Breaking Global Diatropic Current to Tame Diradicaloid Character: Thiele's Hydrocarbon Under Macrocyclic Constraints.

A diradical/biradical character of organic derivatives is one of the key aspects of contemporary research focusing on the fundamental studies followed by potential applicability relying on the unique optical, electronic, or magnetic properties assigned to unpaired electrons. A precise involvement of two p-phenylenes into a cyclophane-like conjugated, diatropic system creates a flexible molecule with the two different characters of both subunits (benzene and quinone) imprinting into the structure a Kekulé delocalized system. A dynamic of both carbocyclic subunits, and their mutual interaction generates a singlet open-shell state (J=-1.25 kcal/mol) as documented spectroscopically (NMR and EPR). The extended theoretical analysis has proved a correlation between dihedral angle and the diradicaloid character that shifts from a closed-shell singlet to an open-shell state, eventually showing the y0=0.86 for 78 degrees and ΔEST=-0.34 kcal/mol.

Bispalladium(II) Complexes of di-p-Pyrirubyrin Derivatives as Promising Near-Infrared Photoacoustic Dyes.

The insertion of palladium(II) into di-p-pyrirubyrin results in mutually convertible bimetallic complexes. Post-synthetic functionalization of one of them yielded bispalladium(II) dioxo-di-p-pyrirubyrin and, after demetallation, dioxo-di-p-pyrirubyrin, introducing for the first time the α,ß'-pyridin-2-one unit into the macrocyclic frame. Bispalladium(II) di-p-pyrirubyrin 6, bispalladium(II) dioxo-di-p-pyrirubyrin 9, and dioxo-di-p-pyrirubyrin 10 absorb and emit light around 1000 nm and are characterized by high photostability. Thus, they are promising candidates for near-infrared photoacoustic dyes, ideally targeting (9) the wavelength of Yb-based fiber lasers. The incorporation of an α,ß'-pyridine moiety into expanded porphyrins opens a highly interesting area of research due to the attractive optical and coordination properties of the resulting molecules.

Peptide Stapling with Boronate Esters- A Reversible Folding of (Artificial) Peptide Chain to α-Helix.

Stabilization of a peptide conformation via stapling strategy may be realized by the reversible or more often irreversible connection of side chains being in mutually appropriate geometry. An incorporation of phenylboronic acid and sugar residues (fructonic or galacturonic acid), attached to two lysine side chains via amide bonds and separated by 2, 3, or 6 other residues in the C-terminal fragment of RNase A introduces the intramolecular interaction stabilizing the α-helical organization. The boronate ester stapling is stabilized in mild basic conditions and may be switched off by acidification leading to unfolded organization of the peptide chain. We investigated the possibility of using switchable stapling by mass spectrometry, NMR and UV-CD spectroscopies, and DFT calculations.

Oxidative insertion of amines into conjugated macrocycles: transformation of antiaromatic norcorrole into aromatic azacorrole.

A new group of aromatic porphyrinoids was obtained by an oxidative insertion of primary amines into the antiaromatic ring of 5,14-dimesityl-norcorrolatonickel(II) activated by iodosobenzene. The substituted 10-azacorroles thus formed were characterized by spectroscopic and electrochemical methods, and XRD analysis. Protonated forms of azacorroles were shown to remain aromatic despite disconnection of the original π-electron delocalization path.

Diagnosing Ring Current(s) in Figure-Eight Skeletons: A 3D Through-Space Conjugation in the Two-Loops Crossing.

The macrocyclic structures with local conjugation readily undergo a redox-triggered change in the diatropic character, leading to a global current-density pathway of the doubly charged systems. The figure-eight geometry of the neutral dimer does not significantly change upon oxidation according to the spectroscopic and computational data. The oxidation leads to 3D cross-conjugation at the intersection of the two ethylene bridges resulting in a global ring current.

B,N-doped PAHs from tridentate 'Defects' - a bottom-up convergent approach for π-extended systems.

