X-ray crystallography, NMR spectroscopy and computations reveal a remarkable diversity of structures in a series of diiridium complexes bridged by μ₂-oxamidato and μ₂-dithioxamidato ligands.

Abstract

Six new diiridium complexes containing 2-methyl-6-phenylpyridyl as the cyclometalating ligand with a μ₂-oxamidato or a μ₂-dithioxamidato ligand as the bridge have been synthesized in 60–73 % yields. These complexes were revealed by multinuclear NMR spectroscopy to contain inseparable mixtures of diastereomers (rac, ΔΔ/ΛΛ and meso, ΔΛ) with bridges in anti and syn configurations. The remarkable variety of isomers present was confirmed by X-ray crystallography on single crystals grown from mixtures of each complex. In one complex with a N,N’-bis(4-trifluoromethylphenyl)-μ₂-oxamidato bridge, two single crystals of anti and syn isomers were structurally determined. Two single crystals of the μ₂-dithioxamidato bridge complex were found to contain rac and meso forms of the syn isomer. Hybrid DFT computations on the four isomers of each diiridium complex revealed negligible energetic preferences for one isomer despite the methyl groups in the 2-methyl-6-phenylpyridyl cyclometalating ligands being close to the neighboring methyl groups and the bridge, thus supporting the experimental findings of isomer mixtures. Two distinct broad emissions with maxima at 522–529 nm and at 689–701 nm observed in these complexes in dichloromethane are attributed to mixed metal-ligand to ligand charge transfer (MLLCT) excited states involving the pyridyl and bridge moieties respectively with the aid of electronic structure computations.

Pyrazole-based PNN(H) Rh^I complexes are active catalysts for the dehydrocoupling of amine boranes to produce cyclic oligomers. Efficient dehydrohalogenation of the Rh chloride precursor complex and ligand deprotonation are essential for the realization of high catalytic activity.

Abstract

Coordination of a pyridine-pyrazole-based PNN(H) ligand to Rh^I produces a family of neutral (1) and cationic (2Cl) Rh^I complexes. Deprotonation of the parent Rh chloride complex with LiNiPr₂ results in formation of a dinuclear LiCl bridged species 3 bearing a pyrazolate fragment. Complexes 1, 2Cl and 3 were tested as precatalyst for the dehydrocoupling of amine boranes. All complexes studied show activity for the formation of cyclic oligomers with N-methylcyclotriborazane as the main product. Base activation of the neutral Rh chloride complex 1 produces catalyst systems that are significantly more active than the parent system, suggesting that dehydrohalogenation of the Rh chloride precatalyst 1 is one of the key steps for catalyst formation.

A series of new octahedral bis-3,6-di-tert-butylcatecholates based on tin(IV) containing metal-coordinated N-donor ligands (substituted iminopyridines and diazabutadienes) has been synthesized and structurally characterized. The compounds, both solid and in solution, are intensely colored. They absorb in the visible region of the spectrum. Charge transfer between the catecholate donor and the diimine acceptor is responsible for the absorption. This observation is in good agreement with the DFT calculations and with the electrochemical studies carried out by CV.

Cyclohexane epoxide with highly active epoxy groups is an important intermediate in the preparation of fine chemicals. However, the epoxidation path of cyclohexene is difficult to be controlled due to the allyl oxidation of cyclohexene and the ring opening of cyclohexane epoxide in the process of cyclohexene epoxidation to cyclohexane oxide. This work mainly reviewed the structure-activity relationships and synthesis processes of a series of heterogeneous transition metal-based catalysts based on cyclohexene epoxidation reaction, including molybdenum(Mo)-based, tungsten(W)-based, vanadium(V)-based, titanium(Ti)-based, cobalt(Co)-based,, etc. catalysts. First, the mechanism of cyclohexene epoxidation by transition metal-based catalysts was collated from the perspective of catalytic active centers. Then, the current status of research on cyclohexene epoxidation catalysts was summarized from the perspective of catalyst support. At the same time, the differences between alkyl hydroperoxide, hydrogen peroxide (H2O2), and oxygen (O2) as oxidants were analyzed. Finally, the main factors affecting the catalytic performance were summarized and the reasonable opinions on the design of catalysts were put forward. The above work provided some scientific support for the development of olefin epoxidation industry.

