Ruy Cervantes

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.

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.

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.

cis and trans influences on [Pt(SRF)(triphos)]+ complexes (SRF = polyfluorobenzothiolate)

Platinum(II) derivatives of the tridentate ligand Ph2PCH2CH2P(Ph)CH2CH2PPh2 (triphos), including [Pt(SRF)(triphos)](CF3SO3) RF = C6F4CF3-4 1, C6F52, C6HF4-4 3, C6H4CF3-4 4, C6H3F2-2,4 5, C6H4F-2 6, C6H4F-4 7 and C6H58, have been prepared and characterised. 1H, 19F and 31P solution NMR has been used to evaluate the cis and trans influences of the fluorinated and non-fluorinated benzothiolate ligands, particularly through the cis and trans1JPt–P values. The crystal and molecular structures of [Pt(SC6F4CF3-4)(triphos)](CF3SO3) 1, with a perfluorinated thiolate ligand, [Pt(SC6HF4-4)(triphos)](CF3SO3) 3, having a fluorinated thiolate, and [Pt(SC6H5)(triphos)](CF3SO3) 8, bearing a non-fluorinated moiety, are discussed as examples of extreme electronegativities.