Crystal structure of an acetylene sorption complex of dehydrated fully Mn(II)-exchanged zeolite X

The crystal structure of an acetylene sorption complex of dehydrated fully Mn(II)-exchanged zeolite X, Mn46Si100Al92O384. 30C(2)H(2) (a=24.705(3) Angstrom) has been determined by single-crystal X-ray diffraction techniques. The structure was solved and refined in the cubic space group Fd (3) over bar at 21(1)degrees C. The complex was prepared by dehydration at 380 degrees C and 2 x 10(-6) Torr for 2 days, followed by exposure to 300 Torr of acetylene gas for 2 h at 24 degrees C. The structure was refined to the final error indices, R-1=0.060 and R-2=0.054 with 383 reflections for which I>3 sigma(I). In the structure, Mn2+ ions are located at two different crystallographic sites; sixteen Mn2+ ions at site I are located at the centers of the double six rings and thirty Mn2+ ions are found at site II in the supercage, respectively. Each of these latter Mn2+ ions is recessed ca. 0.385(2) Angstrom into the supercage from its three-oxygen plane. Thirty acetylene molecules are sorbed per unit cell. Each Mn2+ ion at site II lies on a threefold axis in the supercage of the unit cell, close to three equivalent trigonally arranged zeolite framework oxygen atoms (Mn(II)-O=2.135(9) Angstrom) and symmetrically to both carbon atoms of a C2H2 molecules. At these latter distances, the Mn(II)-C interactions are weak (Mn(II)-C=2.70(5) Angstrom), probably resulting from electrostatic attractions between the divalent cations and the polarizable pi-electron density of the acetylene molecules.