N-Heterocyclic carbene bonding to cobalt porphyrin complexes

Co(III) complexes of the pentadentate NHC ligand PY4Im: carbene-induced trans influences and the non-disappearing 13C NMR peak

Co(III) complexes of the N-heterocyclic carbene ligand PY4Im (PY4Im = (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene)) having the general formula [(PY4Im)Co(X)](ClO4)n (X = NCMe; n = 3: OH-, N3-, NCS-, ONO-, F-; n = 2: O2CO2-, n = 1; (N3-)3, n = 0) were prepared and structurally characterised. X-ray structural data are consistent with the presence of a trans influence due to the coordinated carbene carbon, and this is also supported by computational results. 13C NMR spectra of the complexes did not display peaks corresponding to the carbene carbon, except in the case of the [(PY4Im)Co(O2CO)]+ cation, where a peak at δ = 170.21 ppm was observed. However, HMBC spectra allowed indirect determination of the chemical shifts of the carbene carbon in the remaining complexes, owing to the geometry of the PY4Im ligand. Calculated 13C chemical shifts for the complexes showed very good agreement with the experimental values for all but the carbene carbon atoms in all cases.

Inducing ring distortions in unsubstituted metallophthalocyanines using axial N-heterocyclic carbenes

A series of metallophthalocyanine (PcM) complexes with axial N-heterocyclic carbene ligands (NHC; 1,3-diisopropylimidazol-2-ylidene (DIP) and 1,3-dimethylbenzimidazol-2-ylidene (DMB)) were prepared and structurally characterized. PcCoII(DIP), PcZnII(DIP), and PcZnII(DMB) are five-coordinate complexes with mild dome-type Pc-ring distortions, while PcFeII(DIP)2 is six-coordinate and has a very large ruffle-type ring-distortion with respect to typical PcM(L)2 systems. The distortion is induced by the highly steric axial DIP ligands. The distortions were quantified and classified by their bond lengths and torsion angles, and according to the normal-coordinate structural decomposition (NSD) analysis. Upon ligation of the NHC, the insoluble PcM materials were solublized in common organic solvents, with typical UV-visible Q-band maxima observable between 658 and 677 nm; the increased solubility is rationalized in terms of the reduced solid-state aggregation of the complexes, attributable to the axial ligation.

Synthesis of New Cobalt(III) Meso-Porphyrin Complex, Photochemical, X-ray Diffraction, and Electrical Properties for Photovoltaic Cells

The present work describes the preparation and characterization of a new cobalt(III) porphyrin coordination compound named (chlorido)(nicotinoylchloride)[meso-tetra(para-chlorophenyl)porphyrinato]cobalt(III) dichloromethane monosolvate with the formula [Co^III(TClPP)Cl(NTC)]·CH₂Cl₂ (4). The single-crystal X-ray molecular structure of 4 shows very important ruffling and waving distortions of the porphyrin macrocycle. The Soret and Q absorption bands of 4 are very red-shifted as a consequence of the very distorted porphyrin core. This coordination compound was also studied by fluorescence and cyclic voltammetry. The efficiency of our four porphyrinic compounds-the H₂TClPP (1) free-base porphyrin, the [Co^II(TClPP)] (2) and [Co^III(TClPP)Cl] (3) starting materials, and the new Co(III) metalloporphyrin [Co^III(TClPP)Cl(NTC)]·CH₂Cl₂ (4)-as catalysts in the photochemical degradation was tested on malachite green (MG) dye. The current voltage of complexes 3 and 4 was also studied. Electrical parameters, including the saturation current density (J_s) and barrier height (ϕ_b), were measured.

Transfer Hydrogenation of Ketones Catalyzed by Symmetric Imino-N-heterocyclic Carbene Co(III) Complexes

The synthesis of new moisture-sensitive imine-functionalized N-heterocyclic carbene (NHC) precursor salts [1-(2-[(hydroxyl-benzylidene)-amino]-ethyl)-3-R-3H-imidazole-1-ium bromide; R = methyl (1a), ethyl (1b), and benzyl (1c)] is reported. Subsequent deprotonation of 1a-c and coordination of the in situ generated NHC ligands to CoBr₂ led to the isolation of air-stable six-coordinate Co(III) complexes 2a-c, respectively. All the salts and complexes were fully characterized. Single-crystal X-ray analysis of 2a and 2c showed octahedral Co centers hexacoordinated to two NHC carbons, two imine nitrogen atoms, and two phenolate oxygens in the form [C^N^O(Co³⁺)C^N^O]. The complexes were used in the catalytic transfer hydrogenation (CTH) of a range of ketones in 2-propanol as the solvent and hydrogen donor. Based on a low catalyst concentration of 0.4 mol %, significant conversions in the range of 70-99% were recorded at high turnover frequencies up to 1635 h^-1. A mechanism to account for the steps involved in the CTH of cyclohexanone by complex 2a is proposed and supported by data from cyclic voltammetry, low-resolution mass spectrometry, UV, and IR spectroscopic techniques.

