Preferred dimerization of tetra-tolyl- and tetra-tosylurea derivatives of flexible and rigidified calix[4]arenes

The dimerization of tetratolyl- and tetratosyl-urea derivatives 1 and 2, derived from a tetrapentoxy calix[4]arene in the cone conformation and of the corresponding tetra-urea derivatives 3 and 4, in which the cone conformation is rigidified by the two crown-3 tethers, have been studied. All six possible equimolar mixtures were examined by 1H NMR using CDCl3 and CD2Cl2 as solvents. While no heterodimers are found for the combinations 1/3 and 2/4 in either solvent, all remaining combinations lead to the (exclusive) formation of heterodimers in CD2Cl2. In CDCl3 heterodimers are only observed for the combinations of 3 with 2 or 4. These results are discussed in terms of entropic and enthalpic contributions and compared with MD-simulations in a box of chloroform solvent molecules.     

Formation of the monoanion [Ar*P(BH3)(mu-BH2)2H]- with a symmetrically bridging hydride from the attempted synthesis of the dianion [Ar*P(BH3)3]2-

Addition of an excess of BunLi to the bis(borano)phosphide complex [Ar*PH(BH3)2]-Li+ 1 (Ar* = 2,4,6-tri-tert-butylphenyl) and subsequent treatment with BH3, gives the anionic complex [Ar*P(BH3)(mu-BH2)2H]-Li+ 2 instead of the expected tris(borano)phosphide dilithium, [Ar*P(BH3)3]2-2Li+ 3.     

Fourfold tetraurea calix[4]arenes--potential cores for the formation of self-assembled dendrimers

Wide rim tetraurea calix[4]arenes monofunctionalized at the narrow rim by COOH or NH2 have been synthesized in five steps from t-butylcalix[4]arene tripropylether. Their covalent linkage via the narrow rim to a central calix[4]arene fixed in the 1,3-alternate conformation led to pentacalix[4]arenes 9 bearing four tetraurea derivatives in the cone conformation in a flexible tetrahedral arrangement. Their self-assembly via the formation of hydrogen bonded dimeric capsules has been studied under different conditions. A fourfold heterodimerisation of tetrakis-tetraurea derivatives of type 9 with tetratosylurea 10 has been confirmed by 1H NMR-spectroscopy and dynamic light scattering.     

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

Thiacalix[4]arene 2 , calix[4]arene 3 a and its tetraether fixed in the cone conformation 3 b form homo‐ and heterodimeric capsules in apolar solvents, which are held together by a seam of NH???O=P hydrogen bonds between carbamoylmethyl phospine oxide functions attached to their wide rim. Their internal volume of ～370 ų requires the inclusion of a suitable guest. Although neutral molecules such as adamantane (derivatives) or tetraethylammonium cations form kinetically stable complexes (¹H‐ and ³¹P‐time scale), the included solvent is rapidly exchanged. The internal mobility of the included tetraethylammonium cation is distinctly higher (ΔG=42.5 and 49.7 kJ mol^?1 for 3 a and 3 b ) than that for similar capsules of tetraurea calix[4]arenes 1 . Mixtures of 1 with 2 , 3 a , or 3 b contain only the two homodimers but the heterodimerization occurs with the tetraloop tetraurea 6 , which cannot form homodimers. Two dimers with cationic guests ( 2? (C₅H₅)₂Co⁺ ?2 and 3 a? Et₃NH⁺ ? H₂O ?3 a ) were confirmed by single‐crystal X‐ray analysis.     

The 5v 3 bands of 18O enriched ozone: line positions of 16O16O18O, 16O18O16O, 16O18O18O and 18O16O18O

The four 5v ₃ bands of ¹⁸O enriched ozone have been observed and analysed for the first time. Two species (¹⁶O¹⁸O¹⁶O and ¹⁸O¹⁶O¹⁸O) belong to the C_2v symmetry group and two other (¹⁸O¹⁸O¹⁶O and ¹⁶O¹⁶O¹⁸O) to the C_s symmetry group. They have been recorded at a resolution of 0.008 cm^?1 with a pathlength of 32.16 m. Despite the very weak absorptions observed, almost 250 energy levels have been derived for each of the 4 species, with J ? 35 and K _a ? 13, and suitable sets of Hamiltonian parameters have been determined. For 3 species it has been necessary to account for the resonance between the (005) and (311) states to correctly reproduce the spectra observed. These resonances, anharmonic for C_2v, and hybrid (both anhar-monic and Coriolis) for C_s symmetry confirm the accidentally extremely strong coupling between the (005) and (311) states for ¹⁶O₃, due in that case to the very close distance between unperturbed energy levels. This work also confirms the excellent prediction of band centres of these four species derived from the recently determined isotopically invariant molecular potential function.     

Self-sorting dimerization of tetraurea calix[4]arenes

Calix[4]arenes substituted by four urea functions are self-complementary molecules that spontaneously combine in apolar solvents in the presence of an ammonium salt to form dimeric capsules held together by a belt of hydrogen bonds. In the presence of tetraethylammonium salts, the Et4N+ cation is included as a guest. The sorting between dimeric capsules formed in a mixture of calix[4]arenes directly depends on the steric crowding of the substituents grafted on the urea groups whether aromatic derivatives or aliphatic chains linking urea functions in mono-, di-, or tetraloop structures. Simple rules allow one to anticipate which capsules will be exclusively formed when calix[4]arenes are mixed in different proportions. The stabilization of the dimeric structures by hydrogen bonds is thwarted by the overlaps of aliphatic loops and/or by bulky groups that cannot pass through these loops. Despite the structural similarity of the calixarenes, the exclusive formation of dimers of well-defined compositions and clear titration breaks are observed by electrospray mass spectrometry. This technique yields reliable information on stoichiometries and composition despite measurements in the gas phase rather than in solution and it does not suffer from excessive peak overlaps in contrast with NMR.     

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 ..