Chalcogenides by Reduction of their Dioxides in Ultra‐Alkaline Media

Abstract The reaction of chalcogen dioxides ChO2 (Ch=Se, Te) with As2O3 in a 30 molar KOH hydroflux at about 200 °C yielded crystals of potassium trichalcogenides K2 Ch 3 with dimensions up to 2 cm. Arsenic trioxide acts as electron donor and is oxidized to arsenate(V). The new heterochalcogenide anion (TeSe2)2− formed when starting from SeO2 and TeO2 simultaneously. The compound K2TeSe2 crystallizes isostructural to K2S3 and K2Se3. The unexpected redox reaction as well as the precipitation of hygroscopic compounds from an aqueous solution are attributed to a strongly reduced activity of water. The reactions were studied by Raman and UV/Vis spectroscopy. Depending on the concentration of As2O3, colorless monochalcogenide Ch 2− or orange Se2 2− and purple Te2 2− anions are dominating the solutions.


Introduction
Thehistory of alkali metal selenides and tellurides reaches back at least to the beginning of the last century,w hen the groups of Zintl and Klemm obtained such compounds from reactions in liquid ammonia at À78 8 8C. [1][2][3] They were able to isolate single-crystals and to determine the crystal structures of several alkali metal chalcogenides A 2 Ch (A = Li-K, Ch = Se,T e), which contain closed-shell Ch 2À anions.F orty years later, single-crystals of the triselenide K 2 Se 3 were synthesized under ammonothermal conditions (150 8 8C, 500 bar) in an autoclave starting from the elements. [4] Concurrently,the first tritelluride K 2 Te 3 was obtained from the elements at about 600 8 8Ci naw elded iron autoclave. [5] Thet richalcogenide anions Ch 3 2À are angulated molecules (Se 3 2À 102.58 8;T e 3 2À 104. 48 8) [4,5] with an egative (formal) charge on each of the terminal atoms.U sing similar procedures,s ingle-crystals of several oligoselenides and tellurides were synthesized, for example,K 2 Ch 2 , [6] K 5 Ch 3 (Ch = Se,T e) [7,8] and also the hetero-trichalcogenide K 2 TeSe 3 . [9] Alternatively,p otassium chalcogenides,s uch as b-K 2 Se 2 or K 2 Se 4 ,w ere obtained by solvothermal synthesis in an organic solvent, for example, N,Ndimethylformamide (DMF) or ethane-1,2-diamine. [10] As the starting materials and reaction products are sensitive to oxygen and moisture,a ll described methods necessitate consequent handling under inert gas to exclude water. Accordingly,w ew ere highly surprised to obtain such sensitive compounds with reduced chalcogen species from an aqueous medium starting from chalcogen(IV) oxides.W ehad applied the new hydroflux method, [11] which utilizes ah ighly concentrated mixture of alkali metal hydroxide and water with am olar ratio q(A) = n(H 2 O):n(AOH) close to one (i.e. 30 to 50 molar) as reaction medium. Commonly,s odium or potassium hydroxide are employed. Theh ydroflux provides simple and fast synthesis of crystalline metal oxides and hydroxides in almost quantitative yield. [12,13] Ty pically,t he hydroflux reaction is completed within 10 hours at about 200 8 8Cinastainless steel autoclave with aPTFE inlet, which endures the ultra-alkaline conditions and prevents the loss of water. As the activity of the water is dramatically reduced in such aqueous salt melts,t he pressure evolving during the hydroflux reaction is much lower than under hydrothermal conditions.T he hydroflux medium tends to promote higher oxidations states than expected for adiluted aqueous system. Forexample,the oxidation of arsenic(III) and chromium(III) to their maximum oxidation states was observed. [14,15] In the following,w er eport studies on the reduction of SeO 2 and Te O 2 by As 2 O 3 in aK OH hydroflux, resulting in crystals of K 2 Se 3 ,K 2 Te 3 and the new hetero-trichalcogenide K 2 Te Se 2 .

Results and Discussion
To understand the reactions in the hydroflux systems, abrief review of the chemistry of chalcogens under reductive conditions in various other media is helpful.
