Valentyn Rudzevich

A self-sorting scheme based on tetra-urea calix[4]arenes.

Size and shape do matter: When dimerized in nonpolar solvents, an equimolar mixture of eleven tetra-urea calix[4]arenes with different wide-rim substituents self-sorts into only six out of 35 different homo- and heterodimers (see picture). Since the calixarene scaffold and the four urea units are the same in all cases, the self-sorting process is driven only by the cooperative action of steric requirements and stoichiometry.

Stepwise Synthesis and Selective Dimerisation of Bis‐ and Trisloop Tetra‐urea Calix[4]arenes

Tetra-urea calix[4]arenes substituted with four mono- or bisalkenyl residues have been converted into bis- or tetraloop compounds by intramolecular olefin metathesis, with use of a tetratosylurea calix[4]arene as template. The same strategy has now been used to synthesise trisloop compounds and bisloop compounds with adjacent loops, completing the series of the loop-containing tetra-urea derivatives. A tetra-urea calix[4]arene of the AABB type, where A stands for a bisalkenyl- and B for a monoalkenyl-substituted urea unit, was used as precursor for the three loops. It was easily synthesised from a tetraamino calix[4]arene in which two adjacent amino groups were Boc-protected. The ABCB-type precursor for the two adjacent loops was prepared through protection of two opposite amino functions with trityl groups. The capabilities of the novel macrocyclic tetra-ureas for the selective formation of hydrogen-bonded dimers were studied.

Heterodimeric capsules formed by tetratosyl and tetratolyl urea calix[4]arenes

Predictions made by MD simulations for size, shape and guest inclusion of a heterodimer formed by a tetratosyl and a tetratolyl urea calix[4]arene and for its hydrogen bonding system have been entirely confirmed by a single crystal X-ray structure.

CMPO-calix[4]arenes and the influence of structural modifications on the Eu(III), Am(III), Cm(III) separation

The syntheses of new calix[4]arenes featuring CMPO groups on the wide rim are reported and the extraction of Am(III) and Eu(III) from concentrated HNO3 aqueous phases are discussed with reference to the properties of the symmetric tetra-CMPO derivative 1. All extraction studies were conducted in the same experimental conditions which allows to directly compare the dependence of the distribution coefficients of various calixarenes on the acid concentration (0.1 M < [HNO3] < 5 M). Calix[4]arene 1 becomes a very poor extractant if the length of the aliphatic chain between the amide and phosphine oxide groups of CMPO is increased, if the bridging methylene groups are replaced by sulfur atoms or if the macrocyclic cavity size is increased. By contrast, mixed amide — CMPO calix[4]arenes are nearly as effective than 1. Moreover, Am(III)/Cm(III) separation coefficients between 1.5 and 3 have been obtained with unsymmetrical calix[4]arenes of type 1 with different aliphatic chains grafted on the narrow rim. Guidelines to anticipate the extraction ability of calix[4]arenes remain elusive because of the intricate solution behavior of these compounds.

Dimeric Capsules Formed by Tetra-CMPO Derivatives of (Thia)Calix[4]arenes

Thiacalix[4]arene 2, calix[4]arene 3a and its tetraether fixed in the cone conformation 3b form homo- and heterodimeric capsules in apolar solvents, which are held together by a seam of NH...O=P hydrogen bonds between carbamoylmethyl phosphine oxide functions attached to their wide rim. Their internal volume of approximately 370 A(3) requires the inclusion of a suitable guest. Although neutral molecules such as adamantane (derivatives) or tetraethylammonium cations form kinetically stable complexes ((1)H- and (31)P-time scale), the included solvent is rapidly exchanged. The internal mobility of the included tetraethylammonium cation is distinctly higher (DeltaG=42.5 and 49.7 kJ mol(-1) for 3a and 3b) than that for similar capsules of tetraurea calix[4]arenes 1. Mixtures of 1 with 2, 3a, or 3b contain only the two homodimers but the heterodimerization occurs with the tetraloop tetraurea 6, which cannot form homodimers. Two dimers with cationic guests (2.(C(5)H(5))(2)Co(+).2 and 3a.Et(3)NH(+).H(2)O.3a) were confirmed by single-crystal X-ray analysis.

