Delocalization Of Positive Charge Research Articles

Positive charge delocalization and anti‐aromaticity of cations generated by protonation of benzo[a]fluoranthenes in superacid

AbstractBenzo[a]fluoranthene (4a) is one of non‐alternate polycyclic aromatic hydrocarbons. A reaction of 4a in CF3SO3H yielded a dark yellow solution. Direct NMR observation indicated the exclusive formation of carbocation 4aH+ by protonation at the C(8) position. The most deshielded 1H and 13C signals were observed at 8.99 ppm for H(12) and 8.29 ppm for H(1), 182.6 ppm for C(12b), 155.6 ppm for C(8a), and 149.4 ppm for C(7a). The signals for H(4) and H(5) were significantly more shielded than those of 4a. A reaction of 3‐tert‐butylbenzo[a]fluoranthene (4b) in CF3SO3H afforded carbocation 4bH+ by the protonation at the C(8) position. 4bH+ was gradually converted to 4aH+. The changes in 13C NMR chemical shifts (Δδ13C) suggested that positive charge was delocalized into mainly seven carbons in 4aH+ and 4bH+. The observed cations were found to be the most stable cations among the possible protonation cations by the DFT method. The NICS(1)zz values for the five‐membered rings were calculated to be 35.6 for 4aH+ and 34.4 for 4bH+ by GIAO‐B3LYP/6‐311+G(2d,p). The experimental NMR and the NICS(1)zz data indicated that the five‐membered rings in 4aH+ and 4bH+ exhibited anti‐aromaticity.

Unusual oxidation of methylated diazo dye to 4,4′-dihydroxy-azobenzene: Mechanistic investigations and spectroscopic studies of the isolated product

Here we reported the synthesis of hydroxymethylated diazo compounds through the reduction of corresponding diazo aldehydes and presented our exploration towards oxidation reaction with NaIO4 and PhI(OAc)2, where we were expecting oxidative dearomatization reaction, and the product should be the nature of spiroepoxy cyclohexadienone but surprisingly we isolated a demethylated diazo product, reaction with NaIO4. This unprecedented transformation was believed due to the presence of extended conjugation in the diazo precursor facilitated the delocalization of positive charge and made it difficult for intramolecular epoxidation and dearomatization. The structure of the newly synthesized compound was thoroughly characterized by 1H NMR, FT-IR, and mass spectrometry. A mechanism of demethylation reaction was proposed. This method provided a new protocol for the synthesis of unsymmetrical 4,4′-dihydroxy azobenzene which has many potential uses. Also, we studied its photophysical and acid-base properties and found that the isolated compound shows good fluorescence emission which gets quenched by the addition of acid and base. The acid-base analysis reflects two times proton migration and shows good reversibility.

Electrophilic heterocycles: Functionalization with enol silyl ethers

A series of heterocyclic triaryl methanols were prepared and reacted with enol silyl ethers in the presence of ZnCl2. This reaction gave a variety of functionalized heterocyclic products. A mechanism is proposed involving formation triaryl carbocation electrophiles with delocalization of positive charge to positions in the heterocyclic rings. Nucleophilic attack then occurs at a ring position of the heterocycle.

A Distinctive Pattern for Substituent Effects on Transition Metal Centers: Enhanced Electron-Donating Capacity of Cationic Palladium Species

A Distinctive Pattern for Substituent Effects on Transition Metal Centers: Enhanced Electron-Donating Capacity of Cationic Palladium Species

Efficient synthesis of substituted [1,3]thiazolo[3,2-b][1,2,4]triazol-7-ium hexabromotellurates

