Recent Advances in Design and Synthesis of Diselenafulvenes, Tetraselenafulvalenes, and Their Tellurium Analogs and Application for Materials Sciences

The first organic metals were obtained based on tetrathiafulvalene. The most significant advance in the field of organic metals was the discovery of superconductivity. The first organic superconductors were obtained based on tetramethyltetraselenafulvalene. These facts demonstrate great importance of tetraselenafulvalenes and their precursors, diselenafulvenes, for materials sciences. Derivatives of 1,4-diselenafulvene and 1,4,5,8-tetraselenafulvalene are useful building blocks for organic synthesis and donor units for the preparation of charge-transfer complexes and radical ion salts, the construction of organic metals, superconductors, organic Dirac materials, semiconductors, ferromagnets, and other conductive materials. This review covers the literature on the design, synthesis, and application of 1,4,5,8-tetraselenafulvalenes and 1,4-diselenafulvenes and their tellurium analogs over the past 15–20 years. These two classes of compounds are interconnected, since the main part of methods for the synthesis of tetraselenafulvalenes is based on the diselenafulvene derivatives as starting compounds. Special attention is paid to the development of novel efficient synthetic approaches to these classes of compounds. Conducting properties and distinguishing features of materials based on tetraselenafulvalenes and their tellurium analogs as well as examples of materials with high conductivity are discussed.


Introduction
Historically, the first organic metals based on 1,4,5,8-tetrathiafulvalene (TTF) were discovered. The preparation of a conducting TTF salt was described in 1972 [1] and the synthesis of first organic metal, a complex of TTF with tetracyanoquinodimethane (TCNQ), was reported in 1973 [2]. The TTF-TCNQ complex behaved as a metal over a large temperature range and had by far the largest maximum electrical conductivity of any organic compound known at that time [2].
The discovery of organic metals gave impetus to the development of synthetic approaches to a variety of structural modifications of TTF, which have been carefully studied in search of organic metals with high conductivity [3][4][5]. As the replacement of skeletal sulfur atoms of TTF by more polarizable selenium atoms has been generally recognized as an effective approach to superior electron donors with enhanced intermolecular interactions, selenium analogs of TTF, 1,4,5,8-tetraselenafulvalene (TSF), have received much attention and have been intensively studied as electron donors for the preparation of conducting materials [4][5][6].
The most significant advance in the field of organic metals has been the discovery of their superconductivity. The first organic superconductor was synthesized by Bechgaard et al. based on tetramethyltetraselenafulvalene (TMTSF) [6]. It was obtained in the form of single-crystal salts with a composition of 2:1, (TMTSF) 2 PF 6 , by electrochemical oxidation of TMTSF in the presence of the corresponding tetraalkyl ammonium salts. Later, a number of similar salts, which exhibit superconductivity, were synthesized based on TMTSF and were named Bechgaard's salts [6]. These historical facts demonstrate great importance of TSF derivatives and their precursors, 1,4-diselenafulvene, for materials sciences.
The Bechgaard's salts have the general formula (TMTSF) 2 X, where X − is a monovalent anion. The formation of the charge transfer salts includes the transfer of one electron from two TMTSF molecules to one X molecule.
In 2004, the journal "Chemical Reviews" published an issue devoted to the preparation and properties of organic conductors and superconductors, as well as methods for the synthesis of their molecular components, in which significant place was given to the TSF derivatives [28][29][30]. The tellurium analogs, 1,4-ditellurafulvene and 1,4,5,8-tetratellurafulvalene derivatives, are also of high interest, as it can be seen from a 2003 review [33], which is devoted to the synthesis and physicochemical properties of 1,4-dichalcogenafulvenes and 1,4,5,8-tetrachalcogenafulvalenes.
However, since then and to the present, a number of novel synthetic approaches to 1,4-diselenafulvene and TSF derivatives have been developed and new data for material sciences have been obtained that require processing and rationalization. This article presents a review of the literature on 1,4-diselenafulvene and TSF derivatives, as well as on tellurium analogs of these compounds, mainly for the last 15-20 years.
Molecules 2022, 27, x FOR PEER REVIEW 2 of 34 of single-crystal salts with a composition of 2:1, (TMTSF)2PF6, by electrochemical oxidation of TMTSF in the presence of the corresponding tetraalkyl ammonium salts. Later, a number of similar salts, which exhibit superconductivity, were synthesized based on TMTSF and were named Bechgaard's salts [6]. These historical facts demonstrate great importance of TSF derivatives and their precursors, 1,4-diselenafulvene, for materials sciences. The Bechgaard's salts have the general formula (TMTSF)2X, where X − is a monovalent anion. The formation of the charge transfer salts includes the transfer of one electron from two TMTSF molecules to one X molecule.
In 2004, the journal "Chemical Reviews" published an issue devoted to the preparation and properties of organic conductors and superconductors, as well as methods for the synthesis of their molecular components, in which significant place was given to the TSF derivatives [28][29][30]. The tellurium analogs, 1,4-ditellurafulvene and 1,4,5,8-tetratellurafulvalene derivatives, are also of high interest, as it can be seen from a 2003 review [33], which is devoted to the synthesis and physicochemical properties of 1,4-dichalcogenafulvenes and 1,4,5,8-tetrachalcogenafulvalenes.
However, since then and to the present, a number of novel synthetic approaches to 1,4-diselenafulvene and TSF derivatives have been developed and new data for material sciences have been obtained that require processing and rationalization. This article presents a review of the literature on 1,4-diselenafulvene and TSF derivatives, as well as on tellurium analogs of these compounds, mainly for the last 15-20 years.

Non-Condensed 1,4-Diselenafulvenes
1,4-Diselenafulvene 1 was obtained based on available and inexpensive industrial starting reagents, selenium and acetylene, in the KOH-HMPTA-H2O system (100-140 °C, 10-15 atm). The proposed route for the formation of compound 1 includes the generation of the acetylide anion from acetylene and potassium hydroxide, its insertion reaction with selenium, followed by heterocyclization of the resulting ethyneselenolate anions (Scheme 1) [34].
Lithiation of ethyl propiolate with lithium hexamethyldisilazide in THF followed by selenium insertion reaction gives ethyl 2-(2-ethoxy-2-oxoethylidene)-1,3-diselenole-4-carboxylate in 75% yield as a mixture of Eand Z-isomers (6a,b) (Scheme 4) [44]. The recrystallization of this mixture made it possible to obtain pure Z-isomer 6b suitable for X-ray diffraction analysis. Diselenoles 6a,b turned out to be interesting crystalline compounds that exhibit an unusual coordination between the oxygen atom of the oxoethylidene group and the nearest selenium atom [44]. It is interesting to note that, under the action of the daylight, the E-isomer 6a in solution was completely converted to the Z-isomer 6b (Scheme 4), presumably through a photochemically induced isomerization mechanism. This conversion did not occur in the dark [44]. Scheme 2. The preparation of (E)-2-benzylidene-4-phenyl-1,3-diselenole 3 from phenylacetylene and selenium [38,39].
Diselenole 3 was previously obtained in 94% yield by the one-pot efficient method based on the reaction of phenylacetylene with elemental selenium in the system KOH-HMPA-H 2 O [40].
