Stabilization and Structure of the cis Tautomer of a Free‐Base Porphyrin

Single‐crystal X‐ray analysis of the β‐heptakis(trifluoromethyl)‐meso ‐tetrakis(p ‐fluorophenyl)porphyrin, H₂[(CF₃)₇TpFPP], has revealed the first example of a stable cis tautomer of a free‐base porphyrin, the long‐postulated intermediate of porphyrin tautomerism. The stability of the unique molecule appears to reflect a dual origin: a strongly saddled porphyrin skeleton, which alleviates electrostatic repulsion between the two NH protons, and two polarization‐enhanced, transannular N−H⋅⋅⋅O−H⋅⋅⋅N hydrogen bond chains, each involving a molecule of water. DFT calculations suggest that the observed tautomer has a lower energy than the alternative, doubly hydrated trans tautomer by some 8.3 kcal mol⁻¹. A fascinating prospect thus exists that H₂[(CF₃)₇TpFPP]⋅2 H₂O and cognate structures may act as supramolecular synthons, which, given their chirality, may even be amenable to resolution into optically pure enantiomers.     

Chromatographic characteristics of porphyrine ether glycerides

Summary Efficient TLC and HPLC conditions for the separation of a newly synthesized class of compounds, porphyrine ether glycerides, have been established. Supplementary physico-chemical characteristics are given for this group of solutes.Dedicated to Professor Leslie S. Ettre with best wishes for his 70th birthday anniversary.     

Tetrahedral Pegs in Square Holes: Stereochemistry of Diboron Porphyrazines and Phthalocyanines

The first examples of diboron complexes of the tetrapyrroles octaethylporphyrazine (OEPz) and 2,9,16,23‐tetra‐t‐butyl‐phthalocyanine (Pc) are reported, counterpoints to the better known monoboron tripyrroles, subporphyrazine and subphthalocyanine. Two stereochemical possibilities are observed, with cisoid‐B₂OF₂(OEPz), both cisoid‐B₂OPh₂(OEPz) and transoid‐B₂OPh₂(OEPz), transoid‐B₂OF₂(Pc) and cisoid‐B₂OPh₂(Pc) having been isolated and characterised, including structure determinations for the OEPz complexes. This variation in stereochemistry, which can be extended to include the previously reported transoid‐B₂OF₂(porphyrin), cisoid‐[B₂OF₂(corrole)]^?, and both transoid‐ and cisoid‐B₂OF₂(calixphyrin), prompted a wider DFT study to elucidate the factors influencing the stereochemical preferences. This shows that the cisoid/transoid preference is correlated to the ease with which the macrocycle accommodates a rectangularly distorted N₄ cavity.     

Control of Aromaticity and cis‐/trans‐Isomeric Structure of Non‐Planar Hexaphyrin(2.1.2.1.2.1) and Metal Complexes

Vinylene‐bridged hexaphyrin(2.1.2.1.2.1) was synthesized from dipyrrolyl diphenylethenes by acid‐catalyzed condensation reactions. Freebase hexaphyrin(2.1.2.1.2.1) forms a distorted structure with non‐aromatic characteristics. The aromaticity and molecular configuration of non‐planar hexaphyrin(2.1.2.1.2.1) can be controlled by insertion of metal ions. Freebase and zinc complexes show a distorted structure without macrocyclic aromaticity, whereas copper complexes show a figure‐of‐eight structure with macrocyclic aromaticity. It is the first example of aromaticity conversion of a distorted expanded porphyrin involving vinylene bridges.     

Konformativer Umbau von Porphyrinen als Rezeptoren mit schaltbaren N‐H⋅⋅⋅X‐Bindungsmodi

Die Selektivität und funktionale Variabilität von Porphyrinkofaktoren basiert typischerweise auf der Substratbindung durch Metalloporphyrine, wobei die Pyrrolstickstoffatome nur zur Chelatisierung der Metallionen dienen. In einem ersten Schritt zu Porphyrinzentren mit “enzymähnlicher” Aktivität zeigt eine strukturelle und spektroskopische Untersuchung der Substratbindung im Kern jedoch, dass ein sattelverbogenes Porphyrin mit peripheren Aminorezeptorgruppen ( 1 , 2,3,7,8,12,13,17,18‐Oktaethyl‐5,10,15,20‐tetrakis(2‐aminophenyl)porphyrin), abhängig von der Azidität der Lösung, Analyte in einer schaltbaren Weise koordiniert. Das supramolekulare Ensemble weist eine hohe Affinität und Selektivität für das Pyrophosphatanion (2.26±0.021)×10⁹ m ^?1 auf. ¹H‐NMR‐Spektroskopie liefert Einblicke in den wahrscheinlichen Bindungsmodus und erlaubt die Charakterisierung der Atropisomere, deren Struktur auch durch Röntgenstrukturanalysen aufgeklärt wurde.     

Metal–Organic Frameworks for the Exploitation of Distance between Active Sites in Efficient Photocatalysis

Discoveries of the accurate spatial arrangement of active sites in biological systems and cooperation between them for high catalytic efficiency are two major events in biology. However, precise tuning of these aspects is largely missing in the design of artificial catalysts. Here, a series of metal–organic frameworks (MOFs) were used, not only to overcome the limit of distance between active sites in bio-systems, but also to unveil the critical role of this distance for efficient catalysis. A linear correlation was established between photocatalytic activity and the reciprocal of inter active-site distance; a smaller distance led to higher activity. Vacancies created at selected crystallographic positions of MOFs promoted their photocatalytic efficiency. MOF-525-J33 with 15.6 Å inter active-site distance and 33 % vacancies exhibited unprecedented high turnover frequency of 29.5 h⁻¹ in visible-light-driven acceptorless dehydrogenation of tetrahydroquinoline at room temperature.     

Excited-State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time-Resolved Electronic and Vibrational Spectroscopy

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited-state aromaticity. The coordinated metal allows the excited-state aromaticity in the triplet state to be detected through the heavy-atom effect, but other metalation effects on the excited-state aromaticity were ambiguous. Herein, the excited-state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The S_Q states of gold [26]- and [28]-hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground-state, indicating aromaticity reversal. Furthermore, while the T₁ states of gold [28]-hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand-to-metal charge-transfer state of gold [26]-hexaphyrins contributed by the gold metal showed non-aromatic features arising from the odd-number of π-electrons.