Characterization of HgCl2 Tridentate Amine Complexes by X-ray Crystallography, NMR and ESI-MS

Two new HgCl2 complexes of tridentate nitrogen ligands were characterized by X-ray crystallography, proton NMR spectroscopy and ESI-MS. The five-coordinate complex [Hg(BMPA)Cl-2] (1) (BMPA = bis(2-pyridylmethyl)amine) crystallized from acetonitrile/m-xylene by slow evaporation in the monoclinic space group P2(1)/n with a = 8.3896(8) , b = 12.8020(13) , c = 13.3526(13) , alpha = 90A degrees, beta A = 90.480(2)A degrees, gamma A = 90A degrees and z = 4. The square pyramidal structure (tau = 0.009) has approximate C (s) symmetry. Despite comparable Hg-N bond lengths in 1, inversion of the central nitrogen was rapid on the chemical shift time scale in dilute solution except at very low temperatures. The related complex [Hg(BEPA)Cl-2] (2) (BEPA = bis(2-{pyrid-2-yl}ethyl)amine) crystallized from acetonitrile/ethyl acetate/hexanes by slow diffusion in the orthorhombic space group Pnma with a = 13.424(3) , b = 14.854(3) , c = 8.118(2) , alpha = 90A degrees, beta A = 90A degrees, gamma A = 90A degrees and z = 4. The mixed geometry structure (tau = 0.56) also has crystallographic mirror symmetry as well as C (s) point group symmetry. In dilute acetonitrile solution, 1 was stable while 2 slowly converted to a more thermodynamically stable complex.

Understanding Bile Micelle Chiral Interactions

Bile salts are known to aggregate into micelles in biological systems; however, the fundamental structure and dynamics of bile molecule micelle formation are poorly understood. Previous studies have established that the bile salt cholate is capable of performing chirally selective micellar electrokinetic capillary chromatography (MEKC) separations of model racemic binaphthyl compounds 1,1¿-binaphthyl-2,2¿-diyl hydrogen phosphate (R,S-BNDHP) and 1,1¿-bi-2-naphthol (R,S-BN). Nuclear magnetic resonance (NMR) has been established as a complementary technique for understanding chiral selectivity and micelle formation events based on changes in proton chemical shifts of the probe molecules BNDHP and BN as well as of cholate. This work investigated the effects of the probe molecule, the alkali cation identity and temperature on cholate micelle aggregation and MEKC separations of R,S-BN and R,S-BNDHP. The probe molecule was found to mediate micelle formation by MEKC and proton NMR. A low (0.1 mM) concentration of probe was found to have minimal effects on micellization events detected by proton NMR while higher probe concentration (2.5 mM) was found to mediate micellization causing micellization events to occur at lower cholate concentrations. This work also investigated the effects of alkali counterion on chiral separation. Generally, counterions with larger crystal cationic radius were found to cause greater chiral separation power. NMR data suggest that protons near the surface of the cholate micelle are most sensitive to the cation identity, suggesting a model of improved separation based on the cation sterically inhibiting binding of one isomer. Finally, the effect of temperature on MEKC separation was investigated. Separation power of R,S-BN and R,S-BNDHP appeared to increase linearly with temperature for 22.0 mM to 50.0 mM pH 12.0 cholate. In total, these results indicate that cholate aggregation is dependent on multiple conditions. Understanding the roles that these factors play in influencing cholate micellization can inform better separation in MEKC.