Interannular communication in the radical anions of bis-cyclooctatetraene systems

[reaction: see text] The room-temperature potassium reduction of 1,2-bis-cyclooctatetraeneoxypropane yields two different regio-spin isomer anion radicals in equilibrium (COT-O-C(Me)HCH(2)CH(2)-O-COT(*)(-) = (*)(-)COT-O-C(Me)HCH(2)CH(2)-O-COT) that is shifted far to the right. The presence of the unreduced ring perturbs the spin density on the reduced ring. Addition of more electrons generates the diradical dianion ((*)(-)COT-O-C(Me)HCH(2)CH(2)-O-COT(*)(-)), and the anion radical on the secondary side splits the degeneracy of the psuedo-ortho protons of the anion radical on the primary side.     

Inner reorganization during the radical-biradical transition in a nor-beta-lapachone derivative possessing two redox centers

In this work, the electrochemical behaviour of an antitumoral nitro o-quinone derivative obtained from 3-bromo-nor-beta-lapachone was studied. Cyclic voltammetric experiments, in acetonitrile solution, revealed that both quinone and nitro functions are reduced independently as quasi-reversible one-electron transfer processes in this order. Depending on the reduction potential, a radical anion or a biradical dianion is obtained. The formation of these paramagnetic species was confirmed by performing in situ Electrochemical-Electron Spin Resonance (E-ESR) experiments. Analysis of the kinetics of electron transfer associated to those electron uptake processes, in terms of the Marcus-Hush-Levich model, revealed differences in the reorganization energy (lambda((k))) for both steps (lambda((I)): 1.07-1.11 eV; lambda((II)): 1.21-1.30 eV). By evaluating the conformations of the radical and biradical systems by calculations at the BLYP//TZVP level of theory, it was found that the inner component, for the second reduction process (lambda((II))) was approximately 72% of lambda((II)), reflecting modifications in the molecular structure during the radical anion-biradical dianion transition. This change is also reflected in the differences presented by line widths of the ESR signals of both electrogenerated radical and diradical species.     

Electron-transfer reactions with significant changes in structure. Unsymmetrical crowded ethylenes

The electrochemical reduction mechanisms of xanthylideneanthrone, 6, thioxanthylideneanthrone, 7, 10-(diphenylmethylene)anthrone, 8, and 9-(diphenylmethylene)-9H-fluorene, 9, have been studied in dimethylformamide. The reduction of the first two compounds proceeds from folded forms of the neutral to twisted forms of the anion radical according to a square scheme. The data for reduction of 8 can be well accounted for by the same square scheme. However, one-step reduction with concerted electron transfer and structural change cannot be ruled out. Compound 9, whose fluorene ring system cannot fold, exists only in twisted forms in the neutral, anion radical, and dianion. Consequently, there are no major changes in structure upon reduction, and the compound is reduced in two reversible steps with the second complicated by rapid loss of the dianion that is probably due to protonation by components of the medium.     

Tunable Reduction of 2,4,6‐Tri(4‐pyridyl)‐1,3,5‐Triazine: From Radical Anion to Diradical Dianion to Radical Metal–Organic Framework

The reduction of 2,4,6‐tri(4‐pyridyl)‐1,3,5‐triazine (TPT) with alkali metals resulted in four radical anion salts ( 1 , 2 , 4 and 5 ) and one diradical dianion salt ( 3 ). Single‐crystal X‐ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that 1 contains the monoradical anion TPT^.? stacked in one‐dimensional (1D) with K⁺(18c6) and 2 can be viewed as a 1D magnetic chain of TPT^.?, while 4 and 5 form radical metal‐organic frameworks (RMOFs). 1D pore passages, with a diameter of 6.0 Å, containing solvent molecules were observed in 5 . Variable‐temperature EPR measurements show that 3 has an open‐shell singlet ground state that can be excited to a triplet state, consistent with theoretical calculation. The work suggests that the direct reduction approach could lead to the formation of RMOFs.     

Effects of Strong Ion‐Pairing on the Electrochemical Reduction of Cyclooctatetraene in Tetrahydrofuran in the Presence of Lithium Ion. Peak Coalescence Does Not Imply Potential Inversion

The first study of the voltammetric reduction of cyclooctatetraene (COT) in tetrahydrofuran (THF) in the presence of lithium ion is reported. A single wave is observed at ?2.23 V vs. Ag/0.1 M AgNO₃. Density functional calculations have been carried out on a variety of COT/Li/THF species in order to clarify the nature and role of ion pairing in this system. The dominant species in solution are the COT/Li/(THF)₂ anion radical and the COT/Li₂/(THF)₄ dianion. Computer simulations have been carried out to further understand the effects of ion pairing on the reduction. The simulations show that coalescence of two waves into one can occur in the presence of strong ion pairing even when the second reduction potential is negative of the first.     