In liquid ammonia, solutions of tellurides adapt acharacteristic color depending on the anionic species formed. When adding an alkali metal A to liquid ammonia, an intense blue color caused by solvated electrons appears immediately.T he latter are able to reduce tellurium to form different tellurides: Te 2À anions form awhite precipitate (A 2 Te ), Te 2 2À anions turn the solution deep violet-blue,and Te 3 2À anions have an intense wine-red color. [16] When DMF is used as solvent, no sequence of differently colored solutions occurs,a sf irstly the starting elements are insoluble in DMF and secondly no solvated electrons are present in the solution. When stirring elemental tellurium and an alkali metal in aD MF solution at room temperature,i nt he beginning no reaction is observed visually. [17] After half an hour,t he DMF solution develops af aint pink color that intensifies to deep purple over the course of hours.T he tellurium and alkali metal appear to react by physical contact resulting in the formation of Te 2À anions,w hich are soluble in DMF. [18] Those monotelluride anions react with remaining tellurium metal to form oligotellurides,w hich are linked by the equilibrium (1): When pre-synthesized alkali metal tellurides are dissolved in DMF,t he same deep purple colored solutions form, independent of the A:Ter atio and the nature of the alkali metal A. [18] In contrast to the tellurides,the chemistry of selenides in O 2 -free aqueous systems was investigated in several spectroscopic and electrochemical studies. [19][20][21][22][23] In those experiments, dissolved H 2 Se was oxidized by H 2 O 2 or reacted with elemental selenium. Thee quilibria (2) and (3) between four species Se n 2À with n = 1-4 in low-concentrated alkaline solutions were proposed: [20] At room temperature and at pH 14, the product sides of both equilibria are preferred. [19,20] In the range between pH 7 and pH 5, an approximately equal concentration of diselenide Se 2 2À and triselenide Se 3 2À anions was observed. [21] The protonated selenides species HSe À (pK a = 15.0) and HSe 2 À (pK a = 9.3) were observed even in 1 m KOHs olutions, however, when doubling the base concentration both anions were essentially deprotonated. [19,23] In our experiments,t he syntheses of selenides and tellurides were performed under hydroflux conditions in as tainless-steel autoclave with PTFE inlet. Ther eaction medium consisted of ap otassium hydroxide hydroflux with q(K) = n(H 2 O):n(KOH) = 1.9 (i.e.a bout 30 mol L À1 ). At room temperature this is ac lear solution with as mall residuum of solid KOH. Thes tarting materials SeO 2 ,T eO 2 , and As 2 O 3 are well-soluble in highly alkaline media. Theuse of other reducing agents,e .g., V 2 O 3 ,V O 2 ,o rS b 2 O 3 ,i sa lso possible,b ut they differ in their solubility under hydroflux conditions.F or example,V 2 O 3 has ar ather poor solubility, while its oxidation product VO 4 3À is readily soluble in highly alkaline solutions.F or Sb 2 O 3 ,itisvice versa. As 2 O 3 as well as As 2 O 5 are well-soluble in the hydroflux medium. Therefore, As 2 O 3 was mainly applied as reducing agent. Theo bserved solubility of these oxides under hydroflux conditions is comparable to the one in diluted alkaline solutions. [24][25][26] Fort he synthesis of K 2 Te 3 ,t he molar ratio q(Te) = n(As 2 O 3 ):n(TeO 2 ) = 1.2 was used. After sealing the autoclave, the mixture was reacted for 48 hours at 200 8 8C, before it was cooled down to room temperature within 24 h. Ther eaction product consisted of large black bar-shaped crystals of K 2 Te 3 ( Figure 1) and apale purple solution. Experiments with q(Te) of 1.0 or 0.75 also resulted in the crystallization of K 2 Te 3 . Despite the sub-stoichiometric content of the reducing agent in these experiments (for an equation see below), there was no evidence of the formation of elemental tellurium. In experiments with q(Te) ratios larger than 1.2, K 2 Te 3 did not form.
