A self-folding metallocavitand

Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities

Construction of supramolecular structures with internal functionalities is a promising approach to build enzyme-like cavities. The endo-functionalized [Pd₁₂ L₂₄ ] and [Pd₂ L₄ ] coordination cages represent the most successful systems in this regard. However, these systems mainly contain one type of endo-moiety. We herein provide a solution for the controlled endo-functionalization of [Pd₂ L₄ ] cages. Site-selective introduction of the endo-functional group was achieved through the formation of heteroleptic [Pd₂ (L^A )₂ (L^B )(L^C )] cages. Using two orthogonal steric control elements is the key for the selective formation of the hetero-assemblies. We demonstrated the construction of two hetero-cages with a single internal functional group as well as a hetero-cage with two distinct endohedral functionalities. The endo-functionalized hetero-cages bound sulfonate guests with fast-exchange dynamics. This strategy provides a new solution for the controlled endo-functionalization of supramolecular cavities.

Rhodamine B oxidation promoted by P450-bioinspired Jacobsen catalysts/cellulose systems

P450-bioinspired Jacobsen/Cell(NEt2) catalysts have been applied in RhB dye oxidation, which is used illegally in food industries of some countries.

Binding of ion pairs in a thiourea-functionalized self-folding cavitand

A bioinspired supramolecular container with complementary binding sites displays cooperative binding of ion pairs.

Biomimetic cavity-based metal complexes

The biomimetic association of a metal ion with a cavity allows selective recognition, unusual redox properties and new reactivity patterns.

Electron Paramagnetic Resonance Structure Investigation of Copper Complexation in a Hemicarcerand

The double-bridged hemicarcerand [A,B-(CH2OH)2-cavitand]-(CH2NHCH2)2-[A,B-(CH2OH)2-cavitand] 23 (and several other related compounds) was synthesized by the condensation of the two complementary precursors A,B-(CH2NH2)2(CH2OH)2-cavitand and A,B-(CH2Br)2(CH2OAc)2-cavitand followed by hydrolysis of the acetate groups. This hemicarcerand has nitrogen and oxygen donor atoms located on the interior of the spherical cavity and thus allows endohedral coordination of metal ions. The cavity has a volume of approximately 0.12 nm3, a value obtained by calculating a Connolly-type contact surface and the molecular electrostatic potential. The Cu2+ complex of hemicarcerand 23 was studied in detail by EPR and DFT calculations at the UB3LYP/6-31G level to verify the anticipated endohedral nature of the metal complex. It could be shown that the copper ion is coordinated to four oxygen donor atoms and no deviation from axial symmetry at the copper site could be detected. No direct coordination to nitrogen atoms of the hemicarcerand could be observed; however, complexation with DMF solvent molecules was detected by ESEEM and HYSCORE experiments. The closed structure of the hemicarcerand was also confirmed by an evaluation of proton-copper distances. Results from DFT calculations are in accord with the EPR results, and further support suggested coordination of the Cu(II) within the hemicarcerand cavity by four oxygen donor atoms.

Functional cavitands: chemical reactivity in structured environments

Container-shaped molecules provide structured environments that impart geometric bounds on the motions and conformations of smaller molecular occupants. Moreover, they provide "solvation" that is constrained in time and space. When inwardly directed functional groups are present, they can interact chemically with the occupants. Additionally, the potential for reactivity and catalysis is greatly enhanced. Deep cavitands, derived from resorcinarenes, nearly surround smaller molecules and have been one of the most successful platforms for elaboration with functional groups. Derivatives bearing organic and metal-binding functional groups have been shown to affect recognition properties and selectively accelerate diverse reactions. In this review, we examine recent examples of these systems with an emphasis on how and why ordered nanoenvironments impart changes in the properties and reactivity of their occupants.

Structural Properties of Benzimidazole Cavitand and Its Selective Recognition toward 4-Methylbenzamide over 4-Methylanilide

The conformations and properties of cavitand 5 with four benzimidazole flaps are studied by (1)H NMR. The benzimidazole cavitand 5 can form very stable vase structures with an enforced concave cavity by intermolecular hydrogen bonding with four hydroxyl-containing molecules, X-OH, such as methanol (X = Me), acetic acid (X = CH(3)CO), and trifluoroacetic acid (X = CF(3)CO). The stronger hydrogen bond donor strengths of X-OH are, the stronger hydrogen bonds are formed between the NH and N atoms of the neighboring benzimidazole fragments and the more vase structures of 5.4HOX are stable. The annular tautomerism of 5 in CDCl(3)/CD(3)OD (9:1, v/v) due to the proton exchange between NH and N atoms of the neighboring benzimidazole fragments is observed by 400 MHz (1)H NMR, and the free energy of activation is measured as DeltaG++(210) = 10.2 kcal/mol at a coalescence temperature of 210 K. Cavitand 5 forms inclusion complexes with 4-methylbezamide guests such as 4-methyl-N-p-tolylbenzamide 6 and N,4-dimethylbenzamide 7 in water-saturated CDCl(3). However, an isomorphic 4-methylanilide guest such as N-4-tolylacetamide 8 cannot be recognized in the concave cavity of 5. This high selectivity toward 4-methylbenzamide over 4-methylanilide seems attributable to the hydrogen-binding interaction between the NH proton of 4-methylbezamide guest 7 and the oxygen atom of the closest water molecule.

