N. T. Berberova

The effect of electron deficit on the geometry of sulphide bridged methylcyclopentadienylmetal tetrahedra. Synthesis, physicochemical characteristics, and molecular structures of Cp′3Cr3(μ3-S)4Co(CO), Cp′4Cr4(μ3-O)(μ3-S)3+ ZnCl3·C4H8O−, and Cp′4V4S4 (Cp′ = π-CH3C5H4)

Abstract A diamagnetic electron-saturated cluster, Cp′ 3 Cr 3 (μ 3 -S) 4 Co(CO) (I), was obtained by heating a heterotrinuclear cluster, Cp′ 2 Cr 2 SCMe 3 (μ 3 -S) 2 Co(CO) 2 (Cp′ = π-CH 3 C 5 H 4 ), in heptane in the presence of diphenylacetylene. The treatment of (Cp′CrSCMe 3 ) 2 S with anhydrous ZnCl 2 in THF led to an electron-deficient cationic cluster, Cp′ 4 Cr 4 (μ 3 -O)(μ 3 -S) + 3 (II), with one unpaired electron (ZnCl 3 · thf − is the anion). Interaction between Cp′ 2 V and t-butylmercaptan in boiling heptane gives Cp′ 4 V 4 S 4 (III) which is a new member in the family of cubane clusters, Cp 4 M 4 S 4 , and contains an electron-deficient metal tetrahedral skeleton with two unpaired electrons. The structure of compounds I–III was determined by an X-ray diffraction study. I contains the metal tetrahedral Cr 3 Co skeleton which is flattened down the axis joining Co and the centre of the Cr 3 base (CrCo 2.692(4), CrCr 2.816(3) A). The μ 3 -S bridge is located above each face (CoS average, 2.171(6) A; CrS average, 2.255(5) A). The Co atom is also linked to the terminal CO group (CoC 1.82(2) A). and each Cr atom is bonded to the π-CH 3 C 5 H 4 (CrC average, 2.29(2) A. The cationic cluster in II is a distorted tetrahedron in which the CrCr bonds at the sulphide bridge faces (average, 2.765(2) A) are longer than those at the oxygen bridge face (average, 2.687(2) A). The CrO bonds (average, 1.994(5) A) are shorter than the CrS bonds (average, 2.224(3) A), and both are shorter than their counterparts in the neutral cluster, Cp′ 4 Cr 4 (μ 3 -O)(μ 3 -S) 3 ·CuBr 2 described previously, presumably because of the enhancement of π-bonding in both OCr and SCr bonds. This interaction is particularly noticeable in III, which has a strongly electron-deficient metal V 4 skeleton (VS average, 2.292(3) A) which gives rise to short VV distances (average, 2.867(3) A). The molecule of III is sterically not strained, the VC distances (average, 2.28(1) A) being close to distances expected for ordinary bonds. The physicochemical properties of I–III and the specific features of bonding peculiar to these clusters in relation to the electron deficit of their metal skeletons are discussed.

One-electron transfer in the dehydroaromatization of heterocyclic compounds (survey)

The survey systematizes studies of recent years on the mechanism of the dehydroaromatization of heterocyclic compounds, as well as the dehydrogenation of functionally related systems. The available material is arranged according to types of dehydrogenating agents. On the basis of the literature data and the authors own experimental data, the stepwise mechanism of the oxidative dehydrogenation of heterocyclic compounds is substantiated.

The mechanism of nucleophilic substitution in halopyrylium and halobenzopyrylium salts

The electrochemical behavior of 4-chloro-2,6-diphenylpyrylium and 4-chloro(bromo)flavylium perchlorates has been studied. Their reaction with nucleophiles has also been studied. It was shown that nucleophilic substitution in these compounds occurs through a stage of forming a charge-transfer complex which passes into an ion-radical pair as a result of a one-electron transfer. Heterolytic fission of the C-Hal bond occurs at the stage of the radical or adduct of the substituted pyrylium (flavylium) salt and nucleophile.

Oxidation-reduction characteristics of octahydrochalcogenoxanthenes and octahydroxanthylium cations

Unstable radical-cations, which fragment according to the (−e, −H+, −e) scheme, are formed during the electrochemical oxidation of chalcogenooctahydroxanthenes. Free radicals (identified by ESR) are formed during the electrochemical reduction of the chalcogenooctahydroxanthylium cations.

Molecular complexes ofsym-octahydro-1-chalcogenaanthracene salts with aromatic compounds

Complexes ofsym-octahydro-10-chalogenaanthracene perchlorates with aromatic hydrocarbons, di- and triatomic phenols, naphthols, and aromatic amines can be readily separated in the solid state. Complex formation is clearly shown to lower the oxidative power of the heteroaromatic cations.

OXIDATIVE CLEAVAGE OF THE C-C BOND IN PROCESSES INVOLVING THE HETEROAROMATIZATION OF 9R-SYM-NONAHYDRO-10-OXA(CHALCOGENA)ANTHRACENES

The reaction of 9-(2-methoxyphenyl)- and 9-(2-thienyl)-sym-nonahydro-10-selena(thia)anthracenes with trifluoroacetic acid causes their heteroaromatization with the elimination of substituents from the γ positions of the heterorings. A similar transformation of these compounds, as well as their oxygen and 9-benzyl-substituted analogs, occurs during anode electrochemical oxidation. The stepwise character of the cleavage of the C-C bond, which includes one-electron oxidation of the chalogenapyrans and subsequent fragmentation of the cation-radical intermediates, is substantiated.

Free radicals in redox reactions of 4,4′-di-(2,6-diphenylthiapyrylo)monomethinecyanine fluoroborate

ESR spectroscopic and electrochemical studies of the one-electron reduction and oxidation of 4,4′-di-(2,6-diphenylthiapyrylo)monomethinecyanine have been carried out. Relatively stable cyanine free radicals have been detected and characterized.

Evolution of hydrogen in the reaction of hydroheteroaromatic compounds with dehydrogenating agents

The yields of molecular hydrogen in the reactions of benzimidazolines 1,2-dihydropyrimidines with a number of dehydrogenating agents have been determined. The formation of hydrogen is due to breakdown of the cation-radicals of the hydrohetarenes formed in the first stage of the reactions. The EPR spectrum of the cation-radical from 1,3-dimethyl-2-phenylbenzimidazoline has been obtained.