Dications of benzylidenefluorene and diphenylmethylidene fluorene: the relationship between magnetic and energetic measures of antiaromaticity

Oxidation of m- and p-substituted benzylidene fluorenes to antiaromatic dications was attempted by electrochemical and chemical means. Electrochemical oxidation to dications was successful for benzylidene fluorenes with p-methoxy, p-methyl, p-fluoro, and unsubstituted phenyl rings in the 3-position; attempts to oxidize the m-substituted derivatives via electrochemistry were unsuccessful. Chemical oxidation with SbF(5)/SO(2)ClF gave the dication of 9-[(4-methoxyphenyl)methylene]-9H-fluorene cleanly; oxidation of all other substituted benzylidene fluorenes resulted in mixtures of products. The excellent linear relationship between the chemical shifts calculated by the GIAO method and the experimental shifts for the p-methoxy-substituted benzylidene fluorene dication suggests that the calculations satisfactorily reflect the magnetic properties of this dication and potentially those of the other dications studied. The redox potentials from electrochemical oxidation, a measure of the stability of the dications, showed a good linear relationship with another measure of stability, the calculated difference in energy between each dication and its neutral precursor. The dications of benzylidene fluorenes were less stable than the dications of diphenylmethylidene fluorenes; within each type of compound, dications with p-substituted phenyl rings were more stable than dications with m-substituted phenyl rings and dications with phenyl rings substituted with electron-donating groups were more stable than dications with phenyl rings substituted with electron-withdrawing groups. The antiaromaticity of the fluorenyl system was assessed through the nucleus-independent chemical shift (NICS) that was also calculated by the GIAO method. The plot of the NICS values per square area versus the calculated energy difference for the dications showed a moderate degree of linearity; the plot of NICS values per square area versus the oxidation potentials was less linear. Thus, a suggestive, but not conclusive, relationship between magnetic and energetic measures of antiaromaticity was observed.     

Theoretical study of long oligothiophene dications: bipolaron vs polaron pair vs triplet state

A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication, nT2+, n = 6-50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in some cases, also at BLYP/6-31Gd, was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n = 6-12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.     

Formation of frontier orbitals and the nature of electron transitions in dication-polyenes,analogs of cyanine dyes

By spectral and quantum chemical methods the nature of the electron transitions in three types of linear conjugated compounds including polyenes, their dications, and polymethine cations containing indolenine and 2,6-diphenylthiopyrylium residues as terminal groups were studied. It was shown that in going from neutral molecules to polymethine cations and polyene-dications a regular downward shift of the energy gap occurred and the decrease in interaction of orbitals of the terminal donor residues. Due to that the electron transitions in polyene dications are degenerate and have high energy in contrast to the deeply colored dications of polymethine dyes.     

Catalyst activity in alkylthiophene dealkylation

The transformations of 2-alkylthiophenes on various catalysts were studied. In the presence of oxide and sulfide catalysts, 2-methylthiophenes and 2-isopropylthiophenes underwent dealkylation to thiophene. The reaction was complicated by isomerization to 3-alkylthiophene and cracking. The dealkylation of 2-ethylthiophene occurred only in the presence of oxide catalysts. Side-chain dehydrogenation resulting in the formation of 2-vinylthiophene primarily occurred under the action of sulfide and some oxide catalysts. Acid catalysts (amorphous aluminosilicate and zeolites in the form of hydrogen) were the most active and selective in the dealkylation of 2-alkylthiophenes. It is believed that the dealkylation reaction of 2-alkylthiophenes on these catalysts occurs by means of a protolytic mechanism.Translated from Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 97–103.Original Russian Text Copyright © 2005 by Mashkina, Chernov.     

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.     

Gitonic and distonic alkanonium dications (diprotonated alkane dications C(n)H(2n+4(2+)), n = 1-4)(1)

The structures and stabilities of gitonic and distonic alkanonium dications, i.e., diprotonated alkane dications C(n)H(2n+4)(2+) (n = 1-4), were investigated at the MP4(SDTQ)/6-311G**//MP2/6-31G** level. The global minimum energy structures (2, 4, 7, and 10) of the C(n)H(2n+4)(2+) dications are double C--H protonated alkanes to give structures with two two electron three-center (2e-3c) bonds. Two different dissociation pathways for the dications, viz deprotonation and demethylation, were also computed. Demethylation was found to be the favorable mode of dissociation.     