Substantially shorter reaction times decreased the yield of K 2 Te 3 while the purple color of the solution intensified. The purple solutions are sensitive against water and air, resulting in the precipitation of elemental tellurium. Spectroscopic analysis of this purple solution point towards the ditelluride ion Te 2 2À (see below). Thes yntheses of selenides from SeO 2 under hydroflux conditions followed the same procedure.T osynthesize K 2 Se 3 , the molar ratio q(Se) = n(As 2 O 3 ):n(SeO 2 ) = 1.2 was used with areactant concentration of about c(SeO 2 ) = 1mol L À1 .Hence, c(SeO 2 )w as about ten times higher than c(TeO 2 )needed for the crystallization of K 2 Te 3 .The reaction product consisted of ad eep red solution and large K 2 Se 3 crystals ( Figure 1). Experiments with lower reactant concentrations yielded only in ad eep red solution without any solid product. Ther ed solution contained diselenide anions Se 2 2À (see below). By using al arger excess of reducing agent, the intense color of the dichalcogenide solutions vanished at q(Ch) = 3.0. Furthermore,w hen dissolved As 2 O 3 is added to the colored dichalcogenide solutions at room temperature,c olorless solutions are obtained. Both observations indicate the formation of monochalcogenide anions Ch 2À .E quation (4) summarizes the reduction of the dichalcogenide anions: Am ixture containing TeO 2 and SeO 2 yielded neither amixture of K 2 Se 3 and K 2 Te 3 nor asolid solution with the two anions in one solid, but K 2 Te Se 2 .T he small difference in the electronegativity (Pauling:S e2 .5, Te 2.1) is sufficient to assign the two elements their roles according to the charge distribution in the heteroatomic Ch 3 2À anion. Thei ncreased intramolecular polarity in the diselenotellurate(II) À (Se ÀII À Te II ÀSe ÀII ) À compared to the triselenide À (Se ÀI ÀSe 0 ÀSe ÀI ) À is symbolized, but certainly also overemphasized, by the oxidation states. K 2 Te Se 2 was synthesized under reaction conditions similar to those of the homoatomic trichalcogenides,using SeO 2 and Te O 2 in the molar ratio of 2:1. Adding an excess of about 5% of SeO 2 helped to avoid the co-crystallization of K 2 Te 3 ,which is less soluble in the hydroflux than K 2 Se 3 .A na mount of 1.3 equivalents of the reducing agent As 2 O 3 was added (based on 2 / 3 SeO 2 + 1 / 3 TeO 2 ). Similar to the synthesis of K 2 Se 3 , relatively high reactant concentrations are necessary to obtain crystals of K 2 Te Se 2 .
It was surprising to obtain hygroscopic trichalcogenides from their dioxides in ah ydroflux, which (a) contains water and (b) is usually stabilizing high oxidation states.Inthis case, the As 2 O 3 ,w hich had initially been added for other reasons, acted as the reducing agent and was itself oxidized to arsenic(V). Arsenic is not only the electron donor but binds the oxygen atoms provided by the chalcogen(IV) oxides in AsO 4 3À anions.T his redox chemistry is far from what the standard potentials let expect, but can be rationalized by Equation (5): Theredox reaction is promoted by the high concentration of hydroxide ions on the side of the reactants.Moreover,the hydroflux is highly hygroscopic.The initially contained water but also water formed through the reaction are strongly bonded to hydroxide ions.Thereby,the activity of the water is considerably reduced, which does not only decrease its vapor pressure and drives the reaction but obviously prevents the hydrolysis of the water sensitive trichalcogenides.O nt he other hand, the reaction diluted the hydroflux so that it did not solidify upon cooling to room temperature.W ashing of the reaction product with ap rotic solvent, for example,a n alcohol, strongly increases the activity of water and thereby induces the decomposition of K 2 Ch 3 .Similar observations had been made for other water sensitive products from hydroflux syntheses,f or example,K 2 [Fe 2 O 3 (OH) 2 ]o rT l 3 IO. [27,28] Also several aprotic polar solvents,f or example,D MF,p roved to be unsuitable for rinsing because potassium hydroxide is less soluble in them than the trichalcogenides.T herefore,t he products were filtered under inert conditions by using aSchlenk-frit. Theyields with respect to the used ChO 2 were 90 %for K 2 Te 3 ,60% for K 2 Se 3 and 80 %for K 2 Se 2 Te,related to the solubility of the diverse chalcogenide species.T he adhering KOHtogether with the genuine moisture-sensitivity of K 2 Ch 3 necessitated storage and handling of the crystals under inert conditions (argon). Thepowder X-ray diffraction patterns of the isolated crystals showed single-phase products, but small residuals of the hydroflux were visible in the scanning electron microscope ( Figure S1 to S4, Table S1, Supporting Information).