Electrochemical reactions of organic compounds in liquid ammonia: Part IV. Reduction of cyclooctatetraene

The electroreduction of COT in liquid ammonia at ?38° and ?65°C was investigated by cyclic voltammetric and controlled potential coulometric techniques. The results are interpreted by a two-step two-electron transfer reaction with strong ion-pairing of the COT dianion. Values for the heterogeneous electron transfer rate constants and the equilibrium constant of the disproportionation of the radical anion are estimated, based on a digital simulation of the cyclic voltammogram.     

Reduction of benzylidene dibenzo[a,d]cycloheptenes: over-reduction of antiaromatic dianions to aromatic tetraanions

The antiaromaticity of a series of dianions of p-substituted benzylidene dibenzo[a,d]cycloheptenes was examined through calculated measures of antiaromaticity. The nucleus-independent chemical shifts (NICS) and magnetic susceptibility exaltation both showed substantial antiaromatic character in the benzannulated tropylium anion. When the antiaromaticity was normalized for the area of the ring, these tropylium anions were shown to be among the most antiaromatic anions in the chemical literature. Attempts to make the dianion through reduction with lithium or potassium gave the tetraanion as the only species observable in the (1)H NMR spectrum. Quench of the reaction mixture with trimethylsilyl chloride or D(2)O confirmed the presence of the tetraanion, but only as a small portion of the reaction mixture, with the major product being unreacted starting material. The failure to observe starting material was attributed to similarities in the structures of the starting material and anion radical (first reduction), allowing rapid electron transfer between them. The inability to see the dianion (second reduction) could be the result of the very small HOMO-LUMO gap anticipated for highly antiaromatic species, which would allow access to diradical species. The magnitude of the HOMO-LUMO gap was determined by the difference between the HOMO and LUMO energies from geometry optimization and the lowest energy transition from TD-DFT calculations. The HOMO-LUMO gap for the benzylidene dibenzocycloheptatriene dianions was shown to be much smaller than the HOMO-LUMO gap of species for which (1)H NMR spectra had been observed.     

Tunneling and sterically induced ring puckering in a substituted [8]annulene anion radical

Electron paramagnetic resonance (EPR) studies have revealed that the steric interaction between the methyl hydrogens on a tert-butoxy substituent and the cyclooctatetraene (COT) ring system sterically induces a puckering of the eight-membered ring in the anion radical of tert-butoxy-COT. The induced nonplanarity of the COT ring system causes a large attenuation of the EPR coupling constants. Since the C-D bond length is slightly shorter than is the C-H bond length, replacement of the tert-butyl group with a tert-butyl-d(9) group results in less steric interaction and measurably larger electron proton coupling constants. The oscillation between the two close to planar alternating bond length (ABL) D(2d) conformers of the COT moiety was found to be extremely rapid (k > 10(12) s(-1)) and quantum mechanical tunneling is proposed to be involved.     

A Mixed-Anion System Consisting of a Germyl Anion and Anions Delocalized on Conjugated Carbon Ring Skeletons

A trianion consisting of one germyl anion and two anions delocalized on conjugated carbon ring skeletons was synthesized by trimerization of the germanium analogue of the anthryl anion, which exhibits high germylene character. In addition, the generation of the putative intermediate of this reaction, the radical anion of 9-germaanthracene, was confirmed by the formation of the corresponding dimeric dianion. The isolated trianion and dianion were characterized by X-ray crystallographic analysis together with NMR spectroscopy and theoretical calculations.     

Structure, electron-accepting properties, and reactivity of nitrostilbenes

Nitrostilbenes, nitrostyrenes, nitrotoluenes, and nitrodibenzyls in neutral, radical onion, and dianion forms are studied by quantum chemical, electron spectroscopy, ESR spectroscopy, and cyclic voltammetry methods. The calculated electron affinities of nitroarenes are compared with electrochemical reduction potentials. The dianion form of stilbene is stable provided that it has two nitro groups in the paraposition of the benzene ring. Nitrodibenzyls are converted to the corresponding nitrostilbenes by electrochemical reduction; this is explained by dissociative cleavage of C–H bonds in the trianion radical form of dibenzyl. In reactions of nitroarenes with bases, regioselectivity is determined by spin density distribution and by the conformation composition of the radical onions. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 640–654, July–August, 1998.     

