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 1 b 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) 2 SiF 6 ], [2] 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 60, 2 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 NÀM 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 fr...
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 1 b 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) 2 SiF 6 ], [2] 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 60, 2 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 NÀM 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 ...
Reaction of MeP(O)(OH)2 with tBu3Al at low temperature and subsequent trimethylsilylation of the crude reaction product with Me3SiNMe2 yielded the cyclic dimer [tBu2AlO2P(OSiMe3)Me]2 (1). In contrast, reaction of MeP(O)(OH)2 with tBu3Al in refluxing toluene/THF yielded a mixture of [tBuAlO3PMe]4 (2), [tBuAlO3PMe]6 (3), and [tBuAlO3PMe]10 (4). Crystallization and sublimation of the crude mixture gave compound 2 in 54% yield. Small quantities of pure 3 were obtained by fractional crystallization of the remaining reaction products. Crystalline 3 was also obtained by slow diffusion of a CHCl3 solution of acetylacetone into a solution of 2 in poly(ethylene oxide)/CHCl3 gel. It was not possible to obtain reproducible yields of pure 4 by fractional crystallization of the crude mixture of 2−4. Instead, compound 4 was obtained in 86% yield by an acetylacetone-promoted rearrangement of 2. In the absence of acetylacetone, CDCl3 solutions of 4 rearrange to a mixture of 2, 3, and 4 over a period of days. The role of acetylacetone in the rearrangement of 2 to 4 has not been elucidated. Compounds 1−4 were characterized by multinuclear (1H, 13C, 31P) NMR spectroscopy, infrared spectroscopy, mass spectrometry, and elemental analysis. The molecular structures for compounds 2−4 were determined by X-ray crystallography.
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