A logical construction of monomeric subunits armed with the carbonyl functionality allowing post-synthetic reactivity leads to the convergent formation of π-extended defected units activated with BBr3. The kinetic or thermodynamic control entraps either the pyrido[2,3-g]quinoline or the 4,7-phenanthroline motifs doped with boron(III), which deeply modulates the optical properties.

Edge Decoration of Anthracene Switches Global Diatropic Current That Controls the Acene Reactivity.

The 14π-electron system of anthracene has been merged with the unsaturated Z-1,2-difurylethene to form a macrocycle(s) with the retained local conjugation of all incorporated subunits that were substantially modulated with a redox activation, opening a global delocalization involving all integrated aromatics. In addition, the edge modulation of acene via the attachment of a specific isomer of the conjugated system gives steric confinements that are characteristic of small macrocycles, forcing substantially short C(H)···O electrostatic interactions that are documented spectroscopically with the support of X-ray analysis.

Reductive Dimerization of Macrocycles Activated by BBr3.

A macrocyclic motif composed of carbazole and pyridine subunits linked by a carbonyl bridge (C═O) forms a skeleton with a peripheral reactivity that leads to a pinacol-like coupling activated by BBr3, eventually entrapping a substantially elongated C-C bond. Slightly modified conditions lead to the efficient transformation of the C═O unit to a CH2 linker that, after exposure to air, gives a dimeric molecule with multiple bonds between two macrocyclic units, as documented in spectroscopy and X-ray analysis.

From Antiaromatic Norcorrolatonickel(II) to Aromatic and Nonaromatic Zwitterions: Innocent Ligands with Unbalanced Charge of the Core.

A three-component reaction of antiaromatic meso-mesityl-3-nitronorcorrolatonickel(II) 1-NO2 with dialkyl acetylenedicarboxylate and PBu3 yields aromatic zwitterionic corrole nickel(II) complexes 3-R with one of the meso-substituents comprising a positively charged tributyl phosphonium group and negatively charged coordination core. Reaction of 1-NO2 with PBu3 alone resulted in a nonaromatic chiral adduct 5 of a zwitterionic phlorine character with a -PBu3+ group at a pyrrole ß-position.

(Aza)Acenes Share the C2 Bridge with (Anti)Aromatic Macrocycles: Local vs. Global Delocalization Paths.

A strong conjugation present in fused systems plays a crucial role in tuning of the properties that would be showing a dependence on the efficiency of π-electrons coupling. The π-cloud available in the final structure can be drastically influenced by a side- or a linear fusion of unsaturated and conjugated hydrocarbons. The linear welding of naphthalene/anthracene or quinoxaline/benzo[g]quinoxaline with triphyrin(2.1.1) gives structures where the competition between local and global delocalization is distinguished. The aromatic character observed in skeletons strongly depends on the oxidation state of the macrocyclic flanking and is either extended over the whole system or kept as a composition of local currents (diatropic and paratropic) of incorporated units. The hybrid systems show the properties derived from the π-conjugations that interlace one another but also show a significant independence of (aza)acene subunits reflected in the observed spectroscopic properties.

BODIPY- and Porphyrin-Based Sensors for Recognition of Amino Acids and Their Derivatives.

Molecular recognition is a specific non-covalent and frequently reversible interaction between two or more systems based on synthetically predefined character of the receptor. This phenomenon has been extensively studied over past few decades, being of particular interest to researchers due to its widespread occurrence in biological systems. In fact, a straightforward inspiration by biological systems present in living matter and based on, e.g., hydrogen bonding is easily noticeable in construction of molecular probes. A separate aspect also incorporated into the molecular recognition relies on the direct interaction between host and guest with a covalent bonding. To date, various artificial systems exhibiting molecular recognition and based on both types of interactions have been reported. Owing to their rich optoelectronic properties, chromophores constitute a broad and powerful class of receptors for a diverse range of substrates. This review focuses on BODIPY and porphyrin chromophores as probes for molecular recognition and chiral discrimination of amino acids and their derivatives.

Porphyrin-Amino Acid Conjugates.

