Metal-Switching and Self-Inclusion of Functional Cavitands

Abstract

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.

Conformational Behavior of Pyrazine‐Bridged and Mixed‐Bridged Cavitands: A General Model for Solvent Effects on Thermal “Vase–Kite” Switching

“…The principal dimensions of cavitand 2 are given in Table 1 along with those obtained from the X-ray crystal structure of 1 [3a,b] for comparison. The cylinder, bound on the top by the circular ribbon of the atoms C8 and C9 of the pyrazine and on the bottom by the bridging O1 and O2 atoms, has a diameter D 2 of 8.989(5) , measured at the mouth of the cavity, and a height h 2 of 2.904 (6) . The truncated cone, bound on the top by the circular array of the atoms O1 and O2 (shared with the cylinder) and on the bottom by the atoms C4a-d, has a minor diameter D 1 of 5.173(5) and a height h 1 of 1.734(3) .…”

“…[4] Conformational switching can be reversibly induced by temperature [1] or pH [5] changes, with the kite conformation being preferred at low temperatures and low pH values, or by metal-ion addition. [6] The controllable switching of quin-A C H T U N G T R E N N U N G oxaline-bridged cavitands resembles the movement of a mechanical gripper and implies the synthesis of specifically tuned dynamic molecular receptors, sensors, and molecular machines [7] that feature an addressable, geometrically defined switching between the closed vase and opened kite conformations. Significant efforts of our groups were aimed at preparing new modified cavitand architectures [8] and at a detailed exploration of their switching properties, in particular the dependency of the conformational interconversion on environmental factors such as polarity, polarizability, and size of solvent molecules.…”

Regioselective Synthesis and Structure of New OligophosphorylatedrcttResorcinarenes

“…The 13 C NMR spectra of amidophosphates 8b and 9a (Table 3) agree well with the spectra of corresponding amidophosphites 3b and 4a. § The obtained spectral data show that phospho(III) (3)(4)(5) and phospho(IV) (8-10) resorcinarenes are structural analogues.…”

“…[1][2][3][4][5][6][7][8][9] The acceptor properties of oligophosphorylated resorcinarenes depend on the nature and number of immobilized ionophore groups and their orientation relative to one another and to the macrocycle plane.…”

Ion-mediated conformational switches

“…This behaviour was rationalized on the basis of protonation of the quinoxalines resulting in dipolar repulsion between the cationic arms of the cavitand. Since host molecules can be incorporated into the cavity, 36 such switchable containers offer the promise of targeted delivery of molecular cargo.…”