Mechanistic Insights into Nitrile Activation by Cobalt(III)-Hydroperoxo Intermediates: The Influence of Ligand Basicity

The versatile applications of nitrile have led to the widespread use of nitrile activation in the synthesis of pharmacologically and industrially valuable compounds. We reported the activation of nitriles using mononuclear cobalt(III)-hydroperoxo complexes, [CoIII(Me3-TPADP)(O2H)(RCN)]2+ [R = Me (2) and Ph (2Ph)], to form cobalt(III)-peroxyimidato complexes, [CoIII(Me3-TPADP)(R-C(=NH)O2)]2+ [R = Me (3) and Ph (3Ph)]. The independence of the rate on the nitrile concentration and the positive Hammett value of 3.2(2) indicated that the reactions occur via an intramolecular nucleophilic attack of the hydroperoxide ligand to the coordinated nitrile carbon atom. In contrast, the previously reported cobalt(III)-hydroperoxo complex, [CoIII(TBDAP)(O2H)(CH3CN)]2+ (2TBDAP), exhibited the deficiency of reactivity toward nitrile. The comparison of pKa values and redox potentials of 2 and 2TBDAP showed that Me3-TPADP had a stronger ligand field strength than that of TBDAP. The density functional theory calculations for 2 and 2TBDAP support that the strengthened ligand field in 2 is mainly due to the replacement of two tert-butyl amine donors in TBDAP with methyl groups in Me3-TPADP, resulting in the compression of the Co-Nax bond lengths. These results provide mechanistic evidence of nitrile activation by the cobalt(III)-hydroperoxo complex and indicate that the basicity dependent on the ligand framework contributes to the ability of nitrile activation.

A side-on Mn(III)-peroxo supported by a non-heme pentadentate N3Py2 ligand: synthesis, characterization and reactivity studies

A mononuclear manganese(iii)-peroxo complex [MnIII(N3Py2)(O2)]+ (1a) bearing a non-heme N,N'-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N'-(pyridin-2-ylmethyl)ethane-1,2-diamine (N3Py2) ligand was synthesized by the reaction of [Mn(N3Py2)(H2O)](ClO4)2 (1) with hydrogen peroxide and triethylamine in CH3CN at 25 °C. The reactivity of 1a in aldehyde deformylation using 2-phenyl propionaldehyde (2-PPA) was studied and the reaction kinetics was monitored by UV-visible spectroscopy. A kinetic isotope effect (KIE) = 1.7 was obtained in the reaction of 1a with 2-PPA and α-[D1]-PPA, suggesting nucleophilic character of 1a. The activation parameters ΔH‡ and ΔS‡ were determined using the Eyring plot while Ea was obtained from the Arrhenius equation by performing the reaction between 288 and 303 K. Hammett constants (σp) of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H, and Me) were linear with a slope (ρ) = 3.0. Computational study suggested that the side-on structure of 1a is more favored over the end-on structure and facilitates the reactivity of 1a.

Manganese Catalysts with Tetradentate N-donor Pyridine-Appended Bipiperidine Ligands for Olefin Epoxidation Reactions: Ligand Electronic Effect and Mechanism

In this work, we described an electron-rich manganese mesoPYBP catalyst, Mn-SR-mesoPYBP(ClO4)2, by introducing electron-donating substituents on the mesoPYBP ligand. We optimized the catalytic performance in olefin epoxidation with H2O2 in the presence of acetic acid. The electron paramagnetic resonance (EPR) and cyclic voltammetry (CV) studies supported that an electronic effect could stabilize the high-valent intermediates in the catalytic cycles of the catalyst, which largely improved the catalytic performance and the reactivity of olefin epoxidation.

