The Fluorenyl Cation

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.     

An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air‐Stable 25π Radical

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open‐shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air‐ and water‐stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one‐electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single‐crystal X‐ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.     

Isolation of an Antiaromatic 9-Hydroxy Fluorenyl Cation

Fluorenyl cations are textbook examples of 4π electron antiaromatic five-membered ring systems. So far, they were reported only as short-lived intermediates generated under superacidic conditions or by flash photolysis. Attempts to prepare a m-terphenyl acylium cation by fluoride abstraction from a benzoyl fluoride gave rise to an isolable 9-hydroxy fluorenyl cation that formed by an intramolecular electrophilic attack at a flanking mesityl group prior to a 1,2-methyl shift and proton transfer to oxygen.     

Dications of 3-phenylindenylidenefluorenes: evaluation of antiaromaticity of indenyl and fluorenyl cations by magnetic measures

Dications of p-substituted 3-phenylindenylidenefluorenes were prepared to examine the response of the resulting indenyl and fluorenyl cationic systems to magnetic measures of antiaromaticity. All measures, (1)H NMR shifts, nucleus independent chemical shifts (NICS(1)(zz)), and magnetic susceptibility exaltation, Lambda, supported the antiaromaticity of the dications 3a-f2+. The 1H NMR shifts and NICS(1)(zz) showed that the indenyl ring system was less antiaromatic than the fluorenyl ring system, contrary to the antiaromaticity of indenyl monocations compared to fluorenyl monocations. The presence of a phenyl substituent in the 3-position was able to stabilize the indenylidene cation through resonance, decreasing its antiaromaticity, but even in the absence of the 3-phenyl substituent, the indenyl system of indenylidenefluorene dications was less antiaromatic than the fluorenyl system. The decreased antiaromaticity of the 3-phenylindenylidenefluorene dications over the unsubstituted indenylidenefluorene dication was supported by (anti)aromatic (de)stabilization energy calculations, ASE.     

Dianion and dication of tetrabenzo[5.7]fulvalene. Greater antiaromaticity than aromaticity in comparable systems

The dianion, 5(2-), and dication, 5(2+), of tetrabenzo[5.7]fulvalene represent an aromaticity/antiaromaticity continuum in which the fluorenyl system changes from aromatic in 5(2-) to antiaromatic in 5(2+). Conversely, the antiaromatic dibenzotropylium system of 5(2-) becomes an aromatic system in 5(2+), allowing an examination of aromaticity/antiaromaticity within the same carbon framework. Dianion 5(2-) was prepared and characterized by (1)H NMR spectroscopy. The fluorenyl system of 5(2-) showed the downfield shifts expected for an aromatic system, while the dibenzotropylium system showed the paratropic shifts expected for an antiaromatic system. The conclusions from (1)H NMR spectroscopy were supported by NICS(1) zz calculations for each system. Comparison of the (1)H NMR spectrum and NICS(1) zz of 5(2-) with those of 5(2+) supported the assignments of aromaticity/antiaromaticity for each system. Aromaticity/antiaromaticity were further examined through comparison of the degree of bond length alternation, which showed that the bond length alternation was slightly greater for the antiaromatic ring systems than for the aromatic systems. However, when structures of 5(2-) and 5(2+) with no bond length alternation were examined, there was a dramatic increase in the degree of antiaromaticity for the antiaromatic ring systems as evaluated through NICS. This result suggests that a decrease in bond length alternation results in an increase in antiaromaticity as well as an increase in aromaticity. The magnitude of the antiaromaticity of the fluorenyl system in 5(2+) was greater than the magnitude of the aromaticity in the fluorenyl system of 5(2-), with similar effects shown by the analogous tropylium systems. This is consistent with the behavior of the antiaromatic dication of tetrabenzo[5.5]fulvalene, compared to that of its aromatic dianion, and also with the behavior of the cyclopentadienyl cation/anion and tropylium cation/anion.     

