MOLECULAR ENCAPSULATION OF TRANSITION-METAL COMPLEXES IN CYCLODEXTRINS .2. SYNTHESIS AND CRYSTAL-STRUCTURES OF 2-1 ADDUCTS BETWEEN ALPHA-CYCLODEXTRIN AND METALLOCENIUM HEXAFLUOROPHOSPHATES [ETA-5-C5H5)2M]PF6(M=FE, CO, RH)

The title metallocenium salts form crystalline 2:1 host:guest inclusion compounds with alpha-cyclodextrin, [(eta-5-C5H5)2M]PF6.2-alpha-CD.8H2O (1, M = Fe; 2, M = Co; 3, M = Rh). The X-ray crystal structures of 1 and 3, and the lattice constants, crystal system and space group of 2 have been determined. Crystal data: triclinic, space group P1 (No. 1), Z = 1; 1, a = 13.865 (2) angstrom, b = 13.839 (2) angstrom, c = 15.520 (2) angstrom, alpha = 91.43 (2)degrees, beta = 85.81 (2)degrees, gamma = 120.22 (2)degrees, and R(F) = 0.089 for 4257 observed MoK-alpha reflections [I > 3-sigma(I)]; 2, a = 13.810 (2) angstrom, b = 13.872 (2) angstrom, c = 15.560 (2) angstrom, alpha = 93.99 (2)degrees, beta = 87.06 (2)degrees, gamma = 120.04 (2)degrees; 3, a = 13.756 (1) angstrom, b = 13.863 (1) angstrom, c = 15.561 (2) angstrom, alpha = 94.39 (1)degrees, beta = 86.92 (1)degrees, gamma = 119.89 (1)degree, and R(F) = 0.061 for 11142 observed MoK-alpha reflections [I > 3-sigma(I)]. In the crystals of 1 and 3, two alpha-cyclodextrin molecules are arranged head-to-head to form a dimer by means of intermolecular hydrogen bonding across the secondary hydroxyl faces of adjacent alpha-CD monomers. The dimers are stacked along the crystallographic c axis to form a channel-type structure. The metallocenium cation is encapsulated within the cavity of the dimer, while the PF6- anion is located outside the cavity, being centered between the primary hydroxyl faces of adjacent dimers. Eight water molecules of hydration per asymmetric unit reside in the spaces between the alpha-CD columns. In view of the almost identical crystal data for 2 a similar structure can be assumed for the cobaltocenium adduct.