In Situ Assembly of Octahedral
Fe(II) Complexes for
the Enantiomeric Excess Determination of Chiral Amines Using Circular
Dichroism Spectroscopy
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Abstract
A method for discriminating between α-chiral primary
amine
enantiomers is reported. The method utilizes circular dichroism (CD)
spectroscopy and a sensing ensemble composed of 2-formyl-3-hydroxypyridine
(<b>4</b>) and Fe(II)(TfO)<sub>2</sub>. Aldehyde <b>4</b> reacts rapidly with chiral amines to form chiral imines, which complex
Fe(II) to form a series of diastereomeric octahedral complexes that
are CD-active in both the UV and visible regions of the spectrum.
NMR studies showed that for enantiomerically pure imine complexes,
the Δ-<i>fac</i> isomer is preferred. A statistical
analysis of the distribution of stereoisomers accurately modeled the
calibration curves for enantiomeric excess (ee). CD signals appearing
in the UV region were bisignate, and the nulls of the CD signals were
coincident with maxima in the UV spectrum, consistent with exciton
coupling. Time-dependent density functional theory and semiempirical
calculations confirmed that the CD signals in the UV region arise
from coupling of the π–π* transitions in the imine
chromophores and that they can be used to describe the signs and magnitudes
of the curves accurately. The CD signals in the visible region arise
from metal-to-ligand charge-transfer bands, and these signals can
be used to determine the ee values of chiral amines with an average
absolute error of ±5%. Overall, the strategy presented herein
represents a facile in situ assembly process that uses commercially
available simple reagents to create large optical signals indicative
of ee values