Silica-mimetic polymorphism of the Cd(CN)2 host lattice depending on the guest G in Cd(CN)2·xG clathrates

Abstract

The single-crystal structures have been determined for the Cd(CN)₂ host clathrates Cd(CN)₂·xBuⁿ₂O·yH₂O (x, y≈ 0.5)1, Cd(CN)₂·0.5Buⁱ₂O 2a, Cd(CN)₂·0.5(PrⁱCH₂CH₂)₂O 2b, Cd(CN)₂·PrⁱCl 2o, Cd(CN)₂·CHCl₂CH₂Cl 3a, Cd(CN)₂·PrⁱBr 3b and Cd(CN)₂·PrⁱCN 4, prepared in order to mimic the polymorphism of SiO₂ by Cd(CN)₂. The hexagonal P6₃/mmc host lattice of 1 is isostructural with the high-temperature form of tridymite, Accommodating the dibutyl ether guest in the channel cavity extending along the c axis and the water molecule is hydrogen bonded to the ether in the cage neighbouring the channel. The cubic Fdmm host lattice of 2 in the high-temperature cristobalite structure provides two neighbouring tetrahedral cavities for the respective alkyl ether guests in 2a and 2b; the structure of 2o is the same. The cubic Fd3m lattice is transformed into the tetragonal P4₁2₁2 one in 3 and 4, similar to the deformation from high-to low-temperature cristobalite, the lattice of 4(Z= 8) being more distorted than those of 3a and 3b(Z= 4). The polymorphic behaviour of these Cd(CN)₂ lattices is discussed in terms of the geometry and function of the guest molecules.