Rh(I) and Organo-Rh(III) Complexes of meso-Triarylbiphenylcorrole.

Rhodium complexes of biphenylcorrole are reported, and the molecular structures of the complexes are unambiguously confirmed by single-crystal X-ray analysis. The adj-CCNN core of the dicarbacorrole efficiently stabilizes a rhodium metal ion in its two different oxidation states. It is pertinent to point out that the Rh(I) metal complex attains square-planar geometry while organo-Rh(III) forms an octahedral complex. Furthermore, density functional theory studies corroborate the experimental findings.

Stable meso-meso-Linked 2NH-Corrole Radical Dimers as a Key Intermediate to Corrole Tape.

While oxidation of 5,5',15,15'-tetramesityl-10-10'-linked 3NH-corrole dimer with DDQ gave the corresponding triply linked 2NH-corrole tape, the use of an equimolar amount of p-chloranil as a milder oxidant resulted in the formation of a 10-10'-linked neutral 2NH-corrole radical dimer as a stable product. The stability of this peculiar product is ascribed largely to strong antiferromagnetic interaction of the two spins. Further oxidation of this diradical produced corrole tape, suggesting its involvement as a reaction intermediate to the corrole tape. Oxidation of 10-10'-linked bis-pyridine-coordinated CoIII corrole dimer with DDQ produced a cobalt corrole radical dimer and a doubly linked corrole dimer both as stable compounds bearing pyridine and cyanide axial ligands. This type of oxidative transformation involving neutral diradical intermediates is a unique reaction mechanism specific for corrole dimers.

Cyclophane-Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18-Porphyrinyl Dicyanomethyl Diradicals.

2,18-Bis(dicyanomethyl)-substituted NiII porphyrin 8 and ZnII porphyrin 11 were prepared and subjected to oxidation with PbO2 in CH2 Cl2 at 298 K to give cyclophane-type chlorin dimers (9)2 and (12)2 as a consequence of double recombination of biradicals 9 and 12, respectively. Dimer (9)2 takes a syn-conformation of two distorted NiII chlorins but (12)2 takes an anti-conformation of relatively planar ZnII chlorins. At 298 K, dimer (9)2 is stable and its 1 H NMR spectrum is sharp but becomes broad at high temperature, while the 1 H NMR spectrum of (12)2 is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of (12)2 is much less than that of (9)2 . The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer (16)2 to give homo dimers (9)2 and (15)2 .

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 RhI 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.

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-RhI 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.

Electron transfer reactions in sub-porphyrin-naphthyldiimide dyads.

A series of donor-acceptor compounds based on a sub-porphyrin (SubP) as an electron donor and naphthyldiimide (NDI) as an acceptor has been designed, synthesized and investigated by time-resolved emission and transient absorption measurements. The donor and acceptor are separated by a single phenyl spacer substituted by methyl groups in order to systematically vary the electronic coupling. The electron transfer reactions in toluene are found to be quite fast; charge separation is quantitative and occurs within 5-10 ps and charge recombination occurs in 1-10 ns, depending on the substitution pattern. As expected, when steric bulk is introduced on the adjoining phenyl group, electron transfer rates slow down because of smaller electronic coupling. Quantum mechanical modelling of the potential energy for twisting the dihedral angles combined with a simplified model of the electronic coupling semi-quantitatively explains the observed variation of the electron transfer rates. Investigating the temperature variation of the charge separation in 2-methyltetrahydrofuran (2-MTHF) and analyzing using the Marcus model allow experimental estimation of the electronic coupling and reorganization energies. At low temperature, relatively strong phosphorescence is observed from the donor-acceptor compounds with onset at 660 nm signaling that charge recombination occurs, at least partially, through the sub-porphyrin localized triplet excited state. Finally, it is noted that charge separation in all SubP-NDI dyads is efficient even at cryogenic temperatures (85 K) in 2-MTHF glass.

Rhodium(III) and Iridium(III) Bipyricorrole Complexes: Syntheses, Structures, and Properties.

