Hückel and Möbius Expanded para‐Benziporphyrins: Synthesis and Aromaticity Switching

The four expanded p‐benziporphyrins A,C‐di‐p‐benzi[24]pentaphyrin(1.1.1.1.1), N‐fused A‐p‐benzi[24]pentaphyrin, A,D ‐di‐p‐benzi[28]hexaphyrin(1.1.1.1.1.1), and A,C‐di‐p‐benzi[28]hexaphyrin(1.1.1.1.1.1) were obtained in three‐component Lindsey‐type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY‐like difluoroboron complex was obtained from the A,C‐di‐p‐benzi[24]pentaphyrin, whereas A,C‐di‐p‐benzi[28]hexaphyrin yielded a Möbius‐aromatic Pd^II complex containing fused pyrrole and phenylene subunits. Conformational behavior, tautomerism, and acid‐base chemistry of the new macrocycles were characterized by means of NMR spectroscopy and DFT calculations. Free base N‐fused A‐p‐benzi[24]pentaphyrin showed temperature‐dependent Hückel–Möbius aromaticity switching, whereas the A,C‐di‐p‐benzi[28]hexaphyrin formed a Möbius‐aromatic dication.     

Skeletal Rearrangement of Twisted Thia‐Norhexaphyrin: Multiply Annulated Polypyrrolic Aromatic Macrocycles

A hybrid thia‐norhexaphyrin comprising a directly linked N‐confused pyrrole and thiophene unit ( 1 ) revealed unique macrocycle transformations to afford multiply inner‐annulated aromatic macrocycles. Oxidation with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone triggered a cleavage of the C?S bond of the thiophene unit, accompanied with skeletal rearrangement to afford unique π‐conjugated products: a thiopyrrolo‐pentaphyrin embedded with a pyrrolo[1,2]isothiazole ( 2 ), a sulfur‐free pentaphyrin incorporating an indolizine moiety ( 3 ), and a thiopyranyltriphyrinoid containing a 2H‐thiopyran unit ( 4 ). Furthermore, 2 underwent desulfurization reactions to afford a fused pentaphyrin containing a pyrrolizine moiety ( 5 ) under mild conditions. Using expanded porphyrin scaffolds, oxidative thiophene cleavage and desulfurization of the hitherto unknown N‐confused core‐modified macrocycles would be a practical approach for developing unique polypyrrolic aromatic macrocycles.     

Siamese‐Twin Porphyrin Origami: Oxidative Fusing and Folding

Oxidation of a nonaromatic Siamese‐twin porphyrin, a pyrazole‐containing expanded porphyrin with two porphyrinlike binding pockets, with a stoichiometric amount of the two‐electron, two‐proton oxidizing agent 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzochinone led to the formation of a single N^pz‐C^o‐Ph linkage between the pyrazole unit with a neighboring meso‐phenyl group, forming a pyrazolo‐ [1,5‐a]indole moiety. Repeated treatment with a second equivalent of the oxidant yielded a doubly N‐fused species, involving the second pyrazole moiety. The conversion products were characterized by variable‐temperature and multinuclear 1D and 2D NMR spectroscopy. The fusions strongly alter the conformation of the macrocycles, as shown by X‐ray diffraction analyses of all three compounds, eventually leading to a folded structure. UV/Vis and NMR‐spectroscopic investigations indicated the presence of highly delocalized but nonmacrocycle‐aromatic π systems. This behavior of the Siamese‐twin porphyrin in response to oxidation is in contrast to the behavior of related all‐pyrrole‐based expanded macrocycles that switch, by redox processes and protonation, between Hückel and Möbius aromatic states.     

