Synthesis, acidity and 19F NMR characteristics of imidazoles bearing 1-fluorinated substituents with potential application as probes for intracellular pH determination

Synthesis, Characterization, and Spectroscopic Studies of 2,6-Dimethylpyridyl-Linked Cu(I)-CNC Complexes: The Electronic Influence of Aryl Wingtips on Copper Centers

Six new Cu(I) complexes containing pincer ligands of the type 2,6-bis(3-alkyl/arylimidazol-2-ylidene) methylpyridine I(R/R'Ar) ĈN̂C, where R = trifluoroethyl (TFE) and R' = 4-CF3, 4-NO2, 4-CN, 4-H, and 4-CH3, have been synthesized. These complexes, namely, [Cu(I(TFE)ĈN̂C)]PF6, 1-TFE; [Cu(ICF3Ar ĈN̂C]PF6, 2-CF3; [Cu(INO2Ar ĈN̂C)]PF6, 3-NO2; [Cu(ICNAr ĈN̂C]PF6, 4-CN; [Cu(IHAr ĈN̂C)]2(PF6)2, 5-H; and [Cu(ICH3Ar ĈN̂C)]2(PF6)2, 6-CH3, were fully characterized by 1H, 13C, and HMBC NMR spectroscopy, elemental analysis, electrochemical studies, and single-crystal X-ray crystallography. The crystallographic data revealed different structures and copper nuclearities for the complexes bearing aryl wingtips with electron-withdrawing (2-CF3, 3-NO2, and 4-CN) and electron-donating (5-H and 6-CH3) substituents. The solution-phase conductivity measurements in acetonitrile revealed a mix-electrolyte behavior for these complexes, supporting the presence of both mono- and binuclear forms of each complex. The fast monomer-dimer equilibrium of the Cu-CNC complexes at room temperature is reflected in their simple 1H NMR spectra in acetonitrile. However, both mono- and binuclear forms were identifiable in 1H diffusion-ordered spectroscopy (DOSY) at low temperatures. The dynamic behavior of these complexes in solution was further examined by variable-temperature 1H NMR (VT 1H NMR) experiments, and the relevant thermodynamic parameters were determined. The process was also probed by one-dimensional rotating-frame Overhauser enhancement spectroscopy (1D ROESY) experiments to elucidate the coexisting species in solution. The 2,6-dimethylpyridyl-linked Cu-CNC complexes also presented a quasi-reversible Cu(II)/Cu(I) couple in cyclic voltammetry studies, wherein a clear influence of the aryl wingtips on the E1/2 values was observed. Furthermore, the percent buried volumes (% Vbur) of the complexes were calculated, showing a similar steric hindrance around copper in all complexes. These findings support the importance of electronic effects, induced by the aryl wingtips, on the preferred coordination geometry, copper nuclearity, and redox properties of the Cu-CNC complexes.

Acidity of Frozen Electrolyte Solutions

Ice is selectively intolerant to impurities. A preponderance of implanted anions or cations generates electrical imbalances in ice grown from electrolyte solutions. Since the excess charges are ultimately neutralized via interfacial (H(+)/HO(-)) transport, the acidity of the unfrozen portion can change significantly and permanently. This insufficiently recognized phenomenon should critically affect rates and equilibria in frozen media. Here we report the effective (19)F NMR chemical shift of 3-fluorobenzoic acid as in situ probe of the acidity of extensively frozen electrolyte solutions. The sign and magnitude of the acidity changes associated with freezing are largely determined by specific ion combinations, but depend also on solute concentration and/or the extent of supercooling. NaCl solutions become more basic, those of (NH(4))(2)SO(4) or Na(2)SO(4) become more acidic, while solutions of the 2-(N-morpholino)ethanesulfonic acid zwitterion barely change their acidity upon freezing. We discuss how acidity scales based on solid-state NMR measurements could be used to assess the degree of ionization of weak acids and bases in frozen media.

Imidazole alkaloids from Lepidium meyenii

Two new imidazole alkaloids (lepidiline A and lepidiline B) have been isolated from a root extract of Lepidium meyenii with the common name Maca and identified as 1,3-dibenzyl-4,5-dimethylimidazolium chloride (1) and 1,3-dibenzyl-2,4,5-trimethylimidazolium chloride (2), respectively. The structures of these two new compounds were determined by spectroscopic methods, as well as single-crystal X-ray diffraction performed on compound 1.