Nd 5O 4Cl[AsO 3] 2 and Gd 5O 4Br 3[SeO 3] 2: Two lanthanoid oxide halides with complex “lone-pair” oxoanions

Abstract

Both compounds, neodymium oxide chloride oxoarsenate(III) Nd 5O 4Cl[AsO 3] 2 and gadolinium oxide bromide oxoselenate(IV) Gd 5O 4Br 3[SeO 3] 2, were prepared by solid-state reactions from mixtures of the corresponding binary oxides and halides, and their crystal structures have been determined by X-ray diffraction of single crystals. They crystallize monoclinically ( a = 1241.62(9) pm, b = 565.78(4) pm, c = 902.03(7) pm, β = 116.454(3)° for Nd 5O 4Cl[AsO 3] 2 and a = 1243.70(9) pm, b = 549.91(4) pm, c = 1005.28(8) pm, β = 91.869(3)° for Gd 5O 4Br 3[SeO 3] 2) in space group C2/ m with two formula units per unit cell. The non-isotypic crystal structures contain three crystallographically different M 3+ cations (M = Nd and Gd). The coordination sphere of (M1) 3+ consists of eight oxygen atoms (CN = 8) exclusively, whereas (M2) 3+ carries six oxygen atoms and one X − anion (X = Cl and Br, CN = 7) in each case. For (M3) 3+, however, CN = 8 is realized by six oxygen atoms and two Cl − anions in Nd 5O 4Cl[AsO 3] 2, but five oxygen atoms and three Br − anions in Gd 5O 4Br 3[SeO 3] 2. The isolated pyramidal [AsO 3] 3−/[SeO 3] 2− anions ( d(As 3+–O 2−) = 175–179; d(Se 4+–O 2−) = 165–174 pm) originate from three oxygen atoms (O2 and two O3), which surround the As 3+/Se 4+ cations together with the stereochemically active non-bonding electron pair ( lone pair) Ψ 1-tetrahedrally (∢(O–As–O) = 95–102; ∢(O–Se–O) = 95–96°). Both crystal structures are built up of corrugated two-dimensional lanthanoid–oxygen layers { [ O 4 M 5 ] 7 + } ∞ 2 consisting of edge- and corner-shared [OM 4] 10+ tetrahedra ( d(O 2−–Nd 3+) = 228–242; d(O 2−–Gd 3+) = 226–235 pm). The single Cl − anion in the neodymium and the two crystallographically independent Br − anions in the gadolinium compound reside in between these sheets, where the lone-pair electrons at the As 3+/Se 4+ cations point into the center of channels, which are formed by lanthanoid–oxygen layers and halide chains.