The electrophilic cyclization of methallyl thioethers of 1,2,4-triazoles leads to the formation of condensed [1,3]thiazolo[3,2-b][1,2,4]triazol-7-ium cations. We utilize the potential of this powerful methodology to synthesize hybrid organic-inorganic materials. Hexabromotellurate is chosen as a counter anion. Eight new hexabromotellurate salts of polysubstituted [1,3]thiazolo[3,2-b][1,2,4]triazol-7-ium are synthesized. Theoretical investigations of the lowest unoccupied molecular orbital (LUMO) and LUMO+1 orbitals and atomic partial charges testify the strong delocalization of positive charge over the cations, rather than the [1,3]thiazolo[3,2-b][1,2,4]triazol-7-ium moiety. According to population analysis, an alternative resonance structure with a positive charge over the sulfur atom better represents the delocalization of electron density in the newly synthesized [1,3]thiazolo[3,2-b][1,2,4]triazol-7-ium cations.

A Spiropentasiladiene Radical Cation: Spin and Positive Charge Delocalization across Two Perpendicular Si═Si Bonds and UV-vis-NIR Absorption in the IR-B Region.

Spiroconjugation, that is, through-space orbital interactions between two perpendicular π orbitals, is a key concept in the contemporary molecular design of spirocyclic π-electron systems. We synthesized spiropentasiladiene radical cation salt 1 as a dark-green solid via the one-electron oxidation of the stable spiropentasiladiene 2. Characterization of the molecular structure combined with theoretical studies indicated that the spin and positive charge are delocalized across the two perpendicular Si═Si double bonds of 1. Two π(Si═Si) orbitals are split into HOMO and SOMO with a small energy gap owing to the second-order Jahn-Teller distortion and steric repulsion between bulky alkyl groups upon one-electron oxidation. In the UV-vis-NIR spectrum, the longest-wavelength absorption band of 1 (λmax = 1972 nm) covers the IR-B region (1400-3000 nm; 0.89-0.41 eV) despite having the smallest possible spiroconjugation motif. The unprecedented absorption band in the IR region was assigned to the HOMO → SOMO transition that arises from the delocalized π-orbitals in the spirocyclic Si5 skeleton.

Trisubstituted geminal diazaalkene derived transient 1,2-carbodications.

The coulombic repulsion between two adjacent cation centres of 1,2-carbodications is known to decrease with π- and/or n-donor substituents by a positive charge delocalization. Here we report the delocalization of the positive charge of transient 1,2-carbodications having one H-substituent by an intramolecular base-coordination. N-heterocyclic olefin (NHO) derived 2-pyrrolidinyl appended trisubstituted geminal diazaalkenes were used for the generation of transient 1,2-carbodications through a 2-e chemical oxidation process. We have also studied the 1-e oxidation reaction of trisubstituted geminal diazaalkenes (electrochemically and chemically) and also studied them using in situ EPR spectroscopy.

Zwitterionic Mixed Valence: Internalizing Counteranions into a Biferrocenium Framework toward Molecular Expression of Half-Cells in Quantum Cellular Automata.

Realization of molecular quantum cellular automata (QCA), a promising architecture for molecular computing through current-free processes, requires improved understanding and application of mixed-valence (MV) molecules. In this report, we present an electrostatic approach to creating MV subspecies through internalizing opposite charges in close proximity to MV ionic moieties. This approach is demonstrated by unsymmetrically attaching a charge-responsive boron substituent to a well-known organometallic MV complex, biferrocenium. Guest anions (CN- and F- ) bind to the Lewis acidic boron center, leading to unusual blue-shifts of the intervalence charge-transfer (IVCT) bands. To the best of our knowledge, this is the first reported example of a zwitterionic MV series in which the degree of positive charge delocalization can be varied by changing the bound anions, and serves to clarify the interplay between IVCT parameters. The key underlying factor is the variable zero-level energy difference in the MV states. This work provides new insight into imbuing MV molecules with external charge-responsiveness, a prerequisite of molecular QCA techniques.