An efficient way for the formation of a direct bond between a porphyrin and amino acid through a main chain nitrogen has been optimized and applied in the synthesis of a library of chromophore-amino acid conjugates with slightly modified and adjustable optical properties. The obtained series of hybrid materials shows an optic response characteristic for meso-nitrogen derivatized porphyrinoids. The oxidation with hypervalent iodine shows a regioselective transformation of an amino acid chain changing the character of the final molecule.

Multi-Cation Coordination in Porphyrinoids.

Porphyrinoid macrocycles are unbreakably linked with the coordination of central cations that drastically influence the observed behavior. A specific place has been recorded for structures where more than one cation is entrapped in a single macrocycle. It has been a characteristic feature of expanded porphyrinoids where the increased coordination space allows a multiple coordination. Because of specific steric confinements, an incorporation of more than one cation within regular or contracted porphyrinoids is more demanding but it can be realized on several ways significantly modifying the observed behavior. We will discuss synthetic strategies leading to structures with increased number of cations and the influence on the observed behavior of such modification.

Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles.

Understanding of the aromatic properties and magnetically induced current densities of highly conjugated chromophores is important when designing molecules with strongly delocalized electronic structure. Linear extension of the triphyrin(2.1.1) skeleton with an annelated benzo[b]heterocycle fragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycle subunit. Current-density calculations provide detailed information about the observed pathways and their strengths.

Organoboron Complexes in Edge-Sharing Macrocycles: The Triphyrin(2.1.1)-Tetraphyrin(1.1.1.1) Hybrid.

The formation of a precisely designed environment predefined for stabilizing electron-deficient atoms, such as boron(III), is an important approach for optimizing the properties of a chromophore. A triphyrin(2.1.1) motif built on the extended π-system of a tetraphyrin(1.1.1.1) skeleton creates a new coordination environment, with a CNN set of donors confined in a limited space predefined for binding small cations. The entrapment of boron(III) in the triphyrin(2.1.1) sector, with formation of a direct B-C bond, significantly changes the optical response and the global aromatic character of the compound, leading to an extension of the π-delocalisation.

A route to a cyclobutane-linked double-looped system via a helical macrocycle.

Macrocycles built of carbazole and pyridine fragments with alkene/alkane linkers constitute a helical structure as documented structurally and spectroscopically. The 'solid-state' prearrangement observed for two C[double bond, length as m-dash]C bonds leads to unprecedented transformation to a cyclobutane in a head-to-tail (rtct) geometry with two macrocyclic loops in a perpendicular orientation.

Three-Stage Aromaticity Switching in Boron(III) and Phosphorus(V) N-Fused p-Benziporphyrin.

Insertion of PCl3 or PhBCl2 into 5,10,15,20-tetraaryl-p-benziporphyrin prompted an intramolecular fusion affording anti-aromatic phosphorus(V) and non-aromatic boron(III) complexes of two N-fused dihydro-p-benziporphyrin isomers. These macrocycles are classified as carbatriphyrin due to the common [CNN] coordination. A sequence of direct transformations, triggered by protonation or two-electron redox processes, afforded a set of three mutually convertible N-fused p-benziporphyrinoids, with distinct anti-aromatic, non-aromatic, and aromatic spectroscopic features.

Inversion Triggered by Protonation-A Rubyrin with Embedded α,ß'-Pyridine Moieties.

Incorporation of α,ß'-pyridine into a [26]hexaphyrin(1.1.0.1.1.0) frame resulted in formation of 29,33-diaza-A,D-di-p-benzi[26]hexaphyrin (A,D-di-p-pyrirubyrin), which acquires a strictly planar conformation with two nitrogen pyridine atoms directed to the center of the macrocyclic core. The mode of α,ß'-pyridine incorporation results in efficient 26-electron π-conjugation, as shown by the clear spectroscopic and structural aromatic features. The formation of the trication and tetracation through protonation is accompanied by a conformational flip of one or two pyridinium moieties to result in N-confused or doubly N-confused conformers of the original macrocyclic frame.

Rhodium-Induced Reversible C-C Bond Cleavage: Transformations of Rhodium(III) 22-Alkyl-m-benziporphyrins.