X-ray diffraction on black single-crystals of K 2 Se 3 (Cmc2 1 ) and K 2 Te 3 (Pnma)confirmed the known structures. [4,5] Forthe new compound K 2 Te Se 2 an orthorhombic structure in the non-centrosymmetric space group Cmc2 1 (no.3 6) was found with the lattice parameter a = 783.42(4) pm, b = 1045.64-(6) pm, and c = 777.13(4) pm at 100(1) K. Details on the structure determinations and the atomic parameters of the three compounds can be found in Tables S2 to S8, Supporting  Information. Selected bond lengths and angles are listed in  Table S9, Supporting Information. K 2 Te Se 2 is isostructural to K 2 Se 3 and K 2 S 3 ( Figure 2). The angulate diselenotellurate(II) anion, (TeSe 2 ) 2À ,h as crystallographic C 2v symmetry with two equal SeÀTe bond lengths of 256.2(1) pm and aSe À Te À Se angle of 97.6(1)8 8.T he (TeSe 2 ) 2À anion had previously been found in (2,2,2-crypt-K) 2 (TeSe 2 )·en (en = ethylendiamin) [29] and [Mn(en) 3 ](TeSe 2 ) [30] with Te À Se bond lengths of about 250 pm and 250.3(1) pm as well as SeÀ TeÀSe angles of 111.3(1)8 8 and 102.6(1)8 8,respectively.Inthese structures,the (TeSe 2 ) 2À anions are well separated from each other and interact with hydrogen atoms of the organic ligands. Thew ider Se À Te À Se angles are consistent with the shorter Te À Se bond lengths,which increase the repulsion between the terminal atoms.InK 2 TeSe 2 ,short intermolecular distances of 333.4(1) pm suggest secondary bonds Te II ···Se ÀII ,w hich, together with stronger cation-anion interactions,m ight be responsible for the elongated primary TeÀSe bond. In alkali metal trichalcogenides A 2 Ch 3 (A = K-Cs; Ch = S-Te) with homonuclear anions,w hich have al ower intramolecular polarity than (TeSe 2 ) 2À ,the shortest intermolecular distances range from 344 pm to 386 pm. [31] Besides K 2 Te Se 2 ,t he following compounds crystallize in the K 2 S 3 structure type: A 2 Ch 3 (A = K-Cs; Ch = S, Se), Cs 2 Te 3 and Cs 2 Te S 2 . [4,[31][32][33] Theb ond angle in the Ch 3 2À anions deceases from the sulfides (average angle of 106.08 8)v ia the selenides (average angle of 103.18 8)t othe telluride (100.18 8 in Cs 2 Te 3 ). [31] This can be attributed to ad ecreasing s-orbital contribution to the bonding when proceeding to the heavier main-group elements.Despite its smaller terminal atoms,the anion in Cs 2 Te S 2 has as lightly wider bond angle (99.48 8)t han (TeSe 2 ) 2À in K 2 Te Se 2 ,w hich might be an effect of the higher electronegativity and thus the higher partial charge of the sulfur atoms compared to selenium.  [58] In the crystal structure of K 2 Te Se 2 ,t he (TeSe 2 ) 2À anions form double-layers parallel to (010) (Figure 2) with the tellurium atoms pointing towards the inside of the double layer. Thep otassium atoms separate the double layers.T he polarity of the structure is evident, as all (TeSe 2 ) 2À "arrowheads" point into the same direction along [001] ( Figure S5, Supporting Information). Thet wo potassium atoms K1 and K2 are each coordinated by six selenium atoms in the shape of distorted trigonal prisms with C s symmetry (Figure 3). Within those polyhedra, the K À Se bond lengths range from 338.9(1) pm to 360.9(1) pm and from 338.3(1) pm to 349.6-(1) pm, respectively (Table S8, Supporting Information). The slightly larger polyhedron around K1 involves six (TeSe 2 ) 2À anions,w hereas only five anions form the trigonal prism around K2. Comparing the [KSe 6 ]p olyhedra of K 2 Se 3 and K 2 Te Se 2 ,the sum of their volumes is about 5% larger for the latter.T he [KSe 6 ]prisms of K1 and K2 share the square face that is not capped by at ellurium atom (K···Te3 40.8(1) to 390.7(1) pm). The[ K 2 Se 8 ]d ouble prisms share corners and edges to form at hree-dimensional framework. In K 2 Te Se 2 , the shortest K···K distance is with 366.7(1) pm even shorter than in K 2 Se 3 [369.4(1) pm],b ut not as short as in K 2 S 3 [359.2(2) pm]. [4] In K 2 Te 3 ,w hich crystallizes in its own structure type,t he shortest distance between cations is 441.0(1) pm.
