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Long‐Range Stereocontrol in the Self‐Assembly of Two‐Nanometer‐Dimensioned Triple‐Stranded Dinuclear Helicates
Author(s) -
Albrecht Markus,
Janser Ingo,
Houjou Hirohiko,
Fröhlich Roland
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305602
Subject(s) - ligand (biochemistry) , racemization , chemistry , crystallography , stereochemistry , selectivity , receptor , catalysis , biochemistry
A series of bisimine‐bridged dicatechol ligands 2‐H 4 – 5‐H 4 were synthesized and were used to prepare triple‐stranded dinuclear helicate‐type complexes with a length of up to more than 2 nm. X‐ray structural analyses of Na 4 [( 2 ) 3 V 2 ], Na 4 [( 3 ) 3 Ti 2 ], Na 4 [( 4 ) 3 Ti 2 ], and Na 4 [( 5 ) 3 Ti 2 ], as well as temperature‐dependent NMR investigations of Na 4 [( 4 ) 3 Ti 2 ] and Na 4 [( 5 ) 3 Ti 2 ] show that, in the case of the rigid linear ligands 2 and 3 , and of the ligand 5 , which possesses C 2 h symmetry in its idealized structure, homochiral helicates are diastereoselectively formed. Ligand 4 , on the other hand, with idealized C 2 v symmetry, leads with surprisingly high selectivity to the formation of the heterochiral meso ‐helicate. This is attributed to the ability of ligand 4 to adopt a less‐restricted conformation in the meso compound than in the helical complex. NMR investigations indicate that both complex units of Na 4 [( 4 ) 3 Ti 2 ] invert (ΛΔ→ΔΛ) simultaneously, while in the case of Na 4 [( 5 ) 3 Ti 2 ] a stepwise racemization proceeds.