Charge delocalization mode and change in aromaticity of protonated 7‐phenylbenzo[k]fluoranthenes studied by experimental observation and DFT calculations

Abstract7‐Phenylbenzo[k]fluoranthene (4), which is one of non‐alternant polycyclic aromatic hydrocarbons, was protonated at the C(12) position in neat CF3SO3H to give a persistent carbocation (4aH+). The cation was successfully observed by NMR measurements at rt. The most deshielded protons were observed as doublet signals at 8.92 and 8.71 ppm for H(1) and H(3). The most deshielded 13C signals appeared at 184.4 and 176.6 ppm for C(12a) and C(7). The positive charge was found to be delocalized to four carbons in a 1‐naphthalenium unit and to two other carbons according to Δδ13C from 4 to 4aH+. DFT calculations indicated that 4aH+ was the most stable among possible protonation carbocations. The NICS(1)zz values of 4aH+ suggested that the five‐membered π‐ring is nonaromatic and that the other benzenoid rings are aromatic. The influence of the 7‐phenyl group on aromaticity and on positive charge delocalization was limited. The next most stable cations were computed to be cations (4bH+ and 4cH+) that were generated by the protonation at the C(3) and C(4) positions, respectively. Their Gibbs energies were similar within 0.7 kcal/mol to that of 4aH+. Their five‐membered π‐rings were proved to be anti‐aromatic according to the NICS(1)zz criteria. 4bH+ and 4cH+ can be depicted by canonical structures with a cyclopentadienyl cation unit, which is suggested to be responsible for their anti‐aromaticity.

Delocalization of positive charge in aromatic liquids studied by subnanosecond near-infrared transient absorption spectroscopy

Transient absorption spectra of 1,2,4,5-tetracyanobenzene (TCNB) in aromatic liquids (benzene, toluene, m-xylene, and mesitylene) were measured in subnanosecond time resolution. Upon photoexcitation at 355 nm, exciplexes between TCNB and the aromatic liquids formed quickly, and we observed characteristic absorption bands due to a TCNB radical anion in the visible wavelength range and charge resonance (CR) absorption bands due to aggregate radical cations of the donor molecules in the near-IR wavelength range. The peak positions of the CR bands for the exciplexes differed significantly from those of the dimer radical cations of the corresponding donor molecules, indicating that positive charge was delocalized among more than two donor molecules. The TCNB radical anion decomposed efficiently within the lifetime of the exciplexes, and the positive charge remained in the aromatic liquids after the decomposition. Thus, we could observe CR bands due to positive charge (holes) in the aromatic liquids. From the peak positions of the CR bands, we were able to draw some conclusions regarding the delocalization of positive charge in aromatic liquids.

Interpretation of Both Electron Pushing and Electron Withdrawing Inductive Effect of Alkyl Groups in Terms of Mulliken-Jaffe’s Charge Coefficient Parameters (b)

Alkyl groups can delocalize both the negative and positive charges through ion (charge)-induced dipole interaction with the polarizable alkyl groups. This charge induced polarization of the alkyl group to delocalize the charge is the origin of ‘inductive effect’ of the alkyl groups. The apparent less efficiency in negative charge delocalization compared to positive charge delocalization by a particular alkyl group is attributed to the less polarizing field strength of the anionic charge.

Synthesis and redox properties of sterically crowded triarylphosphine bearing two phenothiazine moieties

The sterically crowded triarylphosphine bearing two phenothiazine moieties as redox sites was synthesized. The cyclic voltammogram of the phosphine exhibits three-step reversible oxidation, and the comparison with that of the triarylphosphine bearing one phenothiazine moiety suggests delocalization of positive charges during redox process.

Basicity of Highly Substituted β-Octaalkyl-meso-aryl- and -meso-thienyl Porphyrins

5-Phenyl-, 5,15-diphenyl-5,15-di(thiophen-2-yl)-, and 5,10,15,20-tetraphenyloctaalkyl-21H,23H-porphyrins in benzene–acetic acid mixtures are moderate bases (pK–0.27 to–2.48). There is no simple correlation between the basicity constants and any electronic or geometric structure parameter of their molecules due to the contributions of three factors to the basicity: distortion of the planar structure in highly substituted macrocycles and their protonated forms, positive charge delocalization over the conjugated bond system, and electronic effects of substituents.