Synthesis of A2B6‐Type [36]Octaphyrins: Copper(II)‐Metalation‐Induced Fragmentation Reactions to Porphyrins and N‐Fusion Reactions of meso‐(3‐Thienyl) Substituents

5,10,15‐Tris(pentafluorophenyl)tetrapyrromethane was efficiently prepared through a route involving stepwise diaroylation of 5‐pentafluorophenyldipyrromethane. A₂B₆‐type [36]octaphyrins were prepared by the cross condensation of the tetrapyrromethane with aryl aldehydes in moderate yields. A₂B₆‐type [36]octaphyrins bearing 2,4,6‐trifluorophenyl, 2,6‐dichlorophenyl, and phenyl substituents underwent Cu^II‐metalation‐induced fragmentation to give two molecules of AB₃‐type Cu^II porphyrins. A₂B₆‐type [36]octaphyrin bearing 3‐thienyl substituents underwent thermal N‐thienyl fusion reactions to provide a modestly aromatic [38]octaphyrin, which, upon treatment with MnO₂, underwent further N‐thienyl fusion and subsequent oxidation to give a nonaromatic doubly N‐thienyl fused [36]octaphyrin.     

The Synthesis of (η3‐C3H5)Pd{Si[N(tBu)CH]2}Cl and the Catalytic Property for Heck Reaction

Treatment of N‐heterocyclic silylene Si[N(^tBu)CH]₂ ( 1 ) and [(η³‐C₃H₅)PdCl]₂ in toluene led to the formation of the mononuclear complex (η³‐C₃H₅)Pd{Si[N(^tBu)CH]₂}Cl ( 3 ), the silicon analogue to N‐heterocyclic carbene complex (η³‐C₃H₅)Pd{C[N(^tBu)CH]₂}Cl ( 2 ). Complex 3 was characterized with ¹H NMR and ¹³C NMR. Investigation shows that (η³‐C₃H₅)Pd{Si[N(^tBu)CH]₂}Cl is an active catalyst for Heck coupling reaction of styrene with aryl bromides.     

SiIV Incorporation into a [28]Hexaphyrin That Triggered Formation of Möbius Aromatic Molecules

Incorporation of Si^IV into an expanded porphyrin has been achieved for the first time. Treatment of [28]hexaphyrin 1 with CH₃SiCl₃ and N,N‐diisopropylethylamine gave Si^IV complex 2 and its N‐fused product 4 that both have Möbius aromatic nature. In both complexes, the coordinated Si atom is satisfied in a typical trigonal bipyramidal coordination. Si^IV incorporation induces conformational rigidification and redshifted absorption profiles due to σ–π conjugation between the Si atom and hexaphyrin macrocycle. Tamao–Fleming oxidation of 2 with H₂O₂ gave β‐hydroxy [28]hexaphyrin 5 , which exists as a ruffled rectangular shape in the solid state, yet it has been revealed to exist predominantly as a twisted Möbius aromatic conformer in CH₂Cl₂.     

Neo‐Fused Hexaphyrin: A Molecular Puzzle Containing an N‐Linked Pentaphyrin

The first neo‐confused hexaphyrin(1.1.1.1.1.0) was synthesized by oxidative ring closure of a hexapyrrane bearing two terminal “confused” pyrroles. The new compound displays a folded conformation with a short interpyrrolic C???N distance of 3.102 Å, and thus it readily underwent ring fusion to afford a neo‐fused hexaphyrin with an unprecedented 5,5,5,7‐tetracyclic ring structure. Furthermore, coordination of Cu^II triggered a ring opening/contracting reaction to afford a Cu^II complex of an N‐linked pentaphyrin derivative. The roles of reactive N? C bonds in the porphyrinoid macrocycles were demonstrated.     

Fused Corrole Dimers Interconvert between Nonaromatic and Aromatic States through Two‐Electron Redox Reactions

A singly linked corrole dimer was synthesized by condensation of a dipyrromethane‐1‐carbinol with 1,1,2,2‐tetrapyrroethane. Oxidation of the dimer gave doubly linked corrole dimers 9 and 10 as the first examples of fused corrole dimers involving a meso–meso linkage. Dimers 9 and 10 exhibit characteristic ¹H NMR spectra, absorption spectra, excited‐state dynamics, and two‐photon absorption (TPA) values, which indicate the nonaromatic nature of 9 and the aromatic nature of 10 . Interestingly, 9 is fairly stable despite its unusual 2H‐corrole structure, which has been ascribed to the presence of two direct connections between the individual corrole units.     

