Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere

The Inhibited Dehydration Polycondensation Tendency of a Titanocene Aqua Complex Attributed to the Formation of Hydrogen Bonds Between the Ligand Water and Py-NO: Mechanism and Application

Titanocene aqua complexes (TACs) demonstrate versatile activities that critically depend on maintaining their molecular integrity in aqueous media. However, the strong oxophilicity of Ti(IV) centers renders these complexes prone to undesirable dehydration-condensation reactions, which severely compromise their structural stability. To address this challenge, we developed a simple yet effective strategy for dramatically stabilizing TACs in aqueous environments through the introduction of pyridine-N-oxides (Py-NOs) as auxiliary ligands. Comprehensive mechanistic investigations, including spectroscopic analyses and DFT calculations, reveal that the enhanced stability originates primarily from the formation of intermolecular hydrogen bonds between TACs and Py-NOs. These interactions effectively suppress Ti(IV)-mediated hydrolysis pathways while preserving the complexes' functionality.

Influence of Asymmetrical Ligand Substitution on the Formation, Stability, and Reactivity of Ruthenium(III)-Hypochlorite Complexes

Hypohalites are commonly generated in biological systems, mostly with functions related to defense and immune system response. These hypohalites can bind to metal centers and are known for their strong oxidizing properties that play crucial roles in various biological processes. Herein, we report the synthesis, characterization and reactivity of novel biomimetic Ru(III)-hypochlorite complexes and focus the work on the electronic effects associated with the incorporation of methyl groups in a pentadentate ligand framework in an asymmetric fashion. The intermediates are stable at room temperature and were characterized by mass spectrometry, UV-vis absorption, resonance Raman and EPR spectroscopy. We show for the first time that the resultant ruthenium-hypochlorite adducts function as an electrophilic oxidant and efficiently catalyze hydrogen atom abstraction reactions from phenol and oxygen atom transfer reactions with dimethylsulfide.

Stereoelectronic Tuning of Bioinspired Nonheme Iron(IV)-Oxo Species by Amide Groups in Primary and Secondary Coordination Spheres for Selective Oxygenation Reactions

Two mononuclear iron(II) complexes, [(6-amide2-BPMEN)FeII](OTf)2 (1) and [(6-amide-Me-BPMEN)FeII(OTf)](OTf) (2), supported by two BPMEN-derived (BPMEN = N1,N2-dimethyl-N1,N2-bis(pyridine-2-yl-methyl)ethane-1,2-diamine) ligands bearing one or two amide functionalities have been isolated to study their reactivity in the oxygenation of C-H and C═C bonds using isopropyl 2-iodoxybenzoate (iPr-IBX ester) as the oxidant. Both 1 and 2 contain six-coordinate high-spin iron(II) centers in the solid state and in solution. The 6-amide2-BPMEN ligand stabilizes an S = 1 iron(IV)-oxo intermediate, [(6-amide2-BPMEN)FeIV(O)]2+ (1A). The oxidant (1A) oxygenates the C-H and C═C bonds with a high selectivity. Oxidant 1A, upon treatment with 2,6-lutidine, is transformed into another oxidant [{(6-amide2-BPMEN)-(H)}FeIV(O)]+ (1B) through deprotonation of an amide group, resulting in a stronger equatorial ligand field and subsequent stabilization of the triplet ground state. In contrast, no iron-oxo species could be observed from complex 2 and [(6-Me2-BPMEN)FeII(OTf)2] (3) under similar experimental conditions. The iron(IV)-oxo oxidant 1A shows the highest A/K selectivity in cyclohexane oxidation and 3°/2° selectivity in adamantane oxidation reported for any synthetic nonheme iron(IV)-oxo complexes. Theoretical investigation reveals that the hydrogen bonding interaction between the -NH group of the noncoordinating amide group and Fe═O core smears out the equatorial charge density, reducing the triplet-quintet splitting, and thus helping complex 1A to achieve better reactivity.

Enhanced Reactivities of Iron(IV)-Oxo Porphyrin Species in Oxidation Reactions Promoted by Intramolecular Hydrogen-Bonding

High-valent iron-oxo species are one of the common intermediates in both biological and biomimetic catalytic oxidation reactions. Recently, hydrogen-bonding (H-bonding) has been proved to be critical in determining the selectivity and reactivity. However, few examples have been established for mechanistic insights into the H-bonding effect. Moreover, intramolecular H-bonding effect on both C-H activation and oxygen atom transfer (OAT) reactions in synthetic porphyrin model system has not been investigated yet. In this study, a series of heme-containing iron(IV)-oxo porphyrin species with or without intramolecular H-bonding are synthesized and characterized. Kinetic studies revealed that intramolecular H-bonding can significantly enhance the reactivity of iron(IV)-oxo species in OAT, C-H activation, and electron-transfer reactions. This unprecedented unified H-bonding effect is elucidated by theoretical calculations, which showed that intramolecular H-bonding interactions lower the energy of the anti-bonding orbital of iron(IV)-oxo porphyrin species, resulting in the enhanced reactivities in oxidation reactions irrespective of the reaction type. To the best of the knowledge, this is the first extensive investigation on the intramolecular H-bonding effect in heme system. The results show that H-bonding interactions have a unified effect with iron(IV)-oxo porphyrin species in all three investigated reactions.