Secododecahedradienes – Syntheses,Reactivity, in‐Plane Homoconjugated 3C/2e Cations, 4C/3e Radical Cations,and σ‐Bishomoaromatic 4C/2e Dications?

Secodecahedradiene 2a , featuring very proximate, perfectly syn‐periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in‐plane(σ)‐homoconjugational electron delocalization, starting from the available pagodane 15b . The response of 2a (and in part its diester 2b ) – in π,π‐distance (average 3.08 Å), olefinic pyramidalization (average 26.9°), and π,π‐split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16‐dodecahedradiene 3a – to selected 4π‐reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in‐plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in‐plane homoconjugated 4C/3e‐species 2a ^.+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two‐electron oxidation in SbF₅/SO₂ClF did not disclose the σ‐bis‐homoaromatic dication 4C/2e (see 2a ^{2 +} ), but a bis‐allylic dication 75 as persistent species. In support of 2a ^{2 +} as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H‐cage σ‐homoconjugated cations.     

Synthesis and characterization of an amethyrin-uranyl complex displaying aromatic character

Abstract

The reaction between amethyrin and non-aqueous uranyl silylamide (UO₂[N(SiMe₃)₂]₂) under anaerobic conditions affords a bench-stable uranyl complex. UV–vis spectroscopy, cyclic voltammetry, as well as proton NMR spectroscopic analyses provide support for the conclusion that all six pyrrole subunits participate in coordination of the uranyl dication and that, upon complexation, the amethyrin-core undergoes a 2-electron oxidation to yield a formal 22 π-electron aromatic species.     

Stable Radical Cation and Dication of an N-Heterocyclic Carbene Stabilized Digallene: Synthesis,Characterization and Reactivity

One- and two-electron oxidation of a digallene stabilized by an N-heterocyclic carbene afforded the first stable gallium-based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃SnH and cyclo-S₈ was studied; a digallium–hydride cation salt containing a Ga−Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder-like Ga₄S₄ core structure were obtained, respectively.     

Stable Radical Cation and Dication of an N‐Heterocyclic Carbene Stabilized Digallene: Synthesis,Characterization and Reactivity

One‐ and two‐electron oxidation of a digallene stabilized by an N‐heterocyclic carbene afforded the first stable gallium‐based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃SnH and cyclo‐S₈ was studied; a digallium–hydride cation salt containing a Ga?Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder‐like Ga₄S₄ core structure were obtained, respectively.     

Bright Luminescence by Combining Chiral [2.2]Paracyclophane with a Boron–Nitrogen-Doped Polyaromatic Hydrocarbon Building Block

Novel BN-doped compounds based on chiral, tetrasubstituted [2.2]paracyclophane and NBN-benzo[f,g]tetracene were synthesized by Sonogashira–Hagihara coupling. Conjugated ethynyl linkers allow electronic communication between the π-electron systems through-bond, whereas through-space interactions are provided by strong π–π overlap between the pairs of NBN-building blocks. Excellent optical and chiroptical properties in racemic and enantiopure conditions were measured, with molar absorption coefficients up to ϵ=2.04×10⁵ M⁻¹ cm⁻¹, fluorescence quantum yields up to Φ_PL=0.70, and intense, mirror-image electronic circular dichroism and circularly polarized luminescence signals of the magnitude of 10⁻³ for the absorption and luminescence dissymmetry factors. Computed g_lum,calcd. values match the experimental ones. Electroanalytical data show both oxidation and reduction of the ethynyl-linked tetra-NBN-substituted paracyclophane, with an overlap of two redox processes for oxidation leading to a diradical dication.     

Oxidation of a P−C Bond under Mild Conditions

The reactivity of phosphenium dication [(Ph₃P)₂C‐P‐NiPr₂]²⁺, 1²⁺ , towards pyridine N‐oxide (O‐py) has been investigated. The resulting oxophosphonium dication [(Ph₃P)₂C(NiPr₂)P(O)(O‐py)]²⁺, 2²⁺ , was surprisingly stabilized by a less nucleophilic O‐py ligand instead of pyridine (py). This compound was then identified as an analogue of the elusive Criegee intermediate as it underwent oxygen insertion into the P?C bond through a mechanism usually observed for Baeyer–Villiger oxidations. This oxygen insertion appears to be the first example of a Baeyer–Villiger oxidation involving O‐py.     