Rhodium(III) and iridium(III) bipyricorrole complexes have been unprecedentedly reported. Single-crystal X-ray diffraction studies unambiguously evinced the molecular structures of metal complexes in octahedral geometry. The monoanionic platform of bipyricorrole effectively stabilizes metal ions in their higher oxidation states. It is worth mentioning that the fluorescence quantum yield of the rhodium(III) complex is 3-fold higher than that of free-base bipyricorrole.

Carbatriphyrin(3.1.1)-A Distinct Coordination Approach of BIII to Generate Organoborane and Weak C-H⋠⋠⋠B Interactions.

Carbatriphyrin(3.1.1) is the structural isomer of biphenylcorrole and achieved by switching the bonding mode of biphenyl unit from 3,3' to 2,3' which turns the corrole into a triphyrin analogue. The presence of a m-arene unit in the framework restricts the overall conjugation and thus leads to a nonaromatic macrocycle. The protonation experiments afford a trifluoroacetate ion complex. The coordination chemistry reveals that the bottom approach of boron(III) ion forms weak C-H⋅⋅⋅B interactions, however a top approach stabilizes the organoborane complex and the product distribution relies on the basicity of the proton scavengers. These results are unambiguously confirmed by single-crystal X-ray analyses.

meso-Aryl [20]π Homoporphyrin: The Simplest Expanded Porphyrin with the Smallest Möbius Topology.

An unstable conjugated homoporphyrin was successfully stabilized by introducing meso-aryl substitutents. It was evident from the moderate diatropic ring current found by NMR analysis that the newly formed 20π conjugated free base and its protonated form exhibited Möbius aromatic character. Furthermore, complexation as a ligand with an RhI ion afforded a unique binding mode and retained the Möbius aromaticity. Overall, these compounds are the smallest Möbius aromatic molecules, as confirmed by spectral and crystal-structure analysis and supported by theoretical studies.

Bipyricorrole: A Corrole Homologue with a Monoanionic Core as a Fluorescence Zn(II) Sensor.

In the corrole homologue, 6,11,16-triarylbipyricorrole, the bipyrrole unit is replaced by a 2,2'-bipyridine unit. This modification effectively alters the corrole N4 coordination sphere from the trianionic [(NH)3 N] to the monoanionic [N3NH] state. The newly formed monoanionic core stabilizes Zn(II) ions with enhanced emission properties. The enhanced emission was further utilized for metal ion sensing studies and exploited for the selective detection of Zn(II) ions.

A 6,11,16-Triarylbiphenylcorrole with an adj-CCNN Core: Stabilization of an Organocopper(III) Complex.

An adj-dicarbacorrole with CCNN in the core is achieved by replacing the bipyrrole moiety by a simple polycyclic aromatic hydrocarbon, such as biphenyl unit. Spectroscopic studies and structural analyzes confirm the absence of macrocyclic aromatization, thus leading to overall nonaromatic character. The trianionic core is effectively utilized to stabilize a copper(III) ion to form an organocopper complex.

Recent Advances in the Design and Syntheses of Porphyrinoids by Embedding Higher Analogues of Arene and Pyridine Units.

Due to enthralling applications in various fields and augmenting fundamental wisdom, π-conjugated macrocycles in general and porphyrin systems in particular are constantly explored. Subtle modifications of porphyrin structure can amend the rudimentary properties. Pursuing innovative properties provides impetus to underpin arene or pyridine moiety embedded porphyrin derivatives. There have been several reviews related to arene incorporated carbaporphyrinoids; however, recent developments of porphyrin analogues by introducing higher analogues of arenes and pyridine units are not adequately inspected. This mini-review mainly focuses on biphenyl, bipyridine, terphenyl, and mixed arene pyridine embedded porphyrin analogues and their coordination chemistry.

3-D Cryptand Like Normal and Doubly N-Confused Calixbenzophyrins: Synthesis and Structural Studies.

The three-dimensional (3-D) cryptand-like normal and doubly-N-confused calixbenzophyrins embedded with phenyl rings in the macrocyclic skeleton are successfully synthesized from single starting materials. These structural isomers are characterized by spectral studies and unambiguously confirmed by crystal analyses.

PtCl2 mediated peripheral transformation of carbatriphyrin(3.1.1) into a meso-fused ß-ß' dimer and its monomer analogue.

