Protic Ruthenium Tris(pyrazol-3-ylmethyl)amine Complexes Featuring a Hydrogen-Bonding Network in the Second Coordination Sphere

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization

We outline the generality and requirements for cooperative N₂H₄ capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH₂ stabilization. While the Lewis acids are required to bind N₂H₄, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

[Cr(pincer2-)]2 as an electron shuttle for reductively promoted hydrazine disproportionation

We describe here delivery of hydrazine to a reducing, low oxidation state chromium bound to a proton responsive ligand which has already been deprotonated. Reaction of PhHNNH₂ at a 4 : 1 mole ratio with the bis-(pyrazolate)pyridyl pincer ligated reducing agent [Cr^IIL]₂ gives prompt conversion to [CrL₂(PhNH₂)₂(μ-PhHNN)], with release of NH₃ and C₆H₆, a new disproportionation of the hydrazine, with trapping of PhHNN as its dianion, bridging the two chromium centers. The redox balance of the reaction is discussed, and participation by Brønsted basic sites on the bis-(pyrazolate)pyridyl pincer ligand L^2- is suggested, but no hydrazine protons remain on the pincer in the product.

Incorporation of redox-inactive cations promotes iron catalyzed aerobic C-H oxidation at mild potentials

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K+ or Ba2+ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen = N,N'-bis(salicylidene)ethylenediamine), and by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect (electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C-H bonds was explored. Prior studies on iron salen complexes modified through conventional electron-donating or withdrawing substituents found that only the most oxidizing derivatives were competent catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active. Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative deactivation through formation of the corresponding Fe2-μ-oxo complexes, which were isolated and characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through traditional substituent effects on the ligand.

Tuning the Fe(II/III) Redox Potential in Nonheme Fe(II)-Hydroxo Complexes through Primary and Secondary Coordination Sphere Modifications

The derivatization of the imino-functionalized tris(pyrrolylmethyl)amine ligand framework, N(_Xpi^R)₃ (_XL^R; X = H, Br; R = cyclohexyl (Cy), phenyl (Ph), 2,6- diisopropylphenyl (DIPP)), is reported. Modular ligand synthesis allows for facile modification of both the primary and secondary coordination sphere electronics. The iron(II)-hydroxo complexes, N(_Xpi^R)(_Xafa^R)₂Fe(II)OH (_XL^RFe^IIOH), are synthesized to establish the impact of the ligand modifications on the complexes' electronic properties, including their chemical and electrochemical oxidation. Cyclic voltammetry demonstrates that the Fe(II/III) redox couple spans a 400 mV range across the series. The origin of the shifted potential is explained based on spectroscopic, structural, and theoretical analyses of the iron(II) and iron(III) compounds.