Bis[1]benzothieno[1,4]thiazines: Planarity,Enhanced Redox Activity and Luminescence by Thieno-Expansion of Phenothiazine

Twofold Buchwald–Hartwig aminations selectively furnish three regioisomers of bis[1]benzothieno[1,4]thiazines; X-ray structure analyses and DFT calculations were corroborated for correlation of their electronic properties. All regioisomers outscore the parent compound phenothiazine with respect to a low-lying oxidation potential and reversible redox activity. The anti-anti bis[1]benzothieno[3,2-b:2′,3′-e][1,4]thiazines possess the lowest oxidation potentials in this series and displayed pronounced green luminescence in solution (Φ_F≈20 %) and in the solid state. Syn-anti regioisomers were only weakly luminescent in solution, but showed aggregation-induced emission enhancement and solid-state luminescence. Most interestingly, X-ray structure analyses revealed that anti-anti derivatives have an amazingly coplanar structure of the pentacyclic anellated 1,4-thiazine system, emphasizing a structural similarity to heteroacenes. The calculated theoretical nucleus-independent chemical shifts additionally suggested that these 8π-electron core systems can be considered as the first electronically unbiased anellated 1,4-thiazines with antiaromatic character.     

Benzene Oxide-Oxepin Valence Tautomerism

Benzene oxide and the potential 8π-electron system oxepin exist in valence-tautomeric equilibrium with each other, to which both components contribute to approximately the same extent. NMR spectroscopic measurements show that the equilibrium is rapidly established (activation energies of the forward and reverse reactions 9.1 and 7.2 kcal mole^?1, respectively). The present knowledge of the properties of oxepin justifies its classification as a “heterotropilidene”. Benzene oxide-oxepin represents a system having fluctuating bonds, the equilibrium of which can be displaced from one extreme to the other by means of suitable substituents. The oxide component determines the reactions of the system with most agents. With 1,6-oxido[10]annulene, which is formally a 2,7-bridged oxepin, the oxepin character is completely suppressed by the formation of a delocalized 10π-electron system extending over the C₁₀ perimeter. The existence and aromatic character of 1,6-oxido[10]-annulene give rise to the conception of a homologous series of oxygen bridged annulenes (1,6; 8,13-bisoxido[14]annulene, 1,6; 8,17; 10,15-trisoxido[18]annulene etc.), which, like the parent acenes, possess a (4n + 2)π-electron system. Molecular models demonstrate that a considerable flattening of the C_4n+2 perimeter is achievable in the case of a syn or all-syn arrangement of the oxygen bridges, and that the requirement for aromaticity is thus satisfied. This is confirmed in a striking manner by the synthesis and properties of syn-1,6; 8,13-bisoxido[14]annulene.     

Modulation of π-Electron Conjugation in Oligo(triacetylene) Chromophores by Incorporation of a Central Spacer

A comprehensive series of trimeric hybrid oligomers 4 – 14 (Fig. 2) was prepared by insertion of different hetero-spacers between two (E)-hex-3-ene-1,5-diyne (=(E)-1,2-diethynylethene, DEE) moieties, and the optical and electrochemical properties of the resulting π-conjugated materials compared to those of the DEE dimer 2 and trimer 3 , which formally contains a DEE moiety as homo-spacer. The hetero-spacers varied from benzenoid (phenylene, naphthalene, biphenylene, anthracene), to π-electron-deficient (pyrazine, pyridine) and π-electron-rich (thiophene, furan) aromatic rings, and to an organometallic trans-Pt(PEt₃)₂ fragment. The hybrid oligomers were synthesized following a general strategy which relied on the Sonogashira cross-coupling between mono-deprotected DEE 15 and the appropriately bis-functionalized spacer (Scheme and Table 1). UV/VIS Studies revealed that the majority of the hetero-spacers were less effective than the homo-spacer DEE in facilitating π-electron delocalization along the linearly conjugated oligomeric backbone (Table 2 and Fig. 3). With increasing degree of benzenoid aromaticity in the hetero-spacer, the electronic communication between the terminal DEE moieties in the hybrid oligomers was reduced. As a remarkable exception, a large bathochromic shift of the longest-wavelength absorption maximum, which is indicative of enhanced π-electron delocalization, was obtained upon introducing an anthracene-9,10-diyl moiety as hetero-spacer into oligomer 7 (Figs. 3 and 6). Electrochemical investigations by cyclic and steady-state voltammetry confirmed the limited extent of π-electron delocalization in the majority of the hybrid oligomers (Table 3). The fluorescence properties of many of the DEE hybrid materials were dramatically enhanced upon incorporation of the hetero-spacers (Table 2). The heterocyclic derivatives 10 – 12 , containing pyridine, pyrazine, or thiophene spacers, respectively, displayed a strong fluorescence emission, which is present to a significant extent neither in DEE homo-oligomers nor in the individual heteroaromatic spacer components, demonstrating the value of combining different repeat units to modulate oligomeric and polymeric properties. The pyridine derivative 10 provided an interesting example of a molecular system, in which both the electronic absorption (Fig. 4) and emission characteristics (Table 2) can be reversibly switched as a function of pH (Fig. 5).     

