Antiaromatic Sapphyrin Isomer: Transformation into Contracted Porphyrinoids with Variable Aromaticity

Sapphyrin is a pentapyrrolic expanded porphyrin with a 22π aromatic character. Herein, we report the synthesis of a 20π antiaromatic sapphyrin isomer 1 by oxidative cyclization of a pentapyrrane precursor P₅ with a terminal β-linked pyrrole. The resulting isomer 1 , containing a mis-linked bipyrrole unit in the skeleton, exhibits a reactivity for further oxidation due to the distinct antiaromatic electronic structure, affording a fused macrocycle 2 , possessing a spiro-carbon-containing [5.6.5.6]-tetracyclic structure. Subsequent treatment with an acid afforded a weakly aromatic pyrrolone-appended N-confused corrole 3 , and thermal fusion gave a [5.6.5.7]-tetracyclic-ring-embedded 14π aromatic triphyrin(2.1.1) analog 4 . The cyclization at the mis-linked pyrrole moiety of P₅ played a crucial role in synthesizing the antiaromatic porphyrinoid susceptible to facile transformation to novel porphyrinoids with variable aromaticity.     

Bipyricorrole: A Corrole Homologue with a Monoanionic Core as a Fluorescence ZnII 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)₃N] 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.     

N‐Confused Phlorin‐Prodigiosin Chimera: meso‐Aryl Oxidation and π‐Extension Triggered by Peripheral Coordination

An N‐confused phlorin isomer bearing a dipyrrin moiety at the α‐position of the confused pyrrole ring ( 1 ) was synthesized. Pd^II and B^III coordination at the peripheral prodigiosin‐like moiety of 1 afforded the corresponding complexes 2 and 3 . Reflux of 2 in triethylamine (TEA) converted the meso‐phenyl into the Pd^II‐coordinating phenoxy group to afford 4 . Under the same reaction conditions, TEA was linked to the α‐position of the dipyrrin unit in 3 as an N,N‐diethylaminovinyl group to afford 5 . Furthermore, peripheral coordination of B^III in 3 and 5 improved the planarity of the phlorin macrocycle and thus facilitated the coordination of Ag^III at the inner cavity to afford 3‐Ag and 5‐Ag , respectively. These results provide an effective approach for developing unique porphyrinoids through peripheral coordination.     

Insights into the Tautomerism in meso‐Substituted Corroles: A Variable‐Temperature 1H, 13C, 15N,and 19F NMR Spectroscopy Study

Tris(pentafluorophenyl)corrole and its ¹⁵N‐enriched isotopomer were studied in [D₈]toluene solution by 1D and 2D variable‐temperature NMR techniques to establish the mechanisms of tautomerization of the NH protons inside the interior of the corrole macrocycle. Three such rate processes could be identified of which two modulate the spectral line shapes at temperatures above 205 K and the third is NMR‐inaccessible as it is very fast. The latter involves the proton engaged in an unsymmetrical proton sponge unit formed by two pyrrole nitrogen atoms. Temperature and concentration dependences of the two remaining processes were determined. One of them is purely intramolecular and the other is intermolecular at low temperatures, with growing contribution of an intramolecular mechanism at elevated temperatures. The proposed microscopic mechanisms of all these processes are semi‐quantitatively confirmed by quantum chemical calculations using density functional theory.     

Triply Linked Corrole Dimers

The oxidation of 10–10′ singly linked corrole dimers with DDQ at low concentration in CHCl₃ afforded meso–meso, β–β, β–β triply linked 2H‐corrole dimers (with two inner NH groups in each corrole unit), which exhibited characteristic ¹H NMR and absorption spectra attributable to their nonaromatic electronic networks. These 2H‐corrole dimers were reduced with NaBH₄ to aromatic 3H‐corrole dimers, which were unstable and easily oxidized back to the 2H‐corrole dimers upon exposure to air. Bis(zinc(II)) complexes of the 2H‐corrole dimers were synthesized and characterized as rare examples of nonaromatic zinc(II) corrole complexes.     

