Synthesis and Crystal Structure of Ag 4 Br 4 Nanoclusters in the Sodalite Cavities of Fully K + -Exchanged Zeolite A (LTA)

Ag 4 Br 4 nanoclusters have been synthesized in about 75% of the sodalite cavities of fully K + -exchanged zeolite A (LTA). An additional KBr molecule is retained in each large cavity as part of a near square-planar K 4 Br 3 + cation. A single crystal of Ag 1 2 -A, prepared by the dynamic ion-exchange of Na 1 2 -A with aqueous 0.05 M AgNO 3 and washed with CH 3 OH, was placed in a stream of flowing 0.05 M KBr in CH 3 OH for two days. The crystal structure of the product (K9(K4Br)Si 1 2 Al 1 2 O 4 8 .0.75Ag 4 Br 4 , a = 12.186(1) A) was determined at 294 K by single-crystal X-ray diffraction in the space group Pm3 m. It was refined with all measured reflections to the final error index R 1 = 0.080 for the 99 reflections for which F o > 4σ(F o ). The thirteen K + ions per unit cell are found at three crystallographically distinct positions: eight K + ions in the large cavity fill the six-ring site, three K' ions fill the eight-rings, and two K' ions are opposite four-rings in the large cavity. One bromide ion per unit cell lies opposite a four-ring in the large cavity, held there by two eight-ring and two six-ring K + ions (K 4 Br 3 + ). Three Ag' and three Br ions per unit cell are found on 3-fold axes in the sodalite unit, indicating the formation of nano-sized Ag 4 Br 4 clusters (interpenetrating tetrahedra; symmetry T d ; diameter ca. 7.9 A) in 75% of the sodalite units. Each cluster (Ag-Br = 2.93(3) A) is held in place by the coordination of its four Ag + ions to the zeolite framework (each Ag' cation is 2.52(3) A from three six-ring oxygens) and by the coordination of its four Br ions to K' ions through six-rings (Br-K = 3.00(4) A).