DFT studies of isomerization in palladium(IV) chemistry and alkyl halide transfer from palladium(IV) to palladium(II)

A DFT study of alkyl halide exchange from Pd(IV) to Pd(II) centers supports proposals for an SN2 process in which nucleophilic Pd(II) square-planar complexes attack the axial alkyl group in electrophilic square-pyramidal Pd(IV) centers, leading to transfer of R+ from Pd(IV) to Pd(II) involving a transition structure of the form [R2(bpy)··R··PdR2 (bpy)]+ (bpy = 2,2′-bipyridine). Computation accounts for the selectivity in benzyl (Bn) over methyl transfer from PhMeBnPdIV(bpy)Br to Me2PdII(bpy), giving PhMePdII(bpy) and BnMe2PdIV(bpy). Transition structures are formed by the interaction of a dz2-like HOMO at the Pd(II) nucleophile and the Pd-C σ* LUMO at the cationic Pd(IV) electrophile. Isomerization of octahedral palladium(IV) complexes via halide loss followed by involvement of trigonal-bipyramidal transition structures to facilitate isomerization is also examined. A lower barrier for Bn+ than Me+ transfer is attributed to stabilisation of the transition structure by delocalisation of positive charge within the benzyl group, reflected in a higher total charge density for the benzyl group than found for the methyl group.

Nature of transients produced on hydrogen atom transfer from capsaicin

Structure and energies of capsaicin and its probable transients formed in oxidation processes (single electron transfer and hydrogen atom transfer) have been investigated using theoretical calculations. Molecular geometries and energies of truncated and complete capsaicin structures have been optimized using density functional theory (DFT) with Becke three-parameter Lee-Yang-Parr (B3LYP) functional and 6–31[Formula: see text]G(d) basis set. The stable geometries have been confirmed by vibrational analysis. The calculations suggest that single-electron transfer takes place at phenolic O-atom in the first step followed by delocalization of positive charge over the whole molecule. Further, the first step of hydrogen atom abstraction should take place at phenolic group due to lowest dissociation energy but post-optimization bond dissociation energy is least for benzylic group in the side chain as compared to other transients. Effect of water as a solvent on the energies has also been studied using self-consistent reaction field calculation. Similar results are obtained for truncated and complete capsaicin structures. The present study also includes Mulliken spin, charge, vibrational frequencies and assignments of frequencies of the transients. The present study provides explanation for the observation of phenoxyl radical in fast kinetic studies using pulse radiolysis study and, formation of breakdown and dimeric products in other studies.

Synthesis, Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes

AbstractMethoxide abstraction from gold acetylide complexes of the form (L)Au[η1‐C≡CC(OMe)ArAr′] (L=IPr, P(tBu)2(ortho‐biphenyl); Ar/Ar′=C6H4X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at −78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr′]+ OTf− in ≥85±5 % yield according to 1H NMR analysis. 13C NMR and IR spectroscopic analysis of these complexes established the arene‐dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh2]+ OTf− reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.

Synthesis, Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes.

Methoxide abstraction from gold acetylide complexes of the form (L)Au[η1 -C≡CC(OMe)ArAr'] (L=IPr, P(t Bu)2 (ortho-biphenyl); Ar/Ar'=C6 H4 X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at -78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr']+ OTf- in ≥85±5 % yield according to 1 H NMR analysis. 13 C NMR and IR spectroscopic analysis of these complexes established the arene-dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh2 ]+ OTf- reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.

Contiguous multiple π-coordination of π-conjugated polyenes: bonding nature and charge delocalization behaviour of polyene-(palladium chain) sandwich clusters.