Heterocycle 1 is an important intermediate in organic synthesis as well as the starting material for preparation of 1,4,5,8-tetraselenafulvalene 2. The latter is the electron donor for the synthesis of organic metals: charge transfer complexes and radical ion salts with high electrical conductivity.
Diselenole 3 was previously obtained in 94% yield by the one-pot efficient method based on the reaction of phenylacetylene with elemental selenium in the system KOH-HMPA-H2O [40].
Lithiation of ethyl propiolate with lithium hexamethyldisilazide in THF followed by selenium insertion reaction gives ethyl 2-(2-ethoxy-2-oxoethylidene)-1,3-diselenole-4-carboxylate in 75% yield as a mixture of Eand Z-isomers (6a,b) (Scheme 4) [44]. The recrystallization of this mixture made it possible to obtain pure Z-isomer 6b suitable for X-ray diffraction analysis. Diselenoles 6a,b turned out to be interesting crystalline compounds that exhibit an unusual coordination between the oxygen atom of the oxoethylidene group and the nearest selenium atom [44]. It is interesting to note that, under the action of the daylight, the E-isomer 6a in solution was completely converted to the Z-isomer 6b (Scheme 4), presumably through a photochemically induced isomerization mechanism. This conversion did not occur in the dark [44]. Scheme 3. The synthesis of 3,5-dibenzoyl-1,4-diselenafulvene (5) by the reaction of selenourea with benzoylbromoacetylene in the presence of triethylamine [41].
Heterocycle 1 is an important intermediate in organic synthesis as well as the starting material for preparation of 1,4,5,8-tetraselenafulvalene 2. The latter is the electron donor for the synthesis of organic metals: charge transfer complexes and radical ion salts with high electrical conductivity.
Diselenole 3 was previously obtained in 94% yield by the one-pot efficient method based on the reaction of phenylacetylene with elemental selenium in the system KOH-HMPA-H2O [40].
Lithiation of ethyl propiolate with lithium hexamethyldisilazide in THF followed by selenium insertion reaction gives ethyl 2-(2-ethoxy-2-oxoethylidene)-1,3-diselenole-4-carboxylate in 75% yield as a mixture of Eand Z-isomers (6a,b) (Scheme 4) [44]. The recrystallization of this mixture made it possible to obtain pure Z-isomer 6b suitable for X-ray diffraction analysis. Diselenoles 6a,b turned out to be interesting crystalline compounds that exhibit an unusual coordination between the oxygen atom of the oxoethylidene group and the nearest selenium atom [44]. It is interesting to note that, under the action of the daylight, the E-isomer 6a in solution was completely converted to the Z-isomer 6b (Scheme 4), presumably through a photochemically induced isomerization mechanism. This conversion did not occur in the dark [44]. The recrystallization of this mixture made it possible to obtain pure Z-isomer 6b suitable for X-ray diffraction analysis. Diselenoles 6a,b turned out to be interesting crystalline compounds that exhibit an unusual coordination between the oxygen atom of the oxoethylidene group and the nearest selenium atom [44]. It is interesting to note that, under the action of the daylight, the E-isomer 6a in solution was completely converted to the Z-isomer 6b (Scheme 4), presumably through a photochemically induced isomerization mechanism. This conversion did not occur in the dark [44].
The new convenient approach to (Z)-2-(2-chloro-5-nitrobenzylidene)-4-(2-chloro-5nitrophenyl)-1,3-diselenole 10 in 78% yield was developed based on available 4-(2-chloro-5nitrophenyl)-1,2,3-selenadiazole 9 (Scheme 6). The nature and position of the substituents in compound 10 make it convenient for further use as a building block for obtaining more complex derivatives. For example, the nitro group in the aryl ring can be reduced to the amino group, while the halogen atom can be used in various cross-coupling reactions [46].
Methylene-and ethylenediselenole derivatives 17a,b are valuable precursor preparation of a wide range of tetraselenafulvalenes.
Diselenafulvene 14 was also obtained by treating a dioxane solution of compound 12 with an ethanolic potassium hydroxide solution [47,48].
The cross-coupling of thione 44 with compound 45 gave the desired unsymmetrical intermediate product 46 in 69% yield. The formation of the selenium-containing ring was carried out in the same way as for the synthesis of compound 39. However, in this case, a mixture of the reaction products, diester 47a and monoester 47b, was formed. The latter compound was a partially deesterified product, the formation of which was probably caused by the action of cesium iodide. Precursors 47a and 47b were converted to product 43 using standard deesterification conditions (Scheme 14) [59].
caused by the action of cesium iodide. Precursors 47a and 47b were converted to product 43 using standard deesterification conditions (Scheme 14) [59].
The obtained one-component molecular conductors with diselenadithiafulvalene skeletons 51a,b showed high three-dimensional conductivity at room temperature [60,61].
The compound dimethyldiselenadithiafulvalene 52, the analog of diselenadithiafulvalene 49, was obtained by a similar way outlined in Scheme 16. The resulting nickel unsymmetrical complex 53 exhibited a strong third-order non-linear optical response in the visible and near-infrared regions of the spectrum and was regarded as a possible photoconductor [62]. Scheme 16. The synthesis of nickel unsymmetrical complex 53 [62].
The compound dimethyldiselenadithiafulvalene 52, the analog of diselenadithiafulvalene 49, was obtained by a similar way outlined in Scheme 16. The resulting nickel unsymmetrical complex 53 exhibited a strong third-order non-linear optical response in the visible and near-infrared regions of the spectrum and was regarded as a possible photoconductor [62]. 43 using standard deesterification conditions (Scheme 14) [59].
The obtained one-component molecular conductors with diselenadithiafulvalene skeletons 51a,b showed high three-dimensional conductivity at room temperature [60,61].
The compound dimethyldiselenadithiafulvalene 52, the analog of diselenadithiafulvalene 49, was obtained by a similar way outlined in Scheme 16. The resulting nickel unsymmetrical complex 53 exhibited a strong third-order non-linear optical response in the visible and near-infrared regions of the spectrum and was regarded as a possible photoconductor [62]. Scheme 16. The synthesis of nickel unsymmetrical complex 53 [62].
Compounds 60 and 61 were used for preparation of electron transfer complexes, which showed a radical cationic character. The charge in the diselenadithiafulvalene structure can be delocalized to the entire molecule [65].
A simple method for the synthesis of tetrathiafulvalene vinylogs, substituted diselenafulvenes, has been developed. Triethylphosphite was added to a solution of the aldehyde and pyrazine-substituted 1,3-diselenole-2-thione in toluene (or benzene) and the mixture was refluxed for 2 h. The target product 62 was isolated in 75% yield (Scheme 18) [66]. This method is applicable to the synthesis of 1,3-diselenoles containing exotic substituents (such as the pyrazine ring), which are sensitive to some highly reactive reagents used in the classical Wittig reaction. Compounds 60 and 61 were used for preparation of electron transfer complexes, which showed a radical cationic character. The charge in the diselenadithiafulvalene structure can be delocalized to the entire molecule [65].