Synthesis and 20π-electrocyclisation of heptahendecafulvadiene - An unusual sequence of pericyclic processes

Upon thermal activation the newly synthesised heptahendecafulvadiene E ( 3) is isomerised into the pentacyclic hydrocarbons 9/11 (xylene,t_1/2(150°C) ca.5min), the formation of which is explained by initial conrotatory 20-electron electrocyclisation followed by a cascade of 10π/6π-pericyclic processes.     

A Cationic NHC-Supported Borole

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF₃)₃]₄} from the NHC-chloroborole adduct yields the first stable NHC-supported 1-(^MeNHC)-2,5-(SiMe₃)₂-3,4-(Ph*)₂-borole cation. Electronically, it features both a five-membered cyclic conjugated 4 π-electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass-spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1-mesityl borole reveals strong Lewis acidity, reduced HOMO–LUMO gaps, and increased anti-aromatic character.     

Phanes with Three- and Four-Membered Rings as Building Elements

The systematic integration of the small 2π- and 4π-electron systems cyclopropenylium ion, cyclopropenone, oxo-cyclobutenylium ion, and cyclobutadiene into phane chemistry was initiated only a few years ago. [n₂]Cyclopropenylophanes, [n₂]cyclopropenonophanes, metal-capped [n₄]cyclobutadienosuperphanes, and other new families of double-decker phane species became available from cycloalkydiynes through special methods of double [2+1]cycloaddition with carbenes and metal complex induced dimerization by double [2+2]cycloaddition. Phane-specific structural features were elucidated by X-ray structural analyses. Cyclic voltammetry and PE spectroscopy as well as MO calculations reealed considerable interactions between closely spaced π-electron systems. Decapping cyclobutadienosuperphanes formally extended the synthetic principle to threefold [2+2]cycloadditions resulting in [n₄]-bridged tricyclo-[4.2.0.0^{3, 5}]octa-3,7-dienes, which represent a new type of cage compounds. Moreover, completion to fourfold [2+2]addition was achieved with the photoinduced transformation of [3₄]bridged tricyclo[4.2.0.0^{3, 5}]octa-3,7-diene into propella[3₄]cubane.     

Miscibility behavior of di(ethyl-2 hexyl) phthalate in binary and ternary chlorinated polymer blends

The miscibility behavior of ternary blends made by the addition of di(ethyl-2 hexyl) phthalate (DOP) to a mixture of chlorinated polymers was investigated by differential scanning calorimetry. Two chlorinated polymer mixtures were selected: polyvinyl chloride (PVC) with a chlorinated polyethylene containing 48 wt% Cl (CPE48), and PVC with a chlorinated PVC containing 67 wt% Cl (CPVC67). Each binary DOP/chlorinated polymer pair is miscible whereas PVC/CPE48 and PVC/CPVC67 blends are immiscible. DOP/CPE48/PVC and DOP/PVC/CPVC67 ternary blends containing, respectively, more than 55 and 20% DOP exhibit a single glass transition temperature (T_g). The spinodal between the one-T_g zone and the two-T_g zone is symmetrical in the two cases. At high DOP concentrations, a quantitative analysis of the results leads to the conclusion of the presence of a true ternary phase. At low DOP concentrations where two T_gs are observed, the DOP is distributed equally between the two chlorinated polymers forming, in the DOP/CPE48/PVC case for instance, two binary DOP/CPE48 and DOP/PVC phases. The broad immiscibility zone observed in the DOP/CPE48/PVC ternary blend as compared to the DOP/PVC/CPVC67 blend appears to be mainly caused by the high molecular weight of CPE48, as compared with PVC and CPVC67. © 1994 John Wiley & Sons. Inc.     

