Equilibrium studies on the protonation and Cu(II) complexation by an hexaaza macrocycle containing p -xylyl spacers. The crystal structure of the hexaprotonated ligand and the kinetics of decomposition of the Cu(II) complexes
Abstract The protonation constants of the ligand 3,6,9,16,19,22-hexaazatricyclo[22.2.2.211,14]triaconta-1(26),11(12),13,24,27, 29-hexaene (L, BPXD) have been determined in water at 25°C by potentiometric procedures and indicate a lower basicity than that corresponding to the analogue ligand with m-xylyl spacers between the diethylenetriamine subunits (BMXD). The crystal structure of a salt of H6L6+ containing Br− and NO3 − as anions has been solved by X-ray diffraction procedures and reveals that the macrocycle adopts an almost planar configuration. The protonated amine groups are involved in a complex network of hydrogen bonds with the anions and water molecules, with two anions being placed close to the cavity of the macrocycle. Despite the lower basicity of BPXD, the stability of the mononuclear CuL2+ and HCuL3+ complexes is several log units higher than that corresponding to the analogous BMXD complexes, which suggests the possibility of a different coordination mode for both closely related ligands. Actually, the whole set of stability constants for the mono and binuclear Cu(II)–L complexes can be rationalised by considering that the macrocycle acts as tetradentate in the mononuclear species. In contrast, the kinetic parameters for decomposition of the mono and binuclear Cu(II)–L complexes in acid solutions are only slightly different from those previously determined for the BMXD complexes.
Basicity properties of a novel azaparacyclophane receptor and its acyclic precursor: a thermodynamic and structural approach
The protonation behaviour of the aza-p-cyclophane receptor 1,4,7,16,19,22-hexamethyl-1,4,7,16,19,22-hexaaza[9.9]paracyclophane (L1) and its acyclic precursor 4,4′-N-methyliminobis(ethylene-N-methyliminomethylene)dibenzoic acid (H2L2) has been studied in aqueous solution by means of potentiometric, calorimetric and 1H and 13C NMR techniques. L1 behaves as a pentaprotic base. NMR experiments have allowed the determination of the stepwise protonation sites. Considering the [H4L1]4+ species, the acidic protons occupy alternate positions in the macrocycle, separated by an unprotonated amino group. The crystal structure of [H4L1](ClO4)4(space group Pbca, a= 16.103(6), b= 22.34(2), c= 23.625(8)A; V= 8499(9)A3, Z= 8, R= 0.0637) confirms the NMR data, showing the four protons located on the amino groups adjacent to the aromatic rings. Two ClO4– anions interact, via hydrogen bonds, with the protonated nitrogens of the macrocycle. L1 is characterized by two N3 binding subunits, each of them interacting with one perchlorate ion. As far as H2L2 is concerned, calorimetric and NMR data allow the proton sites to be determined. In the H2L2 species two protons are located on two amino groups, while the two carboxylate groups are unprotonated, giving rise to an ionic structure.
Identification of Dihydroxy Aromatic Compounds in a Low-Temperature Pyrolysis Coal Tar by Gas Chromatography−Mass Spectrometry (GC−MS) and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS)
Dihydroxy aromatics in coal tar undergo rapid coupling reactions with each other and with some coal-derived species during coal pyrolysis and liquefaction. However, only limited information on the structure of dihydroxy aromatics is known. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and gas chromatography−mass spectrometry (GC−MS) were used to characterize the dihydroxy aromatics in a low-temperature pyrolyzed coal tar and its distillate and residue fractions. Negative-ion electrospray ionization (ESI) FT-ICR MS spectra of coal-tar-derived samples had low molecular masses and narrow mass ranges. Negative-ion ESI is extremely selective for the dihydroxy aromatics present in coal tar. The O2 class species had the highest ion intensity among the species in the negative-ion FT-ICR mass spectrum for each sample. Dihydroxy compounds with an aromatic core structure of benzene, indan, biphenyl (and/or acenaphthene), and naphthalene were identified in the distillate fract...