Sc₂Cl₂[B₂O₅]: an unexpected scandium chloride oxoborate

Abstract

Although there is a plethora of known compositions and crystal structures among the rare-earth metal oxoborates (e.g.REBO3, RE3B5O12, RE1.667B3O7, RE2B4O9, RE4B10O21, REB3O6, RE4B14O27, RE2B8O15, REB5O9 and REB7O12; RE = Sc or Y or La) [1,2], their chloride derivatives are embarrassingly rare.Only LaClB2O4 [3], CeClB2O4 [4], PrClB2O4 [5], Ce3Cl3BO3 [4] and Er2Cl2B2O5 [3] found entry into the literature, if one ignores the oxide chloride oxoborates with the composition RE4O4Cl[BO3] (RE = Eu -Ho) [6].In view of the orthorhombic crystal structure of Er2Cl2[B2O5], one of us immediately predicted a possible analogy to Sc2F2[B2O5], a scandium fluoride oxoborate mentioned for the first time in 1974 [7].Its synthesis and crystal-structure determination occured almost half a century later, but not very much to our surprise Sc2F2[B2O5] (orthorhombic, Pbam; a = 966.66(12),b = 1419.9(2),c = 403.95(6)pm) [8] really crystallizes isostructurally with Er2Cl2[B2O5] [3].A lot more surprising for us was the existence of the isotypic chloride derivative Sc2Cl2[B2O5] (a = 962.59(6),b = 1417.42(8),c = 501.23(3)pm; CSD-number: 2217919), since both Sc2F2[B2O5] and Sc2Cl2[B2O5] contain six-and sevenfold coordinated Sc 3+ cations.Their coordination spheres can be described as tetra-or pentagonal bipyramids [(Sc1)O4X2] and [(Sc2)O5X2] with both halide anions (X = F and Cl) in apical positions (Figure 1).The planar waist consists of four or five oxygen atoms stemming from discrete oxodiborate anions [B2O5] 4-, which strictly reside in the same plane as the Sc 3+ cations and are made up of two [BO3] 3-triangles sharing one oxygen corner.Regardless of the involved halide anion, their size is almost identical (d(B-O t ) = 133 -136 pm, d(B-O b ) = 140 -143 pm, <(B1-O b -B2) ≈ 127°) and even the Sc-O distances (205 -212 pm for Sc1, 208 -218 pm plus 240 pm for Sc2) do not differ significantly.No wonder, since the dimensions of the (001) planes (a = 962 -967 pm and b = 1417 -1420 pm) are very much alike and the parallel 2D-{(Sc2[B2O5]) 2-} layers (Figure 2) can hardly be distinguished.Perpendicular to them, infinite 1D-{[ScX2/2] 2+ or 1D-{[XSc2/2] 2+ chains are propagating along [001] (Figure 3), which make the difference, since the very diverse lattice parameters c (404 pm for X = F vs. 501 pm for X = Cl) cause Sc-X distances such as 202 -203 pm for the fluoride versus 250 -252 pm for the chloride with at least rather similar intrachain angles between 167 and 175°.The emerging gap between Sc2Cl2[B2O5] (a ≈ 963, b ≈ 1417, c ≈ 501 pm) and Er2Cl2[B2O5] (a = 1004.80(6),b = 1489.65(9),c = 524.86(3)pm) [3] suggests that more isostructural rareearth metal chloride oxodiborates RE2Cl2[B2O5] (RE = Y, Ho -Lu) should be accessible by heating appropriate mixtures of the sesquioxides (RE2O3 and B2O3) with the corresponding trichlorides (RECl3) in sealed inert glassware or ceramic ampoules at elevated temperatures (700 -850 °C) under reaction conditions, which have already proven to run successfully.Figure 1.Main structural features in the crystal structure of Sc2Cl2[B2O5].