Observation of gas-phase molecular dications formed from neutral organics in solution via qemical electron-transfer reactions by using electrospray Ionization Mass Spectrometry

The first observation of organic dications formed by multiple electron loss in electrospray mass spectra is reported. The dications of β-carotene, canthaxanthine, cobalt(II) octaethylporphyrin, and nicke(II) octaethylporphyrin were created in solution via chemical electrontransfer reactions and detected in the gas phase by electrospray ionization mass spectrometry (ES-MS) using a flow-injection experiment. The analytes were injected into a flowing solvent-oxidant stream (10 μL/min) composed of dried methylene chloride containing ≈ 0.1% by volume trifluoroacetic acid and 0.1% by volume antimony pentafluoride (SbF₅). The dications created in this oxidizing solvent system were preserved for detection by rapidly transferring them from the reactive solvent-oxidant system to the gas phase, where, in the absence of the solvent system, they were “long-lived” and amenable to mass analysis. This work demonstrates means to produce ions novel to ES-MS and means to detect and study by ES-MS species that are short-lived in solution. In addition, this work shows that electrospray ionization can potentially be used to generate gas-phase dications for mass spectrometric study that are difficult to produce directly from gas-phase neutrals by other ionization techniques (e.g., M²⁺ from β-carotene).     

First synthesis of biquinoliziniun salts: novel example of a chiral azonia dication

[Structure: see text] The 1,1'-, 2,2'-, and 3,3'-biquinolizinium dications are described for the first time and were prepared using palladium-catalyzed homocoupling reactions of the corresponding isomeric bromoquinolizinium bromides. Theoretical calculations show 1,1'- and 4,4'-biquinolizinium dications to be chiral molecules, the latter of which has a high energy of formation, a factor that probably precludes its formation.     

Persistent oxidation dications from twisted fluoranthenes, benzo[k]fluoranthene and dimethyldibenzo[j.l]fluoranthene: charge delocalization mode, tropicity, and formation of novel 8,8'-bifluoranthenyls. An NMR and theoretical study.

First examples of persistent oxidation dications from fluoranthene-PAHs namely 1,3,4,6,7,10-hexamethyl- 2 and 3,4-dichloro-1,6,7,10-tetramethylfluoranthene 3, benzo[k]fluoranthene 6, and 3,6-dimethyldibenzo[j,l]fluoranthene 9 are reported. Charge delocalization mode and tropicity in the resulting nonalternant dications are examined. Quenching of the superacid solutions of the dications resulted in the formation of novel 8,8'-bifluoranthenyls 2a-4a. AM1 was used as an initial guide for dication generation (DeltaDeltaH(f) degrees and ionization potentials) and for probing the structures of the crowded fluoranthene substrates and their bifluoranthenyls. In selected cases, the dications and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level. Charge delocalization mode (difference in NPA charges) and DFT/GIAO-derived NMR chemical shifts were obtained for comparison with experiment; aromaticity was assessed via nucleus independent chemical shift (NICS) calculations.     

Electrophilic and oxidative chemistry of pyrene and its non-alternant isomers: theoretical (DFT, GIAO-NMR, NICS) study of protonation carbocations and oxidation dications from pyrene, azupyrene (dicyclopenta[ef,kl]heptalene) and dicyclohepta[ed,gh]pentalene

Mono- and diprotonated carbocations and the two-electron oxidation dications derived from parent pyrene and its nonalternant isomers "azupyrene"(dicyclopenta[ef,kl]heptalene)(DCPH) and dicyclohepta[ed,gh]pentalene (DCHP) were studied by DFT at the B3LYP/6-31G(d) level. The most likely site(s) for mono- and diprotonation were determined based on relative arenium ion energies and the structures of the energetically most favored carbocations were determined by geometry optimization. The NMR chemical shifts for the protonated mono- and dications and the oxidation dications were computed by GIAO-NMR at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level and their charge delocalization paths were deduced based on magnitude of the computed [capital Delta][small delta](13)C values and the NPA-derived changes in charges. Relative aromaticity/antiaromaticity in various rings in the energetically favored mono- and dications was estimated via NICS and [capital Delta]NICS. Calculated NMR chemical shift data for and were compared with the available experimental NMR values. The available data on chemical and physical properties of DCPH and DCHP are extremely limited and biological activity data are non-existent. The present study provides the first glance into their carbocations and oxidation dications, while augmenting and reinforcing the previous stable ion data on the pyrenium cations.     

