate complexes are presented and demonstrate flexibility in the mode of binding to the acetate ligand concomitantly with the requisite cis-V-configured cross-bridged tetraazamacrocyle. Complexes of the smaller Co3+ metal ion exclusively bind acetate by chelating both oxygens of acetate. Larger Co2+ and Ni2+ metal ions in cross-bridged tetraazamacrocycles show a clear tendency to coordinate acetate in a monodentate fashion with a coordinated water molecule completing the octahedral coordination sphere. However, in unbridged tetraazamacrocycle acetate structures reported in the literature, the coordination preference is to chelate both acetate oxygens. We conclude that the short ethylene cross-bridge restricts the equatorial bulk of the macrocycle, prompting the metal ion to fill the equator with the larger monodentate acetate plus water ligand set. In unbridged ligand examples, the flexible macrocycle expands equatorially and generally only allows chelation of the sterically smaller acetate alone. These results provide insight for generation of optimised bis-macrocyclic CXCR4 antagonists utilising cobalt and nickel ions.

Heterogeneous catalytic water oxidation controlled by coordination geometries of copper(ii) centers with thiolato donors

Heterometallic coordination compounds containing a thiolato dicopper(ii) core increase heterogeneous electrocatalytic activities for water oxidation in proportion to the numbers of vacant dicopper(ii) coordination sites.