An N‐Heterocyclic Silylene‐Stabilized Digermanium(0) Complex

The synthesis of an N‐heterocyclic silylene‐stabilized digermanium(0) complex is described. The reaction of the amidinate‐stabilized silicon(II) amide [LSiN(SiMe₃)₂] ( 1 ; L=PhC(NtBu)₂) with GeCl₂?dioxane in toluene afforded the Si^II–Ge^II adduct [L{(Me₃Si)₂N}Si→GeCl₂] ( 2 ). Reaction of the adduct with two equivalents of KC₈ in toluene at room temperature afforded the N‐heterocyclic carbene silylene‐stabilized digermanium(0) complex [L{(Me₃Si)₂N}Si→ Ge?Ge←Si{N(SiMe₃)₂}L] ( 3 ). X‐ray crystallography and theoretical studies show conclusively that the N‐heterocyclic silylenes stabilize the singlet digermanium(0) moiety by a weak synergic donor–acceptor interaction.     

N-fused pentaphyrins and their rhodium complexes: oxidation-induced rhodium rearrangement

meso-Aryl-substituted pentaphyrins were isolated in the modified Rothemund-Lindsey porphyrin synthesis as a 22-pi-electron N-fused pentaphyrin ([22]NFP5) and a 24-pi-electron N-fused pentaphyrin ([24]NFP5), which were reversibly interconvertible by means of two-electron reduction with NaBH4 or two-electron oxidation with dichlorodicyanobenzoquinone (DDQ). Judging from 1H NMR data, [22]NFP5 is aromatic and possesses a diatropic ring current, while [24]NFP5 exhibits partial anti-aromatic character. Metalation of [22]NFP5 1 with a rhodium(I) salt led to isolation of rhodium complexes 9 and 10, whose structures were unambiguously characterized by X-ray diffraction analyses and were assigned as conjugated 24-pi and 22-pi electronic systems, respectively. In the rhodium(I) metalation of 1, the complex 9 was a major product at 20 degrees C, but the complex 10 became preferential at 55 degrees C. Upon treatment with DDQ, compound 9 was converted to 10 with an unprecedented rearrangement of the rhodium atom.     

Macrocyclic Transformations from Norrole to Isonorrole and an N‐Confused Corrole with a Fused Hexacyclic Ring System Triggered by a Pyrrole Substituent

Three kinds of fused porphyrinoids, L2 – L4 , possessing different types of corrole‐based frameworks were synthesized from a pyrrole‐substituted corrole isomer (norrole L1 ). Oxidation of L1 afforded a unique N‐C_meso‐fused pyrrolyl isonorrole L2 , involving the fusion of an auxiliary pyrrolic NH moiety with a meso‐sp³‐hybridized carbon atom. Subsequently, L2 underwent macrocycle transformations to give singly and doubly N‐C_Ar‐fused N‐confused corroles, L3 and L4 , respectively. L3 and L4 contain fused [5.7.6.5]‐tetra‐ and [5.6.7.7.6.5]‐hexacyclic structures, respectively, prepared through lateral annulation. These skeletal transformation reactions from norrole to its isomer isonorrole and finally to N‐confused corrole indicate that multiply fused porphyrinoids could be readily synthesized from pyrrole‐appended confused porphyrinoids.     

meso‐3,5‐Bis(trifluoromethyl)phenyl‐Substituted Expanded Porphyrins: Synthesis,Characterization, and Optical,Electrochemical, and Photophysical Properties

Trifluoroacetic acid‐catalyzed condensation of pyrrole with electron‐deficient and sterically hindered 3,5‐bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso‐3,5‐bis(trifluoromethyl)phenyl‐substituted expanded porphyrins including [22]sapphyrin 2 , N‐fused [22]pentaphyrin 3 , [26]hexaphyrin 4 , and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20‐tetrakis(3,5‐bis(trifluoromethyl)phenyl)porphyrin 1 . These expanded porphyrins are characterized by mass spectrometry, ¹H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO–LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.     