Regioselective Nucleophilic Functionalization of Antiaromatic Nickel(II) Norcorroles

Treatment of antiaromatic nickel(II) norcorrole with potassium cyanide provided nickel(II) 3‐cyanonorcorrole with perfect regioselectivity without the help of a catalyst. The reaction of the nickel(II) norcorrole with phenol or thiophenol in the presence of a base also yielded substitution products. The antiaromatic 16π conjugation system in the norcorrole core was preserved in the functionalized products. Introduction of phenylthio groups significantly decreased the HOMO–LUMO gap and enhanced the near IR absorption property.     

Aromatic stabilization energy calculations for the antiaromatic fluorenyl cation. Issues in the choice of reference systems for positively charged species

Aromatic stabilization energy (ASE) calculations for the fluorenyl cation show substantial destabilization in comparison to suitable reference systems (16.3 +/- 1.6 kcal/mol), supporting its categorization as an antiaromatic species. The choice of appropriate reference systems is exacting for cationic systems because of the need to match strain energies, convolved with allylic-type resonance terms and other potential structural effects that stabilize charge. Several homodesmotic ASE reaction systems are examined to demonstrate the role played by these factors in the calculation of an ASE value for the fluorenyl cation. The magnitudes of the derived ASE are quite similar for four very different determinative, homodesmotic reaction systems, giving strong support to the inherent accuracy of the final derived ASE value. The results of nucleus independent chemical shift calculations for the components of each one of the ASE reactions add additional weight to this conclusion.     

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.     

Dications of fluorenylidenes. The relationship between redox potentials and antiaromaticity for meta- and para-substituted diphenylmethylidenefluorenes

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.     

[40]π Fused and Nonfused Core‐Modified Nonaphyrins: Syntheses and Structural Diversity

The synthesis of fused and nonfused core‐modified 40π nonaphyrins are reported. Spectroscopic and X‐ray structural studies reveal a twisted figure‐eight conformation in the freebase form that is nonaromatic. Structural changes occur, from figure‐eight to open extended conformation, upon protonation, thereby adopting 4nπ Hückel antiaromatic character, which is reflected in spectroscopic and theoretical studies. Such a structural change also induces ring inversions of specific heterocyclic rings by 180°.     

Solvent-dependent C-OH homolysis and heterolysis in electronically excited 9-fluorenol: the life and solvation time of the 9-fluorenyl cation in water

The primary pathways of the photodecomposition of 9-fluorenol (FOH) were studied in polar and nonpolar solvents by use of laser flash-photolysis with a resolution time of 10 ps. In solvents of high polarity, that is, in 1,1.1,3,3,3-hexafluoroisopropanol (HFIP), 2,2,2-trifluoroethanol (TFE), formamide or water, the fluorenyl cation, F+, forms by heterolytic C-O bond cleavage. In H2O, the initial (10 ps) spectrum of F+ has lambdamax at <460 nm. This absorption red-shifts with T = 25 ps to the "classical" spectrum with lambdamax = 510-515 nm. This process is assigned to the solvation of the initial "naked" cation, or rather, the contact ion pair. The lifetime of the solvated fluorenyl cation in H2O (or D2O) and TFE was measured to be tau 20 ps and 1 ns, respectively. In solvents of lower polarity such as alkanes, ethers and alcohols, the long-lived (tau 1/2 1 micros) fluorenyl radical, F., (lambdamax = 500 nm) forms through homolytic C-O cleavage. In addition to the radical and the cation, the vibrationally relaxed excited singlet state of FOH is seen with its absorption at approximately 640 nm; its lifetime is strongly dependent on the solvent, from 10 ps for formamide to 1.7 ns for cyclohexane. The rate constant for singlet decay increases exponentially with the polarity of the solvent (as expressed by the Dimroth-Reichardt ET value) or with the Gutmann solvent acceptor number. The relaxation of S1 to S0 is accompanied by homolytic C9-O bond cleavage (except in HFIP, TFE, and water, where S1 is not seen).     