Four isomeric copper(II) complexes based on a pair of ligand isomers that each yield a pair of linkage isomers have been synthesized and characterized. The effect of isomerism on the electronic structure and antiproliferation activity of the complexes is described.

Abstract

A series of isomeric bis(alkylthiocarbamate) copper complexes have been synthesized, characterized, and evaluated for antiproliferation activity. The complexes were derived from ligand isomers with 3-methylpentyl (H₂L²) and cyclohexyl (H₂L³) backbone substituents, which each yield a pair of linkage isomers. The thermodynamic products CuL^2a/3a have two imino N and two S donors resulting in three five-member chelate rings (555 isomers). The kinetic isomers CuL^2b/3b have one imino and one hydrazino N donor and two S donors resulting in four-, six-, and five-member rings (465 isomers). The 555 isomers have more accessible Cu^II/I potentials (E_1/2=−811/−768 mV vs. ferrocenium/ferrocene) and lower energy charge transfer bands than their 465 counterparts (E_1/2=−923/-854 mV). Antiproliferation activities were evaluated against the lung adenocarcinoma cell line (A549) and nonmalignant lung fibroblast cell line (IMR-90) using the MTT assay. CuL^2a was potent (^A549EC₅₀=0.080 μM) and selective (^IMR-90EC₅₀/^A549EC₅₀=25) for A549. Its linkage isomer CuL^2b had equivalent A549 activity, but lower selectivity (^IMR-90EC₅₀/^A549EC₅₀=12.5). The isomers CuL^3a and CuL^3b were less potent with ^A549EC₅₀ values of 1.9 and 0.19 M and less selective with ^IMR-90EC₅₀/^A549EC₅₀ ratios of 2.3 and 2.65, respectively. There was no correlation between reduction potential and A549 antiproliferation activity/selectivity.

Compounds belonging to the palmierite structure (Zn_3−xM_x)A₂O₈ (M=Co/Ni/Cu; A=V, P) were prepared and examined for their optical properties. The substitution of bi-valent transition elements in place of Zn²⁺ ions appear to reduce the band gap and, in a way, akin to band gap engineering. The DFT calculations were carried out to support the findings of the study.

Abstract

Compounds belonging to the palmierite structure, (Zn_3−xM_x)A₂O₈ (M=Co, Ni, Cu and A=V, P) have been prepared employing solid state methods. The transition metal substituted compounds of Zn₃V₂O₈ exhibits colors that are unique varying from mint green to forest green for Co²⁺ ions. The observed colors were understood based on the allowed d-d transitions and metal to metal charge transfer (MMCT) transitions. The MMCT transitions involve partially filled d-orbitals of Co²⁺ (3d⁷), Ni²⁺ (3d⁸), and Cu²⁺ (3d⁹) ions and the V⁵⁺ ions (3d⁰). The spinel compounds, Zn_2−xCo_xMO₄ (M=Ti, Sn) were also prepared to understand the MMCT transitions in the compounds. Band structure calculations were carried out to understand the participating orbitals near the Fermi level and the band gap. The calculations support the idea that the substitution of transition elements in the palmierite structure reduces the overall band gap from 3.18 eV for Zn₃V₂O₈ to 2.61 eV Zn_2.5Co_0.5V₂O₈ compound. This indicates the substitution of transition elements provide a tool towards band gap engineering.

In this manuscript, the synthesis, characterization, and crystal structures of Zn^II complexes bearing hydrazone ligands are described. The complexes were evaluated as possible catalysts in the ketone-amine-alkyne coupling reaction. Insights into the catalytic activity of the Zn^II complexes were obtained by electronic structure investigations, using DFT calculations.

Abstract

Two new Zn(II) complexes bearing tridentate hydrazone-based ligands with NNO or NNS donor atoms were synthesised and characterised by elemental analysis, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and single crystal X-ray diffraction methods. These complexes, together with four previously synthesised analogues, having hydrazone ligands with a NNO donor set of atoms, were successfully employed as catalysts in the ketone-amine-alkyne (KA²) coupling reaction, furnishing tetrasubstituted propargylamines, compounds with unique applications in organic chemistry. DFT calculations at the CAM-B3LYP/TZP level of theory were performed to elucidate the electronic structure of the investigated Zn(II) complexes, excellently correlating the structure of the complexes to their catalytic reactivity.

