Extended frameworks encapsulating facial {MoO₃} units: Syntheses and structural characterizations of Ba₃(MoO₃)₃(H₂O)(AsO ₄)₂, SrMoO₃HAsO₄·H₂O, and Sr(MoO₂AsO₄)₂

Three new alkaline earth metal molybdenum(VI) arsenates, Ba₃(MoO₃)₃(H₂O)(AsO ₄)₂ (1), SrMoO₃HAsO₄·H₂O (2), and Sr(MoO₂AsO₄)₂ (3), have been prepared and structurally characterized by single-crystal X-ray diffraction and thermograyimetric analysis. Crystal data: 1, monoclinic, P2₁/c, a = 10.1814(2) Å, b = 17.5553(1) Å, c = 17.0799(1) Å, β= 90.349(1)°, V = 3052.75(7) ų, and Z = 8; 2, monoclinic, C2/c, a = 15.1927(11) Å, b = 10.2590(8) Å, c = 9.6550(7) Å, β= 121.877(1)°, V = 1277.9(3) ų, and Z = 8; 3, monoclinic, P2₁/c, a = 6.5904(3) Å, b = 6.8592(3) Å, c = 10.4472(5) Å, β= 92.106(2)°, V = 471.94(6) A°³, and Z = 2. Crystal 1 adopts a novel open-framework structure which is composed of isolated MoO₆ and Mo(H₂O)O₅ octahedra linked by AsO₄ tetrahedra, resulting in intersecting tunnels where Ba²⁺ cations reside. Crystal 2 has a layer structure in which MoO₆ octahedra are corner-shared with HAsO₄ tetrahedra forming infinite sheets parallel to the (100) planes. The Sr²⁺ cations and water molecules are between the layers. Crystal 3 adopts a tunnel structure which is isotypic with the phosphates A(MoO₂PO₄)₂ (A = Ba, Pb). As a common feature, all of the MoO₆ octahedra possess three terminal O^2- ions in 1 and 2. They are the first extended structures encapsulating facial {MoO₃} units. Thermal analysis and powder X-ray diffraction studies revealed that these units in 2 would progressively dissociate into cis {MoO₂} cores under heating, resulting in the structural transformation into 3. In this paper, the syntheses, crystal structures, thermogravimetric studies, and structural relationship are presented.