Synthesis of Di‐peri‐dinaphthoporphyrins by PtCl2‐Mediated Cyclization of Quinodimethane‐type Porphyrins

Di‐peri‐dinaphthoporphyrins can be regarded as a key and common substructure of fused porphyrinoids. PtCl₂‐mediated cycloisomerization reaction of quinodimethane‐type porphyrins provided these doubly fused porphyrins, which exhibit characteristic paratropic ring currents that presumably arise from 24π antiaromatic circuit as a dominant resonance contributor. UV/Vis absorption spectra, cyclic voltammetry, and excited‐state dynamics as well as theoretical calculation support this conclusion.     

Synthesis and Antiaromatic Properties of Highly Planar Dithiaamethyrin

2,5‐Bis(4‐propyl‐2‐pyrrolyl)thiophene was reacted with aryl aldehydes to afford large porphyrinoids; [24]dithiaamethyrin(1.0.0.1.0.0) and [36]trithianonaphyrin(1.0.0.1.0.0.1.0.0). X‐ray crystallography of the dithiaamethyrin revealed a highly planar ring structure with mean plane deviation (MPD) value of 0.053 Å. A large positive NICS(0) value (+13.9 ppm) calculated for this planar 24π system unambiguously indicates an antiaromatic character that is consistent with a remarkably low field ¹H chemical shift of the inner NH proton at 24.0 ppm. The magnitude of the paratropic ring current effect in the 24π amethyrin framework in the free base form and the diprotonated form was discussed on the basis of the MPD value and the NICS value. The present dithiaamethyrin is much more strongly antiaromatic than the dodecaalkylamethyrin in their free base forms, but the order inversed in their diprotonated forms.     

离子对间电荷转移配合物：阴离子…π作用及近红外吸收

合成并表征了一种离子对配合物1,[Cl₂Bz-1-Apy]₂[Ni(mnt)₂],其中,Cl₂Bz-1-Apy为(E)-1-(3,4-二氯苯亚甲基氨基)吡啶一价阳离子,mnt是马来二睛基二硫烯二价阴离子。在配合物1的晶体中,阴离子和阳离子中吡啶环之间存在阴离子…π相互作用。DFT电荷密度分布分析表明,阴离子…π相互作用主要源自离子对间的Coulomb吸引力。在固体和乙腈溶液电子吸收光谱近红外区,配合物1都有一个宽的弱吸收带,该吸收带可归属为二价阴离子[Ni(mnt)₂]^2-内的d-d电子跃迁和阴阳离子对间的电荷转移跃迁。     

Deciphering Noncovalent Interactions Accompanying 7,7,8,8‐Tetracyanoquinodimethane Encapsulation within Biphene[n]arenes: Nucleus‐Independent Chemical Shifts Approach

Binding of novel biphene[n]arene hosts to antiaromatic 7,7,8,8‐tetracyanoquinodimethane (TCNQ) are investigated by DFT. Biphene[4]arene favors the inclusion complex through noncovalent interactions, such as hydrogen bonding, π–π stacking, C?H???π, and C?H???H?C dihydrogen bonding. Donor–acceptor complexation renders aromatic character to the guest through charge transfer. The formation of TCNQ anionic radicals through supramolecular π stacking significantly influences its chemical and photophysical behavior. Electron density reorganization consequent to encapsulation of TCNQ reflects in the shift of characteristic vibrations in the IR spectra. The accompanying aromaticities arising from the induced ring currents are analyzed by employing nucleus‐independent chemical shifts based profiles.     