Dications of bis-triarylamino-[2.2]paracyclophanes: Evaluation of excited state couplings by GMH analysis

In this paper, we present the absorption properties of a series of bis-triarylamino-[2.2]paracyclophane diradical dications. The localized pi-pi and the charge-transfer (CT) transitions of these dications are explained and analyzed by an exciton coupling model that also considers the photophysical properties of the "monomeric" triarylamine radical cations. Together with AM1-CISD-calculated transition moments, experimental transition moments and transition energies of the bis-triarylamine dications were used to calculate electronic couplings by a generalized Mulliken-Hush (GMH) approach. These couplings are a measure for interactions of the excited mixed-valence CT states. The modification of the diabatic states reveals similarities of the GMH three-level model and the exciton coupling model. Comparison of the two models shows that the transition moment between the excited mixed-valence states mu(ab) of the dimer equals the dipole moment difference Delta of the ground and the excited bridge state of the corresponding monomer.     

Stable ion study of regioisomeric carboxonium-substituted pyrenium ions: directive effects, charge delocalization mode, and conformational aspects

Regioisomeric monoacyl- and monobenzoyl-substituted pyrenes are diprotonated in FSO(3)H.SbF(5) (4:1)/SO(2)ClF to give persistent carboxonium-pyrenium dications, whereas diacetyl- and dibenzoylpyrenes are diprotonated to give dicarboxonium dications. The resulting dications were studied by low-temperature NMR at 500 MHz. Conformational aspects of the carboxonium group in various regioisomers are addressed by a combination of NOED spectra and 2D-NMR and AM1 calculations. Charge delocalization pathways are gauged and compared on the basis of the magnitude of Deltadelta (13)C values.     

Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers

Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaron. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory.     

1,8-bis(diphenylmethylium)naphthalenediyl dication as an organic oxidant: synthesis of benzidines via self-coupling of N,N-dialkylanilines

1,8-Bis(diphenylmethylium)naphthalenediyl dications promoted oxidative coupling of various N,N-dialkylanilines to afford the corresponding para-coupled bisanilines, benzidines, in good to excellent yield. The dications were readily prepared from a cyclic ether precursor, derived from 1,8-dibromonaphthalene, via deoxygenation with a silylating reagent in 1,1,1,3,3,3-hexafluoro-2-propanol under anhydrous conditions. [reaction: see text]     

Ab initio/GIAO-CCSD(T) study of propenoyl (H2C=CH-CO+) and isopentenoyl ((CH3)2C=CH-CO+) cations and their superelectrophilic protonated dications

Structures of superelectrophilic protonated propenoyl (H2C=CH-COH2+) and isopentenoyl ((CH3)2C=CH-COH2+) dications and their parent cations were calculated using ab initio methods at the MP2/6-311+G and MP2/cc-pVTZ levels. Energies were calculated using Gaussian-2 (G2) theory. The alpha-carbon (Calpha) protonated 3 and 7 were found to be the global minima for protonated propenoyl and isopentenoyl dications, respectively. 13C NMR chemical shifts of the cations were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. 13C NMR chemical shifts of the related tert-butyl cation ((CH3)3C+) and protonated tert-butyl dication ((CH3)2CCH4(2+)) were also computed at the same level to compare and explore the effect of the additional charge in dications.     

Exchange interaction of 5,5'-(m- and p-phenylene)bis(10-phenyl-5,10-dihydrophenazine) dications and related analogues

[structures: see text] 5,5'-(m-Phenylene)bis[(10-aryl-5,10-dihydrophenazine) dications, 3a2+ and 3b2+, and their p-analogues 4a2+ and 4b2+, were prepared, and their exchange interaction was investigated. The EPR spectra of these dications at 123 K in a butyronitrile matrix showed the population of a triplet state. The temperature dependence of the EPR signal intensity (absolute value(delta m(s)) = 2) showed that these dications had singlet ground states with deltaE(ST)/k(B) = -27 to -21 K for the m-isomer 3(2+) and with deltaE(ST)/k(B) = -10 to -8 K for the p-isomer 4(2+). Theoretical calculation of the exchange interaction J for these dications at the orthogonal torsion angle geometries was carried out for 3a2+ and 4a2+ and for (m- and p-phenylene)bisphenothiazine dications 1(2+) and 2(2+) using the broken-symmetry approach for the singlet states. A good correlation was observed between the calculated J and a MO-energy term in the triplet state, deltaE(TMO) = absolute value(HOMO(alpha) - (HOMO - 1)(alpha)). The calculated J values were negative in the order of 10 K for the m-dications (J/k(B) = -14.7 K for 1(2+), -11.5 K for 3a(2+)), but much smaller negative values were found for the p-isomers (J/k(B) = -0.9 K for 2(2+), -0.8 K for 4a2+). The smaller absolute value(J) values for the p-dications are qualitatively consistent with the experimental deltaE(ST) (2J) values.     