To obtain further insight into the chemical processes in the hydroflux, especially the prevalent chalcogenide anions Ch n 2À (n = 1, 2, 3), the reacted solutions were analyzed by UV/Vis and Raman spectroscopy.T he reaction conditions were q(K) = 1.9 and 200 8 8C, as for the above syntheses,b ut the reaction time was only five hours.The reactant concentration of the syntheses for the UV/Vis measurements was about 0.01 mol L À1 ,w hile the concentration for the Raman measurements was about 30 times higher. Analyses were per-formed under ambient conditions in air and at room temperature.D uring the UV/Vis measurements,t he lower limit in wavelength was about 240 nm because of strong absorption by the hydroflux medium. Raman and UV-vis spectra of each single reactant dissolved in ahydroflux with q(K) = 1.9 can be found in Figure S6 to S11, Supporting Information.
Purple solutions were obtained starting from amolar ratio of As 2 O 3 and Te O 2 of q(Te) = 1. In this ratio,t he complete oxidation of arsenic(III) to arsenic(V) provides four electrons per tellurium(IV) atom. TheU V/Vis spectrum of such as ample show an absorption band with am aximum at 522 nm (Figure 4), which is slightly shifted to lower frequencies in comparison with the published values for ditelluride anions Te 2 2À of 508 to 512 nm. [34][35][36][37] In diluted alkaline solutions,the presence of the purple Te 2 2À had been observed and analyzed in various experimental setups,e.g., as aproduct of accidental oxidation of monotelluride solutions by oxygen from leakage, [34,35] after the oxidation of monotelluride solution in ap hotochemical cell by irradiation of CdTe, [35] in an electrolysis starting from am onotelluride solution [35] or during an electrolysis generating monotelluride that further reacted with elemental tellurium to form the ditelluride anion. [36,37] Moreover,the absorption spectrum of the isoelectronic iodine molecule I 2 dissolved in hexane exhibits asimilar band with amaximum at about 520 nm. [34,38] TheRaman spectra of experiments with q(Te) = 0.75, 1, or 1.5 (i.e.3 ,4 ,6electrons per Te IV )s how av ibration band at 181 cm À1 (Figure 4), which occurs in an energy range typical  for oligotellurides. [17,[39][40][41] We found no literature data for TeÀ Te vibrations of tellurides in aqueous solutions.K 2 Te 2 dissolved in DMF shows av ibration at 164 cm À1 . [17] The Raman vibrations of the pentatelluride ion Te 5 2À in acetone are located at 170 cm À1 and 195 cm À1 . [39] TheT e ÀTe bands in AsÀTe and SeÀTe glasses were reported to occur at 155 cm À1 and 175 cm À1 . [40,41] Consequently,and as indicated by the UV/ Visspectra, we assign the band at 181 cm À1 to the vibration of the Te 2 2À anion. Thebands observed at 362 cm À1 and 545 cm À1 represent the first and second overtone of the 181 cm À1 vibration band, respectively.T he presence of these overtones and the high intensity of the TeÀTe vibration band compared to the spectra of the selenides are caused by Raman resonance of the 532 nm laser with the absorption band at 522 nm.