Structure and chemistry of 1-silafluorenyl dianion, its derivatives, and an organosilicon diradical dianion.

1-Silafluorene dianion was synthesized by potassium reduction of 1,1-dichloro-1-silafluorene in refluxing THF. The X-ray structure of 1,1-dipotassio-1-silafluorene (3b) shows C-C bond length equalization in the five-membered silole ring and C-C bond length alternation in the six-membered benzene rings, indicating aromatic delocalization of electrons in the silole ring. The downfield (29)Si chemical shift at 29.0 ppm and theoretical calculations also support electron delocalization in the silole ring of 3b. Dianion salt 3b underwent nucleophilic reactions with Me(3)SiCl and EtBr to form the corresponding disubstituted products. Benzaldehyde underwent reductive coupling in the presence of 3b. Slow oxidation of 3b yielded 1,1'-dipotassio-1,1'-bis(silafluorene) (16). The X-ray structure and (29)Si chemical shift (-38.0 ppm) of 16 indicate localized negative charges at the silicon atoms and no aromatic character. Heating a DME/hexane solution of 3b in the presence of 18-crown-6 led to a novel diradical dianion salt.     

Electron Delocalization in a Rigid Cofacial Naphthalene‐1,8:4,5‐bis(dicarboximide) Dimer

Investigating through‐space electronic communication between discrete cofacially oriented aromatic π‐systems is fundamental to understanding assemblies as diverse as double‐stranded DNA, organic photovoltaics and thin‐film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π–π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly‐bridged naphthalene‐1,8:4,5‐bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through‐space interactions between the redox‐active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X‐ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π–π stacking interaction when compared with its neutral analog.     

Electrochemical Reduction of Cyclohex-2-enones

The electrochemical reduction of the cyclohex-2-enones 1a–1e (mercury cathode, CH₃CN, Bu₄NBF₄) was studied by means of cyclic voltammetry, d.c. polarography, coulometry and chemical product analysis. Compounds 1a–1c give a mixture of the hydrodimers 4 and 5 via formation of the radical anion 2 by an irreversible one electron transfer, followed by protonation and dimerization of the allylic radical 3 . The 6-halocyclohex-2-enones 1d and 1e exhibit two distinct reduction waves. The first corresponds to an irreversible two electron transfer with formation of the halide anion and the enolate anion 6 which gives 1b by protonation. The second wave corresponds to a quasi-reversible one electron transfer to 6 to afford the radical dianion 7 (Scheme 2).     

Azulene-substituted aromatic amines. synthesis and amphoteric redox behavior of N,N-Di(6-azulenyl)-p-toluidine and N,N,N',N'-tetra(6-azulenyl)-p-phenylenediamine and their derivatives

[reaction: see text] N,N-Di(6-azulenyl)-p-toluidine (1a) and N,N,N',N'-tetra(6-azulenyl)-p-phenylenediamine (2a) and their derivatives with 1,3-bis(ethoxycarbonyl) substituents on each 6-azulenyl group (1b and 2b) were prepared by Pd-catalyzed amine azulenylation and characterized as a study into new aromatic amines for multistage amphoteric redox materials. The redox behavior of each compound was characterized by cyclic voltammetry. These compounds undergo facile reduction to stable anion radicals and dianion diradicals owing to the resonance stabilization between the 6-azulenyl groups and exhibit electrochemical oxidation depending on the amine subunits. The ESR measurement of anion radicals and a dianion diradical generated by the electrochemical reduction of amine 1b and diamine 2b revealed that the unpaired electron of these radicals delocalizes over the entire azulene ring including the central nitrogen atoms. UV-vis spectral analysis of amines 1a,b and diamines 2a,b, taken during the electrochemical reduction, exhibited a gradual decrease of the absorption bands of the neutral species along with an increase of the new absorption maxima at 625, 605, 640, and 610 nm, respectively, with the development of well-defined isosbestic points at 502, 562, 478, and 545 nm, respectively. As indicated by a combined ESR and UV-vis spectral study, the species giving rise to the new absorption maxima are concluded to be the generation of anion radicals and dianion diradicals of aromatic amines and diamines with high thermodynamic stability.     