A MnII MnIII -Peroxide Complex Capable of Aldehyde Deformylation

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn₂ RNRs superoxide (O₂ ^.- ) was postulated to react with the Mn^II ₂ core to yield a Mn^II Mn^III -peroxide moiety. The reactivity of complex 1 ([Mn^II ₂ (O₂ CCH₃ )₂ (BPMP)](ClO₄ ), where HBPMP=2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol) towards O₂ ^.- was investigated at -90 °C, generating a metastable species, 2. The electronic absorption spectrum of 2 displayed features (λ_max =440, 590 nm) characteristic of a Mn^II Mn^III -peroxide species, representing just the second example of such. Electron paramagnetic resonance and X-ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a Mn^II Mn^III -peroxide complex. Unlike all other previously reported Mn₂ -peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O₂ -activation in Class Ib Mn₂ RNRs, and the highly reactive intermediates in their catalytic cycle.

A putative heme manganese(v)-oxo species in the C–H activation and epoxidation reactions in an aqueous buffer

Synthesis and reactivity studies of manganese(v)-oxo species in the C–H activation of alkyl hydrocarbons and epoxidation of cyclohexene in aqueous conditions are investigated.

Mimicking Class I b Mn2 -Ribonucleotide Reductase: A MnII2 Complex and Its Reaction with Superoxide

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn₂ ) active site in class I b RNRs that requires superoxide anion (O₂^.- ), rather than dioxygen (O₂ ), to access a high-valent Mn₂ oxidant. Complex 1 ([Mn₂ (O₂ CCH₃ )(N-Et-HPTB)](ClO₄ )₂ , N-Et-HPTB=N,N,N',N'-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane) was synthesised in high yield (90 %). 1 was reacted with O₂^.- at -40 °C resulting in the formation of a metastable species (2). 2 displayed electronic absorption features (λ_max =460, 610 nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a Mn^II Mn^III entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of Mn^II₂ in 1 to Mn^II Mn^III for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a Mn^II Mn^III -peroxide complex. 2 was capable of oxidizing ferrocene and weak O-H bonds upon activation with proton donors. Our findings provide support for the postulated mechanism of O₂^.- activation at class I b Mn₂ RNRs.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation

The donor atom type and diazacyclo backbone of the ligands and solvent of coordination dictate the Ni(ii) spin state (4, LS; 1–3, 5, HS) and catalytic activity of complexes.

An amphoteric reactivity of a mixed-valent bis(μ-oxo)dimanganese(III,IV) complex acting as an electrophile and a nucleophile

A mixed-valent bis(μ-oxo)dimanganese(III,IV) complex, [(dpaq)Mn(III)(O)2Mn(IV)(dpaq)](+) (1), was prepared by reacting a hydroxomanganese(III) complex, [(dpaq)Mn(III)(OH)](+), with hydrogen peroxide in the presence of triethylamine. The mixed-valent bis(μ-oxo)dimanganese(III,IV) complex (1) was well characterised by UV-vis, EPR and CSI-MS techniques. The electrophilic reactivity of 1 was investigated in the oxidation of 2,6-di-tert-butylphenol derivatives by 1, in which the relative rate afforded a good Hammett correlation with a ρ value of -1.0. The nucleophilic character of 1 was then investigated in aldehyde deformylation reactions, using 2-phenylpropionaldehyde (2-PPA) and benzaldehyde derivatives as substrates. In contrast to the case of the reaction of 1 with 2,6-di-tert-butylphenol derivatives, a positive ρ value of 0.89 was obtained in the Hammett plot, demonstrating that the bis(μ-oxo)-dimanganese(III,IV) complex is an active nucleophilic oxidant. Thus, 1 exhibited an amphoteric reactivity in both electrophilic and nucleophilic oxidative reactions.

Development of an efficient magnetically separable nanocatalyst: theoretical approach on the role of the ligand backbone on epoxidation capability

The epoxidation property of an asymmetric iron complex has been experimentally and theoretically verified. This catalyst further conjugated with dopamine modified Fe₃O₄ to obtain magnetically separable nano-catalyst.