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The two-electron reduction of tetraphenyl-p-quinodimethane M via its radical anion M⊖ to its dianion M⊖⊖ is explored both by cyclovoltammetry and ESR/ENDOR spectroscopy. Contact of the diglyme solution with added 15-crown-5 under aprotic conditions with a sodium metal mirror yields black crystals of a solvent-separated contact ion triple [M⊖⊖][Na⊕(OCH2CH2)5(H3CO(CH2CH2O)2CH3)]2. The two-electron-insertion into the pquinodimethane derivative R2C⊖=C(HC=CH)2C=CR2 changes its structure drastically to that of a twofold carbanion substituted benzene, R2C⊖ -(C6H4)- ⊖CR2. MNDO calculations provide a rationale for both the tremendous solvation of a Na⊕ center coordinated to seven oxygen centers of 15-crown-5 and of one diglyme molecule and the structural changes as well as the charge distribution in the unique Tetraphenyl-p-quinodimethane dianion (H5C6)2C⊖-(C6H4)- ⊖C(C6H5)2, in which the two negative charges are largely localized at the carbanion center of the benzene -substituents.
Tetraphenylbutatriene is reduced under aprotic conditions to its ESR/ENDOR-spectroscopically characterized radical anion and to its dianion, with both electron transfers quasireversible according to cyclovoltammetric measurements. The alkali cation salts, the red contact ion pair [(H5C6)4C4·⊖][Na⊕ (H3COCH2CH2OCH3)3] and the dark violet contact ion triple [(H5C6)4C4⊖⊖][Li⊕(H3COCH2CH2OCH3)3]2 can be prepared by single electron reduction at a sodium metal mirror or by twofold de-protonation of 1,1,4,4-tetraphenylbutyne-2 using lithium-n-butyl. Their single crystal structures as well as that of the parent acetylene have been determined at low temperatures. The essential structural changes observed are the twisting of both molecular halves (H5C6)2CC relative to each other with increasing negative charge. The simultaneously resulting bond alternancy >C = C = C = C< → >C⊖ - C ≡ C⊖ - C < within the cumulene chain is discussed based on MNDO calculations for the structures determined.
In the pyrolysis of 1,2,3-benzoselenodiazole using a short-distance furnace, a short-lived intermediate is detected photoelectron spectroscopically. Mass spectra recorded under similar conditions suggest an isomer C6H4Se rearranging to the more stable final product 6-fulveneselone. The ionization pattern obtained by computerized spectra stripping is assigned to benzselenirene by molecular radical cation state comparison based on MNDO calculations.
Thermal decompositions of azo compounds in the gas phase under reduced pressure are further investigated using photoelectron spectroscopic gas analysis. Passing diallyl, diphenyl and phenylmethyl derivatives either through a short-pathway pyrolysis (SPP) apparatus or through an external thermal reactor (ETR) results in the following fragmentations: Under nearly unimolecular conditions (SPP, 10-4 mbar pressure), diallyldiazene decomposes above 600 K to N2 and hexadiene-1,5 with the allyl radical as a detectable intermediate. The PE spectra recorded for diphenyldiazene above 1000 K (ETR, 1-2 mbar pressure) show N2, benzene, as well as traces of diphenyl. Phenylmethyldiazene yields above 800 K (SPP) predominantly N2, toluene, diphenyl and ethane with the methyl radical as the only detectable intermediate. Insertion of quartz wool into the pyrolysis tube (ETR) lowers the fragmentation temperatures, and in addition, above 850 K, HCN and aniline are PE spectroscopically identified. Surprisingly, this second reaction channel can be heterogeneously catalyzed: phenylmethyldiazene decomposes under 10-2 mbar pressure at a [Ni/SiO2] catalyst surface selectively to HCN and aniline.
