Bernardo Masci

Uranyl–organic one- and two-dimensional assemblies with 2,2′-bipyridine-3,3′-dicarboxylic, biphenyl-3,3′,4,4′-tetracarboxylic and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acids

The complexes formed by uranyl ions with two aromatic and one alicyclic polycarboxylic acids have been crystallographically characterized. Three complexes were obtained under hydrothermal conditions with 2,2′-bipyridine-3,3′-dicarboxylic acid (H2L1), [UO2(L1)(H2O)]·3H2O (1), [UO2(L1)(DMF)]·0.5H2O (2) and [UO2(L1)(H2L1)]·H2O (3), all of which contain one-dimensional polymeric chains with the ligand being both chelating through one oxygen atom from each carboxylate group and bridging through the remaining donor atoms, the nitrogen atoms being uncoordinated. The last coordination site is occupied by either a water (1), a dimethylformamide (2), or a monodentate, zwitterionic H2L1 molecule (3). Biphenyl-3,3′,4,4′-tetracarboxylic acid (H4L2) gives the complex [UO2(H2L2)(H2O)2]·2H2O (4) under hydrothermal conditions, which is also a one-dimensional coordination polymer with uranyl chelation by only one carboxylate group from each aromatic ring. A two-dimensional assembly is finally obtained in the complex [(UO2)3(HL3)2(H2O)6]·10H2O (5), crystallized at room temperature, in which H4L3 is the all-exo isomer of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid. The three carboxylate groups in 5 are chelating, the ligand being thus a T-shaped node, and the layers are built from a tessellation of oblong twelve-membered rings arranged in herringbone fashion.

Self-assembly of an Octa-uranate Cage Complex with a Rigid bis-Catechol Ligand

2 3 6 7-Tetrahydroxy·9,10·dimethyl·9,10-dihydro-9,10· etha~oanthracene, LH41 reacts with uranyl nitrate in the presence of triethylamine to give a highly symmetric octa-uranate complex with a molecular cage geometry. Four LH2 1igands act as bis-bidentate divergent and rigid bridges between two [(UOz)(Oz)], squares. The resulting molecular container is surrounded by the hydrogen bonded triethylammonium ions and contains hydrogen bonded water molecules. This complex is compared to a previously repo~ed .octa-uranate mole~ular box !nvolving dicarboxyhc ac1ds and JL·peroxo-lons as bndges. Both compounds evidence the potential of uranyl ions as supramolecular building blocks.

Hydrothermal synthesis of uranyl–organic frameworks with pyrazine-2,3-dicarboxylate linkers

The reaction of uranyl nitrate with pyrazine-2,3-dicarboxylic acid (H2PZDC) under hydrothermal conditions and in the presence of different bases gave three novel polymeric assemblages. The complexes [UO2(PZDC)(H2O)2] (1) and [UO2(PZDC)(H2O)] (2), obtained with NMe4OH and pyridine as base, respectively, differ by the coordination mode of the ligand, with O,N-chelation and one monodentate carboxylate group in 1 and both O,N- and O,O-chelation (the latter involving both carboxylate groups) in 2. In both cases, one-dimensional coordination polymers are formed, which are further assembled into three-dimensional networks by hydrogen bonding. In the presence of CsOH as a base, the heterometallic complex [(UO2)2Cs(PZDC)2(OH)(H2O)] (3) was isolated, which is a compact, three-dimensional framework. One uranyl ion is bis-O,N-chelated and the other is bound to three monodentate ligands, whereas each cesium cation is bound to three uranyl oxo groups, one nitrogen and five carboxylate oxygen atoms. The metal atoms are further bridged by μ2-hydroxide and μ2-aqua ligands. One of the PZDC2− ligands in 3 assembles five metal atoms and the other four, thus demonstrating the potential of this molecule for the building of three-dimensional uranyl–organic frameworks.

Synthesis and crystal structure of 1:2 mixed uranyl/alkali metal ions (Li+, Na+, K+, Cs+) complexes of p-tert-butyltetrahomodioxacalix[4]arene

p-tert-Butyltetrahomodioxacalix[4]arene LH4 reacts with uranyl nitrate hexahydrate in the presence of alkali metal hydroxides to give mixed complexes containing the same [UO2(L)]2− central core. All alkali metal ions are bound to the basic uranyl oxo groups, which act as mono- (Li+, Na+) or bidentate (K+, Cs+) ligands. The resulting structures largely depend on the size and predominantly “hard” or “soft” character of the alkali metal ion. Li+ and Na+ are coordinated in both “endo” and “exo” modes, Li+ giving a monomer with one cation bound to each oxo group and Na+ a dimer with a bridging, oxo-coordinated, Na2(CH3OH)8 moiety. Both K+ and Cs+ give polymeric chains, with two cations in the macrocycle cavity, involved in cation-π interactions with the aromatic rings and bound to the oxo groups from two neighbouring molecules in the chain, with differences in further coordination to the calixarenes which are related to the softer nature of Cs+. Unprecedented supramolecular architectures based on coordination bonds are thus obtained.