Electronic Structure of Copper Corroles

The ground state electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. Computational studies formulate neutral Cu corroles with an antiferromagnetically coupled Cu^II corrole radical cation ground state. X‐ray photoelectron spectroscopy, EPR, and magnetometry support this assignment. For comparison, Cu^II isocorrole and [TBA][Cu(CF₃)₄] were studied as authentic Cu^II and Cu^III samples, respectively. In addition, the one‐electron reduction and one‐electron oxidation processes are both ligand‐based, demonstrating that the Cu^II centre is retained in these derivatives. These observations underscore ligand non‐innocence in copper corrole complexes.     

Synthesis and Anticancer Activity Study of Some Novel Substituted and Fused Pyridotriazepine Derivatives

Various new substituted and fused pyridotriazepine analogues have been synthesized via different synthetic pathways. Among which are different heterocyclic compounds consisting of the pyridotriazepine backbone fused to different heterocyclic systems comprising either substituted pyrimidine nucleus such as compounds 3 – 9 or substituted 4‐aminopyridine nucleus such as compounds 10 – 16 . Besides, the tetrahydroquinoline derivative 17 , [1,2,4]triazolopyrimidine derivative 18 , thienodiazocine derivative 19 , dihydrobenzofuropyridine derivative 20 , and the substituted pyrrole derivative 21 were synthesized. In addition, different substituted pyridotriazepine derivatives as indicated in compounds 22 – 25 were designed and synthesized. Twenty‐five of the newly synthesized compounds were subjected to in vitro anticancer screening against mammalian colon carcinoma HCT‐116 cell line using Cisplatin as a reference drug. The anticancer activity screening results revealed that among the tested compounds, the tetrahydropyrido[1,2‐b]pyrimido[4,5‐e][1,2,4]triazepine derivative 4 substituted at C₂ and C₄ positions with S‐methyl and amino moieties, respectively, and the 2,4‐dithioxo analogue 9 and the 2‐thioxodipyrido[1,2‐b:2′,3′‐e][1,2,4]triazepine derivative 11 substituted at C₃ and C₄ with a cyano and amino moieties, respectively, exhibited moderate to strong anticancer activity against mammalian colon carcinoma HCT‐116 cell line.     

N-Confused Phlorin-Prodigiosin Chimera: meso-Aryl Oxidation and π-Extension Triggered by Peripheral Coordination

An N-confused phlorin isomer bearing a dipyrrin moiety at the α-position of the confused pyrrole ring ( 1 ) was synthesized. Pd^II and B^III coordination at the peripheral prodigiosin-like moiety of 1 afforded the corresponding complexes 2 and 3 . Reflux of 2 in triethylamine (TEA) converted the meso-phenyl into the Pd^II-coordinating phenoxy group to afford 4 . Under the same reaction conditions, TEA was linked to the α-position of the dipyrrin unit in 3 as an N,N-diethylaminovinyl group to afford 5 . Furthermore, peripheral coordination of B^III in 3 and 5 improved the planarity of the phlorin macrocycle and thus facilitated the coordination of Ag^III at the inner cavity to afford 3-Ag and 5-Ag , respectively. These results provide an effective approach for developing unique porphyrinoids through peripheral coordination.     

