Identification of a metastable uranium metal–organic framework isomer through non-equilibrium synthesis

Since the structure of supramolecular isomers determines their performance, rational synthesis of a specific isomer hinges on understanding the energetic relationships between isomeric possibilities. To this end, we have systematically interrogated a pair of uranium-based metal–organic framework topological isomers both synthetically and through density functional theory (DFT) energetic calculations. Although synthetic and energetic data initially appeared to mismatch, we assigned this phenomenon to the appearance of a metastable isomer, driven by levers defined by Le Châtelier''s principle. Identifying the relationship between structure and energetics in this study reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs. Additionally, this study demonstrates how defined MOF design rules may enable access to products within the energetic phase space which are more complex than conventional binary (e.g., kinetic vs. thermodynamic) products.

Identifying the relationship between structure and energetics in a uranium MOF isomer system reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs.     

Electronically coupled MM quadruply-bonded complexes (M = Mo or W) employing functionalized terephthalate bridges: toward molecular rheostats and switches

Toluene solutions of M2(O2C(t)Bu)4 (M = Mo, W; 2 equiv) react with a range of functionalized terephthalic acids, HO2CArCO2H (Ar = C6H4, C6F4, C6Cl4, C6H2-2,5-Cl2, C6H2-2,5-(OH)2, C6H3-2-F), to give [(tBuCO2)3M2]2[mu-O2CArCO2]. These compounds show intense ML(bridge)CT absorptions in the visible region of the electronic spectrum, and the terephthalate bridge serves to electronically couple the two M2 units via interactions between the M2 delta and bridge pi orbitals. Electronic structure calculations reveal how the degree of electronic coupling is controlled by the dihedral angles between the terephthalate C6 ring and the two CO2 units and the degree of interaction between the M4 delta MOs and the LUMO of the bridge. Both of these factors are controlled by the aryl substituents, and collectively these determine the thermochromism displayed by these complexes in solution together with the physical properties of the oxidized radical cations as determined by electrochemical studies (CV, DPV), UV-vis-NIR and EPR spectroscopic methods.     

The electron density distribution in CN−, LiCN and LiNC. The use of minimal and extended basis set SCF calculations

Electron density maps are reported for the CN^?ion and the LiCN and LiNC molecules, calculated from molecular wave-functions near the Hartree-Fock limit. The electron density distribution derived from CNDO/ 2 wavefunctions does not resemble the ab initio results. The ultimate ability of a minimal basis set to represent the electron density near the Hartree-Fock limit, has been tested. The requirement of N-representability of the trial electron density has been satisfied. It is found that the molecular valence density cannot be reproduced to a satisfactory extent by a minimal set of Slater orbitals, even when the exponents of the basis orbitals are optimized.     

Infrared spectra of the polymorphs of naphthazarin: Microscopy and low temperature studies

The infrared microscope spectra of theA,B, andC polymorphs of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) can be distinguished clearly. When naphthazarin is dispersed in KBr, such a distinction can only be made when experimental precautions are taken during the preparation of the pellet. Low temperature infrared studies indicate that a phase transition takes place in naphthazarin around 110 K.     

Giant porphyrin disks: control of their self-assembly at liquid-solid interfaces through metal-ligand interactions

The synthesis and self-assembly behaviour of porphyrin dodecamers 1H(2) and Zn-1, which consist of twelve porphyrins that are covalently attached to a central aromatic core, is described. According to STM, 1D and 2D NMR studies, and molecular modelling calculations, the porphyrin dodecamers have a yo-yo-shaped structure. Their large pi surface, in combination with their disk-like shape, allows them to form self-assembled structures, which in the case of Zn-1 can be tuned by adding bidentate ligands. The self-assembly of the molecules at the liquid-solid interface of 1-phenyloctane with highly oriented pyrolytic graphite or Au(111) was imaged by using STM. The porphyrin disks in the self-assembled arrays have an edge-on orientation on the surface. The addition of bidentate axial ligands to the Zn-1 molecules in the arrays allows their intermolecular distance to be precisely controlled.