Photoelektronen-Spektren und Moleküleigenschaften, 110 [1,2]. Tricyanmethan-Derivate X—C(CN)3
(1987)
The photoelectron spectra of tricyanomethane derivatives X-C(CN)3 with substituents X = H, CH3, Br and C6H5 have been recorded and are assigned based on MNDO calculations as well as on radical cation state comparison with the iso(valence)electronic P(CN)3, within the series of cyanomethanes H4-nC(CN)n, and with each other. For HC(CN)3, no traces of the isomeric dicyano, ketimine HN = C=C(CN)2 are detected in the gas phase. Tricyanomethylbenzene, H5C6-C(CN)3, exhibiting the highest first ionization energy of any known singly acceptor substituted phenyl derivative, demonstrates the tremendous electron withdrawing effect of the -C(CN)3 group.
The HCl elimination from β-chloroethyl azide (1-azido-2-chloroethane) over potassium tert. butanolate at 350 K in a low pressure flow system is optimized using PE spectroscopic real-time gas analysis. The highly explosive vinyl azide formed can be purified by cool-trapping the by-products. Its subsequent and virtually hazard-free pyrolysis yields 2H-azirine, which can be isolated at temperatures below 240 K.
In contrast, the direct pyrolysis of β-chloroethyl azide requires temperatures above 710 K and results in a simultaneous split-off of both HCl and N2, yielding acetonitrile as the main thermolysis product. No intermediates such as β-chloroethanimine or ketenimine are observed, a result which is interpreted in terms of chemical activation.
Trifluoromethyl azide decomposes in a low-pressure flow system at rather high temperatures by splitting off N2. The nature of the resulting products depends largely on the wall material of the pyrolysis tube: using molybdenum above 1120 K, FCN is observed exclusively. Neither F2C=NF nor F3C-N=N-CF3 can be detected as intermediates by comparing their PE spectra with those continuously recorded while increasing the temperature. F3C-N = N - CF3 fragments already at 870 K to give N2 and F3C-CF3. The PE spectra of F3CN3 and F2C=NF are assigned based on MNDO calculations.
The PE spectra of the nitrogen-rich title compounds cyanogen azide NC-N3, azodicarbonitrile NC - N = N - CN, azidoacetonitrile NC - H2C - N3, tetrazolo[1,5-a]pyridine (H4C5N)(N )3 and trimethylenetetrazole (H2C)3(CN4) are presented and assigned by radical cation state comparison with related compounds or by Koopmans’ correlation with MNDO eigenvalues. In a low pressure flow system the compounds decompose at higher temperatures, with elimination of the thermodynamically favorable N2 molecule. PE-spectroscopic real-time analysis reveals as further products: NC - N3 → C∞, NC - N = N - CN → NC - CN , NC - H2C - N3 → 2HCN (+ traces NC - HC = NH?) and (H2C)3(CN4) → H2C = N - CN + H2C = CH2. For tetrazolo[1,5-a]pyridine, a preceding ring opening to the corresponding 2-azidopyridine is observed.
The reversible one-electron insertion into mono- and 1,4-di-substituted benzene derivatives is favored by dialkoxyboron and especially by dialkylboron groups. The assumption that it should be the symmetric e2u benzene molecular orbital which is occupied in the resulting radical anions can be supported by comparison of ESR coupling constants.
The sodium salt of the most simple polynitro-substituted hydrocarbon anion. Na⊕⊖C(NO2)3, (for a hazard warning cf. [***]) crystallizes from ether solutions without and with addition of 18-crown-6 either in a polymer band. [(Na⊕⊖C(NO2)3)dioxane]∞, or as a solvent- separated ion pair, [(Na⊕/18-crown-6)(THF2]⊕[(Na⊕/18-crown-6)(O2N-C⊖(NO2)2)2]⊖. The Na⊕ cations are each 8-fold coordinated in hexagonal bipyramidal arrangement. According to extensive quantum-chemical calculations based on the structure coordinates, the formation of these novel salts can be traced back to the charge distribution in the anions ⊖C(NO2)3. which due to negatively charged oxygen centers are favorable complex ligands. The structure determining effects of solvation are discussed.