Homooxacalixarenes. 3. Complexation of quaternary ammonium ions by parent homooxacalixarenes in CDCl3 solution

Abstract Acetylcholine iodide and other quaternary ammonium salts are complexed by the simple calixarene analogues p-tert-butyldihomomonooxacalix[4]arene, p-tert-butyltetrahomodioxacalix[4]arene, and p-tert-butylhexahomotrioxacalix[3]arene in CDCl3 solution. An analysis of the complexation of cations of varying structure showed that the calix[3]arene analogue is the stronger binding agent (-ΔG° up to 2.7 kcal/mol) whilst the limit shielding effects in the 1H NMR spectra of the cation are larger with the two calix[4]arene analogues.

Doubly bridged tetrahomodioxacalix[4]arenes. Synthesis and complexation of quaternary ammonium ions.

Abstract A simple regioselective synthesis is reported for a macrotricyclic receptor related to calix[4]arenes which complexes several quaternary ammonium ions in chloroform and other organic solvents.

Oxa- and Azacalixarenes as Ligands for Uranyl Ions − Evidence for Two Different Complexation Modes

The first crystal structures are reported for uncomplexed parent tetrahomodioxa- and tetrahomodiazacalix[4]arene compounds and their 1:1 uranyl complexes. A 1:1 uranyl complex is also reported for a hexahomotriazacalix[3]arene. While analogous features are observed for the uncomplexed dioxa- and diazacalixarenes, completely different coordination modes are observed in their uranyl complexes. The complex of p-tert-butyltetrahomodioxacalix[4]arene is obtained in the presence of triethylamine and consists of an anionic core in which the cation is located at the centre of the lower rim, as is frequently observed. Both p-chloro-N-benzyl-hexahomotriazacalix[3]arene and p-methyl-N-benzyl-tetrahomodiazacalix[4]arene react with uranyl nitrate without adding a basic agent. The resulting 1:1 complexes are neutral, the cation being located outside the macrocycle and bound to two phenoxide groups of its zwitterionic form and to the nitrate counter-ions. The difference in coordination mode between homooxa- and homoazacalixarene complexes is attributed to the electrostatic repulsion between the uranyl ions and the ammonium groups located around the lower rim in the latter case, which prevents a close approach of the complexed cation.

Hydrogen bonded supramolecular assemblies from uranyl ion complexes of tetrahomodioxacalix[4]arenes with various counterions

Abstract Reaction of the expanded calixarenes p-R-tetrahomodioxacalix[4]arenes (R=methyl, phenyl) with uranyl ions in the presence of amines (butylamine, pyrrolidine, triethylamine) gives complexes in which the metal ion is in a distorted square-planar, tetra-phenoxide, equatorial environment. Changes in para-substituents and/or in the amine lead to different supramolecular assemblies, resulting in sheets or columns held by hydrogen bonds and cation–π interactions. The importance of primary or secondary amines, able to give divergent hydrogen bonds, to promote such structures, is evidenced.

Versatility of {M(30-crown-10)} (M = K+, Ba2+) as a guest in UO22+ complexes of [3.1.3.1]- and [3.3.3]homooxacalixarenes

The reaction between p-R-[3.1.3.1]- or [3.3.3]homooxacalixarenes and uranyl salts in the presence of 30-crown-10 and the alkali or alkaline-earth metal cations K+ or Ba2+ gives various supramolecular assemblages characterized by “complex-within-complex” architectures. These can be of the simple nesting or sandwich types, as in [{Ba(30-crown-10)}{UO2(L1)}]·2H2O·3CHCl3 (L1H4 = p-tert-butyl[3.1.3.1]homooxacalixarene) and [{Ba(30-crown-10)}{UO2(L4)}2]·2CHCl3 (L4H3 = p-bromo[3.3.3]homooxacalixarene), respectively, with the cation held in the cavity of the homooxacalixarene complexes in cone conformation by weak interactions, but more original structures arise when uranyl-cation bonds are present. In [{Ba(30-crown-10)}{UO2(L2)}] (L2H4 = p-phenyl[3.1.3.1]homooxacalixarene), the barium ion included in the crown ether is bound to the uranyl oxo group located out of the calixarene cavity, resulting in the formation of a neutral species which self-organizes to form a columnar assembly by auto-inclusion. In [{K(30-crown-10)}{UO2K(L1)(H2O)3}]2·6H2O, the nesting-type subunit dimerizes around two oxo-bound potassium ions. Finally, the use of the coordinating solvent dimethylsulfoxide leads to the neutral complex [UO2Ba(L3)(dmso)2(MeOH)]2 (L3H4 = p-methyl[3.1.3.1]homooxacalixarene), in which the crown ether is absent and two oxo-, phenoxo- and ether-bound barium atoms ensure the dimerization of the uranyl complex.

Supramolecular assemblies from uranyl ion complexes of hexahomotrioxacalix[3]arenes and protonated [2.2.2]cryptand

The use of [2.2.2]cryptand as a deprotonating agent during the synthesis of the mononuclear, monoanionic, uranyl ion complexes of p-R-hexahomotrioxacalix[3]arenes (R = tert-butyl, phenyl) results in the formation of supramolecular assemblies, which have been characterized by their crystal structures and 1H NMR spectra in solution. In the 1∶1 complex–cryptand assembly, the endo monoprotonated cryptand has its ammonium end embedded in the complex cavity, whereas, in the 2∶1 complexes, the endo diprotonated cryptand is encapsulated in a “sandwich” mode by two complexes with convergent concave surfaces. The latter complexes are the first examples of the use of [2.2.2]cryptand as a “connector” to build calixarene supramolecular systems. The importance of cation–π interactions in this assembling process is discussed.