N‐Fusion Approach in Construction of Contracted Carbaporphyrinoids: Formation of N‐Fused Telluraporphyrin

Insertion of PCl₃ into 5,10,15,20‐tetraaryl‐21‐telluraporphyrin leads to a phosphorus complex of N‐fused dihydrotelluraporphyrin with an inverted tellurophene ring. Its CNN coordination core places the macrocycle in the family of contracted carbaporphyrinoids. A cycle of direct transformations affords an elegant triangle of three mutually convertible N‐fused porphyrinoids, with distinct spectroscopic features: antiaromatic, nonaromatic and aromatic. The nonaromatic species has a dome shaped skeleton which forms in the solid state a ball and socket structure with C₆₀.     

Group 9 Metal Complexes of meso‐Aryl‐Substituted Rubyrin

The metalation of meso‐tetrakis(pentafluorophenyl)‐substituted [26]rubyrin has been explored with Group 9 metal salts (Rh^I, Co^II, Ir^III), affording a Hückel aromatic [26]rubyrin–bis‐Rh^I complex with a highly curved gable‐like structure, a Hückel antiaromatic [24]rubyrin–bis‐Co^II complex that displays intramolecular antiferromagnetic coupling between the two Co^II ions (J=?4.5 cm^?1), and two Cp*‐capped Ir^III complexes; in one, the iridium metal sits on the [26]rubyrin frame with two Ir?N bonds, whereas the other has an additional Ir?C bond, although both Ir^III complexes display moderate aromatic character. This work demonstrates characteristic metalation abilities of this [26]rubyrin toward Group 9 metals.     

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [ 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [ 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [ 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex [Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [ 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating [ 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex [ 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [ 1? F]^?, exposure of [ 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex [Cs ?1? Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.     

A non-fused mono-meso-free pentaphyrin and its rhodium(I) complex

5,10,15,20-Tetrakis(pentafluorophenyl) [22]pentaphyrin(1.1.1.1.1) 7 was synthesised and its bis-rhodium(I) complex 12 has been revealed to be a non-fused, yet planar pentaphyrin with an inverted pyrrole. Both 7 and 12 are aromatic, showing sharp Soret-like bands and diatropic ring currents.     

Conformational Change from a Twisted Figure‐Eight to an Open‐Extended Structure in Doubly Fused 36π Core‐Modified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity

Two examples of core‐modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure‐eight to an open‐extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid‐catalyzed condensation of dipyrrane unit containing an electron‐rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). The single‐crystal X‐ray structure of the octaphyrin (in which Ar=mesityl) shows a figure‐eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π‐cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β‐CH and ? NH protons in the ¹H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two‐photon absorption cross‐section values. Furthermore, nucleus‐independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy‐induced current density (AICD) plots indicate paratropic ring‐currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single‐crystal X‐ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring‐flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso‐carbon atoms and are held by N? H ??? O, N? H ??? F, and C? H ??? F intermolecular hydrogen‐bonding interactions. The extended‐open structure upon protonation allows π‐delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state.     

