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Energy‐Minimized Structures and Calculated and Experimental Isomer distributions in the hexaamine‐cobalt(III) system [Co(L) 2 ] 3+ with the chiral facially‐coordinating triamine (l = butane‐1,2,4‐triamine)
Author(s) -
Comba Peter,
Maeder Marcel,
Zipper Luc
Publication year - 1989
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19890720520
Subject(s) - chemistry , butane , conformational isomerism , ligand (biochemistry) , chiral ligand , stereochemistry , computational chemistry , organic chemistry , enantioselective synthesis , molecule , biochemistry , receptor , catalysis
Butane‐ 1,2,4‐triamine (trab) is the smallest tridentate aliphatic unsubstituted chiral triamine. With optically pure trab, there are three, with racemic trab five isomers of [Co(trab) 2 ] 3+ , One of the five isomers is centrosymmetrical, the others are chiral. For one of the isomers, there are four possible conformations (all combinations of chair and skew boat conformations for the chelate six ring of each ligand), for the others there exist only three independent conformers. All sixteen independent structures have been calculated by strain‐energy minimization. The calculated isomer distribution, based on total strain energies corrected with statistical entropy contributions (21%:16%:16%:4%:43%, and 40%:30%:30%, for racemic and optically pure trab, respectively) are in excellent agreement with the experimental data based on HPLC and 13 C‐NMR analyses of equilibrium solutions of the hexaamine‐Co(III) compounds prepared by oxygenation of aqueous solutions in presence of activated charcoal. The results are also briefly discussed in relation to possible stereoselectivity upon complexation of optically pure trab and a racemic chiral ligand to a transition‐metal center.