Rational Synthesis of 5,5,5‐Tricyclic Fused Thia‐heptaphyrin (1.1.1.1.1.1.0) From a Helical Oligopyrrin Hybrid

Oxidation of a thiophene‐hexapyrrane hybrid S‐P₆ afforded a stable conjugated open‐chain thiaheptapyrrolic helix 1 with the terminal thiophene and confused pyrrole units lying at a long distance that is adverse for further cyclization. Chelation of 1 with copper(II) ion afforded 1‐Cu , which exhibits more distant terminal units. Interestingly, further oxidation of 1 triggered an intramolecular C?N fusion reaction to afford a unique 5,5,5‐tricyclic fused linear thiaheptapyrrin 2 , with the two terminals positioned in proximity, which favors the oxidative ring‐closure reaction to give a unique 5,5,5‐tricyclic fused thiaheptaphyrin (1.1.1.1.1.1.0) 3 under air. The inner‐fusion strategy for positioning the reactive sites in proximity to promote oxidative cyclization offers a new approach for constructing large porphyrinoids through conjugated oligopyrrins without the assistance of metal ions.     

Synthesis and Reactivity of 5,10,15‐Triaryl Doubly N‐Confused Bilanes

A series of 5,10,15‐tris(pentafluorophenyl) doubly N‐confused bilanes were synthesized in a stepwise manner with the aid of sterically demanding N‐protecting groups, in which the difference in reactivity between regular pyrrole and N‐confused pyrrole plays a crucial role in the synthetic strategy. Some doubly N‐confused bilanes were converted into porphyrinoids or a unique 2:2 copper(II) complex with a helical structure. In addition, the conformations and electronic states of the doubly N‐confused bilanes were investigated theoretically, giving fruitful information about the effect of confusion on the bilane skeleton.     

Lewis Base Assisted Magnesium Complexes Incorporating Pyrrolyl and Ketiminate Ligands: Synthesis,Structural Diversity and Characterization

A series of four, five and six‐coordinated magnesium derivatives integrating with substituted pyrrole and ketimine ligands are conveniently synthesized. Reaction of two equiv of 2‐dimethylaminomethyl pyrrole with Mg[N(SiMe₃)₂]₂ in THF affords the monomeric magnesium complex Mg[C₄H₃N(2‐CH₂NMe₂)]₂ (THF)₂ ( 1 ) in high yield along with elimination of two equiv of HN(SiMe₃)₂. Similarly, the reaction between two equiv of 2‐t‐butylaminomethyl pyrrole and Mg[N(SiMe₃)₂]₂ in THF renders the magnesium derivative, Mg[C₄H₃N(2‐CH₂NH^tBu)]₂(THF)2₂( 2 ) in good yield. Interestingly, reaction between two equiv of 2‐t‐butylaminomethyl pyrrole and Mg[N(SiMe₃)₂]₂ in toluene, instead of THF, generates Mg[C₄H₃N(2‐CH2NH^tBu)]₂ ( 3 ), also in high yield. Furthermore, the assembly of two equiv of ketimine ligand, HOCMeCHCMeNAr (Ar = C₆H₃‐2,6‐ⁱPr₂) and Mg[N(SiMe₃)₂]₂, yields five‐coordinated magnesium derivatives, Mg(OCMeCHCMeNAr)₂(THF) ( 4 ) and Mg(OCMeCHCMeNAr)₂(OEt₂) ( 5 ), using THF and diethyl ether, respectively. All the aforementioned derivatives are characterized by ¹H and ¹³C NMR spectroscopy as well as 1 , 3 , 4 and 5 are subjected to X‐ray diffraction analysis in solid state.     

Differentiation of aminomethyl corrole isomers by mass spectrometry

Two new isomeric aminomethyl corrole derivatives of [5,10,15‐tris(pentafluorophenyl)corrolato]gallium(III) were synthesized with pyridine (py) molecules as axial ligands. When investigated by electrospray ionization mass spectrometry, in the positive and the negative ion modes, these compounds showed an unusual gas‐phase behavior that could be used for their differentiation. In the positive ion mode, the differentiation was achieved through the formation of diagnostic fragment ions formed from [M‐py + H]⁺ precursors, by (CH₃)₂NH and HF losses. An unusual addition of water to the main fragment ions provides an alternative route for isomer identification. Semi‐empirical calculations were performed to elucidate the structures and stabilities of the main ionic species formed in the positive ion mode. In the negative ion mode isomer discrimination is accomplished via the fragmentation of the methoxide adduct ions [M‐py + CH₃O]^‐ through (CH₃)₂ N^. and HF losses. Copyright © 2012 John Wiley & Sons, Ltd.     