Electrooxidation of Chlorpromazine in Aqueous and Micellar Media and Spectroscopic Studies of the Derived Cationic Free Radical and Dication Species

Summary.  The electrochemical behaviour of chlorpromazine has been examined in phosphate buffers in aqueous as well as micellar media at a pyrolytic graphite electrode surface. Two oxidation peaks were obtained in linear sweep voltammetry of chlorpromazine. The first peak corresponds to the formation of the cationic free radical, which on further 1e⁻-oxidation gives a dication. The spectroscopic changes and kinetics of the cationic free radical and dication species generated during electrooxidation of chlorpromazine were investigated in both media. The decay of the dication was studied chronoamperometrically and was found to follow first-order kinetics with a half-life of ∼25 ms. Surfactants affect both E _p and i _p values. The anionic surfactant SDS has been found to catalyze the reaction of the free radical cation and the dication.     

A quantum chemical study of [1.1.1.1]pagodane and its related compounds

The PBE0/6-31G** quantum chemical method is used to determine the symmetry and equilibrium structural parameters of the molecules of [1.1.1.1]pagodane (C₂₀H₂₀, D _2h ), two dienes (C₂₀H₂₀, D _2h ), two diradicals (C₂₀H₂₀, C _2ν ), and two dications (C₂₀H₂₀²⁺, D _2h and C _2ν ). The energy of a highly symmetric dication with a rectangular cycle is lower by 36 kcal/mole than that of a low symmetric dication with a trapezoidal cycle. The polarization interaction with liquid methylene chloride causes its decrease by 147 kcal/mole.     

A covalent attraction between two molecular cation TTF·+

The optimized structure of the tetrathiafulvalence radical-cation dimer (TTF·⁺-TTF·⁺) with all-real frequencies is obtained at MP2/6-311G level, which exhibits the attraction between two molecular cation TTF·⁺. The new attraction interaction is a 20-center-2-electron intermolecular covalent π/π bonding with a telescope shape. The covalent π/π bonding has the bonding energy of about ?21 kcal·mol^?1 and is concealed by the Coulombic repulsion between two TTF·⁺ cations. This intermolecular covalent attraction also influences the structure of the TTF·⁺ subunit, i.e., its molecular plane is bent by an angle θ = 5.6°. This work provides new knowledge on intermolecular interaction.

     

Rhodium-Catalyzed Acrylate Synthesis from Carbon Dioxide and Ethylene by using a Guanidine-Based Pincer Ligand: Perturbing Occupied d-Orbitals by pπ-dπ Repulsion Makes a Difference

Rhodium-catalyzed acrylate synthesis from CO₂ and ethylene was accomplished by using a guanidine-based NCN pincer ligand. The repulsion between pπ-electron of guanidine sidearms and occupied dπ orbital of rhodium center raised the level of d-electrons close to those of formerly known d⁸-ruthenium catalyst, thereby promoting the metallalactone formation from carbon dioxide and ethylene. This work fills the absence of group-9 metal based catalyst for the acrylate synthesis and provides a designing approach for pincer-ligated d⁸-metal catalysts to utilize pπ-dπ interaction for promoting desirable redox processes.     

Cluster expansion of the wavefunction: Ionizations and some low-energy excitations of naphthalene

Ionizations and some lower singlet valence and Rydberg excitations of naphthalene are studied by the SAC/SAC CI method with 100 (44 π + 56 σ) active MOs. A systematic assignment of the ionization spectra is given. In particular, assignments are given for the peaks in the 13–20 eV region. A large σ-electron correlation effect is found for the valence excited B_1u state, together with a large effect of the d_π polarization function. The controversy concerning the assignment of the Rydberg nd_π orbitals is resolved: Robin's assignment of the 3d_π orbital at 6.73 eV above the ground state is supported.     

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO–LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.     

The Upper Limit of Luminol's Amphiprotism: The Crystal Structure of 5-Ammonium-2-hydro-1,4-phthalzinediol Sulfate(IV)

Luminol is chemically sufficiently stable to be diprotonated at high proton concentrations as provided by concentrated sulfuric acid. The luminol dication (5-ammonium-2-hydro-1,4-phthalzinediol) sulfate was isolated as macroscopic single crystals and its structure was determined and refined from single-crystal X-ray data collected at 173 K [cell parameters: a = 8.3994(17) Å, b = 6.9985(14) Å, c = 17.486(4) Å, β = 90.85(3)°, V = 1027.8(4) ų, space group P2₁/c]. The structure is comprised of layers stacked along the b axis. Intralayer interactions are accomplished by strong hydrogen bonds of three luminol dications to one central [SO₄]^2– ion. Interlayer interactions are formed by weak hydrogen bonds of one luminol dication to two [SO₄]^2– ions in the adjacent layers, respectively, and alternating sandwich and parallel-displaced π-π-stacking of the 1-hydropyridazine-3,6-diol moieties of luminol dications in adjacent layers, respectively.