An unprecedented formation of a meso-fused ß-ß' carbaporphyrin dimer and its monomer with a keto group was described. These analogues were synthesized from carbatriphyrin(3.1.1.) by a metal assisted strategy using PtCl2 salt in a single step without any prefunctionalized precursors. Upon dimerization, the monomeric ligand with a dianionic core is transformed into a dimeric structure with unique trianionic cores.

Isosmaragdyrin with an N3C2 core: stabilization of Rh(i) and organo-Pt(ii) complexes.

Isosmaragdyrin with an N3C2 core is successfully achieved by introducing a 2,6-di-m-phenylpyridine unit. The core is utilized to stabilize the Rh(i) ion in the N-Rh(i)-N bonding mode and the Pt(ii) ion in the (N^C)N fashion to form an unsymmetrical pincer-type organometallic complex.

Structurally Isomerized Bis-Biphenyl Moieties Embedded in Hexaphyrin(3.1.1.3.1.1) and Octaphyrin(1.1.1.0.1.1.1.0).

An o-p-biphenyl moiety-incorporated 32π hexaphyrin(3.1.1.3.1.1) is successfully achieved. Replacing ortho with meta connectivity in the biphenyl unit of hexaphyrin leads to the formation of its structural isomer, octaphyrin(1.1.1.0.1.1.1.0). Spectral and structural analyses reveal the lack of planarity in hexaphyrin and the presence of an m-arene unit in octaphyrin, thus affording nonaromatic characteristics.

Stable radical versus reversible σ-bond formation of (porphyrinyl)dicyanomethyl radicals.

(Porphyrinyl)dicyanomethyl radicals were produced by oxidation of dicyanomethyl-substituted porphyrins with PbO2. These radicals constitute a rare example displaying stable radical versus dynamic covalent chemistry (DCC) depending upon the substitution position of the dicyanomethyl radical. meso-Dicyanomethyl-substituted radicals exist as stable monomeric species and do not undergo any dimerization processes either in the solid state or in solution. In contrast, ß-dicyanomethyl-substituted radicals are isolated as σ-dimers that are stable in the solid-state but display reversible σ-dimerization behavior in solution; monomeric radical species exist predominantly at high temperatures, while σ-dimerization is favoured at low temperatures. This dynamic behaviour has been confirmed by variable-temperature 1H NMR, UV-vis and EPR measurements. The structures of the stable radical and σ-dimer have been revealed by single-crystal X-ray diffraction analysis. The observed different reactivities of the two (porphyrinyl)dicyanomethyl radicals have been rationalized in terms of their spin delocalization behaviours.

Di-( m- m- m)terphenyl-Embedded Decaphyrin and Its Bis-Rh(I) Complex.

A new rectangular-shaped carbadecaphyrin was successfully synthesized by introducing a terphenylene unit ( m- m- m) in the macrocyclic core. The terphenylene moiety offers an open framework with multiple binding pockets to stabilize two Rh(I) ions in the core. The photophysical and structural studies reveal the non-aromatic character of the ligand and its bis-Rh(I) complex.

Syntheses of Bipyricorroles and Their Meso-Meso Coupled Dimers.

The metal-templated condensation strategy has been developed for the synthesis of meso-free bipyricorrole complexes. The reactive meso-CH in the monomer complex is further treated with various oxidative coupling reagents such as AgPF6, AgOTf, and FeCl3. Unlike Ag(I) salts, the FeCl3 resulted in a meso-meso-linked corrole homologue dimer. The synthetic methodologies successfully introduce the PdII monomer as well as PdII-PdII dimeric complexes in the corrole chemistry.

Chemodosimetric cyanide sensing in a 5,15-porphodimethene Pd(II) complex.

The Pd(II) complex of 5,15-porphodimethene is shown as a chemodosimetric sensor where it selectively senses cyanide ions, monitored through electronic spectral analysis and identified by the naked eye. The possible binding mechanism is also proposed based on the (1)H NMR analyses. Two polymorphs of the mentioned complex are obtained by using different polar solvents and further confirmed by single crystal X-ray analyses.