When the amount of reducing agent is increased, the purple color of the solution fades until acolorless solution is obtained in syntheses starting from q(Te) = 3(i.e.12electrons per Te IV ). These solutions as well as samples for which dissolved As 2 O 3 was added at room temperature to purple Te 2 2À solutions showed as ingle symmetrical absorption band in the UV/Vis with amaximum at 324 nm ( Figure 4). This is in good agreement with the published value of 325 nm for Te 2À in diluted alkaline solutions. [34,36] In the corresponding Raman spectra, no band was detectable in the typical energy range of TeÀTe vibrations,a sc an be expected for the monotelluride anion Te 2À being the predominant species.T he protonated monotelluride HTe À (270 nm) [34] was not observed in our experiments.This can be expected for ultra-alkaline media, as the second acid dissociation constant pK a2 of hydrogen telluride H 2 Te had been reported to be 12.2. [42] In none of the solutions,the tritelluride anion Te 3 2À could be detected spectroscopically.Ithad been reported to have an UV/Vis absorption band at 376 nm in DMF, [18] and K 2 Te 3 dissolved in liquid ammonia or DMF exhibits avibration band at about 162 cm À1 . [17] In the hydroflux medium, the slow crystallization of K 2 Te 3 during the synthesis at 200 8 8Ca nd its insolubility at room temperature suggest an equilibrium between the telluride species similar to Equation (1), in which the tritelluride anion is non-preferential. Thep recipitation of K 2 Te 3 is thus not caused by ahigh concentration of Te 3 2À but av ery small solubility product. These observation are in line with reported electrochemical experiments on the reductive dissolution of at ellurium cathode:w hile at pH 9 mainly Te 2À anions formed, Te 2 2À anions dominated above pH 12. [43] In low-concentrated alkaline solutions,t he optical absorption band of the diselenide anion Se 2 2À had been reported at about 430 nm. [19,20,23] Se 3 2À and Se 4 2À absorb at 530 nm and 470 nm, respectively. [19] TheUV/Vis spectrum ( Figure 5) of an orange solution synthesized with q(Se) = 1i nh ydroflux exhibited as ymmetrical absorption band with its maximum at 440 nm, which we attribute to Se 2 2À with respect to the above cited literature.For higher q(Se) ratios,viz. 2and 3, the formation of the monochalcogenide anion was observed in the UV/Vis,similar as in the case of the tellurides.
Raman spectra of dissolved oligoselenides are scarce.I n acetone,the Se 6 2À anion exhibits vibrations bands at 235 cm À1 , 285 cm À1 and 405 cm À1 . [39] In low-concentrated alkaline sol-utions,the oxidation of H 2 Se with H 2 O 2 had yielded selenium species with average oxidations states of À1, À0.67 and À0.5, that is,S e n 2À (n = 2, 3, 4), and Raman bands at 269 cm À1 and 324 cm À1 . [22] Theb and at 269 cm À1 had been assigned to the Se 4 2À anion based on the Raman resonance of ameasurement with a476 nm laser, [22] since the Se 4 2À anion has an absorption band at 470 nm. [19] AR aman spectrum measured with an 457 nm laser had resulted in an even greater intensity of the 269 cm À1 band. [22] DFT calculations had predicted two Raman active vibration modes at 299 cm À1 and 106 cm À1 for the Se 4 2À anion and one Raman band at 273 cm À1 for Se 2 2À . [22] In glassy selenium, the Se À Se stretching mode had been reported to occur at 250 cm À1 with as houlder at 235 cm À1 , [44] which is similar to AsÀSe (238 cm À1 ,2 52 cm À1 ) [45] and SeÀTe glasses (220 cm À1 to 280 cm À1 ). [41] TheRaman spectra ( Figure 5) of solutions obtained from hydroflux reactions with q(Se) of 0.75, 1, or 1.5 show av ibration band with am aximum at 265 cm À1 ,w hich could indicate higher oligoselenides,although the transmission UV/ Visspectra revealed exclusively Se 2 2À for q(Se) = 1. However, the concentration in the optical spectroscopy had to be about 30 times lower than for the Raman measurements when using standard quartz cuvettes.T oe xclude ac oncentration dependent product formation, we measured an UV/Vis spectrum in reflection mode on the same selenide solution used in the Raman spectroscopy,which showed only one absorption band at 435 nm confirming the presence of mainly Se 2 2À anions ( Figure S12, Supporting Information). Therefore and because of the absence of additional vibration bands [22] we assign the 265 cm À1 band in the Raman spectra of the experiments with q(Se) = 0.75, 1, or 1.5 to the diselenide anion Se 2

2À
,w hich is also in good agreement with the calculated value of 273 cm À1 . [22] In all of our experiments,the diselenide band at 265 cm À1 was accompanied by at iny band at 323 cm À1 ,w hich was proposed to be caused by Se 2 À . [22] Both vibration bands had always the same intensity ratio,despite different q(Se). It had been stated that the Se 2 À radical forms under intense laser light from oligoselenides Se n 2À with n = 2-4. [19,22] However, Se 2 2À is an unlikely precursor,asanelectron would have to be abstracted. Moreover,t he excitation of Se 2 2À with a5 30 nm laser would be very inefficient because of its absorption band at 440 nm. Themost plausible precursor for Se 2 À is Se 4

2À
,since its decomposition involves as ymmetrical bond cleavage and its absorption band at 470 nm is close to the wavelength of the laser. [19,22] Thed issociation of Se 3 2À would result in the Se À radical besides Se 2 À ,w hich had been observed in aqueous solutions. [46] In our experiments,t he presence of small amounts of other oligoselenides than Se 2 2À can be explained by Equation (2) and (3). In addition, the Se 2 À radical is known to have an absorption band between 490 and 520 nm [47][48][49] leading to an intensity enhancement of its vibration band at 323 cm À1 by Raman resonance,sothat the actual Se 2 À radical concentration is expectedly low.Whenchanging the radiation source to a1 064 nm laser, the vibrations band at 323 cm À1 vanishes,w hile the Se 2 2À band remains with no change in intensity ( Figure S13, Supporting Information).