Evaluation of dicationic reagents for their use in detection of anions using positive ion mode ESI-MS via gas phase ion association

Twenty-three different dications were investigated for their effectiveness in pairing with singly charged anions, thereby allowing the electrospray ionization mass spectrometry (ESI-MS) detection of anions as positively charged complexes. Nitrate, iodide, cyanate, monochloroacetate, benzenesulfonate, and perfluoro-octanoate were chosen as representative test anions as they differ in mass, size-to-charge ratio, chaotropic nature, and overall complexity. Detection limits were found using direct injection of the anion into a carrier liquid containing the dication. Detection limits are given for all six anions with each of the 23 dications. Each anion was easily detected at the ppb (microg/L) and often the ppt (ng/L) levels using certain dicationic reagents. The ability of dicationic reagents to pair with anions and produce ESI-MS signals varied tremendously. Indeed, only a few dications can be considered broadly useful and able to produce sensitive results. Liquid chromatography (LC)-ESI-MS also was investigated and used to show how varying the dicationic reagent produced significantly different peak intensities. Also, the use of tandem mass spectrometry can lead to even greater sensitivity when using imidazolium based dications.     

A theoretical (DFT, GIAO-NMR, NICS) study of the carbocations and oxidation dications from azulenes, homoazulene, benzazulenes, benzohomoazulenes, and the isomeric azulenoazulenes

Protonation of parent azulene (1), homoazulene (8), representative isomeric benzazulenes (9, 9A, and 9B), and benzohomoazulenes (10, 10A, and 10B) as well as the mono- and diprotonation of isomeric azulenoazulenes (11-16) were studied by DFT at the B3LYP/6-31G(d) level. The most likely carbocations were identified based on relative protonation energies. For comparison, complete experimental 13C NMR data were obtained for parent azulenium ion 1H+ and guaiazulenium ion 2H+ in TFA. The oxidation dications derived from benzazulenes (9, 9A, and 9B), benzohomoazulenes (10, 10A, and 10B) and azulenoazulenes (11, 16) were also investigated. For azulenoazulene dications the singlet and triplet states are both minima, but except for 11(2+) and 13(2+), the triplet states are higher in energy. Structural/geometrical changes in the carbocations were examined. GIAO-NMR, NPA charges (and changes in charges), and NICS (and delta NICS) were employed to compute the NMR chemical shifts (delta delta 13C values) in order to derive charge delocalization maps and to gauge relative aromaticitylantiaromaticity in the energetically most favored carbocations and oxidation dications. The emerging trends are discussed and compared. Creation of tropylium or homotropylium entities in the carbocations (monoprotonated) and carbodications (diprotonated) is identified as an important driving force governing the protonation outcomes. Consideration of the AM1-derived delta delta Hf values (and the DFT-derived delta delta G values) suggests that the two-electron oxidation of azulenoazulenes should be experimentally feasible. Given their antiaromatic (paratropic) character, azulenoazulene dications represent interesting targets for NMR study. GIAO-derived charge delocalization mapping allows the most likely sites for nucleophilic attack on the dications to be identified.     

Novel examples of three-dimensional aromaticity: 1,3-dehydro-silaadamantane dications. A theoretical (DFT,GIAO NMR,NICS) study

A DFT study of the hitherto elusive 1,3-dehydro-silaadamantane dications 2(2+)-5(2+) has been carried out. Computed nucleus independent chemical shift (NICS) values are strongly indicative of three-dimensional heteroaromaticity in the resulting caged dications (via 2-electron, 4-center homoconjugation). In the optimized structures, silicon is pyramidalized. Although charge calculations (NPA and MKS) indicate significant positive charge build-up at silicon(s), the (29)Si GIAO NMR chemical shifts are unusually shielded. The latter finding agrees with the recent DFT calculations on 7-silanobornadien-7-ylium monocation 10(+), suggesting that silicon shielding is a consequence of unusual bonding and homoconjugation in the dications. Both NICS values and silicon shielding decrease in going from 2(2+) to 5(2+). Natural bond orbital (NBO) analysis was utilized to shed light on the origin of the three-dimensional heteroaromaticity in these dications.     

Synthesis of 2-substituted indoles via a palladium-catalyzed domino Heck reaction and dealkylation

A palladium-catalyzed domino Heck reaction and dealkylation for the preparation of 2-substituted indoles is described. This novel transformation is based on an intramolecular Heck reaction followed by dealkylation.