A simple method for the synthesis of tetrathiafulvalene vinylogs, substituted diselenafulvenes, has been developed. Triethylphosphite was added to a solution of the aldehyde and pyrazine-substituted 1,3-diselenole-2-thione in toluene (or benzene) and the mixture was refluxed for 2 h. The target product 62 was isolated in 75% yield (Scheme 18) [66]. This method is applicable to the synthesis of 1,3-diselenoles containing exotic substituents (such as the pyrazine ring), which are sensitive to some highly reactive reagents used in the classical Wittig reaction. with carbonyldiimidazole. The reaction of compound 58 with 4,5-bis(butylthio)-1,3-dithiol-2-thione in triethylphosphite at 120 °C for 3 h led to 3,6-diethylphthalonitrile 59, containing the dithiadiselenafulvalene moiety. The dithiadiselenafulvalene tetramer-octamethylphthalocyanine 60 was obtained in 33% yield by the treatment of compound 59 with lithium alkoxide at 120 °C. The compound 60 was involved in the reaction with nickel acetate at 155 °C affording a nickel complex 61 (Scheme 17) [65]. Scheme 17. The synthetic approaches to the complexes 60 and 61 [65].
Compounds 60 and 61 were used for preparation of electron transfer complexes, which showed a radical cationic character. The charge in the diselenadithiafulvalene structure can be delocalized to the entire molecule [65].
A simple method for the synthesis of tetrathiafulvalene vinylogs, substituted diselenafulvenes, has been developed. Triethylphosphite was added to a solution of the aldehyde and pyrazine-substituted 1,3-diselenole-2-thione in toluene (or benzene) and the mixture was refluxed for 2 h. The target product 62 was isolated in 75% yield (Scheme 18) [66]. This method is applicable to the synthesis of 1,3-diselenoles containing exotic substituents (such as the pyrazine ring), which are sensitive to some highly reactive reagents used in the classical Wittig reaction.  [67].
(Scheme 19) [67]. A mixture of compounds 64 and 65 was heated in triethylphosphite at 120 °C in a nitrogen atmosphere for 2 h. The copper complexes CuCl2(63)2 and [Cu2Br2. 5(63)] were obtained by the method of vertical diffusion. The first complex exhibited the dielectric properties, whereas the second complex showed the properties of semiconductor (Scheme 19) [67]. Scheme 19. The synthesis of diselenadithiafulvalene 63, condensed with the pyrazine cycle, and its copper complexes [67].
The efficient synthesis of catechol-condensed dithiadiselenafulvalene derivative 68 was developed (Scheme 21) [69]. Compound 68 was a new type of molecular π-electron donor having two phenolic hydroxyl groups, which was promising for the preparation of charge transfer salts. Treatment of compound 69 with NaOMe and ZnCl2 followed by reaction with thiocarbonyldiimidazole under acidic conditions gave 1,3-benzodithiol-2-thione derivative 70 in 60% yield followed by removing benzyl protecting groups by treatment with BF3·Et2O and BuSH. Subsequent re-protection with tert-butyldimethylsilyl group gave compound 71 in 53% yield. Finally, the target compound 68 was obtained in 62% yield by cross-coupling reaction between the product 71 and ketone 34 in triethylphosphite followed by deprotection of the tert-butyldimethylsilyl group in compound 72 (Scheme 21) [69]. semiconductor (Scheme 19) [67]. Scheme 19. The synthesis of diselenadithiafulvalene 63, condensed with the pyrazine cycle, and its copper complexes [67].
The efficient synthesis of catechol-condensed dithiadiselenafulvalene derivative 68 was developed (Scheme 21) [69]. Compound 68 was a new type of molecular π-electron donor having two phenolic hydroxyl groups, which was promising for the preparation of charge transfer salts. Treatment of compound 69 with NaOMe and ZnCl2 followed by reaction with thiocarbonyldiimidazole under acidic conditions gave 1,3-benzodithiol-2-thione derivative 70 in 60% yield followed by removing benzyl protecting groups by treatment with BF3·Et2O and BuSH. Subsequent re-protection with tert-butyldimethylsilyl group gave compound 71 in 53% yield. Finally, the target compound 68 was obtained in 62% yield by cross-coupling reaction between the product 71 and ketone 34 in triethylphosphite followed by deprotection of the tert-butyldimethylsilyl group in compound 72 (Scheme 21) [69]. The efficient synthesis of catechol-condensed dithiadiselenafulvalene derivative 68 was developed (Scheme 21) [69]. Compound 68 was a new type of molecular π-electron donor having two phenolic hydroxyl groups, which was promising for the preparation of charge transfer salts. Treatment of compound 69 with NaOMe and ZnCl 2 followed by reaction with thiocarbonyldiimidazole under acidic conditions gave 1,3-benzodithiol-2-thione derivative 70 in 60% yield followed by removing benzyl protecting groups by treatment with BF 3 ·Et 2 O and BuSH. Subsequent re-protection with tert-butyldimethylsilyl group gave compound 71 in 53% yield. Finally, the target compound 68 was obtained in 62% yield by cross-coupling reaction between the product 71 and ketone 34 in triethylphosphite followed by deprotection of the tert-butyldimethylsilyl group in compound 72 (Scheme 21) [69].

Scheme 19.
The synthesis of diselenadithiafulvalene 63, condensed with the pyrazine cycle, and its copper complexes [67].
Two new donor molecules of the tetrathiapentalene type, compounds 74a,b, containing two selenium atoms and six sulfur atoms in the heterocyclic skeleton, were synthesized by the cross-coupling reaction depicted in Scheme 23 [72]. This combination of the sulfur and selenium atoms in the heterocyclic scaffold results in a particular type of resistivity: flat resistivity over a wide temperature range for the PF 6 and AsF 6 salts of 74a,b, which showed good conductivity [72].
Two new donor molecules of the tetrathiapentalene type, compounds 74a,b, containing two selenium atoms and six sulfur atoms in the heterocyclic skeleton, were synthesized by the cross-coupling reaction depicted in Scheme 23 [72]. This combination of the sulfur and selenium atoms in the heterocyclic scaffold results in a particular type of resistivity: flat resistivity over a wide temperature range for the PF6 and AsF6 salts of 74a,b, which showed good conductivity [72].
Two new donor molecules of the tetrathiapentalene type, compounds 74a,b, containing two selenium atoms and six sulfur atoms in the heterocyclic skeleton, were synthesized by the cross-coupling reaction depicted in Scheme 23 [72]. This combination of the sulfur and selenium atoms in the heterocyclic scaffold results in a particular type of resistivity: flat resistivity over a wide temperature range for the PF6 and AsF6 salts of 74a,b, which showed good conductivity [72]. Two new donor molecules of the tetrathiapentalene type, compounds 74a,b, containing two selenium atoms and six sulfur atoms in the heterocyclic skeleton, were synthesized by the cross-coupling reaction depicted in Scheme 23 [72]. This combination of the sulfur and selenium atoms in the heterocyclic scaffold results in a particular type of resistivity: flat resistivity over a wide temperature range for the PF6 and AsF6 salts of 74a,b, which showed good conductivity [72]. It was found that the PF 6 , AsF 6 , and SbF 6 salts of the product 76b exhibited metallic properties down to 2 K, while the PF 6 and AsF 6 salts of compound 76a showed semiconducting behavior.

