Two-dimensional crystals from reduced symmetry analogues of trimesic acid.

The two-dimensional assembly of multicarboxylated arenes is explored at the liquid-graphite interface using scanning tunneling microscopy. Symmetry variations were introduced via phenylene spacer addition and the influence of these perturbations on the formation of hydrogen-bonded motifs from an alkanoic acid solvent is observed. This work demonstrates the importance of symmetry in 2D crystal formation and draws possible links of this behavior to prediction of coordination modes in three-dimensional coordination polymers.

Two-dimensional crystallization of carboxylated benzene oligomers.

Various carboxylic acid substitution patterns on the 1,3,5-triphenylbenzene nucleus were explored, and their influence on the symmetry of the resulting two-dimensional (2D) crystal structures was assessed. The symmetry of 1,3,5-benzenetribenzoic acid (H(3)BTB) was reduced by modifying the substitution pattern of the arene and/or adding an additional carboxylic acid. Four analogues belonging to various point groups were studied. Comparison of the monolayers of the analogues to that of H(3)BTB shows that plane group symmetry and molecular symmetry are not correlated: H(3)BTB and its analogues exhibit the same plane group p2 at the heptanoic acid/graphite interface. The 2D crystal structure of the H(3)BTB analogues is more strongly controlled by the geometry of hydrogen-bonding interactions rather than molecular symmetry. Other significant observations in this study include porosity, uncommon hydrogen-bonding motifs, and an unusually high number of inquivalent molecules (Z' = 3) present in the 2D crystal of the lowest symmetry analogue. This research demonstrates that reduction of molecular symmetry based on geometric modification of noncovalent interactions allows for control over porosity of the 2D crystals (close-packed structures to nanoporous networks) without changing the core shape of the molecule.

Linker-directed vertex desymmetrization for the production of coordination polymers with high porosity.

Five non-interpenetrated microporous coordination polymers (MCPs) are derived by vertex desymmetrization using linkers with symmetry inequivalent coordinating groups, and these MCPs include properties such as rare metal clusters, new network topologies, and supramolecular isomerism. Gas sorption in polymorphic frameworks, UMCM-152 and UMCM-153 (based upon a copper-coordinated tetracarboxylated triphenylbenzene linker), reveals nearly identical properties with BET surface areas in the range of 3300-3500 m(2)/g and excess hydrogen uptake of 5.7 and 5.8 wt % at 77 K. In contrast, adsorption of organosulfur compounds dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) shows remarkably different capacities, providing direct evidence that liquid-phase adsorption is not solely dependent on surface area or linker/metal cluster identity. Structural features present in MCPs derived from these reduced symmetry linkers include the presence of more than one type of Cu-paddlewheel in a structure derived from a terphenyl tricarboxylate (UMCM-151) and a three-bladed zinc paddlewheel metal cluster in an MCP derived from a pentacarboxylated triphenylbenzene linker (UMCM-154).

Metal-dependent phase selection in coordination polymers derived from a C(2V)-symmetric tricarboxylate.

Reacting biphenyl-3,4',5-tricarboxylic acid (H(3)BHTC) with the appropriate metal salt yields the microporous coordination polymers (MCPs) Mn(3)(BHTC)(2) (1), Mg(3)(BHTC)(2) (2), and Co(3)(BHTC)(2) (3) containing hourglass metal clusters. The addition of Cu to reactions with Co(II), Fe(III), or Mn(II) leads to the formation of heterobimetallic UMCM-150 isostructural analogues Co(1)Cu(2)(BHTC)(2) (4), Fe(1)Cu(2)(BHTC)(2) (5), and Mn(1)Cu(2)(BHTC)(2) (6) containing both paddlewheel and trinuclear metal clusters. X-ray diffraction analysis of the crystals of the heterobimetallic MCPs suggests that Cu on the trinuclear site of UMCM-150 was replaced by the other metal, whereas Cu in paddlewheel sites remains unchanged. N(2) sorption isotherms were measured for the mixed-metal UMCM-150 analogues, and it was confirmed that there is no structural collapse after the metal replacement.

A framework for predicting surface areas in microporous coordination polymers.

A predictive tool termed the linker to metal cluster (LiMe) ratio is introduced as a method for understanding surface area in microporous coordination polymers (MCPs). Calibrated with geometric accessible surface area computations, the LiMe ratio uses molecular weight of building block components to indicate the maximum attainable surface area for a given linker and metal cluster combination. MOF-5 and HKUST-1 are used as prototypical structures to analyze MCPs with octahedral M(4)O(CO(2)R)(6) and paddlewheel M(2)(CO(2)R)(4) metal clusters. Insight into the effects of linker size, geometry, number of coordinating groups, and framework interpenetration is revealed through the LiMe ratio analysis of various MCPs. Experimental surface area deviation provides indication that a material may suffer from incomplete guest removal, structural collapse, or interpenetration. Because minimal data input are required, the LiMe ratio surface area analysis is suggested as a quick method for experimental verification as well as a guide for the design of new materials.