Jorge Tiburcio

Chemically controlled self-assembly of [2]pseudorotaxanes based on 1,2-bis(benzimidazolium)ethane cations and 24-crown-8 macrocycles

Cations derived from 1,2-bis(benzimidazolium)ethane can penetrate the cavity of dibenzo-24-crown-8 macrocycles to produce a new family of [2]pseudorotaxanes. These supramolecular structures are held together by a series of charge-assisted hydrogen bonds (+)N-H[dot dot dot]O, ion-dipole and pi-stacking interactions. These new adducts were fully characterised by NMR spectroscopy, ESI mass spectrometry and single-crystal X-ray diffraction. The effect of electron-donating and electron-withdrawing groups on the association constants was also analyzed. Chemical control of the threading/unthreading process was acheived by the alternate addition of acid and base.

[2]Rotaxanes containing pyridinium-phosphonium axles and 24-crown-8 ether wheels.

A triethylphosphonium group attached to a pyridinium ethane moiety can be used as an axle for the self-assembly of [2]pseudorotaxanes and [2]rotaxanes. Although [2]pseudorotaxane formation is limited due to the bulk of the PR4+ group, [2]rotaxanes can be formed utilising 24-crown-8 ether, benzo-24-crown-8 ether and naphtho-24-crown-8 ether. The synthesis of these [2]rotaxanes and the X-ray structure of the [2]rotaxane containing a 24-crown-8 ether wheel are described. When the crown ether contains an aromatic group two possible conformational isomers exist; these are identified at low temperature by 1H and 31P NMR spectroscopy.

Diastereoselective allylation and crotylation of N-unsubstituted imines derived from ketones

A wide variety of tertiary carbinamines are synthesized in high yields via diastereoselective allylation and crotylation of in situ generated N-unsubstituted ketimines.

A mechanical “flip-switch”. Interconversion between co-conformations of a [2]rotaxane with a single recognition site

[2]Rotaxanes utilising the 1,2-bis(pyridinium)ethane, 24-crown-8 motif can exist in two distinct co-conformations whose relative abundances are solvent dependent.

Complexation of Imidazopyridine-Based Cations with a 24-Crown-8 Ether Host: [2]Pseudorotaxane and Partially Threaded Structures

A new series of linear molecules derived from 1,2-bis(imidazopyridin-2-yl)ethane can fully or partially penetrate the cavity of the dibenzo-24-crown-8 macrocycle to produce a new family of host-guest complexes. Protonation or alkylation of the nitrogen atoms on the pyridine rings led to an increase in the guest total positive charge up to 4+ and simultaneously generated two new recognition sites (pyridinium motifs) that are in competition with the 1,2-bis(benzimidazole)ethane motif for the crown ether. The relative position of the pyridine ring and the chemical nature of the N-substituent determined the preferred motif and the host-guest complex geometry: (i) for linear guests with relatively bulky groups (i.e., a benzyl substituent), the 1,2-bis(benzimidazole)ethane motif is favored, leading to a fully threaded complex with a [2]pseudorotaxane geometry; (ii) for small substituents, such as -H and -CH3 groups, regardless of the guest shape, the pyridinium motifs are preferred, leading to external partially threaded complexes in a 2:1 host to guest stoichiometry.

Optically sensed, molecular shuttles driven by acid–base chemistry

A pair of bistable [2]rotaxane, molecular shuttles were prepared that combine 1,2-bis(pyridinium)ethane and benzylanilinium recognition sites; acid-base controlled shuttling of DB24C8 was accompanied by a change in colour and/or fluorescence intensity.

Co-conformational isomerism in a neutral ion-paired supramolecular system.

Two different counter-ion-free host-guest complexes have been prepared and isolated. These compounds were formed from two equally and opposite doubly-charged species, the viologen guests 1 a(2+) and 1 b(2+) and the anti-disulfodibenzo[24]crown-8 [DSDB24C8](2-) host, which gave rise to the 1:1 neutral complexes [1 a⋅DSDB24C8] and [1 b⋅DSDB24C8]. These species are held together by hydrogen bonding and π stacking, as well as strong electrostatic interactions. The investigation of these neutral ion-paired supramolecular systems in solution and in the solid state allowed us to establish their co-conformational preferences. Compound [1 a⋅DSDB24C8], with small methyl groups as substituents on the viologen unit, may adopt three different geometries, 1) an exo nonthreaded, 2) a partially threaded, and 3) a threaded arrangement, depending on the relative spatial orientation between the host and guest: The partially-threaded structure is preferred in solution and in the solid state. The presence of bulky tert-butylbenzyl groups in the viologen moiety in compound [1 b⋅DSDB24C8] restricts the possible geometrical arrangements to one: The exo nonthreaded arrangement. This structure was confirmed in the solid state by X-ray crystallography. The stability of the neutral complexes in solution was determined by UV/Vis spectrophotometry. The stoichiometry of the complexes was established by continuous variation experiments, and overall equilibrium constants and ΔG° values were determined on the basis of dilution experiments. The results observed are a consequence of only the intrinsic stability of the complexes as there are no additional contributions from counter ions.

Electrostatic Kinetic Barriers in the Threading/Dethreading Motion of a Rotaxane-like Complex

A new rigid cationic thread, based on the 1,2-bis(bipyridinium)ethane motif, and a 24-crown-8 anionic macrocycle self-assemble into a pseudo-rotaxane complex in aqueous solution. The presence of pH-responsive end groups on the thread, remote from the recognition motif, allows controlling the threading/dethreading rate without perturbing the complex structure or stability. The difference in the rate is attributed to the activation or deactivation of electrostatic barriers on the thread during the sliding process at different pH values.

Synthesis and structural characterization of fluorinated thiosemicarbazones.

Six new fluorinated thiosemicarbazones R-C(R′)=N-NH-C(S)NH2 (R = 2,4-C6H3F2, R′ = H (1); R = 2,5-C6H3F2, R′ = H (2); R = 2,6-C6H3F2, R′ = H (3); R = 3,4-C6H3F2, R′ = H (4); R = 3,5-C6H3F2, R′ = H (5) and R = 4-C6H4F, R′ = C6H5, (6)) have been prepared. The molecular structures of compounds 1 to 6 have been determined.

Shuttling Motion in a Host–Guest Complex Triggered by Spiropyran to Merocyanine Reversible Chemical Transformation

A stimulus-responsive guest-containing spiropyran and viologen unit assembles with a 24-membered crown ether into a stable host-guest complex displaying a partially threaded geometry. Acid addition induces guest transformation to a merocyanine species activating a second recognition site, suitable for the formation of a pseudorotaxane. The simultaneous presence of two recognition sites produces a small-amplitude macrocycle shuttling motion, from the viologen to the merocyanine moiety. Base addition returns the guest to its spiropyran form, and concurrently the translation motion stops.