meso‐Free Corroles: Syntheses,Structures, Properties,and Chemical Reactivities

5,10‐Bis(pentafluorophenyl)corrole ( 5 ) and 5,15‐bis(pentafluorophenyl)corrole ( 9 ) have been synthesized as meso‐free corroles by rational synthetic routes. Both the structures of these corroles have been unambiguously revealed by X‐ray diffraction analysis and their optical and electrochemical properties have been studied. Chlorination and oxidative dimerization of 5 and 9 have been explored, which revealed a marked different reactivity of the free meso‐positions in 5 and 9 . 10‐Chlorinated corrole 11 was effectively prepared by the reaction of 9 with Palau‘chlor in the presence of 1 % pyridine whereas 5‐chlorinated corrole 12 was obtained in a trace amount from similar chlorination of 5 . 5,5′‐Linked corrole dimer 13 was produced by reaction of 5 with AgNO₂ in a good yield, whereas 10,10′‐linked corrole dimer 14 was formed in a moderate yield by the reaction of 9 with [bis(trifluoroacetoxy)iodo]benzene. Observed large electronic interaction between the two corroles in 13 as compared with that in 14 has been ascribed mainly to conformational flexibility of the former, which allows more coplanar conformation.     

The Corrole Radical

The reaction of 5,10,15‐trimesitylcorrole (H₃cor) with tungsten hexachloride and tungsten hexacarbonyl resulted in the unexpected formation of the 3,17‐dichloro‐5,10,15‐trimesitylcorrole radical (H₂cor*) as an air‐stable product. X‐ray crystallography proved the planarization of the corrole radical structure, which was rationalized by the reduced steric hindrance of two versus three hydrogen atoms inside the N₄ cavity. Although the aromaticity was lost, no specific changes in C? C or C? N bond distances could be observed. The regioselectivity of the two‐fold chlorination is the result of the nucleophilic attack of chloride ions to an oxidized corrole macrocycle, and is supported by DFT results. The corrole radical acts as a dianionic ligand and allows the insertion of the divalent zinc(II) cation, which usually does not form neutral corrole complexes.     

Synthesis and Photophysical Properties of a Sc3N@C80‐Corrole Electron Donor–Acceptor Conjugate

Embedding endohdedral metallofullerenes (EMFs) into electron donor–acceptor systems is still a challenging task owing to their limited quantities and their still largely unexplored chemical properties. In this study, we have performed a 1,3‐dipolar cycloaddition reaction of a corrole‐based precursor with Sc₃N@C₈₀ to regioselectively form a [5,6]‐adduct ( 1 ). The successful attachment of the corrole moiety was confirmed by mass spectrometry. In the electronic ground state, absorption spectra suggest sizeable electronic communications between the electron acceptor and the electron donor. Moreover, the addition pattern occurring at a [5,6]‐bond junction is firmly proven by NMR spectroscopy and electrochemical investigations performed with 1 . In the electronically excited state, which is probed in photophysical assays with 1 , a fast electron‐transfer yields the radical ion pair state consisting of the one‐electron‐reduced Sc₃N@C₈₀ and of the one‐electron‐oxidized corrole upon its exclusive photoexcitation. As such, our results shed new light on the practical work utilizing EMFs as building blocks in photovoltaics.     