Anion–π Interactions in Salts with Polyhalide Anions: Trapping of I42−

The directionality of interaction of electron‐deficient π systems with spherical anions (e.g,. halides) can be controlled by secondary effects like NH or CH hydrogen bonding. In this study a series of pentafluorophenyl‐substituted salts with polyhalide anions is investigated. The compounds are obtained by aerobic oxidation of the corresponding halide upon crystallization. Solid‐state structures reveal that in bromide 2 , directing NH–anion interactions position the bromide ion in an η¹‐type fashion over but not in the center of the aromatic ring. The same directing forces are effective in corresponding tribromide salt 3 . In the crystal, the bromide ion is paneled by four electron‐deficient aromatic ring systems. In addition, compounds 4 and 6 , which have triiodide and the rare tetraiodide dianion as anions, are described. Computational studies reveal that the latter is highly unstable. In the present case it is stabilized by the crystal lattice, for example, by interaction with electron‐deficient π systems.     

The second triannulenylene: tri-[8]annulenylene

Room-temperature dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with potassium tert-butoxide followed (after a couple of minutes) by alkali metal reduction was used to generate the anion radical of tri-[8]annulenylene [(C8H6*-)3] in HMPA. EPR analysis reveals that the odd electron is primarily located in one of the three eight-membered ring systems, which is rendered planar. Excellent agreement was obtained between spin densities predicted by B3LYP/6-31G* calculations and those observed. The neutral tri-[8]annulenylene system has a propensity toward polymerization, but it can be isolated for NMR and mass spectral analysis via the I2 oxidation of the anion radical. The NMR analysis reveals that two of the eight-membered rings are bent above the plane of the benzene ring and the other is bent below. Tri-[6]annulenylene (triphenylene) is the only other known member of the triannulenylenes.     

Further studies in the acyl-type radical additions promoted by SmI2: mechanistic implications and stereoselective reduction of the keto-functionality

Attempts were made to promote the carbonyl coupling of cyclohexanone to 4-pyridylthioesters of N-carbamate-protected amino acids with the one electron reducing agent, samarium diiodide. Such reactions proved unsuccessful due to the inability of the ketyl-type radical anion intermediate to be reduced to the corresponding dianion at −78°C. Nevertheless, these results explain our recently published work on the high efficiency of the SmI₂-mediated acyl-type radical additions of the same thioesters with electron deficient alkenes [J. Am. Chem. Soc. 2003, 125, 4030]. A study was also undertaken to examine methods for the stereoselective reduction of N-carbamate-protected amino ketones to either the syn- or anti-vicinal amino alcohols. In most cases, LiAl(O-t-Bu)₃H and (S)-Alpine-Hydride were found to effectively provide the anti- and syn-amino alcohols, respectively. The SmI₂-promoted reduction of the same ketones afforded a majority of the syn-isomer with selectivities of approximately 5:1. However, in one case, the SmI₂-promoted reduction was found to be more effective than that of (S)-Alpine-Hydride.     

Mass spectral evidence of alkali metal insertion into C60-cyclooctatetraene complexes: M+@C60-C8H 8 *3-

C60 can be reduced to its trianion anion radical in hexamethylphosphoramide with potassium or cesium metal. The addition of water to these solutions, followed by toluene extraction, yields materials that exhibit the expected mass spectral peaks for the Birch reduction products of C 60 *3- (C60Hn). However, when cyclooctatetraene (COT) is present in the solution, the mass spectral signature for the Birch reduction products of M+@C60-COT*3- and C60-COT*3- are also found. The trianion radical of C60 reacts with COT in HMPA to yield a [2 + 2] cycloaddition product, and subsequent ring opening provides a passageway for the Cs+ or K+ counterion to the interior of the fullerene. Analogous results are not observed when the smaller metals (Na and Li) are used as the reducing agents. Only the larger alkali metal cations form tight ion pairs with the trianion of C60-COT. The tight ion association is necessary to bring the cation into a sufficiently close proximity to the trianion for the cation to proceed to the interior.     

CO2-fixation into organic carbonyl compounds in visible-light-induced photocatalysis of linear aromatic compounds

CO₂ is fixed into benzophenone using poly(p-phenylene) as a photocatalyst and triethylamine as an electron donor under visible light irradiation. The reaction was enhanced by the addition of onium cation with soft character. The enhanced formation of the radical anion and the dianion species in poly(p-phenylene) by sodium reduction was confirmed by the cation addition. The role of the anion species formed in the photocatalytic system is discussed as the electron pool for the photofixation.