Halogenated 1,4-Diselenafulvenes and 1,4,5,8-Dithiadiselenafulvalenes
Halogenated sulfur and selenium-containing fulvalenes have attracted much attention in respect to the unique crystal and electronic structures of their cation radical salts [74][75][76]. In contrast to the other halogenated fulvalenes, iodinated diselenafulvalenes have special ability to construct an intermolecular "iodine bond" by interaction of the iodine atom with other functional groups. The physical properties of materials such as organic conductors depend considerably on the crystal structure properties, and introduction of an "iodine bond" is one of the most effective methods of design and crystal engineering in organic conductors [74][75][76].
A new efficient multistep method for the synthetic preparation of 1,3-diselenole-2thione 16 in 76% yield without the use of toxic carbon diselenide (CSe 2 ) was developed (Scheme 25) [74]. Dicyclopentadienyl dichlorotitanium and readily available elemental selenium were used as starting materials in this synthesis. The resulting diselenafulvene 16 was converted into iodo derivatives 78 and 79. In order to introduce efficiently the iodine atoms and to obtain compounds 78 and 79 in good yields (51% and 97% yields, respectively), a 17-fold excess of perfluorobutyl iodide (PFBI) and a 6-fold excess of lithium diisopropylamide were used (Scheme 25) [74]. The products 78 and 79, in turn, were regarded as valuable starting compounds for the synthesis of various tetraselenafulvalene derivatives.
organic conductors depend considerably on the crystal structure properties, and introduction of an "iodine bond" is one of the most effective methods of design and crystal engineering in organic conductors [74][75][76].
A new efficient multistep method for the synthetic preparation of 1,3-diselenole-2-thione 16 in 76% yield without the use of toxic carbon diselenide (CSe2) was developed (Scheme 25) [74]. Dicyclopentadienyl dichlorotitanium and readily available elemental selenium were used as starting materials in this synthesis. The resulting diselenafulvene 16 was converted into iodo derivatives 78 and 79. In order to introduce efficiently the iodine atoms and to obtain compounds 78 and 79 in good yields (51% and 97% yields, respectively), a 17-fold excess of perfluorobutyl iodide (PFBI) and a 6-fold excess of lithium diisopropylamide were used (Scheme 25) [74]. The products 78 and 79, in turn, were regarded as valuable starting compounds for the synthesis of various tetraselenafulvalene derivatives. engineering in organic conductors [74][75][76].
A new efficient multistep method for the synthetic preparation o 1,3-diselenole-2-thione 16 in 76% yield without the use of toxic carbon diselenide (CSe2 was developed (Scheme 25) [74]. Dicyclopentadienyl dichlorotitanium and readily available elemental selenium were used as starting materials in this synthesis. The re sulting diselenafulvene 16 was converted into iodo derivatives 78 and 79. In order to in troduce efficiently the iodine atoms and to obtain compounds 78 and 79 in good yields (51% and 97% yields, respectively), a 17-fold excess of perfluorobutyl iodide (PFBI) and a 6-fold excess of lithium diisopropylamide were used (Scheme 25) [74]. The products 78 and 79, in turn, were regarded as valuable starting compounds for the synthesis of vari ous tetraselenafulvalene derivatives. New halogenated diselenadithiafulvalenes 82 and 83, containing both chlorine and iodine atoms, were synthesized in 44% and 83% yields, respectively, by the reaction sequence shown in Scheme 27 [76]. Intermediate compound 84 was obtained by successive lithiation by lithium diisopropylamide, chlorination with hexachloroethane, and iodination of the lithium derivative of 1,2-dithiol-2-thione with iodine monochloride. Compound 84 was converted to ketone 85 using the Hg(OAc) 2 -CHCl 3 -AcOH system. Intermediate compounds 84 and 85 were further involved in the cross-coupling reactions with the corresponding ketone 34 and thione 86. Studies of the properties of compounds 82 and 83 showed that the chlorine atom mainly contributes to the electronic properties within one molecule, and the iodine atom to intermolecular interaction through the iodine bond. Appropriate application of the different roles of halogen atoms can be useful for the development of new supermolecular organic conductors based on these compounds (Scheme 27) [76].
Intermediate compounds 84 and 85 were further involved in the cross-coupling reactions with the corresponding ketone 34 and thione 86. Studies of the properties of compounds 82 and 83 showed that the chlorine atom mainly contributes to the electronic properties within one molecule, and the iodine atom to intermolecular interaction through the iodine bond. Appropriate application of the different roles of halogen atoms can be useful for the development of new supermolecular organic conductors based on these compounds (Scheme 27) [76]. The authors noted that bromo derivatives of selenafulvalenes were more available compounds compared to the analogous iodo derivatives. However, the bromo derivatives also display donor abilities and can be used for the preparation of charge-transfer complexes [77].

Non-Condensed 1,4,5,8-Tetraselenafulvalenes
Convenient and practical synthetic procedures for the preparation of diselenafulvene 1 and tetraselenafulvalene 2 based on selenium and sodium acetylide were developed (Scheme 29) [78]. This approach has the advantage of using cheap, non-toxic selenium powder as the selenium source and commercially available sodium acetylide in xylene light mineral oil. with the corresponding ketone 34 and thione 86. Studies of the properties of compounds 82 and 83 showed that the chlorine atom mainly contributes to the electronic properties within one molecule, and the iodine atom to intermolecular interaction through the iodine bond. Appropriate application of the different roles of halogen atoms can be useful for the development of new supermolecular organic conductors based on these compounds (Scheme 27) [76]. The authors noted that bromo derivatives of selenafulvalenes were more available compounds compared to the analogous iodo derivatives. However, the bromo derivatives also display donor abilities and can be used for the preparation of charge-transfer complexes [77].

Non-Condensed 1,4,5,8-Tetraselenafulvalenes
Convenient and practical synthetic procedures for the preparation of diselenafulvene 1 and tetraselenafulvalene 2 based on selenium and sodium acetylide were developed (Scheme 29) [78]. This approach has the advantage of using cheap, non-toxic selenium powder as the selenium source and commercially available sodium acetylide in xylene light mineral oil. The authors noted that bromo derivatives of selenafulvalenes were more available compounds compared to the analogous iodo derivatives. However, the bromo derivatives also display donor abilities and can be used for the preparation of charge-transfer complexes [77].

Non-Condensed 1,4,5,8-Tetraselenafulvalenes
Convenient and practical synthetic procedures for the preparation of diselenafulvene 1 and tetraselenafulvalene 2 based on selenium and sodium acetylide were developed (Scheme 29) [78]. This approach has the advantage of using cheap, non-toxic selenium powder as the selenium source and commercially available sodium acetylide in xylene light mineral oil. The authors emphasized that only two steps were required for the synthesis of the target compound by this method, which was suitable for laboratory-scale preparation, about 7 g of diselenafulvene 1 and more than 2 g of fulvalene 2 can be obtained by these two experiments under laboratory conditions [78]. The authors also noted that in combination with previously developed methods (the functionalization of tetraselenafulvalene 2 as protected thiolate or selenolate moieties followed by their deprotection/realkylation chemistry), the present approach paved a practical way to various heterocycle-fused tetraselenafulvalene type donors, which can be used to produce superconducting radical cation salts (Scheme 29) [78].
