Carbocations Confined in a Thiatriazuliporphyrin Frame.

In the search for tricarbaporphyrinoids, a three-component acid-catalyzed condensation of azulene, 2,5-bis[(p-tolyl)hydroxymethyl]thiophene, and an aryl aldehyde has been elaborated, affording the appropriate thiatriazuliporphyrinogens. The subsequent oxidation yielded a rare example of a macrocyclic organic tetracation, which can be readily and reversibly converted into macrocyclic tri- and dicarbocations by addition of one or two hydroxides bound at the meso position(s). Further insight into the influence of carbocation formation on the geometry, electronic structure, and magnetic manifestation in (1) H NMR spectroscopy has been obtained by using density functional theory calculations. The charge distribution was evaluated by mapping electron density surfaces with electrostatic potential (ESP).

A Rhodium-Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Rhodium(III) m-Benziporphyrin.

The contraction of benzene is one of an exclusive group of reactions where the cleavage of aromatic structure is of fundamental importance. Rhodium(III) m-benziporphyrin undergoes an unprecedented transformation of the built-in m-phenylene in which a perimeter carbon atom is extruded to form rhodium(III) 21-carbaporphyrin, stabilizing the formyl-unit-substituted rhodacyclopropane motif.

From para-Benziporphyrin to Rhodium(III) 21-Carbaporphyrins: Imprinting Rh⋠⋠⋠η(2)-CC, Rh⋠⋠⋠η(2)-CO, and Rh⋠⋠⋠η(2)-CH Coordination Motifs.

Rhodium(III) para-benziporphyrin alters the fundamental reactivity of the built-in para-phenylene moiety. Due to additional macrocyclic stabilization, a sequence of intramolecular rearrangements are triggered to afford rhodium(III) 21-carbaporphyrin, which incorporates the rhodacyclopropane motif. The peculiar reversible transformations of the bridging methylene unit provide an example of selective and reversible aliphatic C-H bond elimination. Rhodium(III) 21-carbaporphyrin can be oxygenated to rhodium(III) 21-oxy-21-carbaporphyrin, whereas the metal ion interacts with the C(21)-O(25) fragment in an η(2) fashion. This species demonstrates a remarkable axial affinity toward alkenes.

A fused meso-aminoporphyrin: a switchable near-IR chromophore.

An aryl amine attached to the meso position of a porphyrin controls the π-delocalization using a redox process or a protonation/deprotonation centered at the meso-nitrogen. An easily accessible modulated motif affords a switchable near-IR chromophore as reflected by significant changes in absorption and fluorescence spectra.

Phenanthriporphyrin: an antiaromatic aceneporphyrinoid as a ligand for a hypervalent organophosphorus(V) moiety.

The incorporation of a phenanthrene moiety into a porphyrin framework results in the formation of a hybrid macrocycle­phenanthriporphyrin­merging the structural features of polycyclic aromatic hydrocarbons and porphyrins. An antiaromatic aceneporphyrinoid, adopting the trianionic {CCNN} core, is suitable for the incorporation of a phosphorus(V) center to form a hypervalent organophosphorus(V) derivative.

Oxatriphyrins(2.1.1) incorporating an ortho-phenylene motif.

An understanding of fundamental aspects of archetypal organic structural motifs remains a key issue faced by the experimental and theoretical chemists. Two possible bonding modes for a disubstituted benzene ring, that is a meta and para, determines the π delocalization for oligomeric structures. When the less abundant ortho-substituted variant is introduced into a triphyrin(2.1.1) skeleton an aromatic molecule is obtained and the carbocyclic ring participates in the conjugation of the macrocycle. The two-electron reduction and introduction of boron(III) changes the aromatic character and results in an anti-aromatic structure which has been confirmed by single-crystal analysis and supported by theoretical calculations.

Synthesis and switching the aromatic character of oxatriphyrins(2.1.1).

Triangularly shaped, contracted porphyrinoids belong to a group of molecules where the geometry significantly modifies the observed electronic properties. The need for a controllable, effective, and widely applicable approach to triphyrins drives extensive research towards macrocyclic materials that act as potential controlling motifs by switching their aromaticity. Two isomeric thiophene-fused triphyrins(2.1.1) were synthesized by applying an innovative approach. Spectroscopic techniques (NMR, UV/Vis) show that both macrocycles are aromatic and quantitatively convert into anti-aromatic structures after reduction with a zinc amalgam. The reduced forms were stabilized through boron(III) coordination, thereby allowing the observation of anti-aromatic 16 π delocalization within a contracted porphyrin.

Towards true carbaporphyrinoids: synthesis of 21-carba-23-thiaporphyrin.

In the search for porphyrinoids with a built-in cyclopentadienyl moiety (true carbaporphyrins), a rational synthesis of carbathiaporphyrin, the synthons, has been elaborated. The donors (C,N,S,N) in the porphyrinic core of carbathiaporphyrinoids are potentially of fundamental importance for generating organometallic complexes, as exemplified through formation of the palladium(II) complex.

Gold(III)-mediated contraction of benzene to cyclopentadiene: from p-benziporphyrin to gold(III) true tetraarylcarbaporphyrin.

