The self-assembly of zinc­(II) acetate tetrahydrate, a flexible tetrapyridyl ligand, tetrakis­(3-pyridyloxymethylene)­methane (3-tpom), a bent dicarboxylic acid, and 4,4′-(dimethylsilanediyl)­bis- benzoic acid (H2L) under solvothermal conditions has resulted in the formation of a microporous zinc­(II)–organic framework, {[Zn2(3-tpom)­(L)2]·2H2O} n (1). The framework exhibits very good thermal stability as evident from the thermogravimetric analysis, which is further supported by variable temperature powder X-ray diffraction analysis. The microporous nature of the framework has been established by the gas adsorption analysis. The framework exhibits exceptionally selective carbon dioxide adsorption in contrast with other gases having comparatively larger kinetic diameters (3.64 Å for N2 and 3.8 Å for CH4) under ambient conditions (298 K and 1 bar pressure). Further, the framework decorated with catalytically active unsaturated metal sites acts as a good catalyst toward the cycloaddition reaction of CO2 with epoxides and the three-component Strecker reaction at ambient conditions and without the requirement of any solvent. The heterogeneous nature along with good catalytic activity at ambient and solvent-free conditions entitles 1 as an excellent catalyst for these organic transformations.

Monosubstituted epoxides were regiospecifically ring-opened from the sterically least hindered side by benzenetellurolate and benzeneselenolate reagents to afford aryl beta-hydroxyalkyl tellurides and selenides, respectively. These materials were O-allylated by treatment with allylic bromides/sodium hydride in tetrahydrofuran and O-prop-2-ynylated when reacted with propargyl bromide/sodium hydride. On photolysis in benzene containing 40 mol % of hexabutylditin, the beta-(allyloxy)alkyl aryl tellurides were found to undergo group transfer cyclization to afford 2-substituted 4-[(aryltelluro)methyl]tetrahydrofurans (cis/trans = 1/3-1/10). The aryl beta-(prop-2-ynyloxy)alkyl tellurides similarly afforded 2-substituted 4-[(aryltelluro)methylene]tetrahydrofurans with an E/Z-ratio close to unity. The beta-(allyloxy)alkyl aryl selenides and aryl beta-(prop-2-ynyloxy)alkyl selenides failed to undergo group transfer cyclization. In the presence of tributyltin hydride and 2,2'-azobisisobutyronitrile, the former compounds were found to undergo reductive radical cyclization in high yields to afford 2-substituted 4-methyltetrahydrofurans (cis/trans = 1/3-1/10). Aryl beta-(prop-2-ynyloxy)alkyl selenides similarly afforded 2-substituted 4-methylenetetrahydrofurans. 2-Alkoxy-2-(allyloxy)ethyl phenyl selenides, prepared by allyloxyselenenation of vinyl ethers, were found to undergo reductive radical cyclization to afford 2-alkoxy-4-methyltetrahydrofurans (cis/trans = 1/3-1/4). The preference for formation of trans-2,4-disubstituted tetrahydrofurans in the group transfer and reductive radical cyclizations was rationalized assuming a chairlike transition state with a preferred adoption of a pseudoequatorial position of the 2-substituent. By carrying out the reactions at lower temperatures (ambient or -45 degrees C), using triethylborane as an initiator, it was possible to further increase the trans selectivity in the reductive cyclizations.

We have designed and synthesized two unprecedented microporous three-dimensional metal–organic frameworks, {[Cd6(TPOM)3(L)6]·12DMF·3H2O} n (1) and {[Zn2(TPOM)­(L)2]·2DMF·H2O} n (2), based on a flexible quadritopic ligand, tetrakis­(4-pyridyloxy­methylene)­methane (TPOM), and a bent dicarboxylic acid, 4,4′-(dimethylsilane­diyl)­bis-benzoic acid (H2L). The networks of 1 and 2 share a 4-c uninodal net NbO topology but exhibit different metal environments due to coordination preferences of Cd­(II) and Zn­(II). The Cd­(II) center in 1 is six-coordinated, whereas the Zn­(II) center in 2 is only four-coordinated, making the latter an unsaturated metal center. Such modulation of coordination atmosphere of metal centers in MOFs with the same topology is possible due to diverse binding of the carboxylate groups of L2–. Both 1 and 2 have relatively high thermal stability and exhibit permanent porosity after the removal of guest solvent molecules based on variable temperature powder X-ray diffraction and gas adsorption analysis. These materials exhibit similar gas adsorption properties, especially highly selective CO2 uptake/capture over other gases (N2 and CH4). However, because of the presence of an unsaturated Lewis acidic metal site, 2 acts as a very efficient heterogeneous catalyst toward the chemical conversion of CO2 to cyclic carbonates under mild conditions, whereas 1 shows very less activity. This work provides experimental evidence for the postulate that an unsaturated metal site in MOFs enhances adsoprtion of CO2 and promotes its conversion via the Lewis-acid catalysis.

The dative-bond representation (L→E) in compounds with main group elements (E) has triggered extensive debate in the recent past. The scope and limits of this nonclassical coordination bond warrant comprehensive exploration. Particularly compounds with (L→N←L')(+) arrangement are of special interest because of their therapeutic importance. This work reports the design and synthesis of novel chemical species with the general structural formula (L→N←L')(+) carrying the unusual ligand cyclohexa-2,5-diene-4-(diaminomethynyl)-1-ylidene. Four species belonging to the (L→N←L')(+) class carrying this unconventional ligand were synthesized. Quantum chemical and X-ray diffraction analyses showed that the electronic and geometric parameters are consistent with those of already reported divalent N(I) compounds. The molecular orbital analysis, geometric parameters, and spectral data clearly support the L→N and N←L' interactions in these species. The newly identified ligand has the properties of a reactive carbene and high nucleophilicity.

Using achiral components, a V-shaped dicarboxylic acid (H2L) and a conformationally flexible bidentate linker (bpp), a thermally stable chiral metal organic framework {[Cd­(bpp)­(L)­(H2O)]·DMF} n (1), where H2L = 4,4′-(dimethylsilanediyl)­bis-benzoic acid, bpp = 1,3-bis­(4-pyridyl)­propane and DMF = N,N-dimethylformamide, has been solvothermally synthesized and crystallographically characterized. It consists of 1D helical chains linked at the cadmium centers resulting in an overall 2D framework. Its microporous nature was confirmed by gas-sorption measurements. Upon thermal activation of 1, where both guest DMF molecules present in the 1D open channels and the coordinated H2O molecules are removed, its active metal site shows Lewis acid character to be an excellent heterogeneous catalyst for the C–C (Knoevenagel condensation reaction) and C–N (Strecker) bond forming reactions.