The authors emphasized that only two steps were required for the synthesis of the target compound by this method, which was suitable for laboratory-scale preparation, about 7 g of diselenafulvene 1 and more than 2 g of fulvalene 2 can be obtained by these two experiments under laboratory conditions [78]. The authors also noted that in combination with previously developed methods (the functionalization of tetraselenafulvalene 2 as protected thiolate or selenolate moieties followed by their deprotection/realkylation chemistry), the present approach paved a practical way to various heterocycle-fused tetrase-lenafulvalene type donors, which can be used to produce superconducting radical cation salts (Scheme 29) [78].
Efficient syntheses of tetramethyltetraselenafulvalene (89) were developed in the last century [4][5][6]. An interesting method for the preparation of tetramethyltetraselenafulvalene, doubly labeled with 13 C isotope at the positions 2 and 2' (4,4',5,5'-tetramethyl ∆ 2,2 -bis-1,3diselenole 89*) was described (Scheme 30) [80]. Labeled with 13 C isotope carbon diselenide was obtained from 13 C-dichloromethane at 580-600 • C. The 13 C-carbon diselenide, after cooling to room temperature and dissolving in pentane, was reacted with piperidine at 0 • C leading to piperidinium 1-piperidine 13 C-diselenocarbamate 90* in 33% yield. target compound by this method, which was suitable for laboratory-scale preparation, about 7 g of diselenafulvene 1 and more than 2 g of fulvalene 2 can be obtained by these two experiments under laboratory conditions [78]. The authors also noted that in combination with previously developed methods (the functionalization of tetraselenafulvalene 2 as protected thiolate or selenolate moieties followed by their deprotection/realkylation chemistry), the present approach paved a practical way to various heterocycle-fused tetraselenafulvalene type donors, which can be used to produce superconducting radical cation salts (Scheme 29) [78].
A series of tetrakis(alkylthio)tetraselenafulvalene compounds 95 (n = 1-15) was synthesized in up to 84% yield by a one-step reaction of dialkyl disulfides 94 with tetralithiated tetraselenafulvalene 2 (Scheme 31) [82]. Compounds 95 were found to be weak electrondonating molecules and to show low dark conductivity. At the same time, as the number of methylene groups increased, the electrical conductivity increased due to the presence of high-dimensional conduction paths. The resulting compounds were highly soluble in organic solvents [82]. The single-site 13 C-enriched tetramethyltetraselenafulvalene was obtained starting from 4,5-dimethyl-1,3-diselenole-2-one [81]. Correlation between non-Fermi-liquid behavior and antiferromagnetic fluctuations in superconducting (TMTSF)2PF6 salt was studied using 13 C-NMR spectroscopy [81].
A series of tetrakis(alkylthio)tetraselenafulvalene compounds 95 (n = 1-15) was synthesized in up to 84% yield by a one-step reaction of dialkyl disulfides 94 with tetralithiated tetraselenafulvalene 2 (Scheme 31) [82]. Compounds 95 were found to be weak electron-donating molecules and to show low dark conductivity. At the same time, as the number of methylene groups increased, the electrical conductivity increased due to the presence of high-dimensional conduction paths. The resulting compounds were highly soluble in organic solvents [82]. Scheme 31. The synthesis of tetrakis(alkylthio)tetraselenafulvalene compounds 95 (n = 1-15) from TSF and dialkyl disulfides 94 [82].
The authors noted that the compounds 95 were good candidates for the field-effect transistor channel based on the advantageous features: low dark conductivity, low donor ability, on-site Coulomb repulsion energy, high-dimensional π-electron structure, and high solubility in organic solvents [82]. Scheme 31. The synthesis of tetrakis(alkylthio)tetraselenafulvalene compounds 95 (n = 1-15) from TSF and dialkyl disulfides 94 [82].
The authors noted that the compounds 95 were good candidates for the field-effect transistor channel based on the advantageous features: low dark conductivity, low donor ability, on-site Coulomb repulsion energy, high-dimensional π-electron structure, and high solubility in organic solvents [82].
The use of one equivalent of phenylselenadiazole 96 and three equivalents of selenadiazole 97 in the reaction of these reagents in a mixture of THF and tert-butanol at 0 • C in the presence of five equivalents of sodium hydride led to diselenafulvene 98 in 46% yield. The latter compound was successfully formylated with the formation of compounds 99 and 100 by the Vilsmeier-Haack reaction (Scheme 32) [83]. Scheme 31. The synthesis of tetrakis(alkylthio)tetraselenafulvalene compounds 95 (n = 1-15) from TSF and dialkyl disulfides 94 [82].
The authors noted that the compounds 95 were good candidates for the field-effect transistor channel based on the advantageous features: low dark conductivity, low donor ability, on-site Coulomb repulsion energy, high-dimensional π-electron structure, and high solubility in organic solvents [82].
The use of one equivalent of phenylselenadiazole 96 and three equivalents of selenadiazole 97 in the reaction of these reagents in a mixture of THF and tert-butanol at 0 °C in the presence of five equivalents of sodium hydride led to diselenafulvene 98 in 46% yield. The latter compound was successfully formylated with the formation of compounds 99 and 100 by the Vilsmeier-Haack reaction (Scheme 32) [83]. Scheme 32. The synthesis of compounds 99-101 [83].
The products 99 and 100, bearing the aldehyde group, can serve as precursors of the vinylogs of tetraselenafulvalene derivatives. The iodine-morpholine reagent was used to convert fulvene 98 to diphenyltetraselenafulvalene 101 (a mixture of E-and Z-isomers) in 28% yield (Scheme 32) [83].
The products 99 and 100, bearing the aldehyde group, can serve as precursors of the vinylogs of tetraselenafulvalene derivatives. The iodine-morpholine reagent was used to convert fulvene 98 to diphenyltetraselenafulvalene 101 (a mixture of Eand Z-isomers) in 28% yield (Scheme 32) [83].

Condensed 1,4,5,8-Tetraselenafulvalenes
The reaction of cyclooctyne with carbon diselenide in the presence of red selenium in boiling dichloromethane afforded cycloocteno[1,2-d]1,3-diselenole-2-selone 102 (59% yield), which was converted into tetraselenafulvalene 103 in 94% yield by the treatment with trimethylphosphite in boiling benzene (Scheme 33) [84]. The formation of selone 102 can be rationalized by addition of carbon diselenide to cyclooctyne to form 1,3-diselenole-2ylidene 104, which then reacts with elemental selenium or can capture selenium atom from the carbon diselenide molecule yielding compound 102 (Scheme 33) [84]. This reaction successfully competed with the carbene dimerization with the formation of tetraselenafulvalene 103 if the process was carried out in the absence of elemental red selenium. The convenient synthesis of bis(ethylenedioxy)tetraselenafulvalene 105 without the use of toxic reagents such as carbon diselenide and hydrogen selenide was developed (Scheme 34) [85]. The key intermediate 106 was synthesized by the reaction between lithium selenolate 107 and N,N-dimethylselenocarbamoyl chloride 108 in THF at 0 • C under argon. Both compounds 107 and 108 can be prepared based on elemental selenium powder. Diselenocarbamate 106 was quantitatively converted to iminium salt 109, which was used to prepare selone 110 in the NaSeH-AcOH system. The synthesis of the target tetraselenafulvalene 105 (30% yield) was carried out by a coupling reaction under very mild conditions in benzene using hexamethylphosphorous triamid (HMPA) at room tem-perature under argon (Scheme 34) [85]. The new donor compound 105 exhibited sufficient solubility in common organic solvents and the ability to form CH . . . O hydrogen bonds. Its electrochemical properties were found to be promising for obtaining new organic metals, including superconductors [85].