The photodecomposition mechanism of tert-butyl-9-methylfluorene-9-percarboxylate: new insight from femtosecond IR spectroscopy

The ultrafast photodissociation of tert-butyl-9-methylfluorene-9-percarboxylate (TBFC) is studied by mid-infrared transient absorption spectroscopy after UV excitation at 266 nm. By means of 13C-labeled TBFC and additional DFT calculations transient IR bands in the fingerprint region were unambiguously assigned to the methylfluorenyl radical. The experiments show that the fragmentation is controlled by the S1-lifetime of TBFC and, dependent on the solvent, within 0.8-2.1 ps leads to tert-butyloxy and methylfluorenyl radicals plus CO2 via concerted bond breakage of the O-O and the fluorenyl-C(carbonyl) bond. In accordance, the CO2 quantum yield is determined to be unity.     

Thienoacene‐Fused Pentalenes: Syntheses,Structures, Physical Properties and Applications for Organic Field‐Effect Transistors

Three soluble and stable thienoacene‐fused pentalene derivatives ( 1 – 3 ) with different π‐conjugation lengths were synthesized. X‐ray crystallographic analysis and density functional theory (DFT) calculations revealed their unique geometric and electronic structures due to the interaction between the aromatic thienoacene units and antiaromatic pentalene moiety. As a result, they all possess a small energy gap and show amphoteric redox behaviour. Time dependent (TD) DFT calculations were used to explain their unique electronic absorption spectra. These new compounds exhibited good thermal stability and ordered packing in solid state and thus their applications in organic field‐effect transistors (OFETs) were also investigated. The highest field‐effect hole mobility of 0.016, 0.036 and 0.001 cm² V^?1 s^?1 was achieved for solution‐processed thin films of 1 – 3 , respectively.     

Dications of fluorenylidenes. The effect of substituent electronegativity and position on the antiaromaticity of substituted tetrabenzo[5.5]fulvalene dications

Oxidation of 3,6-disubstituted tetrabenzo[5.5]fulvalenes by SbF(5) results in the formation of dications that behave like two antiaromatic fluorenyl cations connected by a single bond. Both fluorenyl systems exhibit the paratropic shifts and nucleus independent chemical shifts (NICS) characteristic of antiaromatic species. Comparison with analogous 2,7-disubstituted tetrabenzo[5.5]fulvalenes reveals that the antiaromaticity of the substituted ring system can be altered substantially by changes in the placement of the substituents, possibly due to changes in the delocalization of charge in the system. Substituents in the 3,6-position decrease the antiaromaticity because of the increase in the benzylic resonance compared to 2,7-substituents.     

A Stable Antiaromatic 5,20‐Dibenzoyl [28]Hexaphyrin(1.1.1.1.1.1): Core AuIII Metalation and Subsequent Peripheral BIII Metalation

5,20‐Dibenzoyl [28]hexaphyrin(1.1.1.1.1.1) was synthesized as the first hexaphyrin bearing meso‐aroyl substituents. The meso‐dibenzoyl substituents are hydrogen‐bonded with the pyrrolic protons to stabilize an antiaromatic dumbbell conformer. Core metalation of this hexaphyrin with Au^III afforded rectangular and aromatic [26]hexaphyrin bis‐Au^III complexes, the major isomer of which was reduced with NaBH₄ to give its antiaromatic 28π bis‐Au^III complex. This complex allowed facile peripheral metalation with B^III owing to the peripheral benzoyl substituents.     

Bioinspired Synthesis of (−)‐PF‐1018

The combination of electrocyclizations and cycloadditions accounts for the formation of a range of fascinating natural products. Cascades consisting of 8π electrocyclizations followed by a 6π electrocyclization and a cycloaddition are relatively common. We now report the synthesis of the tetramic acid PF‐1018 through an 8π electrocyclization, the product of which is immediately intercepted by a Diels–Alder cycloaddition. The success of this pericyclic cascade was critically dependent on the substitution pattern of the starting polyene and could be rationalized through DFT calculations. The completion of the synthesis required the instalment of a trisubstituted double bond by radical deoxygenation. An unexpected side product formed through 4‐exo‐trig radical cyclization could be recycled through an unprecedented triflation/fragmentation.