A Dodecalanthanide Wheel Supported by p‐tert‐Butylsulfonylcalix[4]arene

A dodecaholmium wheel of [Ho₁₂(L)₆(mal)₄(AcO)₄(H₂O)₁₄] ( 1 ; mal=malonate) was synthesized by using p‐tert‐butylsulfonylcalix[4]arene (H₄L) as a cluster‐forming ligand. The wheel consists of three fragments of mononuclear A^3? ([Ho(L)(mal)(H₂O)]^3?), trinuclear B^3? ([Ho(H₂O)₂(mal)(Ho(L)(AcO))₂]^3?), and C³⁺ ([Ho(H₂O)₂]³⁺), and an alternate arrangement of these fragments (A^3?? C³⁺? B^3?? C³⁺? A^3?? C³⁺? B^3?? C³⁺? ) results in a wheel structure. The longest and shortest diameters of the core were estimated to be 17.7562(16) and 13.6810(13) Å, respectively, and the saddle‐shaped molecule possesses a pocketlike cavity inside.     

p‐tert‐Butylcalix[8]arene: An Extremely Versatile Platform for Cluster Formation

p‐tert‐Butylcalix[4]arene is a bowl‐shaped molecule capable of forming a range of polynuclear metal clusters under different experimental conditions. p‐tert‐Butylcalix[8]arene (TBC[8]) is a significantly more flexible analogue that has previously been shown to form mono‐ and binuclear lanthanide (Ln) metal complexes. The latter (cluster) motif is commonly observed and involves the calixarene adopting a near double‐cone conformation, features of which suggested that it may be exploited as a type of assembly node in the formation of larger polynuclear lanthanide clusters. Variation in the experimental conditions employed for this system provides access to Ln₁, Ln₂, Ln₄, Ln₅, Ln₆, Ln₇ and Ln₈ complexes, with all polymetallic clusters containing the common binuclear lanthanide fragment. Closer inspection of the structures of the polymetallic clusters reveals that all but one (Ln₈) are in fact based on metal octahedra or the building blocks of octahedra, with the identity and size of the final product dependent upon the basicity of the solution and the deprotonation level of the TBC[8] ligand. This demonstrates both the versatility of the ligand towards incorporation of additional metal centres, and the associated implications for tailoring the magnetic properties of the resulting assemblies in which lanthanide centres may be interchanged.     

Heterometallic [Mn5-Ln4] single-molecule magnets with high anisotropy barriers

The reaction of [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] (1) (py: pyridine, Piv: pivilate) with N-methyldiethanolamine (mdeaH2) and Ln(NO3)3 x 6 H2O in MeCN leads to a series of nonanuclear compounds [Mn5Ln4(O)6(mdea)2(mdeaH)2(Piv)6(NO3)4(H2O)2]2 MeCN (Ln=Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5)). Single-crystal X-ray diffraction shows that compounds 2-5 are isostructural, with the central core composed of two distorted {Mn(IV)Mn(III)Ln2O4} cubanes sharing a Mn(IV) vertex, representing a new heterometallic 3d-4f motif for this class of ligand. The four new compounds display single-molecule magnet (SMM) behaviour, which is modulated by the lanthanide ion used. Moreover, the values found for Delta(eff) and tau(o) for 3 of 38.6 K and 3.0 x 10(-9) s respectively reveal that the complex 3 exhibits the highest energy barrier recorded so far for 3d-4f SMMs. The slow relaxation of the magnetisation for 3 was confirmed by mu-SQUID measurements on an oriented single crystal and the observation of M versus H hysteresis loops below 1.9 K.     

1,3-Diketone Calix[4]arene Derivatives—A New Type of Versatile Ligands for Metal Complexes and Nanoparticles

The present review is aimed at highlighting outlooks for cyclophanic 1,3-diketones as a new type of versatile ligands and building blocks of the nanomaterial for sensing and bioimaging. Thus, the main synthetic routes for achieving the structural diversity of cyclophanic 1,3-diketones are discussed. The structural diversity is demonstrated by variation of both cyclophanic backbones (calix[4]arene, calix[4]resorcinarene and thiacalix[4]arene) and embedding of different substituents onto lower or upper macrocyclic rims. The structural features of the cyclophanic 1,3-diketones are correlated with their ability to form lanthanide complexes exhibiting both lanthanide-centered luminescence and magnetic relaxivity parameters convenient for contrast effect in magnetic resonance imaging (MRI). The revealed structure–property relationships and the applicability of facile one-pot transformation of the complexes to hydrophilic nanoparticles demonstrates the advantages of 1,3-diketone calix[4]arene ligands and their complexes in developing of nanomaterials for sensing and bioimaging.     

