There is provided a crystalline complex comprising a compound of formula (I) ##STR00001## in which the crystal lattice is stabilized by the presence of a guest molecule, characterized in that the crystalline complex is of space group P2.sub.12.sub.12.sub.1 having unit cell dimensions of about 12.5.+-.1.0 .sctn., 15.+-.1.0 .sctn., and 16.2.+-.1.0 .sctn. when determined at either 120K or 150K.