Synthesis and Characterization of Metal Complexes (M = Al,Ti, W,Zn) Containing Pyrrole‐imine Ligands

A series of metal compounds (M = Al, Ti, W, and Zn) containing pyrrole‐imine ligands have been prepared and structurally characterized. The reactions of AlMe₃ with one and three equivs of pyrrole‐imine ligand [C₄H₃NH‐(2‐CH=N? CH₂Ph)] ( 1 ) generated aluminum compounds Al[C₄H₃N‐(2‐CH=N? CH₂Ph)]Me₂ ( 2 ) and Al[C₄H₃N‐(2‐CH=NCH₂Ph)]₃ ( 3 ), respectively, in relatively high yield. Reacting two equivs of 1 with Ti(OⁱPr)₄, W(NH^tBu)₂(=N^tBu)₂, or ZnMe₂ afforded Ti[C₄H₃N‐(2‐CH=NCH₂Ph)]₂(OⁱPr)₂ ( 4 ), W[C₄H₃N‐(2‐CH=NCH₂Ph)]₂(=N^tBu)₂ ( 5 ), and Zn[C₄H₃N‐(2‐CH=NCH₂Ph)]₂ ( 6 ), respectively. All the compounds have been characterized by ¹H and ¹³C NMR spectroscopy. Compounds 3 – 6 have also been characterized by single‐crystal X‐ray structural analysis. The biting angles of pyrrole‐imine ligand with metals decrease and their related M? N_pyrrole and M? N_imine bond lengths increase in the order of 6 , 3 , 4 , and 5 .     

Ultrafast Photoinduced Charge Separation Leading to High‐Energy Radical Ion‐Pairs in Directly Linked Corrole–C60 and Triphenylamine–Corrole‐C60 Donor–Acceptor Conjugates

Closely positioned donor–acceptor pairs facilitate electron‐ and energy‐transfer events, relevant to light energy conversion. Here, a triad system TPACor‐C₆₀ , possessing a free‐base corrole as central unit that linked the energy donor triphenylamine ( TPA ) at the meso position and an electron acceptor fullerene (C₆₀) at the β‐pyrrole position was newly synthesized, as were the component dyads TPA‐Cor and Cor‐C₆₀ . Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady‐state fluorescence studies showed efficient energy transfer from ¹ TPA* to the corrole and subsequent electron transfer from ¹corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron‐transfer products, the corrole radical cation ( Cor^?+ in Cor‐C₆₀ and TPA‐Cor^?+ in TPACor‐C₆₀ ) and fullerene radical anion (C₆₀^??), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, k_CS, was found to be about 10¹¹ s^?1, suggesting the occurrence of an ultrafast charge‐separation process. Interestingly, although an order of magnitude slower than k_CS, the rate of charge recombination, k_CR, was also found to be rapid (k_CR≈10¹⁰ s^?1), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge‐separated species relaxed directly to the ground state in polar solvents while in toluene, formation of ³corrole* was observed, thus implying that the energy of the charge‐separated state in a nonpolar solvent is higher than the energy of ³corrole* being about 1.52 eV. That is, ultrafast formation of a high‐energy charge‐separated state in toluene has been achieved in these closely spaced corrole–fullerene donor–acceptor conjugates.     

The Nitrogen‐Assisted Triphenylphosphine Displacement of 3‐Hydroxypyridine and 3‐Aminopyridine Ligands in Palladium(II) Complexes: Crystal Structures of [Pd(PPh3)Br]2{μ,η2‐C5H3N(OH)}2 and [Pd(PPh3)Br]2{μ,η2‐C5H3N(NH2)}2

Treatment of Pd(PPh₃₎₄ with 2‐bromo‐3‐hydroxypyridine [C₅H₃N(OH)Br] and 3‐amino‐2‐bromopyridine [C₅H₃N(NH₂)Br] in dichloromethane at ambient temperature cause the oxidative addition reaction to produce the palladium complex [Pd(PPh₃₎₂{η¹‐C₅H₃N(OH)}(Br)], 2 and [Pd(PPh₃₎₂{η¹‐C₅H₃N(NH₂)}(Br)], 3 , by substituting two triphenylphosphine ligands, respectively. In dichloromethane solution of complexes 2 and 3 at ambient temperature for 3 days, it undergo displacement of the triphenylphosphine ligand to form the dipalladium complexes [Pd(PPh₃)Br]₂{μ,η²‐C₅H₃N(OH)}₂, 4 and [Pd(PPh₃)Br]₂{μ,η²‐C₅H₃N(NH₂)}₂, 5 , in which the two 3‐hydroxypyridine and 3‐aminopyridine ligands coordinated through carbon to one metal center and bridging the other metal through nitrogen atom, respectively. Complexes 4 and 5 are characterized by X‐ray diffraction analyses.     

