Engineering of copper molybdates: Piperazine dictated pseudopolymorphs

The hydrothermal syntheses, crystal structures and magnetic behavior of two compositionally different piperazine pillared copper molybdates, [{Cu(pip)_0.5}MoO₄] (1) and a hitherto unknown [Cu(pip)MoO₄] (pip = piperazine) (2), are reported. Both 1 and 2 exhibit three-dimensional covalent frameworks constructed from bimetallic oxide layers pillared by piperazine; however, the {CuMoO₄} networks are quite distinct. The Cu-Mo-O layers in 1 are made of edge-shared {CuO₄N} square pyramidal pairs linked through {MoO₄} tetrahedra, in contrast to the sheets in 2 that are built of corner-sharing {MoO₄} tetrahedra and {CuO₃N₂} square pyramids. Self assembly of the two pseudopolymorphs, 1 and 2, is interpreted in terms of molecular recognition between reasonable soluble molecular species in the supramolecular reaction along the mechanistic approach proposed by Ramanan and Whittingham for rationalizing metal-organic framework structures. Crystal data: 1, Triclinic, space group P-1, a = 5.5765(8) Å, b = 6.8304(10) Å, c = 9.2379(14) Å, α = 100.688(2)°, β = 101.462(2)°, γ = 112.624(2)°, Z = 2; 2, Orthorhombic, space group Pbca, a = 11.3899(11) Å, b = 10.7726(10) Å, c = 13.2541(12) Å, Z = 8.