Effect of the coordination of π-acceptor 4-cyanopyridine ligand on the structural and electronic properties ofmeso-tetra(para-methoxy) andmeso-tetra(para-chlorophenyl) porphyrin cobalt(ii) coordination compounds. Application in the catalytic degradation of methylene blue dye

To examine the influence of both the important π-acceptor character of the 4-cyanopyridine ligand and the nature of the para-substituted phenyls of meso-porphyrins on the electronic, electrochemical and structural properties of cobaltous metalloporphyrins, we prepared and fully characterized two coordination compounds: the (4-cyanopyridine)[meso-tetra(para-methoxyphenyl)porphyrinato]cobalt(ii) and the (4-cyanopyridine)[meso-tetra(para-chlorophenyl)porphyrinato]cobalt(ii) with the [Co^II(TMPP)(4-CNpy)] and [Co^II(TClPP)(4-CNpy)] formulas (complexes 1–2). The solution structures of compounds 1–2 were confirmed by ¹H NMR spectroscopy and mass spectrometry methods. They were further characterized by cyclic voltammetry and photoluminescence studies. The X-ray molecular structure data show that the Co-TClPP-4-NCpy derivative (2) exhibits high ruffling deformation compared to that of the Co-TMPP-4-CNpy species (1). Notably, the crystal packing of complex 1 shows the formation of Co⋯Co supramolecular dimers with a distance of 5.663 Å. As an application of our two cobaltous compounds, an investigation involving complexes 1–2 in the degradation of the methylene blue dye in the presence and absence of H₂O₂ in aqueous solutions was carried out. These promising results show that 1–2 can be used as catalysts in the degradation processes of dyes.

Preparation and UV/vis, IR, MS, ¹H NMR, cyclic voltammetry and molecular structures of two new Co(ii) complexes with para-methoxy-phenyl and para-chloro meso-porphyrins and 4-cyanopyridine ligand (1–2). Catalytic oxidation data of MB dye using 1–2.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Luminescent ruffled iridium(iii) porphyrin complexes containing N-heterocyclic carbene ligands: structures, spectroscopies and potent antitumor activities under dark and light irradiation conditions

A panel of iridium(iii) porphyrin complexes containing axial N-heterocyclic carbene (NHC) ligand(s) were synthesized and characterized. X-ray crystal structures of the bis-NHC complexes [Ir^III(ttp)(IMe)₂]⁺ (2a), [Ir^III(oep)(BIMe)₂]⁺ (2d), [Ir^III(oep)(I ⁱ Pr)₂]⁺ (2e) and [Ir^III(F₂₀tpp)(IMe)₂]⁺ (2f) display ruffled porphyrin rings with mesocarbon displacements of 0.483-0.594 Å and long Ir-C_NHC bonds of 2.100-2.152 Å. Variable-temperature ¹H NMR analysis of 2a reveals that the macrocycle porphyrin ring inversion takes place in solution with an activation barrier of 40 ± 1 kJ mol^-1. The UV-vis absorption spectra of Ir^III(por)-NHC complexes display split Soret bands. TD-DFT calculations and resonance Raman experiments show that the higher-energy Soret band is derived from the ¹MLCT dπ(Ir) → π*(por) transition. The near-infrared phosphorescence of Ir^III(por)-NHC complexes from the porphyrin-based ³(π, π*) state features broad emission bands at 701-754 nm with low emission quantum yields and short lifetimes (Φ _em < 0.01; τ < 4 μs). [Ir^III(por)(IMe)₂]⁺ complexes (por = ttp and oep) are efficient photosensitizers for ¹O₂ generation (Φ _so = 0.64 and 0.88) and are catalytically active in the light-induced aerobic oxidation of secondary amines and arylboronic acid. The bis-NHC complexes exhibit potent dark cytotoxicity towards a panel of cancer cells with IC₅₀ values at submicromolar levels. The cytotoxicity of these complexes could be further enhanced upon light irradiation with IC₅₀ values as low as nanomolar levels in association with the light-induced generation of reactive oxygen species (ROS). Bioimaging of [Ir^III(oep)(IMe)₂]⁺ (2c) treated cells indicates that this Ir complex mainly targets the endoplasmic reticulum. [Ir^III(oep)(IMe)₂]⁺ catalyzes the photoinduced generation of singlet oxygen and triggers protein oxidation, cell cycle arrest, apoptosis and the inhibition of angiogenesis. It also causes pronounced photoinduced inhibition of tumor growth in a mouse model of human cancer.

High-Oxidation-State 3d Metal (Ti-Cu) Complexes with N-Heterocyclic Carbene Ligation

High-oxidation-state 3d metal species have found a wide range of applications in modern synthetic chemistry and materials science. They are also implicated as key reactive species in biological reactions. These applications have thus prompted explorations of their formation, structure, and properties. While the traditional wisdom regarding these species was gained mainly from complexes supported by nitrogen- and oxygen-donor ligands, recent studies with N-heterocyclic carbenes (NHCs), which are widely used for the preparation of low-oxidation-state transition metal complexes in organometallic chemistry, have led to the preparation of a large variety of isolable high-oxidation-state 3d metal complexes with NHC ligation. Since the first report in this area in the 1990s, isolable complexes of this type have been reported for titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). With the aim of providing an overview of this intriguing field, this Review summarizes our current understanding of the synthetic methods, structure and spectroscopic features, reactivity, and catalytic applications of high-oxidation-state 3d metal NHC complexes of titanium to copper. In addition to this progress, factors affecting the stability and reactivity of high-oxidation-state 3d metal NHC species are also presented, as well as perspectives on future efforts.

Mesomeric betaine – N-heterocyclic carbene interconversions of 1,2,4-triazolium-phenolates. Sulfur, selenium, and borane adduct formation

Triazolium-phenolates form carbenic tautomers which can be trapped with boranes as a new heterocyclic ring system.

Springloaded porphyrin NHC hybrid rhodium(III) complexes: carbene dissociation and oxidation catalysis

Porphyrin rhodium(III) complexes accommodate one or two NHC ligands in the apical position, which leads to severe porphyrin distortion and dearomatization. The strain in the bis(carbene) complex induces facile carbene dissociation and the formation of a catalytically active site for alcohol oxidation.