Synthesis and Crystal Structure of K₃AsSe₄ and K₄As₂Se₅ Containing [AsSe₄]^3- Anions and a Novel [As₂Se₅]^4- Isomer Featuring an As–As Bond

Abstract

Two novel selenoarsenates, K 3 AsSe 4 and K 4 As 2 Se 5 , were synthesized by the mild solvothermal reaction of potassium carbonate and arsenic and selenium powder in the presence of superheated ethylenediamine and structurally characterized by single-crystal X-ray diffraction. K 3 AsSe 4 crystallizes in the orthorhombic space group Pnma (no. 62) with a = 9.4169(4), b = 10.9695(5), c = 9.7568(8)Å, Z = 4 and V = 1007.86(10)Å 3 and contains [AsSe 4 ] 3− anions with pentavalent arsenic. K 4 As 2 Se 5 crystallizes in the monoclinic space group P2 1 /n (no. 14) with a = 7.6167(2), b = 22.8733(4), c = 8.4687(2)Å, β = 107.241(2) • , Z = 4 and V = 1409.14(5)Å 3 , and shows isolated [As 2 Se 5 ] 4− anions. The most prominent feature of the latter anions is the presence of arsenic in two different formal oxidation states (+4/ + 2) due to the presence of an As-As bond.

Formation of Tetraselenodiarsenate(II) Anions in Ultra‐alkaline Medium

“…[26] The (As 2 Se 4 ) 4À anion had already been found in Cs 4 [Bi(As II 2 Se 4 )(As III Se 3 )] (AsÀ As 246.5(3) pm) [19] and [{Mn(terpy)} 2 (As II 2 Se 4 )] • 2H 2 O (243.3(2) pm), [18] yet slightly distorted as a tetradentate ligand bridging two metal cations. In the related mixed-valence anion (Se 2 As II As IV Se 3 ) 4À , the AsÀ As distance (250.0(1) pm) [16] is slightly longer than in the title compound. The AsÀ Se distance in K 4 (As 2 Se 4 ) • 2H 2 O is 237.6(1) pm, which is within the range known from the two reference compounds (236.4(3) to 239.1(3) pm).…”

“…The AsÀ Se distance in K 4 (As 2 Se 4 ) • 2H 2 O is 237.6(1) pm, which is within the range known from the two reference compounds (236.4(3) to 239.1(3) pm). [16,17] In the Raman spectrum of K 4 (As 2 Se 4 ) • 2H 2 O (Figure S5), strong bands at 269 cm À 1 and 256 cm À 1 lie in the expected energy range of AsÀ As and AsÀ Se stretching vibrations. [22,[27][28][29][30] Few analogs to the (As 2 Se 4 ) 4À anion with other pnicogens or chalcogens are known, e. g. K 4 (As 2 Te 4 ) or Rb 4 (As 2 Te 4 )•en.…”

“…[31,32] Most pnicogen chalcogenides form either polyhedral cages, cyclic structures or extended networks. [16,17] In the crystal structure of K 4 (As 2 Se 4 ) • 2H 2 O, the centers of gravity the (As 2 Se 4 ) 4À anions form an F-centered lattice, but the orientation of the molecules alternates along [001] (Figure 2). The potassium atoms are located between layers of (As 2 Se 4 ) 4À anions parallel to (010) plane.…”

“…A wide variety of compounds containing selenidoarsenate anions (As x Se y ) nÀ with arsenic in different oxidations states are known. [14][15][16] The connectivity patterns of arsenic and selenium atoms are enormously diverse, e.g., there are pyramids of (AsSe 3 ) 3À anions, cyclic (As 4 Se 8 ) 4À anions, chains of 1 1 [As 4 Se 6 ] 2À and layers of 2 1 [As 8 Se 13 ] 2À . [17] In case of selenidoarsenate anions with two arsenic atoms, the following anions are reported: (As 2 II Se 4 ) 4À , [16,18] (As 2 III Se 4 ) 4À , [18,19] (As 2 III Se 5 ) 4À , [18] (As 2 II/IV Se 5 ) 4À , [16] (As 2 III Se 6 ) 2À .…”

“…[14][15][16] The connectivity patterns of arsenic and selenium atoms are enormously diverse, e.g., there are pyramids of (AsSe 3 ) 3À anions, cyclic (As 4 Se 8 ) 4À anions, chains of 1 1 [As 4 Se 6 ] 2À and layers of 2 1 [As 8 Se 13 ] 2À . [17] In case of selenidoarsenate anions with two arsenic atoms, the following anions are reported: (As 2 II Se 4 ) 4À , [16,18] (As 2 III Se 4 ) 4À , [18,19] (As 2 III Se 5 ) 4À , [18] (As 2 II/IV Se 5 ) 4À , [16] (As 2 III Se 6 ) 2À . [20,21] These selenidoarsenate anions were often obtained by solvothermal synthesis in polar solvents, such as methanol, at reaction temperatures of 100 to 200 °C.…”