The Tetracyanopyrrolide Anion as Ligand in Transition Metal Complexes

Two isotypic mononuclear discrete complexes [Co(MeCN)₄(tcp)₂] · 2MeCN ( 1 ) and [Ni(MeCN)₄(tcp)₂] · 2MeCN ( 2 ) containing the tetracyanopyrrolide anion [C₄(CN)₄N]^– (tcp) were synthesized from [Me₄N]tcp and the respective metal perchlorates in acetone/acetonitrile. Tcp coordinates to the transition metal atoms in η¹ fashion via the nitrogen atom of the pyrrole ring. No coordination via the cyano groups is observed. Both complexes show nearly ideal paramagnetic behavior according to the Curie law with magnetic moments of 4.98 μ_B for 1 and 3.09 μ_B for 2 . In the presence of Cu²⁺ ions tcp reacts with traces of water under hydrolysis of one cyano group to tricyanopyrrole‐2‐carboxamide (NC₄(CN)₃C(O)NH₂)^– (tcpc). From solutions in DMF the complex [Cu(tcpc)₂(DMF)₂] ( 3 ) is isolated.     

Fine‐Tuning of Lewis Acidity: The Case of Borenium Hydride Complexes Derived from Bis(phosphinimino)amide Boron Precursors

Reactions of bis(phosphinimino)amines LH and L′H with Me₂S ? BH₂Cl afforded chloroborane complexes LBHCl ( 1 ) and L′BHCl ( 2 ), and the reaction of L′H with BH₃ ? Me₂S gave a dihydridoborane complex L′BH₂ ( 3 ) (LH=[{(2,4,6‐Me₃C₆H₂N)P(Ph₂)}₂N]H and L′H=[{(2,6‐iPr₂C₆H₃N)P(Ph₂)}₂N]H). Furthermore, abstraction of a hydride ion from L′BH₂ ( 3 ) and LBH₂ ( 4 ) mediated by Lewis acid B(C₆F₅)₃ or the weakly coordinating ion pair [Ph₃C][B(C₆F₅)₄] smoothly yielded a series of borenium hydride cations: [L′BH]⁺[HB(C₆F₅)₃]^? ( 5 ), [L′BH]⁺[B(C₆F₅)₄]^? ( 6 ), [LBH]⁺[HB(C₆F₅)₃]^? ( 7 ), and [LBH]⁺[B(C₆F₅)₄]^? ( 8 ). Synthesis of a chloroborenium species [LBCl]⁺[BCl₄]^? ( 9 ) without involvement of a weakly coordinating anion was also demonstrated from a reaction of LBH₂ ( 4 ) with three equivalents of BCl₃. It is clear from this study that the sterically bulky strong donor bis(phosphinimino)amide ligand plays a crucial role in facilitating the synthesis and stabilization of these three‐coordinated cationic species of boron. Therefore, the present synthetic approach is not dependent on the requirement of weakly coordinating anions; even simple BCl₄^? can act as a counteranion with borenium cations. The high Lewis acidity of the boron atom in complex 8 enables the formation of an adduct with 4‐dimethylaminopyridine (DMAP), [LBH ? (DMAP)]⁺[B(C₆F₅)₄]^? ( 10 ). The solid‐state structures of complexes 1 , 5 , and 9 were investigated by means of single‐crystal X‐ray structural analysis.