The reaction of p-benziporphyrin, sodium tetrachloroaurate(III) dihydrate, and potassium carbonate in dichloromethane yielded gold(III) 5,10,15,20-tetraaryl-21-carbaporphyrin owing to the contraction of p-phenylene to cyclopentadiene. This molecule is the very first representative of a true 5,10,15,20-tetraaryl-21-carbaporphyrin complex where four trigonal donor atoms are involved in equatorial coordination. The contraction adds an unprecedented route to numerous organic transformations of aromatic compounds catalyzed by simple gold(III) compounds. p-Benziporphyrin provided the unique environment to alter the fundamental reactivity of the benzene unit facilitating its contraction to cyclopentadiene.

Copper(II) thiaethyneporphyrin and copper(II) 21-phosphoryl N-confused porphyrin hybrids. intramolecular copper(II)-carbon interaction inside of a porphyrinoid surrounding.

Stabilization of unusual organocopper(II) species via the very efficient protection of the copper(II)-carbon bond has been achieved encapsulating the copper(II) center in the coordination core of suitably constructed carbaporphyrinoids. Copper(II) was inserted into hybrid N-confused porphyrins which contain 21-diphenylphosphoryl-, 21-diphenylthiophosphoryl-, or 21-phosphinodithioic substituents or into 20-thiaethyneporphyrin, an aromatic porphyrinoid, which combines two structural motifs of 21-thiaporphyrin and ethyne. Two distinctly different types of the copper(II)-carbon bond have been detected. Copper(II) hybrid N-confused porphyrins reveal the η(1)-C(21) side-on coordination. The unprecedented equatorial metal(II)···Î·(2)-CC interaction has been trapped in a copper(II) thiaethyneporphyrin surrounding.

Regioselective phosphorylation and thiophosphorylation of N-confused porphyrin: a route to hybrid carbaporphyrinoids.

N-confused porphyrin (NCP) undergoes controlled regioselective phosphorylations at the inner, outer or both carbon atoms of the inverted pyrrole ring. Reactivity centered at the internal carbon atom is enhanced in the Ag(III) NCP's whereas the preference for perimeter substitution is characteristic of free base NCP. The addition of S(8) resulted in the formation of thio-derivatives containing 21-diphenylthiophosphoryl or 21-phosphinodithioic substituents.

Toward aceneporphyrinoids: synthesis and transformations of palladium(II) meso-anthriporphyrin.

meso-Anthriporphyrin is a carbaporphyrinoid with an anthracene ring embedded in the tripyrrolic framework. The coordination of palladium(II) results in a specific intramacrocyclic metal(II)-η(2)-CC interaction which facilitates the cleavage to palladium(II) acyclic oligopyrrole with the appended anthracene moiety.

Core-modified hexaphyrin: synthesis and characterization of 31,34-disilahexaphyrinoid.

Condensation of 16-silatripyrrane with pentafluorobenzaldehyde under catalytic conditions followed by DDQ oxidation leads to 31,34-disilahexaphyrinoid--a four times reduced derivative of 31,34-disilahexaphyrin which contains two built-in silole units flanked by four tetrahedrally hybridized meso carbons. In the preferred folded macrocyclic conformation the silole rings remain perpendicular to each other. The steric hindrance of bulky substituents at silicon atoms and ß-positions of siloles prevented aromatization. Only one meso diastereomer (5S, 15S, 20R, 30R) has been isolated and subsequently identified by 1D and 2D NMR techniques. The density functional theory (DFT) has been applied to model the molecular structure of 31,34-disilahexaphyrinoid consistent with constraints imposed by NOE experiments. The total energies calculated at the B3LYP/6-31G**//B3LYP/6-31G** level for four feasible meso diastereomers clearly demonstrated the energetic preference for the meso diastereomer (5S, 15S, 20R, 30R).

Nickel(II) and palladium(II) thiaethyneporphyrins. Intramolecular metal(II)-η2-CC interaction inside a porphyrinoid frame.

3,18-Diphenyl-8,13-di-p-tolyl-20-thiaethyneporphyrin ([18]thiatriphyrin(4.1.1)), which formally contains an C1-C2 ethyne moiety instead of pyrrole embedded in the macrocyclic framework of 21-thiaporphyrin, was obtained in a modification of the "3 + 1" approach using the ethyne analogue of tripyrrane (1,4-diphenyl-1,4-di(pyrrol-2-yl)but-2-yne) and 2,5-bis(p-tolylhydroxymethyl)thiophene. The spectroscopic and structural properties of 20-thiaethyneporphyrin reflect its macrocyclic aromaticity, revealing a combination of the acetylene (≥C-C≡C-C≤) and cumulene (>C═C═C═C<) character of the C18-C1-C2-C3 linker. The magnetic manifestations of aromaticity and antiaromaticity of thiaethyneporphyrin and its two-electron-oxidized derivative were observed using (1)H NMR spectroscopy and were confirmed by density functional theory calculations involving chemical shifts and nucleus-independent chemical shift analysis. Protonation of 20-thiaethyneporphyrin yielded a nonaromatic tautomer of iso-20-thiaethyneporphyrin, locating the saturated meso carbon adjacent to thiophene. Insertion of palladium(II) and nickel(II) into 20-thiaethyneporphyrin afforded planar palladium(II) thiaethyneporphyrin and low-spin diamagnetic nickel(II) 20-thiaethyneporphyrin as determined by X-ray crystallography. 20-Thiaethyneporphyrin acts as a dianionic ligand that coordinates through the two nitrogen and one sulfur donors. Metal(II) ions are uniquely exposed to form an intramolecular metal(II)-η(2)-CC bond, whereas the organometallic fragment is coplanar with the whole macrocycle. Coordination of pyridine converts diamagnetic nickel(II) thiaethyneporphyrin into its paramagnetic counterpart as determined by (1)H NMR.

