We report the synthesis of chiral tetra-armed cyclens, having an asymmetric center at one side-arm, that show selective chiral enhancement with silver(I). When the chiral ligand forms a silver(I) complex, the side-arms cover the silver(I) incorporated into the cyclen. The asymmetric center controls the conformation of the side-arms in one direction, increasing the molar ellipticity of the CD spectrum. Chiral cyclens substituted with biphenyl groups exhibit large molar ellipticity values with strong exciton coupling effects.
Structures of LiSCN, NaSCN, KSCN, RbSCN, and CsSCN complexes with 3',5'-difluoro-4'-hydroxybenzyl-armed monoaza-15-crown-5 ether (5) were investigated. The Li+ and Na+ complexes are (1:1)n polymer-like complexes bridged by hydrogen bonding. On the other hand, the K+, Rb+, and Cs+ complexes are polymer-like complexes bridged by the fluorine atoms of the side arms. The titration calorimetry and 19F NMR titration experiments suggest that one or both fluorine atoms along with the oxygen atom of the phenolic OH group coordinate to the alkali metal ions incorporated in the crown part of a second armed ligand to give polymer-like complexes in solution. The FAB-MS data indicated that larger alkali metal ions form more stable polymer-like complexes.
The
proton signals at the 2′- and 6′-positions of
the aromatic side arms of a silver(I) complex with a chiral tetra-armed
cyclen ((S)-L2) are assigned by comparison
with 1H NMR spectra and X-ray structure of Ag+ complexes with three analogues of (S)-L2: (S)-L2
3D
having
one benzyl group and three deuterium-substituted benzyl groups, (S)-L2
2D
having two
benzyl groups and two deuterium-substituted benzyl groups, and (S)-L2
F
having three
benzyl groups and one 4′-fluorobenzyl group. An interaction
factor is defined by using the X-ray structures. The chemical shift
values of the proton signals in the aromatic side arms can be explained
by using the interaction factors that are defined by the dihedral
angles, the angles between two mean planes, and the CH···π
distances from the X-ray data of the (S)-L2/Ag+ complex.
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