Three nonheme Fe^II complexes with pentadentate electron-rich ligands display an enhanced reactivity with H₂O₂ and improved catalytic properties in the presence of AcOH. The binding of AcOH to the metal center promotes the reaction with the oxidant and leads to the formation of an active species which retains the acetato ligand.

Abstract

Three new amine/pyridine Fe^II complexes bearing pentadentate ligand with one, two or three electron enriched 4-methoxy-3,5-dimethylpyridine were used as catalysts for the oxidation of small organic molecules by hydrogen peroxide. The distribution of products formed suggests that these ligands are not enough electron donating to promote the O−O heterolytic cleavage of the oxidant in order to generate selective Fe^V(O) species. Using acetic acid in the reaction mixtures results in a significant increase of the efficiency of these catalytic systems. Our investigations show that the use of AcOH leads to the protonation/dissociation of a pyridyl moiety and the formation of (N₄ )Fe^II(OAc)(OH) species. These complexes readily react with excess hydrogen peroxide to yield (N₄ )Fe^III(OAc)(OOH) intermediates. These latter intermediates are proposed to evolve into (N₄ )Fe^IV(OAc)(O), which are more efficient than the usual (N₄ )Fe^IV(O) and (N₅ )Fe^IV(O).

Isolation of two large dodecameric {Nb₁₂O₂₁} clusters in the same lattice was achieved by combination of NbCl₄ ⋅ 2THF with anthracene-9-carboxylic acid. The clusters are constituted of octahedral niobium(V) centers associated with rarely seen square pyramidal niobium(V) cations.

Abstract

The reactivity of the complexing anthracene-9-carboxylate ligand has been investigated with a niobium(IV) tetrachloride precursor (NbCl₄ ⋅ 2THF) in isopropanol solvent. This resulted in the crystallization of a molecular assembly containing two distinct {Nb₁₂O₂₁} cores surrounded by multiple isopropanolate and anthracenoate ligands. The compound is formulated [Nb₁₂₍(μ₃-O)₃(μ-O)₁₈(C₁₅H₉O₂)₈(OⁱPr)₁₀] ⋅ [Nb₁₂(μ₃-O)₂(μ-O)₁₉(C₁₅H₉O₂)₈(OⁱPr)₁₀] illustrating the two different dodecameric oxo-clusters, for which the niobium(IV) precursor was oxidized in the niobium(V) state during the reactional process. The two distinct {Nb₁₂O₂₁} units mainly differs by the environment of the niobium centers, which exhibits unexpected five-fold coordination (square pyramid) for some of them, together with the classical six-fold coordination (octahedron) as usually found for niobium(V). In the crystallization process, the. IR spectroscopy was used to analyze the esterification reaction occurring between the anthracene acid an isopropanolate ligands responsible of the production of water used in the oxo-condensation of the niobium centers. ⁹³Nb Solid state NMR was tentatively used to assess the occurrence of the different niobium environments.

Protonation of polyhydride complexes: conjugated base matters! The protonation of a neutral molybdenum(IV) tetrahydride complex was investigated using a range of proton sources. Whereas simple acid-base process was achieved in the case of a phosphonium, subsequent reactivity of the resulting cationic pentahydride species with conjugated base was typically observed.

Abstract

The reaction of [MoH₄(depe)₂] (1) (depe=1,2-bis(diethylphosphino)ethane) with different proton sources (HP^tBu₃ ⁺, C₆H₅COOH, C₆F₅COOH, H₂SO₄) was investigated. Whereas complete conversion of 1 into its protonated form is observed in the presence of the phosphonium salt, [MoH₅(depe)₂]⁺ only transiently forms upon treatment with the other acids. Further reactivity of the pentahydride complex is indeed noticed with the conjugated base, typically resulting in the formation of neutral (C₆F₅COOH, H₂SO₄) or cationic (PhCOOH) molybdenum dihydride complexes. The compounds were characterized by NMR spectroscopy and X-ray crystallographic studies were performed when suitable crystals were obtained.