The contiguous multiple π-coordination (CMπC) bonds are formed at the molecular interface between the extended π-conjugated sp2-carbon framework and metal-metal moieties. However, the static properties including the bonding nature of the CMπC bonds have not been well-understood. Here we report the charge delocalization behavior and bonding nature of the polyene-(palladium chain) sandwich clusters. We observed the chain-length dependent 13C NMR chemical shifts of the terminal phenyl groups in [Pdm{Ph(CH[double bond, length as m-dash]CH)m+1Ph}2][B(ArF)4]2 (m = 3, 4, 5, 6), showing the efficient delocalization of positive charge over the polyene ligands. Furthermore, the detailed molecular orbital analysis of several model clusters provides insights into the bonding nature of the μ-η2:(η2:)nη2- and μ-η3:(η2:)nη3-poylene palladium chain sandwich clusters, where the former type is attributed to the corner-sharing Pd array and the latter to the edge-sharing/corner-sharing hybrid Pd array. The analogy of the μ-η3:(η2:)nη3-type with the carbonyl-bridge coordination is discussed. The Pd string in each mode of sandwich chain clusters has a weak M-M bonding interaction.

Bonding and Electronic Properties of Linear Diethynyl Oligothienoacene-Bridged Diruthenium Complexes and Their Oxidized Forms.

A series of five diruthenium diethynyl complexes based on α,β-fused oligothienoacenes in the core of the bridging ligands [{Ru(dppe)Cp*}2(μ-C≡C-L-C≡C)] [dppe = 1,2-bis(diphenylphosphino)ethane, Cp* = η5-C5Me5; L = thieno[3,2-b]thiophene (4), thieno[2,3-b]thiophene (5), 3,4-dimethylthieno[2,3-b]thiophene (6), dithieno[3,2-b:2',3'-d]thiophene (7), and thieno[3,2-b]thieno[2',3':4,5]thieno[2,3-d]thiophene (8)] have been synthesized and fully characterized electrochemically and spectroscopically. Elongation of the redox noninnocent oligothienoacene bridge core causes a smaller potential difference between the initial two anodic steps, not seen for free dialkyl oligothienoacenes, and increased positive charge delocalization over the conjugated bridge backbone. The highest occupied molecular orbital of the parent complexes resides predominantly on the oligothienoacene core, with strong participation of the ethynyl linkers and slightly smaller contribution from the metallic termini. This bonding character makes the initial one-electron oxidation symmetrical, as revealed by combined voltammetric and spectroscopic (IR, UV-vis-near-IR, and electron paramagnetic resonance) methods as well as density functional theory (DFT) and time-dependent DFT calculations of truncated and selected nontruncated models of the studied series. The remarkable gradual appearance of two C≡C stretching absorptions in the IR spectra of the monocationic diethynyl complexes is ascribed to increasing vibronic coupling of the IR-forbidden νs(C≡C) mode of the oxidized -[C≡C-core-C≡C]+- bridge with a low-lying π-π*(intrabridge)/metal-to-ligand charge-transfer electronic transition in the near-to-mid-IR spectral region.

Efficient Synthesis of Cyclic Carbonates from Atmospheric CO2 Using a Positive Charge Delocalized Ionic Liquid Catalyst

The chemical fixation of CO2 into value-added products has recently received much attention. One of the most well-known methods is the coupling of CO2 with epoxides to form cyclic carbonates. Although this field has progressed considerably, it is still a challenge to perform the reaction with atmospheric CO2 or flue gas. Herein we described the development of 4-(dimethylamino)pyridine hydrobromide ([DMAPH]Br) as a highly efficient and recyclable catalyst for the formation of cyclic carbonates from atmospheric CO2 and epoxides. In the presence of 1 mol % of [DMAPH]Br, excellent conversions and selectivities for a broad range of terminal epoxides were achieved under solvent-free conditions. Furthermore, the catalyst could be recycled over five times without appreciable loss of catalytic activity. The excellent catalytic performance of [DMAPH]Br is attributed to the enhanced synergistic interplay of acidic proton and bromide to epoxides and CO2 through positive charge delocalization on its cation.