Lead and Thallium Tetrakis(imidazolyl)borates: Modifying Structure by Varying Metal and Anion

Charge-Separated and Lewis Paired Metal-Organic Framework for Anion Exchange and CO2 Chemical Fixation

Charge-separated metal-organic frameworks (MOFs) are a unique class of MOFs that can possess added properties originating from the exposed ionic species. A new charge-separated MOF, namely, UNM-6 synthesized from a tetrahedral borate ligand and Co²⁺ cation is reported herein. UNM-6 crystalizes into the highly symmetric P43n space group with fourfold interpenetration, despite the stoichiometric imbalance between the B and Co atoms, which also leads to loosely bound NO₃ ^- anions within the crystal structure. These NO₃ ^- ions can be quantitatively exchanged with various other anions, leading to Lewis acid (Co²⁺ ) and Lewis base (anions) pairs within the pores and potentially cooperative catalytic activities. For example, UNM-6-Br, the MOF after anion exchange with Br^- anions, displays high catalytic activity and stability in reactions of CO₂ chemical fixation into cyclic carbonates.

A charge-separated diamondoid metal–organic framework

Metal–organic framework with diamondoid structure constructed with precisely placed tetrahedral borate anions and copper(i) cations.

Azolylborates for Electrochemical Double Layer Capacitor Electrolytes

Asymmetric tetraalkylammonium salts of azolylborates were synthesized and studied with respect to their suitability as supporting electrolytes in electrochemical double layer capacitors. In contrast to current conducting salts used in this device, azolylborates exhibit an excellent stability towards thermal load and moisture. In addition to good conductivity and stability towards cathodic reduction we found certain limitations when more positive potentials were applied.

catena-Poly[[silver(I)-μ-bis-{2-[(E)-phenyl-diazen-yl]-1H-imidazol-1-yl}methane] trifluoro-methane-sulfonate]

The title compound, {[Ag(C(19)H(16)N(8))](CF(3)SO(3))}(n), is a coordin-ation polymer with cationic chain motif. The Ag(+) cation is coordinated by two unsubstituted imidazolyl N atoms of two independent 2-paBIM ligands [2-paBIM is bis-{2-[(E)-phenyl-diazen-yl]-1H-imidazol-1-yl}methane]. The shortest Ag⋯Ag separation in a cationic chain is 8.841 (2) Å and the dihedral angle between two 2-phenyl-diazenyl-imidazole planes in the same ligand is 74.7 (3)°. Weak C-H⋯O interactions are seen in the crystal.

Control of metal ion size-based selectivity through chelate ring geometry. metal ion complexing properties of 2,2'-biimidazole

Some metal ion complexing properties of 2,2'-biimidazole (BIM) are presented. The ligand BIM forms minimum steric strain complexes with hypothetical metal ions with M-N (metal-nitrogen) bond lengths of 4.2 A, in contrast to more usual ligands such as bipy (2,2'-bipyridyl) that prefer metal ions with M-N bond lengths of 2.51 A. This metal ion size-based preference of BIM suggests that ligands with such architecture could be used to produce selectivity (differences in log K(1)) for very large metal ions. To test this hypothesis, the crystal structure of [Pb(BIM)(2)(ClO(4))(2)](2) (1) was determined as the first example of a complex of BIM with a large metal ion. In addition, formation constants (log K(1)) for BIM with metal ions ranging from the very small Cu(II) to the very large Ba(II) ion were determined to examine the effect of the architecture of BIM on metal ion selectivity. The structure of 1 gave: Triclinic, P1, a = 8.314(2) A, b = 8.677(2) A, c = 14.181(3) (A), alpha = 91.143(3) degrees , beta = 104.066(2) degrees , gamma = 106.044(3) degrees , V = 949.5(4) A(3), Z = 1, R = 0.030. Pb(II) in 1 is eight-coordinate, with relatively short Pb-N bonds to the two BIM ligands ranging from 2.366(5) to 2.665(5) A, while the four Pb-O bonds are very long at 2.826(5) to 3.123(5) A. This is typical of the structure of Pb(II) complexes that have a stereochemically active lone pair of electrons, which is postulated to be situated in the vicinity of the long Pb-O bonds. The geometry of the chelate rings formed by BIM with Pb(II) in 1 is analyzed, and it is shown that these are closer in structure to the minimum-strain chelate ring formed by BIM with a very large metal ion than is the case for structures reported in the literature with smaller metal ions. The formation constants (log K(1)) determined for BIM at 25 degrees C in 0.1 M NaClO(4) by UV-visible spectroscopy are as follows: Cu(II), 6.35; Ni(II), 4.89; Zn(II), 3.42; Cd(II), 3.86; Ca(II), -0.2; Pb(II), 3.2; Ba(II), 0.2. The log K(1) values for BIM complexes show that, as expected from the geometry of the chelate ring formed by BIM, the complexes of BIM with small metal ions such as Cu(II) are considerably weaker than with ligands such as bipy, where the ligand architecture is more favorable for forming chelate rings with small metal ions. In contrast, for very large metal ions such as Pb(II) or Ba(II), the log K(1) values for BIM complexes are larger than for bipy. The use of ligand architecture in BIM-type ligands to engineer selectivity for very large metal ions is discussed. Some fluorescence results for BIM and its complexes are presented. BIM itself fluoresces very strongly, while all of its complexes except for Ca(II) show diminished fluorescence intensity, ranging from small shifts and decreases for Ba(II) to very large decreases for Cd(II), which may be due to the distortion of the ligand geometry in its complexes by metal ions that are too small for low-strain coordination with BIM.

Zeolitic boron imidazolate frameworks

B-hive? A family of crystalline materials analogous to porous AlPO(4) but based on boron imidazolate frameworks (BIFs) can be formed by the crosslinking of various presynthesized boron imidazolates with monovalent cations (Li(+) and Cu(+), see picture). This synthetic method is capable of generating a large variety of open frameworks, ranging from the four-connected zeolitic sodalite type to the three-connected chiral (10,3)-a type.

The assembly of organic radical anions between metal-borate scaffolds

Metal-organic frameworks based on the Pb[B(Im)(4)](+) unit form layered structures analogous to those observed in clays and double layered hydroxide minerals. These layers can act as scaffolds for the organization of anionic organic guests. In this report, we use this scaffold to assemble TEMPO and PROXYL carboxylates in the interlayer spacings of Pb[B(Im)(4)](4-carboxy-TEMPO) 1 and Pb[B(Im)(4)](3-carboxy-PROXYL)(H(2)O)2, respectively. The resultant materials are paramagnetic, and the organization of the radical units differs between the two compounds. This results in changes in electronic structure of the radical unit, as observed by EPR spectroscopy.