Chemistry inside a Porphyrin Skeleton: Platinacyclopentadiene from Tellurophene

Heterobimetallic 21,23-dimetallaporphyrin: activation of metal-metal interactions within the porphyrinoid macrocycle

Two core-modified porphyrins containing metal atoms, namely platinum(II) or platinum(IV) and rhodium(III), in place of two NH units, have been obtained by a post-synthetic modification of the 21,23-ditelluraporphyrin. The products of the tellurium-to-metal exchange, 21-platina-23-rhodaporphyrins, incorporate rhodacyclopentadiene and platinacyclopentadiene units with the metal atoms facing each other. The two molecules exhibit different degrees of metal-metal interaction depending on the oxidation state of platinum, with the NBO bond order being 0.04 for platinum(IV) and 0.15 for platinum(II). Consistently, the Quantum Theory of Atoms in Molecules analysis revealed the presence of the bond determinant, the bond critical point, in the platinum(II) species, in contrast to the platinum(IV) congener. The two porphyrinoids are interconvertible in redox reactions. They both exhibit fluxional behaviour in solution, studied by ¹H NMR, involving alteration in the metal ion coordination sphere accompanied by the macrocyclic skeleton conformation change.

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.

Metalation of Tellurophene: Reactivity of 21,23-Ditelluraporphodimethene toward Palladium(II), Platinum(II), and Rhodium(I)

Ditelluraporphodimethene, a nonaromatic porphyrinoid containing two tellurophene rings, reacted with palladium(II), platinum(II), and rhodium(I) following two different paths. Palladium(II) formed bonds to two tellurium donors of the macrocycle, yielding a side-on coordination compound, with a square planar (Te₂Cl₂) metal ion environment. An alternative reaction path has been observed for ditelluraporphodimethene with platinum(II) or rhodium(I) in high boiling solvents. These conditions led to the profound transformation, that is, one tellurium atom to a metal atom exchange, resulting in the formation of organometallic species containing metallacyclopentadiene rings, that is, 21-platina-23-telluraporphodimethene and 21-rhoda-23-telluraporphodimethene. The substitution reaction proceeded selectively at the tellurophene ring within the conjugated part of the molecule, that is, the tellurophene ring bound to two sp² meso-carbon atoms. In the case of platinum, the exchange was accompanied by one meso-aryl ring fusion with the formed platinacyclopentadiene ring, and the platinum(II) macrocycle underwent reversible oxidation with chlorine. The products are stable and represent first nonaromatic examples of metalloporphyrinoids, with a metallacyclopentadiene ring incorporated into a porphodimethene skeleton.

Two Rhodium(III) Ions Confined in a [18]Porphyrin Frame: 5,10,15,20‐Tetraaryl‐21,23‐Dirhodaporphyrin

Tetraaryl‐21,23‐dirhodaporphyrin and a series of related monorhodaporphyrins have been obtained by tellurium‐to‐rhodium exchange in a reaction of tetraaryl‐21,23‐ditelluraporphyrin with [RhCl(CO)₂]₂. These organometallic metallaporphyrins contain rhodium(III) centers embedded in rhodacyclopentadiene rings, incorporated within the porphyrin frames. The skeletons of 21,23‐dirhodaporphyrin and 21‐rhoda‐23‐telluraporphyrin are strongly deformed in‐plane from the rectangular shape typical for porphyrins, due to rhodium(III) coordination preferences, the large size of the two core atoms, and the porphyrin skeleton constrains. These two metallaporphyrins exhibit fluxional behavior, as studied by ¹H NMR and DFT, involving the in‐plane motion and the switch of the rhodium center(s) between two nitrogen donors. A side product detected in the reaction mixture, 21‐oxa‐23‐rhodaporphyrin, results from tellurium‐to‐oxygen exchange, occurring in parallel to the tellurium‐to‐rhodium exchange. The reaction paths and mechanisms have been analyzed. The title 21,23‐dirhodaporphyrin contains a bridged bimetallic unit, Rh₂Cl₂, in the center of the macrocycle, with two rhodium(III) ions lying approximately in the plane of the porphyrinoid skeleton. The geometry of the implanted Rh₂Cl₂ unit is affected by macrocyclic constrains.

