A topology paradigm for metal-organic zeolites.

Abstract

Coordination polymers that afford controllable cavities and incorporate different guest molecules may provide structural prototypes for the design of porous host lattices for applications such as adsorption,[1, 2] ion exchange[3] and heterogeneous catalysis.[4] The elaboration of such materials is a considerable challenge, as only a few of the previously synthesized solids are stable to the loss of the initially accumulated guests and capable of taking up small organic molecules in the resulting molecular holes.[1] New insights into the development of approaches for the engineering of metal–organic zeolites are possible on the basis of topological considerations.[5] Attractive metal–organic-zeolite models of metal–organic frameworks may be predicted for novel “2D building blocks” of semiregular topology[6] (Figure 1b and c) that are expanded in a third direction by pillaring.[7] In this case, initial plane tiling by a set of different polygons, instead of uniform square grids as in [Cu(bipy)2SiF6], generates closely packed molecular triangles (cf. molecular hexagons) with very open regions within the network. Thus, the free space appears to be concentrated, and the resulting framework of relatively low overall porosity, which is stable and robust, could maintain large cages for guest molecules. Realistic prototypes of such arrays are provided by the structures of purely inorganic materials—tungsten bronzes.[6b] Herein, we report how the combination of the inherent functional features of organic and inorganic counterparts allows an especially effective implementation of this assembly scenario, and the generation of a target coordination network with unprecedented hexagonal tungsten bronze topology. Despite the fact that the angles at the net vertex (2 6 60, 2 6 1208) do not match the demands of a typical coordination environment around octahedrally coordinated transitionmetal ions, the desired connectivity may be tuned by the conformational flexibility of the organic linker. First, the 3,3’,5,5’-tetramethylsubstituted 4,4’-bipyrazolyl ligand (4,4’bpz) acts as an angular linker.[8] Its frame includes two planar pyrazolyl fragments, which have an angle of rotation around 60–808 and the resulting noncollinear orientation of two N M vectors makes possible the assembly of the desired flat coordination net. Second, these coordination layers [M(4,4’bpz)2]n, formed by the octahedrally coordinated metal ions and two equivalents of the bridging 4,4’-bpz ligands, have a rich and versatile functionality for cross-linking into a 3D superstructure. Each four-coordinate point M(pyrazole)4 of the layer provides, in the two axial directions, six binding sites that include coordination positions at the metal atom, and four hydrogen bond donating NH groups of the coordinated pyrazole ligands. Thus, dense interconnection of the layers and generation of a rigid 3D network is feasible by the rational choice of an anionic counterpart to fit perfectly this set of binding sites. The 3D structure of a new family of framework solids [M(4,4’-bpz)2{S2O6}]n 1 (M=Co 1a, Zn 1b, Cd 1c) is supported by cooperation of bridging functions of organic and inorganic linkers. The 2D [M(4,4’-bpz)2]n linkage exists as a triangular so-called Kagom@ network[9] (Figure 2) and dithionate anions are used as pillars. The present 2D metalorganic subunit is a isomer of “square grids” and represents a mode of plane tiling by a combination of molecular triangles and hexagons in 2:1 ratio, instead of a 44 regular tiling topology. The fragments [M(4,4’-bpz)2]6 are distorted and adopt a roughly triangular shape that eliminates possible centers of inversion and the entire 3D structure is chiral. Dithionates are perfectly suited as pillars for the 1:4 metal– pyrazole coordination, as two oxygen atoms complete the octahedral environment of the metal atoms and the remaining four oxygen atoms form relatively strong NH···O hydrogen bonds with all available adjacent pyrazolyl groups (Scheme 1, Figure 3). This complementary set of donor/acceptor interactions, and effective shielding of the S2O6 linkers by eight methyl groups, presumably has an impact on the high thermal stability of the compounds, which remains unchanged in air up to 590 K.[10] Noteworthy, simple inorganic dithionates themselves are much less stable and readily loose SO2 upon heating. Successive planes of [M(4,4’-bpz)2]n are stacked with an interlayer separation of approximately 7.97 F, and the entire Figure 1. Four-connected plane nets of significance for chemical topology: a) regular 44 tiling, b),c) semiregular nets.