The reactions of three bis(viologen) tetraquaternary salts and their reduced radicals

Viologen–cucurbituril host/guest chemistry – redox control of dimerization versus inclusion

Two calix[4]arene systems, C23⁴⁺ and C24⁴⁺ – where 2 corresponds to the number of viologen units and 3–4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core – have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest model compounds, MC3²⁺ and MC⁴⁺, do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.

Two calix[4]arenes with two viologen units separated by 3 or 4 carbon atoms from the macrocyclic core were synthesized and led to the formation of [3]pseudorotaxanes when combined with CB[7] or CB[8].

Flexible Viologen Cyclophanes: Odd/Even Effects on Intramolecular Interactions

The ability of three bis-viologen cyclophanes to act as redox-triggered contractile switches is investigated. Odd/even effects in the formation of cyclic bis-viologens are circumvented by the use of a Zincke salt intermediate and a tetrathiafulvalene template to prepare a flexible cyclophane with hexyl linkers. Comparative spectro-electrochemical studies of this macrocycle with two other pentyl- or heptyl-linked cyclic bis-viologens show that the development of intramolecular interactions in aqueous solution depends on the length of the bridges. This dependence is confirmed by EPR and DFT studies of the magnetic coupling in the diradical dication species. The anti-ferromagnetic or ferromagnetic nature of the coupling depend, respectively, on the odd or even number of methylene groups in the spacer.

Viologen cyclophanes: redox controlled host–guest interactions

Bis(viologen) cyclophanes with alkyl linkers demonstrate redox-dependent host–guest properties, including the formation of a crystalline mixed valence species.

Bridged tetraquaternary salts from N,N'-polyfluoroalkyl-4,4'-bipyridine

4,4'-Bipyridine (1) with excess of polyfluoroalkyl bromide or iodides 2a-d at 100-110 degrees C without solvent gave the monoquaternary salts 3a-d in >90% yields. However, 1 with 2.5 equiv of 2a-c in DMF at 110 degrees C resulted in the diquaternary salts 5a-c in >85% yields. In DMF, 5a-c were obtained in comparable yields when a molar excess of 2a-c reacted with 3a-c. 1,4-Dibromobutane with 3a,b in DMF at 100 degrees C led to the tetraquaternary salts 7a,b in approximately 85% yields. In water or acetone/water as a solvent, salts 3a-d and 5a-c were metathesized with LiN(SO(2)CF(3))(2) and KSO(3)CF(3) to produce monoquaternary ionic liquids 4a-h in >88% yields and diquaternary ionic liquids 6a-f in >86% yields, respectively. Tetraquaternary ionic liquids 8a,b were obtained when LiN(SO(2)CF(3))(2) was reacted with salts 7a,b. These compounds were stable to 340 degrees C as determined by DSC. They are the first N-mono-, N,N'-di-, and N,N,N',N'-tetra-4,4'-polyfluoroalkylbipyridinium quaternary salts and ionic liquids.

Stereoselective Photoinduced Electron-Transfer Reactions of Zinc Myoglobin with Optically Active Viologens

Photoinduced electron-transfer reactions between zinc-substituted myoglobin and optically active viologens and bisviologens, containing ((naphthyl-, ((phenyl-, and ((cyclohexyl)ethyl)carbamoyl)methyl groups, have been studied at 25 degrees C, pH 7.0 (a 0.01 M phosphate buffer), and various ionic strengths. The excited triplet state of zinc myoglobin was preferentially quenched by (S,S)-isomers of optically active viologens; both ratios of the quenching rate constants and back-electron-transfer rate constants, k((S,S))/k((R,R)), range from 1.1 to 1.5 at an ionic strength of 0.02 M with the order naphthyl > phenyl >/= cyclohexyl for the substituents. Stereoselectivity decreased with increasing ionic strengths. The steric bulk of the substituents of viologen may induce the conformational change of zinc myoglobin more effectively due to the steric repulsion between naphthyl groups and the polypeptide chain of zinc myoglobin.