Conformational Landscapes of Metal(II) Porphyrinato, Chlorinato, and Morpholinochlorinato Complexes

Controlling Chirogenic Effects in Porphyrin Based Supramolecular Systems: Theoretical Analysis Versus Experimental Observations

Electronic circular dichroism (ECD) spectroscopy is a widely employed method for studying chiral analysis, requiring the presence of a chromophore close to a chiral centre. Porphyrinoids are found to be one of the best chromophoric systems serving for this purpose and enabling the application of ECD spectroscopy for chirality determination across diverse classes of organic compounds. Consequently, it is crucial to understand the induction mechanisms of ECD in the porphyrin-based complexes. The present study explores systematically the influence of secondary chromophores, bonded to an achiral zinc porphyrin or to chiral guest molecules, on the B-region of ECD spectra using the time-dependent density functional theory (TD-DFT) calculations. The study analyses the impact of change in both the conformation of achiral porphyrin (host) and change in position and conformation of chiral organic molecule (guest) on the B-band of ECD spectra (energy, intensity, sign of Cotton effect). Finally, conclusions made on model complexes are applied to published experimental data, contributing to a deeper understanding of various factors influencing ECD spectra in chiral systems. In addition, a computer program aimed to help rationalise ECD spectra by visualizing corresponding orbital energies, rotatory strengths, electric and magnetic transition moments, and angles between them, is presented.

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

Crystal structure of cis-7,8-dihydroxy-5,10,15,20-tetraphenylchlorin and its zinc(II)–ethylenediamine complex

Normal mode structural decomposition (NSD) shows the title chlorin compounds to have considerable saddling deformation from planarity.

The title chlorin, 2^PhH₂, hydrogen-bonded to di­methyl­amino­pyridine (DMAP), C₄₄H₃₂N₄O₂·C₇H₁₀N₂, and its corresponding zinc(II) complex, 2^PhZn, axially coordinated to ethyl­enedi­amine (EDA), [Zn(C₄₄H₃₀N₄O₂)]·C₂H₈N₂, were isolated and crystallized by adventitious reduction of the corresponding osmate esters by DMAP and EDA, respectively. Known since 1996 and, inter alia, used for the preparation of a wide range of (planar and non-planar) chlorin analogues (so-called pyrrole-modified porphyrins), their conformational analyses in the solid state are important benchmarks. Both macrocycles are only modestly distorted from planarity and both are slightly more non-planar than the corresponding dimeth­oxy-derivative, but less planar than a free-base meso-penta­fluoro­phenyl-based osmate ester. NSD analyses provide qu­anti­tative and qualitative analyses of the distortion modes. One origin of the non-planarity is presumably the avoidance of the eclipsed configuration of the two vic–cis diols on the pyrroline moiety; the resulting deformation of the pyrroline translates in some cases into the macrocycle. The structure of 2^PhH₂ features voids making up ca 26% of the unit-cell volume filled with highly disordered solvate mol­ecules (chloro­form and hexa­nes). 2^PhZn crystallized with a 13.6 (4)% occupied solvate methanol mol­ecule.

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

Collapse

MESH Headings Collapse

Grants

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

Collapse

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

Collapse

Crystal structure of 2,3-dimeth-oxy-meso-tetra-kis(penta-fluoro-phen-yl)morpholino-chlorin methyl-ene chloride 0.44-solvate

The title morpholino-chlorin, C46H16F20N4O3, was crystallized from hexa-ne/methyl-ene chloride as its 0.44 methyl-ene chloride solvate, C46H16F20N4O3·0.44CH2Cl2. The morpholino-chlorin was synthesized by stepwise oxygen insertion into a porphyrin using a 'breaking and mending strategy': NaIO4-induced diol cleavage of the corresponding 2,3-di-hydroxy-chlorin with in situ methanol-induced, acid-catalyzed intra-molecular ring closure of the inter-mediate secochlorins bis-aldehyde. Formally, one of the pyrrolic building blocks was thus replaced by a 2,3-di-meth-oxy-morpholine moiety. Like other morpholino-chlorins, the macrocycle of the title compound adopts a ruffled conformation, and the modulation of the porphyrinic π-system chromophore induces a red-shift of its optical spectrum compared to its corresponding chlorin analog. Packing in the crystal is governed by inter-actions involving the fluorine atoms of the penta-fluoro-phenyl substituents, dominated by C-H⋯F inter-actions, and augmented by short fluorine⋯fluorine contacts, C-F⋯π inter-actions, and one severely slipped π-stacking inter-action between two penta-fluoro-phenyl rings. The solvate methyl-ene chloride mol-ecule is disordered over two independent positions around an inversion center with occupancies of two × 0.241 (5) and two × 0.199 (4), for a total site occupancy of 88%.

Redox behaviour of the β-dihydroporphycene cobalt complex: study on the effect of hydrogenation of the ligand

The dihydrogenated porphycene cobalt(ii) complex was synthesized and electrochemical experiments were carried out. The one-electron reduction of the complex proceeded at the central metal to afford the Co(i) species; in contrast, for the non-hydrogenated porphycene cobalt(ii) complex, the one-electron reduction gave the ligand reduced radical anion species. The reactivity of the one-electron reduced species with alkyl halides showed clear differences between the complexes. Hydrogenation of the β-position of the porphycene makes it possible to generate a central cobalt reduced species possessing a higher reactivity than the ligand reduced radical anion species.

Energetics of Saddling versus Ruffling in Metalloporphyrins: Unusual Ruffled Dodecasubstituted Porphyrins

Presented herein is a first major density functional theory (BP86/D3/STO-TZ2P) survey of the energetics of saddling versus ruffling for a wide range of dodecasubstituted metalloporphyrins with M = Ni, Cu, Zn, Pd, and Pt. For the majority of X₈TPP (i.e., β-octasubstituted-meso-tetraphenylporphyrin), the calculations indicated a clear preference for the saddled conformation, consistent with a large body of experimental data. The preference for the saddled conformation relative to the ruffled conformation was found to vary from about ∼0.3-0.4 eV for Me₈TPP derivatives up to 1 eV for I₈TPP and (CF₃)₈TPP derivatives. For X = Ph, that is, dodecaphenylporphyrins, the saddled and the ruffled conformation are almost equienergetic, with even a slight preference for the ruffled conformation in some cases. This finding provides a satisfactory explanation for the X-ray crystallographic observation of both saddled and ruffled conformations for dodecaphenylporphyrin complexes as well as for spectroscopic evidence for conformational mobility of these complexes in solution. The calculations also indicate near-equienergetic saddled and ruffled conformations for meso-tetraacetylenyltetrabenzoporphyrins, again consonant with key crystallographic findings. By and large, both the energetics and nonplanar distortions of the metalloporphyrin derivatives correlated well with the Charton and Sterimol B₁ steric parameters of the peripheral substituents.