In the UV/Vis spectrum of the experiment with q(Se) = 3 ( Figure 5), the monoselenide anion Se 2À exhibits one absorption band at 262 nm, which is close to the reported value of 270 nm. [19,20] Thesame result was obtained for asolution with higher concentration of the reactants,w hich were used for Raman spectroscopy ( Figure S14, Supporting Information). TheR aman spectra confirmed the absence of species with SeÀSe bond.
By adding at room temperature ah ydroflux solution of As 2 O 3 with the same q(K) to an orange-colored diselenide solution with q(Se) = 1, the color and the absorption band of the Se 2 2À anion vanishes.Inthis case,the strong absorption of the excess AsO 3 3À ions below 300 nm overlays the signal of the monoselenide anion Se 2À ( Figure S6, Supporting Information).
As the vibration bands of AsO 4 3À and SeO 3 2À overlap,we also used Sb 2 O 3 as reducing agent. Thereacted solution with n(Sb 2 O 3 ):n(SeO 2 ) = 0.75 showed the Raman band of Se 2 2À at 265 cm À1 ( Figure S15, Supporting Information). Thea dditional vibrational band at 810 cm À1 is in good agreement with the Raman spectrum of SeO 2 dissolved under hydroflux conditions.T he coexistence of SeO 3 2À and Se 2 2À anions confirmed the observation that no elemental selenium was formed in our experiments.
Theh eterochalcogenide solutions were prepared by starting from SeO 2 and Te O 2 in the molar ratio 2:1, according to the molar fractions of the chalcogens in K 2 Se 2 Te.As 2 O 3 was added as reducing agent in 1, 1.5 and 2equivalents,i .e., q(SeTe) = As 2 O 3 :( 2 / 3 SeO 2 + 1 / 3 Te O 2 ). TheU V/Vis spectrum of the experiment with q(SeTe) = 1s hows the Se 2 2À band at 435 nm as well as the Se 2À band at 262 nm ( Figure 6), while in the spectrum for q(SeTe) = 1.5, one additional band appears at 351 nm. There is very little spectroscopic information on mixed chalcogenides that could be used for comparison. NMR experiments on the mixed chalcogenides (TeSe 2 ) 2À and (TeSe 3 ) 2À in ethylenediamine revealed several equilibria between those anions and tellurium richer phases,f or example,( Te 2 Se) 2À and (Te 3 Se) 2À . [50] In analogy,w ep ropose as imilar equilibrium between the diselenide and ditelluride anions [Eq. (6)].C onsequently,t he absorption band at 351 nm is assigned to the (SeTe) 2À anion, which was not described before.
TheR aman spectra ( Figure 6) for q(SeTe) = 1a nd 1.5 show the Se 2 2À band at 265 cm À1 and one additional band at 231 cm À1 ,w hich occurs in the typical energy range of SeÀTe vibrations and is therefore assigned to the (SeTe) 2À anion. [41,[51][52][53][54] TheS e À Te stretching vibrations in Se À Te glasses had been reported to occur between 205 cm À1 and 216 cm À1 . [52,53] As imilar shift to lower energies had been observed for SeÀSe and TeÀTe vibrations in the solid state compared with Se 2 2À and Te 2 2À anions in aqueous solutions. DFT calculations on mixed trichalcogenides Ch 3 2À (Ch = Se,

Research Articles
Te )had predicted arange from 218 cm À1 to 230 cm À1 for SeÀ Te stretching modes. [41] In [Zn(NH 3 ) 4 ](TeSe 3 ), they occur at 217 cm À1 and 231 cm À1 , [54] in Na 2 Te Se 3 at 213 cm À1 and 238 cm À1 . [51] Thevibrations bands of the Se 2 2À and the (SeTe) 2À anions are clearly detectable.T he Se 2 2À band is the more intense for q(SeTe) = 1, but the weaker for q(SeTe) = 1.5. As indicated by the Raman spectra, TeO 2 is only partly reduced, as small intensities of Te À Ovibrations of the Te O 3 2À anion are visible with q(SeTe) = 1( Figure S16, Supporting Information). The assumption of (SeTe) 2À anions that formed from a2:1 solution of SeO 2 and Te O 2 is indirectly corroborated by the remaining Se 2 2À anions visible in the Raman spectrum. TheR aman spectrum of the experiment with q(SeTe) = 1.5 has an overall lower intensity of the chalcogenide vibration bands than the one with q(SeTe) = 1, since the crystallization of K 2 Se 2 Te has lowered the concentration of dissolved chalcogenide anions.