(Scheme 34) [85]. The key intermediate 106 was synthesized by the reaction between lithium selenolate 107 and N,N-dimethylselenocarbamoyl chloride 108 in THF at 0 °C under argon. Both compounds 107 and 108 can be prepared based on elemental selenium powder. Diselenocarbamate 106 was quantitatively converted to iminium salt 109, which was used to prepare selone 110 in the NaSeH-AcOH system. The synthesis of the target tetraselenafulvalene 105 (30% yield) was carried out by a coupling reaction under very mild conditions in benzene using hexamethylphosphorous triamid (HMPA) at room temperature under argon (Scheme 34) [85]. The new donor compound 105 exhibited sufficient solubility in common organic solvents and the ability to form CH...O hydrogen bonds. Its electrochemical properties were found to be promising for obtaining new organic metals, including superconductors [85]. Scheme 34. The convenient synthetic approach to bis(ethylenedioxy)tetraselenafulvalene 105 [85].
The formation of the dihydrothiophene ring was achieved by a transalkylation reaction at the sulfur atom in the presence of sodium iodide in DMF, which resulted in the desired ethylenethiotetraselenafulvalene 111 in 81% yield (Scheme 35) [86]. The use of electrocrystallization method for compound 111 gave highly conductive radical cationic salts with a number of counter anions such as I3 − , Cl − , Br − , and AuI2 − in a 2:1 donor-acceptor ratio. Scheme 34. The convenient synthetic approach to bis(ethylenedioxy)tetraselenafulvalene 105 [85].
The obtained compounds showed good electron-donating properties. Based on compound 117a, three radical cation salts were obtained. Salts with I2Br − and AsF6 − anions exhibited semiconducting properties, while the PF6 − salt displayed metallic conductivity down to 130 K [52].
The obtained compounds showed good electron-donating properties. Based on compound 117a, three radical cation salts were obtained. Salts with I 2 Br − and AsF 6 − anions exhibited semiconducting properties, while the PF 6 − salt displayed metallic conductivity down to 130 K [52].
Benzoquinone-fused ethylenedithiotetraselenafulvalene 125 was synthesized by the reaction sequence depicted in Scheme 39 [89]. The key intermediate compound 128 was obtained in 73% yield by a four-step synthesis from catechol: protection of two hydroxyl groups with tert-butyldiphenylsilyl groups, diiodination, the Stille cross-coupling reaction with Bu3SnSe(CH2)2CN followed by treatment with NaH and Bu2SnCl2. In the presence of AlMe3, the reaction of compound 128 with methyl ester 129 obtained from ketone 34 and subsequent deprotection of hydroxyl groups gave catechol-condensed tetraselenafulvalene 130 in 52% yield. Electrochemical oxidation of compound 130 in the presence of 2,2'-bipyridine afforded ortho-benzoquinone-fused ethylenedithiotetraselenafulvalene 125 in quantitative yield (Scheme 39) [89]. Scheme 38. The synthesis of tetraselenafulvalenes derivatives 121a-c and 122a-c [88].
The obtained compounds were found to be excellent electron donors for the preparation of organic conductors [88].
Benzoquinone-fused ethylenedithiotetraselenafulvalene 125 was synthesized by the reaction sequence depicted in Scheme 39 [89]. The key intermediate compound 128 was obtained in 73% yield by a four-step synthesis from catechol: protection of two hydroxyl groups with tert-butyldiphenylsilyl groups, diiodination, the Stille cross-coupling reaction with Bu 3 SnSe(CH 2 ) 2 CN followed by treatment with NaH and Bu 2 SnCl 2 . In the presence of AlMe 3 , the reaction of compound 128 with methyl ester 129 obtained from ketone 34 and subsequent deprotection of hydroxyl groups gave catechol-condensed tetraselenafulvalene 130 in 52% yield. Electrochemical oxidation of compound 130 in the presence of 2,2'bipyridine afforded ortho-benzoquinone-fused ethylenedithiotetraselenafulvalene 125 in quantitative yield (Scheme 39) [89]. groups with tert-butyldiphenylsilyl groups, diiodination, the Stille cross-coupling reaction with Bu3SnSe(CH2)2CN followed by treatment with NaH and Bu2SnCl2. In the presence of AlMe3, the reaction of compound 128 with methyl ester 129 obtained from ketone 34 and subsequent deprotection of hydroxyl groups gave catechol-condensed tetraselenafulvalene 130 in 52% yield. Electrochemical oxidation of compound 130 in the presence of 2,2'-bipyridine afforded ortho-benzoquinone-fused ethylenedithiotetraselenafulvalene 125 in quantitative yield (Scheme 39) [89]. Scheme 39. The synthetic route to catechol-condensed tetraselenafulvalene 130 [89].
New tetraselenafulvalenes derivatives find application in the synthesis of organic spin-polarized donors [90].
Various types of highly conductive radical-cationic salts were prepared based on tetraselenafulvalenes 140a,b. For example, the PF6, AsF6, and FeCl4 salts retained metallic properties down to the liquid helium temperature [91,92].
The efficient and practical synthetic method for the preparation of condensed electron donors of the tetraselenafulvalene type 144a,b and 145a,b in up to 94% yield was developed by the approach depicted in Scheme 43 using tetrabutylammonium Various types of highly conductive radical-cationic salts were prepared based on tetraselenafulvalenes 140a,b. For example, the PF 6 , AsF 6 , and FeCl 4 salts retained metallic properties down to the liquid helium temperature [91,92].
Tetraselenafulvalene derivative 141 was obtained in 56% yield using tetrahydropyranylprotected acetylene containing a thioethyl fragment as the starting compound by the approach illustrated in Scheme 42 [92,93].
New tetraselenafulvalenes derivatives find application in the synthesis of organic spin-polarized donors [90].
Various types of highly conductive radical-cationic salts were prepared based on tetraselenafulvalenes 140a,b. For example, the PF6, AsF6, and FeCl4 salts retained metallic properties down to the liquid helium temperature [91,92].
The efficient and practical synthetic method for the preparation of condensed electron donors of the tetraselenafulvalene type 144a,b and 145a,b in up to 94% yield was developed by the approach depicted in Scheme 43 using tetrabutylammonium Scheme 42. Synthesis of tetraselenafulvalene derivative 141 from tetrahydropyranyl-protected acetylene [92,93].
The efficient and practical synthetic method for the preparation of condensed electron donors of the tetraselenafulvalene type 144a,b and 145a,b in up to 94% yield was developed by the approach depicted in Scheme 43 using tetrabutylammonium 4,5-bis(2-selenoxo-1,3diselenole-4,5-diselenolate)zincate 142 (82% yield) as the intermediate compound [94]. The obtained compounds 144a,b and 145a,b served as efficient electron donors for preparation of organic conducting materials [94].
The obtained compounds 144a,b and 145a,b served as efficient electron donors for preparation of organic conducting materials [94].

