Conformational studies by dynamic NMR. 94.1 cogwheel pathway for the stereomutations of durene derivatives containing the mesityl ring

The low-temperature NMR spectra of 1,4-bis(mesitoyl)durene, 1, and of 1,4-bis(mesitylethenyl)durene, 2, reveal the presence of syn and anti rotamers at the equilibrium, their relative proportions depending on the dielectric constant of the solvent. In solution the more stable rotamer of 1 is the anti whereas, in the case of 2, the more stable is the syn. Depending on the crystallization solvent employed the more (anti) and the less stable (syn) rotamers were both observed (X-ray diffraction) in the solid state of 1. On the other hand, only the less stable rotamer (anti) was found to be present in the solid state of 2. As shown by MM calculations, the syn-to-anti interconversion occurs via a correlated process (cogwheel pathway) involving the mesityl-C and durene-C bond rotations: the dynamic NMR technique yields an experimental barrier of 8.2 kcal mol(-)(1) for 1 and 13.1 kcal mol(-)(1) for 2. In the case of derivative 2 a second barrier, due to a second type of correlated rotation process (torsion), was also determined (8.6 kcal mol(-)(1)). As a consequence of the restriction of this second torsional motion the anti rotamer of 2 displays two distinguishable NMR spectra at -133 degrees C, corresponding to a pair of conformers with different symmetry (anti C(i)() and anti C(2)).     

Quantum mechanical calculations of conformationally relevant 1H and 13C NMR chemical shifts of calixarene systems

[graphs: see text] QM GIAO calculations of 13C and 1H chemical shift values of the ArCH2Ar group have been performed, using the hybrid DFT functional MPW1PW91 and the 6-31G(d,p) basis set, on some representative calixarenes and on a series of simplified calixarene models allowing derivation of chemical shift surfaces versus phi and chi dihedral angles. A good reproduction of experimental data was obtained. The applicability of chemical shift surfaces in the study of calixarene conformational features is illustrated.     

Dynamic-mechanical properties and melting points of some aliphatic and partially aromatic polyamides

Some aliphatic and partially aromatic polyamides have been prepared from hexamethylene diamine and the following dicarboxylic acids: deca-, octa-, hexa-methylenedicarboxylic, p-carboxymethylphenoxyacetic, p-carboxyethylphenoxyacetic, p-phenylenedipropionic, p-phenylenediacetic, p-carboxymethoxyphenoxyacetic, β(p-carboxymethyl)phenylpropionic.The dynamic-mechanical properties at 110 Hz have been measured between ?140° and about 200. Three relaxation processes α β and γ have been found: only the main transition α appreciably depends on chemical structure.The influences of the length of repeating unit and of in-chain substitution on melting points, crystallinity and the dynamic-mechanical α transition have been investigated. The results have been discussed in terms of chain flexibility, chain packing and intermolecular forces.     

Mass fragmentation pattern of N- and O-substituted 2-morpholinols

The mass spectra of a series of N- and O-substituted 2-morpholinols were considered and the complex fragmentation pattern explained on the basis of evidences reported in the literature and of experimental data (high resolution, metastable ions). The primary fragmentations are given by inductive cleavage due to the heteroatoms, while ring contractions, through retro Diels-Alder reactions, form most of the secondary pattern with different ions related to N- and O-substituents. Moreover, hemiacetals in the tautomeric hydroxyaldehyde form undergo an α-cleavage.     

On the electronic states of macrocycles of the extended porphyrin family and their coordination compounds

The electronic absorption spectra of Ni, Zn and Mg hemiporphyrazine derivatives are presented and discussed together with theoretical results obtained by INDO/S computations. The absorption spectra of all the metal derivatives show marked red shifts of the lowest energy absorption bands with respect to those of the metal free hemiporphyrazine. The possible explanation that in metal derivatives low lying excited states with a fully conjugated π electron system are present is supported by theoretical computations.     

