Tris(o-phenylenedioxy)cyclotriphosphazene: The Clathration-Induced Monoclinic to Hexagonal Solid-State Transition

Tris(o-phenylenedioxy)cyclotriphosphazene (I) is known to clathrate a variety of small molecules. The clathration process coincides with a solid-state transformation of the host from a monoclinic, guest-free form to a hexagonal, guest-containing modification, a process that involves a 23% volume expansion. The transition occurs in response to improved opportunities for van der Waals attraction as guest molecules enter the lattice. Scanning electron microscopy revealed that considerable crystal disruption occurs during the transition. The resultant microcrystallites were fairly uniform in size and morphology. Crystals of I are monoclinic, space group P2₁/n, with eight formula units in a unit cell of dimensions a = 25.086 (5) Å, b = 5.911 (2) Å, c = 25.913 (7) Å, and β = 95.97 (2)°. The water adduct of I (II), crystallizes in the hexagonal space group P6₃/m with two molecules in a unit cell of dimensions a = 11.606 (4) and c = 10.087 (3) Å. Both structures were solved by direct methods and refined by full-matrix least-squares calculations to R = 0.075 and R_w= 0.082 for 2341 reflections for I and to R = 0.050 and R_w= 0.056 for 287 reflections for II.