Preparation and unique circular dichroism phenomena of urea-functionalized self-folding resorcinarenes bearing chiral termini through asymmetric hydrogen-bonding belts

Chiral macrocycles with eight (R)- and (S)-methylbenzylurea residues on the resorcinarene skeleton linked through a hexyl or dodecyl spacer having amide linkages have been prepared by the reactions of the corresponding octaamine derivative with (R)- and (S)-alpha-methylbenzylisocyanate, respectively. In chloroform, the urea-functionalized resorcinarenes with hexyl spacers form intramolecular hydrogen bonds by bundling the urea and amide residues in a cyclic fashion to give a self-folding cavitand. The urea and amide residues are cooperatively oriented in the same direction to result in asymmetric hydrogen-bonding belts. Unique circular dichroism (CD) bands are induced in the absorption wavelength ranges of the macrocyclic skeleton, caused by a chirality transmission from their chiral urea termini through hexyl spacers in the self-folded conformation. On the other hand, urea-functionalized resorcinarenes with a longer dodecyl spacer do not show such unique CD bands on the macrocycle, because of their weaker propensity for hydrogen bond formation. The characteristic CD bands of the urea-functionalized self-folding macrocycles disappeared upon complexation with anions such as chloride and bromide, reflecting breaking of the intramolecular hydrogen-bonding belts.

Metal-switching and self-inclusion of functional cavitands

Cavitands bearing both eight (5) and two (13) metal-ligating carboxymethylphosphonate groups on their rims were synthesized by Arbuzov reaction of the corresponding bromoacetamido cavitands with trialkyl phosphites. These exist in the vase conformation in CDCl(3) and are stabilized by a cyclic seam of hydrogen bonds. This structure was also found in the solid state for the octabromoacetamide 4a and diphosphonate cavitand 13 by single-crystal X-ray analysis. Cavitands 5 and 13 form caviplexes in CDCl(3), CD(2)Cl(2), and alcohol solutions with adamantane derivatives 15a,b, quinuclidine 15d, ammonium and phosphonium salts 14, and drugs like ibuprofen 15c, all of which are stable on the NMR time scale at 295 K. NMR spectroscopy reveals that at 223 K octaphosphonate 5b exists in two forms: the major C(4)-symmetrical compound is filled with solvent while the minor species shows intramolecular inclusion of a dialkoxyphosphoryl group. In methanol-d(4) 5 and 13 exist in a lower symmetry vase conformation with self-inclusion of one alkyl group. Interaction of these complexes with La(OTf)(3) results in a change in the conformation of the cavitand from vase to kite with concomitant and quantitative release of the encapsulated guests. Two to three equivalents of the lanthanide salt per equivalent of cavitand 5a-d is necessary for the complete decomplexation of the included guest. The kite and the vase conformers equilibrate slowly on the NMR time scale at 295 K. The addition of good ligands for metal cations (nitrate or CMPO calixarene 16) shifts the equilibrium to the vase-shaped caviplex and allows quantitative control of the binding and release of the guest. The lanthanide complexes of octaphosphonates 5 in methanol-d(4) are velcraplex-like dimers held together by four metal cations.

Guest encapsulation and self-assembly of molecular capsules in polar solvents via multiple ionic interactions

Herein we report the formation and characterization of a novel type of capsules resulting from the self-association between oppositely charged complementary building blocks in MeOH/H2O. The assembly is based on the interaction between tetraamidinium calix[4]arenes 1a-d and tetrasulfonato calix[4]arene 2. Evidence for the formation of the expected 1:1 assemblies is provided by proton NMR, ESI-MS, and ITC. The association process is fast on the NMR time scale and strongly entropy driven, with association constants in the range of 10(6) M-1. The system 1a.2 shows binding affinity toward acetylcholine, tetramethylammonium, and N-methylquinuclidinium cations.

Adjusting the binding thermodynamics, kinetics, and orientation of guests within large synthetic hydrophobic pockets

Kinetic analysis of the host guest complexation of a large, open molecular basket and a highly complementary adamantoid guest reveals that for these types of systems a dissociative mechanism is in operation. Hence, the resident adamantyl guest must completely vacate the cavity before another guest molecule can move in to replace it. As a result of the rigid nature of the host, the energy barrier to this process is relatively high, about 16 kcal mol(-1) at room temperature. Modifying the cavity of the host by dangling either a methyl group or a hydroxyl group from the portal rim alters the thermodynamic binding profile of these hosts. (1)H NMR shift data analysis also reveals that these functional groups can adjust the orientation that monosubstituted guests adopt within the cavity. Additionally, (1)H NMR studies of the binding of (E)1,4-dibromoadamantane allow the observation of two energetically similar diastereomeric complexes. An examination of this guest binding to the three hosts reveals that the interchange between the isomers is much faster than the entry and egression rates, and that the functional groups at the rim of each cavity influence both the rates of reorientation and the equilibrium relating the isomers.