Wavelength-dependent electron and energy transfer pathways in a side-to-face ruthenium porphyrin/perylene bisimide assembly

A new side-to-face supramolecular array of chromophores, where a pyridyl-substituted perylene bisimide dye axially binds to two ruthenium porphyrin fragments, has been prepared by self-assembly. The array is formulated as DPyPBI[Ru(TPP)(CO)](2), where DPyPBI = N,N'-di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisimide and TPP = 5,10,15,20-tetraphenylporphyrin. The photophysical behavior of DPyPBI[Ru(TPP)(CO)](2) has been studied by fast (nanoseconds) and ultrafast (femtoseconds) time-resolved techniques. The observed behavior sharply changes with excitation wavelength, depending on whether the DPyPBI or Ru(TPP)(CO) units are excited. After DPyPBI excitation, the strong fluorescence typical of this unit is completely quenched, and time-resolved spectroscopy reveals the occurrence of photoinduced electron transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 5.6 ps) followed by charge recombination (tau = 270 ps). Upon excitation of the Ru(TPP)(CO) fragments, on the other hand, ultrafast (tau < 1 ps) intersystem crossing is followed by triplet energy transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 720 ps). The perylene-based triplet state decays to the ground state on a longer time scale (tau = 9.8 micros). The photophysics of this supramolecular array provides remarkable examples of (i) wavelength-dependent behavior (a small change in excitation wavelength causes a sharp switch from electron to energy transfer) and (ii) intramolecular sensitization (the triplet state of the perylene bisimide, inaccessible in the free dye, is efficiently populated in the array).     

Observation of an Unusual Molecular Switching Device. The Position of One 1,2-Dimethylimidazole Switched "On" or "Off" the Rotation of the Other 1,2-Dimethylimidazole in cis,cis,cis-Ru(II)Cl(2)(Me(2)SO)(2)(1,2-dimethylimidazole)(2)

Some cis,cis,cis-RuX(2)(Me(2)SO)(2)(1,2-Me(2)Im)L complexes [L = 1,2-Me(2)Im (1,2-dimethylimidazole) or Me(3)Bzm (1,5,6-trimethylbenzimidazole), X = Cl or Br, and Me(2)SO = S-bonded DMSO] have been synthesized and their rotamers studied in CDCl(3). From 2D NMR data, cis,cis,cis-RuCl(2)(Me(2)SO)(2)(1,2-Me(2)Im)(Me(3)Bzm) has 1,2-Me(2)Im in position "a" (cis to both Me(2)SO's and cis to "b") and Me(3)Bzm in position "b" (trans to one Me(2)SO and cis to the other). There are two stable atropisomers [head-to-tail (HT, 84%) and head-to-head (HH, 16%), defining the aromatic H of Ru-N-C-H as head for both ligands]. Me(3)Bzm has the same orientation in both atropisomers. In this orientation, the unfavorable interligand steric interactions of Me(3)Bzm with the Me(2)SO and 1,2-Me(2)Im ligands appear to be countered by favorable electrostatic attraction between the delta+ N(2)CH moiety of Me(3)Bzm and the delta- cis Cl ligands. The 1,2-Me(2)Im lacks a delta+ N(2)CH group, and its orientation is dominated by steric effects of the 2-Me group. The NMR spectrum of cis,cis,cis-RuCl(2)(Me(2)SO)(2)(1,2-Me(2)Im)(2) is consistent with four rotamers in restricted rotation about both Ru-N bonds: two HH and two HT. 2D NMR techniques (NOESY and ROESY) afforded complete proton signal assignments. The ligand disposition could be assessed from the large chemical shift dispersion of some 1,2-Me(2)Im ligand signals (Delta 0.86-1.52 ppm) arising from cis-1,2-Me(2)Im shielding modulated by deshielding influences of the cis halides. The relative stability of the four rotamers correlates best with steric interactions between the 2-Me groups and the Me(2)SO ligands. The most favorable conformer (46%) is the HH rotamer with both 2-Me groups pointing away from the Me(2)SO ligands. The least favorable conformer (14%) was also HH, but the methyl groups in this case point toward the Me(2)SO ligands. In the HT conformers of intermediate stability ( approximately 20%), one 2-Me group is toward and the other is away from the Me(2)SO ligands. The exchange cross-peaks in the 2D spectra are unusually informative about the dynamic processes in solution; the spectra provide evidence that the rotamers interchange in a definite pattern of succession. Thus, all conceivable exchange pathways are not available. 1,2-Me(2)Im "b" can rotate regardless of the orientation of 1,2-Me(2)Im "a". 1,2-Me(2)Im "a" can rotate only when "b" has the orientation with its 2-Me group directed away from "a". Thus, 1,2-Me(2)Im "b" can switch 1,2-Me(2)Im "a" rotation on or off.     