Chemistry of tellurium containing macrocycles

The chemistry of Tellurium containing macrocycles has received great attraction and developed rapidly. Recently inorganic chemists are fascinated by ligands containing macrocycles having tellurium as soft donor and N and O as hard donor atoms. The tellurium atom is more electropositive than carbon due to its large size that resulted in polarisation of Te–C bond. So, tellurium containing macrocycles are explored due to their high reactivity and toxicity. Well-designed macrocycles containing different metals is an interesting field of chemistry as macrocycle with mixed donor atoms can bind two different metal atoms with different nature within the same cavity and thereby ion selectivity increases. Chemistry of macrocycles with tellurium as soft donor atoms also gives rise to very interesting coordination behaviour as addition of Tellurium in macrocycle adds an additional probe (125Te NMR help to monitor their structures in solutions). The chemistry of hard and soft donors in macrocyclic framework makes interesting coordination chemistry and need to be explore. The discussion includes different types of tellurium macrocycles and their chemistry.

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

21-Carba-23-oxaporphyrinoids and 21-oxo-21-carba-23-oxaporphyrinoids: macrocyclic π-conjugation involving the carbonyl moiety

Aromaticity of 21-oxo-21-carba-23-oxachlorin resulted from a predominant dipolar contributor. Nonaromaticity of 21-oxo-21-carba-23-oxaporphyrin reflects a participation of canonical aromatic and antiaromatic forms engaged in a peculiar “tug of war”.

The inner oxygen-substituted strategy effects on structure, aromaticity and absorption spectra of corrole isomers: A theoretical study

Corrole and oxaporphyrin have been successfully synthesized and applied in many research fields such as organic photoelectronics and sensors with the unique photophysical and chemical properties. However, the low synthesis yields of oxacorrole drive researchers turning their attention to theoretical studies for more reasonable molecular structure as the appeal of energy conservation and green chemistry. Corroles, oxacorroles (OC) and dioxacorroles (DOC), a total of 14 molecules, are calculated to systematically explore their structures, intramolecular hydrogen bonds, molecular aromatic and absorption spectral properties influenced by the inner O atoms positions with density functional theory (DFT) and time-dependent density functional theory (TDDFT). The smaller NICS(1)_ZZ values of oxacorrole (-35.23 ppm to -33.54 ppm) and dioxacorrole (-34.91 ppm to -33.24) than these of corroles (-32.97 ppm and -33.12 ppm) indicate that the O atoms attendances can increase the molecular aromaticity. The gradually increasing energy gaps of H-8 to H-3 from Corrole1 and Corrole2 to DOC series and the larger charge of C_O (+0.208e-+0.380e) than that of C_N (+0.065e-+0.177e) illustrate that the substitution of O can reduce the degeneracy degree of energy levels and change the charge distributions. With Hirshfeld method, the molecular orbital contributions of H-1, HOMO, LUMO and L+1 exhibit the regular effects of O atoms positions on orbital energy and electron absorption spectra. For series 1, 23O is beneficial to the red shift of electron absorption spectra. These theoretical conclusions manifest that OC1-23 and DOC1-1 possess the excellent absorption characteristics in the visible region, which can be used as potential materials in the fields of photoelectric materials.

Telluracarbaporphyrins and a Related Palladium(II) Complex: Evidence for Hypervalent Interactions

The reaction of a carbatripyrrin with a tellurophene dicarbinol in the presence of BF₃·Et₂O, followed by oxidation with DDQ, afforded the first example of a telluracarbaporphyrin. Although this system exhibits strongly aromatic characteristics, it is prone to air oxidation, giving rise to a hydroxy derivative that was characterized by X-ray crystallography. The initial telluracarbaporphyrin reacted with palladium(II) acetate to give a stable organometallic complex, and X-ray crystallography showed that the palladium cation was coordinated to all four atoms in the CNTeN core. An oxacarbatripyrrin was also reacted with a tellurophene dialcohol to give an air-stable porphyrin analogue with a CNTeO core. Nonmetalated telluracarbaporphyrins showed relatively short Te-N separations that strongly implied the involvement of hypervalent tellurium interactions. Furthermore, despite the presence of a very large tellurium atom, the tellurophene subunit is not strongly pivoted away from the mean macrocyclic plane as would be expected in the absence of these interactions. The aromatic properties of heterocarbaporphyrins were assessed by proton NMR spectroscopy, NICS calculations, and AICD plots. In addition, the relative stability of hydroxytelluraporphyrins in comparison to their tellurophene oxide tautomers was investigated and the aromatic characteristics of these oxidized structures were evaluated.