Reasons for the exclusive formation of heterodimeric capsules between tetra-tolyl and tetra-tosylurea calix[4]arenes

The selective heterodimerization of tetra-tolyl () and tetra-tosylurea () calixarenes, serendipitously found by Rebek et al. (R. K. Castellano, B. H. Kim and J. Rebek, Jr., J. Am. Chem. Soc., 1997, 119, 12671-12672), has been used for the construction of highly sophisticated macrocycles and well-defined supramolecular assemblies. Regrettably, hitherto, neither the exact structure of these heterodimers nor the reason for their exclusive formation is known. We present molecular dynamics simulations using the AMBER force field in explicit chloroform solvent for the two homodimers, the heterodimer and the two uncomplexed tetra-urea calixarenes. The rigid rotation about the C-S-N-C bond of the tosylurea group has been calculated for a model compound (N-mesylformamide) at the RHF/6-31G* level of theory. The calculations suggest that the heterodimer . is energetically favored over the homodimers by a sterically relaxed conformation of the tosylurea hemisphere in ., by a moderate degree of reorganization of the hemispheres from the uncomplexed to the complexed state and by favorable interactions between the hemispheres. The tosylurea S=O groups are involved in the hydrogen bonding system which results in different sizes of the three capsules in increasing order . < . < .. To prove the computational predictions, 1H NMR experiments have been carried out with solvents/guests differing in shape and size. The largest capsule . prefers the larger guests toluene and p-xylene while the latter is not encapsulated in the smallest capsule ..

Hydrogen bonding in dimers of tritolyl and tritosylurea derivatives of triphenylmethanes

The crystal structure of the homodimer formed by the tritolylurea 3a proves the existence of a belt of six bifurcated hydrogen bonds between both NH and the O=C groups of the adjacent urea residues. For the tritosylurea 3b, four additional three-center hydrogen bonds, also involving the SO2 oxygen, are found in the crystalline state. Molecular dynamics simulations in a chloroform box confirm these patterns of the hydrogen bonds and the resulting elongation of the dimer 3b. 3b in comparison to 3a x 3a. The calculated complexation energies for the three dimeric combinations are nearly identical in agreement with the simultaneous formation of heterodimer 3a x 3b in a mixture of 3a and 3b.

Selective dimerisation of tetraurea calix[4]arenes

The formation of hydrogen-bonded dimeric capsules from tetraurea calix[4]arenes is controlled by loops, connecting adjacent urea functions, and more or less bulky substituents. The dimerisation is only possible if loops are not overlapping and if the respective residues can pass the loops. A sorting scheme based on small and bulky residues and one to four loops allows reducing the number of possible dimers from 35 to 6 in a stoichiometric mixture of 11 ureas. With three different loop sizes (O–(CH2) n –O chains with n = 10, 14, 20 connecting adjacent phenylurea functions via their meta positions), it is possible to distinguish four urea residues of different sizes (small, medium, bulky and giant) ranging from tolyl to 4-[tris-(4-t-butylphenyl)methyl]-phenyl. While the smallest residue can pass all loops, the largest is excluded by all loops.

Radiation and chemical stability of calix[4]arene derivatives as prospective liquid-liquid extractants

Abstract Radiation and chemical stabilities of CMPO-functionalised calix[4]arenes, promising ligands for extraction of actinides and lanthanides, were studied both in solution and in the solid state. A comparison of the results showed that the degradation of ligands under radiolytic conditions proceeds indirectly via interactions with the solvent. The radiation stabilities in chloroform and 1-octanol were systematically studied using model compounds, and the main degradation product under extraction conditions (organic phase/conc. HNO3) was characterised.