Scheme 46. Synthesis of bicyclic and polycyclic compounds
The selenoketenes 160 were obtained by deprotonation of aromatic diynes 159 with n-BuLi, subsequent addition of elemental selenium at 0 °C, and reaction with water in the temperature range from -55 °C to room temperature for 3 h (Scheme 47) [98]. The resulting intermediates 160 in situ were subjected to cycloaddition polymerization to pro-Scheme 46. Synthesis of bicyclic and polycyclic compounds 155a-c, 156a-d, 157, and 158 including vinylgues of the dendralene type [97].
The solubility of the polymers depends on their structure. The attachment of long alkyl chains enhances solubility in non-polar solvents. The resulting polymers exhibited electron-donating properties, as did their soluble charge-transfer complexes with tetracyanoquinodimethane [98].
The solubility of the polymers depends on their structure. The attachment of long alkyl chains enhances solubility in non-polar solvents. The resulting polymers exhibited electron-donating properties, as did their soluble charge-transfer complexes with tetracyanoquinodimethane [98].
Two-bridged tetraselenafulvalenophanes 163a,b were efficiently synthesized from trimethylsilylacetylene by the synthetic approach presented in Scheme 48 [99]. Using sequential deprotection and realkylation of the bis-thiolate tetraselenafulvalene building block 162 (51% yield), two-bridged tetraselenafulvalenophanes 163a,b were efficiently synthesized in 25% and 20% yields, respectively (Scheme 48) [99]. The radical cationic salt 163b with the Au(CN) 2 − anion exhibited very high conductivity at room temperature. The cross-coupling of two half-blocks of compound 165 in the presence of triethylphosphite led to the unsymmetrical derivative tetracarbomethoxytriselenathiafulvalene derivative 166 in 40% yield. As a result of the sulfur-selenium "scrambling" in the presence of the electron-withdrawing group in the positions 4 and 5 of the diselenole ring, a mixed thiaselenafulvene core was generated. Unsubstituted triselenathiafulvalene 167 was obtained in 37% yield by decarbomethoxylation of compound 166 (Scheme 49) [100].
The synthesis of 4,5-dicarbomemethoxy-1,3-diselenole-2-thione 165 in 43% yield was developed from the intermediate titanocene pentaselenide 164 and elemental selenium, avoiding the use of highly toxic carbon diselenide (Scheme 49) [100]. The cross-coupling of two half-blocks of compound 165 in the presence of triethylphosphite led to the unsymmetrical derivative tetracarbomethoxytriselenathiafulvalene derivative 166 in 40% yield. As a result of the sulfur-selenium "scrambling" in the presence of the electron-withdrawing group in the positions 4 and 5 of the diselenole ring, a mixed thiaselenafulvene core was generated. Unsubstituted triselenathiafulvalene 167 was obtained in 37% yield by decarbomethoxylation of compound 166 (Scheme 49) [100].
The cross-coupling of two half-blocks of compound 165 in the presence of triethylphosphite led to the unsymmetrical derivative tetracarbomethoxytriselenathiafulvalene derivative 166 in 40% yield. As a result of the sulfur-selenium "scrambling" in the presence of the electron-withdrawing group in the positions 4 and 5 of the diselenole ring, a mixed thiaselenafulvene core was generated. Unsubstituted triselenathiafulvalene 167 was obtained in 37% yield by decarbomethoxylation of compound 166 (Scheme 49) [100].
The charge transfer complex of triselenathiafulvalene 167 with tetracyanoquinodimeth ane showed a high degree of anisotropic conductivity in the polycrystalline sample, which decreased with increasing temperature [100].
In contrast to benzo-1,3-dithiole and its 2-substituted derivatives, which can be lithiated with butyllithium, the C-Te bond in benzo-1,3-ditellurole 178, as well as in its noncyclic analogs, diorganyl tellurides, R 2 Te, is cleaved by butyllithium. As a result of the reaction of heterocycle 178 with n-BuLi, even at low temperatures, a complex mixture of various diorganyl tellurides is formed [103].
The successful synthesis of dendralenic ditellurole-containing compounds including dendralenes was developed based on trimethylsilylacetylene and elemental tellurium (Scheme 53) [104]. Trimethylsilylacetylene was lithiated with butyllithium and lithium trimethylsilylacetylide reacted with tellurium to give tellurolate, which was protonated leading to unstable intermediate 182. Crude compound 182 was subjected to the Vilsmeier-Haack reaction to afford stable desired dialdehyde 183. The latter compound was condensed with malononitrile and carbomethoxymethyl phosphorane to give products 184 and 185 in 45% and 70% yields, respectively. Dialdehyde 183 was efficiently reacted with 4,5-dicarbomethoxy-1,3-dithiole phosphorane in the presence of sodium hydride to produce dendralene 186 in 63% yield. In like fashion, phosphonates 187 and 188 afforded dendralenes 189 and 190 in 38% and 43% yields, respectively, upon condensation with dialdehyde 183 in the presence of a base (Scheme 53) [104].
A systematic study of the synthesis of the π-donor tetratellurafulvalene 191 made it possible to increase the yield of the purified fulvalene product from about 12% to quite reproducible values reaching 26% (if tetrabromoethene is used at the final stage of cyclization). The optimized procedure for the synthesis of tetratellurafulvalene 191 is as follows (Scheme 54) [105]. A solution of n-BuLi in hexane was added to a suspension of distannan 192, tellurium, and LiCl in THF cooled to −78 • C over 30 min in an argon atmosphere. After additional stirring for 45 min, tetrabromoethene in THF was added to this suspension containing ditellurolate 193 over 1 h followed by stirring at −78 • C for 2 h. The crude product, after isolation from the reaction mixture, was purified by column chromatography on silica gel under argon giving tetratellurafulvalene 191 in 26% yield (Scheme 54) [105]. meier-Haack reaction to afford stable desired dialdehyde 183. The latter compound was condensed with malononitrile and carbomethoxymethyl phosphorane to give products 184 and 185 in 45% and 70% yields, respectively. Dialdehyde 183 was efficiently reacted with 4,5-dicarbomethoxy-1,3-dithiole phosphorane in the presence of sodium hydride to produce dendralene 186 in 63% yield. In like fashion, phosphonates 187 and 188 afforded dendralenes 189 and 190 in 38% and 43% yields, respectively, upon condensation with dialdehyde 183 in the presence of a base (Scheme 53) [104]. A systematic study of the synthesis of the π-donor tetratellurafulvalene 191 made it possible to increase the yield of the purified fulvalene product from about 12% to quite reproducible values reaching 26% (if tetrabromoethene is used at the final stage of cyclization). The optimized procedure for the synthesis of tetratellurafulvalene 191 is as follows (Scheme 54) [105]. A solution of n-BuLi in hexane was added to a suspension of distannan 192, tellurium, and LiCl in THF cooled to -78 °C over 30 min in an argon atmosphere. After additional stirring for 45 min, tetrabromoethene in THF was added to this suspension containing ditellurolate 193 over 1 h followed by stirring at -78 °C for 2 h. The crude product, after isolation from the reaction mixture, was purified by column chromatography on silica gel under argon giving tetratellurafulvalene 191 in 26% yield (Scheme 54) [105]. This method of synthesis makes it possible to reliably obtain useful amounts o compound 191 and allowed to contribute to the development of studying the radica cation complexes and charge-transfer salts based on tetratellurafulvalene and to investi gate its potential as a building block for the preparation of functionalized electron dono compounds [105].

