Iron(II) Bis(imidazole) Derivatives of a Binuclear Porphyrin Model: Crystal Structures and Mössbauer Properties

Perspectives on Ligand Properties of N-Heterocyclic Carbenes in Iron Porphyrin Complexes

There has been considerable research interest in the ligand nature of N-heterocyclic carbenes (NHCs). In this work, two six-coordinate NHC iron porphyrin complexes [Fe^II(TTP)(1,3-Me₂Imd)₂] (TTP = tetratolylporphyrin, 1,3-Me₂Imd = 1,3-dimethylimidazol-2-ylidene) and [Fe^III(TDCPP)(1,3-Me₂Imd)₂]ClO₄ (TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) are reported. Single-crystal X-ray characterizations demonstrate that both complexes have strongly ruffled conformations and relatively perpendicular ligand orientations which are forced by the sterically bulky 1,3-Me₂Imd NHC ligands. Multitemperature (4.2-300 K) and high magnetic field (0-9 T) Mössbauer and low-temperature (4.0 K) EPR spectroscopies definitely confirmed the low-spin states of [Fe^II(TTP)(1,3-Me₂Imd)₂] (S = 0) and [Fe^III(TDCPP)(1,3-Me₂Imd)₂]ClO₄ (S = 1/2). The similarity of 1,3-Me₂Imd and imidazole, as well as the well-established correlations between the ligand nature and spectroscopic characteristics of [Fe^II,III(Porph)(L)₂]^0,+ (Porph: porphyrin; L: planar base ligand) species, allowed direct comparisons between the pair of ligands which revealed for the first time that NHC has a stronger π-acceptor ability than imidazoles, in addition to its very strong σ-donation.

Chirogenesis in Zinc Porphyrins: Theoretical Evaluation of Electronic Transitions, Controlling Structural Factors and Axial Ligation

In the present work, sixteen different zinc porphyrins (possessing different meso substituents) with and without a chiral guest were modelled using DFT and TD-DFT approaches in order to understand the influence of various controlling factors on electronic circular dichroism (ECD) spectra. Two major aspects are influenced by these factors: excitation energy of the electronic transitions and their intensity. In the case of excitation energy, the influence increases in the following order: orientation of the peripheral substituents Collapse

Key Words

• circular dichroism
• density functional calculations
• porphyrins
• supramolecular chemistry
• transition metals

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MESH Headings Collapse

Grants

• PUTJD749 Estonian Research Council
• PRG399 Estonian Research Council
• 828779 European Union's H2020-FETOPEN
• Estonian Research Council

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Affiliation(s)

Irina Osadchuk Department of Chemistry and BiotechnologySchool of ScienceTallinn University of Technology AddressAkadeemia tee 1512618TallinnEstonia ICGMUniv MontpellierCNRS, ENSCMMontpellierFrance
Riina Aav Department of Chemistry and BiotechnologySchool of ScienceTallinn University of Technology AddressAkadeemia tee 1512618TallinnEstonia
Victor Borovkov Department of Chemistry and BiotechnologySchool of ScienceTallinn University of Technology AddressAkadeemia tee 1512618TallinnEstonia
Eric Clot ICGMUniv MontpellierCNRS, ENSCMMontpellierFrance

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Bis(1-methyl-imidazole)[meso-α,α,α,α-tetra-kis(o-nicotinamido-phen-yl)porphinato]iron(II)-1-methyl-imidazole-tetra-hydro-furan (1/1/1.5)

In the title compound, [FeII(C68H44N12O4)(C4H6N2)2]·C4H6N2·1.5C4H8O, the central FeII ion is coordinated by four pyrrole N atoms of the porphyrin core and two N atoms of the 1-methyl-imidazole ligands in the axial sites. One 1-methyl-imidazole and one and a half tetra-hydro-furan solvent mol-ecules are also present in the asymmetric unit. The complex exhibits a near planar porphyrin core conformation, in which the iron centre is slightly displaced towards the hindered porphyrin side (0.01 Å). The average Fe-Np (Np refers to the pyrrole nitro-gen atoms in the porphyrin) bond length is 1.990 (9) Å, and the axial Fe-NIm (NIm refers to the imidazole nitro-gen atoms) bond lengths are 1.993 (3) and 2.004 (3) Å. The dihedral angle between the two coordinated 1-methyl-imidazole planes is 56.6 (2)°. The dihedral angles between the 1-methyl-imidazole planes and the planes of the closest Fe-Np vector are 16.8 (2) and 39.8 (2)°. N-H⋯N and N-H⋯O inter-actions are observed in the crystal structure.

Selective Convergence to Atropisomers of a Porphyrin Derivative Having Bulky Substituents at the Periphery

Four kinds of possible atropisomers of a porphyrin derivative (1), having mesityl groups at one of the o-positions of each meso-aryl group, can be selectively converged to targeted atropisomers among the four isomers (αααα, αααβ, αβαβ, and ααββ) under appropriate conditions for each atropisomer. For example, protonation and subsequent neutralization of a free base porphyrin (H₂-1) induces a convergence reaction to the αβαβ atropisomer, H₂-1-αβαβ, from an atropisomeric mixture. The αααα isomer, H₂-1-αααα, was also obtained by heating a solution of H₂-1 in CHCl₃ in 60% isolated yield, probably owing to a template effect of the solvent molecule. Remarkably, when an atropisomeric mixture of its zinc complex, Zn-1, was heated at 70 °C in a ClCH₂CH₂Cl/MeOH mixed solvent, crystals composed of only Zn-1-αααα were formed. The hydrophobic space formed by the four mesityl groups in the αααα isomer can be used for repeatable molecular encapsulation of benzene, and the encapsulation structure was elucidated by powder X-ray diffraction analysis. Heating the solid of an atropisomeric mixture of Zn-1 to 400 °C afforded the ααββ isomer almost quantitatively. On the other hand, the solid of H₂-1-αααα can be converted by heating, successively to H₂-1-αααβ at 286 °C and then to H₂-1-ααββ at 350 °C.

Intermolecular Interactions and Intramolecular Couplings of Binuclear Porphyrin Models for Cytochrome c Oxidase

Cytochrome c oxidase (CcO) has a binuclear active site composed of a high-spin heme group and a tris-histidine-ligated copper ion (Cu_B). By using two different porphyrin models derived by Gunter (H₂TPyPP) and us (H₂TImPP), we have isolated several mono- and binuclear complexes including one carbonyl and three chloride derivatives which are determined by 100 K single-crystal X-ray. Low-temperature (4 K) EPR and multitemperature (295-25 K) Mössbauer investigations on the products not only confirmed the spin states of the two metal ions (S = 5/2 Fe³⁺ and S = 1/2 Cu²⁺) but also revealed the intermolecular interactions and intramolecular couplings which are in accordance with the crystal structural features.

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.