TheUV/Vis spectrum of the experiment with q(SeTe) = 2 shows the absorption bands of Se 2À at 261 nm and Te 2À at 305 nm, indicating that the high amount of As 2 O 3 had reduced the chalcogenide(IV) oxides completely.A ccordingly,n o Raman band is found in the range of Ch À Ch vibrations.
When mixing pre-synthesized Se 2 2À and Te 2 2À solutions at room temperature,t he resulting mixture shows as trong (SeTe) 2À band at 231 cm À1 and as maller Se 2 2À band at 265 cm À1 ( Figure S17, Supporting Information). Thec rystallization of K 2 Te 3 had reduced the Te 2 2À concentration, while the concentration was too low for the precipitation of K 2 Se 3 . This experiment corroborates the equilibrium in Equation (6).
Thec halcogenide solutions were sensitive against atmosphere.T he colorless monotelluride solutions started to oxidize on the slightest contact with oxygen resulting in purple solutions containing Te 2 2À ,from which then elemental tellurium precipitated. Thel atter can be identified by Te À Te vibration bands at 120 cm À1 and 139 cm À1 ( Figure S18, Supporting Information). [55] Ther eaction takes place on the surface of the liquid, which allows handling them in air for as hort period. Upon adding As 2 O 3 solution, the tellurium is again reduced to tellurides ( Figure S19, Supporting Information).
Thes elenide solutions are less reactive.M onoselenide solutions showed the orange color of Se 2 2À only after several hours in air. When diselenide solutions were diluted with water and exposed to air,ared film initially formed on the surface of the liquid, which yielded ag rey powder after several hours that showed Se-Se vibrations bands at 140 cm À1 and 235 cm À1 ( Figure S18, Supporting Information). [56,57] Higher oligochalcogenides could not be detected.
Experiments concerning the reductive potential of As and As 2 O 3 yielded unexpected results,e.g., that elemental arsenic is unable to reduce Ch 2 2À to Ch 2À ,a nd that excess As III O 3 3À anions seem to disproportionate into arsenic and As V O 4 3À (see Supporting Information).

Conclusion
Solutions of mono-and dichalcogenide anions Ch 2À , Ch 2 2À (Ch = Se,T e) and (SeTe) 2À are accessible by reducing the respective ChO 2 with As 2 O 3 under hydroflux conditions. When as ub-stoichiometric amount of As 2 O 3 is added, the reaction product consists of am ixture of ChO 3 2À and Ch 2 2À anions.However,neither elemental chalcogen nor oligochalcogenide anions larger than Ch 2 2À were observed spectroscopically.T he addition of As 2 O 3 in excess yields colorless solutions of monochalcogenide anions Ch 2À .L arge singlecrystals of K 2 Ch 3 were obtained when the amount of As 2 O 3 does not allow an average oxidation state of the chalcogen that is more negative than À0.67 (Ch 3 2À ). Thec rystallization of K 2 Se 3 or K 2 Se 2 Te requires higher reactants concentrations than needed for K 2 Te 3 .
Thep reparation of chalcogenides via the hydroflux method represents an attractive alternative to the hitherto used synthesis routes.T he necessary equipment is cheaper and the procedure is simpler and also safer.T he unexpected formation of chalcogenides from chalcogen dioxides is due to the ultra-alkaline conditions.T he water in the reaction mixture is strongly bonded to hydroxide.I ts reduced activity prevents the hydrolysis of the trichalcogenides,but also shifts the redox equilibria known from dilute alkaline solutions. [24] At ransfer of the approach to other systems should be possible.