Common origin, common fate: regular porphyrin and N-confused porphyrin yield an identical tetrapyrrolic degradation product.

The formation of an identical linear tetrapyrrole observed in the course of photooxidation of meso-tetraarylporphyrin and its N-confused isomer can be explained as a result of 1,2- and 1,3-dioxygen addition, respectively, as substantiated by DFT calculations.

Protonated N-confused porphyrin dimer: formation, structure, and guest binding.

The protonation of 3,3'-bis(meso-tetratolyl-2-aza-21-carbaporphyrin) with various acids was studied. The stepwise formation of mono-, di-, and tetracationic species was shown on the basis of UV-vis-near-IR and low-temperature (1)H NMR. Upon going from di- to tetraprotonated form, the bis(porphyrinoid) skeleton changes its conformation from cisoid to bent-transoid, which was found by single-crystal X-ray analyses, 2D NMR, and density functional theory (DFT) calculations. The formation of cation-anion complexes was established in both the solid state and solution. The substitution of anions was studied by spectrophotometric and (1)H NMR titrations. A pronounced decrease of the HOMO-LUMO gap in the tetraprotonated species was shown by cyclovoltametry and time-dependent DFT calculations.

A flexible porphyrin-annulene hybrid: a nonporphyrin conformation for meso-tetraaryldivacataporphyrin.

An annulene-porphyrin hybrid, the diaaza-deficient porphyrin 5,10,15,20-tetraaryl-21,23-divacataporphyrin, has been synthesized by an extrusion of tellurium atom(s) from 5,10,15,20-tetraaryl-21,23-ditelluraporphyrin under treatment with HCl. In addition, a monoaza-deficient 5,10,15,20-tetraaryl-21-tellura-23-vacataporphyrin was formed in the same reaction. The two new members of the vacataporphyrin family were characterized by X-ray crystallography, as well as UV/Vis and NMR spectroscopy. These aromatic molecules preserve the fundamental structural and spectroscopic features of the parent tetraarylporphyrin. The X-ray crystal structures of 21,23-divacataporphyrin and 21-tellura-23-vacataporphyrin show typical porphyrin patterns. The molecules are not strictly planar and show distortion of the annulene moieties. The N22⋅⋅⋅N24 distances (5.23 and 5.09 Å) are considerably longer than in regular porphyrins. For 21,23-divacataporphyrin, variable-temperature (1)H NMR spectroscopy data allowed the identification of divacataporphyrin stereoisomers differentiated by the geometry of the butadiene bridges. The forms remain in thermodynamic equilibrium.

Enantiomer resolution of intrinsically chiral C21-alkylated N-confused porphyrin complexes.

The resolution of stereoisomers of C21-alkylated nickel(II) complexes of N-confused porphyrin (NCP) was performed by means of chiral-phase HPLC with an effectiveness of above 90 % molar ratio for each isomer. The reverse signs of the Cotton effects in the circular dichroism (CD) spectra of the separated fractions are indicative of the pair of enantiomers. The application of low-temperature 2D NMR methods to the separated diastereomers of the system comprising a chiral 2-(S)-methylbutyl substituent, in connection with the CD spectra and relative HPLC migration rates, allowed the assignment of the absolute configuration of the chiral C21-substituted complexes of NCP. The assignment was confirmed by time-dependent DFT (TDDFT) calculations of CD spectra for the C21-methylated nickel(II) complex. The system remains chiral after removal of the metal ion from the macrocyclic crevice, despite the fact that this demetalation is connected with a change of the C21 hybridization from pyramidal to trigonal. The retention of chirality was established by means of CD spectra and confirmed by TDDFT calculations for a C21-methylated NCP free base. Stereoisomers were also separated for three covalently linked bis(NCP) systems with bridges involving one or two C21 carbon atoms. The occurrence of a pair of enantiomers was established for nonsymmetrical dimers comprising only one stereogenic center. In the case of the 21,21'-(o-xylene)-linked dimer, three stereoisomers, that is, a pair of enantiomers and an optically inactive meso-form, were separated and analyzed by CD and (1) H NMR spectroscopy. The stereoisomers of a diastereoselectively formed nonsymmetrical chloroplatinum(II)-linked dimer, consisting of heterochiral C21-alkylated NCP nickel(II) subunits, after separation displayed a strong optical activity, which can be ascribed to the rigid helical structure of the complex.