The preparation of membranes with high selectivity based on specific chemical properties such as size and charge would impact the efficiency of the world’s energy supply, the production of clean water, and many other separation technologies. We report a flexible synthetic protocol for preparing highly ordered two-dimensional nanoporous polymeric materials (termed covalent organic frameworks or COFs) that allow for placing virtually any function group within the nanopores. We demonstrate that membranes, fabricated with this new family of materials with carboxylated pore walls, are very water permeable, as well as highly charged and size selective.

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Crystallographic evidence that the interstitial atom is Ni(i) rather than Ge(i) (which differ by only four electrons) was provided from separate least-squares refinements, which gave a more reasonable equivalent isotropic thermal parameter when the interstitial atom was designated as Ni(i). The central atom was unambiguously determined to be Ni(i) by LD/FTMS, which revealed the parent-ion peak and its isotopic distribution pattern as well as the fragment-ion pattern to be entirely consistent with the compound's composition.15The following structural-bonding implications emerge from an examination in Table I of the mean molecular parameters in 2 and related clusters: (/) The closely similar geometries of 3 and 4 indicate that replacement of terminal CO with PPh3 ligands does not markedly affect their electronic structures. (2) A consequence of each capping Ge atom in 2 having a 0.16-Á-larger covalent radius16 than each P atom in 3 is that its Ni(s)-E distances are greater by 0.18 Á; thus, the nonbonding Ni(i)-E distances of 2.76 A in 2 (E = Ge) are 0.3 Á larger than the corresponding cube center-E distances in 3 (E = P). It follows that the non bonding trans P-P distances of 4.9 Á in 3 are probably too small to accommodate a Ni(i) within the Ni8(#t4-P)6 cage to give a cluster analogous to 7. The unusually short eight Ni(i)-Ni(s) distances of 2.31 Á in 2 imply strong radial interactions between the Ni(i) AOs and appropriate cage Ni(s) orbitals. (3) Although 2 and 5 contain similar-sized E atoms and have the same number (124) of CVEs, their cage geometries are very different. Whereas the distances in 2 suggest that the Ni(i)-centered Ni8(^4-Ge)6 cage is stabilized by both radial bonding Ni(i)-Ni(s) and tangential (edge-bridged) bonding Ni(s)-Ni(s') interactions, those in 5 signify no edge-bridged bonding Pd(s)-Pd(s') interactions but instead indicate that the Pd(i) is involved in radial bonding interactions with the six capping As atoms as well as with the eight Pd(s) atoms. These geometrical differences are partly attributed to the less contracted valence Pd AOs forming stronger bonding interactions at longer distances. Although similarly large bond-length differences are observed between the Pd(i)-centered Pd8(#t4-Sb)6 cage of the 124-electron 6 and the Ni(i)-centered 8(µ4-ß)6 cage of the 130-electron 7,17•18 their different electron counts prevent an unambiguous qualitative bonding analysis. (4) Both the radial bonding Ni(i)-Ni(s) and edge-bridged bonding Ni(s)-Ni(s') interactions are presumed to be considerably smaller in 7 than in 2 on account of the 0.2-Á-longer distances in 7. (5) From bonding considerations under Oh symmetry, it is proposed that the four "extra" electrons in the 124-electron 2 occupy an additional doubly-degenerate pair of antibonding radial MOs originating from the 3d (eg) AOs of the interstitial Ni(i); the stronger radial interactions of the 3d (t2g) Ni(i) AOs with the cage Ni(s) orbitals are presumed to produce occupied bonding but empty antibonding MOs. This structural-bonding analysis o...

This review provides a synopsis of the formation and chemistry of cyclopropanated carbohydrates. The organization begins with methods for the formation of the cyclopropanated carbohydrate followed by the chemistry of the bicyclic system. Examples of new reactions and rearrangements leading to new products and the synthesis of natural products are presented.

A model for the origin of selectivity in boron-mediated 1,5-anti-aldols is presented. This model involves π-stacking between the boron enolate and a remote aromatic ring. A short, facile method for the synthesis of the C-12 to C-22 segment