New molecular topologies by fourfold metathesis reactions within a hydrogen-bonded calix[4]arene dimer

A calix[4]arene tetrapentyl ether in the cone conformation substituted at its wide rim by four m-(omega-octenyloxy)phenyl urea groups forms hydrogen-bonded dimeric capsules in dichloromethane/benzene (95:5). Metathesis reaction with Grubbs' catalyst under high-dilution conditions (1.1 x 10(-4) M) followed by hydrogenation leads to a covalent connection of all the urea groups within a dimer. Three topologically different products may be expected in such a reaction: a bis[2]catenane, a doubly bridged monocatenane and a tetrabridged capsule. All three possible reaction products could be isolated in an overall yield up to 60 % for the separated and purified compounds. Their identification was based on the NMR patterns which reflect the characteristic symmetry properties of the isomeric products especially in the region of the hydrogen-bonded NH protons and were further confirmed by MALDI-TOF mass spectra. Further structural support for the bis[2]catenane comes from a single-crystal X-ray structure, although severe disorder prevents the localization of all atoms in the aliphatic chains connecting the two calix[4]arenes. Kinetic studies for the guest release/exchange (cyclohexane against the solvent [D(6)]benzene) do not show remarkable differences between the starting dimer and the additionally linked dimers, while the mobility of an included tetraethylammonium cation is obviously more restricted.     

Conformation-controlled luminescent properties of lanthanide clusters containing p-tert-butylsulfonylcalix[4]arene

Dinuclear and cubane-shaped lanthanide cluster complexes containing EuIII)and TbIII were synthesized by step-by-step construction using p-tert-butylsulfonylcalix[4]arene as a cluster-forming ligand. The sulfonylcalixarene adopts a pinched-cone conformation in the dinuclear complexes and a cone conformation in the cubane complexes. Because the calixarene has a large pi-conjugate system expanding over the entire molecule, it behaves as a good antenna chromophore for UV and near-UV light, and a slight conformational change of the calixarene (from cone to pinched-cone and vice versa) has an effect on the energy levels of excited S1 and T1 states. As a result, selectivity is observed in the luminescent properties of dinuclear and cubane-shaped systems of EuIII and TbIII.     

A family of calix[4]arene-supported [Mn(III)2Mn(II)2] clusters

In the cone conformation calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition-metal centres. Reaction of these molecules with manganese salts in the presence of an appropriate base (and in some cases co-ligand) results in the formation of a family of calixarene-supported [Mn(III)(2)Mn(II)(2)] clusters that behave as single-molecule magnets (SMMs). Variation in the alkyl groups present at the upper-rim of the cone allows for the expression of a degree of control over the self-assembly of these SMM building blocks, whilst retaining the general magnetic properties. The presence of various different ligands around the periphery of the magnetic core has some effect over the extended self-assembly of these SMMs.     

Simulations of p-tert-butylcalix[4]arene with multiple occupancies of small guest molecules

Classical molecular dynamics simulations were used to study low-density beta(0)-phase p-tert-butylcalix[4]arene inclusion compounds with multiple calix occupancies of xenon, carbon dioxide, methane, and hydrogen guest molecules with guest-host ratios ranging from 1:4 to 4:1. Custom parameterized force fields were used for the guests and the AMBER force field for the calixarene units was validated in our previous work (Chem. Eur. J. 2006, 12, 5231). The inclusion energy and unit cell volume of the calixarene inclusion compound were determined for various guest occupancies and for occupancies greater than 1:1, strong guest-guest interaction effects are observed. The structure and energetics of the 2:1 CO(2)/beta(0)-phase inclusion compound were compared to those of the low-temperature 2:1 CO(2)/calixarene in which the guest molecules occupy both cage and interstitial sites.     

Molecular dynamics simulations of p-tert-butylcalix[4]arene with small guest molecules

Classical molecular dynamics simulations were used to study p-tert-butylcalix[4]arene inclusion compounds with xenon, nitrogen, hydrogen, methane, and sulfur dioxide guest molecules. The calixarene units were taken to be rigid and the intermolecular molecular interactions were modeled as a sum of the van der Waals interactions with parameters from the AMBER force field and electrostatic interactions. Simulations of the high-density alpha phase and low-density beta0 phase of p-tert-butylcalix[4]arene were used to test the force field. The predicted densities of the two phases were found to agree with experimental measurements at 173 K to within 5 %. Simulations were performed with guests placed inside the calixarene cages of the beta0 phase. Guest-host ratios of 1:1 to 1:4 were considered. Changes in the unit-cell volume and density of the phases with the addition of guest molecules and the inclusion energies for the guests were determined. Finally, the dynamics of the guest motion inside the cages were characterized by determining the root-mean-square displacements and velocity autocorrelation functions of the xenon and nitrogen guests.     

Effect of varying bases on preferential crystallization of C-methylresorcin[4]arene and calix[6]arene

Preferential crystallization from a mixture of C-methylresorcin[4]arene (RsC1) and calix[6]arene (Calix6) in the presence of different bases has been investigated. In the presence of pyridine, a boat conformer of RsC1 crystallizes, whereas in the presence of triethylamine, Calix6 crystallizes in a symmetrically distorted conformation. The packing arrangements of the macrocycles show discrete solvent pockets for calixarenes and channels for resorcinarenes.