Investigation of the stereodynamics of tris‐(α‐diimine)–transition metal complexes by enantioselective dynamic MEKC

Abstract

Enantiomerization of octahedral tris(alpha-diimine)-transition metal complexes was investigated by enantioselective dynamic MEKC. Varying both the transition metal ion (Fe(2+), Fe(3+), and Ni(2+)) and the bidentate diimine ligand (1,10-phenanthroline and 2,2'-bipyridyl), the enantiomer separations were performed either in a 100 mM sodium tetraborate buffer (pH 9.3) or in a 100 mM sodium tetraborate/sodium dihydrogenphosphate buffer (pH 8.0) both containing sodium cholate as chiral surfactant. The unified equation of dynamic chromatography was employed to determine apparent reaction rate constants from the electropherograms showing distinct plateau formation. Apparent activation parameters DeltaH(double dagger) and DeltaS(double dagger) were calculated from temperature-dependent measurements between 10.0 and 35.0 degrees C in 2.5 K steps. It was found that the nature of the central metal ion and the ligand strongly influence the enantiomerization barrier. Surprisingly, complexes containing the 2,2'-bipyridyl ligand show highly negative activation entropies between -103 and -116 J (K mol)(-1) while the activation entropy of tris(1,10-phenanthroline) complexes is positive indicating a different mechanism of interconversion. Furthermore, it was found that the Ni(2+) complexes are stereostable under the conditions investigated here making them a lucent target as enantioselective catalysts.

Controlling the Transmission of Stereochemical Information through Space in Terphenyl-Edged Fe4L6 Cages

“…The mechanism of racemization of Fe(diimine) 3 centers has been reported to involve a combination of predominantly intramolecular (Rây-Dutt twist or Bailar twist) and minor intermolecular (dissociative) rearrangements. The interconversion barriers observed in the present systems are consistent with these prior reports, suggesting a predominantly intramolecular isomerization process. ,− The molar percentages of each diastereomer for each cage at equilibrium were calculated and compared at different temperatures. As the temperature was increased, the population of 5- T decreased gradually as the populations of both 5- C 3 and 5- S 4 were observed to increase (Figure ); in contrast, no significant changes in diastereomer populations were observed for cages 1 , 6 , and 7 .…”

Organic-soluble optically pure anionic metal complexes PPh4[MIII(S,S-EDDS)]·2H2O (M = Fe, Co, Cr)

“…Analogous transition metal systems, such as D 3 tris(oxalato) complexes generally suffer from lability at the metal, leading to racemisation and limiting their application. [18][19][20][21] The diastereomeric purity of homochiral A key structural difference between EDDS and the widely used chelating agent ethylenediaminetetraacetic acid (EDTA) is the presence of two stereogenic carbon atoms in the former. As a result, three possible isomers exist; S,S-, R,R-and meso R,S-EDDS.…”

Element Speciation Analysis Using Capillary Electrophoresis: Twenty Years of Development and Applications

“…In the case of hydrophobic or zwitterionic analytes, micellar, ,,,,, nonaqueous, ,, or cyclodextrin-modified , electrolytes can all be utilized for improving separability. Research has also been devoted to the enantioselective separation of metal–ligand complexes using electrolytes containing chiral additives, such as various tartrate derivatives, cyclodextrins, synthetic DNA polynucleotide, sodium cholate or other bile salt surfactants . Other examples are based on a cationic polymer or a microemulsion .…”