The structurally prearranged carbaporphyrins 22-methyl- and 22-ethyl-m-benziporphyrins provide the platform stabilizing aromatic rhodium(III) 22-(µ-methylene-m-benziporphyrin) and rhodium(III) 22-(µ-ethylidene-m-benziporphyrin). An intramolecular conversion facilitated by the m-phenylene reactivity and observed for both aromatic complexes efficiently leads to rhodium(III) 21-(µ-methylene)-21-carbaporphyrin and rhodium(III) 21-(µ-ethylidene)-21-carbaporphyrin. The distinctive macrocyclic environment of rhodium(III) 21-carbaporphyrin created an opportunity to trap unique organometallic transformations of inner core substituents affording the fulvene-like bond pattern or the rearrangement to 21-vinyl substituent. The one-electron reduction of the rhodium(III) carbaporphyrin anion π-radical with a (dxy )2 (dxz )2 (dyz )2 -(P.- ) electronic configuration is demonstrated. The further process of reduction of paramagnetic species triggers the ethyl migration from carbon(22) to rhodium(III), affording the diamagnetic rhodium(III) meta-benziporphyrin containing the apically coordinated σ-ethyl ligand providing an example of reversible C(sp2 )-C(sp3 ) bond cleavage.

Ruthenium(II) and Ruthenium(III) Complexes of p-Benziporphyrin: Merging Equatorial and Axial Organometallic Coordination.

A diamagnetic ruthenium(II) complex of 5,10,15,20-tetraphenyl-p-benziporphyrin [RuII(p-BzP)(CO)Cl] was obtained via the insertion of ruthenium into p-benziporphyrin using triruthenium(0) dodecacarbonyl [Ru3(CO)12] as the metal source. The procedure applying dichloro(cycloocta-1,5-diene)ruthenium(II) (polymer, [Ru(COD)Cl2]n) afforded the paramagnetic six-coordinate ruthenium(III) p-benziporphyrin [RuIII(p-BzP)Cl2]. As shown by X-ray crystallography, the p-phenylene ring in both complexes is sharply tilted out of the N3 plane, as reflected by the respective N3 (pyrrole)-C6 (p-phenylene) dihedral angle [RuII(p-BzP)(CO)Cl, 52.5°; RuIII(p-BzP)Cl2, 53.7°]. p-Phenylene is bound to the ruthenium cation in an η2 fashion, revealing the shortest ever Ru-C distance in the series of p-benziporphyrin complexes [RuII(p-BzP)(CO)Cl, 2.275(2) Å; RuIII(p-BzP)Cl2, 2.324(5) Å]. The reaction of RuII(p-BzP)(CO)Cl with ArMgCl or AlkMgCl results in the formation of diamagnetic six-coordinate ruthenium(II) p-benziporphyrin complexes containing the apically coordinated σ-alkyl or σ-aryl ligands, where the metal ion simultaneously coordinates to three carbon centers respectively accommodating η2 (phenylene) and σ (aryl and alkyl) modes. Reactions of σ-aryl (alkyl) carbanions with paramagnetic RuIII(p-BzP)Cl2 have been followed by 1H NMR spectroscopy. The procedure afforded the six-coordinate paramagnetic ruthenium(III) p-benziporphyrin [RuIII(p-BzP)(Ph)Cl], which binds one σ-aryl ligand, as reflected by the characteristic 1H NMR spectra spread within the +120 to -120 ppm range. Both paramagnetic complexes RuIII(p-BzP)(Ph)Cl and RuIII(p-BzP)(p-Tol)Cl are formed as a mixture of two stereoisomers differentiated by two nonequivalent locations of σ-aryl with respect to the puckered macrocyclic ring. The paramagnetic shifts of σ-aryls are indicative of π-spin delocalization patterns. Analysis of the contact shifts and parallel density functional theory calculations of the spin density distribution in RuIII(p-BzP)Cl2, RuIII(p-BzP)(Ar)Cl, and RuIII(p-BzP)(Alk)Cl reflect the features of the dxy2(dxzdyz)3 electronic ground state.