Synthesis of π-Ring-Fused Porphyrin(2.1.1.1)s and Their Rh(I) Complexes

Stable and simplest expanded porphyrins, π-ring-fused porphyrin(2.1.1.1)s and Rh(I) complexes, have been obtained for the first time. Two free bases show chair-shaped molecular conformations, as if reassembled by the halves of porphyrin(1.1.1.1) and porphyrin(2.1.2.1). The insertion of Rh(CO)2 groups induced more twisted molecular conformations. The NMR spectra, X-ray structure, NICS, and ACID of obtained molecules all support their nonaromaticity due to chair-shaped molecular conformations. The protonated and Rh(I) coordination of porphyrin(2.1.1.1)s process red-shifted absorptions in the NIR region.

One Carbon Ring Expansion of Bipyrrole to Bipyridine Enables Access to a π-Extended, Non-innocent, Corrole-like Ligand

A π-extended, diaza-triphenylene embedded, mono-anionic corrole analogue and its Ni^II complex were synthesized from a diaza-triphenylene precursor, which was obtained from a double one-carbon insertion into a naphthobipyrrole diester. Following conversion to the corresponding activated diol and acid-catalyzed condensation with pyrrole, subsequent reaction with pentafluorobenzaldehyde afforded mono-anionic, π-extended bipyricorrole-like macrocycle. Attempted Ni^II insertion with Ni(OAc)₂  ⋅ 4H₂ O resulted an ESR active, Ni^II bipyricorrole radical complex, which was converted to a stable cationic Ni^II complex upon treatment with [(Et₃ O)⁺ (SbCl₆ )^- ]. Both complexes were characterized by ¹ H and ¹³ C NMR, UV/Vis spectroscopy and single crystal X-ray diffraction analysis. The Ni^II bipyricorrole radical complex is converted to a cationic Ni^II complex by single-electron reduction using cobaltocene. Both the cationic Ni^II complex and the radical Ni^II complex exhibited ligand-centered redox behavior, whereas the Ni^II remains in the +2 oxidation state.

Synthesis and Structure of meso-Substituted Dibenzihomoporphyrins

Bench-stable meso-substituted di(p/m-benzi)homoporphyrins were synthesized through acid-catalyzed condensation of dipyrrole derivatives with aryl aldehydes. The insertion of a 1,1,2,2-tetraphenylethene (TPE) or but-2-ene-2,3-diyldibenzene unit in the porphyrin framework results in the formation of dibenzihomoporphyrins, merging the features of hydrocarbons and porphyrins. Single crystal X-ray analyses established the non-planar structure of these molecules, with the phenylene rings out of the mean plane, as defined by the dipyrromethene moiety and the two meso-carbon atoms. Spectroscopic and structural investigations show that the macrocycles exhibit characteristics of both TPE or but-2-ene-2,3-diyldibenzene and dipyrromethene units indicating the non-aromatic characteristics of the compounds synthesized. Additionally, the dibenzihomoporphyrins were found to generate singlet oxygen, potentially allowing their use as photosensitizers.

Dicarba[26]hexaporphyrinoids(1.1.1.1.1.1) with an Embedded Cyclopentene Moiety-Conformational Switching

Incorporation of cyclopentene fragments into a skeleton of parental [26]hexaphyrin(1.1.1.1.1.1) afforded extended carbaporphyrinoids: 31,34-dicarbahexa[26]chlorin and its derivatives: the first externally substituted by ethoxy and 2,4,6-trimethylbenzylidene groups and the second one formed by selective oxidation of one cyclopentene ring. Macrocycles adopt dumbbell-shaped conformations with two meso hydrogen atoms located inside the macrocyclic cavity. Protonation of 31,34-dicarba[26]hexachlorins provided dications existing in dumbbell-shaped and rectangular conformations.

Expanded Carbaporphyrinoids

This Review outlines the progress in the field of synthetic expanded carbaporphyrinoids. The evolution of this topic is demonstrated with expanded porphyrin-inspired systems with a variety of incorporated entities that introduce one or more carbon atoms into the cavity. The discussion starts with platyrins-the macrocycles that were identified as parent molecules of not only the expanded carbaporphyrinoids, but the carbaporphyrinoid class in general. After historic considerations, the plethora of expanded porphyrin-like macrocycles containing N-confused or neo-confused pyrrole motifs and different carbocyclic subunits are presented. Special emphasis is given to applications of expanded carbaporphyrinoids in different areas, including organometallic chemistry, switching systems, or aromaticity, concluding with the demonstration of a covalent cage based on an expanded carbaporphyrinoid.

An Adaptable N-Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I-+III) in a series of structurally characterized complexes (1-3). Redox interconversion of [LCu]+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E1/2 =-0.45 V and +0.82 V (vs. Fc/Fc+ )). A linear CNHC -Cu-CNHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of CuI versus CuII /CuIII . Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1-3 are textbook examples for CuI , CuII , and CuIII species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced CuI form suggest that [LCu]+/2+/3+ may operate in catalysis via coupled 2 e- /2 H+ transfer.

Bis-4,4'-biphenyl Ring Embedded Octaphyrin with Three Distinct Conformational Structures

Three distinct conformational structures of carbaoctaphyrins were prepared by incorporating bis-4,4'-biphenyl units in the macrocyclic core. The free-base form adopts a figure-eight conformation, whereas the protonation triggers a conformational change with a pyrrole ring inversion and acquires an open-framework structure. The insertion of bis-Rh^I metal ion in the macrocyclic core affords a singly twisted conformational structure. Furthermore, the local aromaticity in the bis-4,4'-biphenyl ring dominates the overall macrocyclic aromaticity in all three forms, and thus adopts nonaromatic characteristics. These results are supported by spectral as well as theoretical studies, and they are unambiguously confirmed by X-ray crystal analyses.

28-Hetero-2,7-Naphthiporphyrins: Horizontal Expansion of the m-Benziporphyrin Macrocycle

Replacement of the m-phenylene moiety of m-benziporphyrins with the 2,7-naphthalenyl subunit yielded 28-hetero-2,7-naphthiporphyrins-macrocycles that can be considered as expanded carbaporphyrinoids. This group retains some features of parent m-benziporphyrins, but due to larger size and different shape of the macrocyclic cavity, their coordination properties are different. Upon reduction and conformational rearrangement the 28-thia- and 28-selena-2,7-naphthiporphyrin form organophosphorus(V) complexes.

Homocarbaporphyrinoids: The m-o-m and p-o-p Terphenyl Embedded Expanded Porphyrin Analogues and Their RhI Complexes

Stable homocarbaporphyrinoids were successfully synthesized by incorporating the m-o-m and p-o-p terphenyl units into the porphyrin core. The distinct bonding modes of terphenyl in the macrocycle generated two structural isomers with two and four carbon atoms in the macrocyclic environment. The core was utilized to stabilize the Rh^I ion. The spectral and structural analyses revealed that the restricted (m-o-m) and allowed (p-o-p) conjugation in the macrocyclic core provide overall non-aromatic characteristics both to the free bases and their complexes.