Metal-induced assembling/disassembling of fluorescent naphthalenediimide derivatives signalled by excimer emission

The new quadridentate bischelating ligands 2 and 3 display in solution the typical absorption and emission properties expected for naphthalenediimide derivatives. Spectrophotometric studies show that systems 2 and 3 interact with Zn(II), Cd(II) and Cu(I) in CHCl3 or MeCN according to an apparent 1:1 stoichiometry. Molecular modelling, ESI-MS and 1H NMR experiments indicate that the complex species formed in the presence of stoichiometric amounts of metal ion are nonhelical [2 + 2] adducts. The metal-induced self-assembling process is signalled by an intense excimer-type emission caused by the intramolecular interaction of two naphthalenediimide subunits that face each other in the [2 + 2] adduct, as shown by molecular modelling studies. In the presence of excess metal ion, a disassembling process takes place, leading to a dinuclear complex with a 2:1 metal/ligand stoichiometry, in which the intramolecular excimer is no longer allowed to form because the interaction between the naphthalenediimide subunits has been lost. Thus, the overall metal-induced assembling/disassembling process is signalled by the appearance and disappearance of the excimer band in the emission spectrum.     

Electrochemically switchable hydrogen-bonded molecular shuttles

A series of [2]rotaxanes containing succinamide and naphthalimide hydrogen-bonding stations for a benzylic amide macrocycle is described. Electrochemical reduction and oxidation of the naphthalimide group alters its ability to form hydrogen bonds to the macrocycle to such a degree that redox processes can be used to switch the relative macrocycle-binding affinities of the two stations in a rotaxane by over 8 orders of magnitude. The structure of the neutral [2]rotaxane in solution is established by (1)H NMR spectroscopy and shows that the macrocycle exhibits remarkable positional integrity for the succinamide station in a variety of solvents. Cyclic voltammetry experiments allow the simultaneous stimulation and observation of a redox-induced dynamic process in the rotaxane which is both reversible and cyclable. Model compounds in which various conformational and co-conformational changes are prohibited demonstrate unequivocally that the redox response is the result of shuttling of the macrocycle between the two stations. At room temperature in tetrahydrofuran the electrochemically induced movement of the macrocycle between the two stations takes approximately 50 micros.     

Functionalization of montmorillonite by acrylamide polymers containing side-chain ammonium cations

Stable polyacrylamide (polyAA)–montmorillonite adducts were prepared by two distinct processes: (1) free radical copolymerization of AA with alkaline clay previously treated with 2-(N-methyl-N,N-diethyl ammonium iodide)ethylacrylate (QD1) and (2) direct interactions of alkaline montmorillonite with various preformed copolymers of AA with QD1. The structure of the adducts as determined by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry and X-ray diffraction was shown to consist of AA macromolecules inserted between lamellar layers whose spacing was larger than in the polymer-free clay. The polymer was strongly attached to the inorganic surfaces, probably due to cooperative formation of electrostatic bonding. The thermal stability of the organic polymers in the resulting complexes was substantially enhanced while the mobility of macromolecules decreased.