Application of 1,4,5,8-Tetraselenafulvalene Derivatives and Their Tellurium Analogs for Materials Sciences
As shown by the literature data, 1,4-diselenafulvene derivatives serve as main building blocks for construction of 1,4,5,8-tetraselenafulvalenes, which find augmenting application in the preparation of materials with varying degrees of conductivity and various properties.
Compound 33 (BEDT-STF) is a very important unsymmetrical donor that was used to obtain the organic charge-transfer complex (33)2I3 exhibiting unique properties to form This method of synthesis makes it possible to reliably obtain useful amounts of compound 191 and allowed to contribute to the development of studying the radical cation complexes and charge-transfer salts based on tetratellurafulvalene and to investigate its potential as a building block for the preparation of functionalized electron donor compounds [105].

Application of 1,4,5,8-Tetraselenafulvalene Derivatives and Their Tellurium Analogs for Materials Sciences
As shown by the literature data, 1,4-diselenafulvene derivatives serve as main building blocks for construction of 1,4,5,8-tetraselenafulvalenes, which find augmenting application in the preparation of materials with varying degrees of conductivity and various properties.
The well-known and frequently used tetraselenafulvalene π-donors molecules are bis(ethylenedithio)diselenadithiafulvalene 33, tetramethyltetraselenafulvalene 89, bis(ethyl enedithio)tetraselenafulvalene 122b, and dimethyl(ethylenedioxy)tetraselenafulvalene 194 (Scheme 55). This method of synthesis makes it possible to reliably obtain useful amounts of compound 191 and allowed to contribute to the development of studying the radical cation complexes and charge-transfer salts based on tetratellurafulvalene and to investigate its potential as a building block for the preparation of functionalized electron donor compounds [105].

Application of 1,4,5,8-Tetraselenafulvalene Derivatives and Their Tellurium Analogs for Materials Sciences
As shown by the literature data, 1,4-diselenafulvene derivatives serve as main building blocks for construction of 1,4,5,8-tetraselenafulvalenes, which find augmenting application in the preparation of materials with varying degrees of conductivity and various properties.
Recently, much attention has been paid to Dirac electronic systems. However, most of these studies are theoretical (e.g., quantum chemical calculations by DFT methods) due to the limited availability of the relevant materials. The Dirac electron systems (DES) are characterized by massless electrons with relativistic behavior and high speed Scheme 55. Well-known tetraselenafulvalenes and diselenadithiafulvalene derivatives, which are effective as electron-donor compounds and commonly used for the preparation of conductive materials.
Recently, much attention has been paid to Dirac electronic systems. However, most of these studies are theoretical (e.g., quantum chemical calculations by DFT methods) due to the limited availability of the relevant materials. The Dirac electron systems (DES) are characterized by massless electrons with relativistic behavior and high speed (1/100-1/1000th of the velocity of light). Previously, similar relativistic behavior of electrons was observed in graphene. In fact, DES were initially found in graphene and some inorganic compounds [22,55]. It should be noted that organic DES exhibit important advantages over their inorganic counterparts. For example, in contrast to inorganic DES, most organic DES are characterized by a clearly defined crystal structure and chemical stoichiometry. However, the majority of organic DES shows the properties of Dirac electron states only under high pressure. In contrast to this observation, the organic charge-transfer complex (33) 2 I 3 exhibits unique properties to form Dirac electron states under ambient pressure [32,[53][54][55][56][57]. Based on studies of the electrical, magnetic, optical, and structural properties of (33) 2 I 3 under ambient pressure, it was established that this salt possesses a band structure characterized with Dirac cones, which was in good agreement with the quantum chemical calculations [20][21][22]31,32,[53][54][55][56][57]. In fact, this salt is a unique object for research, which can provide an important insight into the properties of the Dirac electrons by measurements of various physical properties under ambient pressure.
The compound 89 is also used in multisensor matrices for the detection of analytes in the gas or liquid phase, which includes, along with tetraselenafulvalene 89, tetrahalogenated tetraselenafulvalene and other tetrachalcogenafulvalene derivatives that can individually change their physicochemical properties when exposed to analytes or to mixtures of analytes, and these changes can be detected by a sensor or by a set of sensors [116].
The well-known tellurium-containing fulvalenes effective as π-donors are tetratellurafulvalene 191, hexamethylenetetratellurafulvalene 195, and tetrachlorotetratellurafulvalene 196 (Scheme 56). Scheme 56. Well-known tetratellurafulvalenes, which are effective as electron-donor compounds and used for the preparation of conductive materials.
Tetratellurafulvalene 191 in the form of a charge transfer complex with tetracyanoquinodimethane is used in the production of modified electrically conductive thin graphene films [133].
Tetratellurafulvalene 191 and hexamethylenetetratellurafulvalene 195 are used in photoelectric conversion functional devices as substrates in the form of charge transfer complexes that exhibit near-IR absorption [134]. In addition, fulvalenes 191, 195, and tetramethylthiotetratellurafulvalene are used in electroluminescent devices as electron carriers and electron donors. Such organic electroluminescent devices show low operating voltage and high luminescence intensity due to the low resistance of the organic layer [135].
The complex of fulvalene 195 with 2,5-diethyltetracyanoquinodimethane, Scheme 56. Well-known tetratellurafulvalenes, which are effective as electron-donor compounds and used for the preparation of conductive materials.
Tetratellurafulvalene 191 in the form of a charge transfer complex with tetracyanoquinodimethane is used in the production of modified electrically conductive thin graphene films [133].
Tetratellurafulvalene 191 and hexamethylenetetratellurafulvalene 195 are used in photoelectric conversion functional devices as substrates in the form of charge transfer complexes that exhibit near-IR absorption [134]. In addition, fulvalenes 191, 195, and tetramethylthiotetratellurafulvalene are used in electroluminescent devices as electron carriers and electron donors. Such organic electroluminescent devices show low operating voltage and high luminescence intensity due to the low resistance of the organic layer [135].
Tetrachlorotetratellurafulvalene 196 is used in multisensor matrices for the detection of analytes in the gas or liquid phase [116].
However, some efficient methods deserve to be mentioned. The method for preparation of diselenafulvene 1 and tetraselenafulvalene 2 based on selenium and sodium acetylide is a convenient and practical synthetic approach (Scheme 29) [78]. This approach, as well as the method depicted in Scheme 1 [34], have the advantages of using cheap, non-toxic selenium powder as the selenium source and commercially available acetylene or sodium acetylide.
A valuable synthesis of tetratellurafulvalene 191 in 26% yield by the optimized procedure with the use of tetrabromoethene at the final stage of cyclization is also very important (Scheme 54) [105]. This approach makes it possible to reliably obtain sufficient amounts of compound 191, which can be used for investigation of its potential as a building block for the preparation of functionalized electron donor compounds and the preparation of novel radical cation complexes and charge-transfer salts based on tetratellurafulvalene [105].
Obviously, the possibilities of 1,4-diselenafulvenes, 1,4,5,8-tetraselenafulvalenes, and their tellurium analogs for use in organic synthesis and in the preparation of conductive materials are far from being exhausted. Further research will lead to the synthesis of previously unknown complex compounds and preparation of novel conductive materials with new useful combinations of physicochemical properties.