ESI‐MS/MS of expanded porphyrins: a look into their structure and aromaticity

Electrospray mass spectrometry/mass spectrometry was used to investigate the gas‐phase properties of protonated expanded porphyrins, in order to correlate those with their structure and conformation. We have selected five expanded meso‐pentafluorophenyl porphyrins, respectively, a pair of oxidized/reduced fused pentaphyrins (22 and 24 π electrons), a pair of oxidized/reduced regular hexaphyrins (26 and 28 π electrons) and a regular doubly N‐fused hexaphyrin (28 π electrons). The gas‐phase behavior of the protonated species of oxidized and reduced expanded porphyrins is different. The oxidized species (aromatic Hückel systems) fragment more extensively, mainly by the loss of two HF molecules. The reduced species (Möbius aromatic or Möbius‐like aromatic systems) fragment less than their oxidized counterparts because of their increased flexibility. The protonated regular doubly fused hexaphyrin (non‐aromatic Hückel system) shows the least fragmentation even at higher collision energies. In general, cyclization through losses of HF molecules decreases from the aromatic Hückel systems to Möbius aromatic or Möbius‐like aromatic systems to non‐aromatic Hückel systems and is related to an increase in conformational distortion. Copyright © 2016 John Wiley & Sons, Ltd.     

Ligand‐Centred Reactivity of Bis(picolyl)amine Iridium: Sequential Deprotonation,Oxidation and Oxygenation of a “Non‐Innocent” Ligand

Treatment of [Ir(bpa)(cod)]⁺ complex [ 1 ]⁺ with a strong base (e.g., tBuO^?) led to unexpected double deprotonation to form the anionic [Ir(bpa?2H)(cod)]^? species [ 3 ]^?, via the mono‐deprotonated neutral amido complex [Ir(bpa?H)(cod)] as an isolable intermediate. A certain degree of aromaticity of the obtained metal–chelate ring may explain the favourable double deprotonation. The rhodium analogue [ 4 ]^? was prepared in situ. The new species [M(bpa?2H)(cod)]^? (M=Rh, Ir) are best described as two‐electron reduced analogues of the cationic imine complexes [M^I(cod)(Py‐CH₂‐N?CH‐Py)]⁺. One‐electron oxidation of [ 3 ]^? and [ 4 ]^? produced the ligand radical complexes [ 3 ]^. and [ 4 ]^.. Oxygenation of [ 3 ]^? with O₂ gave the neutral carboxamido complex [Ir(cod)(py‐CH₂‐N‐CO‐py)] via the ligand radical complex [ 3 ]^. as a detectable intermediate.     

3‐Rhoda‐1,2‐diazacyclopentanes: A Series of Novel Metallacycle Complexes Derived From CN Functionalization of Ethylene

Rh‐containing metallacycles, [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NR)₂‐]Cl; TPA=N,N,N,N‐tris(2‐pyridylmethyl)amine have been accessed through treatment of the Rh^I ethylene complex, [(TPA)Rh(η²‐CH₂CH₂)]Cl ([ 1 ]Cl) with substituted diazenes. We show this methodology to be tolerant of electron‐deficient azo compounds including azo diesters (RCO₂N?NCO₂R; R=Et [ 3 ]Cl, R=iPr [ 4 ]Cl, R=tBu [ 5 ]Cl, and R=Bn [ 6 ]Cl) and a cyclic azo diamide: 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD), [ 7 ]Cl. The latter complex features two ortho‐fused ring systems and constitutes the first 3‐rhoda‐1,2‐diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N–N coordination followed by insertion of ethylene into a [Rh]?N bond. In terms of reactivity, [ 3 ]Cl and [ 4 ]Cl successfully undergo ring‐opening using p‐toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh^III(Cl)(κ¹‐(C)‐CH₂CH₂(NCO₂R)(NHCO₂R)]OTs; [ 13 ]OTs and [ 14 ]OTs. Deprotection of [ 5 ]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end‐on coordinated diazene [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NH)₂‐]⁺ [ 16 ]Cl, a hitherto unreported motif. Treatment of [ 16 ]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NAc)₂‐]⁺, [ 17 ]Cl. Treatment of [ 1 ]Cl with AcN?NAc did not give the Rh?N insertion product, but instead the N,O‐chelated complex [(TPA)Rh^I ( κ²‐(O,N)‐CH₃(CO)(NH)(N?C(CH₃)(OCH?CH₂))]Cl [ 23 ]Cl, presumably through insertion of ethylene into a [Rh]?O bond.